Breast cancer metastasis to the bone: mechanisms of bone loss

The active role of osteoporosis in the interaction between osteoblasts and bone metastases

INTRODUCTION

To minimize the severity of bone metastases and to delay their onset, it is important to analyze the underlying biological mechanisms. The present study focused on the link between OP and metastatic cells, with particular attention to osteoblast behavior.

METHODS

Osteoblasts (OB) were isolated from the trabecular bone of iliac crest of healthy (SHAM) and ovariectomized (OVX) adult female rats and co-cultured with MRMT-1 rat breast carcinoma cells as conditioned medium (CM) or alone (CTR) for 24h, 7 and 14 days and tested for cell viability, morphology and synthetic activity, i.e. C-terminal procollagen type I, alkaline phosphatase, osteoprotegerin, receptor activator for nuclear factor KB ligand and interleukin-8.

RESULTS

Osteoblast morphology showed a reduced organization in the OVX group, in particular in the CM condition. Conversely, the analysis of cell viability revealed significantly higher values in the OVXCM group with respect to the SHAMCM group at all experimental times, whereas the OVXCTR group had significantly lower values at 7 and 14 days in comparison to those of the SHAM group. ALP release was significantly lower in the CM condition than that of CTR at all timepoints, and so was procollagen type I at 7 and 14 days. The RANKL/OPG ratio showed significantly higher values in OVX osteoblasts in comparison with those of the SHAM group, both in CTR and in CM conditions at each experimental time. Finally, OVXCM showed significantly higher values of IL-8 than those of SHAMCM at 7 and 14 days.

CONCLUSIONS

The results clearly indicate an influence of the metastatic cells on the osteoblastic physiology at different levels: morphology, viability, release of typical proteins, and also IL-8 as a proinflammatory cytokine, especially marked by osteoporosis. Further investigations might highlight the relationship between osteoblasts and breast cancer cells, which might be useful to improve common drugs used against osteoporosis and bone metastases, by enhancing the bone deposition/tumor progression ratio.

The effect of zinc and phytoestrogen supplementation on the changes in mineral content of the femur of rats with chemically induced mammary carcinogenesis

The aim of this study was to assess skeletal effects of zinc or zinc with phytoestrogen (resveratrol or genistein) supplementation in an animal model of rats with DMBA-induced mammary carcinogenesis. The changes in bone parameters such as the length and mass were examined, as well as the changes in concentrations of selected minerals: calcium, magnesium, zinc, iron and phosphorus. Moreover, the investigations focused on finding the differences between the levels of iron and zinc in other tissues: the liver, spleen and serum of the examined rats. Fifty-six female Sprague-Dawley rats, 40 days old, were divided into four groups, regardless of the diets: standard (77mg Zn kg/food), zinc (4.6mg/mL via gavage), zinc (4.6mg/mL) plus resveratrol (0.2mg/kgbw), and zinc (4.6mg/mL) plus genistein (0.2mg/kgbw) for a period from 40 days until 20 weeks of age. The study rats were also treated with 7,12-dimethyl-1,2-benz[a]anthracene (DMBA) to induce mammary carcinogenesis. The applied diet and the advanced mammary cancer did not affect macrometric parameters of the rats' bones, but they strongly affected their mineral content. It was found that mammary cancer, irrespectively of the applied diet, significantly modified the iron level in the femur, liver, spleen and serum of the examined rats. In addition, zinc supplementation significantly lowered the levels of calcium, magnesium and phosphorus in the femur of rats with mammary cancer as compared with respective levels in the control group. So, it was found that additional supplementation with zinc, which is generally considered to be an antioxidant, with the co-existing mammary carcinoma, increased the unfavorable changes as concerns the stability of bone tissue. The appropriate combination of zinc and phytoestrogens (resveratrol or genistein) could help prevent or slow bone loss associated with a range of skeletal disorders in breast cancer.

ODAM inhibits RhoA-dependent invasion in breast cancer

Odontogenic ameloblast-associated protein (ODAM) contributes to cell adhesion. In human cancer, ODAM is down-regulated, and the overexpression of ODAM results in a favourable prognosis; however, the molecular mechanisms underlying ODAM-mediated inhibition of cancer invasion and metastasis remain unclear. Here, we identify a critical role for ODAM in inducing cancer cell adhesion. ODAM induced RhoA activity and the expression of downstream factors, including Rho-associated kinase (ROCK). ODAM-mediated RhoA signalling resulted in actin filament rearrangement by activating PTEN and inhibiting the phosphorylation of AKT. When ODAM is overexpressed in MCF7 breast cancer cells and AGS gastric cancer cells that activate RhoA at high levels, it decreases motility, increases adhesion and inhibits the metastasis of MCF7 cells. Conversely, depletion of ODAM in cancer cells inhibits Rho GTPase activation, resulting in increased cancer migration and invasion. These results suggest that ODAM expression in cells maintains their adhesion, resulting in the prevention of their metastasis via the regulation of RhoA signalling in breast cancer cells. SIGNIFICANCE Breast cancer represents the first most frequent cancer, and the ratio of mortality is high in women. Of utmost importance for reducing risk by breast cancer are their anti-invasion mechanisms, particularly in the non-invasive cancer cells because metastasis is the principal cause of death among cancer patients. ODAM induced RhoA activity. ODAM-mediated RhoA signalling resulted in actin filament rearrangement, increased cell adhesion and inhibited the migration/invasion of MCF7 cells. These results suggest that ODAM expression maintains their adhesion, resulting in the prevention of their metastasis via the regulation of RhoA signalling in breast cancer cells.

Eph receptor and ephrin function in breast, gut, and skin epithelia

Epithelial cells are tightly coupled together through specialized intercellular junctions, including adherens junctions, desmosomes, tight junctions, and gap junctions. A growing body of evidence suggests epithelial cells also directly exchange information at cell-cell contacts via the Eph family of receptor tyrosine kinases and their membrane-associated ephrin ligands. Ligand-dependent and -independent signaling via Eph receptors as well as reverse signaling through ephrins impact epithelial tissue homeostasis by organizing stem cell compartments and regulating cell proliferation, migration, adhesion, differentiation, and survival. This review focuses on breast, gut, and skin epithelia as representative examples for how Eph receptors and ephrins modulate diverse epithelial cell responses in a context-dependent manner. Abnormal Eph receptor and ephrin signaling is implicated in a variety of epithelial diseases raising the intriguing possibility that this cell-cell communication pathway can be therapeutically harnessed to normalize epithelial function in pathological settings like cancer or chronic inflammation.

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.

Transcriptional regulation of tenascin-W by TGF-beta signaling in the bone metastatic niche of breast cancer cells

Tenascin‐W is a matricellular protein with a dynamically changing expression pattern in development and disease. In adults, tenascin‐W is mostly restricted to stem cell niches, and is also expressed in the stroma of solid cancers. Here, we analyzed its expression in the bone microenvironment of breast cancer metastasis. Osteoblasts were isolated from tumor‐free or tumor‐bearing bones of mice injected with MDA‐MB231‐1833 breast cancer cells. We found a fourfold upregulation of tenascin‐W in the osteoblast population of tumor‐bearing mice compared to healthy mice, indicating that tenascin‐W is supplied by the bone metastatic niche. Transwell and co‐culture studies showed that human bone marrow stromal cells (BMSCs) express tenascin‐W protein after exposure to factors secreted by MDA‐MB231‐1833 breast cancer cells. To study tenascin‐W gene regulation, we identified and analyzed the tenascin‐W promoter as well as three evolutionary conserved regions in the first intron. 5′RACE analysis of mRNA from human breast cancer, glioblastoma and bone tissue showed a single tenascin‐W transcript with a transcription start site at a noncoding first exon followed by exon 2 containing the ATG translation start. Site‐directed mutagenesis of a SMAD4‐binding element in proximity of the TATA box strongly impaired promoter activity. TGFβ1 induced tenascin‐W expression in human BMSCs through activation of the TGFβ1 receptor ALK5, while glucocorticoids were inhibitory. Our experiments show that tenascin‐W acts as a niche component for breast cancer metastasis to bone by supporting cell migration and cell proliferation of the cancer cells.

What's new?

Once breast cancer metastasizes, it is generally incurable. Proteins in the extracellular matrix play a crucial role in launching the tumor cells to a new site. These authors investigated one such protein, tenascin‐W, which can be found surrounding not only tumor cells but also in bone tissue. Among other things, they studied how breast cancer cells affected tenascin‐W expression. The tumor cells induced bone marrow stromal cells to make more tenascin‐W, suggesting that the protein may pave the way for the cancer to spread to the bone.

Interpretation of immunohistochemistry data of tumor should consider microenvironmental factors

The influence of tumor surrounding microenvironment is often neglected when immunohistochemistry is performed to investigate tumor properties and search biomarkers of cancer. This study was designed to evaluate whether the influence of tumor microenvironment on biological features of tumor cells should be taken into account for interpretation of the immunohistochemistry data of tumor specimens. In this study, we showed an example by using three tumor cell lines (HeLa, WSU-HN6, and Tca83) to establish tumor-caused bone destruction models in nude mice and then to investigate the influence of bone marrow microenvironment (BMM) on biological features of tumor cells. Immunohistochemistry results showed that, compared with tumor cells located outside of BMM, tumor cells located inside of BMM presented huge differences in the expression of inflammation-related proteins including tumor necrosis factor-α (TNF-α), TNF receptor-associated factor protein-6 (TRAF-6), phosphorylated-NF-κB p65 (p-p65), interleukin (IL)-6 and IL-11, matrix metalloproteinases including MMP-1, MMP-2, MMP-9, and MMP-13; and osteogenesis-related proteins including runt-related transcription factor 2 (RUNX2), bone sialoprotein (BSP), and osteocalcin (OCN) in all the models. However, when we compared the cell line pair derived from different sites (outside and inside of BMM, respectively) of the same HeLa tumor sample by real-time PCR, Western blot, and immunocytochemistry, the differences aforementioned in tumor tissues were not found. In addition, we verified that normal human bone marrow could not cause the above changes detected in vivo. Our results suggested that tumor-modified microenvironment could give the new biological features of the invaded tumor cells. Therefore, we should consider the influence of the surrounding microenvironment on tumor cells when we analyze tumor properties using immunohistochemistry.

OPG-Fc inhibits ovariectomy-induced growth of disseminated breast cancer cells in bone

Dormant disseminated tumour cells can be detected in the bone marrow of breast cancer patients several years after resection of the primary tumour. The majority of these patients will remain asymptomatic, however, ∼ 15% will go on to develop overt bone metastases and this condition is currently incurable. The reason why these dormant cells are stimulated to proliferate and form bone tumours in some patients and not others remains to be elucidated. We have recently shown that in an in vivo model, increasing bone turnover by ovariectomy stimulated proliferation of disseminated tumour cells, resulting in formation of bone metastasis. We now show for the first time that osteoclast mediated mechanisms induce growth of tumours from dormant MDA-MB-231 cells disseminated in the bone. We also show that disruption of RANK-RANKL interactions following administration of OPG-Fc inhibits growth of these dormant tumour cells in vivo. Our data support early intervention with anti-resorptive therapy in a low-oestrogen environment to prevent development of bone metastases.

Wedelolactone inhibits breast cancer-induced osteoclastogenesis by decreasing Akt/mTOR signaling

The bone is the most common metastatic site of breast cancer. Bone metastasis causes pain, pathologic fractures, and severely reduces the quality of life. Breast cancer causes osteolytic bone metastasis, which is dependent on osteoclast-mediated bone resorption. While current treatments rely on palliative anti-resorptive agents, there is a need to develop a drug based on potential alternative therapies. This study is the first to determine that wedelolactone (WDL), a natural coumarin isolated from plants, can inhibit breast cancer-mediated osteoclastogenesis. Osteoclasts were generated from human CD14(+) monocytes cultured with M-CSF/RANKL and WDL suppressed human osteoclast differentiation and activity in vitro in a dose-dependent manner. Moreover, WDL inhibited the upregulation of osteoclasts stimulated by MDA‑MB‑231 breast cancer cells. The activity of WDL on osteoclasts and breast cancer-mediated osteoclastogenesis was associated with the inhibition of Akt/mammalian target of the rapamycin signaling pathway (mTOR). Blocking Akt and mTOR by specific inhibitors significantly decreased osteoclast differentiation and bone resorption. Furthermore, WDL regulated breast cancer-enhanced interaction of osteoblasts and osteoclasts by decreasing M-CSF expression in MDA‑MB‑231-stimulated osteoblasts. Thus, this study suggests that WDL may be a potential natural agent for preventing and treating bone destruction in patients with bone metastasis due to breast cancer.

Early growth response 4 is involved in cell proliferation of small cell lung cancer through transcriptional activation of its downstream genes

Small cell lung cancer (SCLC) is aggressive, with rapid growth and frequent bone metastasis; however, its detailed molecular mechanism remains poorly understood. Here, we report the critical role of early growth factor 4 (EGR4), a DNA-binding, zinc-finger transcription factor, in cell proliferation of SCLC. EGR4 overexpression in HEK293T cells conferred significant upregulation of specific splice variants of the parathyroid hormone-related protein (PTHrP) gene, resulting in enhancement of the secretion of PTHrP protein, a known mediator of osteolytic bone metastasis. More importantly, depletion of EGR4 expression by siRNA significantly suppressed growth of the SCLC cell lines, SBC-5, SBC-3 and NCI-H1048. On the other hand, introduction of EGR4 into NIH3T3 cells significantly enhanced cell growth. We identified four EGR4 target genes, SAMD5, RAB15, SYNPO and DLX5, which were the most significantly downregulated genes upon depletion of EGR4 expression in all of the SCLC cells examined, and demonstrated the direct recruitment of EGR4 to their promoters by ChIP and luciferase reporter analysis. Notably, knockdown of the expression of these genes by siRNA remarkably suppressed the growth of all the SCLC cells. Taken together, our findings suggest that EGR4 likely regulates the bone metastasis and proliferation of SCLC cells via transcriptional regulation of several target genes, and may therefore be a promising target for the development of anticancer drugs for SCLC patients.

The prognostic significance of RUNX2 and miR-10a/10b and their inter-relationship in breast cancer

Background

The major cancer related mortality is caused by metastasis and invasion. It is important to identify genes regulating metastasis and invasion in order to curtail metastatic spread of cancer cells.

Methods

This study investigated the association between RUNX2 and miR-10a/miR-10b and the risk of breast cancer relapse. Expression levels of RUNX2 and miR-10a/b in108 pairs of tumor and non-tumor tissue of breast cancer were assayed by quantitative PCR analysis and evaluated for their prognostic implications.

Results

The median expression levels of RUNX2 and miR-10b in tumor tissue normalized using adjacent non-tumor tissue were significantly higher in relapsed patients than in relapse-free patients. Higher expression of these three genes were significantly correlated with the hazard ratio for breast cancer recurrence (RUNX2: 3.02, 95% CI = 1.50 ~ 6.07; miR-10a: 2.31, 95% CI = 1.00 ~ 5.32; miR-10b: 3.96, 95% CI = 1.21 ~ 12.98). The joint effect of higher expression of all three genes was associated with a hazard ratio of 12.37 (95% CI = 1.62 ~ 94.55) for relapse. In a breast cancer cell line, RUNX2 silencing reduced the expression of miR-10a/b and also impaired cell motility, while RUNX2 overexpression elicited opposite effects.

Conclusions

These findings indicate that higher expression of RUNX2 and miR-10a/b was associated with adverse outcome of breast cancer. Expression levels of RUNX2 and miR-10a/b individually or jointly are potential prognostic factors for predicting breast cancer recurrence. Data from in vitro studies support the notion that RUNX2 promoted cell motility by upregulating miR-10a/b.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-014-0257-3) contains supplementary material, which is available to authorized users.

RANK, RANKL, and OPG in recurrent solid/multicystic ameloblastoma: their distribution patterns and biologic significance

OBJECTIVES

To determine the distribution patterns of bone resorption regulators, receptor activator of nuclear factor κ-B (RANK), RANK ligand (RANKL), and osteoprotegerin (OPG) in recurrent ameloblastoma (RAs) and to clarify their impact on the biologic behavior of these neoplasms.

MATERIALS AND METHODS

Fifteen paraffin-embedded RA cases were subjected to immunohistochemistry for expression of RANK, RANKL, and OPG.

RESULTS

The RANK-RANKL-OPG triad was heterogeneously detected in RA samples. RANK, essential for osteoclast differentiation, was strongly expressed in tumoral epithelium. Conversely, RANKL, an osteoclast activator, was markedly underexpressed, and protein localization was predominantly stromal. OPG, an osteoclastogenesis inhibitory factor, was detected in neoplastic epithelium more than in stroma, suggesting functional inactivation of RANKL. Most RA (n = 12/15; 80%) exhibited a bimolecular spatial expression pattern, the most common being RANK-positive/OPG-positive (n = 8/15; 53.3%). All three proteins showed no significant correlation with the clinical/histopathologic parameters in RA patients (P > .05).

CONCLUSIONS

The RANK(+)/RANKL(low/-)/OPG(+) phenotype observed in RA suggests an altered local bone metabolism characterized by low bone resorptive activity in these recurrent tumors.

Potential role of the OPG/RANK/RANKL axis in prostate cancer invasion and bone metastasis

Receptor activator of NF-κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) are key regulators of bone metabolism under both normal and pathological conditions, including prostate cancer (PCa) bone metastases. However, little is known concerning the expression and function of these regulators in prostate tumor samples and PCa cells and their correlation with invasion and bone metastasis. In the present study, we determined the expression of RANK, RANKL and OPG in 3 human PCa cell lines and 40 PCa patient samples by immunohistochemistry and enzyme-linked immunosorbent assay (ELISA). As controls, samples from 20 patients with benign prostate hyperplasia (BPH) and normal prostate epithelial RWPE2 cells were also included in the analyses. The effects of soluble RANKL (sRANKL) and OPG as well as RANK knockdown on PCa invasion were examined in Transwell assays. Immunohistochemical staining detected little RANK, OPG and RANKL expression in hyperplasia prostate while the percentages of positivity were increased to 50, 45 and 52.5%, respectively, in prostate tumor tissues. OPG and sRANKL levels in the prostate tumor samples as measured by ELISA were ~10-fold that in the BPHs (P<0.01) and the levels were higher in aggressive tumors than non-aggressive ones (P<0.05). The sRANKL level in the serum of PCa patients was the same as that in the patients with BPH, yet the serum OPG levels correlated with the tissue levels (R2=0.620, P<0.01, which both showed a 10-fold increase in PCa over BPH (P<0.01) with higher levels in aggressive PCa than non-aggressive ones (P<0.05). Consistent with the tissue analyses, expression levels of RANK mRNA and protein were detected in multiple human PCa cell lines by RT-PCR and western blotting, respectively. The treatment of PCa cells with RANKL significantly increased the number of invaded cells (P<0.01), which was suppressed by the decoy receptor OPG. RANK siRNA transfection dramatically dampened the stimulatory effect of RANKL on PCa cell invasion. Our findings indicate that the expression of RANK, RANKL and OPG may be used as diagnostic markers to identify patients at high risk for aggressive PCa and that the effective suppression of PCa cell migration by OPG via the blockage of RANKL activity represents a potential therapeutic strategy for interfering with prostate tumor metastasis and progression to bone.

The role of tight junctions in cancer metastasis

Over the last decade, it has become apparent that the tight junction (TJ) is a key component in tumour progression and metastasis. In addition to its role in the control of paracellular diffusion of ions and certain molecules, the TJ has a vital role in maintaining cell to cell adhesion and tissue integrity. Changes in the expression and/or distribution of TJ proteins can result in loss in cohesion of the TJ structure, which in turn results in the ability of cancer cells to become invasive and then ultimately lead to the metastasis of cancer cells. This review will discuss recent insights into how TJ are involved in the process of tumour metastasis.

BMP9 regulates cross-talk between breast cancer cells and bone marrow-derived mesenchymal stem cells

PURPOSE

Breast cancer cells frequently metastasize to distant organs, including bone. Interactions between breast cancer cells and the bone microenvironment are known to enhance tumor growth and osteolytic damage. Here we investigated whether BMP9 (a secretary protein) may change the bone microenvironment and, by doing so, regulate the cross-talk between breast cancer cells and bone marrow-derived mesenchymal stem cells.

METHODS

After establishing a co-culture system composed of MDA-MB-231 breast cancer cells and HS-5 bone marrow-derived mesenchymal stem cells, and exposure of this system to BMP9 conditioned media, we assessed putative changes in migration and invasion capacities of MDA-MB-231 cells and concomitant changes in osteogenic marker expression in HS-5 cells and metastases-related genes in MDA-MB-231 cells.

RESULTS

We found that BMP9 can inhibit the migration and invasion of MDA-MB-231 cells, and promote osteogenesis and proliferation of HS-5 cells, in the co-culture system. We also found that the BMP9-induced inhibition of migration and invasion of MDA-MB-231 cells may be caused by a decreased RANK ligand (RANKL) secretion by HS-5 cells, leading to a block in the AKT signaling pathway.

CONCLUSIONS

From our data we conclude that BMP9 inhibits the migration and invasion of breast cancer cells, and promotes the osteoblastic differentiation and proliferation of bone marrow-derived mesenchymal stem cells by regulating cross-talk between these two types of cells through the RANK/RANKL signaling axis.

CSF-1 blockade impairs breast cancer osteoclastogenic potential in co-culture systems

Metastatic bone disease has a major impact on the morbidity and mortality of breast cancer patients, and studies on bone metastasis biology have led to the development of the most widely used drugs for bone metastases treatment: zoledronate (Zol) and denosumab (Den). The aim of the present study was to assess the effect of soluble mediators produced by breast cancer cells on human osteoclast maturation in a co-culture model. We also tested the ability of zoledronate, denosumab and 5H4, an antibody directed against CSF-1, to interfere with the osteoclastogenic potential of breast cancer. The study was performed on the triple negative cell line MDA-MB-231 and on human osteoclasts obtained from the differentiation of peripheral blood monocytes of a healthy volunteer. Osteoclastogenesis was evaluated by TRAP assay after 14days of differentiation with 10% MDA-MB-231-conditioned media or with CSF-1 and RANKL. Den, Zol and 5H4 were administered after 7days of differentiation. MDA-MB-231-conditioned media doubled the differentiation of monocytes into osteoclasts. MDA-MB-231 secreted CSF-1, especially when cells were cultured to confluence. Induced osteoclasts were sensitive to bone-targeted drugs: Den and 5H4 blocked osteoclast differentiation and survival, while Zol induced osteoclast apoptosis. Osteoclasts differentiated by breast cancer cells were less sensitive to Zol than those induced by differentiation factors, whereas sensitivity to Den was similar. Conversely, breast cancer-induced osteoclast activation resulted in a higher sensitivity to 5H4. A significant increase in CSF-1 secretion was observed in osteoclast precursors after treatment with the highest concentration of Den. Further research is ongoing to evaluate the efficacy of 5H4 combination with Den.

Selenoproteins and selenium status in bone physiology and pathology

BACKGROUND

Emerging evidence supports the view that selenoproteins are essential for maintaining bone health.

SCOPE OF REVIEW

The current state of knowledge concerning selenoproteins and Se status in bone physiology and pathology is summarized.

MAJOR CONCLUSIONS

Antioxidant selenoproteins including glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), as a whole, play a pivotal role in maintaining bone homeostasis and protecting against bone loss. GPx1, a major antioxidant enzyme in osteoclasts, is up-regulated by estrogen, an endogenous inhibitor of osteoclastogenesis. TrxR1 is an immediate early gene in response to 1α,25-dihydroxyvitamin D3, an osteoblastic differentiation agent. The combination of 1α,25-dihydroxyvitamin D3 and Se generates a synergistic elevation of TrxR activity in Se-deficient osteoblasts. Of particular concern, pleiotropic TrxR1 is implicated in promoting NFκB activation. Coincidentally, TrxR inhibitors such as curcumin and gold compounds exhibit potent osteoclastogenesis inhibitory activity. Studies in patients with the mutations of selenocysteine insertion sequence-binding protein 2, a key trans-acting factor for the co-translational insertion of selenocysteine into selenoproteins have clearly established a causal link of selenoproteins in bone development. Se transport to bone relies on selenoprotein P. Plasma selenoprotein P concentrations have been found to be positively correlated with bone mineral density in elderly women.

GENERAL SIGNIFICANCE

A full understanding of the role and function of selenoproteins and Se status on bone physiology and pathology may lead to effectively prevent against or modify bone diseases by using Se.

Clinical potential of novel therapeutic targets in breast cancer: CDK4/6, Src, JAK/STAT, PARP, HDAC, and PI3K/AKT/mTOR pathways

Breast cancers expressing estrogen receptor α, progesterone receptor, or the human epidermal growth factor receptor 2 (HER2) proto-oncogene account for approximately 90% of cases, and treatment with antiestrogens and HER2-targeted agents has resulted in drastically improved survival in many of these patients. However, de novo or acquired resistance to antiestrogen and HER2-targeted therapies is common, and many tumors will recur or progress despite these treatments. Additionally, the remaining 10% of breast tumors are negative for estrogen receptor α, progesterone receptor, and HER2 (“triple-negative”), and a clinically proven tumor-specific drug target for this group has not yet been identified. Therefore, the identification of new therapeutic targets in breast cancer is of vital clinical importance. Preclinical studies elucidating the mechanisms driving resistance to standard therapies have identified promising targets including cyclin-dependent kinase 4/6, phosphoinositide 3-kinase, poly adenosine diphosphate–ribose polymerase, Src, and histone deacetylase. Herein, we discuss the clinical potential and status of new therapeutic targets in breast cancer.

Runx2-Smad signaling impacts the progression of tumor-induced bone disease

Runx2, a master regulator of osteogenesis, is abnormally expressed in advanced prostate cancer. Here, we addressed Runx2 contribution to formation of prostate cancer-related osteolytic and osteoblastic bone lesions by mediating TGFβ/BMP signaling through direct interaction with Smads. Further, we examined involvement of the Runx2-Smad complex in mediating tumor growth and distal metastasis. To identify Runx2-Smad-specific mechanisms of prostate tumor activity in bone, we generated PC3 prostate cancer cell lines expressing Runx2-WT or one of two mutant proteins (Runx2-HTY and Runx2-ΔC) that each disrupt the Runx2-Smad interaction, either directly through a point mutation or by deletion of the functional C-terminus, respectively. Intratibial tumors generated from these cells revealed that Runx2-WT-expressing cells resulted in predominantly osteolytic disease, whereas cells expressing mutant proteins exhibited tumors with mixed osteolytic/osteoblastic lesions. Extent of bone loss and woven bone formation was assessed by radiography and micro-computed tomography. Bioluminescent imaging showed the presence of labeled prostate cancer cells in the lung at the latest time point examined, with Runx2-WT group exhibiting increased incidence of tumor cells in lung. Notably, disruption of the Runx2-Smad interaction significantly reduced incidence and size of lung tumors. Altered expression of Runx2 target genes involved in invasion, growth, adhesion and metastasis supported our findings. Thus, our studies demonstrate that Runx2 in prostate cancer cells plays a significant role in intratibial prostate cancer-related tumor growth and bone loss through mechanisms mediated by the Runx2-Smad signaling pathway. This work expands upon the potential importance of Runx2 as a therapeutic target in cancer.

Curcumin analogue UBS109 prevents bone loss in breast cancer bone metastasis mouse model: involvement in osteoblastogenesis and osteoclastogenesis

Bone metastasis of breast cancer typically leads to osteolysis, which causes severe pathological bone fractures and hypercalcemia. Bone homeostasis is skillfully regulated through osteoblasts and osteoclasts. Bone loss with bone metastasis of breast cancer may be due to both activation of osteoclastic bone resorption and suppression of osteoblastic bone formation. This study was undertaken to determine whether the novel curcumin analogue UBS109 has preventive effects on bone loss induced by breast cancer cell bone metastasis. Nude mice were inoculated with breast cancer MDA-MB-231 bone metastatic cells (10(6) cells/mouse) into the head of the right and left tibia. One week after inoculation, the mice were treated with control (vehicle), oral administration (p.o.) of UBS109 (50 or 150 mg/kg body weight), or intraperitoneal administration (i.p.) of UBS109 (10 or 20 mg/kg body weight) once daily for 5 days per week for 7 weeks. After UBS109 administration for 7 weeks, hind limbs were assessed using an X-ray diagnosis system and hematoxylin and eosion staining to determine osteolytic destruction. Bone marrow cells obtained from the femurs and tibias were cultured to estimate osteoblastic mineralization and osteoclastogenesis ex vivo and in vitro. Remarkable bone loss was demonstrated in the tibias of mice inoculated with breast cancer MDA-MB-231 bone metastatic cells. This bone loss was prevented by p.o. administration of UBS109 (50 and 150 mg/kg body weight) and i.p. treatment of UBS109 (10 and 20 mg/kg) in vivo. Culture of bone marrow cells obtained from the bone tissues of mice with breast cancer cell bone metastasis showed suppressed osteoblastic mineralization and stimulated osteoclastogenesis ex vivo. These changes were not seen after culture of the bone marrow cells obtained from mice treated with UBS109. Moreover, UBS109 was found to stimulate osteoblastic mineralization and suppress lipopolysaccharide (LPS)-induced osteoclastogenesis in bone marrow cells obtained from normal nude mice in vitro. These findings suggest that the novel curcumin analogue UBS109 prevents breast cancer cell bone metastasis-induced bone loss by stimulating osteoblastic mineralization and suppressing osteoclastogenesis.

MicroRNAs in the control of metastatic bone disease

Bone metastasis is a common and devastating complication of late-stage breast and prostate cancer. Complex interactions between tumor cells, bone cells, and a milieu of components in their microenvironment contribute to the osteolytic, osteoblastic, or mixed lesions present in patients with metastasis to bone. In the past decade microRNAs (miRNAs) have emerged as key players in cancer progression, but the importance of miRNAs in regulating cancer metastasis to bone is only now being appreciated. We emphasize here important concepts of bone biology and miRNAs in the context of breast and prostate cancer, and focus on recent advances that have improved our understanding of the role of specific miRNAs with direct involvement in metastatic bone disease.

The Therapeutic Potential of Anti-Interleukin-20 Monoclonal Antibody

Interleukin (IL)-20, a member of the IL-10 family of cytokines, was discovered in 2001. IL-20 acts on multiple cell types by activating on a heterodimer receptor complex of either IL-20R1–IL-20R2 or IL-22R1–IL-20R2. Recent evidence indicates that IL-20's interaction with its receptors might have proinflammatory effects on chronic inflammatory diseases, particularly rheumatoid arthritis (RA), osteoporosis, and breast cancer. Updated information about IL-20, such as its identification, expression, receptors, signaling, and biological activities, is illustrated in this review based on our research and the data available in the literature. IL-20 is a pleiotropic cytokine, which promotes inflammation, angiogenesis, and chemotaxis. IL-20 also regulates osteoclast differentiation by altering the receptor activator of NF-κB (RANK) and RANK ligand (RANKL) axis. Inflammation, angiogenesis, and osteoclastogenesis are critical for the pathogenesis of RA, osteoporosis, and breast cancer-induced osteolysis. Based on the in vitro and in vivo data and clinical samples, we demonstrated that IL-20 plays pivotal roles in these three diseases. In experimental models, anti-IL-20 monoclonal antibody ameliorates arthritis severity, protects against ovariectomized-induced bone loss, and inhibits breast tumor-induced osteolysis. This review presents the clinical implications of IL-20, which will lead to a better understanding of the biological functions of IL-20 in these diseases and provide new therapeutic options in the future.

Therapeutic strategies for treating osteolytic bone metastases

The recent progress in oncologic management of patients with localized cancer or metastatic disease has permitted a significant improvement in life expectancy. Nevertheless, bone metastases and their consequent skeletal-related events (SREs) are still associated with unfavorable prognosis and greatly affect quality of life. Global management of these bone metastases includes traditional local approaches (surgery, radiotherapy, etc.) and systemic administration of chemotherapeutic agents. This review focuses on treatments specific for bone metastases and, in particular, on inhibitors of bone resorption that are effective for preventing and delaying the development of SREs.

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

Bone is a dynamic tissue constantly renewed through a regulated balance between bone formation and resorption. Excessive bone degradation by osteoclasts leads to pathological decreased bone density characteristic of osteolytic diseases such as post-menopausal osteoporosis or bone metastasis. Osteoclasts are multinucleated cells derived from hematopoietic stem cells via a complex differentiation process. Their unique ability to resorb bone is dependent on the formation of the actin-rich sealing zone. Within this adhesion structure, the plasma membrane differentiates into the ruffled border where protons and proteases are secreted to demineralize and degrade bone, respectively. On the bone surface, mature osteoclasts alternate between stationary resorptive and migratory phases. These are associated with profound actin cytoskeleton reorganization, until osteoclasts die of apoptosis. In this review, we highlight the role of Rho GTPases in all the steps of osteoclasts differentiation, function, and death and conclude on their interest as targets for treatment of osteolytic pathologies.

Electrolyte disorders associated with cancer

Patients with malignancies commonly experience abnormalities in serum electrolytes, including hyponatremia, hypokalemia, hyperkalemia, hypophosphatemia, and hypercalcemia. In many cases, the causes of these electolyte disturbances are due to common etiologies not unique to the underlying cancer. However, at other times, these electrolyte disorders signal the presence of paraneoplastic processes and portend a poor prognosis. Furthermore, the development of these electrolyte abnormalities may be associated with symptoms that can negatively affect quality of life and may prevent certain chemotherapeutic regimens. Thus, prompt recognition of these disorders and corrective therapy is critical in the care of the patient with cancer.

Changes in the peripheral blood and bone marrow from untreated advanced breast cancer patients that are associated with the establishment of bone metastases

Bone metastasis is an incurable complication of breast cancer affecting 70-80 % of advanced patients. It is a multistep process that includes tumour cell mobilisation, intravasation, survival in the circulation, extravasation, migration and proliferation in the bone marrow/bone. Although novel findings demonstrate the bone marrow microenvironment significance in bone metastatic progression, a majority of studies have focused on end-stage disease and little is known about how the pre-metastatic niche arises in the bone marrow/bone tissues. We demonstrated a significant increase in patients' peripheral blood plasma ability to induce transendothelial migration of MCF-7 cells compared with healthy volunteers. Moreover, high RANKL, MIF and OPG levels in patients' peripheral blood could play a role in the intravasation, angiogenesis, survival and epithelial-mesenchymal transition of circulating tumour cells. Also, we observed a significant increase in patients' bone marrow plasma capacity to induce transendothelial migration of MDA-MB231 and MCF-7 cells compared with healthy volunteers. Furthermore, patients' bone marrow mesenchymal stem cells could control the recruitment of tumour cells, modifying the MCF-7 and MDA-MB231 cell migration. In addition, we found a significantly higher MDA-MB231 cell proliferation when we used patients' bone marrow plasma compared with healthy volunteers. Interestingly, PDGF-AB, ICAM-1 and VCAM-1 levels in patients' bone marrow were significantly higher than the values of healthy volunteers, suggesting that they could be involved in the cancer cell extravasation, bone resorption and cancer cell proliferation. We believe that these results can reveal new information about what alterations happen in the bone marrow of advanced breast cancer patients before bone colonisation, changes that create optimal soil for the metastatic cascade progression.

Dietary selenium supplementation modifies breast tumor growth and metastasis

The survival rate for breast cancer drops dramatically once the disease progresses to the metastatic stage. Selenium (Se) is an essential micronutrient credited with having high anticancer and chemopreventive properties. In our study, we investigated if dietary Se supplementation modified breast cancer development in vivo. Three diets supplemented with sodium selenite, methylseleninic acid (MSA) or selenomethionine (SeMet), as well as a Se-deficient and a Se-adequate diet were fed to mice before mammary gland inoculation of 4T1.2 cells. The primary tumor growth, the numbers of cancer cells present in lungs, hearts, livers, kidneys and femurs and several proinflammatory cytokines were measured. We found that inorganic selenite supplementation provided only short-term delay of tumor growth, whereas the two organic SeMet and MSA supplements provided more potent growth inhibition. These diets also affected cancer metastasis differently. Mice fed selenite developed the most extensive metastasis and had an increased incidence of kidney and bone metastasis. On the other hand, mice fed the SeMet diet showed the least amount of cancer growth at metastatic sites. The MSA diet also provided some protection against breast cancer metastasis although the effects were less significant than those of SeMet. The cytokine profiles indicated that serum levels of interlukin-2, interleukin-6, interferon γ and vascular endothelial growth factor were elevated in SeMet-supplemented mice. There was no significant difference in tumor growth and the patterns of metastasis between the Se-deficient and Se-adequate groups. Our data suggest that organic Se supplementation may reduce/delay breast cancer metastasis, while selenite may exacerbate it.

Metastatic bone disease as seen in our clinical practice--experience at a tertiary care cancer center in Pakistan

AIM

Metastatic tumor of bone is the most common malignancy involving bone and is an important predictor of prognosis in advanced cancers. The prognosis depends upon the primary site of origin and the extent of disease. In current study, we present the pattern and distribution of metastatic bone disease seen in the leading cancer care center of Pakistan, Shaukat Khanum Cancer Hospital and Research Center (SKMCH and RC), Lahore.

MATERIALS AND METHODS

All cases of bony metastatic disease were included that presented in the Pathology Department , from Jan 2005 to July 2011. Patients of all ages and both sexes were included. Primary bone tumors, lymphomas, sarcomas and other malignancies were excluded. The data were recorded and analyzed with SPSS 16.0.

RESULTS

A total of 146 cases of metastatic bone disease were included in the study. Out of the total cases, 79 were male and 67 were female. Age range 25-82 years (median 52). Hip bone was the most frequent bone involved, with femur and vertebrae as second and third in the list. The commonest bone involved in males was vertebrae with 23 cases and in females was hip bone with 22 cases. Regarding primary site, cancers of breast, prostate and gastrointestinal tract were at the top of the list with prostate and breast being the most frequent primary sites of metastasis in males and females respectively.

CONCLUSION

Bone metastasis is an important entity to consider in the differential diagnosis whenever a bony tumor especially carcinoma present in older age. Our data are comparable with international findings and the literature available regarding the site and distribution of skeletal metastatic lesions. A slight deviation noted was more common bony metastatic lesions with ovarian primaries in females and gastrointestinal tract cancers in males in our study.

Development and characterization of murine models of medulloblastoma extraneural growth in bone

Medulloblastoma is a malignant pediatric brain neoplasm with an unusual predilection for metastasis to the skeleton. The objective of this study was to generate and characterize murine models of medulloblastoma extraneural growth in bone as 'discovery tools' for the identification of unrecognized signal transduction pathways and factors driving metastatic bone disease. To this end, the human Daoy and D283 medulloblastoma cell lines were inoculated into the intratibial medullary space of athymic nude mice. Daoy injected mice developed a primarily osteolytic radiographic and histological phenotype. In contrast, both areas of osteolytic and osteosclerotic activity were evident in D283 inoculated bones. D283 and Daoy cell conditioned media increased in vitro osteoblast differentiation and is consistent with the enhanced bone turnover characteristic of bone metastases. Daoy cells also significantly increased bone marrow osteoclast formation, consistent with the robust in vivo osteolytic phenotype. A survey of secreted factors implicated in bone metastasis and expressed by D283 and Daoy was performed. High expression of the bone-homing factor, CXCR4, was observed in both Daoy and D283 tissues. Consistent with the skeletal phenotypes, Daoy cells, while secreting the osteoblastic factor ET-1, abundantly produced the osteolytic factors RANKL, PTHrP and TNFα. D283 cells produced high levels of both RANKL and ET-1. These newly described animal models of medulloblastoma bone metastasis are expected to serve as platforms to aid in the elucidation of novel bone metastasis signaling cascades and to test therapeutics that target both medulloblastoma metastasis and the primary tumor.

ETV5 as a regulator of matrix metalloproteinase 2 in human chondrosarcoma

Chondrosarcoma is a unique type of bone cancer in that it does not respond to chemotherapy or radiation therapy, and therefore many affected patients die from metastatic disease. Metastasis has been correlated with the upregulation of the matrix metalloproteinase (MMP) family of proteases, which can degrade extracellular components. ETV5 is a transcription factor which has shown to be overexpressed in various types of invasive tumors. We hypothesized that ETV5 regulates MMP2 in human chondrosarcoma with the protease acting as a downstream effector. Gene knock-down of ETV5 in human chondrosarcoma cells reduces MMP2 mRNA expression as well as decreased protein production and significantly decreased MMP2 activity. With plasmid transfected ETV5 upregulation, MMP2 expression is similarly upregulated at the gene expression and protein levels. Data from our bone resorption studies revealed that when a matrix metalloproteinase-2 inhibitor is added to the growth media of chondrosarcoma cells, collagen released from bone chips incubated with the cells decreased by 27%. This data suggests that ETV5 has a significant role in regulating MMP2 expression and therefore matrix resorption in human chondrosarcoma, and thus may be a targetable upstream effector of the metastatic cascade in this cancer.

MRMT-1 rat breast carcinoma cells and models of bone metastases: improvement of an in vitro system to mimic the in vivo condition

Breast cancer frequently metastasizes to the skeleton thus interrupting the normal bone remodeling process and causing bone degradation. Having suitable in vitro and in vivo models is important for understanding the pathogenesis and developing treatment strategies for bone metastasis in humans. In order to improve and characterize an in vitro model of bone metastasis from breast cancer an MRMT-1 rat breast carcinoma cell line or their conditioned medium were directly co-cultured with rat monocytes. To confirm the in vitro results, an in vivo model, in which MRMT-1 cells were inoculated into the proximal surface of the tibia, was also adopted. Osteoclast viability, activity and differentiation showed a significant increase (p<0.05, p<0.0005, p<0.0005, respectively) between co-culture with MRMT-1 cells and the other culture conditions. Moreover, scanning electron microscopy analysis, phalloidin staining and 4'-6-diamidino-2-phenylindole (DAPI) nuclear acid staining confirmed that co-culture with MRMT-1 cells also induced a greater differentiation in osteoclast structure and morphology. Finally, the in vivo outcome at 3 weeks showed the presence of a severe osteolytic lesion, thus confirming the effectiveness of the present in vitro results. These results demonstrated an improvement of an in vitro model of bone metastases from breast cancer in which co-culture with MRMT-1 cells was shown to be a dynamic system that closely mimics the in vivo situation. The present study may help improve our understanding of the complex "vicious cycle" between osteoblasts, osteoclasts and tumor cells.

ABCC5 supports osteoclast formation and promotes breast cancer metastasis to bone

Introduction

Bone is the most common site of breast cancer metastasis, and complications associated with bone metastases can lead to a significantly decreased patient quality of life. Thus, it is essential to gain a better understanding of the molecular mechanisms that underlie the emergence and growth of breast cancer skeletal metastases.

Methods

To search for novel molecular mediators that influence breast cancer bone metastasis, we generated gene-expression profiles from laser-capture microdissected trephine biopsies of both breast cancer bone metastases and independent primary breast tumors that metastasized to bone. Bioinformatics analysis identified genes that are differentially expressed in breast cancer bone metastases compared with primary, bone-metastatic breast tumors.

Results

ABCC5, an ATP-dependent transporter, was found to be overexpressed in breast cancer osseous metastases relative to primary breast tumors. In addition, ABCC5 was significantly upregulated in human and mouse breast cancer cell lines with high bone-metastatic potential. Stable knockdown of ABCC5 substantially reduced bone metastatic burden and osteolytic bone destruction in mice. The decrease in osteolysis was further associated with diminished osteoclast numbers in vivo. Finally, conditioned media from breast cancer cells with reduced ABCC5 expression failed to induce in vitro osteoclastogenesis to the same extent as conditioned media from breast cancer cells expressing ABCC5.

Conclusions

Our data suggest that ABCC5 functions as a mediator of breast cancer skeletal metastasis. ABCC5 expression in breast cancer cells is important for efficient osteoclast-mediated bone resorption. Hence, ABCC5 may be a potential therapeutic target for breast cancer bone metastasis.

Onco-nephrology: the pathophysiology and treatment of malignancy-associated hypercalcemia

Hypercalcemia complicates the course of 10%-30% of all patients with malignancies and can be a sign of very poor prognosis and advanced malignancy. Prompt recognition of the nonspecific signs and symptoms of hypercalcemia and institution of therapy can be lifesaving, affording the opportunity to address the underlying etiology. The mechanisms of malignancy-associated hypercalcemia generally fall into three categories: humoral hypercalcemia due to secreted factors (such as parathyroid-related hormone), local osteolysis due to tumor invasion of bone, and absorptive hypercalcemia due to excess vitamin D produced by malignancies. The mainstays of therapy for hypercalcemia are aggressive intravenous volume expansion with saline, bisphosphonate therapy, and perhaps loop diuretics. Adjunctive therapy may include calcitonin and corticosteroids. In refractory cases, gallium nitrate and perhaps denosumab are alternatives. In patients presenting with severe AKI, hemodialysis with a low-calcium bath can be effective. In most cases, therapy normalizes calcium levels and allows for palliation or curative therapy of the malignancy.

Pathophysiology of Bone Remodelling and Current Therapeutic Approaches

Different metabolic bone parameters such as RANK (receptor activator of nuclear factor-κB), RANK ligand (receptor activator of nuclear factor-κB ligand) and OPG (osteoprotegerin) control physiological bone remodelling. The pathophysiology of these factors in bone diseases and osseous metastases is becoming clearer. In metastatic breast cancer, osteolytic bone metastases are the result of increased osteoclastic activity caused either by increased RANK ligand or decreased OPG expression of metastatic osseous tumour cells. These findings may lead to new therapeutic options for the treatment of breast cancer patients. The aim of this work is to provide an overview of physiological bone remodelling and of the interaction between tumour cells and bone environment. Current therapy approaches and the mechanisms of action of drugs are described.

Effect of Id1 knockdown on formation of osteolytic bone lesions by prostate cancer PC3 cells in vivo

The formation of osteolytic bone lesions is a key process for osteolytic cancer to metastasize to the bone and is under the control of a set of transcription factors. Recently, the inhibitor of differentiation 1 (Id1) has been linked with angiogenesis, tumorigenesis, metastasis and bone formation. However, the function of Id1 during the process of bone destruction caused by cancer in vivo has not yet been elucidated. We, therefore, examined whether and how Id1 affects the ability of cancer to form osteolytic lesion in vivo. The study used a lentiviral vector overexpressing short hairpin RNA (shRNA) targeting Id1 gene. PC3 cells, a prostate cancer cell line, were transduced with Id1 shRNA or negative control (NC) shRNA before implantation in BALB/c mice. Cells were implanted in a tibial injection model. Tumor formation in bone was monitored by X-ray. The relationship between parathyroid hormone-related protein (PTHrP), an osteolytic factor, and Id1 was analyzed by using immunohistochemistry in tissue sections from osteolytic lesion of the BALB/c mice. Our results showed that Id1 shRNA delivery to PC3 cells by lentivirus caused efficient and stable Id1 gene silencing. In the intratibial model, PC3 cells produced primarily osteolytic lesions in the bone. Eleven of 14 mice in Id1 shRNA group but only 4 of 14 mice in the NC shRNA group developed osteolytic lesions with cortical destruction at 4th week. Mice treated with Id1 shRNA had larger tumor volume in the bone and larger cortical destruction. The expression of PTHrP protein in PC3 cells was not affected by Id1 knockdown in vivo. These results indicate that Id1 may down-regulate the ability of PC3 cells to form osteolytic lesions in vivo and the signal pathway needs to be further investigated.

Hedgehog signaling in tumor cells facilitates osteoblast-enhanced osteolytic metastases

The remodeling process in bone yields numerous cytokines and chemokines that mediate crosstalk between osteoblasts and osteoclasts and also serve to attract and support metastatic tumor cells. The metastatic tumor cells disturb the equilibrium in bone that manifests as skeletal complications. The Hedgehog (Hh) pathway plays an important role in skeletogenesis. We hypothesized that the Hh pathway mediates an interaction between tumor cells and osteoblasts and influences osteoblast differentiation in response to tumor cells. We have determined that breast tumor cells have an activated Hh pathway characterized by upregulation of the ligand, IHH and transcription factor GLI1. Breast cancer cells interact with osteoblasts and cause an enhanced differentiation of pre-osteoblasts to osteoblasts that express increased levels of the osteoclastogenesis factors, RANKL and PTHrP. There is sustained expression of osteoclast-promoting factors, RANKL and PTHrP, even after the osteoblast differentiation ceases and apoptosis sets in. Moreover, tumor cells that are deficient in Hh signaling are compromised in their ability to induce osteoblast differentiation and consequently are inefficient in causing osteolysis. The stimulation of osteoblast differentiation sets the stage for osteoclast differentiation and overall promotes osteolysis. Thus, in the process of developing newer therapeutic strategies against breast cancer metastasis to bone it would worthwhile to keep in mind the role of the Hh pathway in osteoblast differentiation in an otherwise predominant osteolytic phenomenon.

P2X7 Receptor Function in Bone-Related Cancer

Modulation of tumor microenvironment by different mediators is central in determining neoplastic formation and progression. Among these molecules extracellular ATP is emerging as a good candidate in promoting cell growth, neovascularization, tumor-host interactions, and metastatization. This paper summarizes recent findings on expression and function of P2X7 receptor for extracellular ATP in primary and metastatic bone cancers. Search of mRNA expression microchip databases and literature analysis demonstrate a high expression of P2X7 in primary bone tumors as well as in other malignancies such as multiple myeloma, neuroblastoma, breast, and prostate cancer. Evidence that P2X7 triggers NFATc1, PI3K/Akt, ROCK, and VEGF pathways in osteoblasts promoting either primary tumor development or osteoblastic lesions is also reported. Moreover, P2X7 receptor is involved in osteoclast differentiation, RANKL expression, matrix metalloproteases and cathepsin secretion thus promoting bone resorption and osteolytic lesions. Taken together these data point to a pivotal role for the P2X7 receptor in bone cancer biology.

Metastatic breast cancer cells inhibit osteoblast differentiation through the Runx2/CBFβ-dependent expression of the Wnt antagonist, sclerostin

INTRODUCTION

Breast cancers frequently metastasise to the skeleton where they cause osteolytic bone destruction by stimulating osteoclasts to resorb bone and by preventing osteoblasts from producing new bone. The Runt-related transcription factor 2, Runx2, is an important determinant of bone metastasis in breast cancer. Runx2 is known to mediate activation of osteoclast activity and inhibition of osteoblast differentiation by metastatic breast cancer cells. However, while Runx2-regulated genes that mediate osteoclast activation have been identified, how Runx2 determines inhibition of osteoblasts is unknown.

METHODS

The aim of this study was to determine how Runx2 mediates the ability of metastatic breast cancer cells to modulate the activity of bone cells. We have previously demonstrated that Runx2 requires the co-activator core binding factor beta (CBFβ) to regulate gene expression in breast cancer cells. We, therefore, performed independent microarray analyses to identify target genes whose expression is dependent upon both Runx2 and CBFβ. Common target genes, with a role in modulating bone-cell function, were confirmed using a combination of siRNA, quantitative reverse transcriptase PCR (qRT-PCR), ELISA, promoter reporter analysis, Electrophoretic Mobility Shift Assay (EMSA) and chromatin immunoprecipitation (ChIP) assays. The function of Runx2/CBFβ-regulated genes in mediating the ability of MDA-MB-231 to inhibit osteoblast differentiation was subsequently established in primary bone marrow stromal cell cultures and MC-3T3 osteoblast cells.

RESULTS

We show that Runx2/CBFβ mediates inhibition of osteoblast differentiation by MDA-MB-231 cells through induction of the Wnt signaling antagonist, sclerostin. We demonstrate that MDA-MB-231 cells secrete sclerostin and that sclerostin-expression is critically dependent on both Runx2 and CBFβ. We also identified the osteoclast activators IL-11 and granulocyte-macrophage colony-stimulating factor (GM-CSF) as new target genes of Runx2/CBFβ in metastatic breast cancer cells.

CONCLUSIONS

This study demonstrates that Runx2 and CBFβ are required for the expression of genes that mediate the ability of metastatic breast cancer cells to directly modulate both osteoclast and osteoblast function. We also show that Runx2-dependent inhibition of osteoblast differentiation by breast cancer cells is mediated through the Wnt antagonist, sclerostin.