Mechanisms of tumor metastasis to the bone: challenges and opportunities

Artificial Intelligence in Bone Metastasis Imaging: Recent Progresses from Diagnosis to Treatment - A Narrative Review

The introduction of artificial intelligence (AI) represents an actual revolution in the radiological field, including bone lesion imaging. Bone lesions are often detected both in healthy and oncological patients and the differential diagnosis can be challenging but decisive, because it affects the diagnostic and therapeutic process, especially in case of metastases. Several studies have already demonstrated how the integration of AI-based tools in the current clinical workflow could bring benefits to patients and to healthcare workers. AI technologies could help radiologists in early bone metastases detection, increasing the diagnostic accuracy and reducing the overdiagnosis and the number of unnecessary deeper investigations. In addition, radiomics and radiogenomics approaches could go beyond the qualitative features, visible to the human eyes, extrapolating cancer genomic and behavior information from imaging, in order to plan a targeted and personalized treatment. In this article, we want to provide a comprehensive summary of the most promising AI applications in bone metastasis imaging and their role from diagnosis to treatment and prognosis, including the analysis of future challenges and new perspectives.

Pogostone attenuates osteolysis in breast cancer by inhibiting the NF-kB and JNK signaling pathways of osteoclast

AIMS

Breast cancer is the most prevalent cancer in females, and approximately 70 % of all patients have evidence of metastatic bone disease, which substantially affects the quality of life and survival rate of breast cancer patients. Osteoporosis has become a global public health problem, and the abnormal activation of osteoclasts is the key to the progression of osteoporosis and the key to both diseases lies in the osteoclasts. Effective drug treatments are lacking and there is an urgent need to explore new drugs.

MATERIALS AND METHODS

We observed the effects of pogostone (PO) on osteoclast differentiation, bone resorption function and other indicators, and F-actin ring formation by using Trap staining, SEM and immunofluorescence, and further explored the targets of pogostone in regulating osteoclast differentiation and function using qPCR and Western Blot. In addition, we used CCK 8, Transwell, and flow cytometry to study the effects of pogostone on proliferation, invasion, migration, and apoptosis of MDA-MB-231 cells. Animal models were also constructed for in vivo validation.

KEY FINDINGS

Pogostone inhibits osteoclast differentiation, bone resorption, formation of F-actin ring, and the expression of specific genes by attenuated NF-kB degradation and phosphorylation of JNK. In vitro, pogostone suppresses invasion of breast cancer cells, migration, and promotes their apoptosis. In mouse models, pogostone attenuated osteoclast formation and bone resorption, blocked breast cancer cells migration, and supprsed breast cancer-induced osteolysis and ovariectomized (OVX)-mediated osteoporosis.

SIGNIFICANCE

These biological functions of pogostone make it a potential drug for treatment of breast cancer-associated bone metastasis in the future.

Attenuation by HSP90 inhibitors of EGF-elicited migration of osteoblasts: involvement of p44/p42 MAP kinase

BACKGROUND

We have demonstrated that epidermal growth factor (EGF)-induced migration of osteoblast-like MC3T3-E1 cells is mediated through p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase, stress-activated protein kinase/ c-Jun N-terminal kinase (SAPK/JNK), and Akt.The molecular chaperone heat shock protein 90 (HSP90) is abundantly expressed in osteoblasts. However, the role of HSP90 in osteoblast migration remains obscure.

OBJECTIVE

In this study, we investigated the effect of HSP90 inhibitors on the EGF-induced migration of MC3T3-E1 cells and the mechanism.

METHODS

Clonal osteoblast-like MC3T3-E1 cells were treated with the HSP90 inhibitors geldanamycin or onalespib and then stimulated with EGF. Cell migration was evaluated using the transwell cell migration assay and wound-healing assay. The viability of MC3T3-E1 cells was analyzed using the Cell Counting Kit-8. The phosphorylation of p44/p42 MAP kinase, p38 MAP kinase, SAPK/JNK, Akt, and protein kinase-like endoplasmic reticulum kinase (PERK) was evaluated by western blot analysis.

RESULTS

EGF-induced migration was significantly suppressed by geldanamycin and onalespib, evaluated by both transwell cell migration assay and wound-healing assay. Geldanamycin and onalespib did not significantly alter cell viability. Geldanamycin and onalespib markedly reduced the EGF-induced phosphorylation of p44/p42 MAP kinase, but not p38 MAP kinase or Akt. By contrast, geldanamycin and onalespib increased the EGF-induced phosphorylation of SAPK/JNK. PERK phosphorylation was not significantly affected by geldanamycin or onalespib.

CONCLUSION

Our results strongly suggest that HSP90 inhibitors reduce the EGF-induced osteoblast migration through the p44/p42 MAP kinase.

Understanding Drug Sensitivity and Tackling Resistance in Cancer

Decades of research into the molecular mechanisms of cancer and the development of novel therapeutics have yielded a number of remarkable successes. However, our ability to broadly assign effective, rationally targeted therapies in a personalized manner remains elusive for many patients, and drug resistance persists as a major problem. This is in part due to the well-documented heterogeneity of cancer, including the diversity of tumor cell lineages and cell states, the spectrum of somatic mutations, the complexity of microenvironments, and immune-suppressive features and immune repertoires, which collectively require numerous different therapeutic approaches. Here, we describe a framework to understand the types and biological causes of resistance, providing translational opportunities to tackle drug resistance by rational therapeutic strategies.

Overview of Ca2+ signaling in lung cancer progression and metastatic lung cancer with bone metastasis

Intracellular Ca²⁺ ions that are thought to be one of the most important second messengers for cellular signaling, have a substantial diversity of roles in regulating a plethora of fundamental cellular physiology such as gene expression, cell division, cell motility and apoptosis. It has been suggestive of the Ca²⁺ signaling-dependent cellular processes to be tightly regulated by the numerous types of Ca²⁺ channels, pumps, exchangers and sensing receptors. Consequently, dysregulated Ca²⁺ homeostasis leads to a series of events connected to elevated malignant phenotypes including uncontrolled proliferation, migration, invasion and metastasis, all of which are frequently observed in advanced stage lung cancer cells. The incidence of bone metastasis in patients with advanced stage lung cancer is estimated in a range of 30% to 40%, bringing about a significant negative impact on both morbidity and survival. This review dissects and summarizes the important roles of Ca²⁺ signaling transduction in contributing to lung cancer progression, and address the question: if and how Ca²⁺ signaling might have been engaged in metastatic lung cancer with bone metastasis, thereby potentially providing the multifaceted and promising solutions for therapeutic intervention.

Resveratrol suppresses insulin-like growth factor I-induced osteoblast migration: attenuation of the p44/p42 MAP kinase pathway

Resveratrol is a natural polyphenol with beneficial antioxidant properties. It suppresses the migration of osteoblast-like MC3T3-E1 cells induced by epidermal growth factor, via SIRT1-mediated inhibition of SAPK/JNK and Akt. Moreover, insulin-like growth factor-I (IGF-I) stimulates the migration involving the pathways of p44/p42 mitogen-activated protein (MAP) kinase and Akt. Therefore, we investigated the effects of resveratrol on IGF-I-induced cell migration. Resveratrol and SRT1720, an activator of SIRT1, suppressed IGF-I-induced migration. Inauhzin, a SIRT1 inhibitor, significantly rescued the inhibition of IGF-I-induced cell migration by resveratrol. Resveratrol inhibited IGF-I-induced phosphorylation of p44/p42 MAP kinase but not Akt. SRT1720 inhibited IGF-I-induced phosphorylation of p44/p42 MAP kinase. Furthermore, PD98059, p44/p42 MAP kinase inhibitor, alone suppressed IGF-I-induced osteoblast migration, but did not affect the suppressive effect of resveratrol when administered concomitantly. These findings strongly suggest that resveratrol suppresses IGF-I-induced osteoblast migration via SIRT1 activation at least partially by attenuating the p44/p42 MAP kinase pathway.

Food for Bone: Evidence for a Role for Delta-Tocotrienol in the Physiological Control of Osteoblast Migration

Bone remodeling and repair require osteogenic cells to reach the sites that need to be rebuilt, indicating that stimulation of osteoblast migration could be a promising osteoanabolic strategy. We showed that purified δ-tocotrienol (δ-TT, 10 μg/mL), isolated from commercial palm oil (Elaeis guineensis) fraction, stimulates the migration of both MC3T3-E1 osteoblast-like cells and primary human bone marrow mesenchymal stem cells (BMSC) as detected by wound healing assay or Boyden chamber assay respectively. The ability of δ-TT to promote MC3T3-E1 cells migration is dependent on Akt phosphorylation detected by Western blotting and involves Wnt/β-catenin signalling pathway activation. In fact, δ-TT increased β-catenin transcriptional activity, measured using a Nano luciferase assay and pretreatment with procaine (2 µM), an inhibitor of the Wnt/β-catenin signalling pathway, reducing the wound healing activity of δ-TT on MC3T3-E1 cells. Moreover, δ-TT treatment increased the expression of β-catenin specific target genes, such as Osteocalcin and Bone Morphogenetic Protein-2, involved in osteoblast differentiation and migration, and increased alkaline phosphatase and collagen content, osteoblast differentiation markers. The ability of δ-TT to enhance the recruitment of BMSC, and to promote MC3T3-E1 differentiation and migratory behavior, indicates that δ-TT could be considered a promising natural anabolic compound.

Incretin accelerates platelet-derived growth factor-BB-induced osteoblast migration via protein kinase A: The upregulation of p38 MAP kinase

Incretins, including glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), secreted from enteroendocrine cells after food ingestion, are currently recognized to regulate glucose metabolism through insulin secretion. We previously demonstrated that platelet-derived growth factor-BB (PDGF-BB) induces the migration of osteoblast-like MC3T3-E1 cells through mitogen-activated protein (MAP) kinases, including p38 MAP kinase. In the present study, we investigated whether or not incretins affect the osteoblast migration. The PDGF-BB-induced cell migration was significantly reinforced by GLP-1, GIP or cAMP analogues in MC3T3-E1 cells and normal human osteoblasts. The upregulated migration by GLP-1 or cAMP analogues was suppressed by H89, an inhibitor of protein kinase A. The amplification by GLP-1 of migration induced by PDGF-BB was almost completely reduced by SB203580, a p38 MAP kinase inhibitor in MC3T3-E1 cells and normal human osteoblasts. In addition, GIP markedly strengthened the PDGF-BB-induced phosphorylation of p38 MAP kinase. Exendin-4, a GLP-1 analogue, induced Rho A expression and its translocation from cytoplasm to plasma membranes in osteoblasts at the epiphyseal lines of developing mouse femurs in vivo. These results strongly suggest that incretins accelerates the PDGF-BB-induced migration of osteoblasts via protein kinase A, and the up-regulation of p38 MAP kinase is involved in this acceleration. Our findings may highlight the novel potential of incretins to bone physiology and therapeutic strategy against bone repair.

Development of Diagnostic and Therapeutic Probes with Controlled Pharmacokinetics for Use in Radiotheranostics

The word "theranostics," a portmanteau word made by combining "therapeutics" and "diagnostics," refers to a personalized medicine concept. Recently, the word, "radiotheranostics," has also been used in nuclear medicine as a term that refer to the use of radioisotopes for combined imaging and therapy. For radiotheranostics, a diagnostic probe and a corresponding therapeutic probe can be prepared by introducing diagnostic and therapeutic radioisotopes into the same precursor. These diagnostic and therapeutic probes can be designed to show equivalent pharmacokinetics, which is important for radiotheranostics. As imaging can predict the absorbed radiation dose and thus the therapeutic and side effects, radiotheranostics can help achieve the goal of personalized medicine. In this review, I discuss the use of radiolabeled probes targeting bone metastases, sigma-1 receptor, and αVβ3 integrin for radiotheranostics.

Accuracy of iodine density thresholds for the separation of vertebral bone metastases from healthy-appearing trabecular bone in spectral detector computed tomography

PURPOSE

To evaluate quantitative iodine density mapping (IDM) with spectral detector computed tomography (SDCT) as a quantitative biomarker for separation of vertebral trabecular bone metastases (BM) from healthy-appearing trabecular bone (HTB).

MATERIALS AND METHODS

IRB-approved retrospective single-center-study of portal venous SDCT datasets acquired between June 2016 and March 2017. Inclusion of 43 consecutive cancer patients with BM and 40 without. Target lesions and non-affected control vertebrae were defined using follow-up imaging, MRI, and/or bone scintigraphy. ID and standard deviation were determined with ROI measures by two readers in (a) bone metastases, (b) HTB of BM patients and controls, and (c) ID of various vessels. Volumetric bone mineral density (vBMD) of the lumbar spine and age were recorded. Multivariate ROC analyses und Wilcoxon test were used to determine thresholds for separation of BM and HTB. p < 0.05 was considered significant.

RESULTS

ID measurements of 40 target lesions and 83 reference measurements of HTB were acquired. Age (p < 0.0001) and vBMD (p < 0.05) affected ID measurements independently in multivariate models. There were significant differences of ID between metastases (n = 43) and HTB ID (n = 124; mean 5.5 ± 0.9 vs. 3.5 ± 0.9; p < 0.0001), however, with considerable overlap. In univariate analysis, increased ID discriminated bone lesions (AUC 0.90) with a maximum combined specificity/sensitivity of 77.5%/90.7% when applying a threshold of 4.5 mg/ml. Multivariate regression models improved significantly when considering vBMD, the noise of ID, and vertebral venous ID (AUC 0.98).

CONCLUSION

IDM of SDCT yielded a statistical separation of vertebral bone lesions and HTB. Adjustment for confounders such as age and lumbar vBMD as well as for vertebral venous ID and lesion heterogeneity improved discrimination of trabecular lesions.

KEY POINTS

• SDCT iodine density mapping provides the possibility for quantitative analysis of iodine uptake in tissue, which allows to differentiate bone lesions from healthy bone marrow. • Age and vBMD have a significant impact on iodine density measurements. • Iodine density measured in SDCT yielded highest sensitivity and specificity for the statistical differentiation of vertebral trabecular metastases and healthy trabecular bone using an iodine density threshold of 4.5 mg/ml (most performant)-5.0 mg/ml (optimized for specificity).

HSP70 Inhibitor Suppresses IGF-I-Stimulated Migration of Osteoblasts through p44/p42 MAP Kinase

Heat shock protein 70 (HSP70) is a ubiquitously expressed molecular chaperone in a variety of cells including osteoblasts. We previously showed that insulin-like growth factor-I (IGF-I) elicits migration of osteoblast-like MC3T3-E1 cells through the activation of phosphatidylinositol 3-kinase/Akt and p44/p42 mitogen-activated protein (MAP) kinase. In the present study, we investigated the effects of HSP70 inhibitors on the IGF-I-elicited migration of these cells and the mechanism involved. The IGF-I-stimulated osteoblast migration evaluated by a wound-healing assay and by a transwell cell migration was significantly reduced by VER-155008 and YM-08, which are both HSP70 inhibitors. VER-155008 markedly suppressed the IGF-I-induced phosphorylation of p44/p42 MAP kinase without affecting that of Akt. In conclusion, our results strongly suggest that the HSP70 inhibitor reduces the IGF-I-elicited migration of osteoblasts via the p44/p42 MAP kinase.

Repression of IGF-I-induced osteoblast migration by (-)-epigallocatechin gallate through p44/p42 MAP kinase signaling

Polyphenolic compounds in beverages may have benefits in the prevention of osteoporosis. It has been demonstrated previously that insulin-like growth factor-I (IGF-I) could stimulate the migration of osteoblasts. In the present study, it was investigated whether chlorogenic acid, a major polyphenol in coffee, and (-)-epigallocatechin gallate (EGCG), a major polyphenol in green tea, could affect this IGF-I-stimulated migration of osteoblast-like MC3T3-E1 cells. The IGF-I-stimulated osteoblast migration, evaluated by Transwell cell migration and wound-healing assays, was inhibited by EGCG but not chlorogenic acid. IGF-I induced the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase, p70 S6 kinase and Akt. The IGF-I-induced migration was suppressed by PD98059, a MAP kinase kinase 1/2 inhibitor, and deguelin, an Akt inhibitor, but not rapamycin, an inhibitor of the upstream kinase of p70 S6 kinase (mammalian target of rapamycin). EGCG attenuated the IGF-I-induced phosphorylation of p44/p42 MAP kinase but not Akt. Taken together, the present results suggest that EGCG inhibits IGF-I-induced osteoblast migration via p44/p42 MAP kinase.

(-)-Epigallocatechin gallate but not chlorogenic acid suppresses EGF-stimulated migration of osteoblasts via attenuation of p38 MAPK activity

Phenolic compounds provide health benefits in humans. A previous study by our group has indicated that the epidermal growth factor (EGF)‑induced migration of osteoblast‑like MC3T3‑E1 cells is mediated by the phosphorylation of p44/p42 mitogen‑activated protein (MAPK), p38 MAPK, stress‑activated protein kinase (SAPK)/c‑Jun N‑terminal kinase (JNK) and Akt, and that resveratrol, a major polyphenol in grape skin, suppresses the EGF‑induced migration by attenuating Akt and SAPK/JNK activation. In the present study, the effects of chlorogenic acid, a major phenolic acid in coffee, and (‑)‑epigallocatechin gallate (EGCG), a major flavonoid in green tea, on the EGF‑induced migration of MC3T3‑E1 cells were investigated. EGCG significantly reduced the EGF‑induced migration as evaluated by a Transwell migration assay and by a wound healing assay. However, chlorogenic acid failed to affect the EGF‑induced migration. The phosphorylation of p38 MAPK induced by EGF was significantly suppressed by EGCG; however, the EGF‑induced phosphorylation of p44/p42 MAP kinase, SAPK/JNK or Akt was not affected by EGCG. These results suggest that EGCG, but not chlorogenic acid, suppresses EGF‑induced osteoblast migration through inhibiting p38 MAPK activation.

The Osteoclast in Bone Metastasis: Player and Target

Bone metastases are frequently the final fate of breast and prostate cancer patients. According to the definition of metastasis as an incurable disease, to date there are no effective treatments for tumor-associated bone metastases and this represents a real challenge for the researchers in the field. The bone is a heterogeneous environment that represents a fertile soil for tumor cells, supporting their growth. Among the different cell types present in the bone, in this review we will focus our attention on the osteoclasts, which are crucial players in the so called “vicious cycle”, a phenomenon triggered by tumor cells eventually leading to both tumor proliferation as well as bone deregulation, thus fueling the development of bone metastasis. The complex network, linking tumor cells to the bone by activating osteoclasts, represents a fruitful target for the treatment of bone metastases. In this review we will describe how tumor cells perturb the bone microenvironment by actively influencing osteoclast formation and activity. Moreover, we will describe the current antiresorptive drugs employed in the treatment of bone metastases as well as new, targeted therapies able to affect both cancer cells and osteoclasts.

Atypical mandibular metastasis as the first presentation of a colorectal cancer

We describe a case of a 70-year-old man presenting with a mandibular metastasis as the first sign of a mucinous adenocarcinoma of the rectum. After 6 months of a protracted toothache, the patient presented with a palpable mandibular mass and trismus, precluding adequate clinical evaluation. A CT scan was performed, and imaging findings suggested an aggressive primary jawbone tumour, most likely an osteosarcoma. However, biopsy and further patient's management proved to be a metastasis. Metastasis to the oral cavity account for only 1%-3% of all malignant oral tumours, and the mandible is the most frequent site. Clinical presentation can be quite variable, and most often a primary malignancy is already known. Jawbone metastases are a sign of disseminated malignant neoplasms, with poor prognosis and usually an indication for palliative therapy.

Attenuation of TGFBR2 expression and tumour progression in prostate cancer involve diverse hypoxia-regulated pathways

Background

Dysregulation of transforming growth factor β (TGF-β) signaling and hypoxic microenvironment have respectively been reported to be involved in disease progression in malignancies of prostate. Emerging evidence indicates that downregulation of TGFBR2, a pivotal regulator of TGF-β signaling, may contribute to carcinogenesis and progression of prostate cancer (PCa). However, the biological function and regulatory mechanism of TGFBR2 in PCa remain poorly understood. In this study, we propose to investigate the crosstalk of hypoxia and TGF-β signaling and provide insight into the molecular mechanism underlying the regulatory pathways in PCa.

Methods

Prostate cancer cell lines were cultured in hypoxia or normoxia to evaluate the effect of hypoxia on TGFBR2 expression. Methylation specific polymerase chain reaction (MSP) and demethylation agents was used to evaluate the methylation regulation of TGFBR2 promoter. Besides, silencing of EZH2 via specific siRNAs or chemical inhibitor was used to validate the regulatory effect of EZH2 on TGFBR2. Moreover, we conducted PCR, western blot, and luciferase assays which studied the relationship of miR-93 and TGFBR2 in PCa cell lines and specimens. We also detected the impacts of hypoxia on EZH2 and miR-93, and further examined the tumorigenic functions of miR-93 on proliferation and epithelial-mesenchymal transition via a series of experiments.

Results

TGFBR2 expression was attenuated under hypoxia. Hypoxia-induced EZH2 promoted H3K27me3 which caused TGFBR2 promoter hypermethylation and contributed to its epigenetic silencing in PCa. Besides, miR-93 was significantly upregulated in PCa tissues and cell lines, and negatively correlated with the expression of TGFBR2. Ectopic expression of miR-93 promoted cell proliferation, migration and invasion in PCa, and its expression could also be induced by hypoxia. In addition, TGFBR2 was identified as a bona fide target of miR-93.

Conclusions

Our findings elucidate diverse hypoxia-regulated pathways including EZH2-mediated hypermethylation and miR-93-induced silencing contribute to attenuation of TGFBR2 expression and promote cancer progression in prostate cancer.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0764-9) contains supplementary material, which is available to authorized users.

Radium-223 international early access program: results from the Spanish subset

AIM

To report results from the Spanish subset included in the radium-223 international early access program (iEAP).

PATIENTS & METHODS

Ninety patients with castration-resistant prostate cancer and bone metastases received radium-223 55 kBq/kg every 4 weeks for six cycles.

RESULTS

The median time to disease progression was 8 months and to prostate-specific antigen progression was 4 months. The percentage of patients with ≥50% confirmed declines in prostate-specific antigen was 9%. The median overall survival was 14 months. Grade 3 or 4 treatment emergent adverse events (TEAEs) occurred in 34% of patients (serious TEAEs 28%, TEAEs leading to discontinuation 27%).

CONCLUSION

Outcomes of the Spanish subset are consistent with the iEAP. Radium-223 was generally well tolerated with no safety concerns.

Numb chin syndrome as a sign of mandibular metastasis: A case report

INTRODUCTION

Metastasis to the oral cavity can be located in both the soft and bony tissues and comprise only 1% of all oral malignancies; however, it is clinically significant because it indicates widespread metastatic disease and an unfavorable prognosis. A numb chin is an important presentation of oral metastasis, but other dental and systemic pathology may be involved.

PRESENTATION OF CASE

We present the case of a 54-year-old woman who presented with numb chin syndrome 8 years after a diagnosis of primary breast carcinoma. The former was caused by mandibular ramal metastasis of the tumor, which also spread to the spinal canal, ribs, pelvic bones, sacrum, and proximal femur. Because of widespread metastasis, palliative treatment was administered.

DISCUSSION

Despite its low incidence, oral metastasis should be considered as a diagnostic option when patients present with numb chin syndrome. Bony metastasis may produce symptoms late, in contrast to soft tissue metastasis. Pain is the first presenting symptom in most cases, but paresthesia, hypoesthesia and anesthesia in the facial region are possible symptoms too. Most oral bony metastasis are located in the mandible, especially in the molar region. Despite treatment, the average survival after diagnosis of mandibular metastasis is 6-7 months.

CONCLUSION

In most cases with oral metastasis, palliative treatment is indicated. Early detection of oral lesions could improve treatment outcome and survival. A full diagnostic work-up is therefore of great importance.

Key Factors in Breast Cancer Dissemination and Establishment at the Bone: Past, Present and Future Perspectives

Bone metastases associated with breast cancer remain a clinical challenge due to their associated morbidity, limited therapeutic intervention and lack of prognostic markers. With a continually evolving understanding of bone biology and its dynamic microenvironment, many potential new targets have been proposed. In this chapter, we discuss the roles of well-established bone markers and how their targeting, in addition to tumour-targeted therapies, might help in the prevention and treatment of bone metastases. There are a vast number of bone markers, of which one of the best-known families is the bone morphogenetic proteins (BMPs). This chapter focuses on their role in breast cancer-associated bone metastases, associated signalling pathways and the possibilities for potential therapeutic intervention. In addition, this chapter provides an update on the role receptor activator of nuclear factor-κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) play on breast cancer development and their subsequent influence during the homing and establishment of breast cancer-associated bone metastases. Beyond the well-established bone molecules, this chapter also explores the role of other potential factors such as activated leukocyte cell adhesion molecule (ALCAM) and its potential impact on breast cancer cells' affinity for the bone environment, which implies that ALCAM could be a promising therapeutic target.

Bone: A Fertile Soil for Cancer Metastasis

Bone is one of the most common and most dangerous sites for metastatic growth across cancer types, and bone metastasis remains incurable. Unfortunately, the processes by which cancers preferentially metastasize to bone are still not well understood. In this review, we summarize the morphological features, physical properties, and cell signaling events that make bone a unique site for metastasis and bone remodeling. The signaling crosstalk between the tumor cells and bone cells begins a vicious cycle - a self-sustaining feedback loop between the tumor cells and the bone microenvironment composed of osteoclasts, osteoblasts, other bone marrow cells, bone matrix, and vasculature to support both tumor growth and bone destruction. Through this crosstalk, bone provides a fertile microenvironment that can harbor dormant tumor cells, sometimes for long periods, and support their growth by releasing cytokines as the bone matrix is destroyed, similar to providing nutrients for a seed to germinate in soil. However, few models exist to study the late stages of bone colonization by metastatic tumor cells. We describe some of the current methodologies used to study bone metastasis, highlighting the limitations of these methods and alternative future strategies to be used to study bone metastasis. While <i>in vivo</i> animal and patient studies may provide the gold standard for studying metastasis, <i>ex vivo</i> models can be used as an alternative to enable more controlled experiments designed to study the late stages of bone metastasis.

Heterogeneity of tumor cells in the bone microenvironment: Mechanisms and therapeutic targets for bone metastasis of prostate or breast cancer

Bone is the most common target organ of metastasis of prostate and breast cancers. This produces considerable morbidity due to skeletal-related events, SREs, including bone pain, hypercalcemia, pathologic fracture, and compression of the spinal cord. The mechanism of bone metastasis is complex and involves cooperative reciprocal interaction among tumor cells, osteoblasts, osteoclasts, and the mineralized bone matrix. The interaction between the metastatic tumor and bone stromal cells has been commonly referred to as the "vicious cycle". Tumor cells stimulate osteoblasts, which in turn stimulate osteoclasts through the secretion of cytokines such as the TNF family member receptor activator of nuclear κB ligand (RANKL). Activated osteoclasts degrade the bone matrix by producing strong acid and proteinases. Bone degradation by osteoclasts releases TGFβ and other growth factors stored in the bone matrix, that further stimulate tumor cells. Bone modifying agents, targeting osteoclast activity, such as bisphosphonate and RANKL antibodies are considered as the standard of care for reducing SREs of patients with bone metastatic diseases. These agents decrease osteoclast activity and delay worsening of skeletal pain and aggravation of bone metastatic diseases. While the management of SREs by these agents may improve patients' lives, this treatment does not address the specific issues of the patients with bone metastasis such as tumor dormancy, drug resistance, or improvement of survival. Here, we review the mechanisms of bone metastasis formation, tumor heterogeneity in the bone microenvironment, and conventional therapy for bone metastatic diseases and discuss the potential development of new therapies targeting tumor heterogeneity in the bone microenvironment.

The effects of lung and prostate cancer bone metastasis on serum osteoprotegerin levels: a meta-analysis

Bone metastasis leads to skeletal-related events in final-stage cancer patients. The incidence of prostate and lung cancers increases yearly; these cancers readily invade the bone. Some recent studies have found that serum osteoprotegerin (OPG) levels may be altered in patients with bone metastasis, whereas other reports have produced inconsistent findings. Hence, we conducted a meta-analysis to examine the effects of lung and prostate cancer on serum OPG levels. A systematic literature search was conducted using PubMed, Medline, and CNKI to identify relevant studies. A total of 11 studies were included. The standardized mean difference (SMD) and 95% confidence interval (95% CI) of the bone metastasis (BM) group, the non-bone metastasis (BM-) group and healthy controls were calculated. In prostate cancer, serum OPG levels in the BM group were higher than in the BM- group and healthy controls. Additionally, no significant difference in serum OPG levels was found between the BM- group and healthy controls. In lung cancer, serum OPG levels in the BM and BM- groups were significantly increased compared with healthy controls. However, no significant difference in serum OPG levels was found between the BM and BM- groups. Studies with larger sample sizes are required to confirm these findings.

Myeloid-specific TGF-β signaling in bone promotes basic-FGF and breast cancer bone metastasis

Breast cancer (BCa) bone metastases cause osteolytic bone lesions, which result from the interactions of metastatic BCa cells with osteoclasts and osteoblasts. Osteoclasts differentiate from myeloid lineage cells. To understand the cell-specific role of transforming growth factor beta (TGF-β) in the myeloid lineage, in BCa bone metastases, MDA-MB-231 BCa cells were intra-tibially or intra-cardially injected into LysM(Cre)/Tgfbr2(floxE2/floxE2) knockout (LysM(Cre)/Tgfbr2 KO) or Tgfbr2(floxE2/floxE2) mice. Metastatic bone lesion development was compared by analysis of both lesion number and area. We found that LysM(Cre)/Tgfbr2 knockout significantly decreased MDA-MB-231 bone lesion development in both the cardiac and tibial injection models. LysM(Cre)/Tgfbr2 knockout inhibited the tumor cell proliferation, angiogenesis and osteoclastogenesis of the metastatic bones. Cytokine array analysis showed that basic fibroblast growth factor (bFGF) was downregulated in MDA-MB-231-injected tibiae from the LysM(Cre)/Tgfbr2 KO group, and intravenous injection of the recombinant bFGF to LysM(Cre)/Tgfbr2 KO mice rescued the inhibited metastatic bone lesion development. The mechanism by which bFGF rescued the bone lesion development was by promotion of tumor cell proliferation through the downstream mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK)-cFos pathway after binding to the FGF receptor 1 (FGFR1). Consistent with animal studies, we found that in human BCa bone metastatic tissues, TGF-β type II receptor (TβRII) and p-Smad2 were expressed in osteoclasts and tumor cells, and were correlated with the expression of FGFR1. Our studies suggest that myeloid-specific TGF-β signaling-mediated bFGF in the bone promotes BCa bone metastasis.

Tumor-induced pressure in the bone microenvironment causes osteocytes to promote the growth of prostate cancer bone metastases

Cross-talk between tumor cells and their microenvironment is critical for malignant progression. Cross-talk mediators, including soluble factors and direct cell contact, have been identified, but roles for the interaction of physical forces between tumor cells and the bone microenvironment have not been described. Here, we report preclinical evidence that tumor-generated pressure acts to modify the bone microenvironment to promote the growth of prostate cancer bone metastases. Tumors growing in mouse tibiae increased intraosseous pressure. Application of pressure to osteocytes, the main mechanotransducing cells in bone, induced prostate cancer growth and invasion. Mechanistic investigations revealed that this process was mediated in part by upregulation of CCL5 and matrix metalloproteinases in osteocytes. Our results defined the critical contribution of physical forces to tumor cell growth in the tumor microenvironment, and they identified osteocytes as a critical mediator in the bone metastatic niche.

Palliative treatment of bone metastases with samarium-153 EDTMP at onset of pain

We evaluated the pain response and daily discomfort in patients suffering from a borderline degree of bone pain due to breast or lung cancer bone metastases, who had undergone early palliative radionuclide treatment. The results were compared with those from patients who had received standard analgesic therapy. Twenty-one patients (65.7 ± 3 years; 17 women) with metastatic bone cancer underwent samarium-153 (Sm-153) ethylene diamine tetramethylene phosphonate (EDTMP) administration (group A) and 18 patients (64.3 ± 8 years; 16 women)continued to receive standard analgesics (group B; control group). The patients kept a daily pain diary assessing both their discomfort and the pain at specific sites by means of a visual analog scale, rating from 0 (no discomfort–no pain)to 10 (worst discomfort–pain). These diaries were reviewed weekly for 2 months and three physicians rated the pain response on a scale from -2 (considerable deterioration) to +2 (considerable improvement). Baseline characteristics were similar in both groups. The reduction of total discomfort and of bone pain in group A was significantly greater compared to group B (p < 0.0001). A significant improvement of clinical conditions was observed in group A, where the physician rate changed from -1 to 1, compared to group B in which the rate changed from -1 to 0. Sm-153 EDTMP therapy can be considered for patients with bone pain from breast and lung cancer in advance, i.e.,before the establishment of severe pain syndrome.

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.

Sclerotic osseous metastases from renal cell carcinoma

This case series describes and illustrates three cases of sclerotic osseous metastases from untreated renal cell carcinoma (RCC). RCC is commonly metastatic to the skeleton but almost always produces lytic metastases, with only three prior reports of sclerotic metastases identified in the literature. Sclerotic metastasis causing low back pain was the initial disease presentation in two of the three patients in this case series and the first manifestation of metastatic disease in one. The most common metastatic sites of RCC, i.e., retroperitoneal lymph nodes, lung, and liver, were not identified in any of the cases, and skeletal involvement with epidural extension was the only site of metastasis in two. Pathologic specimens from all three cases revealed RCC of high nuclear grade.

Variability of the paracrine-induced osteoclastogenesis by human breast cancer cell lines

Breast cancer frequently metastasizes to the bone, often leading to the formation of osteolytic lesions. This work compares the paracrine-induced osteoclastogenesis mediated by four human breast cancer cell lines, the estrogen-receptor positive T47D and MCF-7 and the estrogen-negative SK-BR-3 and Hs-578T cell lines. Human osteoclast precursor cells were cultured in the presence of conditioned media from the breast cancer cell lines (10% and 20%), collected at different culture periods (48 h, 7 days, and 14 days). Cultures performed in the absence or the presence of M-CSF and RANKL served as negative and positive control, respectively. Results showed that the cell lines differentially expressed several osteoclastogenic genes. All cell lines exhibited a significant osteoclastogenic potential, evidenced by a high TRAP activity and number of osteoclastic cells, expression of several osteoclast-related genes, and, particularly, a high calcium phosphate resorption activity. Differences among the osteoclastogenic potential of the cell lines were noted. T47D and MCF-7 cell lines displayed the highest and the lowest osteoclastogenic response, respectively. Despite the variability observed, MEK and NF-κB signaling pathways, and, at a lesser extent, PGE2 production, seemed to have a central role on the observed osteoclastogenic response. In conclusion, the tested breast cancer cell lines exhibited a high osteoclastogenic potential, although with some variability on the cell response profile, a factor to be considered in the development of new therapeutic approaches for breast cancer-induced bone metastasis.

Clinical features and prognostic factors for patients with bone metastases from prostate cancer

To identify the clinical features and independent predictors of survival in patients with bone metastases from prostate cancer (PCa). We retrospectively analysed 115 PCa patients with bone metastases between 1997 and 2009. The overall survival rate after bone metastases was calculated using the Kaplan-Meier method. The prognostic factors were identified by univariate analysis using a log-rank test and by multivariate analysis using Cox proportional hazards regression models. The follow-up rate was 100%, the follow-up cases during 1, 3 and 5 years were 103, 79 and 55, respectively. The 1-, 3- and 5-year survival rates were 89.1%, 60.9% and 49.8%, respectively, with a median survival time of 48.5 months for patients with bone metastases from PCa. In univariate analysis, age, Gleason score, clinical stage, the number of bone lesions, alkaline phosphatase (ALP) level, invasion of neighbouring organs and non-regional lymph node metastases were correlated with prognosis. By multivariate analysis using Cox regression, ALP level, Gleason score and non-regional lymph node metastases were independent prognostic factors. These prognostic factors will help us to determine the appropriate dose and fraction of radiotherapy for these patients.

A novel animal model for bone metastasis in human lung cancer

Metastases account for 90% of lung cancer mortalities, frequently target the skeleton and lead to rapid deterioration in quality of life. The molecular mechanism underlying bone metastases is largely unknown. Development of xenograft mouse models, such as the severe combined immunodeficient (SCID) CB-17 mouse and the non-obese diabetic (NOD)/SCID mouse, both of which lack functional B- and T-cells and are able to host allogeneic or xenogeneic tumor cells, has made great contributions in this area. However, residual natural killer (NK) cells in these models are able to significantly modify local tumor growth and metastasis. Treatment with anti-murine IL-2 receptor β chain Ab (TM-β1) antibody can abrogate NK cell activity in vivo; however, the antibody treatment may result in unexpected effects and the stability is hard to control. To overcome these shortcomings, we evaluated xenografts in NOD-scid IL2Rγ(null) immunodeficient mice that lacked mature T cells, B cells and functional NK cells. We compared the target tissue distribution of the human small cell lung cancer cell lines SBC-5 and SBC-3. Gross necropsy and whole skeletal X-ray film examination of the host mice were conducted 30 days post-tail vein injection. The SBC-5 cells colonized bone and formed lytic lesions. The cells also colonized liver, spleen and, less frequently, the pancreas, ovary and kidney. The SBC-3 cell xenografts formed easily visible tumor foci in the liver, pancreas, ovary/uterus and kidney, but not bone metastases. Our results showed that SBC-5 cells in NOD-scid IL2Rγ(null) immunodeficient mice provide a suitable xenograft model system for bone metastasis of human lung cancer. This novel animal model may therefore be used to study the molecular pathway of bone metastases and to evaluate targets for effective therapies.

A cross-species analysis of a mouse model of breast cancer-specific osteolysis and human bone metastases using gene expression profiling

Background

Breast cancer is the second leading cause of cancer-related death in women in the United States. During the advanced stages of disease, many breast cancer patients suffer from bone metastasis. These metastases are predominantly osteolytic and develop when tumor cells interact with bone. In vivo models that mimic the breast cancer-specific osteolytic bone microenvironment are limited. Previously, we developed a mouse model of tumor-bone interaction in which three mouse breast cancer cell lines were implanted onto the calvaria. Analysis of tumors from this model revealed that they exhibited strong bone resorption, induction of osteoclasts and intracranial penetration at the tumor bone (TB)-interface.

Methods

In this study, we identified and used a TB microenvironment-specific gene expression signature from this model to extend our understanding of the metastatic bone microenvironment in human disease and to predict potential therapeutic targets.

Results

We identified a TB signature consisting of 934 genes that were commonly (among our 3 cell lines) and specifically (as compared to tumor-alone area within the bone microenvironment) up- and down-regulated >2-fold at the TB interface in our mouse osteolytic model. By comparing the TB signature with gene expression profiles from human breast metastases and an in vitro osteoclast model, we demonstrate that our model mimics both the human breast cancer bone microenvironment and osteoclastogenesis. Furthermore, we observed enrichment in various signaling pathways specific to the TB interface; that is, TGF-β and myeloid self-renewal pathways were activated and the Wnt pathway was inactivated. Lastly, we used the TB-signature to predict cyclopenthiazide as a potential inhibitor of the TB interface.

Conclusion

Our mouse breast cancer model morphologically and genetically resembles the osteoclastic bone microenvironment observed in human disease. Characterization of the gene expression signature specific to the TB interface in our model revealed signaling mechanisms operative in human breast cancer metastases and predicted a therapeutic inhibitor of cancer-mediated osteolysis.

Paracrine factors produced by bone marrow stromal cells induce apoptosis and neuroendocrine differentiation in prostate cancer cells

BACKGROUND

Preferential bony metastasis of human prostate cancer (PCa) cells contributes to disease mortality and morbidity. Local factors in bone stromal extracellular matrix microenvironment affect tumor growth through paracrine interactions between tumor and stromal cells.

METHODS

Using co-culture and medium transfer, we used several methods to assess interactions between PCa and bone stromal cells using three PCa cell lines: PC3, LNCaP, and the LNCaP derivative, C4-2B.

RESULTS

Co-culture of LNCaP and C4-2B cells with bone marrow stromal cell lines, HS27a and HS5, decreased cell number, as did culture with conditioned medium (CM) harvested from these two cell lines suggesting a soluble paracrine factor was responsible. PC3 cell growth was unaffected. CM harvested from bone stromal cell lines triggered apoptosis in LNCaP and C4-2B cell lines, but not in PC3 cells. Surviving C4-2B cells grown in bone stromal cell CM over several days were growth arrested, suggesting presence of a growth inhibitor. Apoptosis induced by CM was dose-dependent. Flow cytometry demonstrated that over a 5-day culture period in stromal cell CM, LNCaP, and C4-2B cell lines, but not PC3 cells, underwent greater apoptosis than parallel cultures in SF medium. The LNCaP and C4-2B cells showed morphology and biomarker expression consistent with transdifferentiation towards a neuroendocrine phenotype after exposure to stromal cell CM.

CONCLUSIONS

The reactive bone stromal microenvironment initially is hostile to PCa cells producing widespread apoptosis. Activation of transdifferentiation in a subset of apoptotic resistant cells may support phenotypic adaptation during disease progression in bone, eventually favoring lethal disease.

Breast cancer at bone metastatic sites: recent discoveries and treatment targets

Breast carcinoma is the most common cancer of women. Bones are often involved with breast carcinoma metastases with the resulting morbidity and reduced quality of life. Breast cancer cells arriving at bone tissues mount supportive microenvironment by recruiting and modulating the activity of several host tissue cell types including the specialized bone cells osteoblasts and osteoclasts. Pathologically activated osteoclasts produce osteolytic lesions associated with bone pain, pathological fractures, cord compression and other complications of metastatic breast carcinoma at bone. Over the last decade there has been enormous growth of knowledge in the field of osteoclasts biology both in the physiological state and in the tumor microenvironment. This knowledge allowed the development and implementation of several targeted therapeutics that expanded the armamentarium of the oncologists dealing with the metastases-associated osteolytic disease. While the interactions of cancer cells with resident bone cells at the established metastatic gross lesions are well-studied, the preclinical events that underlie the progression of disseminated tumor cells into micrometastases and then into clinically-overt macrometastases are just starting to be uncovered. In this review, we discuss the established information and the most recent discoveries in the pathogenesis of osteolytic metastases of breast cancer, as well as the corresponding investigational drugs that have been introduced into clinical development.

Interleukin-6 in bone metastasis and cancer progression

The bone and bone marrow are among the most frequent sites of cancer metastasis. It is estimated that 350,000 patients die with bone metastases annually in the United States. The ability of tumor cells to colonize the bone marrow and invade the bone is the result of close interactions between tumor cells and the bone marrow microenvironment. In this article, we review the contribution of interleukin-6 (IL-6) produced in the bone marrow microenvironment to bone metastasis. This cytokine has a strong pro-tumorigenic activity due to its multiple effects on bone metabolism, tumor cell proliferation and survival, angiogenesis, and inflammation. These effects are mediated by several signaling pathways, in particular the Janus kinase/signal transducer and transcription activator (JAK/STAT-3), Ras/mitogen activated protein kinase (MAPK), and phosphoinositol-3 kinase (PI3K)-protein kinase B/Akt (PkB/Akt), which are activated by IL-6 and amplified in the presence of soluble IL-6 receptor (sIL-6R). Supporting the role of IL-6 in human cancer is the observation of elevated serum levels of IL-6 and sIL-6R in patients with bone metastasis and their association with a poor clinical outcome. Over the last decade several large (monoclonal antibodies) and small (inhibitors of IL-6 mediated signaling) molecules that inhibit IL-6 activity in preclinical models have been developed. Several of these inhibitors are now undergoing phases I and II clinical trials, which will determine their inclusion in the list of effective targeted agents in the fight against cancer.

Enhanced expression and shedding of receptor activator of NF-kappaB ligand during tumor-bone interaction potentiates mammary tumor-induced osteolysis

The bone microenvironment plays a critical role in tumor-induced osteolysis and osteolytic metastasis through tumor-bone (TB)-interaction. Receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL) is one of the critical signaling molecules involved in osteolysis and bone metastasis. However, the regulation and functional significance of RANKL at the TB-interface in tumor-induced osteolysis remains unclear. In this report, we examined the role of tumor-stromal interaction in the regulation of RANKL expression and its functional significance in tumor-induced osteolysis. Using a novel mammary tumor model, we identified that RANKL expression was upregulated at the TB-interface as compared to the tumor alone area. We demonstrate increased generation of sRANKL at the TB-interface, which is associated with tumor-induced osteolysis. The ratio of RANKL to osteoprotegrin (OPG), a decoy receptor for RANKL, at the TB-interface was also increased. Targeting RANKL expression with antisense oligonucleotides (RANKL-ASO), significantly abrogated tumor-induced osteolysis, decreased RANKL expression and the RANKL:OPG ratio at the TB-interface. Together, these results demonstrate that upregulation of RANKL expression and sRANKL generation at the TB-interface potentiates tumor-induced osteolysis.

Applications of transgenics in studies of bone sialoprotein

Bone sialoprotein (BSP) is a major non-collagenous protein in mineralizing connective tissues such as dentin, cementum and calcified cartilage tissues. As a member of the Small Integrin-Binding Ligand, N-linked Glycoprotein (SIBLING) gene family of glycoproteins, BSP is involved in regulating hydroxyapatite crystal formation in bones and teeth, and has long been used as a marker gene for osteogenic differentiation. In the most recent decade, new discoveries in BSP gene expression and regulation, bone remodeling, bone metastasis, and bone tissue engineering have been achieved with the help of transgenic mice. In this review, we discuss these new discoveries obtained from the literatures and from our own laboratory, which were derived from the use of transgenic mouse mutants related to BSP gene or its promoter activity.

Mechanism of bone metastasis: the role of osteoprotegerin and of the host-tissue microenvironment-related survival factors

Osteoprotegerin (OPG), member of tumor necrosis factor (TNF) receptor superfamily, has various biological functions including bone remodeling. OPG binds to receptor activator of nuclear factor-kB ligand (RANKL) and prevents osteoclastic bone resorption. Recently, OPG has gained more clinical interest as its role in cancer-mediated bone destruction and the potential of RANKL inhibition could act as a novel treatment in tumor-induced bone disease. OPG protects prostate cancer cells from apoptotic effects of TRAIL and therefore provides tumor cells producing OPG with survival advantages. Additionally, the increased RANKL/OPG ratio in metastatic breast cancer results in severe osteolysis. Thus, bone formation and resorption are the crux of cancer metastasis, resulting in bone pain and pathological fractures. This review provides an overview of the role of OPG in cancer-induced bone disease.

Transforming growth factor-beta signaling at the tumor-bone interface promotes mammary tumor growth and osteoclast activation

Understanding the cellular and molecular changes in the bone microenvironment is important for developing novel therapeutics to control breast cancer bone metastasis. Although the underlying mechanism(s) of bone metastasis has been the focus of intense investigation, relatively little is known about complex molecular interactions between malignant cells and bone stroma. Using a murine syngeneic model that mimics osteolytic changes associated with human breast cancer, we examined the role of tumor-bone interaction in tumor-induced osteolysis and malignant growth in the bone microenvironment. We identified transforming growth factor-beta receptor 1 (TGF-betaRI) as a commonly upregulated gene at the tumor-bone (TB) interface. Moreover, TGF-betaRI expression and activation, analyzed by nuclear localization of phospho-Smad2, was higher in tumor cells and osteoclasts at the TB interface as compared to the tumor-alone area. Furthermore, attenuation of TGF-beta activity by neutralizing antibody to TGF-beta or TGF-betaRI kinase inhibitor reduced mammary tumor-induced osteolysis, TGF-betaRI expression and its activation. In addition, we demonstrate a potential role of TGF-beta as an important modifier of receptor activator of NF-kappaB ligand (RANKL)-dependent osteoclast activation and osteolysis. Together, these studies demonstrate that inhibition of TGF-betaRI signaling at the TB interface will be a therapeutic target in the treatment of breast cancer-induced osteolysis.

Targeted overexpression of BSP in osteoclasts promotes bone metastasis of breast cancer cells

Bone is one of the most common sites of breast cancer metastasis while bone sialoprotein (BSP) is thought to play an important role in bone metastasis of malignant tumors. The objective of this study is to determine the role of BSP overexpression in osteolytic metastasis using two homozygous transgenic mouse lines in which BSP expression is elevated either in all the tissues (CMV-BSP mice) or only in the osteoclasts (CtpsK-BSP mice). The results showed that skeletal as well as systemic metastases of 4T1 murine breast cancer cells were dramatically increased in CMV-BSP mice. In CtpsK-BSP mice, it was found that targeted BSP overexpression in osteoclasts promoted in vitro osteoclastogenesis and activated osteoclastic differentiation markers such as Cathepsin K, TRAP and NFAT2. MicroCT scan demonstrated that CtpsK/BSP mice had reduced trabecular bone volume and bone mineral density (BMD). The real-time IVIS Imaging System showed that targeted BSP overexpression in osteoclasts promoted bone metastasis of breast cancer cells. The osteolytic lesion area was significantly larger in CtpsK/BSP mice than in the controls as demonstrated by both radiographic and histomorphometric analyses. TRAP staining demonstrated a twofold increase in the number of osteoclasts in the bone lesion area from CtpsK/BSP mice compared with that from wild type mice. We conclude that host tissue-derived BSP also plays important roles in breast cancer metastasis through inducing tumor cell seeding into the remote host tissues. Furthermore, osteoclast-derived BSP promotes osteoclast differentiation in an autocrine manner and consequently promotes osteolytic bone metastasis of breast cancer.

Tumor cell dormancy: implications for the biology and treatment of breast cancer

Despite progress made in the therapy of solid tumors such as breast cancer, the prognosis of patients even with small primary tumors is still limited by metastatic relapse often long after removal of the primary tumor. Therefore, it has been hypothesized that primary tumors shed tumor cells already at an early stage into the blood circulation. A subset of these disseminated tumor cells may persist in a state of so-called "dormancy". Based on cell culture and animal models, dormancy can occur at two different stages. Single dormant cells are defined as cells with a lack of proliferation and apoptosis with the cells undergoing cell cycle arrest. The micrometastasis model defines tumor cell dormancy as a state of balanced apoptosis and proliferation of micrometastasis resulting in no net increase of tumor mass. Mechanisms leading to a growth activation of dormant tumor cells and the outgrowth of manifest metastases are not completely understood. Genetic predisposition of the dormant cells as well as immunological and angiogenetic influences of the surrounding environment may contribute to this phenomenon. In this review, we summarize findings on different factors for tumor cell dormancy and potential therapeutic implications that should help to reduce metastatic relapse in cancer patients.

Prostate cancer induces bone metastasis through Wnt-induced bone morphogenetic protein-dependent and independent mechanisms

Prostate cancer (PCa) is frequently accompanied by osteosclerotic (i.e., excessive bone production) bone metastases. Although bone morphogenetic proteins (BMP) and Wnts are mediators of PCa-induced osteoblastic activity, the relation between them in PCa bone metastases is unknown. The goal of this study was to define this relationship. Wnt3a and Wnt5a administration or knockdown of DKK-1, a Wnt inhibitor, induced BMP-4 and 6 expression and promoter activation in PCa cells. DKK-1 blocked Wnt activation of the BMP promoters. Transfection of C4-2B cells with axin, an inhibitor of canonical Wnt signaling, blocked Wnt3a but not Wnt5a induction of the BMP promoters. In contrast, Jnk inhibitor I blocked Wnt5a but not Wnt3a induction of the BMP promoters. Wnt3a, Wnt5a, and conditioned medium (CM) from C4-2B or LuCaP23.1 cells induced osteoblast differentiation in vitro. The addition of DKK-1 and Noggin, a BMP inhibitor, to CM diminished PCa CM-induced osteoblast differentiation in a synergistic fashion. However, pretreatment of PCa cells with DKK-1 before collecting CM blocked osteoblast differentiation, whereas pretreatment with Noggin only partially reduced osteoblast differentiation, and pretreatment with both DKK-1 and Noggin had no greater effect than pretreatment with DKK-1 alone. Additionally, knockdown of BMP expression in C4-2B cells inhibited Wnt-induced osteoblastic activity. These results show that PCa promotes osteoblast differentiation through canonical and noncanonical Wnt signaling pathways that stimulate both BMP-dependent and BMP-independent osteoblast differentiation. These results show a clear link between Wnts and BMPs in PCa-induced osteoblast differentiation and provide novel targets, including the noncanonical Wnt pathway, for therapy of PCa.

Breast adenocarcinoma MCF-7 cell line induces spontaneous osteoclastogenesis via a RANK-ligand-dependent pathway

The metastasis of breast cancer to the skeleton is a serious clinical problem resulting in hypercalcemia, bone fragility and insurmountable pain. The invasion of bony tissue by neoplastic cells usually very rapidly affects the balance between bone apposition and bone resorption. In order to elucidate a mechanism for cancer-induced osteoclastogenesis, cells from a human breast cancer line, MCF-7, were directly co-cultured with murine monocytes RAW 264.7 type CRL 2278. Compared with controls, co-culture of MCF-7 induced differentiation of multinucleated cells by membrane-bound and soluble receptor activator of NF-kB ligand (RANKL) as quantified by ELISA, Western blot analysis, transmission electron microscopy (TEM), and immunocytochemistry. The aim of this study was to determine an in vitro model system of MCF-7 human breast cancer cells grown together with monocytes to show that expression of RANKL promotes osteoclastogenesis, which may indicate a mechanism for the development of osteolytic lesions in breast cancer bone metastasis.