Skeletal metastasis of prostate adenocarcinoma in rats: morphometric analysis and role of parathyroid hormone-related protein

Dosimetry of bone seeking beta emitters for bone pain palliation metastases

Amongst cancer patients, bone pain due to skeletal metastases is a major cause of morbidity. A number of beta-emitting radiopharmaceuticals have been used to provide internal radiotherapy of bone metastases and provide palliative pain relief. In this article we describe the different physical characteristics of the various beta emitting radionuclides which have been used in this clinical setting and the potential impact of differences in dose-rate on radiobiological outcomes. A detailed review of the biodistribution of these treatments, based on both in-vivo clinical investigations and post mortem autoradiography assessments is provided. These treatments result in physiological delivery of radiation doses to the target disease as well as to critical healthy organs. Particular attention is paid to the radiation doses received by normal bone tissue, bone marrow as well as metastatic bone disease. The underlying models of radiation transport within bone and bone marrow are reviewed alongside the practical steps that must be taken to acquire and analyse the information require for clinical dosimetry assessments. The role of whole body measurements, blood and faecal assays as well as both planar and tomographic gamma camera imaging are considered. In addition we review the rationale for allocating measured bone uptake between trabecular and cortical bone tissue. The difference between bone volume and bone surface seeking radiopharmaceuticals are also discussed. This review also extends to the development of preclinical models of bone metastases which may inform future dosimetric calculations. Finally, we also present a comprehensive review of the dosimetry of the established treatments ⁸⁹Strontium-chloride; ³²Phosphorus; ¹⁸⁸Rhenium-hydroxyethylidine disphosphonate; ¹⁸⁶Rhenium-1,1-hydroxyethylidene disphosphonate (¹⁸⁶Re-HEDP); ¹⁵³Samarium-ethylenediaminetetramethylene phosphonate; as well as the emerging treatments ¹⁸⁸Rhenium-zoledronic acid; ¹⁸⁸Rhenium-ibedronat; ¹⁷⁷Lutetium-zoledronic acid; and ¹⁷⁷Lutetium ethylenediaminetetramethylene phosphonate. This review highlights not only the inter treatment differences in the radiation absorbed doses delivered to metastatic disease by different radiopharmaceuticals but also the intra treatment differences which result in a large range of observed doses between patients.

Conventional finite element models estimate the strength of metastatic human vertebrae despite alterations of the bone's tissue and structure

INTRODUCTION

Pathologic vertebral fractures are a major clinical concern in the management of cancer patients with metastatic spine disease. These fractures are a direct consequence of the effect of bone metastases on the anatomy and structure of the vertebral bone. The goals of this study were twofold. First, we evaluated the effect of lytic, blastic and mixed (both lytic and blastic) metastases on the bone structure, on its material properties, and on the overall vertebral strength. Second, we tested the ability of bone mineral content (BMC) measurements and standard FE methodologies to predict the strength of real metastatic vertebral bodies.

METHODS

Fifty-seven vertebral bodies from eleven cadaver spines containing lytic, blastic, and mixed metastatic lesions from donors with breast, esophageal, kidney, lung, or prostate cancer were scanned using micro-computed tomography (μCT). Based on radiographic review, twelve vertebrae were selected for nanoindentation testing, while the remaining forty-five vertebrae were used for assessing their compressive strength. The μCT reconstruction was exploited to measure the vertebral BMC and to establish two finite element models. 1) a micro finite element (μFE) model derived at an image resolution of 24.5 μm and 2) homogenized FE (hFE) model derived at a resolution of 0.98 mm. Statistical analyses were conducted to measure the effect of the bone metastases on BV/TV, indentation modulus (E_it), ratio of plastic/total work (W_Pl/W_tot), and in vitro vertebral strength (F_exp). The predictive value of BMC, μFE stiffness, and hFE strength were evaluated against the in vitro measurements.

RESULTS

Blastic vertebral bodies exhibit significantly higher BV/TV compared to the mixed (p = 0.0205) and lytic (p = 0.0216) vertebral bodies. No significant differences were found between lytic and mixed vertebrae (p = 0.7584). Blastic bone tissue exhibited a 5.8% lower median E_it (p< 0.001) and a 3.3% lower median W_pl/W_tot (p<0.001) compared to non-involved bone tissue. No significant differences were measured between lytic and non-involved bone tissues. F_exp ranged from 1.9 to 13.8 kN, was strongly associated with hFE strength (R²=0.78, p< 0.001) and moderately associated with BMC (R²=0.66, p< 0.001) and μFE stiffness (R²=0.66, p< 0.001), independently of the lesion type.

DISCUSSION

Our findings show that tumour-induced osteoblastic metastases lead to slightly, but significantly lower bone tissue properties compared to controls, while osteolytic lesions appear to have a negligible impact. These effects may be attributed to the lower mineralization and woven nature of bone forming in blastic lesions whilst the material properties of bone in osteolytic vertebrae appeared little changed. The moderate association between BMC- and FE-based predictions to fracture strength suggest that vertebral strength is affected by the changes of bone mass induced by the metastatic lesions, rather than altered tissue properties. In a broader context, standard hFE approaches generated from CTs at clinical resolution are robust to the lesion type when predicting vertebral strength. These findings open the door for the development of FE-based prediction tools that overcomes the limitations of BMC in accounting for shape and size of the metastatic lesions. Such tools may help clinicians to decide whether a patient needs the prophylactic fixation of an impending fracture.

Prostate Cancer and Bone Metastases: The Underlying Mechanisms

Patients with advanced prostate cancer often develop bone metastases, leading to bone pain, skeletal fracture, and increased mortality. Bone provides a hospitable microenvironment to tumor cells. The disease manifestation is driven by the interaction between invading tumor cells, bone-forming osteoblasts, and bone-resorbing osteoclasts. The increased level of osteoclast-activating factor (parathyroid hormone-related peptide, PTHrP) is believed to induce bone resorption by upregulating receptor activator of nuclear factor-kappa B ligand (RANKL) and the release of various growth factors into the bone microenvironment to enhance cancer cell growth. However, the underlying molecular mechanisms remain poorly understood. This review outlines the possible molecular mechanisms involved in governing bone metastases driven by prostate cancer, which further provide the basis in searching for new molecular targets for the development of potential therapy.

A loss of host-derived MMP-7 promotes myeloma growth and osteolytic bone disease in vivo

Matrix metalloproteinases (MMPs) play a critical role in cancer pathogenesis, including tumor growth and osteolysis within the bone marrow microenvironment. However, the anti-tumor effects of MMPs are poorly understood, yet have significant implications for the therapeutic potential of targeting MMPs. Host derived MMP-7 has previously been shown to support the growth of bone metastatic breast and prostate cancer. In contrast and underscoring the complexity of MMP biology, here we identified a tumor-suppressive role for host MMP-7 in the progression of multiple myeloma in vivo. An increase in tumor burden and osteolytic bone disease was observed in myeloma-bearing MMP-7 deficient mice, as compared to wild-type controls. We observed that systemic MMP-7 activity was reduced in tumor-bearing mice and, in patients with multiple myeloma this reduced activity was concomitant with increased levels of the endogenous MMP inhibitor, tissue inhibitor of metalloproteinases-1 (TIMP-1). Our studies have identified an unexpected tumour-suppressive role for host-derived MMP-7 in myeloma bone disease in vivo, and highlight the importance of elucidating the effect of individual MMPs in a disease-specific context.

Animal Models of Bone Metastasis

Bone is one of the most common sites of cancer metastasis in humans and is a significant source of morbidity and mortality. Bone metastases are considered incurable and result in pain, pathologic fracture, and decreased quality of life. Animal models of skeletal metastases are essential to improve the understanding of the molecular pathways of cancer metastasis and growth in bone and to develop new therapies to inhibit and prevent bone metastases. The ideal animal model should be clinically relevant, reproducible, and representative of human disease. Currently, an ideal model does not exist; however, understanding the strengths and weaknesses of the available models will lead to proper study design and successful cancer research. This review provides an overview of the current in vivo animal models used in the study of skeletal metastases or local tumor invasion into bone and focuses on mammary and prostate cancer, lymphoma, multiple myeloma, head and neck squamous cell carcinoma, and miscellaneous tumors that metastasize to bone.

Tumor microenvironment regulates metastasis and metastasis genes of mouse MMTV-PymT mammary cancer cells in vivo

Metastasis is the primary cause of death in breast cancer patients, yet there are challenges to modeling this process in vivo. The goal of this study was to analyze the effects of injection site on tumor growth and metastasis and gene expression of breast cancer cells in vivo using the MMTV-PymT breast cancer model (Met-1 cells). Met-1 cells were injected into 5 sites (subcutaneous, mammary fat pad, tail vein, intracardiac, and intratibial), and tumors and metastases were monitored using bioluminescent imaging and confirmed with gross necropsy and histopathology. Met-1 tumors were analyzed based on morphology and changes in gene expression in each tissue microenvironment. There were 6 permissible sites of Met-1 tumor growth (mammary gland, subcutis, lung, adrenal gland, ovary, bone). Met-1 cells grew faster in the subcutis compared to mammary fat pad tumors (highest Ki-67 index). Morphologic differences were evident in each tumor microenvironment. Finally, 7 genes were differentially expressed in the Met-1 tumors in the 6 sites of growth or metastasis. This investigation demonstrates that breast cancer progression and metastasis are regulated by not only the tumor cells but also the experimental model and unique molecular signals from the tumor microenvironment.

In vivo animal models of spinal metastasis

The vertebral column is the commonest site for skeletal metastases, with breast, prostate and lung cancers being the most common primary sources. The spine has structural and neural-protective properties thus involvement by metastatic cancer often causes bony instability and fracture, intractable pain and neurological deficit. In vivo animal models which resemble the human condition are essential in order to improve understanding of the pathophysiology behind the spread of metastatic cancer to the spine and its subsequent local growth and invasion, to enable in-depth analysis of the interaction between host and tumour cells and the molecular processes behind local cancer invasion and barriers to invasion as well as to allow assessment of novel treatment modalities for spinal metastases. This review summarizes the current status of the animal models specifically used for the study of spinal metastasis, their relevance, advantages and limitations, and important considerations for the development of future in vivo animal models.

Evaluating the effects of mixed osteolytic/osteoblastic metastasis on vertebral bone quality in a new rat model

Spinal metastases often show mixed areas of enhanced (osteoblastic) bone growth adjacent to areas of thinning (osteolytic) bone. This study aims to quantitatively characterize bone quality and tumor burden within a new rat model of mixed osteolytic/osteoblastic spinal metastases. Mixed vertebral metastases were analyzed in nude rats 21-days post intracardiac injection of Ace-1 canine prostate cancer cells. Vertebral micro-architecture was assessed in µCT images. Histologic processing quantified tumor burden (PTHrP), osteoclast activity (TRAP), and osteoid formation (Goldner's Trichrome) in ½ of all samples. Remaining samples were mechanically tested to failure in compression. Metastatically involved vertebrae exhibited extreme osteolysis, evident through an increase in osteoclasts leading to significantly reduced trabecular bone volume. Metastatically involved vertebrae also exhibited increased osteoid characteristic of osteoblastic lesions. While mechanical properties in tumor-bearing vertebrae were not significantly decreased compared to controls, a strong correlation was found between trabecular volumetric BMD and ultimate force. The highly aggressive Ace-1 skeletal metastases demonstrated predominant osteolysis with some areas of immature, new osteoblastic bone formation. Bone quality resulting from these lesions consisted of decreased structural properties, but without a significant impact on mechanical integrity.

Local treatment of mixed osteolytic/osteoblastic spinal metastases: is photodynamic therapy effective?

The widespread use of systemic and local therapies aimed at spinal metastatic lesions secondary to breast cancer has increased the incidence of mixed osteolytic/osteoblastic patterns of bony disease. The complex structure of these lesions requires novel therapeutic approaches to both reduce tumor burden and restore structural stability. In photodynamic therapy (PDT), a minimally invasive approach can be used to employ light to activate a photosensitizing agent that preferentially accumulates in tumor tissue, leading to cell toxicity and death. Previous work in an osteolytic rat model (MT-1) demonstrated that PDT effectively ablates tumor and improves vertebral structural properties. The aim of this study was to assess the efficacy of PDT in a rat model of mixed osteolytic/osteoblastic spinal metastases. Mixed spinal metastases were generated through intracardiac injection of Ace-1 canine prostate cancer cells into female athymic rats (day 0). A single PDT treatment was applied to lumbar vertebra L2 of tumor-bearing and healthy control rats (day 14). PDT-treated and untreated control rats were euthanized and excised spines imaged with μCT to assess bone quality (day 21). Spines were mechanically tested or histologically processed to assess mechanical integrity, tumor burden, and remodelling properties. Untreated tumor-bearing vertebrae showed large areas of osteolysis and areas of immature, new bone formation. The overall bone quality resulting from these lesions consisted of decreased structural properties but without a significant reduction in mechanical integrity. PDT was shown to significantly decrease tumor burden and osteoclastic activity, thereby improving vertebral bone structural properties. While non-tumor-bearing vertebrae exhibited significantly more new bone formation following PDT, the already heightened level of new bone formation in the mixed tumor-bearing vertebrae was not further increased. As such, the effect of PDT on mixed metastases may be more influenced by suppression of osteoclastic resorption as opposed to the triggering of new bone formation.

PSA affects prostate cancer cell invasion in vitro and induces an osteoblastic phenotype in bone in vivo

BACKGROUND

Patients with advanced prostate cancer frequently have a poor prognosis as a result of metastasis and present with high serum PSA levels. There is evidence suggesting that the serine protease activity of PSA could be involved in the invasion and metastasis of prostate cancer. In this study, we determined the effects of PSA and its precursor, pro-PSA, on invasion and the type of bone metastasis.

METHODS

We stably transfected prostate adenocarcinoma cells, human DU-145 and rat MatLyLu, with either the full-length prepro-PSA sequence or pre-PSA DNA, to generate subclones of cells that secrete pro-PSA or free PSA, respectively. Secretion of PSA was measured by western blot analysis and enzyme-linked immunosorbent assay (ELISA). The invasive and migratory properties of the cells were determined using a basement membrane extract and were compared with corresponding empty vector control cells. Twelve days after injection of PSA-secreting MatLyLu cells into the femora of nude mice, bone tumor burden and histomorphometry were determined using a stereological technique.

RESULTS

The transfected cells secreted 0.15-2.23 ng PSA/10(6) cells/day. Pro-PSA-secreting subclones increased invasion and migration by 24-263%. Conversely, the PSA-secreting subclones significantly reduced both invasion and migration by 59-70%. The divergent effects on invasion and migration observed in pro-PSA- and PSA-secreting subclones indicate that different forms of PSA may have different functions. Intrafemoral injections with PSA-secreting MatLyLu cells resulted in an increase in osteoblastic parameters when compared with non-PSA-secreting subclones as measured by bone histomorphometry. Concomitantly, a decrease in osteoclasts and eroded surface was observed.

CONCLUSIONS

Our in vitro data suggest that PSA, dependent on the predominant form secreted, may decrease or increase invasive properties of prostate cancer cells. The in vivo results indicate that PSA in the bone microenvironment may contribute to the osteoblastic phenotype of bone metastasis frequently observed in prostate cancer.

Clinical significance of interleukin (IL)-6 in cancer metastasis to bone: potential of anti-IL-6 therapies

Metastatic events to the bone occur frequently in numerous cancer types such as breast, prostate, lung, and renal carcinomas, melanoma, neuroblastoma, and multiple myeloma. Accumulating evidence suggests that the inflammatory cytokine interleukin (IL)-6 is frequently upregulated and is implicated in the ability of cancer cells to metastasize to bone. IL-6 is able to activate various cell signaling cascades that include the STAT (signal transducer and activator of transcription) pathway, the PI3K (phosphatidylinositol-3 kinase) pathway, and the MAPK (mitogen-activated protein kinase) pathway. Activation of these pathways may explain the ability of IL-6 to mediate various aspects of normal and pathogenic bone remodeling, inflammation, cell survival, proliferation, and pro-tumorigenic effects. This review article will discuss the role of IL-6: 1) in bone metabolism, 2) in cancer metastasis to bone, 3) in cancer prognosis, and 4) as potential therapies for metastatic bone cancer.

A single pretreatment by zoledronic acid converts metastases from osteolytic to osteoblastic in the rat

Bone metastases are severe complications of cancers associated with increased morbidity, pain, risk fracture, and reduced life span for patients. Bisphosphonates emerged as a relief treatment in bone metastases. A single dose of zoledronic acid (78 microg/kg) was injected into six Copenhagen rats 4 days before receiving an intraosseous inoculation of metastatic anaplastic tumor of lymph node and lung cell (MLL) prostate cancer cells. Rat femurs were analyzed for changes by microCT and histomorphometry; trabecular volume, trabecular characteristics, osteoid parameters, osteoblastic surfaces, and osteoclast number were measured. Values were compared to a group of SHAM animals, a group of SHAM animals having received zoledronic acid and animals inoculated with MLL cells. All rats were euthanized after 1 month. MLL cells induced osteolysis in the metaphysis with extension of the tumor to soft tissues through cortical perforations. Zoledronic acid induced a marked osteosclerosis in the primary spongiosa in both SHAM and rats inoculated with MLL. Osteosclerosis was obtained in the secondary spongiosa of MLL rats. The bisphosphonate preserved cortical integrity in all animals, and no extension to soft tissues was observed in most animals. The number of osteoclasts was elevated, indicating that there was no apoptosis of osteoclasts but they became inactive. Osteosclerosis was associated with increased osteoblastic surfaces. A single zoledronic acid injection turned osteolytic metastases into osteosclerotic and preserved cortical integrity.

EB1089 inhibits the parathyroid hormone-related protein-enhanced bone metastasis and xenograft growth of human prostate cancer cells

Parathyroid hormone-related protein (PTHrP) plays a major role in prostate carcinoma progression and bone metastasis. Once prostate cancers become androgen-independent, treatment options become limited. Vitamin D analogues represent a potentially valuable class of agents in this clinical context. Using the prostate cancer cell line C4-2 as a model, we studied the effects of PTHrP and the noncalcemic vitamin D analogue EB1089 on markers of prostate cancer cell progression in vitro and in vivo. C4-2 is a second-generation androgen-independent LNCaP subline that metastasizes to the lymph nodes and bone when injected into nude mice and produces mixed lytic/blastic lesions, mimicking the in vivo situation. We report that PTHrP increases cell migration and invasion, and that a pathway via which EB1089 inhibits these processes is through down-regulation of PTHrP expression. PTHrP also increases anchorage-independent cell growth in vitro and xenograft growth in vivo; EB1089 reverses these effects. The in vivo PTHrP effects are accompanied by increased tumor cell proliferation and survival. Treatment with EB1089 reverses the proliferative but not the antiapoptotic effects of PTHrP. PTHrP also increases intratumor vessel density and vascular endothelial growth factor expression; EB1089 reverses these effects. Intracardially injected C4-2 cells produce predominantly osteoblastic lesions; PTHrP overexpression decreases the latency, increases the severity and alters the bone lesion profile to predominantly osteolytic. EB1089 largely reverses these PTHrP effects. A direct correlation between PTHrP immunoreactivity and increasing tumor grade is observed in human prostate cancer specimens. Thus, decreasing PTHrP production by treatment with vitamin D analogues may prove therapeutically beneficial for prostate cancer.

A destructive cascade mediated by CCL2 facilitates prostate cancer growth in bone

Monocyte chemoattractant protein 1 (CCL2) is a recently identified prominent regulator of prostate cancer growth and metastasis. The purpose of this study was to investigate the mechanistic role of CCL2 in prostate cancer growth in bone. The present study found that CCL2 was up-regulated in osteoblasts (3-fold by PC-3 and 2-fold by VCaP conditioned medium) and endothelial cells (2-fold by PC-3 and VCaP conditioned medium). Parathyroid hormone-related protein (PTHrP) treatment of osteoblastic cells up-regulated CCL2 and was blocked by a PTHrP antagonist, suggesting that prostate cancer-derived PTHrP plays an important role in elevation of osteoblast-derived CCL2. CCL2 indirectly increased blood vessel formation in endothelial cells through vascular endothelial growth factor-A, which was up-regulated 2-fold with administration of CCL2 in prostate cancer cells. In vivo, anti-CCL2 treatment suppressed tumor growth in bone. The decreased tumor burden was associated with decreased bone resorption (serum TRAP5b levels were decreased by 50-60% in anti-CCL2-treated animals from VCaP or PC-3 cell osseous lesions) and microvessel density was decreased by 70% in anti-CCL2-treated animals with bone lesions from VCaP cells. These data suggest that a destructive cascade is driven by tumor cell-derived, PTHrP-mediated induction of CCL2, which facilitates tumor growth via enhanced osteoclastic and endothelial cell activity in bone marrow. Taken together, CCL2 mediates the interaction between tumor-derived factors and host-derived chemokines acting in cooperation to promote skeletal metastasis.

Mesenchymal stem cell secretion of chemokines during differentiation into osteoblasts, and their potential role in mediating interactions with breast cancer cells

Over 70% of patients with advanced breast cancer will develop bone metastases for which there is no cure. Mesenchymal Stem Cells (MSCs) and their derivative osteoblasts are subpopulations of cells within the bone marrow environment, postulated as potential interacting targets for disseminating cancer cells because of their ability to secrete a range of chemokines. This study aimed to investigate chemokine secretion throughout MSC differentiation into osteoblasts and their effect on the breast cancer cells. Primary MSCs and osteoblast progenitors were cultured in appropriate conditions to induce differentiation into mature osteoblasts. Chemokines secreted throughout differentiation were detected using ChemiArray and ELISA. Migration of breast cancer cells in response to the bone-derived cells was quantified using Transwell inserts. Breast cancer cells were cocultured with MSCs, retrieved using magnetic beads, and changes in CCL2 expression were analyzed. MSCs secreted a range of factors including IL-6, TIMP-1 and CCL2, the range and level of which changed throughout differentiation. CCL2 secretion by MSCs increased significantly above control cells as they differentiated into mature osteoblasts (p<0.05). The bone-derived cells stimulated migration of breast cancer cells, and this was inhibited (21-50%) in the presence of a CCL2 antibody. CCL2 gene expression in breast cancer cells was upregulated following direct coculture with MSCs. The varying levels of chemokines secreted throughout MSC differentiation may play an important role in supporting tumor cell homing and progression. These results further highlight the distinct effect MSCs have on breast cancer cells and their potential importance in supporting development of metastases.

Relevance of a new rat model of osteoblastic metastases from prostate carcinoma for preclinical studies using zoledronic acid

Animal models that mimic osteoblastic metastases associated with prostate carcinoma are required to improve the therapeutic options in humans. A new model was then developed and characterized in immunocompetent rats. The bisphosphonate zoledronic acid (ZOL) was tested to validate this model as a therapeutic application. Rat AT6-1 prostate tumor cells were characterized in vitro at the transcriptional (bone and epithelial markers) and functional (induction of mineralized nodules) levels. The bone lesions induced after their direct injection into the femur bone marrow were characterized by radiography, microscanner and histology analyses. ZOL effects were studied in vivo on bone lesion development and in vitro on AT6-1 cell proliferation, apoptosis and cell cycle analysis. Apart from epithelial markers, AT6-1 cells express an osteoblast phenotype as they express osteoblastic markers and are able to induce mineralized nodule formation in vitro. A disorganization of the trabecular bone at the growth zone level was observed in vivo after intraosseous AT6-1 cell injection as well as cortical erosion. The tumor itself is associated with bone formation as revealed by SEM analysis and polarized light microscopy. ZOL prevents the development of such osteoblastic lesions, related to a direct inhibitory effect on tumor cell proliferation independent of caspase 3 activation, but associated with cell cycle arrest. A new rat model of osteoblastic bone metastases was validated in immunocompetent rats and used to show the relevance of using ZOL in such lesions, as this compound shows bifunctional effects on both bone remodelling and tumor cell proliferation.

Multimodality imaging for vertebral metastases in a rat osteolytic model

Imaging modalities facilitate the detection of early bony metastases. Few studies specifically address vertebral metastases in animal models for preclinical (early, asymptomatic) disease. We performed intracardiac injection of human breast cancer (MT-1) cells in 35 athymic nude rats. We evaluated potential temporal differences in appendicular versus axial metastases as detectable by longitudinal in vivo conventional radiography (ie, fine detail radiography and two-dimensional fluoroscopy). We compared bioluminescent reporter imaging with conventional radiographs in the detection of vertebral metastasis, and compared bioluminescent imaging with subsequent ex vivo microcomputed tomography analysis of osteolysis. The mean survival was 25 days in the animals that had metastases develop. Conventional radiographs identified appendicular osteolysis by 14 days; however, vertebral osteolysis was identified late in the metastatic spread (Days 25-28). Bioluminescence imaging was more sensitive in earlier detection of vertebral lesions in all imaged animals at Day 21, which corresponded to microcomputed tomography evaluation of osteolysis. Conventional radiographs do not appear useful for early detection of vertebral metastasis. Early identification of metastasis is important when considering the use of this model to evaluate therapeutic outcomes directed toward vertebral metastasis.

Combination therapy using LHRH and somatostatin analogues plus dexamethasone in androgen ablation refractory prostate cancer patients with bone involvement: a bench to bedside approach

The development of resistance to anticancer therapies is a major hurdle in preventing long-lasting clinical responses to conventional therapies in hormone-refractory prostate cancer. Herein, the molecular evidence documenting that bone metastasis microenvironment survival factors (mainly the paracrine growth hormone-independent, urokinase-type plasminogen activator-mediated increase of IGF-1 and the endocrine production of growth hormone-dependent IGF-1, mainly liver-derived IGF-1 production) produce an epigenetic form of prostate cancer cells that are resistant to proapoptotic therapies is reviewed. Consequently, the authors present the conceptual framework of a novel antibone microenvironment survival factor, mainly an anti-IGF-1 hormonal manipulation for androgen ablation refractory prostate cancer (a combination of conventional androgen ablation therapy [luteinising hormone-releasing hormone agonist-A or orchiectomy]) with dexamethasone plus somatostatin analogue, which yielded durable objective responses and major improvement of bone pain and performance status in stage D3 prostate cancer patients.

Assessing Tumor Growth and Distribution in a Model of Prostate Cancer Metastasis using Bioluminescence Imaging

Bioluminescence imaging (BLI) has greatly facilitated the development of animal models of cancer, allowing sensitive detection of luciferase-expressing cancer cells in living mice. Previous efforts characterizing such models have involved small numbers of animals, limiting understanding of their performance features. We employed BLI to serially image the growth and distribution of a prostate cancer cell line, 22Rv1, after intracardiac injection into scid mice (n = 85). This approach models hematogenous dissemination of cancer cells and allows inquiry of the process of metastatic colonization at various organ sites, although accurately injecting cancer cells into the left ventricle remains challenging. Therefore, to predict injection success we measured the ratio of the thoracic bioluminescence signal to the whole body bioluminescence signal (T/WB ratio) immediately following intracardiac injection. A T/WB ratio less than 0.50 predicted the development of tumors outside of the thoracic cavity while a T/WB greater than 0.50 predicted the development of tumors entirely within the thoracic cavity, suggestive of a failed injection. Progressive tumor growth was quantified using BLI. Tumors colonized multiple organ sites including bone, liver, and adrenal glands resembling the spectrum of metastases in autopsy studies of patients with prostate cancer. Tumors growing in bone exhibited mixed osteolytic and osteoblastic features, eliciting a spiculated periosteal response. With the ability to more accurately predict injection success, we can now monitor efficacy of intracardiac injections facilitating the performance of this model.

Regulation of SDF-1 (CXCL12) production by osteoblasts; a possible mechanism for stem cell homing

Stromal derived factor-1 (SDF-1 or CXCL12) controls many aspects of stem cell function including trafficking and proliferation. Previously, it was demonstrated that DNA-damaging agents such as irradiation, cyclophosphamide or 5-fluorouracil increase the expression of SDF-1 by osteoblasts in murine marrow. Here, the production of SDF-1 by osteoblasts in vitro in response to cytokines known to be particularly important in bone physiology was examined using primary human osteoblasts (HOBs), mixed marrow stromal cells (BMSCs), and by, mouse, rat and human osteoblast-like cell lines. From these studies, it was determined that the expression of SDF-1 is an early feature of osteoblastic induction that may be modulated by IL-1beta, PDGF-BB, VEGF, TNF-alpha and PTH. Each of these factors increased SDF-1 synthesis, while TGF-beta1 decreased SDF-1 secretion. Of note, the biodistribution of SDF-1 in culture was equally distributed between the medium and detergent-soluble and -insoluble fractions of the cultures. Immunohistochemistry of developing bones demonstrated that SDF-1 was also a feature of early bone development first beginning in the perichondrium and moving into the marrow cavity of the developing bone analogue. As SDF-1 expression increases in response to PTH in vitro, animals were treated with an anabolic regime of PTH for 21 days. Under these conditions, significant increases in SDF-1 mRNA expression were observed near the growth plate and epiphysis regions of the long bones. Yet, in serum, immunodetectable SDF-1 levels were significantly reduced (24%) in the PTH-treated animals (Vehicle: 408 +/- 25 vs. PTH 308 +/- 20 SDF-1 pg/ml). Together, these data suggest a possible mechanism for localizing stem cells into a developing marrow where increased expression of SDF-1 in the local marrow environment along with decreased SDF-1 in the serum may create a homing gradient.

The use of micro-CT to evaluate cortical bone geometry and strength in nude rats: correlation with mechanical testing, pQCT and DXA

In both clinical and experimental settings, access to quantitative methods enabling the objective evaluation of cortical bone mass, structure, geometry and strength are essential for the assessment of efficacy and safety of different treatments aimed to improve bone strength. The ability of non-invasive methodologies (DXA, pQCT and micro-CT) to assess and quantify cortical bone mass and geometry was tested in a nude rat model in which bone loss was induced by surgical castration. Treatment with a bone antiresorptive (alendronate) or a bone forming (PTH) drug was used to: (A) validate the nude rat model in terms of bone metabolism, (B) test the ability of each technology to detect change in cortical bone geometry and (C) correlate cortical bone geometry with bone strength data obtained by 3-point bending method. Our observations regarding effect of castration and treatment with PTH and alendronate on cortical bone parameters in nude rats is in general agreement with previously published data obtained in immunocompetent male rats under similar experimental conditions. Data presented here support the hypothesis that nude rats have similar bone physiology and response to known bone therapies to that observed in normal rats and therefore could be effectively used to predict skeletal response in humans. All three technologies deployed in this study (DXA, pQCT and micro-CT) proved useful in describing cancellous and/or cortical bone parameters and positive correlations were demonstrated between data obtained by different methods. The cross-sectional area of a bone structure is crucial for resisting loads in bending or torsion and is described as "areal moment of inertia" for bending, and as "polar moment of inertia" in torsion. Novel, three-dimensional micro-CT methodology used in this study to assess geometry of cortical bone provides data that accurately describes cortical bone geometry and parallels cortical bone strength results obtained by the 3-point bending method. Our micro-CT data meet the criteria of providing quick, reproducible and accurate answers regarding cortical bone geometry as a predictor of cortical bone strength.

Bone morphogenetic protein-6 promotes osteoblastic prostate cancer bone metastases through a dual mechanism

Prostate cancer frequently metastasizes to bone where it forms osteoblastic lesions through unknown mechanisms. Bone morphogenetic proteins (BMP) are mediators of skeletal formation. Prostate cancer produces a variety of BMPs, including BMP-6. We tested the hypothesis that BMP-6 contributes to prostate cancer-induced osteosclerosis at bone metastatic sites. Prostate cancer cells and clinical tissues produced BMP-6 that increased with aggressiveness of the tumor. Prostate cancer-conditioned medium induced SMAD phosphorylation in the preosteoblast MC3T3 cells, and phosphorylation was diminished by anti-BMP-6 antibody. Prostate cancer-conditioned medium induced mineralization of MC3T3 cells, which was blocked by both the BMP inhibitor noggin and anti-BMP-6. Human fetal bones were implanted in severe combined immunodeficient mice and after 4 weeks, LuCaP 23.1 prostate cancer cells were injected both s.c. and into the bone implants. Anti-BMP-6 or isotype antibody administration was then initiated. Anti-BMP-6 reduced LuCaP 23.1-induced osteoblastic activity, but had no effect on its osteolytic activity. This was associated with increased osteoblast numbers and osteoblast activity based on bone histomorphometric evaluation. As endothelin-1 has been implicated in bone metastases, we measured serum endothelin-1 levels but found they were not different among the treatment groups. In addition to decreased bone production, anti-BMP-6 reduced intraosseous, but not s.c., tumor size. We found that BMP-2, BMP-4, BMP-6, and BMP-7 had no direct effect on prostate cancer cell growth, but BMP-2 and BMP-6 increased the in vitro invasive ability of prostate cancer cell. These data show that prostate cancer promotes osteoblastic activity through BMP-6 and that, in addition to its bone effects, suggest that BMPs promote the ability of the prostate cancer cells to invade the bone microenvironment.

Prostate secretory protein of 94 amino acids (PSP-94) and its peptide (PCK3145) as potential therapeutic modalities for prostate cancer

This review focuses on the promising roles of prostate secretory protein of 94 amino acids (PSP-94) and one of its derived peptides (PCK3145) as potential therapeutic modalities for prostate cancer and its associated complications. Evaluation of these compounds was carried out in vitro and in vivo using syngeneic models of rat prostate cancer. Overproduction of parathyroid hormone-related protein (PTHrP) results in the development of hypercalcemia of malignancy in several malignancies including prostate cancer. In order to evaluate the effect of PSP-94 and PCK3145 on prostate cancer progression, the rat Dunning R3227 MatLyLu cell line transfected with full-length cDNA encoding PTHrP (MatLyLu-PTHrP) was used. As the main pathogenetic factor of hypercalcemia of malignancy, overexpression of PTHrP was aimed at mimicking the hypercalcemic nature seen in patients suffering from late-stage cancer. In vitro studies showed that PSP-94 and PCK3145 can cause a dose-dependent inhibition in the growth of MatLyLu-PTHrP cells. For in vivo studies, male Copenhagen rats were inoculated either s.c. into the right flank or directly into the left ventricle via intracardiac (i.c.) inoculation with MatLyLu-PTHrP cells. In these models, s.c. injection of MatLyLu cells results in the development of primary tumor growth, whereas i.c. inoculation routinely results in the development of experimental skeletal metastases in the lumbar vertebrae causing hind-limb paralysis. Administration of PSP-94 and PCK3145 into tumor-bearing animals resulted in a dose-dependent inhibition of primary tumor growth, and tumoral and plasma PTHrP levels, and in the reduction of plasma calcium levels. Additionally, treatment with PSP-94 or PCK3145 caused an inhibition of skeletal metastases resulting in a significant delay in the development of hind-limb paralysis. Interestingly, equimolar concentrations of PCK3145 were shown to be more effective in delaying the development of experimental skeletal metastases as compared to PSP-94. One of the possible mechanisms of action of these modalities is through the induction of apoptosis which was observed by both in-vitro and in-vivo analyses of MatLyLu-PTHrP cells and tumors. Several intracellular mechanisms can also be involved in inhibiting PTHrP production and anti-tumor effects of PSP-94 and PCK3145. Collectively, these studies warrant the continued clinical development of these agents as therapeutic agents for patients with hormone-refractory prostate cancer.

Bisphosphonates Treatment in Metastatic Prostate Cancer

The skeleton is the third most common site for cancer to spread after the liver and lungs. Malignancies that can cause destruction of skeletal bones include multiple myeloma and metastatic diseases of the lung, prostate and other solid cancers. The clinical complications include pain, fractures, compression of the spinal cord and hypercalcemia of malignancy. Bisphosphonates are bone-seeking agents originally designed to treat loss of bone density. Accumulating data show that they are effective in diseases in which there is upregulation of osteoclastic or osteolytic activity. Bisphosphonates can reduce skeletal-related events and bone pain, as well as reduce the adverse effects of androgen deprivation therapy on skeletal integrity. However, it is clear that bisphosphonates do not represent a decisive treatment in the care of metastases but a therapeutic choice in synergy with regular anti-tumor drugs. The preclinical and clinical data to support this are reviewed here.

A synthetic 15-mer peptide (PCK3145) derived from prostate secretory protein can reduce tumor growth, experimental skeletal metastases, and malignancy-associated hypercalcemia

In previous studies, we have shown that prostate secretory protein (PSP-94) can reduce prostate cancer growth in vivo. In the current study, we identified the amino acid sequence of PSP-94 that is required for eliciting this response. For these studies, we used rat prostate cancer Mat Ly Lu cells overexpressing parathyroid hormone-related protein (PTHrP), which is the main pathogenetic factor responsible for hypercalcemia of malignancy. Synthetic peptides corresponding to amino acids 7-21 (PCK721), 31-45 (PCK3145), and 76-94 (PCK7694) of PSP-94 were synthesized. Only PCK3145 showed a significant reduction in tumor cell proliferation. For in vivo studies, syngenic male Copenhagen rats were inoculated s.c. with Mat Ly Lu cells overexpressing PTHrP into the right flank or into the left ventricle via intracardiac injection, which results in experimental metastases to the lumbar vertebrae causing hind-limb paralysis. Animals were infused with different doses (1, 10, and 100 microg/kg/day) of peptides for 15 days, and the effect of these treatments on tumor volume, skeletal metastases, or development of hind-limb paralysis was determined. Treatment with PCK3145 resulted in a dose-dependent decrease in tumor volume and delay in the development of skeletal metastases. Bone histomorphometry showed that after intracardiac inoculation of tumor cells, the highest dose of PCK3145 (100 microg/kg/day) resulted in reducing skeletal tumor burden, which delayed the development of hind-limb paralysis. Treatment with PCK3145 led to reduction of plasma calcium and PTHrP levels and a significant decrease in PTHrP levels in the primary tumors and in vertebrae of experimental animals. These effects of PCK3145 were due to its ability to promote tumor cell apoptosis. Collectively, the results of these studies have demonstrated the ability of a small peptide derived from PSP-94 to reduce tumor volume and experimental skeletal metastases-results that will be highly beneficial in the continued development of this peptide as a novel therapeutic agent for patients with hormone refractory, late-stage prostate cancer.

Endothelins in bone cancer metastases

Most evidence indicates that osteoblastic bone metastases are due to tumor-produced factors that stimulate the osteoblast. This review supports a causal role for ET-1. Based on our results, we propose a model to explain the tumor cell and bone interactions that are responsible for the osteoblastic response (Figure 2). Tumor cells housed in bone produce factors, such as ET-1, stimulate osteoblast activity. This results in the abundant and disorganized new bone formation that is characteristic of osteoblastic metastases. The effects of ET-1 to stimulate bone formation are mediated by ETA receptors on the osteoblast. ETA receptor inhibition successfully blocked osteoblastic bone metastases in a mouse model. These receptor antagonists are currently in clinical trials for advanced prostate cancer and bone metastases (Stephenson, 2001; Carducci et al., 2002; 2003). Therefore, the molecular mechanisms responsible for osteoblastic metastases are complex and involve bi-directional interactions between tumor cells and bone. Elucidation of the interactions at a molecular level can identify therapeutic targets for osteoblastic metastases. Although ET-1 and ETA receptors are potential targets for this devastating complication of cancer (Remuzzi et al., 2003), they are certainly not the only ones. The rapid pace of metastasis research, will not only expand our therapeutic armamentarium against bone metastases, but will also provide insight into achieving the ultimate goal: the prevention of cancer metastases to bone.

Prostate cancer bone metastases promote both osteolytic and osteoblastic activity

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

Animal models of bone metastasis

Animal models will continue to be indispensable to investigate the pathogenesis of bone metastasis in vivo, conduct preclinical chemotherapeutic, chemoprevention and genetic therapy studies, test gene delivery mechanisms, and identify metastasis suppressor and inducer genes. It is likely that the bone marrow microenvironment, such as the endothelial cells, stromal cells, hematopoietic cells, bone cells, and the intercellular matrix play important roles in the localization and clonal growth of cancer cells in bone. Given the complexity of bone metastasis, many genes are expected to be involved in the pathogenesis and few are likely indispensable. The use of genomic and proteomic approaches to study these animal models will identify key targets for therapeutic intervention. As we further refine these models and use imaging for real-time evaluation of cells, and eventually target genes, these models will more closely mirror human disease and will hopefully become more predictive of the human response to therapy.

Mechanisms of osteoblastic metastases: role of endothelin-1

Certain solid tumors metastasize to bone, causing an osteoblastic response. The mechanisms by which tumor cells stimulate this new bone formation are not understood completely. We identified three breast cancer lines that cause osteoblastic metastases in female nude mice and provide evidence that tumor-produced endothelin-1 (ET-1) mediates the osteoblastic response. Tumor-conditioned media and exogenous ET-1 stimulated osteoblast proliferation and new bone formation in cultures of calvarias from mice. These effects were blocked by endothelin A (ETA) but not by ETB receptor antagonists. Mice inoculated with the ZR-75-1 breast cancer line and treated with a selective ETA receptor antagonist (ABT-627) had significantly fewer osteoblastic bone metastases and less tumor burden compared with untreated mice. In contrast, there was no effect of ABT-627 on osteolytic bone metastases caused by ET-1-negative breast cancer, MDA-MB-231. ABT-627 had no effect on cell growth in vitro or at the orthotopic site (mammary fat pad) of ZR-75-1, or MDA-MB-231 cells. Collectively, the data suggest that tumor-produced ET-1 mediates osteoblastic bone metastases by stimulating osteoblast proliferation and new bone formation. Endothelin A receptor blockade may be useful for the prevention and treatment of osteoblastic bone metastases attributable to breast or prostate cancer.

Rationale for the use of bisphosphonates in osteoblastic and osteolytic bone lesions

Various primary malignancies develop bone metastases, and the resultant skeletal complications cause significant morbidity/mortality in advanced cancer patients. Bone lesions associated with metastases are traditionally classified radiologically as either osteolytic or osteoblastic, and both types of lesions are associated with elevated levels of specific bone resorption markers. Some common aspects in the pathophysiology of bone lesions have prompted speculation that treatments for osteolytic metastases might also be effective for predominantly osteoblastic metastases, such as in prostate cancer. Potent osteoclast activity inhibitors, bisphosphonates have been successful in the treatment of osteolytic tumor bone disease. Zoledronic acid is the first bisphosphonate shown to have a direct clinical benefit in the treatment of osteoblastic bone metastases, reducing the number and rate of skeletal events in prostate cancer patients with metastatic bone disease. Moreover, the shorter, more convenient infusion time and similar safety profile of 4 mg zoledronic acid compared with 90 mg pamidronate presently make zoledronic acid the preferred therapy for treatment of bone metastases in patients with all types of advanced malignancy.

Animal models of bone metastasis

BACKGROUND

Animal models are important tools to investigate the pathogenesis and develop treatment strategies for bone metastases in humans. However, there are few spontaneous models of bone metastasis despite the fact that rodents (rats and mice) and other animals (dogs and cats) often spontaneously develop cancer. Therefore, most experimental models of bone metastasis in rodents require injection or implantation of neoplastic cells into orthotopic locations, bones, or the left ventricle of the heart.

METHODS

The current study reviews the natural incidence and clinical manifestation of bone metastases of mammary and prostate carcinoma in animals, as well as the experimental models developed in mice using animal and human-derived neoplasms.

RESULTS

Rats, mice, dogs, and cats often develop spontaneous mammary carcinoma, but bone metastases are rare. Intact and neutered dogs develop prostate carcinoma that is usually androgen independent and may be associated with regional bone invasion or distant bone metastasis. Normal dog prostate tissue induces new bone formation in vivo and can serve as a model of osteoblastic metastasis without concurrent bone destruction. Experimental models of osteolytic, osteoblastic, and mixed osteolytic/osteoblastic bone metastases include syngeneic rodent neoplasms or human xenografts implanted at orthotopic sites (e.g., breast or prostate glands) in immunodeficient mice, injection of cancer cells into the left ventricle of the heart, or direct injection into bones. New transgenic mouse models of cancer have a low incidence of spontaneous bone metastasis, but cell lines derived from these tumors can be selected in vivo for increased incidence of bone metastasis. It is essential to validate and correctly interpret the lesions in models of bone metastasis to accurately correlate the data from animal models to human disease. Animal models have provided support for the "seed and soil" hypothesis of bone metastasis. However, the roles of vascular patterns in the metaphyses of long bones and rapid bone turnover in young animals in the pathogenesis of metastasis in experimental models are uncertain. Improvements in the imaging of experimental animals in vivo using fluorescent markers or light emitted from luciferase have led to increased sensitivity of detection and more accurate quantification of bone metastases. For example, imaging of human prostate carcinoma PC-3M cells transfected with luciferase, following injection into the left ventricle, has demonstrated that there is rapid localization of tumor cells to bones and other organs, such as the kidneys and lungs.

CONCLUSIONS

Animal models of metastasis have supported drug development and have been useful for identification of metastasis suppressor and promoter genes as novel targets for the development of novel therapies. Further refinement of these models will involve spatiotemporal analysis of the metastatic process by imaging and use of image data to stage disease and guide tissue sampling for gene expression profiling via gene array technology. In the future, integrated analyses of these models will be needed to understand the complexities of this important disease process.

New bone formation in nude mouse calvaria induced by canine prostate tissue

Osteoblastic metastases are common in patients with advanced prostate cancer. The pathophysiology of the new bone formation at metastatic sites is not currently known, but it is hypothesized that growth factors secreted by the prostate may be involved. Unfortunately, most rodent models of prostate cancer with metastasis to bone are osteolytic and not osteoblastic. Significant osteolysis by tumor cells at metastatic sites also may lead to fractures or bone instability. Misinterpretation of new periosteal bone due to bone instability as tumor-cell osteo-induction is another disadvantage of the osteolytic models. To circumvent these problems, we have developed a model system of new bone formation in the calvaria of nude mice stimulated by normal canine prostate tissue. Collagenase-digested normal prostate tissue was implanted adjacent to the calvaria of nude mice. Calvaria were examined at 2 weeks post-implantation for changes in the bone microenvironment by histology, calcein uptake at sites of bone mineralization, and tartrate-resistant acid phosphatase staining for osteoclasts. The prostate tissue remained viable and induced abundant new woven bone formation on the adjacent periosteal surface. In some cases new bone formation also was induced on the distant or concave calvarial periosteum. The new bone stained intensely with calcein, which demonstrated mineralization of the bone matrix. The new bone formation on prostate-implanted calvaria significantly increased (1.7-fold) the thickness of the calvaria compared with control calvaria. New bone formation was not induced in calvaria of mice implanted with normal canine kidney, urinary bladder, spleen, or skeletal muscle tissue, or mice with surgically-induced disruption of the periosteum. Osteoclast numbers in the medullary spaces and periosteum of calvaria were mildly increased (61%) in mice with implanted prostate tissue. In conclusion, this animal model will be useful for investigating the roles of prostate-derived growth factors on new bone formation in vivo.

Canine prostate induces new bone formation in mouse calvaria: A model of osteoinduction by prostate tissue

BACKGROUND

Osteoblastic metastases are common in patients with advanced prostate cancer. The pathophysiology of the new bone formation at metastatic sites is not currently known, but it is hypothesized that growth factors secreted by the prostate may be involved. Unfortunately, most rodent models of prostate cancer with metastasis to bone are osteolytic and not osteoblastic. Significant osteolysis by tumor cells at metastatic sites may also lead to fractures or bone instability. Misinterpretation of new periosteal bone due to bone instability as tumor-cell osteoinduction is another disadvantage of the osteolytic models. To circumvent these problems, we have developed a model system of new bone formation in the calvaria of nude mice stimulated by normal canine prostate tissue.

METHODS

Collagenase-digested normal prostate tissue was implanted adjacent to the calvaria of nude mice. Calvaria were examined at 2 weeks post-implantation for changes in the bone microenvironment by histology, calcein uptake at sites of bone mineralization, and tartrate-resistant acid phosphatase staining for osteoclasts.

RESULTS

The prostate tissue remained viable and induced abundant new woven bone formation on the adjacent periosteal surface. In some cases new bone formation also was induced on the distant or concave calvarial periosteum. The new bone stained intensely with calcein, which demonstrated mineralization of the bone matrix. The new bone formation on prostate-implanted calvaria significantly increased (1.7-fold) the thickness of the calvaria compared with control calvaria. New bone formation was not induced in calvaria of mice implanted with normal canine kidney, urinary bladder, spleen, or skeletal muscle tissue, or mice with surgically-induced disruption of the periosteum. Osteoclast numbers in the medullary spaces and periosteum of calvaria were mildly increased (61%) in mice with implanted prostate tissue.

CONCLUSIONS

This animal model will be useful for investigating the roles of prostate-derived growth factors on new bone formation in vivo.