Osteocytes and Paget's Disease of Bone.

Purpose Of Review: To describe the contributions of osteocytes to the lesions in Paget's disease, which are characterized by locally overactive bone resorption and formation.

Recent Findings: Osteocytes, the most abundant cells in bone, are altered in Paget's disease lesions, displaying increased size, decreased canalicular length, incomplete differentiation, and less sclerostin expression compared to controls in both patients and mouse models. Pagetic lesions show increased senescent osteocytes that express RANK ligand, which drives osteoclastic bone resorption.

Pharmacologic targeting of the p62 ZZ domain enhances both anti-tumor and bone-anabolic effects of bortezomib in multiple myeloma.

Multiple myeloma (MM) is a malignancy of plasma cells whose antibody secretion creates proteotoxic stress relieved by the N-end rule pathway, a proteolytic system that degrades N-arginylated proteins in the proteasome. When the proteasome is inhibited, protein cargo is alternatively targeted for autophagic degradation by binding to the ZZ-domain of p62/ sequestosome-1. Here, we demonstrate that XRK3F2, a selective ligand for the ZZ-domain, dramatically improved two major responses to the proteasome inhibitor bortezomib (Btz) by increasing: i) killing of human MM cells by stimulating both Btz-mediated apoptosis and necroptosis, a process regulated by p62; and ii) preservation of bone mass by stimulating osteoblast differentiation and inhibiting osteoclastic bone destruction.

Osteoclast-derived IGF1 induces RANKL production in osteocytes and contributes to pagetic lesion formation.

We previously reported that measles virus nucleocapsid protein (MVNP) expression in osteoclasts (OCLs) of patients with Paget disease (PD) or targeted to the OCL lineage in MVNP-transgenic mice (MVNP mice) increases IGF1 production in osteoclasts (OCL-IGF1) and leads to development of PD OCLs and pagetic bone lesions (PDLs). Conditional deletion of Igf1 in OCLs of MVNP mice fully blocked development of PDLs. In this study, we examined whether osteocytes (OCys), key regulators of normal bone remodeling, contribute to PD.

Osteoclast-derived IGF1 is required for pagetic lesion formation in vivo.

We report that transgenic mice expressing measles virus nucleocapsid protein (MVNP) in osteoclasts (OCLs) (MVNP mice) are Paget's disease (PD) models and that OCLs from patients with PD and MVNP mice express high levels of OCL-derived IGF1 (OCL-IGF1). To determine OCL-IGF1's role in PD and normal bone remodeling, we generated WT and MVNP mice with targeted deletion of Igf1 in OCLs (Igf1-cKO) and MVNP/Igf1-cKO mice, and we assessed OCL-IGF1's effects on bone mass, bone formation rate, EphB2/EphB4 expression on OCLs and osteoblasts (OBs), and pagetic bone lesions (PDLs). A total of 40% of MVNP mice, but no MVNP/Igf1-cKO mice, had PDLs.

Small molecules inhibit ex vivo tumor growth in bone.

Bone is a common site of metastasis for breast, prostate, lung, kidney and other cancers. Bone metastases are incurable, and substantially reduce patient quality of life. To date, there exists no small-molecule therapeutic agent that can reduce tumor burden in bone.

The Role of Semaphorin 4D in Bone Remodeling and Cancer Metastasis.

Semaphorin 4D (Sema4D; CD100) is a transmembrane homodimer 150-kDa glycoprotein member of the Semaphorin family. Semaphorins were first identified as chemorepellants that guide neural axon growth. Sema4D also possesses immune regulatory activity.

Halofuginone inhibits TGF-β/BMP signaling and in combination with zoledronic acid enhances inhibition of breast cancer bone metastasis.

More efficient therapies that target multiple molecular mechanisms are needed for the treatment of incurable bone metastases. Halofuginone is a plant alkaloid-derivative with antiangiogenic and antiproliferative effects. Here we demonstrate that halofuginone is an effective therapy for the treatment of bone metastases, through multiple actions that include inhibition of TGFβ and BMP-signaling.

Bidirectional Notch Signaling and Osteocyte-Derived Factors in the Bone Marrow Microenvironment Promote Tumor Cell Proliferation and Bone Destruction in Multiple Myeloma.

In multiple myeloma, an overabundance of monoclonal plasma cells in the bone marrow induces localized osteolytic lesions that rarely heal due to increased bone resorption and suppressed bone formation. Matrix-embedded osteocytes comprise more than 95% of bone cells and are major regulators of osteoclast and osteoblast activity, but their contribution to multiple myeloma growth and bone disease is unknown. Here, we report that osteocytes in a mouse model of human MM physically interact with multiple myeloma cells in vivo, undergo caspase-3-dependent apoptosis, and express higher RANKL (TNFSF11) and sclerostin levels than osteocytes in control mice.

The TGF-β Signaling Regulator PMEPA1 Suppresses Prostate Cancer Metastases to Bone.

Transforming growth factor-β (TGF-β) regulates the expression of genes supporting breast cancer cells in bone, but little is known about prostate cancer bone metastases and TGF-β. Our study reveals that the TGFBR1 inhibitor SD208 effectively reduces prostate cancer bone metastases. TGF-β upregulates in prostate cancer cells a set of genes associated with cancer aggressiveness and bone metastases, and the most upregulated gene was PMEPA1.

FGF23 is elevated in multiple myeloma and increases heparanase expression by tumor cells.

Multiply myeloma (MM) grows in and destroys bone, where osteocytes secrete FGF23, a hormone which affects phosphate homeostasis and aging. We report that multiple myeloma (MM) cells express receptors for and respond to FGF23. FGF23 increased mRNA for EGR1 and its target heparanase, a pro-osteolytic factor in MM.

Tumor-expressed adrenomedullin accelerates breast cancer bone metastasis.

Introduction: Adrenomedullin (AM) is secreted by breast cancer cells and increased by hypoxia. It is a multifunctional peptide that stimulates angiogenesis and proliferation. The peptide is also a potent paracrine stimulator of osteoblasts and bone formation, suggesting a role in skeletal metastases-a major site of treatment-refractory tumor growth in patients with advanced disease.

Halofuginone inhibits the establishment and progression of melanoma bone metastases.

TGF-β derived from bone fuels melanoma bone metastases by inducing tumor secretion of prometastatic factors that act on bone cells to change the skeletal microenvironment. Halofuginone is a plant alkaloid derivative that blocks TGF-β signaling with antiangiogenic and antiproliferative properties. Here, we show for the first time that halofuginone therapy decreases development and progression of bone metastasis caused by melanoma cells through the inhibition of TGF-β signaling.

Regulation of postnatal trabecular bone formation by the osteoblast endothelin A receptor.

Endothelin-1 (ET-1) is a potent vasoconstrictor that also stimulates cells in the osteoblast lineage by binding to the endothelin A receptor (ETAR). ET-1 ligand is widely secreted, particularly by the vasculature. However, the contributions of ETAR signaling to adult bone homeostasis have not been defined.

Hypoxia and TGF-beta drive breast cancer bone metastases through parallel signaling pathways in tumor cells and the bone microenvironment.

Background: Most patients with advanced breast cancer develop bone metastases, which cause pain, hypercalcemia, fractures, nerve compression and paralysis. Chemotherapy causes further bone loss, and bone-specific treatments are only palliative. Multiple tumor-secreted factors act on the bone microenvironment to drive a feed-forward cycle of tumor growth.

Agents targeting prostate cancer bone metastasis.

Bone is the most common site for metastasis of advanced prostate cancers. Once housed in the skeleton, tumors are incurable and cause protracted morbidity, and bone metastases may contribute to mortality through unknown mechanisms. Bone provides a unique microenvironment whose local interactions with tumor cells offer novel targets for therapeutic interventions.

Assessing new bone formation in neonatal calvarial organ cultures.

Bone formation is a complex process, and testing anabolic effects on the skeleton of agents is slow and expensive in animals. Neonatal mouse calvariae cultured ex vivo show strong anabolic or catabolic bone responses to 1-week treatments and can be analyzed by quantitative histomorphometry. Changes in new bone area and osteoblast number caused by added proteins, drugs, or transfected genes can be quantified and analyzed for statistical significance.

Molecular biology of bone metastasis.

Metastasis is a final stage of tumor progression. Breast and prostate cancer cells preferentially metastasize to bone, wherein they cause incurable osteolytic and osteoblastic lesions. The bone matrix is rich in factors, such as transforming growth factor-beta and insulin-like growth factors, which are released into the tumor microenvironment by osteolysis.

Dickkopf homolog 1 mediates endothelin-1-stimulated new bone formation.

Tumor-produced endothelin-1 (ET-1) stimulates osteoblasts to form new bone and is an important mediator of osteoblastic bone metastasis. The anabolic actions of ET-1 in osteoblasts were investigated by gene microarray analyses of murine neonatal calvarial organ cultures. Targets of ET-1 action were validated by real-time RT-PCR in murine primary osteoblast cultures.

Basic mechanisms responsible for osteolytic and osteoblastic bone metastases.

Certain solid tumors metastasize to bone and cause osteolysis and abnormal new bone formation. The respective phenotypes of dysregulated bone destruction and bone formation represent two ends of a spectrum, and most patients will have evidence of both. The mechanisms responsible for tumor growth in bone are complex and involve tumor stimulation of the osteoclast and the osteoblast as well as the response of the bone microenvironment.