Margaux Bernard
email: margaux.bernard@inserm.fr

Paola Di Mauro
email: paola.di-mauro@inserm.fr

Caroline Reynaud
email: caroline.reynaud@inserm.fr

Chantal Diaz-Latoud

Philippe Clézardin
email: philippe.clezardin@inserm.fr

Chantal Diaz
email: chantal.diaz@univ-lyon1.fr

Bone metastasis: basic aspects

Keywords: Pre-metastatic niche, exosome, cancer niche, osteoclast, osteoblast, lysyl oxidase, integrin, miRNA, chemokine, DTC, RANKL, dormancy, osteomimicry Glossary BMP-7: bone morphogenetic protein 7 CaSR: calcium-sensing receptor CDH11: cadherin 11 CTGF: connective tissue growth factor CTSK: cathepsin K CX43: connexin 43 CXCL12: C-X-C motif chemokine ligand 12 CXCR4: Couple chemokine

Bone metastases, the spread of cancer to the bones, occur frequently in patients with advanced disease and contribute substantially to morbidity and mortality in cancer patients. Bone metastasis formation is a stepwise sequence of events that include bone colonization by tumor cells, construction of a cancer niche in the bone marrow, tumor dormancy and, following dormant cell reactivation, tumor cell interactions with bone-resorbing (osteoclasts) and bone-forming (osteoblasts) cells. By disrupting the physiological balance between bone resorption and bone formation, tumor cells promote bone destruction and, in turn, growth factors and calcium released from the resorbed bone matrix promote skeletal tumor growth. Thus, there is a vicious cycle whereby tumor cells disrupt bone homeostasis and resorbed bone promotes tumor growth.

Introduction

Bone metastases, the spread of cancer to the bones, occur in more than 1.5 million patients with cancer world-wide and are most commonly associated with cancers of the prostate, lung, and breast, with incidence rates as high as 75 percent of patients with metastatic disease [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Metastatic cancer cells residing in the bone marrow alter the functions of bone-resorbing (osteoclasts) and bone-forming (osteoblasts) cells [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. By disrupting the physiological balance between bone resorption and bone formation, metastatic cells promote skeletal destruction [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Weakened bones due to skeletal metastases can then lead to occurrence of skeletal-related events, such as fractures and compression of the spinal cord, bone pain and disability, contributing substantially to morbidity and mortality in patients with advanced cancer [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Here we provide an overview of the mechanisms of bone metastasis formation.

Bone metastasis is a stepwise sequence of events that include tumor cell colonization of the bone marrow, construction of a cancer niche, tumor cell interactions with bone cells (osteoclasts, osteoblasts) and release of signals from the resorbed bone matrix that promote skeletal tumor growth. Molecular mechanisms that mediate each of these events are described below.

Bone colonization by tumor cells

Pre-metastatic niche. Experimental studies suggest that, even before becoming clinically detectable, primary tumors release soluble factors into the bloodstream that induce the formation of a tumor growthfavoring microenvironment (called "pre-metastatic niche") in distant organs in order to attract tumor cells to these sites [START_REF] Peinado | Pre-metastatic niches: organ-specific homes for metastases[END_REF]. Among these soluble factors, exosomes may play a prominent role. Exosomes are small vesicles (30-120 nm) containing DNA, RNA [messenger RNA (mRNA), microRNA (miRNA) and other noncoding RNAs], lipids and proteins that are secreted by all types of cells in culture, including tumor cells. Exosomes are also found in body fluids (blood, urine, saliva and breast milk) [START_REF] Vlassov | Exosomes: current knowledge of their composition, biological functions, and diagnostic and therapeutic potentials[END_REF]. In cancer, tumor-derived exosomes express distinct cell surface receptors (integrins) that are addressing them to specific organs [START_REF] Hoshino | Tumour exosome integrins determine organotropic metastasis[END_REF]. Specifically, tumorderived exosomal integrins α6β4 and α6β1 are associated with lung metastasis, while exosomal integrin αvβ5 is linked to liver metastasis in animals [START_REF] Hoshino | Tumour exosome integrins determine organotropic metastasis[END_REF]. These tumor-derived exosomes are up-taken by organ-specific cells (S100A4-positive fibroblasts and Kupffer cells in lungs and liver, respectively), which then produce pro-migratory and pro-inflammatory molecules within pulmonary and hepatic tissues, thereby preparing the soil to facilitate the arrival of tumor cells at these sites [START_REF] Hoshino | Tumour exosome integrins determine organotropic metastasis[END_REF]. We, and others, showed that tumor cell integrins ITGBL1, 21, αvβ3 and v6 are associated with breast or prostate cancer bone metastasis [START_REF] Zhao | Tumor alphavbeta3 integrin is a therapeutic target for breast cancer bone metastases[END_REF][START_REF] Sottnik | Integrin alpha2beta 1 (α2β1) promotes prostate cancer skeletal metastasis[END_REF][START_REF] Dutta | Integrin αvβ6 promotes an osteolytic program in cancer cells by upregulating MMP2[END_REF][START_REF] Li | ITGBL1 Is a Runx2 Transcriptional Target and Promotes Breast Cancer Bone Metastasis by Activating the TGFβ Signaling Pathway[END_REF], suggesting that they could address exosomes to bone. Integrin-independent exosomal mechanisms are also involved. In melanoma, the transfer of the MET oncoprotein from tumorderived exosomes to bone marrow progenitor cells promotes the metastatic process in lungs and bone [START_REF] Peinado | Melanoma exosomes educate bone marrow progenitor cells toward a pro-metastatic phenotype through MET[END_REF]. Tumor-derived exosomal miRNAs can also reprogram or educate target cells that take up exosomes towards a pro-metastatic and pro-inflammatory phenotype, thereby creating a pre-metastatic niche [START_REF] Peinado | Pre-metastatic niches: organ-specific homes for metastases[END_REF]. The main action of miRNAs is the negative regulation of gene expression by base-pairing to the 3'-UTR of target mRNAs, resulting in translational repression or mRNA degradation [START_REF] Beermann | Non-coding RNAs in Development and Disease: Background, Mechanisms, and Therapeutic Approaches[END_REF]. In cancer, miRNAs function either as tumor suppressors or as oncogenes, and their dysregulation contributes to metastasis in specific organs, including the skeleton [START_REF] Beermann | Non-coding RNAs in Development and Disease: Background, Mechanisms, and Therapeutic Approaches[END_REF]. With regards to bone metastasis, there are evidence that exosomal miR-141 and miR-940 produced by prostate cancer cells promote the osteogenic differentiation of mesenchymal cells in the bone marrow microenvironment, which then facilitates the formation of bone metastases with an osteoblastic phenotype [START_REF] Hashimoto | Cancer-secreted hsa-miR-940 induces an osteoblastic phenotype in the bone metastatic microenvironment via targeting ARHGAP1 and FAM134A[END_REF][START_REF] Ye | Exosomal miR-141-3p regulates osteoblast activity to promote the osteoblastic metastasis of prostate cancer[END_REF].

Another important class of molecules involved in the formation of the pre-metastatic niche is the lysyl oxidase (LOX) family whose primary function is to drive collagen crosslinking and extracellular matrix stiffness [START_REF] Peinado | Pre-metastatic niches: organ-specific homes for metastases[END_REF]. Hypoxia-inducible factor-1 (HIF-1) induces the expression of LOX in breast tumors and secreted LOX catalyses collagen crosslinking in the lungs, facilitating the recruitment of bone marrow-derived cells and the subsequent colonization of the pulmonary tissue by breast cancer cells [START_REF] Erler | Hypoxia-induced lysyl oxidase is a critical mediator of bone marrow cell recruitment to form the premetastatic niche[END_REF]. In bone, we and others have shown that tumorderived LOX from breast and colon cancer cells generates pre-metastatic osteolytic lesions in animals by impairing bone homeostasis through the stimulation of osteoclast formation induced by RANKL and the inhibition of osteoblast formation [START_REF] Cox | The hypoxic cancer secretome induces premetastatic bone lesions through lysyl oxidase[END_REF][START_REF] Reynaud | Lysyl Oxidase Is a Strong Determinant of Tumor Cell Colonization in Bone[END_REF]. Thus, by acting systemically, LOX contributes to the formation of a pre-metastatic niche in the bone marrow.

Factors promoting the homing of tumor cells to the bone marrow. Chemokines, which are small proteins (8-14 kDa) belonging to the cytokine family, play a crucial role in promoting the migration of tumor cells to the bone marrow and, more specifically, to the pre-metastatic niche [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. They are produced by stromal cells, which are associated with the pre-metastatic niche, and released in the bloodstream. For instance, the chemokine C-X-C motif chemokine ligand 12 (CXCL-12) is produced in large amounts in the bone marrow, lungs, and liver. Metastatic cells from breast or prostate cancers that express CXCR-4, the CXCL-12 receptor, migrate preferentially to these three organs due to the chemotactic properties of CXCL-12 [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Several other chemokines (CXCL-10, CXCL-13, CXCL-16, CX3CL-1, CCL22) are also produced by the bone microenvironment and have been implicated in the homing of tumor cells to bone [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Lee | CXCL10 promotes osteolytic bone metastasis by enhancing cancer outgrowth and osteoclastogenesis[END_REF][START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF].

Construction of a cancer niche in the bone marrow

The fate of tumor cells after they enter the bone marrow remains the most elusive aspect of metastasis formation. Disseminated tumor cells (DTCs) in the bone marrow may be non-proliferative and remain dormant for several years, therefore never develop into overt bone metastases [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. What enables DTCs in the bone marrow to remain dormant and how do they acquire the competence to form overt metastases? These questions come under intense scrutiny. Current hypotheses gaining ground are that DTCs must build a cancer niche by producing their own niche components to anchor and survive in the bone microenvironment, or bind to extracellular matrix components of the pre-metastatic niche that are produced by recruited stromal cells and/or occupy native bone marrow niches in which hematopoietic stem cells reside [START_REF] Oskarsson | Metastatic stem cells: sources, niches, and vital pathways[END_REF]. By building this cancer niche, DTCs can enter dormancy and thus survive treatment with chemotherapeutics, which only target proliferating tumor cells.

Extracellular matrix niche and tumor survival

The extracellular matrix components tenascin C and periostin have been shown to play important roles in the construction of cancer niche in mouse models of cancer. Tenascin C is a hexameric protein produced by tumor cells that acts in an autocrine manner to support tumor cell survival. Periostin is a protein produced by stromal cells from the pre-metastatic niche that supports in a paracrine manner tumor cell survival. Tenascin C and periostin bind tightly to each other and to tumor cell surface integrins 91 and v3, respectively, which leads to the activation of Wnt-and Notch-dependent intracellular signaling pathways [START_REF] Oskarsson | Metastatic stem cells: sources, niches, and vital pathways[END_REF][START_REF] San Martin | Tenascin-C and Integrin α9 Mediate Interactions of Prostate Cancer with the BoneMicroenvironment[END_REF]. Other extracellular matrix components, including SIBLINGS (osteopontin, bone sialoprotein, dentin matrix protein 1), osteonectin, connective tissue growth factor (CTGF, CCN2), and nephroblastoma overexpressed (Nov, CCN3), have also been associated with bone metastasis formation [START_REF] Trotter | Matricellular proteins as regulators of cancer metastasis to bone[END_REF]. Osteopontin has been involved in early bone colonization through binding to tumor cell surface integrin v3 [START_REF] Trotter | Matricellular proteins as regulators of cancer metastasis to bone[END_REF]. Other SIBLINGs, osteonectin, CTGF and Nov are involved at a late stage in the bone metastatic disease, promoting tumor cell invasion and growth [START_REF] Trotter | Matricellular proteins as regulators of cancer metastasis to bone[END_REF][START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF].

Bone niches and tumor dormancy

Osteogenic and vascular niches are crucial regulators of hematopoietic stem cell (HSC) behavior in the bone marrow. During homeostasis, signals coming from the osteogenic niche keep HSCs dormant, which preserves their long-term self-renewal potential, whereas the vascular niche is oxygenated and stimulates proliferation and differentiation of HSCs to maintain hematopoiesis [START_REF] Winkler | Vascular niche E-selectin regulates hematopoietic stem cell dormancy, self renewal and chemoresistance[END_REF]. A current hypothesis gaining ground is that common mechanisms are likely to be involved in maintaining HSC and DTC dormancy in the osteogenic niche. "Dormancy is a stage in cancer progression where the cells cease dividing but survive in a quiescent state while waiting for appropriate environmental conditions to begin proliferation again. Quiescence is the state where cells are not dividing but at arrest in the cell cycle in G0-G1 » (https://en.wikipedia.org/wiki/Cancer_dormancy). Quiescent DTCs are therefore protected against chemotherapy-induced apoptosis.

The osteogenic niche is home to a particular spindle-shaped N-cadherin+ osteoblast (SNO) cell population. SNO cells regulate dormancy of HSCs through interaction with N-cadherin. It has been shown that E-cadherin-expressing breast cancer cells home to SNO cells through interactions with N-cadherin [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF][START_REF] Kan | Cancer Cell Colonisation in the Bone Microenvironment[END_REF]. Another molecule that regulates HSC dormancy is growth arrest-specific 6 (GAS6), which is an osteoblast-derived ligand of the MER, TYRO3 and AXL (also known as UFO) tyrosine kinase receptors. When prostate cancer cells bind to osteoblasts in the niche they increase their expression level of AXL and GAS6 signaling inhibits tumor cell proliferation [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF][START_REF] Kan | Cancer Cell Colonisation in the Bone Microenvironment[END_REF]. Osteoblasts also express the chemokine CXCL-12, which mediates adhesive interactions with HSCs by binding to CXCR4, the receptor for CXCL-12. It has been observed that prostate cancer cells, which express CXCR4, can directly compete with HSCs for osteogenic niche support [START_REF] Shiozawa | Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow[END_REF].

The vascular niche is composed of endothelial cells that express specific cell adhesion molecules (E-and P-selectins), E-selectin being a crucial component that mediates HSC attachment and proliferation [START_REF] Winkler | Vascular niche E-selectin regulates hematopoietic stem cell dormancy, self renewal and chemoresistance[END_REF]. Using real-time in vivo microscopy of breast tumor xenografts, it has been shown that dormant tumor cells were preferentially located in E-selectin-rich vascular regions in which they were anchored through SDF-1/CXCR4 interactions [START_REF] Price | Dormant breast cancer micrometastases reside in specific bone marrow niches that regulate their transit to and from bone[END_REF]. These results [START_REF] Price | Dormant breast cancer micrometastases reside in specific bone marrow niches that regulate their transit to and from bone[END_REF] are in contrast with previous findings showing that dormant prostate cancer cells, which express CXCR4, compete with HSCs for osteogenic niche support [START_REF] Shiozawa | Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow[END_REF]. [START_REF] Ghajar | The perivascular niche regulates breast tumour dormancy[END_REF] also reported that the bone marrow microvasculature might constitute a dormant niche for breast cancer cells. Specifically, they showed that the endothelium-derived extracellular matrix protein thrombospondin-1 (TSP-1) induces sustained dormancy of breast cancer cells in vivo. Overall, these experimental data suggest the concept of niche support for DTCs is still in the early stages of investigation, and warrants further investigation.

Other factors produced by osteoblasts or bone marrow mesenchymal stem cells, including bone morphogenetic protein 7 (BMP-7), transforming growth factor-β2 (TGFβ-2), leukemia inhibitory factor (LIF), and exosomal miR-23b, can also induce tumor dormancy in different types of cancer [START_REF] Ono | Exosomes from bone marrow mesenchymal stem cells contain a microRNA that promotes dormancy in metastatic breast cancer cells[END_REF][START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF][START_REF] Johnson | Induction of LIFR confers a dormancy phenotype in breast cancer cells disseminated to the bone marrow[END_REF].

Osteomimicry

During the time DTCs are in the bone marrow, they are able to adapt, exiting and re-entering a dormant state, and undergoing further selection to acquire a full complement of metastasiscolonization functions that they did not express before. In this context, tumor cells in the bone marrow express bone-associated genes (RUNX2, CTSK, OPG, SPARC, CDH11, CX43, DKK1), which are normally expressed by osteoblasts or osteoclasts (Bellahcène, 2007;[START_REF] Gall | A cathepsin K inhibitor reduces breast cancer induced osteolysis and skeletal tumor burden[END_REF][START_REF] Fradet | Dual function of ERRα in breast cancer and bone metastasis formation: implication of VEGF and osteoprotegerin[END_REF][START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. This process is called osteomimicry. For instance, the disruption of RUNX2 activity in breast cancer cells abolishes their ability to form osteolytic lesions in vivo [START_REF] Pratap | Metastatic bone disease: Role of transcription factors and future targets[END_REF]. Similarly, the incidence of prostate cancer bone metastasis is reduced greatly when cadherin-11 (CDH11) expression by tumor cells is silenced [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Moreover, paired immunochemistry on primary breast tumor samples and associated liver, lung or bone metastases showed that only bone metastatic cancer cells express bone proteins such as cathepsin K, osteonectin, cadherin-11, connexin-43 and RUNX2, demonstrating the clinical relevance of this process (Bellahcène, 2007;[START_REF] Gall | A cathepsin K inhibitor reduces breast cancer induced osteolysis and skeletal tumor burden[END_REF][START_REF] Fradet | Dual function of ERRα in breast cancer and bone metastasis formation: implication of VEGF and osteoprotegerin[END_REF].

Dormant cell reactivation

Two mitogen-activated kinases (MAPKs), namely p38 and extracellular signal-regulated kinase (ERK), control the switch of tumor cells between dormancy and active growth, respectively [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF]. Some of the molecular mechanisms that regulate this switch in the bone marrow have been identified. For example, mitogen-and stress-activated kinase 1 (MSK1), a downstream effector of the p38 and ERK1/2 signaling pathways, has been involved in the regulation of tumor dormancy in estrogen receptor (ER)-positive breast cancer cells that are metastatic to bone [START_REF] Gawrzak | MSK1 regulates luminal cell differentiation and metastatic dormancy in ER + breast cancer[END_REF]. Specifically, p38 depletion in tumor cells decreases MSK1 expression, and MSK1 depletion increases the capacity of poorly metastatic ER-positive breast cancer cells to form overt metastasis in animals [START_REF] Gawrzak | MSK1 regulates luminal cell differentiation and metastatic dormancy in ER + breast cancer[END_REF]. Thus, MSK1 is a dormancy enforcer and a negative regulator of metastasis initiation.

Bone might also provide an environment that enables dormant cell reactivation. Within a mouse bone metastasis model, indolent breast cancer micrometastases were found to overexpress adhesion protein vascular cell adhesion protein 1 (VCAM-1), which promotes the recruitment of osteoclast precursors by binding to osteoclast integrin α4β1, leading to osteoclast formation and osteoclastic bone resorption [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF]. The mechanisms underlying VCAM-1 silencing and reexpression in these indolent bone marrow micrometastases are unknown. However, bone resorption likely creates an environment that promotes tumor cell reactivation [START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF]. In this respect, TGFβ1 and TGFβ2 exhibit competing functions on behavior of tumor cells in the bone marrow [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF]. TGFβ2 promotes tumor cell dormancy through Smad1/5 and p38 signaling, whereas TGFβ1 switches off dormancy, leading to rapid tumor growth in vivo [START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF]. Thus, TGFβ1 released from resorbed bone could contribute to dormant cell reactivation [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF].

As exemplified with VCAM-1 and TGFβ1, there is a growing body of evidence suggesting that osteoclasts may be responsible for the reactivation of dormant DTCs [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Sosa | Mechanisms of disseminated cancer cell dormancy: an awakening field[END_REF][START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF]. It is well known that manipulating the bone microenvironment in mice by stimulating osteoclast activity through vitamin D deficiency, or through estrogen or androgen deprivation enhances skeletal tumor burden and bone destruction [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF]. In this context, TGFβ1 that is released from resorbed bone not only switches off DTC dormancy, but also plays a central role in most of the events leading to tumor expansion in bone and bone destruction [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. For instance, bone-derived TGFβ1 stimulates the expression of the Notch ligand Jagged-1 in bone metastatic tumor cells, which in turn promotes bone metastasis by activating the Notch pathway in supporting bone cells. Specifically, Jagged-Notch signaling directly promotes osteoclast differentiation and bone resorption, and it stimulates the release of IL-6 from osteoblasts to promote tumor cell proliferation [START_REF] Sethi | Tumor-derived JAGGED1 promotes osteolytic bone metastasis of breast cancer by engaging notch signaling in bone cells[END_REF]. In addition, the therapeutic targeting of Jagged-1 with a monoclonal antibody inhibits bone metastasis formation in animals [START_REF] Zheng | Therapeutic Antibody Targeting Tumor-and Osteoblastic Niche-Derived Jagged1 Sensitizes Bone Metastasis to Chemotherapy[END_REF]. Thus, osteoclastic bone resorption is likely to play an important role at an early stage in the establishment of bone metastasis.

Tumor-derived factors mediating osteolytic or osteoblastic bone metastases

At a late stage in the progression of skeletal lesions, the patterns of bone metastases ranged from mostly destructive or osteolytic (in breast cancer, lung cancer, or myeloma), to mostly boneforming or osteoblastic (in prostate cancer). There is however always an imbalance between bone formation and bone resorption. Bone metastasis is therefore a spectrum between these two extremes where, at one end, predominantly osteolytic lesions are associated with high osteoclast activity and reduced osteoblast activity and, at the other end, bone metastases that are predominantly osteoblastic have a high osteoblast activity and a reduced osteoclast activity [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Different molecular mechanisms associated with these different patterns of bone metastasis have been identified and are described below.

Osteolytic lesions

Several factors secreted by tumor cells during the course of development of malignant osteolytic lesions stimulate osteoclast activity and bone resorption. Among them, parathyroid hormone-related peptide (PTHrP) was the first to be recognized as involved in malignant osteolysis. PTHrP stimulates in osteoblasts the expression of the cytokine receptor activator of nuclear factor-κB ligand (RANKL), which binds to its receptor RANK on osteoclast precursors, leading to the formation of new osteoclasts and therefore to osteoclastic bone resorption [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. The transcription factor RUNX2 contributes to the ability of tumor cells to activate osteoclasts by upregulating Indian Hedgehog (IHH) expression, which in turn stimulates PTHrP-induced RANKL in osteoblasts and subsequently promotes RANK/RANKL-dependent activation of osteoclasts [START_REF] Pratap | Metastatic bone disease: Role of transcription factors and future targets[END_REF]. Likewise, the transcription factor MAF mediates breast cancer bone metastasis through the control of PTHrP [START_REF] Pavlovic | Enhanced MAF Oncogene Expression and Breast Cancer Bone Metastasis[END_REF]. Subsequent studies, described below, showed that TGF released from resorbed bone induces PTHrP production from tumor cells that results in enhanced bone resorption [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Overall, these studies [START_REF] Pratap | Metastatic bone disease: Role of transcription factors and future targets[END_REF][START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Pavlovic | Enhanced MAF Oncogene Expression and Breast Cancer Bone Metastasis[END_REF] demonstrate the importance of PTHrP in the pathogenesis of osteolytic lesions. Several other molecules produced by tumor cells, including interleukins and granulocyte macrophage-colony stimulating factor (GM-CSF), stimulate osteoclast activity through the activation of the RANK/RANKL pathway [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Platelet-derived lysophosphatidic acid (LPA) also supports progression of osteolytic bone metastases in breast cancer. We showed that LPA, by binding to its receptor LPA1 at the tumor cell surface, promotes tumor cell proliferation and the LPA1-dependent secretion of IL-6 and IL-8, which in turn promotes osteoclast-mediated bone resorption [START_REF] Boucharaba | Platelet-derived lysophosphatidic acid supports the progression of osteolytic bone metastases in breast cancer[END_REF]. Moreover, the functional blockade of LPA action on its receptor, using a LPA1 antagonist, substantially reduces progression of osteolytic bone metastases in animals [START_REF] Boucharaba | The type 1 lysophosphatidic acid receptor is a target for therapy in bone metastases[END_REF].

Tumor cells not only stimulate osteoclast activity, but also inhibit osteoblast activity, thereby worsening the imbalance between bone formation and bone resorption, and promoting bone destruction [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Specifically, tumor cells secrete activin A (a member of the TGF- superfamily of growth factors), noggin (a BMP antagonist), and dickkopf-1 (DKK-1) and sclerostin (SOST-1) [two Wingless/int (Wnt) protein antagonists], all of them suppressing osteoblast differentiation [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Zhu | Sclerostin induced tumor growth, bone metastasis and osteolysis in breast cancer[END_REF]. For instance, MDA-MB-231 and MCF-7 breast cancer cells express SOST-1 and an anti-SOST antibody reduces tumor cell migration and invasion in vitro, and prevents formation of osteolytic lesions in animals [START_REF] Zhu | Sclerostin induced tumor growth, bone metastasis and osteolysis in breast cancer[END_REF].

Osteoblastic lesions

One of the most well studied mediators is endothelin-1 (ET-1), which stimulates osteoblast proliferation and inhibits osteoclast activity and motility. Among patients with prostate cancer, those with bone metastases have far higher circulating levels of ET-1, compared to those with localized cancer [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Gene expression analysis in mouse primary osteoblasts revealed that treatment with ET1 up-regulates multiple osteoblast-stimulatory factors (e.g., IL-6 and RANKL) and down-regulates DKK-1 [START_REF] Rosano | Endothelin 1 in cancer: biological implications and therapeutic opportunities[END_REF]. Stimulatory effects of ET-1 on osteoblasts are mediated by two receptors, ETAR and ETBR, which activate similar signaling pathways. Osteoblast proliferation and bone metastasis are both inhibited by ETAR antagonists atrasentan and zibotentan, as well as by the dual ETAR and ETBR antagonist bosentan, highlighting the prominent role played by ET-1 in the formation of osteoblastic lesions [START_REF] Rosano | Endothelin 1 in cancer: biological implications and therapeutic opportunities[END_REF]. Other osteoblast-stimulatory factors (VEGF, BMP-6) have also been shown to participate to the formation of osteoblastic lesions (Dai, 2005;[START_REF] Kitagawa | Vascular endothelial growth factor contributes to prostate cancer-mediated osteoblastic activity[END_REF]. Osteoblasts express VEGF membrane receptors Flt1 and KDR and the treatment of animals with the Flt1 receptor inhibitor PTK787 blocks the formation of prostate cancer osteoblastic lesions in animals [START_REF] Kitagawa | Vascular endothelial growth factor contributes to prostate cancer-mediated osteoblastic activity[END_REF]. Likewise, the treatment of animals with an antibody to BMP-6 decreases the formation of osteoblastic bone metastases induced by prostate cancer cells (Dai, 2005). The osteoblast-stimulatory factors BMP-2, adrenomedullin, Wnt, and fibroblast growth factor 9 (FGF9) have been also involved in the formation of osteoblastic lesions [START_REF] Logothetis | Osteoblasts in prostate cancer metastasis to bone[END_REF]. In addition, prostate-specific antigen (PSA), as a serine protease that cleaves PTHrP in the PTH-like domain, could also indirectly contribute to the formation of osteoblastic lesions by neutralizing osteoclaststimulatory effect of PTHrP [START_REF] Cramer | Prostate specific antigen cleaves parathyroid hormonerelated protein in the PTH-like domain: inactivation of PTHrP-stimulated cAMP accumulation in mouse osteoblasts[END_REF].

Osteoprotegerin (OPG) is a soluble RANKL inhibitor produced by osteoblasts that inhibits RANKL/RANK interaction. OPG levels are increased in patients with prostate cancer bone metastases. In addition, OPG can be expressed in breast and prostate cancer cells. OPG expressed by prostate cancer cells inhibits bone resorption, thereby contributing to the osteoblastic trait of most prostate cancer bone lesions [START_REF] Corey | Osteoprotegerin in prostate cancer bone metastasis[END_REF]. In addition, OPG can promote tumor cell survival by binding to tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) [START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF].

Contribution of bone to metastasis progression

Bone tissue is an abundant reservoir for growth factors, including TGF, insulin-like growth factors (IGFs) and platelet-derived growth factors (PDGFs) [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. When released from the resorbed bone matrix, TGF acts on tumor cells, via SMAD-and COX2-dependent signaling pathways, which stimulates the expression of factors such as PTHrP, Jagged-1, IL-11 and prostaglandin E2 [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Sethi | Tumor-derived JAGGED1 promotes osteolytic bone metastasis of breast cancer by engaging notch signaling in bone cells[END_REF][START_REF] Croucher | Bone metastasis: the importance of the neighbourhood[END_REF]. Bone-derived IGFs and PDGFs stimulate tumor growth [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF]. Calcium is another factor released from bone during osteoclastic resorption. Calcium binds on tumor cells (breast, prostate) via a calcium-sensing receptor (CaSR) and promotes tumor cell proliferation and migration and secretion of PTHrP and epiregulin [START_REF] Weilbaecher | Cancer to bone: a fatal attraction[END_REF][START_REF] Boudot | Overexpression of a functional calcium-sensing receptor dramatically increases osteolytic potential of MDA-MB-231 cells in a mouse model of bone metastasis through epiregulin-mediated osteoprotegerin downregulation[END_REF]. Epiregulin is an EGF-like ligand whose expression is increased in osteolytic lesions in vivo. It decreases OPG expression in osteoblasts, thereby contributing to the progression of osteolytic lesions [START_REF] Boudot | Overexpression of a functional calcium-sensing receptor dramatically increases osteolytic potential of MDA-MB-231 cells in a mouse model of bone metastasis through epiregulin-mediated osteoprotegerin downregulation[END_REF].

Summary and conclusion

In this review, we described early events that allow tumor cells to disseminate and colonize in the bone marrow and we highlighted the prominence of pre-metastatic and metastatic niches in mediating homing and dormancy of tumor cells, respectively. We also discussed the importance of the osteoclast activity for reactivation of dormant tumor cells. Finally, we explained how, at a later stage, tumor cells induce osteolytic or osteoblastic lesions. To date, no effective treatments for bone metastasis exist, only therapies that aim to limit the bone degradation such as denosumab (anti-RANKL monoclonal antibody) and bisphosphonates. The future challenge will lie in our ability to develop therapeutics targeting dormancy in order to address residual disease in patients.