*3.3.1 The "seed and soil" in bone tissue adaptation*

Breast cancer cells migration to bone is innately related to the molecular and cellular components provided by the pre-metastatic niche, in sequential and distinct phases [137, 172, 176]. In fact, in the nineteenth century, Stephen Paget proposed that tumor cells ("seeds") only grow in specific and permissive microenvironments ("fertile soil") [177]. Of note, BM is a fertile microenvironment, composed of hematopoietic cells, MSCs, endothelial cells, OBs, OCs, molecules secreted by breast primary tumor, either as soluble or contained in extracellular vesicles (EVs) or exosomes, and immune migrating cells [137, 175]. However, how and when these factors, produced locally or systemically, regulate the crucial mechanisms behind the establishment of this site remains less clear [175].

Recent studies suggest that bone pre-metastatic niche exists prior to metastatic colonization; however, disseminated breast cancer cells are detectable in BM prior to clinically detectable bone metastases [175, 178]. Interestingly, patients without any metastases harbored disseminated breast cancer cells with less genetic heterogeneity compared with the primary tumor or those disseminated cells isolated from bone metastatic patients [175, 178]. Of note, less than 0.1% of disseminated breast cancer cells survive during circulation and homing [179–181]. Based on these *Perspective Chapter: Breast-Tumor-Derived Bone Pre-Metastatic Disease – Interplay... DOI: http://dx.doi.org/10.5772/intechopen.107278*

findings, we can speculate that bone/BM stromal cellular and molecular components probably play roles in supporting these mutations, for further licensing and selection of the best "seeds" to adapt in the pre-metastatic niche, until their overt bone colonization.

Additionally, a recent study identified LOX-derived by hypoxia condition, a factor significantly associated with bone tropism and relapse. LOX induces an intense osteoclastogenesis, through NFATc1, before, and independent of breast tumor cells arrival at BM [174]. Therefore, this study identified a previous step in bone metastases development, triggered by these osteolytic lesions, opening new opportunities for therapeutic intervention [174]. In fact, in a previous study using an intracardiac mouse model of breast-cancer-derived bone metastases, animals treated with a nonspecific LOX inhibitor—β-aminopropionitrile—reduce bone colonization when administered at the time of tumor inoculation [182].

As mentioned in the last section, recent evidence suggests that breast-cancerderived miRNAs play key roles in tumor development and progression via exosomes transfer, regulating the outgrowth and metastases of breast cancer [183, 184]. Of note, it was described that miR-21, a highly conserved oncomicro RNA, is expressed in serum of breast cancer patients, significantly higher as compared with healthy controls [185]. Moreover, it was demonstrated that miR-21 induces OCs differentiation, by directly binding programmed cell death 4 (PDCD4), upregulation of NFATc1, and suppression of c-Fos transactivation [186, 187]. Indeed, it was showed that breast cancer cell–secreted exosomes containing miR-21 lead to an exacerbated osteoclastogenesis, which contributes to the generation of a pre-metastatic niche and further enhancing bone metastases development [188]. Importantly, the expression level of miR-21 was detected at higher level in serum exosomes of breast cancer patients with bone metastases, as compared with patients without bone metastases [188].

Almost 20 years ago, a pioneer study challenged the molecular basis for bone metastases. Using human breast cancer cell lines with elevated metastatic activity, it was determined a breast-cancer-derived bone metastases gene signature, which included genes involved in: (i) BM homing (CXCR4); (ii) extracellular matrix alteration (Matrix Metallopeptidase 1 (MMP1), ADAM metallopeptidase with thrombospondin type 1 motif 1 (ADAMTS1), and proteoglycan-1); (iii) angiogenesis (Fibroblast growth factor 5 (FGF5), and Connective tissue growth factor (CTGF); and osteoclastogenesis (IL-11) [189]. Moreover, the overexpression of this gene set is superimposed on a poor prognosis already present in the parental breast cancer population, suggesting that metastases require a set of functions beyond those underlying the emergence of the primary tumor [189]. Thereafter, several other bone metastases gene signatures were proposed, such as Src-dependent [190] or Irf7-regulated genes [191]. To date, it remains unclear the clinical significance and applicability of these gene signatures described, either by tumor heterogeneity in primary and secondary sites or by differences in tumor sources.
