**3. Breast-cancer-derived bone metastases: molecular interactions within the BM**

### **3.1 Preclinical and clinical implications**

Breast cancer is the most frequent cancer in women, with increasing incidence and high mortality rates [125]. Breast-cancer-induced bone metastases are a frequent complication of advanced disease, with up to 70% of incidence, associated with skeletal complications, including pain, osteopenia and bone loss, pathological fracture, hypercalcemia spinal cord compression, BM aplasia, demanding surgery, and radiotherapy for bone complications, and change of antineoplastic therapy for bone pain [126–129]. Collectively, these comorbidities are defined as skeletal-related events (SREs) that dramatically impair the patient's quality of life and reduce overall survival [127, 128, 130].

The preservation of bone mass has been achieved using bone anti-resorptive bisphosphonates, such as zoledronic acid, and denosumab, an anti-RANKL monoclonal antibody, which block OC-mediated bone resorption and are approved for use in patients with cancer metastatic to bone [128]. However, these drugs only alleviate SREs complications, the development of bone metastases remains an incurable condition, and mortality rates are kept at elevated level [131]. Metaanalyses studies showed a statistically significant overall survival benefit with women treated with bisphosphonates [132–134]. It is not surprising, however, that bone-targeted therapies also display systemic immunological effects, regarding

the interactions between immune and bone cells, which can partially cause eliminatory anti-tumor effects. Indeed, zoledronic acid and denosumab can modulate immune cells activity, such as γδ T cells, macrophages, and CD4<sup>+</sup> Tregs, in many different types of cancer, including breast cancer, leading to an increase in T-cellmediated anti-tumor cytotoxic effects [135]. Moreover, the knowledge about how current treatments affect the immune landscape in bone metastatic microenvironment is scarcely known. This fact could be due to our limited understanding of osteoimmunological interactions for tumor growth, the low availability of biopsies from bone metastases, and appropriate metastatic models for preclinical studies.

The risk factors for predicting breast-cancer-derived bone metastases are still controversial. In a recent study, a total of 2133 patients, including 327 with bone metastases (15.33%) and 1806 without bone metastases (84.67%), were retrospectively reviewed and showed that the spine is the most common site for bone metastases, including thoracic spine (63.61%) and lumbar spine (53.82%), followed by ribs (57.5%), pelvis (54.1%), and sternum (44.3%) [136]. The results also indicated that combined axillary LN metastases, high serum concentrations of cancer Ag 15-3 (CA15-3), alkaline phosphatase (ALP), and low level of hemoglobin have the highest predictive accuracy for bone metastases in breast cancer [136].

Breast-cancer-derived bone metastases give rise predominantly to most aggressive osteolytic lesions, although 15–20% of clinical cases present an osteoblastic pattern, resulting in a dysregulated bone deposition [126, 128, 137]. Notably, it has been shown that breast cancer osteolytic lesions may also lead to skeletal muscle atrophy and weakness, through bone-muscle cross talk, which in turn leads to a feed-forward cycle of musculoskeletal degradation [138, 139]. Osteoclastic bone resorption releases transforming growth factor-β (TGF-β), which causes oxidative stress and skeletal muscle Ca2+ leak and weakness, via the TGFβ-Nox4-RyR1 axis, inducing a muscle atrophy program [138, 139]. Interestingly, the same pattern was shown in both immunodeficient and immunocompetent mice, suggesting that adaptive immune system may be excluded from this pathological aspect [140, 141]. Moreover, it has been suggested that muscle dysfunction occurs prior to the loss of muscle mass—cachexia [142]. In addition, experimental strategies are being analyzed for skeletal muscle mass preservation, including: (i) the blocking of myostatin signaling [143, 144]; and (ii) antagonizing the growth hormone secretagogue receptor (GHSR)-1a [145, 146]. Both strategies showed improved survival in mice with cancer cachexia [147].

Recently, it was reported that breast cancer cell lines and human breast cancer tissue express sclerostin, suggesting that breast cancer cells impair bone formation while promoting bone resorption [140]. In a mouse model of bone metastases, the pharmacological inhibition of sclerostin by setrusumab—an anti-sclerostin monoclonal antibody, reduced bone metastatic burden and destruction, without increasing metastases at other sites [140]. Moreover, this treatment protected from induction of muscle atrophy and loss of function, leading to prolonged life span [148]. Accordingly, the expanding and maintenance of OBs functional properties were then proposed as an approach to restore bone and muscle integrity, in the context of metastases-induced osteolytic disease [140]. In parallel, it was reported that homeodomain protein TG-interacting factor-1 (Tgif1)—an inducer of osteoblastogenesis acting at Wnt and PTH1R-dependent signaling pathways, is increased in OBs upon stimulation by metastatic breast cancer cells [141]. High levels of Tgif1 were associated with poor patient survival in breast cancer [147]. The lack of Tgif1 in

### *Perspective Chapter: Breast-Tumor-Derived Bone Pre-Metastatic Disease – Interplay... DOI: http://dx.doi.org/10.5772/intechopen.107278*

OBs increases Semaphorin 3E (Sema3E) expression and attenuates breast cancer cell migration as well as metastases formation, indicating that Tgif1 plays a role during the early stages of bone metastases establishment [141]. Therefore, the mechanisms driving the early steps of bone metastatic process are still not sufficiently understood and the induction of osteoblastogenesis should be analyzed with caution, since OBs and their molecules seem to play contradictory roles in breast-cancer-derived bone disease.

Finally, preclinical studies suggested that non-coding RNAs (ncRNAs) such as long ncRNAs, microRNAs, and circular RNAs are crucial regulators of breast-cancerinduced bone metastases [149–151]. Indeed, unique miRNA expression patterns were reported in different breast cancer subtypes, displaying both pro- and anti-tumorigenic functional properties [150]. In fact, lower levels of miR-34a were observed in patients suffering from later stages of breast cancer in comparison to benign breast disease and healthy controls [131], while higher expression of miR10b was observed in breast cancer patients with LN and bone metastases [148, 152]. Furthermore, lower levels of miR-124 in primary breast cancer correlate with shorter bone-metastasesfree survival [153], and miR-218 serum levels are higher in patients with breast cancer bone metastases when compared with patients without metastases [154]. Currently, since altered expression of miRNAs has been associated with disease progression and clinical outcome, these molecules are emerging as potential therapeutic targets and prognostic biomarkers in the context of bone metastases.
