**7. Conclusion**

544 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

osteoclastogenesis (180) and has been successful in combating the osteolytic effects of multiple myeloma (181), making it an attractive candidate for the prevention and treatment

Fig. 3. Breast cancer cells alter normal bone homeostasis. Breast cancer cells maintain osteoblasts in an immature state and stimulate RANKL production by osteoblasts, while inhibiting OPG. Breast cancer cells stimulate osteoclastogenesis directly through TGF M-CSF and CCN3 production. Increased bone resorption by activated osteoclasts releases matrix-derived growth factors TGF, IGF, FGF, PDGF, which act back on breast cancer cells

VEGF represents an interesting target potentially affecting breast cancer cell homing, development of pre-metastatic niche and new vasculature formation. Many anti-VEGF therapies exist to prevent vascularization of tumours and inhibit their growth (182). There have been several hindrances in the progress of this therapy due to drug resistance and toxicity (183), and the increased incidence of osteonecrosis of the jaw in combined bisphosphonate-antiangiogenic agent therapy (184). Notwithstanding, the use of VEGF-A monoclonal antibody Bevacizumab in combination with chemotherapy has proven beneficial in reducing breast cancer growth (185) and osteolysis (186). Other targets based on the in vitro and in vivo studies, such as TGF, GPNMB, and CXCR4 are being explored in preclinical and clinical studies, providing the basis for the next generation of treatments. Osteoclasts are commonly targeted therapeutically for osteolytic disease, with one of the most widely used drugs being bisphosphonates. Analogs of mineralization-inhibiting pyrophosphate (187), bisphosphonates are a class of synthetic compounds composed of two phosphate groups covalently linked to carbon with a P-C-P backbone and side groups that vary their properties and pharmacokinetics. Bisphosphonates attach selectively to bone and induce osteoclast apoptosis when they are ingested during resorption. In osteoporosis studies, all bisphosphonates given daily have been shown to reduce osteoporotic vertebral

of breast cancer-induced osteolysis.

to stimulate their growth and survival.

Breast cancer is the most commonly diagnosed cancer in women, which may lead to bone metastasis resulting in altered mineral homeostasis, the disruption of bone microarchitecture, pain and pathological fractures. Recent studies have demonstrated that breast cancer cells start affecting the bone microenvironment prior to their dissemination from the primary tumour by secreting circulating soluble factors that prepare bone for the future arrival of metastasizing cancer cells, a process that likely involves mediators of the hematopoietic stem cell niche. Multiple mediators of directional migration of breast cancer cells have been identified, as well as mediators of breast cancer cells anti-osteoblastic and pro-osteoclastic actions. Breast cancer-stimulated RANKL, M-CSF, PTHrP, TGF, GPNMB, Runx2 and CXCR4 remain among the most critical mediators of cancer-induced osteoclastic bone resorption. Yet, they are not the whole picture, and new players are being identified, providing more complex and comprehensive description of the events leading from the formation of primary tumour to the establishment of progressive osteolytic bone lesions. However, while considering the multitude of molecular mediators, it is important to remember the heterogeneity of breast cancer disease in patients, suggesting that treatments targeting different molecular mediators should develop in parallel with the testing capabilities able to implicate a particular mediator in disease progression in a specific patient. An alternative approach is to target the processes and cellular targets similarly altered through different molecular mediators. An example of such approach is the clinical success of bisphosphonates, which broadly target osteoclast formation and activity. Nevertheless, both approaches need to be developed to provide clinicians with the set of tools for broad preventive measures, as well as for targeted personalized medicine for nonresponsive or atypical cases.
