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Bone metastasis is an incurable complication of breast cancer affecting 70-80 % of advanced patients. It is a multistep process that includes tumour cell mobilisation, intravasation, survival in the circulation, extravasation, migration and proliferation in the bone marrow/bone. Although novel findings demonstrate the bone marrow microenvironment significance in bone metastatic progression, a majority of studies have focused on end-stage disease and little is known about how the pre-metastatic niche arises in the bone marrow/bone tissues. We demonstrated a significant increase in patients' peripheral blood plasma ability to induce transendothelial migration of MCF-7 cells compared with healthy volunteers. Moreover, high RANKL, MIF and OPG levels in patients' peripheral blood could play a role in the intravasation, angiogenesis, survival and epithelial-mesenchymal transition of circulating tumour cells. Also, we observed a significant increase in patients' bone marrow plasma capacity to induce transendothelial migration of MDA-MB231 and MCF-7 cells compared with healthy volunteers. Furthermore, patients' bone marrow mesenchymal stem cells could control the recruitment of tumour cells, modifying the MCF-7 and MDA-MB231 cell migration. In addition, we found a significantly higher MDA-MB231 cell proliferation when we used patients' bone marrow plasma compared with healthy volunteers. Interestingly, PDGF-AB, ICAM-1 and VCAM-1 levels in patients' bone marrow were significantly higher than the values of healthy volunteers, suggesting that they could be involved in the cancer cell extravasation, bone resorption and cancer cell proliferation. We believe that these results can reveal new information about what alterations happen in the bone marrow of advanced breast cancer patients before bone colonisation, changes that create optimal soil for the metastatic cascade progression.

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Tumour cells can find in bone marrow (BM) a niche rich in growth factors and cytokines that promote their self-renewal, proliferation and survival. In turn, tumour cells affect the homeostasis of the BM and bone, as well as the balance among haematopoiesis, osteogenesis, osteoclastogenesis and bone-resorption. As a result, growth and survival factors normally sequestered in the bone matrix are released, favouring tumour development. Mesenchymal stem cells (MSCs) from BM can become tumour-associated fibroblasts, have immunosuppressive function, and facilitate metastasis by epithelial-to-mesenchymal transition. Moreover, MSCs generate osteoblasts and osteocytes and regulate osteoclastogenesis. Therefore, MSCs can play an important pro-tumorigenic role in the formation of a microenvironment that promotes BM and bone metastasis. In this study we showed that BM MSCs from untreated advanced breast and lung cancer patients, without bone metastasis, had low osteogenic and adipogenic differentiation capacity compared to that of healthy volunteers. In contrast, chondrogenic differentiation was increased. Moreover, MSCs from patients had lower expression of CD146. Finally, our data showed higher levels of Dkk-1 in peripheral blood plasma from patients compared with healthy volunteers. Because no patient had any bone disorder by the time of the study we propose that the primary tumour altered the plasticity of MSCs. As over 70 % of advanced breast cancer patients and 30-40 % of lung cancer patients will develop osteolytic bone metastasis for which there is no total cure, our findings could possibly be used as predictive tools indicating the first signs of future bone disease. In addition, as the MSCs present in the BM of these patients may not be able to regenerate bone after the tumour cells invasion into BM/bone, it is possible that they promote the cycle between tumour cell growth and bone destruction.