4. Discussion
Bone metastases of breast cancer typically lead to a severe osteolysis due to an excessive osteoclast activity. Considering its bone affinity, as well as its inhibitory action on bone resorption, we decided to explore the in vitro Ga effects on osteoclastogenesis in a bone metastatic environment. Experiments were conducted using the breast tumour cell line model MDA-MB-231, and its bone-seeking clone MDA-231BO. We stimulated RAW 264.7 cells with breast cancer-derived factors by using conditioned medium (CM) isolated from breast tumour cell culture and, similarly to Guo et al. [29], we observed that factors derived from breast cancer cells supported mature osteoclasts formation from RANKL-primed precursors in the absence of supporting cell types. According to Gallet et al., MDA-MB-231 cells release several soluble factors such as macrophage colony-stimulating factor (M-CSF) and parathyroid hormone-related protein (PTHLH), which both act on osteoclastogenesis and mature osteoclast survival [33]. We demonstrated here that, under these conditions, Ga inhibited in a dose-dependent manner the tumour cell-induced formation of mature osteoclasts. With a view to mimic a more aggressive environment where pro-tumourigenic factors are released from bone matrix due to osteoclastic resorption, metastatic breast tumour cells were stimulated with TGF-b. In these conditions, we observed that Ga still inhibited cancer cells-driven osteoclast differentiation. To summarize, our results clearly evidence that Ga may disturb the vicious circle between bone cells and breast cancer cells by preventing the release of pro-tumourigenic factors from bone matrix (Fig. 5). We were next interested in deciphering whether Ga could impact directly metastatic breast cancer cells; and we focussed our experiments on the bone-seeking clone MDA-231BO. We first confirmed previous results published by Yoneda et al. [32] concerning the greater production of PTHLH by MDA-231BO as compared to the parental MDA-MB-231 clone (data not shown). Further supporting the interest of using this cell clone in experiments related to a bone metastases context, this phenotypic change allows breast cancer cells to survive, proliferate into bone and promote osteoclastic resorption, which in turn leads to the establishment of severe osteolytic bone metastases. Importantly, we observed that Ga inhibited in a dose-dependent manner the proliferation and the viability of these bone metastatic breast tumour cells.
