Fibroblast growth factor 8 induces osteoclast differentiation in vitro and increases experimental bone metastasis of MDA-MB-231 breast cancer cells in vivo in nude mice

Proc Amer Assoc Cancer Res, Volume 47, 2006 3973 Fibroblast growth factor 8 (FGF8) is involved in regulation of growth of breast and prostate cancer cells which preferentially metastasize to bone. We have previously shown that overexpression of FGF8 (isoform b) in breast cancer cells increases tumor growth and angiogenesis in nude mice. Here we studied the role of FGF8 in formation of breast cancer metastasis in bone. MDA-MB-231 human breast cancer cells forming osteolytic bone metastases were transfected with FGF8 isoforms a, b and e. The in vitro proliferation and invasion of FGF8-transfected MDA-MB-231 cells did not differ from those of vector-transfected control cells (MDA-MB-231-Mock cells), nor did the growth rate of the orthotopical tumors in nude mice. However, when the cells were injected intracardially into nude mice micro x-ray analysis in vivo and histomorphometry of long bones ex vivo showed that FGF8-transfected MDA-MB-231 cells, especially MDA-MB-231-FGF8e cells, formed larger tumor colonies than MDA-MB-231-Mock cells. We studied the possibility that this could be explained by stimulation of osteoclast formation by FGF8 using co-cultures of MDA-MB-231-FGF8 cells with mouse bone marrow cells. We found that in co-cultures MDA-MB-231-FGF8 cells were able to induce increased osteoclast differentiation compared to MDA-MB-231-Mock and MDA-MB-231 wild type cells. Furthermore, FGF8e and FGF8b recombinant proteins were able to stimulate osteoclast differentiation alone when added to mouse bone marrow cultures on bone slices in vitro. Addition of osteoprotegerin (OPG) to the cultures together with FGF8 inhibited stimulation of osteoclast differentiation suggesting that the osteoclastogenic effect of FGF8 was RANKL-mediated. In conclusion, our results show for the first time that FGF8 potently induces osteoclast differentiation in vitro. In addition, it increased formation of experimental bone metastases by MDA-MB-231 cells in nude mice in vivo. Taken together, our results suggest that breast cancer cell produced FGF8 may have a role in the promotion of bone metastasis and formation of lytic lesions of breast cancer by enhancing bone marrow stromal cell -mediated induction of osteoclast differentiation.