Imatinib mesylate suppresses bone metastases of breast cancer by inhibiting osteoclasts through the blockade of c-Fms signals.
Imatinib mesylate (imatinib) is a potent and selective inhibitor of the tyrosine kinases, Bcr-Abl, c-Kit and platelet-derived growth factor receptors (PDGFRs). Recently, it has been reported that imatinib also targets the macrophage colony-stimulating factor (M-CSF) receptor c-Fms. M-CSF signals are essential for the differentiation of osteoclasts. Bone metastases of breast cancer are frequently associated with osteoclastic bone destruction. Furthermore, several lines of evidence suggest that osteoclasts play central roles in the development and progression of bone metastases. Thus, in the present study, we examined the effects of imatinib on bone metastases of breast cancer. Coimmunoprecipitation assays showed that imatinib inhibited the M-CSF-induced phosphorylation of c-Fms in osteoclast precursor cells as well as the PDGF-induced PDGFR phosphorylation in MDA-MB-231 human breast cancer cells. Imatinib also markedly reduced osteoclast formation in vitro. In contrast, those concentrations of imatinib did not affect osteoblast differentiation. We then examined the effects of imatinib on bone metastases of MDA-MB-231 cells in a nude mouse model. Radiographic and histomorphometric analyses demonstrated that imatinib significantly decreased bone metastases associated with the reduced number of osteoclasts. In support of the notion that the inhibition of c-Fms acts to suppress the development of bone metastases, we found that a specific inhibitor of c-Fms Ki20227 also decreased bone metastases. In conclusion, these results collectively suggest that imatinib reduced bone metastases, at least in part, by inhibiting osteoclastic bone destruction through the blockade of c-Fms signals. Our results also suggest that imatinib may have a protective effect against cancer treatment-induced bone loss.