Abstract: Objective To investigate the effect of bone morphogenetic protein 9(BMP9) on osteogenic differentiation of HS-5 cells and the expressions of metastasis-related factors of MDA-MB-231 cells. Methods The co-culture system with Transwell chamber was exposed to the conditioned media containing BMP9. Then the changes in the activity and expression of alkaline phosphatase (ALP) and calcium deposition in HS-5 cells were observed by chemiluminescence assay、cytochemical staining and Alizarin Red S staining respectively. RT-PCR and Western blot were carried out to detect the mRNA and protein expressions of osteocalcin(OCN) in HS-5 cells、parathyroid hormone-related protein (PTH-rP) and interleukin-6(IL-6) in MDA-MB-231 cells. Results The ALP activity was increased in a time-dependent manner and reached the peak on the ninth day in HS-5 cells，and the cytochemical staining further confirmed that. Compared with the control group,BMP9 increased calcium deposition and the mRNA and protein expressions of OCN in HS-5 cells（P<0.05）,but decreased the mRNA and protein expressions of PTH-rP and IL-6 in MDA-MB-231 cells（P<0.05）.Conclusion BMP9 could regulate the cross-talk between breast cancer cells and bone marrow mesenchymal stem cells.

Key words: Key words: bone morphogenetic protein 9, breast cancer, bone marrow mesenchymal stem cells