OBJECTIVE: To explore the role of Sema4D in the bone metastasis of breast cancer. STUDY DESIGN: The expression of Sema4D in breast cancer MCF-7 cells was downregulated by siRNA transfection and detected by RT-PCR. Proliferation and invasion were detected by MTT and Transwell assays, respectively. A co-culture system of osteoblasts and breast cancer cells was constructed. Bone mineralization was detected by alizarin red staining. The expressions of AKT and p-AKT proteins were detected by western blot. RESULTS: After transfection, the expression of the Sema4D siRNA group was significantly lower than those of the Control and Negative Control (NC) groups (p<0.05). The proliferation rate and number of invading cells in the Sema4D siRNA group from 36 to 72 hours were significantly lower than those of the NC and Control groups (p<0.05). The bone nodule area of the Sema4D siRNA+osteogenic medium (OM) group was significantly larger than that of the MCF-7+OM group (p<0.05). The expression level of p-AKT protein in the Sema4D siRNA+OM group significantly exceeded that of the MCF-7+OM group (p<0.05). CONCLUSION: The Sema4D expression in MCF-7 cells was significantly upregulated. Downregulating this expression attenuated the inhibition of osteoblast differentiation, probably being associated with enhanced AKT phosphorylation. © Science Printers and Publishers, Inc.