Over-expression of bone sialoprotein enhances bone metastasis of human breast cancer cells in a mouse model.

Bone sialoprotein (BSP) is a major non-collagenous protein found almost exclusively in bone and other mineralized tissues including enamel, dentin and cementum. Although a role for BSP in mineralization has been indicated, BSP also appears to function in patho-physiological processes, including the metastasis of breast and prostate cancer cells to bone. The purpose of this study was to determine the role of BSP in the homing of cancer cells and to provide insights into the role of BSP in physiological as well as pathological processes. We established cultures of MDA-231 breast cancer cells stably transfected with DNA constructs of pIRES2-EGFP (green fluorescent protein) expressing human BSP (hBSP) cDNA (231BSP) under a CMV promoter, or with an antisense sequence of hBSP cDNA (231BSPAS), or with an empty vector as a control (231EV). These 3 cell groups were selected for neomycin resistance using G418 and analyzed by flow cytometry for GFP expression. The resultant cultured cells expressed different levels of hBSP as detected by RT-PCR and Western blot. Among the three, 231BSP expressed the highest levels of hBSP while 231BSPAS expressed the lowest. The capacity of the tumor cells to metastasize to bone was determined in nude mice (5 in each group) by intra-cardiac injection of the cells from the 3 different groups. Four weeks after inoculation, radiological examination revealed that all the 5 mice in the 231BSP cell group had developed osteolytic bone metastases. In the 231BSPAS group only 1 mouse demonstrated metastatic bone lesions while 3 out of 5 mice in the control group (231EV) developed metastatic lesions in the bone. These results strongly suggest that BSP over-expression in human tumor cells can enhance bone metastasis of MDA-231 cells whereas repressed expression of BSP, using antisense BSP cDNA, inhibits this effect in a mouse model.