Expression of bone sialoprotein and osteopontin in breast cancer bone metastases

Bone sialoprotein (BSP) and osteopontin (OPN) are prominent, mineral-associ ated proteins in the extracellular matrix of bone that have been implicated in the metastatic activity of cancer cells. The expression of BSP, which i s normally restricted to mineralizing tissues, has been observed in cancers with a high propensity for forming bone metastases. To investigate the rel ationship between BSP expression and the formation of bone metastases we ha ve conducted an initial study of the expression of BSP in 10 intraductal br east carcinoma bone metastases using immunostaining and in situ hybridizati on, and compared the expression with OPN. The metastases were characterized by the infiltration of tumour cells into bone with extensive bone resorpti on evident. Moderate to strong staining for BSP was observed in all (100%) carcinomas, which also expressed BSP mRNA as determined by in situ hybridiz ation. Variable staining for BSP was also observed in the mineralized bone and expression of BSP mRNA could be observed in osteoblastic cells on the b one surface and in some osteocytes at sites of bone remodelling. Contrary t o a previous report, BSP expression could be demonstrated by PCR in three b reast cancer cell lines, MCF-7, T47-D and MDA-MB-231. Moreover, in sub-cuta neous tumours formed by MDA-MB-231 breast cancer cells injected into athymi c mice, higher immunostaining for BSP was seen in large ulcerating tumours in which mineral deposits were formed. In contrast to BSP, staining for OPN in bone metastases was generally restricted to the interface between tumor cells and bone surface of the carcinomas. While OPN staining was also obse rved in the cytoplasm of osteoclasts, which showed strong hybridization to a digoxygenin-labelled OPN cRNA probe, expression of OPN was not clearly de tectable in the tumour cells. These studies provide the first demonstration of BSP expression by tumour cells in bone metastases and support the conce pt that BSP may have a role in targeting metastatic cells to bone. Expressi on of OPN in bone metastases appears to be related to increased bone resorp tive activity by osteoclasts.