Bone sialoprotein (BSP) is a secreted
glycoprotein found in mineralized tissues however, BSP is aberrantly expressed in a variety of osteotropic
tumors. Elevated BSP expression in breast and prostate primary
carcinomas is directly correlated with increased bone
metastases and
tumor progression. In this study, the intracellular signaling pathways responsible for BSP-induced migration and
tumor survival were examined in breast and
prostate cancer cells (MDA-MB-231, Hs578T and PC3). Additionally, the effects of exogenous
TGF-beta1 and
EGF,
cytokines associated with
tumor metastasis and present in high-levels in the bone microenvironment, were examined in BSP-expressing
cancer cells. Expression of BSP but not an
integrin-binding mutant (BSP-KAE) in tumor cell lines resulted in increased levels of alpha(v)-containing
integrins and number of mature focal adhesions. Adhesion of cells to recombinant BSP or the expression of BSP stimulated
focal adhesion kinase and ERK phosphorylation, as well as activated AP-1-family
proteins. Activation of these pathways by BSP expression increased the expression of the
matrix metalloproteinases MMP-2, MMP-9, and MMP-14. The BSP-mediated activation of the FAK-associated pathway resulted in increased
cancer cell invasion in a
Matrigel-coated Boyden-chamber assay and increased cell survival upon withdrawal of serum. Addition of
EGF or
TGF-beta1 to the BSP-expressing cell lines significantly increased ERK phosphorylation,
AP-1 activation, MMP-2 expression, cell migration and survival compared to untreated cells expressing BSP. This study thus defines the cooperative mechanisms by which BSP can enhance specific factors associated with a metastatic phenotype in tumor cell lines, an effect that is increased by circulating
TGF-beta1 and
EGF.