Bone sialoprotein (BSP) expression is detected in a variety of human osteotropic
cancers. High expression of BSP in breast and prostate primary
carcinomas is associated with progression and bone
metastases development. In this study, we examined the transcriptional regulation of BSP gene expression in MDA-MB-231 and MCF-7 human
breast cancer cells compared with Saos-2 human osteoblast-like cells. BSP human promoter deletion analyses delineated a -56/-84 region, which comprises a cAMP response element (CRE) that was sufficient for maximal promoter activity in
breast cancer cell lines. We found that the
basic fibroblast growth factor response element (FRE) also located in the proximal promoter was a crucial regulator of human BSP promoter activity in Saos-2 but not in
breast cancer cells. Promoter activity experiments in combination with
DNA mobility shift assays demonstrated that BSP promoter activity is under the control of the CRE
element, through CREB-1, JunD and Fra-2 binding, in MDA-MB-231, MCF-7 and in Saos-2 cells.
Forskolin, a
protein kinase A pathway activator, failed to enhance BSP transcriptional activity suggesting that CRE site behaves as a constitutive rather than an inducible
element in these cell lines. Over-expression of JunD and Fra-2 increased BSP promoter activity and upregulated endogenous BSP
protein expression in MCF-7 and Saos-2 cells while
siRNA-mediated inhibition of both factors expression significantly reduced BSP
protein level in MDA-MB-231. Collectively, these data provide with new transcriptional mechanisms, implicating CREB and
AP-1 factors, that control BSP gene expression in
breast cancer cells.