Analysis of human bone sialoprotein in normal and pathological tissues using a monoclonal antibody (BSP 1.2 mab)

Deletion of OPN in BSP knockout mice does not correct bone hypomineralization but results in high bone turnover

The two SIBLING (Small Integrin Binding Ligand N-linked Glycoproteins), bone sialoprotein (BSP) and osteopontin (OPN) are expressed in osteoblasts and osteoclasts. In mature BSP knockout (KO, ^-/-) mice, both bone formation and resorption as well as mineralization are impaired. OPN^-/- mice display impaired resorption, and OPN is described as an inhibitor of mineralization. However, OPN is overexpressed in BSP^-/- mice, complicating the understanding of their phenotype. We have generated and characterized mice with a double KO (DKO) of OPN and BSP, to try and unravel their respective contributions. Despite the absence of OPN, DKO bones are still hypomineralized. The SIBLING, matrix extracellular phosphoglycoprotein with ASARM motif (MEPE) is highly overexpressed in both BSP^-/- and DKO and may impair mineralization through liberation of its ASARM (Acidic Serine-Aspartate Rich MEPE associated) peptides. DKO mice also display evidence of active formation of trabecular, secondary bone as well as primary bone in the marrow-ablation repair model. A higher number of osteoclasts form in DKO marrow cultures, with higher resorption activity, and DKO long bones display a localized and conspicuous cortical macroporosity. High bone formation and resorption parameters, and high cortical porosity in DKO mice suggest an active bone modeling/remodeling, in the absence of two key regulators of bone cell performance. This first double KO of SIBLING proteins thus results in a singular, non-trivial phenotype leading to reconsider the interpretation of each single KO, concerning in particular matrix mineralization and the regulation of bone cell activity.

BSP gene silencing inhibits migration, invasion, and bone metastasis of MDA-MB-231BO human breast cancer cells

Bone sialoprotein (BSP) has been implicated in a variety of physiological and pathophysiological events, including tumor cell invasion, bone homing, adhesion, and matrix degradation. To explore the potential involvement of BSP in human breast cancer cell invasion and metastasis, we used retrovirus-mediated RNAi to deplete BSP levels in the human bone-seeking breast cancer cell line MDA-MB-231BO (231BO) and established the 231BO-BSP27 and 231BO-BSP81 cell clones. Cell proliferation, colony formation, wound healing, and the ability to invade into matrigel of these BSP-depleted clones were all decreased. Both 231BO-BSP27 cells and 231BO-BSP81 cells showed a significant (15.4% and 28.6% respectively) reduction of bone metastatic potential following intracardiac injection as determined by X-ray detection and by hematoxylin and eosin staining. Moreover, the expression of integrins αvβ3 and β3 was decreased in the BSP-silenced cells whereas ectopic BSP expression increased the integrins αvβ3 and β3 levels. These results together suggest that BSP silencing decreased the integrin αvβ3 and β3 levels, in turn inhibiting cell migration and invasion and decreasing the ability of the cells to metastasize to bone.

Small integrin-binding ligand N-linked glycoproteins (SIBLINGs): multifunctional proteins in cancer

Numerous components and pathways are involved in the complex interplay between cancer cells and their environment. The family of glycophosphoproteins comprising osteopontin, bone sialoprotein, dentin matrix protein 1, dentin sialophosphoprotein and matrix extracellular phosphoglycoprotein - small integrin-binding ligand N-linked glycoproteins (SIBLINGs) - are emerging as important players in many stages of cancer progression. From their detection in various human cancers to the demonstration of their key functional roles during malignant transformation, invasion and metastasis, the SIBLINGs are proteins with potential as diagnostic and prognostic tools, as well as new therapeutic targets.