Role of the metastasis-promoting protein osteopontin in the tumour microenvironment.

Osteopontin (OPN) is a secreted protein present in bodily fluids and tissues. It is subject to multiple post-translational modifications, including phosphorylation, glycosylation, proteolytic cleavage and crosslinking by transglutamination. Binding of OPN to integrin and CD44 receptors regulates signalling cascades that affect processes such as adhesion, migration, invasion, chemotaxis and cell survival. A variety of cells and tissues express OPN, including bone, vasculature, kidney, inflammatory cells and numerous secretory epithelia. Normal physiological roles include regulation of immune functions, vascular remodelling, wound repair and developmental processes. OPN also is expressed in many cancers, and elevated levels in patients' tumour tissue and blood are associated with poor prognosis. Tumour growth is regulated by interactions between tumour cells and their tissue microenvironment. Within a tumour mass, OPN can be expressed by both tumour cells and cellular components of the tumour microenvironment, and both tumour and normal cells may have receptors able to bind to OPN. OPN can also be found as a component of the extracellular matrix. The functional roles of OPN in a tumour are thus complex, with OPN secreted by both tumour cells and cells in the tumour microenvironment, both of which can in turn respond to OPN. Much remains to be learned about the cross-talk between normal and tumour cells within a tumour, and the role of multiple forms of OPN in these interactions. Understanding OPN-mediated interactions within a tumour will be important for the development of therapeutic strategies to target OPN.

Osteopontin increases breast cancer cell sensitivity to specific signaling pathway inhibitors in preclinical models.

Src kinase and Hsp90 play important roles in malignancy-promoting signaling pathways in a variety of cancers and related targeting agents are presently in clinical trials. To help improve the success of Src kinase and Hsp90 inhibitor therapies, identifying sensitive patient populations will be essential. Osteopontin (OPN), a secreted integrin-binding glycophosphoprotein, is associated with progression and metastasis in a variety of cancers and has been studied as a prognostic marker. Previous work of ours and other groups has indicated that OPN-induced signal transduction involves activation of both Src and Hsp90-dependent pathways. We thus hypothesized that over-expression of OPN could make tumor cells more vulnerable to these classes of inhibitors. This study used multiple in vitro assays to determine if OPN levels could predict breast cancer cell sensitivity to Hsp90 and/or Src kinase inhibitors. We used multiple derivatives of two unrelated of human breast cancer cell lines, high vs. low levels of OPN, to determine if OPN affects the response to two specific inhibitors, an Hsp90 inhibitor and a Src kinase inhibitor, in in vitro migration and colony formation assays. Cells had greater decreases in migration and colony forming ability after Hsp90 and Src kinase inhibitor treatments when OPN was present (either endogenous or exogenous). Decreasing OPN levels via shRNA knockdown decreased inhibitor effects. In rescue experiments, adding exogenous OPN to non-expressing cells increased inhibitor effects. These results suggest that OPN could potentially be useful clinically as a predictive marker in identifying patients who will benefit from either Hsp90 or Src kinase inhibitor therapy.

Pre- and post-translational regulation of osteopontin in cancer.

Osteopontin (OPN) is a matricellular protein that binds to a number of cell surface receptors including integrins and CD44. It is expressed in many tissues and secreted into body fluids including blood, milk and urine. OPN plays important physiological roles in bone remodeling, immune response and inflammation. It is also a tumour-associated protein, and elevated OPN levels are associated with tumour formation, progression and metastasis. Research has revealed a promising role for OPN as a cancer biomarker. OPN is subject to alternative splicing, as well as post-translational modifications such as phosphorylation, glycosylation and proteolytic cleavage. Functional differences have been revealed for different isoforms and post-translational modifications. The pattern of isoform expression and post-translational modification is cell-type specific and may influence the potential role of OPN in malignancy and as a cancer biomarker.