The function of SPARC in tumor cell biology: SPARC as a modulator of cell-extracellular matrix interaction

Although alterations in the level of SPARC (secreted protein, acidic and rich in cysteine; also known as osteonectin) expression have been associated with a large number of studies on tumor tissue from various anatomical locations, the mechanisms by which SPARC influences tumor progression are not well defined. The capacity of SPARC to affect cellular proliferation, adhesion, migration, and invasion in a cell and tissue-specific manner further complicates the analysis of the function of SPARC in tumor biology. In this chapter, an effort is made to bring together results generated from a number of different studies that highlight functional consequences of SPARC expression. Emphasis has been placed on cell-extracellular matrix interactions, in particular collagen-binding and collagen receptor activity. Likely, it is the contextual nature of divergent functions associated with SPARC, based in unique tissue microenvironments, that give rise to what are seen as seemingly contradictory effects of SPARC expression in different tumors.