Secreted Protein Acidic, Rich in Cysteine (SPARC), Mediates Cellular Survival of Gliomas through AKT Activation*

Secreted protein acidic, rich in cysteine (SPARC), is an extracellular matrix protein expressed in many advanced cancers, including malignant gliomas. We and others have previously shown that human glioma cell lines engineered to overexpress SPARC adopt an invasive phenotype. We now show that SPARC expression increases cell survival under stress initiated by serum withdrawal through a decrease in apoptosis. Phosphatidylinositol 3-OH kinase/AKT is a potent pro-survival pathway that contributes to the malignancy of gliomas. Cells expressing SPARC display increased AKT activation with decreased caspase 3/7 activity. Exogenous SPARC rapidly induces AKT phosphorylation, an effect that is blocked by a neutralizing SPARC antibody. Furthermore, AKT activation is essential for the anti-apoptotic effects of SPARC as the decreased apoptosis and caspase activity associated with SPARC expression can be blocked with dominant-negative AKT or a specific AKT inhibitor. As tumor cells face stressful microenvironments particularly during the process of invasion, these results suggest that SPARC functions, in part, to promote tumor progression by enabling tumor cells to survive under stressful conditions.