SPARC/Osteonectin expression induces glioma invasion and survival under serum-free conditions

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Tumor invasion into normal brain is a major contributor to the failure of glioma treatment. Osteonectin, also known as secreted protein acidic and rich in cysteine (SPARC), is overexpressed by gliomas at the tumor-brain interface, reactive astrocytes, and neoangiogenic vasculature implying a role in invasion. We have shown that the expression of osteonectin in human malignant glioma cell lines induces an invasive phenotype through extracellular matrix and into normal brain with expression of specific matrix metalloproteinases (MMPs) [Rich JN, et al., J. Biol. Chem. 278: 15951-7, 2003]. Despite gaps in the understanding of the invasion process, specific cell signaling pathways have been linked to regulation of MMP expression and cellular invasion. We now show that treatment of malignant glioma cultures with exogenous osteonectin acutely induces phosphorylation of FAK, JNK, and Akt/PKB and increases activity of the small G-protein, Rac1. Glioma lines constitutively expressing osteonectin also have activation of these same pathways. As invading cells may experience restriction of nutrients relative to the primary tumor, we examined the survival of polyclonal glioma cultures expressing either vector alone or osteonectin under prolonged serum starvation. Cells expressing osteonectin did not differ from vector control in cell cycle fraction under normal serum conditions, but osteonectin expression dramatically lowered the level of apoptosis (4.91 ± 0.01% vs. 0.18 ± 0.02%) and increased G₂ cell cycle fraction (4.16 ± 1.70% vs. 51.85 ± 0.36%) with serum starvation. Combination treatment of serum starved glioma cultures with both serum and osteonectin induced an increase in the phosphorylation of both FAK and Akt suggesting cooperation between the mechanisms by which osteonectin and serum activate these pathways. Thus, glioma expression of osteonectin may promote tumor invasion through induction of cell motility and survival under stress through the activation of specific intracellular pathways involved in cell movement, protease expression, and cell survival.