Purpose: Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase that has been shown to promote proliferation, migration, and invasion of several cell types in vitro, and we have shown recently that FAK promotes proliferation of malignant astrocytoma cells in vivo. To determine the role of FAK in angiogenesis in malignant astrocytic tumors, we investigated the expression and function of FAK in brain endothelial cells.
Experimental design: We characterized the expression of FAK and activated FAK in endothelial cells by immunohistochemistry. We also determined the function of FAK in brain microvascular endothelial cells by transfecting these cells with a dominant interfering form of FAK [FAK-related nonkinase (FRNK)] or a mutant FRNK (Leu-1034 to Ser) and assessed the effect on capillary tube formation and cell migration.
Results: We found that FAK was expressed in the endothelial cells of grade III (4 of 9 samples) and IV (9 of 10 samples) astrocytoma biopsies but not in the endothelial cells of normal brain (0 of 9 samples) and not in grade I (0 of 5 samples) or II (0 of 4 samples) astrocytoma biopsies. Furthermore, we found that both FAK and activated FAK were expressed in the endothelial cells in malignant astrocytoma tumors propagated intracerebrally in the severe combined immunodeficient mouse brain. As expected, immunofluorescence analysis showed FRNK protein to localize to focal adhesions, whereas mutant FRNK protein did not. FRNK-transfected endothelial cells showed a 55% reduction in branched tube formation and a 40% reduction in tube length when propagated in three-dimensional collagen gels, compared with cells transfected with the mutant FRNK construct. Furthermore, FRNK-transfected cells showed a 35-50% reduction in haptotactic migration toward fibronectin and collagen, compared with mutant FRNK-transfected cells.
Conclusions: These data suggest that FAK promotes angiogenesis and that this occurs, at least in part, through the promotion of endothelial cell migration.