R-Ras Promotes Focal Adhesion Formation through Focal Adhesion Kinase and p130Cas by a Novel Mechanism That Differs from Integrins

摘要

R-Ras regulates integrin function, but its effects on integrin signaling pathways have not been well described. We demonstrate that activation of R-Ras promoted focal adhesion formation and altered localization of the α2β1 integrin from cell-cell to cell-matrix adhesions in breast epithelial cells. Constitutively activated R-Ras(38V) dramatically enhanced focal adhesion kinase (FAK) and p130^Cas phosphorylation upon collagen stimulation or clustering of the α2β1 integrin, even in the absence of increased ligand binding. Signaling events downstream of R-Ras differed from integrins and K-Ras, since pharmacological inhibition of Src or disruption of actin inhibited integrin-mediated FAK and p130^Cas phosphorylation, focal adhesion formation, and migration in control and K-Ras(12V)-expressing cells but had minimal effect in cells expressing R-Ras(38V). Therefore, signaling from R-Ras to FAK and p130^Cas has a component that is Src independent and not through classic integrin signaling pathways and a component that is Src dependent. R-Ras effector domain mutants and pharmacological inhibition suggest a partial role for phosphatidylinositol 3-kinase (PI3K), but not Raf, in R-Ras signaling to FAK and p130^Cas. However, PI3K cannot account for the Src-independent pathway, since simultaneous inhibition of both PI3K and Src did not completely block effects of R-Ras on FAK phosphorylation. Our results suggest that R-Ras promotes focal adhesion formation by signaling to FAK and p130^Cas through a novel mechanism that differs from but synergizes with the α2β1 integrin.