Activation of Rac1 by Src-dependent phosphorylation of Dock [180.sup.Y1811] mediates PDGFRα-stimulated glioma tumorigenesis in mice and humans

Two hallmarks of glioblastoma multiforme, the most common malignant brain cancer in humans, are aggressive growth and the ability of single glioma cells to disperse throughout the brain. These characteristics render tumors resistant to current therapies and account for the poor prognosis of patients. Although it is known that oncogenic signaling caused by overexpression of genes such as PDGFRA is responsible for robust glioma growth and cell infiltration, the mechanisms underlying glioblastoma malignancy remain largely elusive. Here, we report that PDGFRαsignaling in glioblastomas leads to Src-dependent phosphorylation of the guanine nucleotide exchange factor Dock180 at tyrosine 1811 (Dock[180.sup.Y1811]) that results in activation of the GTPase Rac1 and subsequent cell growth and invasion. In human glioma cells, knockdown of Dock180 and reversion with an RNAi-resistant Dock[180.sup.Y1811F] abrogated, whereas an RNAi-resistant Dock[180.sup.WT] rescued, PDGFRα-promoted glioma growth, survival, and invasion. Phosphorylation of Dock[180.sup.Y1811] enhanced its association with CrkII and p130.sup.Cas], causing activation of Rac1 and consequent cell motility. Dock180 also associated with PDGFRαto promote cell migration. Finally, phosphorylated Dock[180.sup.Y1811] was detected in clinical samples of gliomas and various types of human cancers, and coexpression of phosphorylated Dock[180.sup.Y1811], phosphorylated [Src.sup.Y418], and PDGFRα was predictive of extremely poor prognosis of patients with gliomas. Taken together, our findings provide insight into PDGFRα-stimulated gliomagenesis and suggest that phosphorylated Dock[180.sup.Y1811] contributes to activation of Rac1 in human cancers with PDGFRA amplification.