Tumor-stimulated angiogenesis involves endothelial cell migration and reorganization into vessel structures, a process that is regulated by the balance between kinases and phosphatases. We have shown that the principal motility-stimulatory factors produced by several tumors are PGE2 and TGFbeta which, in turn, inhibit the activity of the serine/threonine protein phosphatase PP-2A. Pharmacological inhibition of PP-2A increases cellular motility. Our hypothesis is that by producing the motility-stimulatory factors PGE2 and TGFbeta, tumors inhibit endothelial cell PP-2A activity, which stimulates endothelial cell motility through two interconnected pathways: (i) serine/threonine phosphorylation of paxillin, which destabilizes FAK/Src/paxillin complexes and triggers Src activation of the p130(Cas)/Crk and PI3K motility-stimulatory pathways, (ii) serine/threonine phosphorylation of PTEN, thus inhibiting its ability to limit activation of the PI3K and p130(Cas)/Crk pathways. Defining these motility-stimulatory pathways will also identify signaling components that can be targeted to interrupt endothelial cell motility and, in turn, the neovascularization that is required for tumor growth. Our hypothesis will be tested in vitro and in vivo in a murine Lewis lung carcinoma (LLC) model by assessing if the inhibition of PP-2A by PGE2/TGFbeta stimulates: (i) serine/threonine phosphorylation of paxillin, dissolution of FAK/Src/paxillin complexes and activation of Src as one arm of a motility-stimulatory pathway; (ii) serine/threonine phosphorylation and, consequently, inactivation of PTEN is the second arm of a motility-stimulatory pathway; (iii) activation of the motility-stimulatory p130(Cas)/Crk and PI3K pathways by the two arms of the of the cascade that yield active Src and inactive PTEN. Upon completion, these studies will identify how tumor inhibition of PP-2A increases endothelial cell motility, a requirement for the process of angiogenesis. These studies will also identify signaling components that can be targeted to block the motility-stimulatory pathways. Although being conducted in the LLC model, the proposed studies are applicable to solid cancers in general, as their growth is dependent on angiogenesis.
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