Protein kinase B activation and lamellipodium formation are independent phosphoinositide 3-kinase-mediated events differentially regulated by endogenous Ras

Regulation of phosphoinositide 3-kinase (PI 3-kinase) can occur by binding of the regulatory p85 subunit to tyrosine-phosphorylated proteins and by binding of the p110 catalytic subunit to activated Ras. However, the way in which these regulatory mechanisms act to regulate PI 3-kinase in vivo is unclear, Were we show that several growth factors (basic fibroblast growth factor [bFGF], platelet-derived growth factor [PDGF], and epidermal growth factor [EGF; to activate an EGF receptor-Ret chimeric receptor]) all activate PI3-kinase in vivo in the neuroectoderm-derived cell line SKF5. However, these growth factors differ in their ability to activate PI 3-kinase-dependent signaling. PDGF and EGF(Ret) treatment induced PI 3-kinase-dependent lamellipodium formation and protein kinase B (PKB) activation. In contrast, bFGF did not induce lamelli-podium formation but activated PKB, albeit to a small extent. PDGF and EGF(Ret) stimulation resulted in binding of p85 to tyrosine-phosphorylated proteins and strong Ras activation, bFGF, however, induced only strong activation of Ras, In addition, while Ras(Asn17) abolished bFGF activation of PKB, PDGF-and EGF(Rea)-induced PKB activation was only partially, inhibited and lamelli-podium formation was unaffected. Interestingly, in contrast to activation of only endogenous Ras (bFGF), ectopic expression of activated Ras did result in lamelli-podium formation, From this we conclude that, in vivo, p85 and Ras synergize to activate PI 3-kinase and that strong activation of only endogenous Bas exerts a small effect on PI 3-kinase activity, sufficient for PKB activation but not lamelli-podium formation. This differential sensitivity to PI3-kinase activation could be explained by our Ending that PKB activation and lamelli-podium formation are independent PI 3-kinase-induced events.