Activation of phosphatidylinositol 3-kinase by epidermal growth factor, basic fibroblast growth factor, and nerve...

Epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and nerve growth factor (NGF), which stimulate the phosphorylation of proteins on tyrosine in PC12 cells, initiate these modifications through ligand-specific cell surface receptors that contain the causative tyrosine kinases. One apparent substrate for these enzymes is phosphatidylinositol 3-kinase (PI 3-kinase), an enzyme that phosphorylates the D-3 position of the inositol ring and associates with several protein tyrosine kinases, as indicated by the fact that it is immunoprecipitated from EGF-, bFGF-, and NGF-stimulated PC12 cells by an anti-phosphotyrosine antibody. All three growth factors increase immunoprecipitable PI 3-kinase activity after 2 min of addition at concentrations able to stimulate either mitogenic or neurotrophic responses in PC12 cells. The level of stimulation of PI 3-kinase activity by EGF, bFGF, and NGF is 15- to 20-fold, 2- to 3-fold, and 8- to 10-fold, respectively. Moreover, tyrosine phosphorylation of PI 3-kinase was detected in EGF-, bFGF-, and NGF-stimulated PC12 cells, and the amount of the phosphorylation correlated with the level of stimulation of enzyme activity. In contrast, phosphatidylinositol 4-kinase, which produces the inositol phospholipids cleaved by phospholipase C-gamma to yield diacylglycerol and inositol-1,4,5-trisphosphate, is not affected by these growth factors. The pattern of stimulation of PI 3-kinase does not correlate with the induction of neurite outgrowth but rather with the mitotic responses, suggesting that PI 3-kinase and its products may be more important for signaling in cell division than in trophic processes. However, the levels of phosphatidylinositol 3-phosphate do not coincide with the stimulation of [3H]thymidine incorporation by these growth factors, rendering its role in mitotic functions, at least in PC12 cells, also uncertain.