Specific requirement for the p85-p110α phosphatidylinositol 3-kinase during epidermal growth factor-stimulated actin nucleation in breast cancer cells

We have studied the role of phosphatidylinositol 3-kinases (PI 3-kinases) in the regulation of the actin cytoskeleton in MTLn3 rat adenocarcinoma cells. Stimulation of MTLn3 cells with epidermal growth factor (EGF) induced a rapid increase in actin polymerization, with production of lamellipodia within 3 min. EGF-stimulated lamellipodia were blocked by 100 naa wortmannin, suggesting the involvement of a class Ia PI 3-kinase, MTLn3 cells contain equal amounts of p110 alpha and p110 beta, and do not contain p110 delta. Injection of specific inhibitory antibodies to p110 alpha induced cell rounding and blocked EGF-stimulated lamellipod extension, whereas control or anti-p110 beta antibodies had no effect. In contrast, both antibodies inhibited EGF-stimulated DNA synthesis. An in situ assay for actin nucleation showed that EGF-stimulated formation of new barbed ends was blocked by injection of anti-p110 alpha antibodies. In summary, the p110 alpha isoform of PI 3-kinase is specifically required for EGF-stimulated actin nucleation during lamellipod extension in breast cancer cells.