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 nM wortmannin, suggesting the involvement of a class Ia PI 3-kinase. MTLn3 cells contain equal amounts of p110α and p110β, and do not contain p110δ. Injection of specific inhibitory antibodies to p110α induced cell rounding and blocked EGF-stimulated lamellipod extension, whereas control or anti- p110β 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- p110α antibodies. In summary, the p110α isoform of PI 3-kinase is specifically required for EGF-stimulated actin nucleation during lamellipod extension in breast cancer cells.