Specific requirement for the p85-p110 alpha phosphatidylinositol 3-kinase during epidermal growth factor-stimulated actin nucleation in breast cancercells

We have studied the role of phosphatidylinositol 3-kinases (PI 3-kinases) i n the regulation of the actin cytoskeleton in MTLn3 rat adenocarcinoma cell s. Stimulation of MTLn3 cells with epidermal growth factor (EGF) induced a rapid increase in actin polymerization, with production of lamellipodia wit hin 3 min. EGF-stimulated lamellipodia were blocked by 100 naa wortmannin, suggesting the involvement of a class Ia PI 3-kinase, MTLn3 cells contain e qual amounts of p110 alpha and p110 beta, and do not contain p110 delta. In jection of specific inhibitory antibodies to p110 alpha induced cell roundi ng 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-k inase is specifically required for EGF-stimulated actin nucleation during l amellipod extension in breast cancer cells.