Macropinocytosis results from the closure of lamellipodia generated by membrane 
ruffling, thereby reflecting cortical actin dynamics. Both transformation of 
Rat-1 fibroblasts by v-Src or K-Ras and stable transfection for expression of 
dominant-positive, wild-type phosphoinositide 3-kinase (PI3K) regulatory subunit 
p85 alpha constitutively led to stress fiber disruption, cortical actin 
recruitment, extensive ruffling, and macropinosome formation, as measured by a 
selective acceleration of fluid-phase endocytosis. These alterations closely 
correlated with activation of PI3K and phosphatidylinositol-specific 
phospholipase C (PI-PLC), as assayed by 3-phosphoinositide synthesis in situ and 
in vitro and inositol 1, 4,5 trisphosphate steady-state levels, respectively; 
they were abolished by stable transfection of v-Src-transformed cells for 
dominant-negative truncated p85 alpha expression and by pharmacological 
inhibitors of PI3K and PI-PLC, indicating a requirement for both enzymes. 
Whereas PI3K activation resisted PI-PLC inhibition, PI-PLC activation was 
abolished by a PI3K inhibitor and dominant-negative transfection, thus placing 
PI-PLC downstream of PI3K. Together, these data suggest that permanent 
sequential activation of both PI3K and PI-PLC is necessary for the dramatic 
reorganization of the actin cytoskeleton in oncogene-transformed fibroblasts, 
resulting in constitutive ruffling and macropinocytosis.