Activated Gαq inhibits p110α phosphatidylinositol 3-kinase and Akt

Some G(q)-coupled receptors have been shown to antagonize growth factor activation of phosphatidylinositol 3-kinase (PI3K) and its downstream effector, Akt. We used a constitutively active Galpha(q)(Q209L) mutant to explore the effects of Galpha(q) activation on signaling through the PI3K/Akt pathway. Transient expression of Galpha(q)(Q209L) in Rat-1 fibroblasts inhibited Akt activation induced by platelet-derived growth factor or insulin treatment. Expression of Galpha(q)(Q209L) also attenuated Akt activation promoted by coexpression of constitutively active PI3K in human embryonic kidney 293 cells. Galpha(q)(Q209L) had no effect on the activity of an Akt mutant in which the two regulatory phosphorylation sites were changed to acidic amino acids. Inducible expression of Galpha(q)(Q209L) in a stably transfected 293 cell line caused a decrease in PI3K activity in p110alpha (but not p110beta) immunoprecipitates. Receptor activation of Galpha(q) also selectively inhibited PI3K activity in p110alpha immunoprecipitates. Active Galpha(q) still inhibited PI3K/Akt in cells pretreated with the phospholipase C inhibitor U73122. Finally, Galpha(q)(Q209L) co-immunoprecipitated with the p110alpha-p85alpha PI3K heterodimer from lysates of COS-7 cells expressing these proteins, and incubation of immunoprecipitated Galpha(q)(Q209L) with purified recombinant p110alpha-p85alpha in vitro led to a decrease in PI3K activity. These results suggest that agonist binding to G(q)-coupled receptors blocks Akt activation via the release of active Galpha(q) subunits that inhibit PI3K. The inhibitory mechanism seems to be independent of phospholipase C activation and might involve an inhibitory interaction between Galpha(q) and p110alpha PI3K.