Activated Gα_q 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 Gα_q(Q209L) mutant to explore the effects of Gα_q activation on signaling through the PI3K/Akt pathway. Transient expression of Gα_q(Q209L) in Rat-1 fibroblasts inhibited Akt activation induced by platelet-derived growth factor or insulin treatment. Expression of Gα_q(Q209L) also attenuated Akt activation promoted by coexpression of constitutively active PI3K in human embryonic kidney 293 cells. Gα_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 Gα_q(Q209L) in a stably transfected 293 cell line caused a decrease in PI3K activity in p110α (but not p110β) immunoprecipitates. Receptor activation of Gα_q also selectively inhibited PI3K activity in p110α immunoprecipitates. Active Gα_q still inhibited PI3K/Akt in cells pretreated with the phospholipase C inhibitor U73122. Finally, Gα_q(Q209L) co-immunoprecipitated with the p110α-p85α PI3K heterodimer from lysates of COS-7 cells expressing these proteins, and incubation of immunoprecipitated Gα_q(Q209L) with purified recombinant p110α-p85α in vitro led to a decrease in PI3K activity. These results suggest that agonist binding to Gqcoupled receptors blocks Akt activation via the release of active Gα_q subunits that inhibit PI3K. The inhibitory mechanism seems to be independent of phospholipase C activation and might involve an inhibitory interaction between Gα_q and p110α PI3K.