Membrane-targeted phosphatidylinositol 3-kinase mimics insulin actions andinduces a state of cellular insulin resistance

Phosphatidylinositol (PI) 3-kinase plays an important role in various insul in-stimulated biological responses including glucose transport, glycogen sy nthesis, and protein synthesis. However, the molecular link between PI 3-ki nase and these biological responses is still unclear. We have investigated whether targeting of the catalytic p110 subunit of PI 3-kinase to cellular membranes is sufficient and necessary to induce PI 3-kinase dependent signa ling responses, characteristic of insulin action. We overexpressed Myc-tagg ed, membrane-targeted p110 (p110(CAAX)), and wild-type p110 (p110(WT)) in 3 T3-L1 adipocytes by adenovirus-mediated gene transfer. Overexpressed p110(C AAX) exhibited similar to 2-fold increase in basal kinase activity in p110 immunoprecipitates, that further increased to similar to 4-fold with insuli n. Even at this submaximal PI 3-kinase activity, p110(CAAX) fully stimulate d p70 S6 kinase, Akt, 2-deoxyglucose uptake, and Ras, whereas, p110(WT) had little or no effect on these downstream effects, Interestingly p110(CAAX) did not activate MAP kinase, despite its stimulation of p21(ras). Surprisin gly, p110(CAAX) did not increase basal glycogen synthase activity, and inhi bited insulin stimulated activity, indicative of cellular resistance to thi s action of insulin, p110(CAAX) also inhibited insulin stimulated, but not platelet-derived growth factor-stimulated mitogen-activated protein kinase phosphorylation, demonstrating that the p110(CAAX) induced inhibition of mi togen-activated protein kinase and insulin signaling is specific, and not d ue to some toxic or nonspecific effect on the cells. Moreover, p110(CAAX) s timulated IRS-1 Ser/Thr phosphorylation, and inhibited IRS-1 associated PI 3-kinase activity, without affecting insulin receptor tyrosine phosphorylat ion, suggesting that it may play an important role as a negative regulator for insulin signaling. In conclusion, our studies show that membrane-target ed PI S-kinase can mimic a number of biologic effects normally induced by i nsulin. In addition, the persistent activation of PI 3-kinase induced by p1 10(CAAX) expression leads to desensitization of specific signaling pathways . Interestingly, the state of cellular insulin resistance is not global, in that some of insulin's actions are inhibited, whereas others are intact.