Differential Effects of Phosphatidylinositol 3-Kinase Inhibition on Intracellular Signals Regulating GLUT4 Translocation and Glucose Transport

Abstract

Phosphatidylinositol (PI) 3-kinase is required for insulin-stimulated translocation of GLUT4 to the surface of muscle and fat cells. Recent evidence suggests that the full stimulation of glucose uptake by insulin also requires activation of GLUT4, possibly via a p38 mitogen-activated protein kinase (p38 MAPK)-dependent pathway. Here we used L6 myotubes expressing Myc-tagged GLUT4 to examine at what level the signals regulating GLUT4 translocation and activation bifurcate. We compared the sensitivity of each process, as well as of signals leading to GLUT4 translocation (Akt and atypical protein kinase C) to PI 3-kinase inhibition. Wortmannin inhibited insulin-stimulated glucose uptake with an IC(50) of 3 nm. In contrast, GLUT4myc appearance at the cell surface was less sensitive to inhibition (IC(50) = 43 nm). This dissociation between insulin-stimulated glucose uptake and GLUT4myc translocation was not observed with LY294002 (IC(50) = 8 and 10 microm, respectively). The sensitivity of insulin-stimulated activation of PKC zeta/lambda, Akt1, Akt2, and Akt3 to wortmannin (IC(50) = 24, 30, 35, and 60 nm, respectively) correlated closely with inhibition of GLUT4 translocation. In contrast, insulin-dependent p38 MAPK phosphorylation was efficiently reduced in cells pretreated with wortmannin, with an IC(50) of 7 nm. Insulin-dependent p38 alpha and p38 beta MAPK activities were also markedly reduced by wortmannin (IC(50) = 6 and 2 nm, respectively). LY294002 or transient expression of a dominant inhibitory PI 3-kinase construct (Delta p85), however, did not affect p38 MAPK phosphorylation. These results uncover a striking correlation between PI 3-kinase, Akt, PKC zeta/lambda, and GLUT4 translocation on one hand and their segregation from glucose uptake and p38 MAPK activation on the other, based on their wortmannin sensitivity. We propose that a distinct, high affinity target of wortmannin, other than PI 3-kinase, may be necessary for activation of p38 MAPK and GLUT4 in response to insulin.