EFFECT OF THE ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE BY A THIOPHOSPHOTYROSINE PEPTIDE ON GLUCOSE-TRANSPORT IN 3T3-L1 ADIPOCYTES

Insulin causes the activation of phosphatidylinositol 3-kinase (PI 3-k inase) through complexation of tyrosine-phosphorylated YMXM motifs on insulin receptor substrate 1 with the Src homology 2 domains of PI 3-k inase. Previous studies with inhibitors have indicated that activation of PI 3-kinase is necessary for the stimulation of glucose transport in adipocytes. Here, we investigate whether this activation is suffici ent for this effect. Short peptides containing two tyrosine-phosphoryl ated or thiophosphorylated YMXM motifs potently activated PI 3-kinase in the cytosol from 3T3-L1 adipocytes. Introduction of the phosphatase -resistant thiophosphorylated peptide into 3T3-L1 adipocytes through p ermeabilization with Staphylococcus aureus alpha-toxin stimulated PI 3 -kinase as strongly as insulin. However, under the same conditions the peptide increased glucose transport into the permeabilized cells only 20% as well as insulin. Determination of the distribution of the gluc ose transporter isotype GLUT4 by confocal immunofluorescence showed th at GLUT4 translocation to the plasma membrane can account for the effe ct of the peptide. These results suggest that one or more other insuli n-triggered signaling pathways, besides the PI 3-kinase one, participa te in the stimulation of glucose transport.