| Insulin stimulates facilitated glucose transport in adipose tissues. One protein activated by insulin is phosphatidylinositol 3-kinase (PI 3-kinase), which has been shown to play a role in glucose transport. Akt kinase is a serine/threonine kinase that also carries a pleckstrin homology (PH) domain. Since ligand-binding of the insulin receptor activates ser/thr kinases, a ser/thr kinase like Akt could participate in biological responses initiated by the insulin receptor.;In the present studies, insulin is shown to rapidly stimulate Akt kinase activity. Insulin activation was accompanied by a shift in molecular weight of Akt and the activated kinase was deactivated by treatment with a phosphatase, indicating that insulin activated the enzyme by stimulating its phosphorylation. The insulin-stimulated increase in Akt activity was inhibited by wortmannin (an inhibitor of PI 3-kinase). These results indicate that Akt activity can be regulated by the insulin receptor and suggest that Akt functions downstream of PI 3-kinase.;To explore the role of the PH domain in Akt stimulation, a mutant was made in which the entire PH domain was excised. In three cell types, Akt does not require its PH domain to respond to insulin. Targeting Akt, without its PH domain, to the membrane constitutively activates Akt by causing an increase in its basal level of phosphorylation. This constitutively active form of Akt can also activate p70;The constitutively active Akt was expressed in 3T3-L1 cells that can be differentiated into adipocytes. The constitutively active Akt induced glucose uptake into adipocytes in the absence of insulin by stimulating translocation of the insulin-responsive glucose transporter GLUT4 to the plasma membrane and by increasing the synthesis of the ubiquitously expressed glucose transporter, GLUT1. The increased glucose influx in the 3T3-L1 adipocytes directed lipid but not glycogen synthesis. These results indicate that Akt can regulate glucose uptake and metabolism. |
