Evidence that Akt mediates platelet-derived growth factor-dependent increases in activity and surface expression of...

Previously we have shown that platelet-derived growth factor (PDGF) rapidly increases the activity of the neuronal glutamate transporter, EAAC1. This increase in activity is associated with a rapid (within minutes) redistribution of transporter from a subcellular compartment to the plasma membrane and is blocked by inhibitors of phosphatidylinositol 3-kinase (PI3K). Similar effects of PI3K inhibitors have been observed for insulin-dependent up-regulation of the GLUT4 subtype of glucose transporter. Although GLUT4 regulation also depends on the serine-threonine kinase (Akt/protein kinase B), a downstream target of PI3K, the downstream effectors responsible of PDGF-dependent regulation of EAAC1 have not been identified. In the present study, PDGF increased the level of Akt phosphorylation (Ser 473) in C6 glioma cells, a cell line that has been used to study regulated trafficking of endogenous EAAC1. Two inhibitors of PI3K blocked this effect. In transient transfection studies, a dominant negative mutant of Akt-1 blocked PDGF-induced redistribution of epitope-tagged EAAC1 (myc-EAAC1). Conversely, constitutively active Akt-1 (CA Akt-1) increased the cell surface expression of myc-EAAC1. A lentiviral vector engineered to express CA Akt-1 increased Akt activation, cell surface expression of endogenous EAAC1, and Na(+)-dependent glutamate transport activity. Together, these studies suggest that Akt is required for PDGF-induced regulation of EAAC1.