Constitutive activation of protein kinase Bα by membrane targeting promotes glucose and system A amino acid transport, protein synthesis, and inactivation of glycogen synthase kinase 3 in L6 muscle cells

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated in the regulation of numerous cellular processes, including the insulin-induced regulation of glycogen synthase kinase 3 (GSK-3) and glucose transport. The hormonal-induced inactivation of GSK-3 is mediated by protein kinase B (PKB), a downstream target of PI 3-kinase, whose involvement in other insulin-stimulated responses remains poorly defined at present. In this study, me investigated whether the uptake of glucose, system A amino acid transport, and cellular protein synthesis are regulated by PKB alpha in L6 skeletal muscle cells. L6 cells stably overexpressing wild-type PKB alpha (wtPKB alpha) or a constitutively active membrane-targeted PKBa (mPKB alpha) showed a 3- and 15-fold increase in PRE activity, respectively. Both wtPKB alpha and mPKB alpha expression led to a significant increase in the basal uptake of glucose and methyl-aminoisobutyric acid (a substrate for the system A amino acid transporter), at least to a level seen in control cells treated with insulin. The stimulation in glucose transport was facilitated, in part, by the increased translocation of GLUT4 to the plasma membrane and also through an increase in the cellular synthesis of GLUT3. In the absence of insulin, only muscle cells expressing the constitutively active PKB alpha showed a significant increase in protein synthesis and an inhibition in GSK-3. Our results indicate that constitutive activation of PKBa in skeletal muscle stimulates the uptake of glucose, system A amino acids, and protein synthesis and promotes the inactivation of GSK-3. These observations imply that PKB alpha may have a role in the insulin-regulated control of these processes in skeletal muscle.