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

Abstract

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, we investigated whether the uptake of glucose, system A amino acid transport, and cellular protein synthesis are regulated by PKBalpha in L6 skeletal muscle cells. L6 cells stably overexpressing wild-type PKBalpha (wtPKBalpha) or a constitutively active membrane-targeted PKBalpha (mPKBalpha) showed a 3- and 15-fold increase in PKB activity, respectively. Both wtPKBalpha and mPKBalpha 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 PKBalpha showed a significant increase in protein synthesis and an inhibition in GSK-3. Our results indicate that constitutive activation of PKBalpha 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 PKBalpha may have a role in the insulin-regulated control of these processes in skeletal muscle.

Lipoic acid protects against oxidative stress induced impairment in insulin stimulation of protein kinase B and glucose transport in 3T3-L1 adipocytes

“…1A and 2B, respectively). These are consistent with the notion that PKB activation has a role in the induction of GLUT4 translocation by insulin [22,23,25,26] and thus in the protective mechanism against oxidative stress exerted by LA pretreatment. Whether the insulin stimulation of PKB activity is in itself a reduced sulphydryl dependent process or rather reflects upstream oxidative sensitive steps, such as the translocation and activation of PI 3-kinase in the low density microsomes, remains to be evaluated.…”

Expression of rainbow trout glucose transporters GLUT1 and GLUT4 during in vitro muscle cell differentiation and regulation by insulin and IGF-I

“…In contrast, GLUT4 expression dramatically decreased with dedifferentiation of adult rat cardiomyocytes (39). For this reason the GLUT4 gene is often used as a marker of muscle differentiation (19,24), and the results of the present study would support the notion that the increase in the expression of GLUT4 throughout the process of muscle cell differentiation represents an evolutionary conserved feature of skeletal muscle in vertebrates.…”

Engagement of the insulin-sensitive pathway in the stimulation of glucose transport by α-lipoic acid in 3T3-L1 adipocytes

“…One of the downstream effectors of PI3K is the protein serine/threonine kinase Akt/protein kinase B (PKB). Activation of Akt causes GLUT4 translocation in L6 skeletal muscle cells [13,14], 3T3-L1 adipocytes [15] and isolated rat adipocytes [16]. A role for Akt in the stimulation of glucose transport by insulin in L6 muscle cells has now been confirmed using a dominant negative mutant construct [17].…”