Potential Role of Protein Kinase B in Glucose Transporter 4 Translocation in Adipocytes*

Abstract

Phosphatidylinositol 3-kinase (PI 3-kinase) activation promotes glucose transporter 4 (Glut 4) translocation in adipocytes. In this study, we demonstrate that protein kinase B, a serine/threonine kinase stimulated by PI 3-kinase, is activated by both insulin and okadaic acid in isolated adipocytes, in parallel with their effects on Glut 4 translocation. In 3T3-L1 adipocytes, platelet-derived growth factor activated PI 3-kinase as efficiently as insulin but was only half as potent as insulin in promoting protein kinase B (PKB) activation. To look for a potential role of PKB in Glut 4 translocation, adipocytes were transfected with a constitutively active PKB (Gag-PKB) together with an epitope tagged transporter (Glut 4 myc). Gag-PKB was associated with all membrane fractions, whereas the endogenous PKB was mostly cytosolic. Expression of Gag-PKB led to an increase in Glut 4 myc amount at the cell surface. Our results suggest that PKB could play a role in promoting Glut 4 appearance at the cell surface following exposure of adipocytes to insulin and okadaic acid stimulation.

Protein kinase B (c-Akt): a multifunctional mediator of phosphatidylinositol 3-kinase activation

“…Inhibitors of PI-3K have been reported to block the ability of insulin to stimulate glucose uptake [64][65][66], while overexpression of active forms of PI-3K at least partially mimic the effect of insulin [67][68][69][70]. Several groups have recently provided a possible link between activation of PKB and the stimulation of GLUT4 translocation [21,71,72]. As mentioned above, expression of constitutively active membrane-targeted variants of PKB in 3T3-L1 fibroblasts results in spontaneous differentiation into adipocytes [21,22].…”

“…This may be due to the activation of p70 S6-kinase (p70 S'K ), which has also been implicated in this process [24,73]. Subcellular fractionation of 3T3-L1 adipocytes reveals that GLUT4 is mostly located in the low-and highdensity microsomes [71]. The largest concentration of endogenous PKB is found in the cytosol, while gag-PKB and the p85 subunit of PI-3K are enriched in membrane fractions.…”

“…The largest concentration of endogenous PKB is found in the cytosol, while gag-PKB and the p85 subunit of PI-3K are enriched in membrane fractions. Furthermore, constitutively active PKB induces glucose uptake associated with increased GLUT1 expression and GLUT4 translocation to the plasma membrane [21,71]. The increased glucose influx is associated with lipogenesis.…”

GLUT4 vesicle dynamics in living 3T3 L1 adipocytes visualized with green-fluorescent protein

“…One of the potential targets for PtdIns 3-kinase activation is protein kinase B, and there is indirect evidence that this enzyme may also be involved in the translocation response. In particular, a constitutively active membrane-targeted form of protein kinase B promotes GLUT4 translocation in the absence of insulin, when over-expressed in rat adipocytes or 3T3 L1 cells [10,11].…”

Molecular mechanism of insulin resistance

“…Binding of insulin to the heterotetrameric membrane receptor results in insulin receptor substrate-1 (IRS-1) phosphorylation and IRS-1-associated phosphatidylinositol 3 phosphate kinase (PI3 kinase) activation (Dresner et al 1999;Yu et al 2002;Chavez et al 2003). This affects downstream effectors such as Akt/ PKB which activates the glucose transporter Glut4; Glut4 is then translocated to the membrane and imports glucose into the cell (Kohn et al 1996;Tanti et al 1997;Hill et al 1999). This suggests that the impairment of insulin activity leading to insulin resistance is linked to insulin signalling defects.…”