A Role for Protein Kinase Bβ/Akt2 in Insulin-Stimulated GLUT4 Translocation in Adipocytes

Abstract

Insulin stimulates glucose uptake into muscle and fat cells by promoting the translocation of glucose transporter 4 (GLUT4) to the cell surface. Phosphatidylinositide 3-kinase (PI3K) has been implicated in this process. However, the involvement of protein kinase B (PKB)/Akt, a downstream target of PI3K in regulation of GLUT4 translocation, has been controversial. Here we report that microinjection of a PKB substrate peptide or an antibody to PKB inhibited insulin-stimulated GLUT4 translocation to the plasma membrane by 66 or 56%, respectively. We further examined the activation of PKB isoforms following treatment of cells with insulin or platelet-derived growth factor (PDGF) and found that PKBbeta is preferentially expressed in both rat and 3T3-L1 adipocytes, whereas PKBalpha expression is down-regulated in 3T3-L1 adipocytes. A switch in growth factor response was also observed when 3T3-L1 fibroblasts were differentiated into adipocytes. While PDGF was more efficacious than insulin in stimulating PKB phosphorylation in fibroblasts, PDGF did not stimulate PKBbeta phosphorylation to any significant extent in adipocytes, as assessed by several methods. Moreover, insulin, but not PDGF, stimulated the translocation of PKBbeta to the plasma membrane and high-density microsome fractions of 3T3-L1 adipocytes. These results support a role for PKBbeta in insulin-stimulated glucose transport in adipocytes.

The role of Akt on Arsenic trioxide suppression of 3T3-L1 preadipocyte differentiation

“…This indicates that the repression effect of arsenic trioxide on the differentiation was depended on the activation of Akt/PKB. This is in agreement with previous report that Akt/PKB-α plays an important role in the regulation of preadipocyte growth and proliferation, and it is down-regulated upon differentiation [16]. Our results are also in agreement with wealth information that Akt/PKB functions to suppress apoptosis.…”

Impaired phosphorylation and insulin-stimulated translocation to the plasma membrane of protein kinase B/Akt in adipocytes from Type II diabetic subjects

“…Furthermore, similar to recent reports in rat fat cells and differentiated 3T3-L1 cells [19,21], PKBb/Akt 2 is highly expressed but PKBa/ Akt 1 is almost absent in human fat cells [23]. We have preliminary evidence that PKBg/Akt 3 is also expressed in human fat cells but its abundance and role are under current investigation.…”

“…Although PKB/Akt was expressed as a single band in PM, similar to recent results in differentiated 3T3-L1 adipocytes [19], at least two bands were found in cytosol and LDM; the subcellular fractions where insulin-stimulated threonine phosphorylation was predominant. The reason for this discrepancy is not clear but could relate to the constitutive phosphorylation of PKBb/Akt 2.…”

“…Similarly, introduction of dominant-negative and kinase-dead PKB/Akt constructs in L6 cells inhibited insulin-stimulated glucose transport [18]. Furthermore, microinjection of a PKB/Akt substrate peptide or an antibody to PKB/ Akt also inhibited the effect of insulin [19]. In contrast, another PKB/Akt dominant-negative mutant, where the two phosphorylation sites had been mutated, did not prevent insulin-stimulated GLUT-4 translocation although other inhibitory effects were found [20].…”

Nelfinavir-induced insulin resistance is associated with impaired plasma membrane recruitment of the PI 3-kinase effectors Akt/PKB and PKC-?

“…5d). Since the β isoform of PKB (or Akt2) was recently linked to insulin stimulation of GLUT4 translocation in adipocytes [38,39,40], we also assessed its recruitment to the plasma membrane in response to insulin using an isoform-specific antibody. In control cells, insulin stimulation resulted in a nearly six-fold increase in Akt2/PKB-β plasma membrane content (Fig.…”