Differential effects of phosphatidylinositol 3-kinase inhibition on intracellular signals regulating GLUT4 translocation and glucose transport

Resistin inhibits glucose uptake in L6 cells independently of changes in insulin signaling and GLUT4 translocation

Elevated levels of resistin have been proposed to cause insulin resistance and therefore may serve as a link between obesity and type 2 diabetes. However, its role in skeletal muscle metabolism is unknown. In this study, we examined the effect of resistin on insulin-stimulated glucose uptake and the upstream insulin-signaling components in L6 rat skeletal muscle cells that were either incubated with recombinant resistin or stably transfected with a vector containing the myc-tagged mouse resistin gene. Transfected clones expressed intracellular resistin, which was released in the medium. Incubation with recombinant resistin resulted in a dose-dependent inhibition of insulin-stimulated 2-deoxyglucose (2-DG) uptake. The inhibitory effect of resistin on insulin-stimulated 2-DG uptake was not the result of impaired GLUT4 translocation to the plasma membrane. Furthermore, resistin did not alter the insulin receptor (IR) content and its phosphorylation, nor did it affect insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation, its association with the p85 subunit of phosphatidylinositol (PI) 3-kinase, or IRS-1-associated PI 3-kinase enzymatic activity. Insulin-stimulated phosphorylation of Akt/protein kinase B-alpha, one of the downstream targets of PI 3-kinase and p38 MAPK phosphorylation, was also not affected by resistin. Expression of resistin also inhibited insulin-stimulated 2-DG uptake when compared with cells expressing the empty vector (L6Neo) without affecting GLUT4 translocation, GLUT1 content, and IRS-1/PI 3-kinase signaling. We conclude that resistin does not alter IR signaling but does affect insulin-stimulated glucose uptake, presumably by decreasing the intrinsic activity of cell surface glucose transporters.

Intracellular segregation of phosphatidylinositol-3,4,5-trisphosphate by insulin-dependent actin remodeling in L6 skeletal muscle cells

Insulin stimulates glucose uptake by recruiting glucose transporter 4 (GLUT4) from an intracellular pool to the cell surface through a mechanism that is dependent on phosphatidylinositol (PI) 3-kinase (PI3-K) and cortical actin remodeling. Here we test the hypothesis that insulin-dependent actin filament remodeling determines the location of insulin signaling molecules. It has been shown previously that insulin treatment of L6 myotubes leads to a rapid rearrangement of actin filaments into submembrane structures where the p85 regulatory subunit of PI3-K and organelles containing GLUT4, VAMP2, and the insulin-regulated aminopeptidase (IRAP) colocalize. We now report that insulin receptor substrate-1 and the p110alpha catalytic subunit of PI3-K (but not p110beta) also colocalize with the actin structures. Akt-1 was also found in the remodeled actin structures, unlike another PI3-K effector, atypical protein kinase C lambda. Transiently transfected green fluorescent protein (GFP)-tagged pleckstrin homology (PH) domains of general receptor for phosphoinositides-1 (GRP1) or Akt (ligands of phosphatidylinositol-3,4,5-trisphosphate [PI-3,4,5-P(3)]) migrated to the periphery of the live cells; in fixed cells, they were detected in the insulin-induced actin structures. These results suggest that PI-3,4,5-P(3) is generated on membranes located within the actin mesh. Actin remodeling and GLUT4 externalization were blocked in cells highly expressing GFP-PH-GRP1, suggesting that PI-3,4,5-P(3) is required for both phenomena. We propose that PI-3,4,5-P(3) leads to actin remodeling, which in turn segregates p85alpha and p110alpha, thus localizing PI-3,4,5-P(3) production on membranes trapped by the actin mesh. Insulin-stimulated actin remodeling may spatially coordinate the localized generation of PI-3,4,5-P(3) and recruitment of Akt, ultimately leading to GLUT4 insertion at the plasma membrane.

Vanadate and rapamycin synergistically enhance insulin-stimulated glucose uptake

Tyrosine dephosphorylation, serine phosphorylation, and proteasomal degradation of insulin receptor substrates (IRSs) are implicated in the negative regulation of insulin action. Here we show that simultaneous inhibition of IRS-1 tyrosine dephosphorylation and proteasomal degradation synergistically augments insulin-responsive glucose uptake. L6 skeletal muscle cells (L6 cells) were treated with inhibitors of protein-tyrosine phosphatases, proteasomal degradation, and mammalian target of rapamycin (mTOR), and the effects of insulin on glucose uptake, IRS-1 tyrosine phosphorylation, phosphatidylinositol (PI) 3-kinase activity, and IRS-1 mass were examined. Pretreatment of L6 cells with sodium orthovanadate (Na(3)VO(4)) plus the mTOR inhibitor rapamycin caused a 5-fold increase in insulin-responsive glucose uptake at 2 hours when compared to insulin alone. Evaluation of IRS-1 associated PI 3-kinase activity, IRS-1-associated p85 mass, and IRS-1 tyrosine phosphorylation showed that 2 hours after insulin addition they were reduced by 70% from maximal activity. Likewise, IRS-1 mass was reduced by 50%. When L6 cells were pretreated with Na(3)VO(4) plus the proteasome inhibitor MG-132 or the mTOR inhibitor rapamycin prior to insulin addition, IRS-1 mass loss as well as IRS-1/PI-3 kinase complex decay was blocked at 2 hours and PI 3-kinase activity was increased 2.5-fold and 4-fold, respectively, over insulin alone. Finally, treatment of L6 cells with subtherapeutic amounts of vanadyl sulfate and rapamycin induced a synergistic 3-fold increase in insulin-induced glucose uptake at 2 hours. These findings indicate that vanadium and rapamycin synergize to enhance glucose uptake by preventing IRS-1 mass loss and IRS-1/PI 3-kinase complex decay and may offer a new approach to enhance glucose transport in diabetes.

Stimulation of glucose uptake by chronic vanadate pretreatment in cardiomyocytes requires PI 3-kinase and p38 MAPK activation

Vanadate, an inhibitor of tyrosine phosphatases, has insulin-mimetic properties. It has been shown that acute vanadate administration enhances glucose uptake independently of phosphatidylinositol (PI) 3-kinase and p38 MAPK. However, therapeutic vanadate use requires chronic administration, and this could potentially involve a different signaling pathway(s). Thus, we examined the mechanisms by which chronic vanadate exposure (16 h) stimulates glucose uptake in primary cultures of adult cardiomyocytes. The effect of vanadate on the activation of insulin-signaling molecules was evaluated 60 min after its withdrawal and in the absence of insulin. We therefore evaluated the persistent effect of vanadate on the insulin-signaling cascade. Our results demonstrate that preincubation with low vanadate concentrations (25-75 microM) induces a dose-dependent increase in glucose uptake. The augmentation of this process was not due to alterations in GLUT1 or GLUT4 protein levels, transcription, or de novo protein synthesis. Chronic vanadate exposure was associated with activation of the insulin receptor, insulin receptor substrate-1 (IRS-1), PKB/Akt, and p38 MAPK. Furthermore, inhibition of PI 3-kinase or p38 MAPK by wortmannin and PD-169316, respectively, significantly inhibited vanadate-mediated glucose uptake in cardiomyocytes. Thus, over time, different (albeit overlapping) signaling cascades may be activated by vanadate.

Maturation of the regulation of GLUT4 activity by p38 MAPK during L6 cell myogenesis

Insulin stimulates glucose uptake in skeletal muscle cells and fat cells by promoting the rapid translocation of GLUT4 glucose transporters to the plasma membrane. Recent work from our laboratory supports the concept that insulin also stimulates the intrinsic activity of GLUT4 through a signaling pathway that includes p38 MAPK. Here we show that regulation of GLUT4 activity by insulin develops during maturation of skeletal muscle cells into myotubes in concert with the ability of insulin to stimulate p38 MAPK. In L6 myotubes expressing GLUT4 that carries an exofacial myc-epitope (L6-GLUT4myc), insulin-stimulated GLUT4myc translocation equals in magnitude the glucose uptake response. Inhibition of p38 MAPK with SB203580 reduces insulin-stimulated glucose uptake without affecting GLUT4myc translocation. In contrast, in myoblasts, the magnitude of insulin-stimulated glucose uptake is significantly lower than that of GLUT4myc translocation and is insensitive to SB203580. Activation of p38 MAPK by insulin is considerably higher in myotubes than in myoblasts, as is the activation of upstream kinases MKK3/MKK6. In contrast, the activation of all three Akt isoforms and GLUT4 translocation are similar in myoblasts and myotubes. Furthermore, GLUT4myc translocation and phosphorylation of regulatory sites on Akt in L6-GLUT4myc myotubes are equally sensitive to insulin, whereas glucose uptake and phosphorylation of regulatory sites on p38 MAPK show lower sensitivity to the hormone. These observations draw additional parallels between Akt and GLUT4 translocation and between p38 MAPK and GLUT4 activation. Regulation of GLUT4 activity by insulin develops upon muscle cell differentiation and correlates with p38 MAPK activation by insulin.

Insulin improves postischemic recovery of function through PI3K in isolated working rat heart

Insulin improves contractile function after ischemia, but does not increase glucose uptake in the isolated working rat heart. We tested the hypothesis that the positive inotropic effect of insulin is independent of the signaling pathway responsible for insulin-stimulated glucose uptake. We inhibited this pathway at the level of phosphatidyl inositol 3-kinase (PI3K) with wortmannin. Hearts were perfused for 70 min at physiological workload with Krebs-Henseleit buffer containing [2-(3)H] glucose (5 mM, 0.05 microCi/ml) and oleate (0.4 mM, 1% BSA) in the presence (WM, n = 5) or absence (control, n = 7) of wortmannin (WM, 3 micromol/L). After 20 min, hearts were subjected to 15 min of total global ischemia followed by 35 min of reperfusion. Insulin (1 mU/ml) was added at the beginning of reperfusion (WM + insulin n = 8, insulin n = 8). Cardiac power before ischemia was 8.1 +/- 0.7 mW. Recovery of contractile function after ischemia was significantly increased in the presence of insulin (73.5 +/- 8.9% vs. 38.5 +/- 6.7%, p < 0.01). The addition of wortmannin completely abolished the effect of insulin on recovery (32.6 +/- 6.4%). Glucose uptake was 1.84 +/- 0.32 micromol/min/g dry before ischemia and was slightly elevated during reperfusion (2.68 +/- 0.35 micromol/min/g dry, n.s.). Insulin did not affect postischemic glucose uptake. In the presence of wortmannin, glucose uptake was lowest during reperfusion (n.s.). The results suggest that PI3K is involved in the insulin-induced improvement in postischemic recovery of contractile function. This effect of insulin is independent of its effect on glucose uptake.

Activation of the glucose transporter GLUT4 by insulin

The transport of glucose into cells and tissues is a highly regulated process, mediated by a family of facilitative glucose transporters (GLUTs). Insulin-stimulated glucose uptake is primarily mediated by the transporter isoform GLUT4, which is predominantly expressed in mature skeletal muscle and fat tissues. Our recent work suggests that two separate pathways are initiated in response to insulin: (i) to recruit transporters to the cell surface from intracellular pools and (ii) to increase the intrinsic activity of the transporters. These pathways are differentially inhibited by wortmannin, demonstrating that the two pathways do not operate in series. Conversely, inhibitors of p38 mitogen-activated protein kinase (MAPK) imply that p38 MAPK is involved only in the regulation of the pathway leading to the insulin-stimulated activation of GLUT4. This review discusses the evidence for the divergence of GLUT4 translocation and activity and proposed mechanisms for the regulation of GLUT4.

Insulin modulates PC-1 processing and recruitment in cultured human cells

We evaluated whether insulin signaling modulates plasma cell glycoprotein (PC-1) plasma membrane recruitment, posttranslational processing, and gene expression in human cultured cell lines. Insulin induced a fourfold increase (P < 0.01) of membrane PC-1 expression by rapid and sensitive mechanism(s). This effect was reduced (P < 0.05-0.01) by inhibition of phosphatidylinositol 3-kinase (200 nmol/l wortmannin) and S6 kinase (50 nmol/l rapamycin) activities and intracellular trafficking (50 micromol/l monensin) and was not accompanied by PC-1 gene expression changes. Moreover, at Western blot, insulin elicited the appearance, in both plasma membrane and cytosol, of a PC-1-related 146-kDa band (in addition to bands of 163, 117, 106, and 97 kDa observed also in absence of insulin) that was sensitive to endoglycosidase H. Finally, inhibition of PC-1 translocation to plasma membrane, by wortmannin pretreatment, increases insulin-stimulated receptor autophosphorylation. Our data indicate that insulin stimulates PC-1 posttranslational processing and translocation to the plasma membrane, which in turn impairs insulin receptor signaling. Bidirectional cross talk between insulin and PC-1, therefore, takes place, which may be part of the hormone self-desensitization mechanism.

Methylglyoxal induces apoptosis through activation of p38 mitogen-activated protein kinase in rat mesangial cells

BACKGROUND

The formation of methylglyoxal (MG), a highly reactive dicarbonyl compound, is accelerated through several pathways, including the glycation reaction under diabetic conditions, presumably contributing to tissue injury in diabetes. On the other hand, apoptotic cell death of glomerular cells has been suggested to play a role in the development of glomerulosclerosis in various types of glomerular injuries. We therefore examined whether MG was capable of inducing apoptosis in rat mesangial cells to address the possible mechanism by which hyperglycemia-related products accelerated pathologic changes in diabetic glomerulosclerosis.

METHODS

Rat mesangial cells were incubated with 0 to 400 micromol/L MG, followed by the detection of apoptosis by both TUNEL method and electrophoretic analysis for DNA fragmentation. In addition, we investigated intracellular mechanisms mediating MG-induced apoptosis, focusing especially on the p38 mitogen-activated protein kinase (MAPK) pathway.

RESULTS

MG induced apoptosis in rat mesangial cells in a dose-dependent manner and was accompanied by the activation of p38alpha isoform. Aminoguanidine and N-acetyl-l-cysteine inhibited the MG-induced p38 MAPK activation, as well as apoptosis in rat mesangial cells, suggesting the involvement of oxidative stress in these phenomena. SB203580, a specific inhibitor of p38 MAPK also suppressed the MG-induced apoptosis in rat mesangial cells.

CONCLUSIONS

These results suggest a potential role for MG in glomerular injury through p38 MAPK activation under diabetic conditions and may serve as a novel insight into the therapeutic strategies for diabetic nephropathy.

A long search for Glut4 activation

Insulin stimulates glucose transport in its target cells by translocation of the glucose transporter isoform 4 (Glut4) from an intracellular storage pool to the plasma membrane. A large body of evidence indicates that activity of Glut4 at the plasma membrane may vary. Recent findings suggest that p38 MAPK may be involved in regulation of the intrinsic activity of the transporter.

A dominant-negative p38 MAPK mutant and novel selective inhibitors of p38 MAPK reduce insulin-stimulated glucose uptake in 3T3-L1 adipocytes without affecting GLUT4 translocation

Participation of p38 mitogen-activated protein kinase (p38) in insulin-induced glucose uptake was suggested using pyridinylimidazole p38 inhibitors (e.g. SB203580). However, the role of p38 in insulin action remains controversial. We further test p38 participation in glucose uptake using a dominant-negative p38 mutant and two novel pharmacological p38 inhibitors related to but different from SB203580. We present the structures and activities of the azaazulene pharmacophores A291077 and A304000. p38 kinase activity was inhibited in vitro by A291077 and A304000 (IC(50) = 0.6 and 4.7 microm). At higher concentrations A291077 but not A304000 inhibited JNK2alpha (IC(50) = 3.5 microm). Pretreatment of 3T3-L1 adipocytes and L6 myotubes expressing GLUT4myc (L6-GLUT4myc myotubes) with A291077, A304000, SB202190, or SB203580 reduced insulin-stimulated glucose uptake by 50-60%, whereas chemical analogues inert toward p38 were ineffective. Expression of an inducible, dominant-negative p38 mutant in 3T3-L1 adipocytes reduced insulin-stimulated glucose uptake. GLUT4 translocation to the cell surface, immunodetected on plasma membrane lawns of 3T3-L1 adipocytes or on intact L6-GLUT4myc myotubes, was not altered by chemical or molecular inhibition of p38. We propose that p38 contributes to enhancing GLUT4 activity, thereby increasing glucose uptake. In addition, the azaazulene class of inhibitors described will be useful to decipher cellular actions of p38 and JNK.

Insulin signal transduction pathways and insulin-induced gene expression

Insulin regulates metabolic activity, gene transcription, and cell growth by modulating the activity of several intracellular signaling pathways. Insulin activation of one mitogen-activated protein kinase cascade, the MEK/ERK kinase cascade, is well described. However, the effect of insulin on the parallel p38 pathway is less well understood. The present work examines the effect of inhibiting the p38 signaling pathway by use of specific inhibitors, either alone or in combination with insulin, on the activation of ERK1/2 and on the regulation of gene transcription in rat hepatoma cells. Activation of ERK1/2 was induced by insulin and was dependent on the activation of MEK1, the kinase upstream of ERK in this pathway. Treatment of cells with p38 inhibitors also induced ERK1/2 activation/phosphorylation. The addition of p38 inhibitors followed by insulin addition resulted in a greater than additive activation of ERK1/2. The two genes studied, c-Fos and Pip92, are immediate-early genes that are dependent on the ERK1/2 pathway for insulin-regulated induction because the insulin effect was inhibited by pretreatment with a MEK1 inhibitor. The addition of p38 inhibitors induced transcription of both genes in a dose-dependent manner, and insulin stimulation of both genes was enhanced by prior treatment with p38 inhibitors. The ability of the p38 inhibitors to induce ERK1/2 and gene transcription, both alone and in combination with insulin, was abolished by prior inhibition of MEK1. These data suggest possible cross-talk between the p38 and ERK1/2 signaling pathways and a potential role of p38 in insulin signaling.

Gliclazide increases insulin receptor tyrosine phosphorylation but not p38 phosphorylation in insulin-resistant skeletal muscle cells

Sulfonylurea drugs are used in the treatment of type 2 diabetes. The mechanism of action of sulfonylureas is to release insulin from pancreatic cells and they have been proposed to act on insulin-sensitive tissues to enhance glucose uptake. The goal of the present study was to test the hypothesis that gliclazide, a second-generation sulfonylurea, could enhance insulin signaling in insulin-resistant skeletal muscle cells. We demonstrated that gliclazide enhanced insulin-stimulated insulin receptor tyrosine phosphorylation in insulin-resistant skeletal muscle cells. Although insulin receptor substrate-1 tyrosine phosphorylation was unaffected by gliclazide treatment, phosphatidylinositol 3-kinase activity was partially restored by treatment with gliclazide. No increase in 2-deoxyglucose uptake in insulin-resistant cells by treatment with gliclazide was observed. Further investigations into the mitogen-activated protein kinase (MAPK) pathway revealed that insulin-stimulated p38 phosphorylation was impaired, as compared with extracellular-signal-regulated kinase (ERK) and c-Jun N-terminal kinase(JNK), which were phosphorylated normally in insulin-resistant cells. Treatment with gliclazide could not restore p38 phosphorylation in insulin-resistant cells. We propose that gliclazide can regulate part of the insulin signaling in insulin-resistant skeletal muscle, and p38 could be a potential therapeutic target for glucose uptake to treat insulin resistance.

Need for GLUT4 activation to reach maximum effect of insulin-mediated glucose uptake in brown adipocytes isolated from GLUT4myc-expressing mice

There is a need to understand whether the amount of GLUT4 at the cell surface determines the extent of glucose uptake in response to insulin. Thus, we created a heterozygous mouse expressing modest levels of myc-tagged GLUT4 (GLUT4myc) in insulin-sensitive tissues under the control of the human GLUT4 promoter. Insulin stimulated 2-deoxyglucose uptake 6.5-fold in isolated brown adipocytes. GLUT1 did not contribute to the insulin response. The stimulation by insulin was completely blocked by wortmannin and partly (55 +/- 2%) by the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580. Insulin increased surface exposure of GLUT4myc twofold (determined by fluorescent or enzyme-linked myc immunodetection in intact adipocytes). Such increase was completely blocked by wortmannin but insensitive to SB203580. Insulin increased the kinase activity of the p38 MAPK beta-isoform 1.9-fold without affecting p38-alpha. In summary, the GLUT4myc mouse is a promising model for measuring GLUT4 translocation in intact primary cells. It affords direct comparison between GLUT4 translocation and glucose uptake in similar cell preparations, allowing one to study the regulation of GLUT4 activity. Using this animal model, we found that stimulation of glucose uptake into brown adipocytes involves both GLUT4 translocation and activation.

Phosphatidylinositol-3 kinase inhibitors enhance the anti-leukemia effect of STI571

BCR/ABL fusion tyrosine kinase is responsible for the initiation and maintenance of the Philadelphia chromosome (Ph(1))-positive chronic myelogenous leukemia (CML) and a cohort of acute lymphocytic leukemias (ALL). STI571 (Gleevec), a novel anti-leukemia drug targeting BCR/ABL kinase can induce remissions of the Ph(1)-positive leukemias. STI571 was recently combined with the standard cytostatic drugs to achieve better therapeutic results and to overcome emerging drug resistance mechanisms. We decided to search for a more specific partner compound for STI571. Our previous studies showed that a signaling protein phosphatidylinositol-3 kinase (PI-3k) is essential for the growth of CML cells, but not of normal hematopoietic cells (Blood, 86:726,1995). Therefore the anti- Ph(1)-leukemia effect of the combination of BCR/ABL kinase inhibitor STI571 and PI-3k inhibitor wortmannin (WT) or LY294002 (LY) was tested. We showed that STI571+WT exerted a synergistic effect against the Ph(1)-positive cell lines, but did not affect the growth of Ph(1)-negative cell line. Moreover, the combinations of STI571+WT or STI571+LY were effective in the inhibition of clonogenic growth of CML-chronic phase and CML-blast crisis patient cells, while sparing normal bone marrow cells. Single colony RT-PCR assay showed that colonies arising from the mixture of CML cells and normal bone marrow cells after treatment with STI571+WT were selectively depleted of BCR/ABL-positive cells. Biochemical analysis of the CML cells after the treatment revealed that combination of STI571+WT caused a more pronounced activation of caspase-3 and induced massive apoptosis, in comparison to STI571 and WT alone. In conclusion, combination of STI571+WT or STI571+LY may represent a novel approach against the Ph(1)-positive leukemias.

Sustained exposure of L6 myotubes to high glucose and insulin decreases insulin-stimulated GLUT4 translocation but upregulates GLUT4 activity

Hyperglycemia and hyperinsulinemia are cardinal features of acquired insulin resistance. In adipose cell cultures, high glucose and insulin cause insulin resistance of glucose uptake, but because of altered GLUT4 expression and contribution of GLUT1 to glucose uptake, the basis of insulin resistance could not be ascertained. Here we show that GLUT4 determines glucose uptake in L6 myotubes stably overexpressing myc-tagged GLUT4. Preincubation for 24 h with high glucose and insulin (high Glc/Ins) reduced insulin-stimulated GLUT4 translocation by 50%, without affecting GLUT4 expression. Insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation, phosphatidylinositol 3-kinase activation, and Akt phosphorylation also diminished, as did insulin-mediated glucose uptake. However, basal glucose uptake rose by 40% without any gain in surface GLUT4. High Glc/Ins elevated basal p38 mitogen-activated protein kinase (MAPK) phosphorylation and activity, and a short inhibition of p38 MAPK with SB202190 corrected the rise in basal glucose uptake, suggesting that p38 MAPK activity contributes to this rise. We propose that in a cellular model of skeletal muscle, chronic exposure to high Glc/Ins reduced the acute, insulin-elicited GLUT4 translocation. In addition, basal state GLUT4 activity was augmented to partially compensate for the translocation defect, resulting in a more robust glucose uptake than what would be predicted from the amount of cell surface GLUT4 alone.

A method to identify serine kinase substrates. Akt phosphorylates a novel adipocyte protein with a Rab GTPase-activating protein (GAP) domain

This study describes a method for the identification of the substrates of specific serine kinases. An antibody specific for the phosphomotif generated by the kinase is used to isolate phosphorylated substrates by immunoprecipitation, and the isolated proteins are identified by tandem mass spectrometry of peptides. This method was applied to the identification of substrates for the protein kinase Akt, which specifically phosphorylates the RXRXXS/T motif. 3T3-L1 adipocytes were treated with insulin to activate Akt, and the putative Akt substrate proteins were isolated by immunoprecipitation with an antibody against the phospho form of this motif. This led to the identification of a novel 160-kDa substrate for Akt. The 160-kDa substrate for Akt, which was designated AS160, has a Rab GAP domain. Recombinant AS160 was shown to be a substrate for Akt, and two sites of phosphorylation, both in RXRXXS/T motifs, were identified by mass spectrometry and mutation. Insulin treatment of adipocytes caused AS160 to redistribute from the low density microsomes to the cytosol.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.