GLUT4 translocation: the last 200 nanometers

Sildenafil promotes adipogenesis through a PKG pathway

Sildenafil is the first oral PDE5 inhibitor for the treatment of erectile dysfunction and pulmonary arterial hypertension. In the present study, we investigated the effect of sildenafil on adipogenesis in 3T3L1 preadipocytes. Treatment with sildenafil for 8 days significantly promoted adipogenesis characterized by increased lipid droplet and triglyceride content in 3T3L1 cells. Meanwhile, sildenafil induced a pronounced up-regulation of the expression of adipocyte-specific genes, such as aP2 and GLUT4. The results by RT-PCR and Western blotting further showed that sildenafil increased the sequential expression of C/EBP beta, PPAR gamma and C/EBP alpha. Additionally, we found that the other two PDE5 inhibitors (vardenafil and tadalafil) and the cGMP analog 8-pCPT-cGMP also increased adipogenesis. Likewise, 8-pCPT-cGMP could up-regulate the expression of adipogenic and adipocyte-specific genes. Importantly, the PKG inhibitor Rp-8-pCPT-cGMP was able to inhibit both sildenafil and 8-pCPT-cGMP-induced adipogenesis. Furthermore, sildenafil promoted basal and insulin-mediated glucose uptake in 3T3L1 cells, which was counteracted by Rp-8-pCPT-cGMP. These results indicate that sildenafil could promote adipogenesis accompanied by increased glucose uptake through a PKG pathway at least partly.

GLUT4 is sorted to vesicles whose accumulation beneath and insertion into the plasma membrane are differentially regulated by insulin and selectively affected by insulin resistance

GLUT4 is sorted to vesicles whose accumulation beneath and insertion into the plasma membrane are differentially regulated by insulin and selectively affected by insulin resistance.

Insulin stimulates glucose transport by recruiting the GLUT4 glucose transporter to the plasma membrane. Here we use total internal reflection fluorescence microscopy to show that two trafficking motifs of GLUT4, a FQQI motif and a TELE-based motif, target GLUT4 to specialized vesicles that accumulate adjacent to the plasma membrane of unstimulated adipocytes. Mutations of these motifs redistributed GLUT4 to transferrin-containing recycling vesicles adjacent to the plasma membrane, and the degree of redistribution correlated with the increases of the GLUT4 mutants in the plasma membrane of basal adipocytes. These results establish that GLUT4 defaults to recycling endosomes when trafficking to specialized vesicles is disrupted, supporting the hypothesis that the specialized vesicles are derived from an endosomal compartment. Insulin stimulates both the accumulation of GLUT4 in the evanescent field and the fraction of this GLUT4 that is inserted into the plasma membrane. Unexpectedly, these two steps are differentially affected by the development of insulin resistance. We ascribe this selective insulin resistance to inherent differences in the sensitivities of GLUT4 vesicle accumulation and insertion into the plasma membrane to insulin. Differences in insulin sensitivities of various processes may be a general mechanism for the development of the physiologically important phenomenon of selective insulin resistance.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

Nuclear factor-kappaB decoy oligodeoxynucleotides ameliorate impaired glucose tolerance and insulin resistance in mice with cecal ligation and puncture-induced sepsis

OBJECTIVE

Insulin-resistant hyperglycemia is commonly observed in septic patients and may actually lead to some of adverse outcomes. We examined the changes in insulin signaling and glucose uptake regulation in sepsis and the involvement of the nuclear factor-kappaB pathway.

DESIGN

Controlled animal study.

SETTING

University research laboratory.

SUBJECTS

One hundred fifty-four BALB/c mice (8-12 wks of age).

INTERVENTIONS

The following four experimental groups were studied: sham-operated control, cecal ligation and puncture-induced sepsis, sepsis + nuclear factor-kappaB decoy oligodeoxynucleotide treatment, and sepsis + scrambled decoy oligodeoxynucleotide treatment.

MEASUREMENTS AND MAIN RESULTS

Septic mice were markedly hyperinsulinemic with apparently normal blood glucose levels in the fasted state, suggesting they are insulin-resistant. In fact, glucose clearance in response to insulin was markedly impaired in septic mice. They had impaired GLUT4 membrane translocation resulting from impaired insulin signaling as indicated by the decreased amount of insulin receptor substrate protein and the reduced activation of phosphatidylinositol 3-kinase and Akt. Interestingly, injection of nuclear factor-kappaB decoy oligodeoxynucleotide into the skeletal muscle dramatically improved all of the changes, including glucose clearance and insulin signaling. We also found that the Cbl-associated protein to TC10 pathway, another pathway regulating GLUT4 translocation, was up-regulated in septic mice in a nuclear factor-kappaB-dependent manner. This pathway may be one of the compensatory mechanisms to translocate GLUT4 because silencing of the individual components of the pathway with small interfering RNAs further reduced GLUT4 translocation in muscles of septic mice.

CONCLUSIONS

In sepsis, skeletal muscle GLUT4 translocation is impaired as a result of the reduced phosphatidylinositol 3-kinase/Akt pathway associated with insulin receptor substrate down-regulation through nuclear factor-kappaB activation.

Sac3 is an insulin-regulated phosphatidylinositol 3,5-bisphosphate phosphatase: gain in insulin responsiveness through Sac3 down-regulation in adipocytes

Insulin-regulated stimulation of glucose entry and mobilization of fat/muscle-specific glucose transporter GLUT4 onto the cell surface require the phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) pathway for optimal performance. The reduced insulin responsiveness observed under ablation of the PtdIns(3,5)P(2)-synthesizing PIKfyve and its associated activator ArPIKfyve in 3T3L1 adipocytes suggests that dysfunction of the PtdIns(3,5)P(2)-specific phosphatase Sac3 may yield the opposite effect. Paradoxically, as uncovered recently, in addition to turnover Sac3 also supports PtdIns(3,5)P(2) biosynthesis by allowing optimal PIKfyve-ArPIKfyve association. These opposing inputs raise the key question as to whether reduced Sac3 protein levels and/or hydrolyzing activity will produce gain in insulin responsiveness. Here we report that small interfering RNA-mediated knockdown of endogenous Sac3 by approximately 60%, which resulted in a slight but significant elevation of PtdIns(3,5)P(2) in 3T3L1 adipocytes, increased GLUT4 translocation and glucose entry in response to insulin. In contrast, ectopic expression of Sac3(WT), but not phosphatase-deficient Sac3(D488A), reduced GLUT4 surface abundance in the presence of insulin. Endogenous Sac3 physically assembled with PIKfyve and ArPIKfyve in both membrane and soluble fractions of 3T3L1 adipocytes, but this remained insulin-insensitive. Importantly, acute insulin markedly reduced the in vitro C8-PtdIns(3,5)P(2) hydrolyzing activity of Sac3. The insulin-sensitive Sac3 pool likely controls a discrete PtdIns(3,5)P(2) subfraction as the high pressure liquid chromatography-measurable insulin-dependent elevation in total [(3)H]inositol-PtdIns(3,5)P(2) was minor. Together, our data identify Sac3 as an insulin-sensitive phosphatase whose down-regulation increases insulin responsiveness, thus implicating Sac3 as a novel drug target in insulin resistance.

New insights into myocardial glucose metabolism: surviving under stress

PURPOSE OF REVIEW

Myocardial glucose uptake and metabolism are essential for maintaining myocardial energetics under circumstances of stress, such as myocardial ischemia or hypertrophy. We will discuss new information as to the mechanisms of altered glucose uptake as a consequence of impaired insulin action as well as emerging alternative cellular signaling mechanisms that lead to increased noninsulin dependent glucose uptake and metabolism. We will also review provocative clinical data that demonstrate the limitations of tight glycemic control as a mechanism to confer myocardial protection in patients with type 2 diabetes and discuss potential mechanistic explanations for the paradoxical results.

RECENT FINDINGS

New studies have shed important light on the distal mechanisms involved in insulin-mediated Glut 4 translocation. There are also important new insights into the role of AMP kinase and hypoxia-induced factor-1alpha in mediating myocardial glucose uptake while conferring cardioprotection.

SUMMARY

The importance of understanding the link between glucose uptake and cardioprotection is underscored by recent clinical trials that have failed to demonstrate a benefit between tight glycemic control and reduced cardiovascular events. These data underscore the need for additional agents that affect both outcomes.

Increased AS160 phosphorylation, but not TBC1D1 phosphorylation, with increased postexercise insulin sensitivity in rat skeletal muscle

A single exercise bout can increase insulin-independent glucose transport immediately postexercise and insulin-dependent glucose transport (GT) for several hours postexercise. Akt substrate of 160 kDa (AS160) and TBC1D1 are paralog Rab GTPase-activating proteins that have been proposed to contribute to these exercise effects. Previous research demonstrated greater AS160 and Akt threonine phosphorylation in rat skeletal muscle at 3-4 h postexercise concomitant with enhanced insulin-stimulated GT. To further probe whether these signaling events or TBC1D1 phosphorylation were important for the enhanced postexercise insulin-stimulated GT, male Wistar rats were studied using four experimental protocols (2-h swim exercise, differing with regard to timing of muscle sampling and whether food was provided postexercise) that were known to vary in their influence of insulin-independent and insulin-dependent GT postexercise. The results indicated that, in isolated rat epitrochlearis muscle, 1) elevated phosphorylation of AS160 (measured using anti-phospho-Akt substrate, PAS-AS160, and phosphospecific anti-Thr(642)-AS160, pThr(642)-AS160) consistently tracked with elevated insulin-stimulated GT; 2) PAS-TBC1D1 was not different from sedentary values at 3 or 27 h postexercise, when insulin sensitivity was increased; 3) insulin-stimulated Akt activity was not increased postexercise in muscles with increased insulin sensitivity; 4) PAS-TBC1D1 was increased immediately postexercise, when insulin-independent GT was elevated, and reversed at 3 and 27 h postexercise, when insulin-independent GT was also reversed; and 5) there was no significant effect of exercise or insulin on total abundance of AS160, TBC1D1, Akt, or GLUT4 protein with any of the protocols. The results are consistent with increased AS160 phosphorylation (PAS-AS160 or pThr(642)-AS160) but not increased PAS-TBC1D1 or Akt activity, which is important for increased postexercise insulin-stimulated GT in rat skeletal muscle. They also support the idea that increased TBC1D1 phosphorylation may play a role in the insulin-independent increase in GT postexercise.

How insulin regulates glucose transport in adipocytes

Insulin stimulates glucose storage and metabolism by the tissues of the body, predominantly liver, muscle and fat. Storage in muscle and fat is controlled to a large extent by the rate of facilitative glucose transport across the plasma membrane of the muscle and fat cells. Insulin controls this transport. Exactly how remains debated. Work presented in this review focuses on the pathways responsible for the regulation of glucose transport by insulin. We present some historical work to show how the prevailing model for regulation of glucose transport by insulin was originally developed, then some more recent data challenging this model. We finish describing a unifying model for the control of glucose transport, and some very recent data illustrating potential molecular machinery underlying this regulation. This review is meant to give an overview of our current understanding of the regulation of glucose transport through the regulation of the trafficking of Glut4, highlighting important questions that remain to be answered. A more detailed treatment of specific aspects of this pathway can be found in several excellent recent reviews (Brozinick et al., 2007 Hou and Pessin, 2007; Huang and Czech, 2007;Larance et al., 2008 Sakamoto and Holman, 2008; Watson and Pessin, 2007; Zaid et al., 2008)One of the main objectives of this review is to discuss the results of the experiments measuring the kinetics of Glut4 movement between subcellular compartments in the context of our emerging model of the Glut4 trafficking pathway.

Development of a novel GLUT4 translocation assay for identifying potential novel therapeutic targets for insulin sensitization

GLUT4 (glucose transporter 4) plays important roles in glucose homoeostasis in vivo. GLUT4 expression and function are diminished in diabetic human and animal subjects. The goal of the present study is to develop a cell-based assay for identifying negative regulators of GLUT4 translocation as potential targets for the treatment of Type 2 diabetes. Traditional GLUT4 translocation assays performed in differentiated myocytes or adipocytes are difficult to perform, particularly in HTS (high-throughput screening) mode. In the present study, we stably co-expressed c-Myc and eGFP [enhanced GFP (green fluorescent protein)] dual-tagged recombinant GLUT4 with recombinant IRS1 (insulin receptor substrate 1) in HEK-293 cells (human embryonic kidney cells) (HEK-293.IRS1.GLUT4 cells). Insulin treatment stimulated both glucose uptake and GLUT4 translocation in these cells. GLUT4 translocation is quantified by a TRF (time-resolved fluorescence) assay in a 96-well HTS format. TRF assays confirmed insulin-stimulated GLUT4 translocation, which can be inhibited by PI3K (phosphoinositide 3-kinase) or Akt [also called PKB (protein kinase B)] inhibitors. Treatment with palmitate increased IRS1 serine phosphorylation and reduced insulin-stimulated Akt phosphorylation and GLUT4 translocation, indicating insulin resistance. Knockdown of PTEN (phosphatase and tensin homologue deleted on chromosome 10) and PTP1B (protein tyrosine phosphatase 1B) gene expression by siRNA (small interfering RNA) treatment significantly increased GLUT4 translocation only in cells treated with palmitate but not in untreated cells. Similar results were obtained on treatment with siRNA of JNK1 (c-Jun N-terminal kinase 1), S6K1 (ribosomal protein S6 kinase, 70 kDa, polypeptide 1) and PKCθ (protein kinase C θ). In summary, we have established and validated a novel GLUT4 translocation assay that is optimal for identifying negative regulators of GLUT4 translocation. In combination with more physiologically relevant secondary assays in myotubes and adipocytes, this assay system can be used to identify potential novel therapeutic targets for the treatment of Type 2 diabetes.

The measurement of GLUT4 translocation in 3T3-L1 adipocytes

Type 2 diabetes (T2D) is one of the fastest growing threats to human health in westernised and developing countries and is associated with central obesity, atherosclerosis, dyslipidaemia, hyperinsulinaemia and hypertension. Insulin resistance, defined as a diminished response to ordinary levels of circulating insulin in one or more peripheral tissues, is an integral feature of T2D pathophysiology. This includes an impairment of insulin to inhibit hepatic glucose output and to stimulate glucose disposal into muscle and fat. While insulin is responsible for a number of specific biological responses, stimulation of glucose transport is critical for the maintenance of glucose homeostasis. The primary mechanism for insulin stimulation of glucose uptake into muscle and fat is the translocation of glucose transporter 4 (GLUT4) to the cell surface from intracellular storage vesicles within the cell. A major advantage in focussing on insulin regulation of glucose transport is that this represents the endpoint of multiple upstream signalling pathways. This chapter describes the measurement of GLUT4 translocation in cultured cells and its potential application for both mechanistic and therapeutic studies.

Ready, set, internalize: mechanisms and regulation of GLUT4 endocytosis

The facilitative glucose transporter GLUT4, a recycling membrane protein, is required for dietary glucose uptake into muscle and fat cells. GLUT4 is also responsible for the increased glucose uptake by myofibres during muscle contraction. Defects in GLUT4 membrane traffic contribute to loss of insulin-stimulated glucose uptake in insulin resistance and Type 2 diabetes. Numerous studies have analysed the intracellular membrane compartments occupied by GLUT4 and the mechanisms by which insulin regulates GLUT4 exocytosis. However, until recently, GLUT4 internalization was less well understood. In the present paper, we review: (i) evidence supporting the co-existence of clathrin-dependent and independent GLUT4 internalization in adipocytes and muscle cells; (ii) the contrasting regulation of GLUT4 internalization by insulin in these cells; and (iii) evidence suggesting regulation of GLUT4 endocytosis in muscle cells by signals associated with muscle contraction.

Munc18c is not rate-limiting for glucose and long-chain fatty acid uptake in the heart

The role of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)- and SNARE-associated proteins have not yet been assessed in regulation of cardiac glucose uptake, nor in the regulation of long-chain fatty acid (LCFA) uptake in any tissue. Munc18c is a SNARE-associated protein that regulates GLUT4 translocation in skeletal muscle and adipose tissue. Using cardiomyocytes from Munc18c(-/+) mice (with 56% reduction of Munc18c protein expression), we investigated whether this syntaxin4-associated protein is involved in regulation of cardiac substrate uptake. Basal, insulin- and oligomycin (a 5' AMP-activated protein kinase-activating agent)-stimulated glucose and LCFA uptake were not altered significantly in Munc18c(-/+) cardiomyocytes compared to wild-type cells. We conclude, therefore, that Munc18c is not rate-limiting for cardiac substrate uptake, neither under basal conditions nor when maximally stimulated metabolically.

Triglyceride containing lipid droplets and lipid droplet-associated proteins

PURPOSE OF REVIEW

Cytosolic lipid droplets are now recognized as dynamic organelles. This review summarizes our current understanding of the mechanisms involved in the formation of lipid droplets, the importance of lipid droplet-associated proteins and the link between lipid droplet accumulation and development of insulin resistance.

RECENT FINDINGS

Lipid droplets are formed as primordial droplets and they increase in size by fusion. This fusion process requires the alpha-soluble N-ethylmaleimide-sensitive factor adaptor protein receptor SNAP23, which is also involved in the insulin-dependent translocation of a glucose transporter to the plasma membrane. Recent data suggest that SNAP23 is the link between increased lipid droplet accumulation and development of insulin resistance. Lipid droplets also form tight interactions with other organelles. Furthermore, additional lipid droplet-associated proteins have been identified and shown to play a role in droplet assembly and turnover, and in sorting and trafficking events.

SUMMARY

Recent studies have identified a number of key proteins that are involved in the formation and turnover of lipid droplets, and SNAP23 has been identified as a link between accumulation of lipid droplets and development of insulin resistance. Further understanding of lipid droplet biology could indicate potential therapeutic targets to prevent accumulation of lipid droplets and associated complications.

Design, synthesis, and structure-activity relationships of novel insulin receptor tyrosine kinase activators

A novel series of symmetrical ureas of [(7-amino(2-naphthyl))sulfonyl]phenylamines were designed, synthesized, and tested for their ability to increase glucose transport in mouse 3T3-L1 adipocytes, a surrogate readout for activation of the insulin receptor (IR) tyrosine kinase (IRTK). A structure-activity relationship was established that indicated glucose transport activity was dependent on the presence of two acidic functionalities, two sulfonamide linkages, and a central urea or 2-imidazolidinone core. Compound 30 was identified as a potent and selective IRTK activator. At low concentrations, 30 was able to increase the tyrosine phosphorylation of the IR stimulated by submaximal insulin. At higher concentrations, 30 was able to increase tyrosine the phosphorylation levels of the IR in the absence of insulin. When administered intraperitoneally (ip) and orally (po), 30 improved glucose tolerance in hypoinsulinemic, streptozotocin-treated rats. These data provide pharmacological validation that small molecule IRTK activators represent a potential new class of antidiabetic agents.

Phosphoinositides in insulin action on GLUT4 dynamics: not just PtdIns(3,4,5)P3

Accumulated evidence over the last several years indicates that insulin regulates multiple steps in the overall translocation of GLUT4 vesicles to the fat/muscle cell surface, including formation of an intracellular storage pool of GLUT4 vesicles, its movement to the proximity of the cell surface, and the subsequent docking/fusion with the plasma membrane. Insulin-stimulated formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3); and in some cases, of its catabolite PtdIns(3,4)P(2)] plays a pivotal role in this process. PtdIns(3,4,5)P(3) is synthesized by the activated wortmannin-sensitive class IA phosphoinositide (PI) 3-kinase and controls the rate-limiting cell surface terminal stages of the GLUT4 journey. However, recent research is consistent with the conclusion that signals by each of the remaining five PIs, i.e., PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(4,5)P(2), may act in concert with that of PtdIns(3,4,5)P(3) in integrating the insulin receptor-issued signals with GLUT4 surface translocation and glucose transport activation. This review summarizes the experimental evidence supporting the complementary function of these PIs in insulin responsiveness of fat and muscle cells, with particular reference to mechanistic insights and functional significance in the regulation of overall GLUT4 vesicle dynamics.

Lipid droplets as dynamic organelles connecting storage and efflux of lipids

Neutral lipids are stored in the cytosol in so-called lipid droplets. These are dynamic organelles with neutral lipids as the core surrounded by a monolayer of amphipathic lipids (phospholipids and cholesterol) and specific proteins (PAT proteins and proteins involved in the turnover of lipids and in the formation and trafficking of the droplets). Lipid droplets are formed at microsomal membranes as primordial droplets with a diameter of 0.1-0.4 microm and increase in size by fusion. In this article, we review the assembly and fusion of lipid droplets, and the processes involved in the secretion of triglycerides. Triglycerides are secreted from cells by two principally different processes. In the mammary gland, lipid droplets interact with specific regions of the plasma membrane and bud off with an envelope consisting of the membrane, to form milk globules. In the liver and intestine, very low-density lipoproteins (VLDL) and chylomicrons are secreted by using the secretory pathway of the cell. Finally, we briefly review the importance of lipid droplets in the development of insulin resistance and atherosclerosis.

GLUT4 vesicle recruitment and fusion are differentially regulated by Rac, AS160, and Rab8A in muscle cells

Insulin increases glucose uptake into muscle by enhancing the surface recycling of GLUT4 transporters. In myoblasts, insulin signals bifurcate downstream of phosphatidylinositol 3-kinase into separate Akt and Rac/actin arms. Akt-mediated Rab-GAP AS160 phosphorylation and Rac/actin are required for net insulin gain of GLUT4, but the specific steps (vesicle recruitment, docking or fusion) regulated by Rac, actin dynamics, and AS160 target Rab8A are unknown. In L6 myoblasts expressing GLUT4myc, blocking vesicle fusion by tetanus toxin cleavage of VAMP2 impeded GLUT4myc membrane insertion without diminishing its build-up at the cell periphery. Conversely, actin disruption by dominant negative Rac or Latrunculin B abolished insulin-induced surface and submembrane GLUT4myc accumulation. Expression of non-phosphorylatable AS160 (AS160-4P) abrogated membrane insertion of GLUT4myc and partially reduced its cortical build-up, an effect magnified by selective Rab8A knockdown. We propose that insulin-induced actin dynamics participates in GLUT4myc vesicle retention beneath the membrane, whereas AS160 phosphorylation is essential for GLUT4myc vesicle-membrane docking/fusion and also contributes to GLUT4myc cortical availability through Rab8A.

Alpha-actinin-4 is selectively required for insulin-induced GLUT4 translocation

Insulin induces GLUT4 translocation to the muscle cell surface. Using differential amino acid labeling and mass spectrometry, we observed insulin-dependent co-precipitation of actinin-4 (ACTN4) with GLUT4 (Foster, L. J., Rudich, A., Talior, I., Patel, N., Huang, X., Furtado, L. M., Bilan, P. J., Mann, M., and Klip, A. (2006) J. Proteome Res. 5, 64-75). ACTN4 links F-actin to membrane proteins, and actin dynamics are essential for GLUT4 translocation. We hypothesized that ACTN4 may contribute to insulin-regulated GLUT4 traffic. In L6 muscle cells insulin, but not platelet-derived growth factor, increased co-precipitation of ACTN4 with GLUT4. Small interfering RNA-mediated ACTN4 knockdown abolished the gain in surface-exposed GLUT4 elicited by insulin but not by platelet-derived growth factor, membrane depolarization, or mitochondrial uncoupling. In contrast, knockdown of alpha-actinin-1 (ACTN1) did not prevent GLUT4 translocation by insulin. GLUT4 colocalized with ACTN4 along the insulin-induced cortical actin mesh and ACTN4 knockdown prevented GLUT4-actin colocalization without impeding actin remodeling or Akt phosphorylation, maintaining GLUT4 in a tight perinuclear location. We propose that ACTN4 contributes to GLUT4 traffic, likely by tethering GLUT4 vesicles to the cortical actin cytoskeleton.

Impaired insulin-stimulated phosphorylation of Akt and AS160 in skeletal muscle of women with polycystic ovary syndrome is reversed by pioglitazone treatment

OBJECTIVE

Insulin resistance in skeletal muscle is a major risk factor for type 2 diabetes in women with polycystic ovary syndrome (PCOS). However, the molecular mechanisms underlying skeletal muscle insulin resistance and the insulin-sensitizing effect of thiazolidinediones in PCOS in vivo are less well characterized.

RESEARCH DESIGN AND METHODS

We determined molecular mediators of insulin signaling to glucose transport in skeletal muscle biopsies of 24 PCOS patients and 14 matched control subjects metabolically characterized by euglycemic-hyperinsulinemic clamps and indirect calorimetry, and we examined the effect of 16 weeks of treatment with pioglitazone in PCOS patients.

RESULTS

Impaired insulin-mediated total (R(d)) oxidative and nonoxidative glucose disposal (NOGD) was paralleled by reduced insulin-stimulated Akt phosphorylation at Ser473 and Thr308 and AS160 phosphorylation in muscle of PCOS patients. Akt phosphorylation at Ser473 and Thr308 correlated positively with R(d) and NOGD in the insulin-stimulated state. Serum free testosterone was inversely related to insulin-stimulated R(d) and NOGD in PCOS. Importantly, the pioglitazone-mediated improvement in insulin-stimulated glucose metabolism, which did not fully reach normal levels, was accompanied by normalization of insulin-mediated Akt phosphorylation at Ser473 and Thr308 and AS160 phosphorylation. AMPK activity and phosphorylation were similar in the two groups and did not respond to pioglitazone in PCOS patients.

CONCLUSIONS

Impaired insulin signaling through Akt and AS160 in part explains insulin resistance at the molecular level in skeletal muscle in PCOS, and the ability of pioglitazone to enhance insulin sensitivity involves improved signaling through Akt and AS160. Moreover, our data provide correlative evidence that hyperandrogenism in PCOS may contribute to insulin resistance.