Immunoelectron microscopic evidence that GLUT4 translocation explains the stimulation of glucose transport in isolated rat white adipose cells

Insulin-Mimetic Activity of Herbal Extracts Identified with Large-Scale Total Internal Reflection Fluorescence Microscopy

Diabetes mellitus is a spreading global pandemic. Type 2 diabetes mellitus (T2DM) is the predominant form of diabetes, in which a reduction in blood glucose uptake is caused by impaired glucose transporter 4 (GLUT4) translocation to the plasma membrane in adipose and muscle cells. Antihyperglycemic drugs play a pivotal role in ameliorating diabetes symptoms but often are associated with side effects. Hence, novel antidiabetic compounds and nutraceutical candidates are urgently needed. Phytogenic therapy can support the prevention and amelioration of impaired glucose homeostasis. Using total internal reflection fluorescence microscopy (TIRFM), 772 plant extracts of an open-access plant extract library were screened for their GLUT4 translocation activation potential, resulting in 9% positive hits. Based on commercial interest and TIRFM assay-based GLUT4 translocation activation, some of these extracts were selected, and their blood glucose-reducing effects in ovo were investigated using a modified hen's egg test (Gluc-HET). To identify the active plant part, some of the available candidate plants were prepared in-house from blossoms, leaves, stems, or roots and tested. Acacia catechu (catechu), Pulmonaria officinalis (lungwort), Mentha spicata (spearmint), and Saponaria officinalis (common soapwort) revealed their potentials as antidiabetic nutraceuticals, with common soapwort containing GLUT4 translocation-activating saponarin.

GLUT4 dispersal at the plasma membrane of adipocytes: a super-resolved journey

In adipose tissue, insulin stimulates glucose uptake by mediating the translocation of GLUT4 from intracellular vesicles to the plasma membrane. In 2010, insulin was revealed to also have a fundamental impact on the spatial distribution of GLUT4 within the plasma membrane, with the existence of two GLUT4 populations at the plasma membrane being defined: (1) as stationary clusters and (2) as diffusible monomers. In this model, in the absence of insulin, plasma membrane-fused GLUT4 are found to behave as clusters. These clusters are thought to arise from exocytic events that retain GLUT4 at their fusion sites; this has been proposed to function as an intermediate hub between GLUT4 exocytosis and re-internalisation. By contrast, insulin stimulation induces the dispersal of GLUT4 clusters into monomers and favours a distinct type of GLUT4-vesicle fusion event, known as fusion-with-release exocytosis. Here, we review how super-resolution microscopy approaches have allowed investigation of the characteristics of plasma membrane-fused GLUT4 and further discuss regulatory step(s) involved in the GLUT4 dispersal machinery, introducing the scaffold protein EFR3 which facilitates localisation of phosphatidylinositol 4-kinase type IIIα (PI4KIIIα) to the cell surface. We consider how dispersal may be linked to the control of transporter activity, consider whether macro-organisation may be a widely used phenomenon to control proteins within the plasma membrane, and speculate on the origin of different forms of GLUT4-vesicle exocytosis.

A high-content endogenous GLUT4 trafficking assay reveals new aspects of adipocyte biology

Insulin-induced GLUT4 translocation to the plasma membrane in muscle and adipocytes is crucial for whole-body glucose homeostasis. Currently, GLUT4 trafficking assays rely on overexpression of tagged GLUT4. Here we describe a high-content imaging platform for studying endogenous GLUT4 translocation in intact adipocytes. This method enables high fidelity analysis of GLUT4 responses to specific perturbations, multiplexing of other trafficking proteins and other features including lipid droplet morphology. Using this multiplexed approach we showed that Vps45 and Rab14 are selective regulators of GLUT4, but Trarg1, Stx6, Stx16, Tbc1d4 and Rab10 knockdown affected both GLUT4 and TfR translocation. Thus, GLUT4 and TfR translocation machinery likely have some overlap upon insulin-stimulation. In addition, we identified Kif13A, a Rab10 binding molecular motor, as a novel regulator of GLUT4 traffic. Finally, comparison of endogenous to overexpressed GLUT4 highlights that the endogenous GLUT4 methodology has an enhanced sensitivity to genetic perturbations and emphasises the advantage of studying endogenous protein trafficking for drug discovery and genetic analysis of insulin action in relevant cell types.

Fluorescence Microscopy-Based Quantitation of GLUT4 Translocation

Postprandial insulin-stimulated glucose uptake into target tissue is crucial for the maintenance of normal blood glucose homeostasis. This step is rate-limited by the number of facilitative glucose transporters type 4 (GLUT4) present in the plasma membrane. Since insulin resistance and impaired GLUT4 translocation are associated with the development of metabolic disorders such as type 2 diabetes, this transporter has become an important target of antidiabetic drug research. The application of screening approaches that are based on the analysis of GLUT4 translocation to the plasma membrane to identify substances with insulinomimetic properties has gained global research interest in recent years. Here, we review methods that have been implemented to quantitate the translocation of GLUT4 to the plasma membrane. These methods can be broadly divided into two sections: microscopy-based technologies (e.g., immunoelectron, confocal or total internal reflection fluorescence microscopy) and biochemical and spectrometric approaches (e.g., membrane fractionation, photoaffinity labeling or flow cytometry). In this review, we discuss the most relevant approaches applied to GLUT4 thus far, highlighting the advantages and disadvantages of these approaches, and we provide a critical discussion and outlook into new methodological opportunities.

Positive Reinforcing Mechanisms between GPR120 and PPARγ Modulate Insulin Sensitivity

G protein-coupled receptor 120 (GPR120) and PPARγ agonists each have insulin sensitizing effects. But whether these two pathways functionally interact and can be leveraged together to markedly improve insulin resistance has not been explored. Here, we show that treatment with the PPARγ agonist rosiglitazone (Rosi) plus the GPR120 agonist Compound A leads to additive effects to improve glucose tolerance and insulin sensitivity, but at lower doses of Rosi, thus avoiding its known side effects. Mechanistically, we show that GPR120 is a PPARγ target gene in adipocytes, while GPR120 augments PPARγ activity by inducing the endogenous ligand 15d-PGJ2 and by blocking ERK-mediated inhibition of PPARγ. Further, we used macrophage- (MKO) or adipocyte-specific GPR120 KO (AKO) mice to show that GRP120 has anti-inflammatory effects via macrophages while working with PPARγ in adipocytes to increase insulin sensitivity. These results raise the prospect of a safer way to increase insulin sensitization in the clinic.

Insulin acutely increases glucose transporter 1 on plasma membranes and glucose uptake in an AKT-dependent manner in chicken adipocytes

Avian glucose transporters (GLUT) responsible for insulin-responsive glucose uptake into adipocytes remain poorly characterized. We aimed to identify the insulin-responsive GLUT using primary culture of chicken adipocytes. Acute stimulation with 1 μM insulin for 20 min increased 2-deoxyglucose uptake, AKT protein phosphorylation, and GLUT1 protein levels on the plasma membrane of the chicken adipocytes, whereas pretreatment with 10 μM triciribine, an AKT inhibitor, canceled these effects. Furthermore, the insulin stimulation did not affect GLUT12 protein levels on the plasma membrane of the chicken adipocytes. Our results suggest that GLUT1 is an insulin-responsive GLUT in chicken adipocytes.

Sterol structure dependence of insulin receptor and insulin-like growth factor 1 receptor activation

The plasma membrane is a dynamic environment with a complex composition of lipids, proteins, and cholesterol. Areas enriched in cholesterol and sphingolipids are believed to form lipid rafts, domains of highly ordered lipids. The unique physical properties of these domains have been proposed to influence many cellular processes. Here, we demonstrate that the activation of insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) depends critically on the structures of membrane sterols. IR and IGF1R autophosphorylation in vivo was inhibited by cholesterol depletion, and autophosphorylation was restored by the replacement with exogenous cholesterol. We next screened a variety of sterols for effects on IR activation. The ability of sterols to support IR autophosphorylation was strongly correlated to the propensity of the sterols to form ordered domains. IR autophosphorylation was fully restored by the incorporation of ergosterol, dihydrocholesterol, 7-dehydrocholesterol, lathosterol, desmosterol, and allocholesterol, partially restored by epicholesterol, and not restored by lanosterol, coprostanol, and 4-cholesten-3-one. These data support the hypothesis that the ability to form ordered domains is sufficient for a sterol to support ligand-induced activation of IR and IGF1R in intact mammalian cells.

A high-throughput chemical-genetics screen in murine adipocytes identifies insulin-regulatory pathways

Pathways linking activation of the insulin receptor to downstream targets of insulin have traditionally been studied using a candidate gene approach. To elucidate additional pathways regulating insulin activity, we performed a forward chemical-genetics screen based on translocation of a glucose transporter 4 (Glut4) reporter expressed in murine 3T3-L1 adipocytes. To identify compounds with known targets, we screened drug-repurposing and natural product libraries. We identified, confirmed, and validated 64 activators and 65 inhibitors that acutely increase or rapidly decrease cell-surface Glut4 in adipocytes stimulated with submaximal insulin concentrations. These agents were grouped by target, chemical class, and mechanism of action. All groups contained multiple hits from a single drug class, and several comprised multiple structurally unrelated hits for a single target. Targets include the β-adrenergic and adenosine receptors. Agonists of these receptors increased and inverse agonists/antagonists decreased cell-surface Glut4 independently of insulin. Additional activators include insulin sensitizers (thiazolidinediones), insulin mimetics, dis-inhibitors (the mTORC1 inhibitor rapamycin), cardiotonic steroids (the Na⁺/K⁺-ATPase inhibitor ouabain), and corticosteroids (dexamethasone). Inhibitors include heterocyclic amines (tricyclic antidepressants) and 21 natural product supplements and herbal extracts. Mechanisms of action include effects on Glut4 trafficking, signal transduction, inhibition of protein synthesis, and dissipation of proton gradients. Two pathways that acutely regulate Glut4 translocation were discovered: de novo protein synthesis and endocytic acidification. The mechanism of action of additional classes of activators (tanshinones, dalbergiones, and coumarins) and inhibitors (flavonoids and resveratrol) remains to be determined. These tools are among the most sensitive, responsive, and reproducible insulin-activity assays described to date.

EsGLUT4 and CHHBP are involved in the regulation of glucose homeostasis in the crustacean Eriocheir sinensis

Glucose is an essential energy source for both vertebrates and invertebrates. In mammals, glucose uptake is mediated primarily by glucose transporters (GLUTs), members of the major facilitator superfamily (MFS) of passive transporters. Among the GLUTs, GLUT4 is the main glucose transporter in muscles and adipocytes. In skeletal muscle cells, GLUT4 interacts with the lipid raft protein flotillin to transport glucose upon stimulation by insulin. Although several studies have examined GLUT4 function in mammals, few have been performed in crustaceans, which also use glucose as their main energy source. Crustacean hyperglycemic hormone (CHH) is a multifunctional neurohormone found only in arthropods, and one of its roles is to regulate glucose homeostasis. However, the molecular mechanism that underlies CHH regulation and whether GLUT4 is involved in its regulation in crustaceans remain unclear. In the present study, we identified a full-length GLUT4 cDNA sequence (defined herein as EsGLUT4) from the Chinese mitten crab Eriocheir sinensis and analyzed its tissue distribution and cellular localization. By the ForteBio Octet system, two large hydrophilic regions within EsGLUT4 were found to interact with the CHH binding protein (CHHBP), an E. sinensis flotillin-like protein. Interestingly, live-cell imaging indicated that EsGLUT4 and CHHBP responded simultaneously upon stimulation by CHH, resulting in glucose release. In contrast to insulin-dependent GLUT4, however, EsGLUT4 and CHHBP were present within cytoplasmic vesicles, both translocating to the plasma membrane upon CHH stimulation. In conclusion, our results provide new evidence for the involvement of EsGLUT4 and CHHBP in the regulation of glucose homeostasis in crustacean carbohydrate metabolism.

Summary: Here we identified that Glucose transporter 4 (GLUT4) could interact with CHH binding protein (CHHBP) to regulate CHH-stimulated glucose release in Eriocheir sinensis.

Rab14 limits the sorting of Glut4 from endosomes into insulin-sensitive regulated secretory compartments in adipocytes

Insulin increases glucose uptake by increasing the rate of exocytosis of the facilitative glucose transporter isoform 4 (Glut4) relative to its endocytosis. Insulin also releases Glut4 from highly insulin-regulated secretory compartments (GSVs or Glut4 storage vesicles) into constitutively cycling endosomes. Previously it was shown that both overexpression and knockdown of the small GTP-binding protein Rab14 decreased Glut4 translocation to the plasma membrane (PM). To determine the mechanism of this perturbation, we measured the effects of Rab14 knockdown on the trafficking kinetics of Glut4 relative to two proteins that partially co-localize with Glut4, the transferrin (Tf) receptor and low-density-lipoprotein-receptor-related protein 1 (LRP1). Our data support the hypothesis that Rab14 limits sorting of proteins from sorting (or 'early') endosomes into the specialized GSV pathway, possibly through regulation of endosomal maturation. This hypothesis is consistent with known Rab14 effectors. Interestingly, the insulin-sensitive Rab GTPase-activating protein Akt substrate of 160 kDa (AS160) affects both sorting into and exocytosis from GSVs. It has previously been shown that exocytosis of GSVs is rate-limited by Rab10, and both Rab10 and Rab14 are in vitro substrates of AS160. Regulation of both entry into and exit from GSVs by AS160 through sequential Rab substrates would provide a mechanism for the finely tuned 'quantal' increases in cycling Glut4 observed in response to increasing concentrations of insulin.

Glut4 Is Sorted from a Rab10 GTPase-independent Constitutive Recycling Pathway into a Highly Insulin-responsive Rab10 GTPase-dependent Sequestration Pathway after Adipocyte Differentiation

The RabGAP AS160/TBC1D4 controls exocytosis of the insulin-sensitive glucose transporter Glut4 in adipocytes. Glut4 is internalized and recycled through a highly regulated secretory pathway in these cells. Glut4 also cycles through a slow constitutive endosomal pathway distinct from the fast transferrin (Tf) receptor recycling pathway. This slow constitutive pathway is the only Glut4 cycling pathway in undifferentiated fibroblasts. The α2-macroglobulin receptor LRP1 cycles with Glut4 and the Tf receptor through all three exocytic pathways. To further characterize these pathways, the effects of knockdown of AS160 substrates on the trafficking kinetics of Glut4, LRP1, and the Tf receptor were measured in adipocytes and fibroblasts. Rab10 knockdown decreased cell surface Glut4 in insulin-stimulated adipocytes by 65%, but not in basal adipocytes or in fibroblasts. This decrease was due primarily to a 62% decrease in the rate constant of Glut4 exocytosis (kex), although Rab10 knockdown also caused a 1.4-fold increase in the rate constant of Glut4 endocytosis (ken). Rab10 knockdown in adipocytes also decreased cell surface LRP1 by 30% by decreasing kex 30-40%. There was no effect on LRP1 trafficking in fibroblasts or on Tf receptor trafficking in either cell type. These data confirm that Rab10 is an AS160 substrate that limits exocytosis through the highly insulin-responsive specialized secretory pathway in adipocytes. They further show that the slow constitutive endosomal (fibroblast) recycling pathway is Rab10-independent. Thus, Rab10 is a marker for the specialized pathway in adipocytes. Interestingly, mathematical modeling shows that Glut4 traffics predominantly through the specialized Rab10-dependent pathway both before and after insulin stimulation.

Resveratrol induces cell apoptosis in adipocytes via AMPK activation

Resveratrol is identified as polyphenolic compound with anti-inflammatory, antioxidant, anti-insulin resistance characteristics. Moreover, resveratrol exerts pro-apoptotic effects in varieties of cancer cell lines. However, effects and mechanisms of resveratrol on the regulation of adipocytes apoptosis remain largely unknown. In this study, we found that resveratrol treatment could induce cell apoptosis in murine 3T3-L1 adipocytes. Furthermore, resveratrol activated the mitochondrial apoptotic signaling pathway with the decrease in the mitochondrial membrane potential (MMP), and the activation of caspase 3. Mechanistically, we found that phosphorylation level of AMP-activated protein kinase α (AMPKα) was elevated, accompany with reduced level of phosphorylation of protein kinase B (AKT) when cells were exposed to resveratrol. By using small interfering RNAs of AMPKα and specific inhibitor for p-AKT, it was shown that activation of AMPKα could inhibit downstream of p-AKT, consequently activating mitochondrion-mediated apoptotic pathway. Additionally, we observed similar pro-apoptotic effects of Res on mouse primary adipocytes. Our findings clarified the apoptotic effects and underlying mechanisms of resveratrol in adipocytes, suggesting its potential therapeutic application in the treatment or prevention of obesity and related metabolic symptoms.

Development of a quantitative Correlative Light Electron Microscopy technique to study GLUT4 trafficking

Correlative Light Electron Microscopy (CLEM) combines advantages of light microscopy and electron microscopy in one experiment to deliver information above and beyond the capability of either modality alone. There are many different CLEM techniques, each having its own special advantages but also its technical challenges. It is however the biological question that (should) drive(s) the development and application of a specific CLEM technique in order to provide the answer. Here we describe the development of a CLEM technique that is based on the Tokuyasu cryo immuno-gold labelling technique that has allowed us to quantitatively study GLUT4 trafficking.

Insulin regulates Glut4 confinement in plasma membrane clusters in adipose cells

Insulin-stimulated delivery of glucose transporter-4 (GLUT4) to the plasma membrane (PM) is the hallmark of glucose metabolism. In this study we examined insulin’s effects on GLUT4 organization in PM of adipose cells by direct microscopic observation of single monomers tagged with photoswitchable fluorescent protein. In the basal state, after exocytotic delivery only a fraction of GLUT4 is dispersed into the PM as monomers, while most of the GLUT4 stays at the site of fusion and forms elongated clusters (60–240 nm). GLUT4 monomers outside clusters diffuse freely and do not aggregate with other monomers. In contrast, GLUT4 molecule collision with an existing cluster can lead to immediate confinement and association with that cluster. Insulin has three effects: it shifts the fraction of dispersed GLUT4 upon delivery, it augments the dissociation of GLUT4 monomers from clusters ∼3-fold and it decreases the rate of endocytic uptake. All together these three effects of insulin shift most of the PM GLUT4 from clustered to dispersed states. GLUT4 confinement in clusters represents a novel kinetic mechanism for insulin regulation of glucose homeostasis.

Mitigation of isolation-associated adipocyte interleukin-6 secretion following rapid dissociation of adipose tissue

Primary adipocyte isolation by collagenase digestion is a widely used technique to study metabolic regulation and insulin action in adipocytes. However, induction of a proinflammatory response characterized by enhanced secretion of interleukin (IL)-6 has been tightly linked to the isolation process itself. To test the hypothesis that the shaking mechanical force exerted on adipocytes stimulates inflammation during isolation, rat primary adipocytes were prepared by collagenase digestion in orbital shaking incubators maintained at varying speeds. Contrary to expectation, the isolation-induced release of IL-6 was attenuated by increasing the rotational speed of digestion and the concentration of collagenase, both of which resulted in rapid dissociation of adipocytes from the vasculature. In addition, the attenuation of IL-6 secretion was associated with decreased phosphorylation of the stress-related p38 mitogen-activated protein kinase (p38 MAPK) and preserved insulin action. The data suggest that optimization of parameters including, but not limited to, mincing technique, time of digestion, and collagenase concentration will make it possible to isolate primary adipocytes without activation of a proinflammatory response leading to elevated secretion of IL-6.

Metabolic effects of intermittent hypoxia in mice: steady versus high-frequency applied hypoxia daily during the rest period

Intermittent hypoxia (IH) is a frequent occurrence in sleep and respiratory disorders. Both human and murine studies show that IH may be implicated in metabolic dysfunction. Although the effects of nocturnal low-frequency intermittent hypoxia (IH(L)) have not been extensively examined, it would appear that IH(L) and high-frequency intermittent hypoxia (IH(H)) may elicit distinct metabolic adaptations. To this effect, C57BL/6J mice were randomly assigned to IH(H) (cycles of 90 s 6.4% O(2) and 90 s 21% O(2) during daylight), IH(L) (8% O(2) during daylight hours), or control (CTL) for 5 wk. At the end of exposures, some of the mice were subjected to a glucose tolerance test (GTT; after intraperitoneal injection of 2 mg glucose/g body wt), and others were subjected to an insulin tolerance test (ITT; 0.25 units Humulin/kg body wt), with plasma leptin and insulin levels being measured in fasting conditions. Skeletal muscles were harvested for GLUT4 and proliferator-activated receptor gamma coactivator 1-α (PGC1-α) expression. Both IH(H) and IH(L) displayed reduced body weight increases compared with CTL. CTL mice had higher basal glycemic levels, but GTT kinetics revealed marked differences between IH(L) and IH(H), with IH(L) manifesting the lowest insulin sensitivity compared with either IH(H) or CTL, and such findings were further confirmed by ITT. No differences emerged in PGC1-α expression across the three experimental groups. However, while cytosolic GLUT4 protein expression remained similar in IH(L), IH(H), and CTL, significant decreases in GLUT4 membrane fraction occurred in hypoxia and were most pronounced in IH(L)-exposed mice. Thus IH(H) and IH(L) elicit differential glucose homeostatic responses despite similar cumulative hypoxic profiles.

GLUT4 exocytosis

GLUT4 is an insulin-regulated glucose transporter that is responsible for insulin-regulated glucose uptake into fat and muscle cells. In the absence of insulin, GLUT4 is mainly found in intracellular vesicles referred to as GLUT4 storage vesicles (GSVs). Here, we summarise evidence for the existence of these specific vesicles, how they are sequestered inside the cell and how they undergo exocytosis in the presence of insulin. In response to insulin stimulation, GSVs fuse with the plasma membrane in a rapid burst and in the continued presence of insulin GLUT4 molecules are internalised and recycled back to the plasma membrane in vesicles that are distinct from GSVs and probably of endosomal origin. In this Commentary we discuss evidence that this delivery process is tightly regulated and involves numerous molecules. Key components include the actin cytoskeleton, myosin motors, several Rab GTPases, the exocyst, SNARE proteins and SNARE regulators. Each step in this process is carefully orchestrated in a sequential and coupled manner and we are beginning to dissect key nodes within this network that determine vesicle-membrane fusion in response to insulin. This regulatory process clearly involves the Ser/Thr kinase AKT and the exquisite manner in which this single metabolic process is regulated makes it a likely target for lesions that might contribute to metabolic disease.

Regulation of glucose transport by insulin: traffic control of GLUT4

Despite daily fasting and feeding, plasma glucose levels are normally maintained within a narrow range owing to the hormones insulin and glucagon. Insulin increases glucose uptake into fat and muscle cells through the regulated trafficking of vesicles that contain glucose transporter type 4 (GLUT4). New insights into insulin signalling reveal that phosphorylation events initiated by the insulin receptor regulate key GLUT4 trafficking proteins, including small GTPases, tethering complexes and the vesicle fusion machinery. These proteins, in turn, control GLUT4 movement through the endosomal system, formation and retention of specialized GLUT4 storage vesicles and targeted exocytosis of these vesicles. Understanding these processes may help to explain the development of insulin resistance in type 2 diabetes and provide new potential therapeutic targets.

Role of LXR in trout adipocytes: target genes, hormonal regulation, adipocyte differentiation and relation to lipolysis

In the present study, we describe an initial approach to investigate the role of LXR in fish adipose tissue. Rainbow trout (Oncorhynchus mykiss) isolated adipocytes were incubated with LXR agonists, unsaturated fatty acids, tumour necrosis factor-α (TNFα), insulin or growth hormone (GH) for 6h and LXR expression was analyzed. Lipolysis was measured after incubation with one of the LXR agonists and LXR expression was compared with levels of lipolysis. LXR expression was also analyzed during the differentiation of adipocytes in culture. The incubations with agonists in isolated adipocytes indicated that ATP-binding cassette transporter A1 (ABCA1) is an LXR target gene, but lipoprotein lipase (LPL), fatty acid synthase (FAS), hormone-sensitive lipase (HSL) and peroxisome proliferator-activated receptor (PPARs) are not. LXR agonists also induced LXR expression and raised lipolysis levels. Besides, LXR expression was upregulated in parallel with basal lipolysis. LXR mRNA expression was regulated by unsaturated fatty acids, insulin, TNFα and GH in isolated adipocytes. Besides, LXR showed an upregulation during adipocyte differentiation. All these data indicate that LXR is involved in orchestrating the transcriptional regulatory network in trout adipocyte lipid metabolism, specifically, in cholesterol transport, adipocyte differentiation and lipolysis.

Regulation of HIF-1{alpha} activity in adipose tissue by obesity-associated factors: adipogenesis, insulin, and hypoxia

The transcription factor HIF-1α activity is increased in adipose tissue to contribute to chronic inflammation in obesity. However, its upstream and downstream events remain to be characterized in adipose tissue in obesity. We addressed this issue by investigating adipocyte HIF-1α activity in response to obesity-associated factors, such as adipogenesis, insulin, and hypoxia. In adipose tissue, both HIF-1α mRNA and protein were increased by obesity. The underlying mechanism was investigated in 3T3-L1 adipocytes. HIF-1α mRNA and protein were augmented by adipocyte differentiation. In differentiated adipocytes, insulin further enhanced HIF-1α in both levels. Hypoxia enhanced only HIF-1α protein, not mRNA. PI3K and mTOR activities are required for the HIF-1α expression. Function of HIF-1α protein was investigated in the regulation of VEGF gene transcription. ChIP assay shows that HIF-1α binds to the proximal hypoxia response element in the VEGF gene promoter, and its function is inhibited by a corepressor composed of HDAC3 and SMRT. These observations suggest that of the three obesity-associated factors, all of them are able to augment HIF-1α protein levels, but only two (adipogenesis and insulin) are able to enhance HIF-1α mRNA activity. Adipose tissue HIF-1α activity is influenced by multiple signals, including adipogenesis, insulin, and hypoxia in obesity. The transcriptional activity of HIF-1α is inhibited by HDAC3-SMRT corepressor in the VEGF gene promoter.

Downregulation of adipose glutathione S-transferase A4 leads to increased protein carbonylation, oxidative stress, and mitochondrial dysfunction

OBJECTIVE

Peripheral insulin resistance is linked to an increase in reactive oxygen species (ROS), leading in part to the production of reactive lipid aldehydes that modify the side chains of protein amino acids in a reaction termed protein carbonylation. The primary enzymatic method for lipid aldehyde detoxification is via glutathione S-transferase A4 (GSTA4) dependent glutathionylation. The objective of this study was to evaluate the expression of GSTA4 and the role(s) of protein carbonylation in adipocyte function.

RESEARCH DESIGN AND METHODS

GSTA4-silenced 3T3-L1 adipocytes and GSTA4-null mice were evaluated for metabolic processes, mitochondrial function, and reactive oxygen species production. GSTA4 expression in human obesity was evaluated using microarray analysis.

RESULTS

GSTA4 expression is selectively downregulated in adipose tissue of obese insulin-resistant C57BL/6J mice and in human obesity-linked insulin resistance. Tumor necrosis factor-alpha treatment of 3T3-L1 adipocytes decreased GSTA4 expression, and silencing GSTA4 mRNA in cultured adipocytes resulted in increased protein carbonylation, increased mitochondrial ROS, dysfunctional state 3 respiration, and altered glucose transport and lipolysis. Mitochondrial function in adipocytes of lean or obese GSTA4-null mice was significantly compromised compared with wild-type controls and was accompanied by an increase in superoxide anion.

CONCLUSIONS

These results indicate that downregulation of GSTA4 in adipose tissue leads to increased protein carbonylation, ROS production, and mitochondrial dysfunction and may contribute to the development of insulin resistance and type 2 diabetes.

Characterization of GLUT4-containing vesicles in 3T3-L1 adipocytes by total internal reflection fluorescence microscopy

Insulin-responsive GLUT4 (glucose transporter 4) translocation plays a major role in regulating glucose uptake in adipose tissue and muscle. Whether or not there is a specialized secretory GSV (GLUT4 storage vesicle) pool, and more importantly how GSVs are translocated to the PM (plasma membrane) under insulin stimulation is still under debate. In the present study, we systematically analyzed the dynamics of a large number of single GLUT4-containing vesicles in 3T3-L1 adipocytes by TIRFM (total internal reflection fluorescence microscopy). We found that GLUT4-containing vesicles can be classified into three groups according to their mobility, namely vertical, stable, and lateral GLUT4-containing vesicles. Among these groups, vertical GLUT4-containing vesicles exclude transferrin receptors and move towards the PM specifically in response to insulin stimulation, while stable and lateral GLUT4-containing vesicles contain transferrin receptors and show no insulin responsiveness. These data demonstrate that vertical GLUT4-containing vesicles correspond to specialized secretory GSVs, which approach the PM directly and bypass the constitutive recycling pathway.

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.

Oxidized LDL impair adipocyte response to insulin by activating serine/threonine kinases

Oxidized LDL (oxLDL) increase in patients affected by type-2 diabetes, obesity, and metabolic syndrome. Likewise, insulin resistance, an impaired responsiveness of target tissues to insulin, is associated with those pathological conditions. To investigate a possible causal relationship between oxLDL and the onset of insulin resistance, we evaluated the response to insulin of 3T3-L1 adipocytes treated with oxLDL. We observed that oxLDL inhibited glucose uptake (-40%) through reduced glucose transporter 4 (GLUT4) recruitment to the plasma membrane (-70%), without affecting GLUT4 gene expression. These findings were associated to the impairment of insulin signaling. Specifically, in oxLDL-treated cells insulin receptor (IR) substrate-1 (IRS-1) was highly degraded likely because of the enhanced Ser(307)phosphorylation. This process was largely mediated by the activation of the inhibitor of kappaB-kinase beta (IKKbeta) and the c-Jun NH(2)-terminal kinase (JNK). Moreover, the activation of IKKbeta positively regulated the nuclear content of nuclear factor kappaB (NF-kappaB), by inactivating the inhibitor of NF-kappaB (IkappaBalpha). The activated NF-kappaB further impaired per se GLUT4 functionality. Specific inhibitors of IKKbeta, JNK, and NF-kappaB restored insulin sensitivity in adipocytes treated with oxLDL. These data provide the first evidence that oxLDL, by activating serine/threonine kinases, impaired adipocyte response to insulin affecting pathways involved in the recruitment of GLUT4 to plasma membranes (PM). This suggests that oxLDL might participate in the development of insulin resistance.

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.

Application of immunocytochemistry and immunofluorescence techniques to adipose tissue and cell cultures

When isolated from tissue, white adipose cells are round, and their interior is filled with a large (80-120 microm) droplet of stored triglyceride, leaving a thin (1-2-microm) layer of cytoplasm between the lipid droplet and the plasma membrane. Their three-dimensional architecture, together with the fact that these cells ordinarily float in medium, have created major challenges when one attempts to perform microscopy techniques with these cells. Adipocytes serve as the principal energy reservoir in the body, and it is essential to overcome these difficulties to be able to study hormone-mediated responses in real adipose cells, which convey physiological significance that cannot be readily duplicated by the use of cultured model adipocytes. This chapter focuses on the use of confocal microscopy optical sectioning and computer-assisted image reconstruction in the whole adipose cell in the study of insulin-regulated protein trafficking. In addition, we illustrate the possibility to image whole-mount preparations of living adipose tissue, opening new ways to probe adipose cells in situ without disrupting their cellular interactions within living adipose tissue. Confocal microscopy constitutes an effective morphological approach to investigating adipose cell physiology and pathophysiology.

Selective regulation of the perinuclear distribution of glucose transporter 4 (GLUT4) by insulin signals in muscle cells

Insulin regulates glucose transporter 4 (GLUT4) availability at the surface of muscle and adipose cells. In L6 myoblasts, stably expressed GLUT4myc is detected mostly in a perinuclear region. In unstimulated cells, about half of perinuclear GLUT4myc colocalizes with the transferrin receptor (TfR). Insulin stimulation selectively decreased the perinuclear colocalization of GLUT4myc with TfR determined by 3D-reconstruction of fluorescence images. Perinuclear GLUT4myc adopted two main distributions defined morphometrically as 'conical' and 'concentric'. Insulin rapidly reduced the proportion of cells with conical in favor of concentric perinuclear GLUT4myc distributions in association with the gain in surface GLUT4myc. Upon removal of insulin, the GLUT4myc perinuclear distribution and surface levels reversed in parallel. In contrast, hypertonicity (which like insulin elevates surface GLUT4myc) did not elicit perinuclear GLUT4myc redistribution. Insulin also caused redistribution of perinuclear vesicle-associated membrane protein-2 (VAMP2), without alteration of perinuclear TfR and VAMP3. Inhibitory mutants of phosphatidylinositol-3 kinase (Deltap85) or Akt substrate AS160 (AS160-4P) prevented insulin-mediated perinuclear GLUT4myc redistribution. Tetanus toxin expression did not prevent the perinuclear GLUT4myc redistribution, suggesting that redistribution is independent of GLUT4myc fusion with the plasma membrane. We propose that insulin causes selective, dynamic relocalization of perinuclear GLUT4myc and VAMP2 and perinuclear GLUT4myc redistribution is a direct target of insulin-derived signals.

Identification of amino acid residues within the C terminus of the Glut4 glucose transporter that are essential for insulin-stimulated redistribution to the plasma membrane

The Glut4 glucose transporter undergoes complex insulin-regulated subcellular trafficking in adipocytes. Much effort has been expended in an attempt to identify targeting motifs within Glut4 that direct its subcellular trafficking, but an amino acid motif responsible for the targeting of the transporter to insulin-responsive intracellular compartments in the basal state or that is directly responsible for its insulin-stimulated redistribution to the plasma membrane has not yet been delineated. In this study we define amino acid residues within the C-terminal cytoplasmic tail of Glut4 that are essential for its insulin-stimulated translocation to the plasma membrane. The residues were identified based on sequence similarity (LXXLXPDEXD) between cytoplasmic domains of Glut4 and the insulin-responsive aminopeptidase (IRAP). Alteration of this putative targeting motif (IRM, insulin-responsive motif) resulted in the targeting of the bulk of the mutant Glut4 molecules to dispersed membrane vesicles that lacked detectable levels of wild-type Glut4 in either the basal or insulin-stimulated states and completely abolished the insulin-stimulated translocation of the mutant Glut4 to the plasma membrane in 3T3L1 adipocytes. The bulk of the dispersed membrane vesicles containing the IRM mutant did not contain detectable levels of any subcellular marker tested. A fraction of the total IRM mutant was also detected in a wild-type Glut4/Syntaxin 6-containing perinuclear compartment. Interestingly, mutation of the IRM sequence did not appreciably alter the subcellular trafficking of IRAP. We conclude that residues within the IRM are critical for the targeting of Glut4, but not of IRAP, to insulin-responsive intracellular membrane compartments in 3T3-L1 adipocytes.

Involvement of vesicular H+-ATPase in insulin-stimulated glucose transport in 3T3-F442A adipocytes

In secretory cells, osmotic swelling of secretory granules is proposed to be an intermediate step in exocytic fusion of the granules with the plasma membrane. For osmotic swelling of the granule, a H (+) gradient generated by vacuolar-type H (+) -ATPase (V-ATPase) may be a driving force for accumulation of K (+) via its exchange with H (+) , concurrent with accumulation of Cl (-) and H(2)O. Here, we investigated whether a similar chemiosmotic mechanism is involved in the insulin-stimulated recruitment of GLUT4 to the plasma membrane in 3T3-F442A adipocytes. Incubating cells in a hypo-osmotic medium significantly increased 2-deoxy glucose (2-DG) uptake and the plasma membrane GLUT4 content (possibly via induction of osmotic swelling of GLUT4-containing vesicles (G4V)) and also potentiated the insulin-stimulated 2-DG uptake. Promotion of the G4V membrane ionic permeability using nigericin, an electroneutral K (+) /H (+) exchange ionophore, increased 2-DG uptake and the plasma membrane GLUT4 content. However, co-treatment with nigericin and insulin did not show an additive effect. Bafilomycin A(1), a diagnostically specific inhibitor of V-ATPase, inhibited insulin- and nigericin-stimulated 2-DG uptake. Immunoadsorption plus immunoblotting demonstrated that GLUT4 and V-ATPase co-localize in the same intracellular membranes. Together, these results indicate that V-ATPases in the G4V membrane may play an important role in the insulin-stimulated exocytic fusion of G4V with the plasma membrane via its participation in osmotic swelling of the vesicle.

Hypoxia is a potential risk factor for chronic inflammation and adiponectin reduction in adipose tissue of ob/ob and dietary obese mice

Chronic inflammation and reduced adiponectin are widely observed in the white adipose tissue in obesity. However, the cause of the changes remains to be identified. In this study, we provide experimental evidence that hypoxia occurs in adipose tissue in obese mice and that adipose hypoxia may contribute to the endocrine alterations. The adipose hypoxia was demonstrated by a reduction in the interstitial partial oxygen pressure (Po(2)), an increase in the hypoxia probe signal, and an elevation in expression of the hypoxia response genes in ob/ob mice. The adipose hypoxia was confirmed in dietary obese mice by expression of hypoxia response genes. In the adipose tissue, hypoxia was associated with an increased expression of inflammatory genes and decreased expression of adiponectin. In dietary obese mice, reduction in body weight by calorie restriction was associated with an improvement of oxygenation and a reduction in inflammation. In cell culture, inflammatory cytokines were induced by hypoxia in primary adipocytes and primary macrophages of lean mice. The transcription factor NF-kappaB and the TNF-alpha gene promoter were activated by hypoxia in 3T3-L1 adipocytes and NIH3T3 fibroblasts. In addition, adiponectin expression was reduced by hypoxia, and the reduction was observed in the gene promoter in adipocytes. These data suggest a potential role of hypoxia in the induction of chronic inflammation and inhibition of adiponectin in the adipose tissue in obesity.

The GLUT4 code

Despite being one of the first recognized targets of insulin action, the acceleration of glucose transport into muscle and fat tissue remains one of the most enigmatic processes in the insulin action cascade. Glucose transport is accomplished by a shift in the distribution of the insulin-responsive glucose transporter GLUT4 from intracellular compartments to the plasma membrane in the presence of insulin. The complexity in deciphering the molecular blueprint of insulin regulation of glucose transport arises because it represents a convergence of two convoluted biological systems-vesicular transport and signal transduction. Whereas more than 60 molecular players have been implicated in this orchestral performance, it has been difficult to distinguish between mainly passive participants vs. those that are clearly driving the process. The maze-like nature of the endosomal system makes it almost impossible to dissect the anatomical nature of what appears to be a medley of many overlapping and rapidly changing transitions. A major limitation is technology. It is clear that further progress in teasing apart the GLUT4 code will require the development and application of novel and advanced technologies that can discriminate one molecule from another in the living cell and to superimpose this upon a system in which the molecular environment can be carefully manipulated. Many are now taking on this challenge.

The SUMO conjugating enzyme Ubc9 is a regulator of GLUT4 turnover and targeting to the insulin-responsive storage compartment in 3T3-L1 adipocytes

The small ubiquitin-related modifier (SUMO) conjugating enzyme Ubc9 has been shown to upregulate GLUT4 in L6 myoblast cells, although the mechanism of action has remained undefined. Here we investigated the physiological significance of Ubc9 in GLUT4 turnover and subcellular targeting by adenovirus vector-mediated overexpression and by small interfering RNA (siRNA)-mediated gene silencing of Ubc9 in 3T3-L1 adipocytes. Overexpression of Ubc9 resulted in an inhibition of GLUT4 degradation and promoted its targeting to the unique insulin-responsive GLUT4 storage compartment (GSC), leading to an increase in GLUT4 amount and insulin-responsive glucose transport in 3T3-L1 adipocytes. Overexpression of Ubc9 also antagonized GLUT4 downregulation and its selective loss in GSC induced by long-term insulin stimulation. By contrast, siRNA-mediated depletion of Ubc9 accelerated GLUT4 degradation and decreased the amount of the transporter, concurrent with its selective loss in GSC, which resulted in attenuated insulin-responsive glucose transport. Intriguingly, overexpression of the catalytically inactive mutant Ubc9-C93A produced effects indistinguishable from those with wild-type Ubc9, suggesting that Ubc9 regulates GLUT4 turnover and targeting to GSC by a mechanism independent of its catalytic activity. Thus, Ubc9 is a pivotal regulator of the insulin sensitivity of glucose transport in adipocytes.

Cellular spelunking: exploring adipocyte caveolae

It has been known for decades that the adipocyte cell surface is particularly rich in small invaginations we now know to be caveolae. These structures are common to many cell types but are not ubiquitous. They have generated considerable curiosity, as manifested by the numerous publications on the topic that describe various, sometimes contradictory, caveolae functions. Here, we review the field from an "adipocentric" point of view and suggest that caveolae may have a function of particular use for the fat cell, namely the modulation of fatty acid flux across the plasma membrane. Other functions for adipocyte caveolae that have been postulated include participation in signal transduction and membrane trafficking pathways, and it will require further experimental scrutiny to resolve controversies surrounding these possible activities.

The luminal Vps10p domain of sortilin plays the predominant role in targeting to insulin-responsive Glut4-containing vesicles

In fat and skeletal muscle cells, insulin-responsive vesicles, or IRVs, deliver glucose transporter Glut4 and several associated proteins to the plasma membrane in response to hormonal stimulation. Although the protein composition of the IRVs is well studied, the mechanism of their formation is unknown. It is believed, however, that the cytoplasmic tails of the IRV component proteins carry targeting information to this compartment. To test this hypothesis, we have studied targeting of sortilin, one of the major IRV constituents. We have found that the reporter protein consisting of the cytoplasmic tail of sortilin and EGFP is co-localized with ectopically expressed Glut4 in the perinuclear compartment of undifferentiated 3T3-L1 cells that do not form insulin-responsive vesicles. Upon cell differentiation, this reporter protein does not enter the IRVs; moreover, it loses its perinuclear localization and becomes randomly distributed throughout the whole intracellular space. In contrast, the tagged luminal Vps10p domain of sortilin demonstrates partial co-localization with Glut4 in both undifferentiated and differentiated cells and is targeted to the IRVs upon cell differentiation. Using chemical cross-linking and the yeast two-hybrid system, we show that sortilin interacts with Glut4 and IRAP in the vesicular lumen. Our results suggest that luminal interactions between component proteins play an important role in the process of IRV biogenesis.

Dissecting multiple steps of GLUT4 trafficking and identifying the sites of insulin action

Insulin-stimulated GLUT4 translocation is central to glucose homeostasis. Functional assays to distinguish individual steps in the GLUT4 translocation process are lacking, thus limiting progress toward elucidation of the underlying molecular mechanism. Here we have developed a robust method, which relies on dynamic tracking of single GLUT4 storage vesicles (GSVs) in real time, for dissecting and systematically analyzing the docking, priming, and fusion steps of GSVs with the cell surface in vivo. Using this method, we have shown that the preparation of GSVs for fusion competence after docking at the surface is a key step regulated by insulin, whereas the docking step is regulated by PI3K and its downstream effector, the Rab GAP AS160. These data show that Akt-dependent phosphorylation of AS160 is not the major regulated step in GLUT4 trafficking, implicating alternative Akt substrates or alternative signaling pathways downstream of GSV docking at the cell surface as the major regulatory node.

Activation of the Cbl insulin signaling pathway in cardiac muscle; Dysregulation in obesity and diabetes

In adipocytes, the Cbl/CAP dependent signaling pathway has been involved in regulating insulin-stimulated glucose uptake. We investigated activation of Cbl and its downstream effector TC10 in cardiac and skeletal muscle of Balb/C mice. Insulin administration resulted in Cbl phosphorylation in cardiac, skeletal muscle, and adipose tissue. Subsequent TC10 activation was detected only in heart and adipose tissue. c-Cbl and CAP gene expression was significantly reduced in the heart tissue of streptozotocin-induced diabetic animals, whereas no change was observed for other components of the pathway. No changes in Cbl expression were detected in hindlimb muscle. In leptin-/- obese mice Cbl expression in heart and adipose tissue was maintained, although insulin-mediated Cbl phosphorylation and subsequent TC10 activation were significantly reduced. In conclusion, our data demonstrate that Cbl/CAP/TC10 insulin signaling pathway is active in cardiac muscle and impaired during obesity and insulin deficiency.

Spatial and temporal regulation of GLUT4 translocation by flotillin-1 and caveolin-3 in skeletal muscle cells

Skeletal muscle tissue is one of the main sites where glucose uptake occurs in response to insulin. The glucose transporter type-4 (GLUT4) is primarily responsible for the insulin-stimulated increase in glucose uptake. Upon insulin stimulation, GLUT4 is recruited from intracellular reserves to the plasma membrane. The molecular mechanisms that regulate the translocation of GLUT4 to the sarcolemma remain to be fully identified. Here, we demonstrate that GLUT4 is localized to perinuclear stores that contain flotillin-1, a marker of lipid rafts, in skeletal muscle cells. Stimulation with insulin for 10 min results in the translocation of flotillin-1/GLUT4-containing domains to the plasma membrane in a PI3K- and PKCzeta-dependent manner. We also demonstrate that caveolin-3, a marker of caveolae, is required for the insulin receptor-mediated activation of the PI3K-dependent pathway, which occurs 2 min after insulin stimulation. In fact, we demonstrate that lack of caveolin-3 significantly reduces insulin-stimulated glucose uptake in caveolin-3 null myotubes by inhibiting both PI3K and Akt, as well as the movement of GLUT4 to the plasma membrane. Interestingly, caveolin-3 moves away from the plasma membrane toward the cytoplasm 5 min after insulin stimulation and temporarily interacts with flotillin-1/GLUT4-containing domains before they reach the sarcolemma, with the consequent movement of the insulin receptor from caveolin-3-containing domains to flotillin-1-containing domains. Such translocation temporally matches the insulin-stimulated movement of Cbl and CrkII in flotillin-1/GLUT4-containing domains, as well as the activation of the GDP-GTP exchange factor C3G. Disruption of flotillin-1-based domains prevents the activation of C3G, movement of GLUT4 to the sarcolemma, and glucose uptake in response to insulin. Thus, the activation of the Cbl/C3G/TC10-dependent pathway, which occurs before flotillin-1/GLUT4-containing domains reach the plasma membrane, is flotillin-1 mediated and follows the activation of the PI3K-mediated signaling. Taken together, these results indicate that flotillin-1 and caveolin-3 may regulate muscle energy metabolism through the spatial and temporal segregation of key components of the insulin signaling.

Glucose transporter 4: cycling, compartments and controversies

Insulin promotes glucose uptake into muscle and adipose tissues through glucose transporter 4 (GLUT4). In unstimulated cells, rapid endocytosis, slow exocytosis and dynamic or static retention cause GLUT4 to concentrate in early recycling endosomes, the trans-Golgi network and vesicle-associated protein 2-containing vesicles. The coordinated action of phosphatidylinositol 3-kinase effectors, protein kinase Akt, atypical protein kinase C (aPKC) and Akt substrate of 160-kDa (AS160), regulates the GLUT4 cycle by affecting its translocation, fusion with the plasma membrane, internalization and sorting. We review the evidence that supports such cycling, evaluate current models proposing static or dynamic retention, and highlight how distinct steps of GLUT4 transport are regulated by insulin signals. In particular, fusion seems to be regulated by aPKC (via munc18) and Akt (via syntaxin4-interacting protein (synip)). AS160 participates in GLUT4 intracellular retention, and possibly fusion, through candidate ras-related GTP-binding protein (Rab)2, Rab8, Rab10 and/or Rab14. The localization of the insulin-sensitive GLUT4 compartment and the precise target of insulin-derived signals remain open for future investigation.

Characterization of the Role of the Rab GTPase-activating Protein AS160 in Insulin-regulated GLUT4 Trafficking

Insulin stimulates the translocation of the glucose transporter GLUT4 from intracellular vesicles to the plasma membrane. In the present study we have conducted a comprehensive proteomic analysis of affinity-purified GLUT4 vesicles from 3T3-L1 adipocytes to discover potential regulators of GLUT4 trafficking. In addition to previously identified components of GLUT4 storage vesicles including the insulin-regulated aminopeptidase insulin-regulated aminopeptidase and the vesicle soluble N-ethylmaleimide factor attachment protein (v-SNARE) VAMP2, we have identified three new Rab proteins, Rab10, Rab11, and Rab14, on GLUT4 vesicles. We have also found that the putative Rab GTPase-activating protein AS160 (Akt substrate of 160 kDa) is associated with GLUT4 vesicles in the basal state and dissociates in response to insulin. This association is likely to be mediated by the cytosolic tail of insulin-regulated aminopeptidase, which interacted both in vitro and in vivo with AS160. Consistent with an inhibitory role of AS160 in the basal state, reduced expression of AS160 in adipocytes using short hairpin RNA increased plasma membrane levels of GLUT4 in an insulin-independent manner. These findings support an important role for AS160 in the insulin regulated trafficking of GLUT4.

Sortilin Is Essential and Sufficient for the Formation of Glut4 Storage Vesicles in 3T3-L1 Adipocytes

Impaired translocation of the glucose transporter isoform 4 (Glut4) to the plasma membrane in fat and skeletal muscle cells may represent a primary defect in the development of type 2 diabetes mellitus. Glut4 is localized in specialized storage vesicles (GSVs), the biological nature and biogenesis of which are not known. Here, we report that GSVs are formed in differentiating 3T3-L1 adipocytes upon induction of sortilin on day 2 of differentiation. Forced expression of Glut4 prior to induction of sortilin leads to rapid degradation of the transporter, whereas overexpression of sortilin increases formation of GSVs and stimulates insulin-regulated glucose uptake. Knockdown of sortilin decreases both formation of GSVs and insulin-regulated glucose uptake. Finally, we have reconstituted functional GSVs in undifferentiated cells by double transfection of Glut4 and sortilin. Thus, sortilin is not only essential, but also sufficient for biogenesis of GSVs and acquisition of insulin responsiveness in adipose cells.

Insulin regulates the membrane arrival, fusion, and C-terminal unmasking of glucose transporter-4 via distinct phosphoinositides

Insulin increases glucose uptake into muscle via glucose transporter-4 (GLUT4) translocation to the cell membrane, but the regulated events in GLUT4 traffic are unknown. Here we focus on the role of class IA phosphatidylinositol (PI) 3-kinase and specific phosphoinositides in the steps of GLUT4 arrival and fusion with the membrane, using L6 muscle cells expressing GLUT4myc. To this end, we detected the availability of the myc epitope at the cell surface or intravesicular spaces and of the cytosol-facing C-terminal epitope, in cells and membrane lawns derived from them. We observed the following: (a) Wortmannin and LY294002 at concentrations that inhibit class IA PI 3-kinase reduced but did not abate the C terminus gain, yet the myc epitope was unavailable for detection unless lawns or cells were permeabilized, suggesting the presence of GLUT4myc in docked, unfused vesicles. Accordingly, GLUT4myc-containing vesicles were detected by immunoelectron microscopy of membranes from cells pretreated with wortmannin and insulin, but not insulin or wortmannin alone. (b) Insulin caused greater immunological availability of the C terminus than myc epitopes, suggesting that C terminus unmasking had occurred. Delivering phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) to intact cells significantly increased lawn-associated myc signal without C terminus gain. Conversely, phosphatidylinositol 3-phosphate (PI3P) increased the detection of C terminus epitope without any myc gain. We propose that insulin regulates GLUT4 membrane arrival, fusion, and C terminus unmasking, through distinct phosphoinositides. PI(3,4,5)P(3) causes arrival and fusion without unmasking, whereas PI3P causes arrival and unmasking without fusion.

Insulin stimulates the halting, tethering, and fusion of mobile GLUT4 vesicles in rat adipose cells

Glucose transport in adipose cells is regulated by changing the distribution of glucose transporter 4 (GLUT4) between the cell interior and the plasma membrane (PM). Insulin shifts this distribution by augmenting the rate of exocytosis of specialized GLUT4 vesicles. We applied time-lapse total internal reflection fluorescence microscopy to dissect intermediates of this GLUT4 translocation in rat adipose cells in primary culture. Without insulin, GLUT4 vesicles rapidly moved along a microtubule network covering the entire PM, periodically stopping, most often just briefly, by loosely tethering to the PM. Insulin halted this traffic by tightly tethering vesicles to the PM where they formed clusters and slowly fused to the PM. This slow release of GLUT4 determined the overall increase of the PM GLUT4. Thus, insulin initially recruits GLUT4 sequestered in mobile vesicles near the PM. It is likely that the primary mechanism of insulin action in GLUT4 translocation is to stimulate tethering and fusion of trafficking vesicles to specific fusion sites in the PM.

Molecular mechanisms and drug development in aquaporin water channel diseases: water channel aquaporin-2 of kidney collecting duct cells

Aquaporin-2 (AQP2) is one of the membrane water channel proteins expressed in principal cells of the kidney collecting ducts. In the basal state, AQP2 resides in the storage vesicles localized in the subapical cytoplasm. Upon stimulation with vasopressin, AQP2 is translocated to the apical plasma membrane by the exocytic fusion of the storage vesicles with the apical membrane. This translocation enables the transepithelial reabsorption of water from the lumen to the interstitium via AQP2 at the apical membrane and AQP3/AQP4 at the basolateral membrane. AQP2-storage vesicles are distinct from the endoplasmic reticulum, Golgi apparatus, trans-Golgi network, and lysosomes. The early endosomal marker EEA1 is colocalized with some of AQP2 vesicles. Further analyses in Madin-Darby canine kidney (MDCK) cells transfected with AQP2 revealed that subapical Rab11-positive/EEA1-negative smaller vesicles constitute part of the AQP2 storage vesicles for the translocation to the apical membrane. Termination of stimulation results in the retrieval of AQP2 to the larger EEA1-positive early endosomal compartment. AQP2 is then transferred to the subapical storage compartment in a PI3-kinase-dependent manner. GLUT4 is an isoform of glucose transporters whose localization is also regulated by vesicular trafficking induced by insulin stimulation. Comparison of the intracellular localization of AQP2 with GLUT4 suggests distinct regulation of AQP2 trafficking.

Golgi-localized, gamma-ear-containing, Arf-binding protein adaptors mediate insulin-responsive trafficking of glucose transporter 4 in 3T3-L1 adipocytes

Small glucose transporter 4 (Glut4)-containing vesicles represent the major insulin-responsive compartment in fat and skeletal muscle cells. The molecular mechanism of their biogenesis is not yet elucidated. Here, we studied the role of the newly discovered family of monomeric adaptor proteins, GGA (Golgi-localized, gamma-ear-containing, Arf-binding proteins), in the formation of small Glut4 vesicles and acquisition of insulin responsiveness in 3T3-L1 adipocytes. In these cells, all three GGA isoforms are expressed throughout the differentiation process. In particular, GGA2 is primarily present in trans-Golgi network and endosomes where it demonstrates a significant colocalization with the recycling pool of Glut4. Using the techniques of immunoadsorption as well as glutathione-S-transferase pull-down assay we found that Glut4 vesicles (but not Glut4 per se) interact with GGA via the Vps-27, Hrs, and STAM (VHS) domain. Moreover, a dominant negative GGA mutant inhibits formation of Glut4 vesicles in vitro. To study a possible role of GGA in Glut4 traffic in the living cell, we stably expressed a dominant negative GGA mutant in 3T3-L1 adipocytes. Formation of small insulin-responsive Glut4-containing vesicles and insulin-stimulated glucose uptake in these cells were markedly impaired. Thus, GGA adaptors participate in the formation of the insulin-responsive vesicular compartment from the intracellular donor membranes both in vivo and in vitro.

A simple method for comparing immunogold distributions in two or more experimental groups illustrated using GLUT1 labelling of isolated trophoblast cells

Colloidal gold-labelling, combined with transmission electron microscopy, is a valuable technique for high-resolution immunolocalization of identified antigens in different subcellular compartments. Whilst the technique has been applied to placental tissues, few quantitative studies have been made. Subcellular compartments exist in three main categories (viz. organelles, membranes, filaments/tubules) and this affects the possibilities for quantification. Generally, gold particles are counted in order to compare either (a) compartments within an experimental group or (b) compartmental labelling distributions between groups. For the former, recent developments make it possible to test whether or not there is differential (nonrandom) labelling of compartments. The methods (relative labelling index and labelling density) are ideally suited to analysing label in one category of compartment (organelle or membrane or filament) but may be adapted to deal with a mixture of categories. They also require information about compartment size (e.g. profile area or trace length). Here, a simple and efficient method for drawing between-group comparisons of labelling distributions is presented. The method does not require information about compartment size or specimen magnification. It relies on multistage random sampling of specimens and unbiased counting of gold particles associated with different compartments. Distributions of observed gold counts in different experimental groups are compared by contingency table analysis with degrees of freedom for chi-squared (chi(2)) values being determined by the numbers of compartments and experimental groups. Compartmental values of chi(2)which contribute substantially to total chi(2)identify the principal subcellular sites of between-group differences. The method is illustrated using datasets from immunolabelling studies on the localization of GLUT1 glucose transporters in cultured human trophoblast cells exposed to different treatments.

V-type ATPase is involved in biogenesis of GLUT4 vesicles

Proton pumps participate in several aspects of endocytic protein trafficking. However, their involvement specifically in the GLUT4 pathway has been a matter of great controversy. Here, we report that incubation of 3T3-L1 adipocytes with specific inhibitors of V-type ATPase, concanamycin A and bafilomycin A1, inhibits insulin-regulated glucose transport and results in accumulation of GLUT4 in heavy, rapidly sedimenting intracellular membranes. Correspondingly, the amount of small responsive GLUT4 vesicles in concanamycin A- and bafilomycin A1-treated cells is decreased. We conclude that these drugs block translocation of GLUT4 in adipose cells by inhibiting formation of small insulin-responsive vesicles on donor intracellular membranes. At the same time, proton pump inhibitors do not affect insulin-dependent translocation of preexisting vesicles or GLUT4 sorting in recycling endosomes. On the contrary, wortmannin acutely inhibits insulin-dependent translocation of the preexisting vesicles but has no effect on vesicle formation.

Applications of an efficient method for comparing immunogold labelling patterns in the same sets of compartments in different groups of cells

Quantitative immunoelectron microscopy often involves determining the distributions of gold label in different intracellular compartments and then drawing comparisons between compartments in the same sample of cells or between experimental groups of cells. In the case of within-group comparisons, recent developments in the estimation of relative labelling index and labelling density make it possible to test whether or not particular compartments are preferentially labelled. These methods are ideally suited to analysing gold label restricted to volume (organelle) or surface (membrane) compartments but may be modified to analyse label localised in mixtures of both. Here, a simple and efficient approach to drawing between-group comparisons for label associated with organelles and/or membranes is presented. The method relies on multistage random sampling of specimens (via blocks and microscopic fields) followed by simply counting gold particles associated with different compartments. The distributions of raw gold counts in different groups are then compared by contingency table analysis with statistical degrees of freedom for chi-squared values being determined by the number of compartments and the number of experimental groups of cells. Compartmental chi-squared values making substantial contributions to the total chi-squared values then identify where the main between-group differences reside. The method requires no information about compartment size (for example, organelle profile area or membrane trace length) and does not even depend critically on standardising between-group magnification. Its application is illustrated using datasets from immunolabelling studies designed to localise the KDEL receptor, phosphatidyl-inositol 4,5-bisphosphate, GLUT4 and rab4 at the electron microscopic level.