Protein kinase-zeta interacts with munc18c: role in GLUT4 trafficking

Combined Hyperglycemia- and Hyperinsulinemia-Induced Insulin Resistance in Adipocytes Is Associated With Dual Signaling Defects Mediated by PKC-ζ

A hyperglycemic and hyperinsulinemic environment characteristic of type 2 diabetes causes insulin resistance. In adipocytes, defects in both insulin sensitivity and maximum response of glucose transport have been demonstrated. To investigate the molecular mechanisms, freshly isolated rat adipocytes were incubated in control (5.6 mM glucose, no insulin) and high glucose (20 mM)/high insulin (100 nM) (HG/HI) for 18 hours to induce insulin resistance. Insulin-resistant adipocytes manifested decreased sensitivity of glucose uptake associated with defects in insulin receptor substrate (IRS)-1 Tyr phosphorylation, association of p85 subunit of phosphatidylinositol-3-kinase, Akt Ser473 and Thr308 phosphorylation, accompanied by impaired glucose transporter 4 translocation. In contrast, protein kinase C (PKC)-ζ activity was augmented by chronic HG/HI. Inhibition of PKC-ζ with a specific cell-permeable peptide reversed the signaling defects and insulin sensitivity of glucose uptake. Transfection of dominant-negative, kinase-inactive PKC-ζ blocked insulin resistance, whereas constitutively active PKC-ζ recapitulated the defects. The HG/HI incubation was associated with stimulation of IRS-1 Ser318 and Akt Thr34 phosphorylation, targets of PKC-ζ. Transfection of IRS-1 S318A and Akt T34A each partially corrected insulin signaling, whereas combined transfection of both completely normalized insulin signaling. In vivo hyperglycemia/hyperinsulinemia in rats for 48 hours similarly resulted in activation of PKC-ζ and increased phosphorylation of IRS-1 Ser318 and Akt Thr34. These data indicate that impairment of insulin signaling by chronic HG/HI is mediated by dual defects at IRS-1 and Akt mediated by PKC-ζ.

Disruption of protein complexes containing protein phosphatase 2B and Munc18c reduces the secretion of von Willebrand factor from endothelial cells

Essentials Endothelial secretion of von Willebrand factor (VWF) promotes inflammation and thrombosis. We studied the role of protein phosphatase 2B (PP2B) and Munc18c protein complex in VWF secretion. Disruption of PP2B-Munc18c complex in endothelial cells reduced agonist-induced VWF secretion. PP2B-Munc18c complex represents a potential target for thrombotic and inflammatory conditions.

SUMMARY

Background Aberrant secretion of von Willebrand factor (VWF) from endothelial cells contributes to inflammation and vascular thrombosis. Agonist-induced VWF secretion is facilitated by protein kinase and phosphatase-mediated signaling. Although the catalytic subunit of protein phosphatase 2B (PP2B-Aα) is targeted to the secretory machinery via an interaction with the vesicle trafficking protein Munc18c in endothelial cells, the functional relevance of this phosphatase complex is unclear. Objective To assess the contribution of the PP2B-Aα-Munc18c complex to endothelial VWF secretion. Results Here, we show that amino acids 120-130 of PP2B-Aα are important to support an interaction with Munc18c. A synthetic myristylated cell-permeable peptide, which is derived from amino acids 121-130 of PP2B-Aα, disrupted endogenous PP2B-Aα-Munc18c complexes in human umbilical vein endothelial cells, and decreased low-dose histamine-stimulated and thrombin-stimulated VWF secretion. Conclusion These studies indicate that PP2B-Aα-Munc18c complex supports agonist-induced VWF secretion, and suggest the potential of targeting this phosphatase complex in thrombotic and inflammatory conditions.

Insulin-stimulated glucose uptake in healthy and insulin-resistant skeletal muscle

Skeletal muscle is the largest tissues in the human body and is considered the primary target for insulin-stimulated glucose disposal. In skeletal muscle, binding of the insulin to insulin receptor (IR) initiates a signaling cascade that results in the translocation of the insulin-sensitive glucose transporter protein 4 (GLUT4) to the plasma membrane which leads to facilitated diffusion of glucose into the cell. Understanding the precise signaling events guiding insulin-stimulated glucose uptake is pivotal, because impairment in these signaling events leads to development of insulin resistance and type 2 diabetes. This review summarizes current understanding of insulin signaling pathways mediating glucose uptake in healthy and insulin-resistant skeletal muscle.

Unacylated ghrelin restores insulin and autophagic signaling in skeletal muscle of diabetic mice

Impairment of insulin signaling in skeletal muscle detrimentally affects insulin-stimulated disposal of glucose. Restoration of insulin signaling in skeletal muscle is important as muscle is one of the major sites for disposal of blood glucose. Recently, unacylated ghrelin (UnAG) has received attention in diabetic research due to its favorable actions on improving glucose tolerance, glycemic control, and insulin sensitivity. The investigation of UnAG has entered phase Ib clinical trial in type 2 diabetes and phase II clinical trial in hyperphagia in Prader-Willi syndrome. Nonetheless, the precise mechanisms responsible for the anti-diabetic actions of UnAG remain incompletely understood. In this study, we examined the effects of UnAG on restoring the impaired insulin signaling in skeletal muscle of db/db diabetic mice. Our results demonstrated that UnAG effectively restored the impaired insulin signaling in diabetic muscle. UnAG decreased insulin receptor substrate (IRS) phosphorylation, increased protein kinase B (Akt) phosphorylation, and, hence, suppressed mTOR signaling. Consequently, UnAG enhanced Glut4 localization and increased PDH activity in the diabetic skeletal muscle. Intriguingly, our data indicated that UnAG normalized the suppressed autophagic signaling in diabetic muscle. In conclusion, our findings illustrated that UnAG restored the impaired insulin and autophagic signaling in skeletal muscle of diabetic mice, which are valuable to understand the underlying mechanisms of the anti-diabetic action of UnAG at peripheral skeletal muscle level.

Mathematical modeling of the insulin signal transduction pathway for prediction of insulin sensitivity from expression data

Mathematical models of biological pathways facilitate a systems biology approach to medicine. However, these models need to be updated to reflect the latest available knowledge of the underlying pathways. We developed a mathematical model of the insulin signal transduction pathway by expanding the last major previously reported model and incorporating pathway components elucidated since the original model was reported. Furthermore, we show that inputting gene expression data of key components of the insulin signal transduction pathway leads to sensible predictions of glucose clearance rates in agreement with reported clinical measurements. In one set of simulations, our model predicted that glycerol kinase knockout mice have reduced GLUT4 translocation, and consequently, reduced glucose uptake. Additionally, a comparison of our extended model with the original model showed that the added pathway components improve simulations of glucose clearance rates. We anticipate this expanded model to be a useful tool for predicting insulin sensitivity in mammalian tissues with altered expression protein phosphorylation or mRNA levels of insulin signal transduction pathway components.

Abi3bp is a multifunctional autocrine/paracrine factor that regulates mesenchymal stem cell biology

Mesenchymal stem cells (MSCs) transplanted into injured myocardium promote repair through paracrine mechanisms. We have previously shown that MSCs over-expressing AKT1 (Akt-MSCs) exhibit enhanced properties for cardiac repair. In this study, we investigated the relevance of Abi3bp toward MSC biology. Abi3bp formed extracellular deposits with expression controlled by Akt1 and ubiquitin-mediated degradation. Abi3bp knockdown/knockout stabilized focal adhesions and promoted stress-fiber formation. Furthermore, MSCs from Abi3bp knockout mice displayed severe deficiencies in osteogenic and adipogenic differentiation. Knockout or stable knockdown of Abi3bp increased MSC and Akt-MSC proliferation, promoting S-phase entry via cyclin-d1, ERK1/2, and Src. Upon Abi3bp binding to integrin-β1 Src associated with paxillin which inhibited proliferation. In vivo, Abi3bp knockout increased MSC number and proliferation in bone marrow, lung, and liver. In summary, we have identified a novel extracellular matrix protein necessary for the switch from proliferation to differentiation in MSCs.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Effects of exercise on AMPK signaling and downstream components to PI3K in rat with type 2 diabetes

Exercise can increase skeletal muscle sensitivity to insulin, improve insulin resistance and regulate glucose homeostasis in rat models of type 2 diabetes. However, the potential mechanism remains poorly understood. In this study, we established a male Sprague–Dawley rat model of type 2 diabetes, with insulin resistance and β cell dysfunction, which was induced by a high-fat diet and low-dose streptozotocin to replicate the pathogenesis and metabolic characteristics of type 2 diabetes in humans. We also investigated the possible mechanism by which chronic and acute exercise improves metabolism, and the phosphorylation and expression of components of AMP-activated protein kinase (AMPK) and downstream components of phosphatidylinositol 3-kinase (PI3K) signaling pathways in the soleus. As a result, blood glucose, triglyceride, total cholesterol, and free fatty acid were significantly increased, whereas insulin level progressively declined in diabetic rats. Interestingly, chronic and acute exercise reduced blood glucose, increased phosphorylation and expression of AMPKα1/2 and the isoforms AMPKα1 and AMPKα2, and decreased phosphorylation and expression of AMPK substrate, acetyl CoA carboxylase (ACC). Chronic exercise upregulated phosphorylation and expression of AMPK upstream kinase, LKB1. But acute exercise only increased LKB1 expression. In particular, exercise reversed the changes in protein kinase C (PKC)ζ/λ phosphorylation, and PKCζ phosphorylation and expression. Additionally, exercise also increased protein kinase B (PKB)/Akt1, Akt2 and GLUT4 expression, but AS160 protein expression was unchanged. Chronic exercise elevated Akt (Thr³⁰⁸) and (Ser⁴⁷³) and AS160 phosphorylation. Finally, we found that exercise increased peroxisome proliferator-activated receptor-γ coactivator 1 (PGC1) mRNA expression in the soleus of diabetic rats. These results indicate that both chronic and acute exercise influence the phosphorylation and expression of components of the AMPK and downstream to PIK3 (aPKC, Akt), and improve GLUT4 trafficking in skeletal muscle. These data help explain the mechanism how exercise regulates glucose homeostasis in diabetic rats.

Cardiac PI3K-Akt impairs insulin-stimulated glucose uptake independent of mTORC1 and GLUT4 translocation

Impaired insulin-mediated glucose uptake characterizes cardiac muscle in humans and animals with insulin resistance and diabetes, despite preserved or enhanced phosphatidylinositol 3-kinase (PI3K) and the serine-threonine kinase, Akt-signaling, via mechanisms that are incompletely understood. One potential mechanism is PI3K- and Akt-mediated activation of mechanistic target of rapamycin (mTOR) and ribosomal protein S6 kinase (S6K), which may impair insulin-mediated activation of insulin receptor substrate (IRS)1/2 via inhibitory serine phosphorylation or proteasomal degradation. To gain mechanistic insights by which constitutive activation of PI3K or Akt may desensitize insulin-mediated glucose uptake in cardiomyocytes, we examined mice with cardiomyocyte-restricted, constitutive or inducible overexpression of a constitutively activated PI3K or a myristoylated Akt1 (myrAkt1) transgene that also expressed a myc-epitope-tagged glucose transporter type 4 protein (GLUT4). Although short-term activation of PI3K and myrAkt1 increased mTOR and S6 signaling, there was no impairment in insulin-mediated activation of IRS1/2. However, insulin-mediated glucose uptake was reduced by 50-80%. Although longer-term activation of Akt reduced IRS2 protein content via an mTORC1-mediated mechanism, treatment of transgenic mice with rapamycin failed to restore insulin-mediated glucose uptake, despite restoring IRS2. Transgenic activation of Akt and insulin-stimulation of myrAkt1 transgenic cardiomyocytes increased sarcolemmal insertion of myc-GLUT4 to levels equivalent to that observed in insulin-stimulated wild-type controls. Despite preserved GLUT4 translocation, glucose uptake was not elevated by the presence of constitutive activation of PI3K and Akt. Hexokinase II activity was preserved in myrAkt1 hearts. Thus, constitutive activation of PI3K and Akt in cardiomyocytes impairs GLUT4-mediated glucose uptake via mechanisms that impair the function of GLUT4 after its plasma-membrane insertion.

Sequestosome 1/p62, a scaffolding protein, is a newly identified partner of IRS-1 protein

Defects in the insulin-signaling pathway may lead to the development of skeletal muscle insulin resistance, which is one of the earliest abnormalities detected in individuals with the metabolic syndrome and predisposes them to develop type 2 diabetes. Previous studies have shown that deletion of the mouse sequestosome 1/p62 gene results in mature-onset obesity that progresses to insulin and leptin resistance and, ultimately, type 2 diabetes. Sequestosome 1/p62 is involved in receptor-mediated signal transduction and functions as an intracellular signal modulator or adaptor protein. Insulin receptor substrate-1 (IRS-1) plays a central role in transducing the insulin signal via phosphorylation, protein-protein interactions, and protein modifications. Mapping studies demonstrated that the SH(2) domain at the amino terminus of sequestosome 1/p62 interacts with IRS-1 upon insulin stimulation. Further, IRS-1 interacts with p62 through its YMXM motifs at Tyr-608, Tyr-628, and/or Tyr-658 in a manner similar to its interaction with p85 of phosphoinositol 3-kinase. Overexpression of p62 increased phosphorylation of Akt, GLUT4 translocation, and glucose uptake, providing evidence that p62 participates in the insulin-signaling pathway through its interactions with IRS-1.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

Protein expression of PKCZ (Protein Kinase C Zeta), Munc18c, and Syntaxin-4 in the insulin pathway in endometria of patients with polycystic ovary syndrome (PCOS)

BACKGROUND

Polycystic Ovary Syndrome (PCOS) is an endocrine-metabolic disorder commonly associated with insulin resistance (IR). Previous studies indicate about the expression of molecules involved in the insulin pathway in endometria of women with PCOS-IR. Therefore, the aim of the present study was to evaluate the effect of insulin and testosterone in the expression of these proteins in the endometria and immortal endometrial stromal cell line (T-HESCs).

METHODS

We examined the protein levels of Munc18c, PKC zeta, phospho-PKC Zeta, and Syntaxin-4. Protein levels were assessed by Western Blot and/or immunohistochemistry in proliferative endometria (NPE = 6) and in PCOS endometria with insulin resistance (PCOSE-IR = 6). We also evaluated whether high concentrations of insulin (100 nM) and/or testosterone (100 nM), during a 24 h stimulatory period, affected the expression of these proteins in an immortal endometrial stromal cell line (T-HESCs). Once stimulated, proteins were extracted from cells and were assessed by Western Blot analysis. Immunocytochemistry was performed to detect AR in T-HESC cells.

RESULTS

Western Blot data showed decreased expression (p < 0,05) of Munc18c and phospho-PKC Zeta in PCOS-IR endometria (PCOSE-IR) with respect to the control (NPE). In the in vitro study, Western Blot analysis showed decreased levels of Munc18c, PKC Zeta and phospho-PKC Zeta with the different hormonal treatments when compared to the control condition (no hormonal stimulation) (p < 0,05). The AR was present in the endometrial stromal cell line (T-HESC).

CONCLUSION

The conditions of hyperinsulinism and hyperandrogenism present in PCOS-IR patients modulate the expression and/or phosphorylation of the proteins involved in the insulin pathway at the endometrial level. These data extend to the T-HESCs cells results, where insulin and testosterone exert an effect on both the expression and phosphorylation of proteins present in the pathway.

Signaling, cytoskeletal and membrane mechanisms regulating GLUT4 exocytosis

Solving how insulin regulates glucose transport into skeletal muscle and adipose tissue remains a fundamental challenge in biology and a significant issue in medicine. A central feature of this process is the coordinated accumulation of the glucose transporter GLUT4 into the plasma membrane. New signaling and cytoskeletal mechanisms of insulin-stimulated GLUT4 exocytosis are of emerging interest, particularly those at or just beneath the plasma membrane. This review examines signals that functionally engage GLUT4 exocytosis, considers cytoskeletal regulation of the stimulated GLUT4 itinerary, and appraises the involvement of plasma membrane parameters in GLUT4 control. We also explore how these newly-defined signaling, cytoskeletal and membrane mechanisms could be of therapeutic interest in the treatment and/or prevention of GLUT4 dysregulation in disease.

Compartmentalization and regulation of insulin signaling to GLUT4 by the cytoskeleton

One of the early events in the development of Type 2 diabetes appears to be an inhibition of insulin-mediated GLUT4 redistribution to the cell surface in tissues that express GLUT4. Understanding this process, and how it begins to breakdown in the development of insulin resistance is quite important as we face treatment and prevention of metabolic diseases. Over the past few years, and increasing number of laboratories have produced compelling data to demonstrate a role for both the actin and microtubule networks in the regulation of insulin-mediated GLUT4 redistribution to the cell surface. In this review, we explore this process from insulin-signal transduction to fusion of GLUT4 membrane vesicles, focusing on studies that have implicated a role for the cytoskeleton. We see from this body of work that both the actin network and the microtubule cytoskeleton play roles as targets of insulin action and effectors of insulin signaling leading to changes in GLUT4 redistribution to the cell surface and insulin-mediated glucose uptake.

Protein phosphatase 2B inhibition promotes the secretion of von Willebrand factor from endothelial cells

BACKGROUND

Secretion of Weibel-Palade body (WPB) contents is regulated, in part, by the phosphorylation of proteins that constitute the endothelial exocytotic machinery. In comparison to protein kinases, a role for protein phosphatases in regulating endothelial exocytosis is undefined.

OBJECTIVE AND METHOD

In this study, we investigated the role of protein phosphatase 2B (PP2B) in the process of endothelial exocytosis using pharmacological and gene knockdown approaches.

RESULTS

We show that inhibition of protein phosphatase 2B (PP2B) activity by cyclosporine A (CsA), tacrolimus or a cell-permeable PP2B autoinhibitory peptide promotes the secretion of ultralarge von Willebrand factor (ULVWF) from human umbilical vein endothelial cells (HUVECs) in the absence of any other endothelial cell-stimulating agent. PP2B inhibitor-induced secretion and anchorage of ULVWF strings from HUVECs mediate platelet tethering. In support of a role for PP2B in von Willebrand factor (VWF) secretion, the catalytic subunit of PP2B interacts with the vesicle trafficking protein, Munc18c. Serine phosphorylation of Munc18c, which promotes granule exocytosis in other secretory cells, is increased in CsA-treated HUVECs, suggesting that this process may be involved in CsA-mediated WPB exocytosis. Furthermore, the plasma VWF antigen level is also enhanced in CsA-treated mice, and small interfering RNA-mediated knockdown of the alpha and beta isoforms of the PP2B-A subunit in HUVECs enhanced VWF secretion.

CONCLUSIONS

These observations suggest that CsA promotes VWF release, in part by inhibition of PP2B activity, and are compatible with the clinically observed association of CsA treatment and increased plasma VWF levels in humans.

Activation of atypical protein kinase Czeta toward TC10 is regulated by high-fat diet and aerobic exercise in skeletal muscle

We determined whether sustained aerobic exercise reverses high-fat diet-induced impairments in the c-Cbl associated protein (CAP)/Casitas b-lineage lymphoma (c-Cbl) signaling cascade in rodent skeletal muscle. Sprague-Dawley rats were placed into either control (n = 16) or high-fat-fed (n = 32) diet groups for 4 weeks. During a subsequent 4-week experimental period, 16 high-fat-fed rats remained sedentary, 16 high-fat-fed rats completed 4 weeks of exercise training, and control animals were sedentary and remained on the control diet. After the intervention period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. In the plasma membrane fractions, neither high-fat feeding nor exercise training altered adaptor protein with PH and SH2 domains, (APS), c-Cbl, or TC10 protein concentrations. In contrast, CAP protein concentration and insulin-stimulated plasma membrane c-Cbl tyrosine phosphorylation were reduced by high-fat feeding; but exercise training reversed these impairments. Of note was that insulin-stimulated atypical protein kinase Czeta kinase activity toward TC10 was reduced by high-fat feeding but normalized by exercise training. We conclude that sustained (4 weeks) exercise training can reverse high-fat diet-induced impairments on the CAP/c-Cbl pathway in high-fat-fed rodent skeletal muscle. We also provide the first evidence that the CAP/c-Cbl insulin signaling cascade in skeletal muscle may directly interact with components of the classic (phosphoinositide 3-kinase dependent) insulin signaling cascade.

Substrate uptake and metabolism are preserved in hypertrophic caveolin-3 knockout hearts

Caveolin-3 (Cav3), the primary protein component of caveolae in muscle cells, regulates numerous signaling pathways including insulin receptor signaling and facilitates free fatty acid (FA) uptake by interacting with several FA transport proteins. We previously reported that Cav3 knockout mice (Cav3KO) develop cardiac hypertrophy with diminished contractile function; however, the effects of Cav3 gene ablation on cardiac substrate utilization are unknown. The present study revealed that the uptake and oxidation of FAs and glucose were normal in hypertrophic Cav3KO hearts. Real-time PCR analysis revealed normal expression of lipid metabolism genes including FA translocase (CD36) and carnitine palmitoyl transferase-1 in Cav3KO hearts. Interestingly, myocardial cAMP content was significantly increased by 42%; however, this had no effect on PKA activity in Cav3KO hearts. Microarray expression analysis revealed a marked increase in the expression of genes involved in receptor trafficking to the plasma membrane, including Rab4a and the expression of WD repeat/FYVE domain containing proteins. We observed a fourfold increase in the expression of cellular retinol binding protein-III and a 3.5-fold increase in 17beta-hydroxysteroid dehydrogenase type 11, a member of the short-chain dehydrogenase/reductase family involved in the biosynthesis and inactivation of steroid hormones. In summary, a loss of Cav3 in the heart leads to cardiac hypertrophy with normal substrate utilization. Moreover, a loss of Cav3 mRNA altered the expression of several genes not previously linked to cardiac growth and function. Thus we have identified a number of new target genes associated with the pathogenesis of cardiac hypertrophy.

An intracellular motif of GLUT4 regulates fusion of GLUT4-containing vesicles

Background

Insulin stimulates glucose uptake by adipocytes through increasing translocation of the glucose transporter GLUT4 from an intracellular compartment to the plasma membrane. Fusion of GLUT4-containing vesicles at the cell surface is thought to involve phospholipase D activity, generating the signalling lipid phosphatidic acid, although the mechanism of action is not yet clear.

Results

Here we report the identification of a putative phosphatidic acid-binding motif in a GLUT4 intracellular loop. Mutation of this motif causes a decrease in the insulin-induced exposure of GLUT4 at the cell surface of 3T3-L1 adipocytes via an effect on vesicle fusion.

Conclusion

The potential phosphatidic acid-binding motif identified in this study is unique to GLUT4 among the sugar transporters, therefore this motif may provide a unique mechanism for regulating insulin-induced translocation by phospholipase D signalling.

Insulin-triggered repositioning of munc18c on syntaxin-4 in GLUT4 signalling

One of the most important actions of insulin is the stimulation of the uptake of glucose into fat and muscle cells. Crucial to this response is the translocation of GLUT4 (glucose transporter-4) to the plasma membrane. The insulin-stimulated GLUT4 vesicle docking at the plasma membrane requires an interaction between VAMP-2 (vesicle-associated membrane protein-2) on the GLUT4 vesicle and syntaxin-4 in the plasma membrane. In the basal state, munc18c is thought to preclude GLUT4 vesicle docking by inhibiting this interaction. Here, we have used FCS (fluorescence correlation spectroscopy) in single living cells to show that munc18c binds to syntaxin-4 in both the basal and insulin-stimulated states. We show that munc18c contains two binding sites for syntaxin-4, one of which is disrupted by insulin, while the other is activated by insulin. Insulin-triggered repositioning of munc18c on syntaxin-4 in this way in turn allows syntaxin-4 to adopt its 'open' conformation and bind VAMP-2, resulting in the docking of the GLUT4 vesicle at the cell surface. The results also demonstrate the utility of using FCS in intact single living cells to elucidate cell signalling events.

Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle

The aims of this investigation were 1) to determine whether endurance exercise training could reverse impairments in insulin-stimulated compartmentalization and/or activation of aPKCzeta/lambda and Akt2 in skeletal muscle from high-fat-fed rodents and 2) to assess whether the PPARgamma agonist rosiglitazone could reverse impairments in skeletal muscle insulin signaling typically observed after high-fat feeding. Sprague-Dawley rats were placed on chow (NORCON, n = 16) or high-fat (n = 64) diets for 4 wk. During a subsequent 4-wk experimental period, high-fat-fed rats were allocated (n = 16/group) to either sedentary control (HFC), exercise training (HFX), rosiglitazone treatment (HFRSG), or a combination of both exercise training and rosiglitazone (HFRX). Following the 4-wk experimental period, animals underwent hindlimb perfusions. Insulin-stimulated plasma membrane-associated aPKCzeta and -lambda protein concentration, aPKCzeta/lambda activity, GLUT4 protein concentration, cytosolic Akt2, and aPKCzeta/lambda activities were reduced (P < 0.05) in HFC compared with NORCON. Cytosolic Akt2, aPKCzeta, and aPKClambda protein concentrations were not affected in HFC compared with NORCON. Exercise training reversed the deleterious effects of the high-fat diet such that insulin-stimulated compartmentalization and activation of components of the insulin-signaling cascade in HFX were normalized to NORCON. High-fat diet-induced impairments to skeletal muscle glucose metabolism were not reversed by rosiglitazone administration, nor did rosiglitazone augment the effect of exercise. Our findings indicate that chronic exercise training, but not rosiglitazone, reverses high-fat diet induced impairments in compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

Ins (endocytosis) and outs (exocytosis) of GLUT4 trafficking

Glucose transporter 4 (GLUT4) is the major insulin-regulated glucose transporter expressed mainly in muscle and adipose tissue. GLUT4 is stored in a poorly characterized intracellular vesicular compartment and translocates to the cell surface in response to insulin stimulation resulting in an increased glucose uptake. This process is essential for the maintenance of normal glucose homeostasis and involves a complex interplay of trafficking events and intracellular signaling cascades. Recent studies have identified sortilin as an essential element for the formation of GLUT4 storage vesicles during adipogenesis and Golgi-localized gamma-ear-containing Arf-binding protein (GGA) as a key coat adaptor for the entry of newly synthesized GLUT4 into the specialized compartment. Insulin-stimulated GLUT4 translocation from this compartment to the plasma membrane appears to require the Akt/protein kinase B substrate termed AS160 (Akt substrate of 160kDa). In addition, the VPS9 domain-containing protein Gapex-5 in complex with CIP4 appears to function as a Rab31 guanylnucleotide exchange factor that is necessary for insulin-stimulated GLUT4 translocation. Here, we attempt to summarize recent advances in GLUT4 vesicle biogenesis, intracellular trafficking and membrane fusion.

The role of phosphoinositide 3-kinase C2alpha in insulin signaling

The members of the class II phosphoinositide 3-kinase (PI3K) family can be activated by several stimuli, indicating that these enzymes can regulate many intracellular processes. Nevertheless, to date, there has been no definitive identification of their in vivo product, their mechanism(s) of activation, or their precise intracellular roles. By metabolic labeling, we here identify phosphatidylinositol 3-phosphate as the sole in vivo product of the insulin-dependent activation of PI3K-C2alpha, confirming the emerging role of such a phosphoinositide in signaling. We demonstrate that activation of PI3K-C2alpha involves its recruitment to the plasma membrane and that activation is mediated by the GTPase TC10. This is the first report showing a membrane targeting-mediated mechanism of activation for PI3K-C2alpha and that a small GTP-binding protein can activate a class II PI3K isoform. We also demonstrate that PI3K-C2alpha contributes to maximal insulin-induced translocation of the glucose transporter GLUT4 to the plasma membrane and subsequent glucose uptake, definitely assessing the role of this enzyme in insulin signaling.

Munc18c interaction with syntaxin 4 monomers and SNARE complex intermediates in GLUT4 vesicle trafficking

In the process of insulin-stimulated GLUT4 vesicle exocytosis, Munc18c has been proposed to control SNARE complex formation by inactivating syntaxin 4 in a self-associated conformation. Using in vivo fluorescence resonance energy transfer in 3T3L1 adipocytes, co-immunoprecipitation, and in vitro binding assays, we provide data to indicate that Munc18c also associates with nearly equal affinity to a mutant of syntaxin 4 in a constitutively open (unfolded) state (L173A/E174A; LE). To bind to the open conformation of syntaxin 4, we found that Munc18c requires an interaction with the N terminus of syntaxin 4, which resembles Sly1 interaction with the N terminus of ER/Golgi syntaxins. However, both N and C termini of syntaxin 4 are required for Munc18c binding, since a mutation in the syntaxin 4 SNARE domain (I241A) reduces the interaction, irrespective of syntaxin 4 conformation. Using an optical reporter for syntaxin 4-SNARE pairings in vivo, we demonstrate that Munc18c blocks recruitment of SNAP23 to wild type syntaxin 4 yet associates with syntaxin 4LE-SNAP23 Q-SNARE complexes. Fluorescent imaging of GLUT4 vesicles in 3T3L1 adipocytes revealed that syntaxin 4LE expressed with Munc18c bypasses the requirement of insulin for GLUT4 vesicle plasma membrane docking. This effect was attenuated by reducing the Munc18c-syntaxin 4LE interaction with the I241A mutation, indicating that Munc18c facilitates vesicle docking. Therefore, in contradiction to previous models, our data indicates that the conformational "opening" of syntaxin 4 rather than the dissociation of Munc18c is the critical event required for GLUT4 vesicle docking.

Insulin signaling diverges into Akt-dependent and -independent signals to regulate the recruitment/docking and the fusion of GLUT4 vesicles to the plasma membrane

Insulin modulates glucose disposal in muscle and adipose tissue by regulating the cellular redistribution of the GLUT4 glucose transporter. Protein kinase Akt/PKB is a central mediator of insulin-regulated translocation of GLUT4; however, the GLUT4 trafficking step(s) regulated by Akt is not known. Here, we use acute pharmacological Akt inhibition to show that Akt is required for insulin-stimulated exocytosis of GLUT4 to the plasma membrane. Our data also suggest that the AS160 Rab GAP is not the only Akt target required for insulin-stimulated GLUT4 translocation. Using a total internal reflection microscopy assay, we show that Akt activity is specifically required for an insulin-mediated prefusion step involving the recruitment and/or docking of GLUT4 vesicles to within 250 nm of the plasma membrane. Moreover, the insulin-stimulated fusion of GLUT4 vesicles with the plasma membrane can occur independently of Akt activity, although based on inhibition by wortmannin, it is dependent on phosphatidylinositol 3' kinase activity. Hence, to achieve full redistribution of GLUT4 into the plasma membrane, insulin signaling bifurcates to independently regulate both fusion and a prefusion step(s).

How many signals impinge on GLUT4 activation by insulin?

GLUT4 is the main glucose transporter activated by insulin in skeletal muscle cells and adipocytes. GLUT4 storage vesicles (GSVs) traffic in endocytic and exocytic compartments. In the basal state, GLUT4 compartments are preferentially sequestered in perinuclear deposits wherein stimuli including insulin and non-insulin factors can increase GLUT4 vesicle formation, its exocytosis, and fusion to plasma membrane. In addition to well-established effectors of insulin signaling pathway, such as PKCzeta and Akt, the cytoskeletal network is implicated in GLUT4 translocation. This review will discuss the mechanisms and activation of GLUT4 trafficking and incorporating to PM from three aspects: known molecules of the insulin signaling pathway; Rho and Rab family proteins and cytoskeletal molecules.

Perturbed skeletal muscle insulin signaling in the adult female intrauterine growth-restricted rat

To determine the molecular mechanism(s) linking fetal adaptations in intrauterine growth restriction (IUGR) to adult maladaptations of type 2 diabetes mellitus, we investigated the effect of prenatal seminutrient restriction, modified by early postnatal ad libitum access to nutrients (CM/SP) or seminutrient restriction (SM/SP), vs. early postnatal seminutrient restriction alone (SM/CP) or control nutrition (CM/CP) on the skeletal muscle postreceptor insulin-signaling pathway in the adult offspring. The altered in utero hormonal/metabolic milieu was associated with no change in basal total IRS-1, p85, and p110beta subunits of PI 3-kinase, PKCtheta, and PKCzeta concentrations but an increase in basal IRS-2 (P < 0.05) only in the CM/SP group and an increase in basal phospho (p)-PDK-1 (P < 0.05), p-Akt (P < 0.05), and p-PKCzeta (P < 0.05) concentrations in the CM/SP and SM/SP groups. Insulin-stimulated increases in p-PDK-1 (P < 0.05) and p-Akt (P < 0.0007), with no increase in p-PKCzeta, were seen in both CM/SP and SM/SP groups. SHP2 (P < 0.03) and PTP1B (P < 0.03) increased only in SM/SP with no change in PTEN in CM/SP and SM/SP groups. Aberrations in kinase and phosphatase moieties in the adult IUGR offspring were initiated in utero but further sculpted by the early postnatal nutritional state. Although the CM/SP group demonstrated enhanced kinase activation, the SM/SP group revealed an added increase in phosphatase concentrations with the net result of heightened basal insulin sensitivity in both groups. The inability to further respond to exogenous insulin was due to the key molecular distal roadblock consisting of resistance to phosphorylate and activate PKCzeta necessary for GLUT4 translocation. This protective adaptation may become maladaptive and serve as a forerunner for gestational and type 2 diabetes mellitus.