Push/pull mechanisms of GLUT4 traffic in muscle cells

TMF1 is upregulated by insulin and is required for a sustained glucose homeostasis

Insulin-regulated glucose homeostasis is a critical and intricate physiological process, of which not all regulatory components have been deciphered. One of the key players in modulating glucose uptake by cells is the glucose transporter-GLUT4. In this study, we aimed to explore the regulatory role of the trans-Golgi-associated protein-TATA Element Modulatory Factor (TMF1) in the GLUT4 mediated, insulin-directed glucose uptake. By establishing and using TMF1-/- myoblasts and mice, we examined the effect of TMF1 absence on the insulin driven functioning of GLUT4. We show that TMF1 is upregulated by insulin in myoblasts, and is essential for the formation of insulin responsive, glucose transporter GLUT4-containing vesicles. Absence of TMF1 leads to the retention of GLUT4 in perinuclear compartments, and to severe impairment of insulin-stimulated GLUT4 trafficking throughout the cytoplasm and to the cell plasma membrane. Accordingly, glucose uptake is impaired in TMF1-/- cells, and TMF1-/- mice are hyperglycemic. This is reflected by the mice impaired blood glucose clearance and increased blood glucose level. Correspondingly, TMF1-/- animals are leaner than their normal littermates. Thus, TMF1 is a novel effector of insulin-regulated glucose homeostasis, and dys-functioning of this protein may contribute to the onset of a diabetes-like disorder.

GLUT4 Translocation in Single Muscle Cells in Culture: Epitope Detection by Immunofluorescence

GLUT4 is the major glucose transporter in skeletal muscle. GLUT4 cycles to and from the plasma membrane and its exocytic rate is accelerated by insulin and muscle contraction to achieve a new steady state with more GLUT4 proteins at the muscle cell surface. To gain a better understanding of the molecular and cellular mechanisms that govern GLUT4 protein recycling, we developed an in vitro model in which myc-epitope-tagged GLUT4 or GLUT4-GFP is expressed in L6 skeletal muscle cells. The myc-epitope is inserted into an exofacial domain that is accessible to anti-myc antibodies from the outside of non-permeabilized cells, allowing one to count the number of transporters at the cell surface. This enables one to perform single-cell analysis using confocal fluorescence microscopy to quantify cell surface GLUT4myc or GLUT4myc-GFP in cells co-transfected with diverse cDNA constructs, treated with siRNAs, or co-stained with antibodies for other proteins of interest. Herein, we describe the methodology to perform these experimental approaches in insulin-stimulated L6 muscle cells.

Electrical pulse stimulation of cultured skeletal muscle cells as a model for in vitro exercise - possibilities and limitations

The beneficial health-related effects of exercise are well recognized, and numerous studies have investigated underlying mechanism using various in vivo and in vitro models. Although electrical pulse stimulation (EPS) for the induction of muscle contraction has been used for quite some time, its application on cultured skeletal muscle cells of animal or human origin as a model of in vitro exercise is a more recent development. In this review, we compare in vivo exercise and in vitro EPS with regard to effects on signalling, expression level and metabolism. We provide a comprehensive overview of different EPS protocols and their applications, discuss technical aspects of this model including critical controls and the importance of a proper maintenance procedure and finally discuss the limitations of the EPS model.

Serotonin improves glucose metabolism by Serotonylation of the small GTPase Rab4 in L6 skeletal muscle cells

BACKGROUND

Serotonin (5-HT) improves insulin sensitivity and glucose metabolism, however, the underlying molecular mechanism has remained elusive. Previous studies suggest that 5-HT can activate intracellular small GTPases directly by covalent binding, a process termed serotonylation. Activated small GTPases have been associated with increased GLUT4 translocation to the cell membrane. Therefore, we investigated whether serotonylation of small GTPases may be involved in improving Insulin sensitivity and glucose metabolism.

METHODS

Using fully differentiated L6 rat skeletal muscle cells, we studied the effect of 5-HT in the absence or presence of insulin on glycogen synthesis, glucose uptake and GLUT4 translocation. To prove our L6 model we additionally performed preliminary experiments in C2C12 murine skeletal muscle cells.

RESULTS

Incubation with 5-HT led to an increase in deoxyglucose uptake in a concentration-dependent fashion. Accordingly, GLUT4 translocation to the cell membrane and glycogen content were increased. These effects of 5-HT on Glucose metabolism could be augmented by co-incubation with insulin and blunted by co incubation of 5-HT with monodansylcadaverine, an inhibitor of protein serotonylation. In accordance with this observation, incubation with 5-HT resulted in serotonylation of a protein with a molecular weight of approximately 25 kDa. We identified this protein as the small GTPase Rab4, the activity of which has been shown to be stimulated by both insulin signalling and serotonylation.

CONCLUSION

Our data suggest that 5-HT elicits its beneficial effects on Glucose metabolism through serotonylation of Rab4, which likely represents the converging point between the insulin and the 5-HT signalling cascades.

Polymeric carrier-mediated intracellular delivery of phosphatidylinositol-3,4,5-trisphosphate to overcome insulin resistance

BACKGROUND

Phosphatidylinositol-3,4,5-trisphosphate (PIP3) is a major lipid second messenger in insulin-mediated signalling towards the metabolic actions of this hormone in muscle and fat.

PURPOSE

Assessing the intracellular transport of exogenous PIP3 attached to a polymeric carrier in an attempt to overcome cellular insulin resistance.

METHODS

Artificial chromatic bio-mimetic membrane vesicles composed of dimyristoylphosphatidylcholine and polydiacetylene were applied to screen the polymeric carriers. PIP3 cellular localization and bio-activity was assessed by fluorescent and live-cell microscopy in L6 muscle cells and in 3T3-L1 adipocytes.

RESULTS AND DISCUSSION

We demonstrate that a specific-branched polyethylenimine (PEI-25, 25 kDa) carrier forms complexes with PIP3 that interact with the bio-mimetic membrane vesicles in a manner predictive of their interaction with cells: In L6 muscle cells, PEI-25/fluorescent-PIP3 complexes are retarded at the cell perimeter. PEI-25/PIP3 complexes retain their bio-activity, engaging signalling steps downstream of PIP3, even in muscle cells rendered insulin resistant by exposure to high glucose/high insulin.

CONCLUSIONS

Inducing insulin actions by intracellular PIP3 delivery (PEI-25/PIP3 complexes) in some forms of insulin-resistant cells provides the first proof-of-principle for the potential therapeutic use of PIP3 in a "second-messenger agonist" approach. In addition, utilization of an artificial bio-mimetic membrane platform to screen for highly efficient PIP3 delivery predicts biological function in cells.

Mitochondrial fragmentation impairs insulin-dependent glucose uptake by modulating Akt activity through mitochondrial Ca2+ uptake

Insulin is a major regulator of glucose metabolism, stimulating its mitochondrial oxidation in skeletal muscle cells. Mitochondria are dynamic organelles that can undergo structural remodeling in order to cope with these ever-changing metabolic demands. However, the process by which mitochondrial morphology impacts insulin signaling in the skeletal muscle cells remains uncertain. To address this question, we silenced the mitochondrial fusion proteins Mfn2 and Opa1 and assessed insulin-dependent responses in L6 rat skeletal muscle cells. We found that mitochondrial fragmentation attenuates insulin-stimulated Akt phosphorylation, glucose uptake and cell respiratory rate. Importantly, we found that insulin induces a transient rise in mitochondrial Ca(2+) uptake, which was attenuated by silencing Opa1 or Mfn2. Moreover, treatment with Ruthenium red, an inhibitor of mitochondrial Ca(2+) uptake, impairs Akt signaling without affecting mitochondrial dynamics. All together, these results suggest that control of mitochondrial Ca(2+) uptake by mitochondrial morphology is a key event for insulin-induced glucose uptake.

PI3 kinase directly phosphorylates Akt1/2 at Ser473/474 in the insulin signal transduction pathway

Insulin stimulated translocation of the glucose transporter GLUT4 from the cytosol to the plasma membrane in a concentration (1  nM-1  μM)-dependent manner and increased glucose uptake in 3T3-L1 adipocytes. Insulin-induced GLUT4 translocation to the cell surface was prevented by the phosphoinositide 3 kinase (PI3K) inhibitor wortmannin, the 3-phosphoinositide-dependent protein kinase 1 (PDK1) inhibitor BX912 or the Akt1/2 inhibitor MK2206, and by knocking-down PI3K, PDK1 or Akt1/2. Insulin increased phosphorylation of Akt1/2 at Thr308/309 and Ser473/474, to activate Akt1/2, in the adipocytes. Insulin-induced phosphorylation of Akt1/2 was suppressed by wortmannin and knocking-down PI3K, while no significant inhibition of the phosphorylation was obtained with BX912 or knocking-down PDK1. In the cell-free Akt assay, PI3K phosphorylated Akt1 both at Thr308 and Ser473 and Akt2 at Ser474 alone. In contrast, PDK1 phosphorylates Akt1 at Thr308 and Akt2 at Thr309. The results of this study indicate that PI3K activates Akt1, independently of PDK1, and Akt2 by cooperating with PDK1 in the insulin signal transduction pathway linked to GLUT4 translocation.

Muscle-specific Pikfyve gene disruption causes glucose intolerance, insulin resistance, adiposity, and hyperinsulinemia but not muscle fiber-type switching

The evolutionarily conserved kinase PIKfyve that synthesizes PtdIns5P and PtdIns(3,5)P₂ has been implicated in insulin-regulated GLUT4 translocation/glucose entry in 3T3-L1 adipocytes. To decipher PIKfyve's role in muscle and systemic glucose metabolism, here we have developed a novel mouse model with Pikfyve gene disruption in striated muscle (MPIfKO). These mice exhibited systemic glucose intolerance and insulin resistance at an early age but had unaltered muscle mass or proportion of slow/fast-twitch muscle fibers. Insulin stimulation of in vivo or ex vivo glucose uptake and GLUT4 surface translocation was severely blunted in skeletal muscle. These changes were associated with premature attenuation of Akt phosphorylation in response to in vivo insulin, as tested in young mice. Starting at 10-11 wk of age, MPIfKO mice progressively accumulated greater body weight and fat mass. Despite increased adiposity, serum free fatty acid and triglyceride levels were normal until adulthood. Together with the undetectable lipid accumulation in liver, these data suggest that lipotoxicity and muscle fiber switching do not contribute to muscle insulin resistance in MPIfKO mice. Furthermore, the 80% increase in total fat mass resulted from increased fat cell size rather than altered fat cell number. The observed profound hyperinsulinemia combined with the documented increases in constitutive Akt activation, in vivo glucose uptake, and gene expression of key enzymes for fatty acid biosynthesis in MPIfKO fat tissue suggest that the latter is being sensitized for de novo lipid anabolism. Our data provide the first in vivo evidence that PIKfyve is essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle.

Regulated recycling of mutant CFTR is partially restored by pharmacological treatment

Efficient trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) to and from the cell surface is essential for maintaining channel density at the plasma membrane (PM) and ensuring proper physiological activity. The most common mutation, F508del, exhibits reduced surface expression and impaired function despite treatment with currently available pharmacological small molecules, called correctors. To gain more detailed insight into whether CFTR enters compartments that allow corrector stabilization in the cell periphery, we investigated the peripheral trafficking itineraries and kinetics of wild type (WT) and F508del in living cells using high-speed fluorescence microscopy together with fluorogen activating protein detection. We directly visualized internalization and accumulation of CFTR WT from the PM to a perinuclear compartment that colocalized with the endosomal recycling compartment (ERC) markers Rab11 and EHD1, reaching steady-state distribution by 25 minutes. Stimulation by protein kinase A (PKA) depleted this intracellular pool and redistributed CFTR channels to the cell surface, elicited by reduced endocytosis and active translocation to the PM. Corrector or temperature rescue of F508del also resulted in targeting to the ERC and exhibited subsequent PKA-stimulated trafficking to the PM. Corrector treatment (24 hours) led to persistent residence of F508del in the ERC, while thermally destabilized F508del was targeted to lysosomal compartments by 3 hours. Acute addition of individual correctors, C4 or C18, acted on peripheral trafficking steps to partially block lysosomal targeting of thermally destabilized F508del. Taken together, corrector treatment redirects F508del trafficking from a degradative pathway to a regulated recycling route, and proteins that mediate this process become potential targets for improving the efficacy of current and future correctors.

Subcellular trafficking of the substrate transporters GLUT4 and CD36 in cardiomyocytes

Cardiomyocytes use glucose as well as fatty acids for ATP production. These substrates are transported into the cell by glucose transporter 4 (GLUT4) and the fatty acid transporter CD36. Besides being located at the sarcolemma, GLUT4 and CD36 are stored in intracellular compartments. Raised plasma insulin concentrations and increased cardiac work will stimulate GLUT4 as well as CD36 to translocate to the sarcolemma. As so far studied, signaling pathways that regulate GLUT4 translocation similarly affect CD36 translocation. During the development of insulin resistance and type 2 diabetes, CD36 becomes permanently localized at the sarcolemma, whereas GLUT4 internalizes. This juxtaposed positioning of GLUT4 and CD36 is important for aberrant substrate uptake in the diabetic heart: chronically increased fatty acid uptake at the expense of glucose. To explain the differences in subcellular localization of GLUT4 and CD36 in type 2 diabetes, recent research has focused on the role of proteins involved in trafficking of cargo between subcellular compartments. Several of these proteins appear to be similarly involved in both GLUT4 and CD36 translocation. Others, however, have different roles in either GLUT4 or CD36 translocation. These trafficking components, which are differently involved in GLUT4 or CD36 translocation, may be considered novel targets for the development of therapies to restore the imbalanced substrate utilization that occurs in obesity, insulin resistance and diabetic cardiomyopathy.

Observing GLUT4 translocation in live L6 cells using quantum dots

The glucose transporter 4 (GLUT4) plays a key role in maintaining whole body glucose homeostasis. Tracking GLUT4 in space and time can provide new insights for understanding the mechanisms of insulin-regulated GLUT4 translocation. Organic dyes and fluorescent proteins were used in previous studies for investigating the traffic of GLUT4 in skeletal muscle cells and adipocytes. Because of their relative weak fluorescent signal against strong cellular autofluorescence background and their fast photobleaching rate, most studies only focused on particular segments of GLUT4 traffic. In this study, we have developed a new method for observing the translocation of GLUT4 targeted with photostable and bright quantum dots (QDs) in live L6 cells. QDs were targeted to GLUT4myc specifically and internalized with GLUT4myc through receptor-mediated endocytosis. Compared with traditional fluorescence dyes and fluorescent proteins, QDs with high brightness and extremely photostability are suitable for long-term single particle tracking, so individual GLUT4-QD complex can be easily detected and tracked for long periods of time. This newly described method will be a powerful tool for observing the translocation of GLUT4 in live L6 cells.

Fatty acids inhibit insulin-mediated glucose transport associated with actin remodeling in rat L6 muscle cells

In skeletal muscle cells, insulin stimulates cytoskeleton actin remodeling to facilitate the translocation of glucose transporter GLUT4 to plasma membrane. Defect of insulin-induced GLUT4 translocation and actin remodeling may cause insulin resistance. Free fatty acids cause insulin resistance in skeletal muscle. The aim of this study was to investigate the effects of fatty acids on glucose transport and actin remodeling. Differentiated L6 muscle cells expressing c-myc epitope-tagged GLUT4 were treated with palmitic acid, linoleic acid and oleic acid. Surface GLUT4 and 2-deoxyglucose uptake were measured in parallel with the morphological imaging of actin remodeling and GLUT4 immunoreactivity with fluorescence, confocal and transmission electron microscopy. Differentiated L6 cells showed concentration responses of insulin-induced actin remodeling and glucose uptake. The ultrastructure of insulin-induced actin remodeling was cell projections clustered with actin and GLUT4. Acute and chronic treatment with the 3 fatty acids had no effect on insulin-induced actin remodeling and GLUT4 immunoreactivity. However, insulin-mediated glucose uptake significantly decreased by palmitic acid (25, 50, 75, 100 μmol/L), oleic acid (180, 300 μmol/L) and linoleic acid (120, 180, 300 μmol/L). Oleic acid (120, 300 μmol/L) and linoleic acid (300 μmol/L), but not palmitic acid, significantly decreased insulin-mediated GLUT4 translocation. These data suggest that fatty acids inhibit insulin-induced glucose transport associated with actin remodeling in L6 muscle cells.

Measuring GLUT4 translocation in mature muscle fibers

Skeletal muscle is the major tissue for postprandial glucose disposal. Facilitated glucose uptake into muscle fibers is mediated by increases in surface membrane levels of the glucose transporter GLUT4 via insulin- and/or muscle contraction-mediated GLUT4 translocation. However, the regulatory mechanisms controlling GLUT4 translocation in skeletal muscle have been difficult to characterize at the cell biology level due to muscle tissue complexity. Muscle cell culture models have improved our understanding of GLUT4 translocation and glucose transport regulation, but in vitro muscle models lack many of the characteristics of mature muscle fibers. Thus, the molecular and cellular details of GLUT4 translocation in mature skeletal muscle are deficient. The objective of this review is to highlight how advances in recent experimental approaches translate into an enhanced understanding of the regulation of GLUT4 translocation and glucose transport in mature skeletal muscle.

Contraction-related stimuli regulate GLUT4 traffic in C2C12-GLUT4myc skeletal muscle cells

Muscle contraction stimulates glucose uptake acutely to increase energy supply, but suitable cellular models that faithfully reproduce this complex phenomenon are lacking. To this end, we have developed a cellular model of contracting C(2)C(12) myotubes overexpressing GLUT4 with an exofacial myc-epitope tag (GLUT4myc) and explored stimulation of GLUT4 traffic by physiologically relevant agents. Carbachol (an acetylcholine receptor agonist) induced a gain in cell surface GLUT4myc that was mediated by nicotinic acetylcholine receptors. Carbachol also activated AMPK, and this response was sensitive to the contractile myosin ATPase inhibitor N-benzyl-p-toluenesulfonamide. The gain in surface GLUT4myc elicited by carbachol or by the AMPK activator 5-amino-4-carboxamide-1 beta-ribose was sensitive to chemical inhibition of AMPK activity by compound C and partially reduced by siRNA-mediated knockdown of AMPK catalytic subunits or LKB1. In addition, the carbachol-induced gain in cell surface GLUT4myc was partially sensitive to chelation of intracellular calcium with BAPTA-AM. However, the carbachol-induced gain in cell surface GLUT4myc was not sensitive to the CaMKK inhibitor STO-609 despite expression of both isoforms of this enzyme and a rise in cytosolic calcium by carbachol. Therefore, separate AMPK- and calcium-dependent signals contribute to mobilizing GLUT4 in response to carbachol, providing an in vitro cell model that recapitulates the two major signals whereby acute contraction regulates glucose uptake in skeletal muscle. This system will be ideal to further analyze the underlying molecular events of contraction-regulated GLUT4 traffic.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Labeling and imaging of GLUT4 in live L6 cells with quantum dots

GLUT4 is sequestered in intracellular storage compartments in a basal state and is rapidly translocated to the cell surface in response to insulin stimulation. Regulation of GLUT4 distribution is key for maintaining whole-body glucose homeostasis. To investigate the complicated intracellular movement of GLUT4 vesicles and their interactions with organelles in detail, new probes suitable for long-term tracking of cellular events are required. In this study, we introduce for the first time quantum dots (QDs) as a superior probe into the research of the mechanisms of GLUT4 translocation. QDs are light-emitting semiconductor nanoparticles with unique optical and spectroscopic properties, such as broad absorption, narrow and tunable emission, resistance to photobleaching, strong luminescence, and long luminescent lifetimes. Owing to their remarkable photophysical properties and relatively small size, QDs are emerging as an alternative to conventional dyes for fluorescence-based applications. We have developed a procedure for labeling and imaging GLUT4 in live cells with streptavidin-conjugated quantum dot (QD-SA) and demonstrated that QDs contained in cytoplasm have no obvious negative influence on L6 cells. This study provides a sensitive, nontoxic, long-term imaging platform for observing the dynamics and regulated characteristics of GLUT4 transport.

Insulin-stimulated translocation of glucose transporter (GLUT) 12 parallels that of GLUT4 in normal muscle

CONTEXT

GLUT4 is the predominant glucose transporter isoform expressed in fat and muscle. In GLUT4 null mice, insulin-stimulated glucose uptake into muscle was diminished but not eliminated, suggesting that another insulin-sensitive system was present.

OBJECTIVE

This study was intended to determine whether insulin caused GLUT12 translocation in muscle.

DESIGN

Six normal volunteers had muscle biopsies before and after euglycemic insulin infusions.

SETTING

Infusions and biopsies were performed in an outpatient clinic.

PARTICIPANTS

Subjects were nonobese, young adults with no family history of diabetes.

MAIN OUTCOME MEASURES

GLUT12, GLUT4, and GLUT1 proteins were quantified in muscle biopsy fractions. Cultured myoblasts were used to determine whether GLUT12 translocation was phosphatidyl inositol-3 kinase (PI3-K)-dependent.

INTERVENTION

Insulin was infused at 40 mU/m(2) x min for 3 h.

RESULTS

In human muscle, insulin caused a shift of a portion of GLUT12 from intracellular low-density microsomes to the plasma membrane (PM) fraction (17% in PM at baseline, 38% in PM after insulin). Insulin increased GLUT4 in PM from 13 to 42%. GLUT1 was predominantly in the PM fractions at baseline and did not change significantly after insulin. L6 myoblasts in culture also expressed and translocated GLUT12 in response to insulin, but inhibiting PI3-K prevented the translocation of GLUT12 and GLUT4.

CONCLUSIONS

Insulin causes GLUT12 to translocate from an intracellular location to the plasma membrane in normal human skeletal muscle. Translocation of GLUT12 in cultured myoblasts was dependent on activation of PI3-K. GLUT12 may have evolutionarily preceded GLUT4 and now provides redundancy to the dominant GLUT4 system in muscle.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

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

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

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

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

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

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

Regulation of sarcolemmal glucose and fatty acid transporters in cardiac disease

Circulating long-chain fatty acids (LCFA) and glucose are the main sources for energy production in the heart. In the healthy heart the ratio of glucose and LCFA oxidation is sensitively balanced and chronic alterations in this substrate mix are closely associated with cardiac dysfunction. While it has been accepted for several years that cardiac glucose uptake is mediated by facilitated transport, i.e. by means of the glucose transport proteins GLUT1 and GLUT4, only in the last few years it has become clear that proteins with high-affinity binding sites to LCFA, referred to as LCFA transporters, are responsible for bulk LCFA uptake. Similar to the GLUTs, the LCFA transporters CD36 and FABP(pm) can be recruited from an intracellular storage compartment to the sarcolemma to increase the rate of substrate uptake. Permanent relocation of LCFA transporters, mainly CD36, from intracellular stores to the sarcolemma is accompanied by accumulation of lipids and lipid metabolites in the heart. As a consequence, insulin signalling and glucose utilization are impaired, leading to decreased contractile activity of the heart. These observations underline the particular role and interplay of substrate carriers for glucose and LCFA in modulating cardiac metabolism, and the development of heart failure. The signalling and trafficking pathways and subcellular machinery regulating translocation of glucose and LCFA transporters are beginning to be unravelled. More knowledge on substrate transporter recycling, especially the similarities and differences between glucose and LCFA transporters, is expected to enable novel therapies aimed at changing the subcellular distribution of glucose and LCFA transporters, thereby manipulating the substrate preference of the diseased heart to help restore cardiac function.

Functional role of sortilin in myogenesis and development of insulin-responsive glucose transport system in C2C12 myocytes

Sortilin has been implicated in the formation of insulin-responsive GLUT4 storage vesicles in adipocytes by regulating sorting events between the trans-Golgi-network and endosomes. We herein show that sortilin serves as a potent myogenic differentiation stimulator for C2C12 myocytes by cooperatively functioning with p75NTR, which subsequently further contributes to development of the insulin-responsive glucose transport system in C2C12 myotubes. Sortilin expression was up-regulated upon C2C12 differentiation, and overexpression of sortilin in C2C12 cells significantly stimulated myogenic differentiation, a response that was completely abolished by either anti-p75NTR- or anti-nerve growth factor (NGF)-neutralizing antibodies. Importantly, small interference RNA-mediated suppression of endogenous sortilin significantly inhibited C2C12 differentiation, indicating the physiological significance of sortilin expression in the process of myogenesis. Although sortilin overexpression in C2C12 myotubes improved insulin-induced 2-deoxyglucose uptake, as previously reported, this effect apparently resulted from a decrease in the cellular content of GLUT1 and an increase in GLUT4 via differentiation-dependent alterations at both the gene transcriptional and the post-translational level. In addition, cellular contents of Ubc9 and SUMO-modified proteins appeared to be increased by sortilin overexpression. Taken together, these data demonstrate that sortilin is involved not only in development of the insulin-responsive glucose transport system in myocytes, but is also directly involved in muscle differentiation via modulation of proNGF-p75NTR.

Large GLUT4 vesicles are stationary while locally and reversibly depleted during transient insulin stimulation of skeletal muscle of living mice: imaging analysis of GLUT4-enhanced green fluorescent protein vesicle dynamics

OBJECTIVE

Insulin stimulates glucose transport in skeletal muscle by GLUT4 translocation from intracellular compartments to sarcolemma and t-tubules. We studied in living animals the recruitment of GLUT4 vesicles in more detail than previously done and, for the first time, analyzed the steady-state recycling and subsequent re-internalization of GLUT4 on an insulin bolus.

RESEARCH DESIGN AND METHODS

A confocal imaging technique was used in GLUT4-enhanced green fluorescent protein-transfected superficial muscle fibers in living mice.

RESULTS

During the first 30 min of insulin stimulation, very few superficially or deeply located GLUT4 storage vesicles (>1 microm) moved in toto. Rather, big vesicles were stationary in their original position at sarcolemma or t-tubules and were locally depleted of GLUT4 by budding off of smaller vesicles. Photobleaching experiments revealed that during initial translocation and steady-state recycling, GLUT4 microvesicles (<1 microm) move from perinuclear GLUT4 depots out along the plasma membrane. Furthermore, after photobleaching of t-tubule areas, recovery of GLUT4 was slow or absent, indicating no recycling of GLUT4 from perinuclear or adjacent (1 microm) or more distant (20 microm) t-tubule areas. During waning of insulin effect, GLUT4 was re-internalized to basal stores with a delay in t-tubules compared with sarcolemma, probably reflecting delayed disappearance of insulin from t-tubules.

CONCLUSIONS

In skeletal muscle, insulin reversibly stimulates local depletion of GLUT4 storage vesicles at sarcolemma and t-tubules rather than inducing movement of intact storage vesicles. During steady-state stimulation, recycling of GLUT4-containing microvesicles over longer distances (10-20 microm) takes place between perinuclear depots and sarcolemma but not at t-tubules.

Small G proteins in insulin action: Rab and Rho families at the crossroads of signal transduction and GLUT4 vesicle traffic

Insulin stimulates glucose uptake into muscle and adipose tissues through glucose transporter 4 (GLUT4). GLUT4 cycles between the intracellular compartments and the plasma membrane. GLUT4 traffic-regulating insulin signals are largely within the insulin receptor-insulin receptor substrate-phosphatidylinositol 3-kinase (IR-IRS-PI3K) axis. In muscle cells, insulin signal bifurcates downstream of the PI3K into one arm leading to the activation of the Ser/Thr kinases Akt and atypical protein kinase C, and another leading to the activation of Rho family protein Rac1 leading to actin remodelling. Activated Akt inactivates AS160, a GTPase-activating protein for Rab family small G proteins. Here we review the roles of Rab and Rho proteins, particularly Rab substrates of AS160 and Rac1, in insulin-stimulated GLUT4 traffic. We discuss: (1) how distinct steps in GLUT4 traffic may be regulated by discrete Rab proteins, and (2) the importance of Rac1 activation in insulin-induced actin remodelling in muscle cells, a key element for the net gain in surface GLUT4.

Fish glucose transporter (GLUT)-4 differs from rat GLUT4 in its traffic characteristics but can translocate to the cell surface in response to insulin in skeletal muscle cells

In mammals, glucose transporter (GLUT)-4 plays an important role in glucose homeostasis mediating insulin action to increase glucose uptake in insulin-responsive tissues. In the basal state, GLUT4 is located in intracellular compartments and upon insulin stimulation is recruited to the plasma membrane, allowing glucose entry into the cell. Compared with mammals, fish are less efficient restoring plasma glucose after dietary or exogenous glucose administration. Recently our group cloned a GLUT4-homolog in skeletal muscle from brown trout (btGLUT4) that differs in protein motifs believed to be important for endocytosis and sorting of mammalian GLUT4. To study the traffic of btGLUT4, we generated a stable L6 muscle cell line overexpressing myc-tagged btGLUT4 (btGLUT4myc). Insulin stimulated btGLUT4myc recruitment to the cell surface, although to a lesser extent than rat-GLUT4myc, and enhanced glucose uptake. Interestingly, btGLUT4myc showed a higher steady-state level at the cell surface under basal conditions than rat-GLUT4myc due to a higher rate of recycling of btGLUT4myc and not to a slower endocytic rate, compared with rat-GLUT4myc. Furthermore, unlike rat-GLUT4myc, btGLUT4myc had a diffuse distribution throughout the cytoplasm of L6 myoblasts. In primary brown trout skeletal muscle cells, insulin also promoted the translocation of endogenous btGLUT4 to the plasma membrane and enhanced glucose transport. Moreover, btGLUT4 exhibited a diffuse intracellular localization in unstimulated trout myocytes. Our data suggest that btGLUT4 is subjected to a different intracellular traffic from rat-GLUT4 and may explain the relative glucose intolerance observed in fish.

IGF-1 controls GLUT3 expression in muscle via the transcriptional factor Sp1

Glucose transporter 3 (GLUT3), while first found in human fetal muscle, is predominantly expressed in brain and neural tissue. By several independent techniques we have previously shown that GLUT3 is expressed in human skeletal muscle cells. The structure of the human GLUT3 gene has not been previously reported nor has there been any evaluation of the 5'-untranslated region (UTR). To this end, we have cloned and sequenced the human GLUT3 gene. Insulin-like growth factor-1 (IGF-1) increased endogenous Glut3 protein in cultured L6 myotubes, and similarly stimulated luciferase activity in a construct of the human GLUT3 5'-UTR linked to a luciferase reporter gene. Actinomycin D, an inhibitor of mRNA synthesis, prevented IGF-1 stimulation of Glut3 protein. Transfection of L6 cells with Sp1 increased Glut3 and augmented IGF-1 stimulation of Glut3 expression. Knockdown of Glut3 expression in cultured L6 muscle cells using small interference RNA (siRNA) specific for Glut3 significantly reduced myocyte glucose uptake. DNAse footprinting and gel shift assays showed Sp1 specifically bound to the human GLUT3 5'-UTR. Substitution mutants of the human GLUT3 5'-UTR luciferase construct indicated that only one of three Sp1 site clusters was involved in IGF-1 action. These data, using both a human GLUT3 5'-UTR construct and L6 cells' endogenous promoter, suggest that IGF-1 plays a role in maintaining muscle GLUT3 expression and basal glucose uptake via the transcriptional factor Sp1.

Protein kinase C-zeta regulation of GLUT4 translocation through actin remodeling in CHO cells

Actin remodeling plays a crucial role in insulin-induced translocation of glucose transporter 4 (GLUT4) from the cytoplasm to the plasma membrane and subsequent glucose transport. Protein kinase C (PKC) zeta has been implicated in this translocation process, although the exact mechanism remains unknown. In this study, we investigated the effect of PKCzeta on actin cytoskeleton and translocation of GLUT4 in CHO-K1 cells expressing myc-tagged GLUT4. Insulin stimulated the phosphorylation of PKCzeta at Thr410 with no apparent effect on its protein expression. Moreover, insulin promoted colocalization of PKCzeta and actin that could be abolished by Latrunculin B. The overexpression of PKCzeta mimicked the insulin-induced change in actin cytoskeleton and translocation of GLUT4. These effects were also completely abrogated by Latrunculin B treatment. Using cell-permeable pseudosubstrate (PS) inhibitor of PKCzeta, the response to insulin could be alleviated. Our results strongly suggest that PKCzeta mediates the stimulatory effect of insulin on GLUT4 translocation through its interaction with actin cytoskeleton.

Rabs 8A and 14 are targets of the insulin-regulated Rab-GAP AS160 regulating GLUT4 traffic in muscle cells

Insulin causes translocation of glucose transporter GLUT4 to the membrane of muscle and fat cells, a process requiring Akt activation. The Rab GTPase-activating protein (Rab-GAP) AS160 is inhibited upon phosphorylation by insulin-activated Akt, thereby allowing GLUT4 translocation. Although several Rab proteins are detected on GLUT4 vesicles, the target Rabs of AS160 involved in the GLUT4 translocation have not been identified. We test whether Rabs 8A, 10, and 14 (in vitro targets of AS160) rescue the inhibition of GLUT4 translocation caused by 'constitutively active' 4P-AS160 in L6 muscle cells. Coexpression of GFP-tagged Rabs 8A or Rab14 with 4P-AS160 prevented the inhibition of GLUT4 translocation imposed by 4P-AS160. GFP-tagged, constitutively active Rab8A also elicited this rescue. In contrast, neither wild-type nor constitutively active GFP-tagged Rab10 restored GLUT4 translocation. These results suggest that Rab8A and possibly Rab14 may be targets of AS160 leading to GLUT4 translocation in L6 muscle cells.

The Rab GTPase-activating protein AS160 integrates Akt, protein kinase C, and AMP-activated protein kinase signals regulating GLUT4 traffic

Insulin-dependent phosphorylation of Akt target AS160 is required for GLUT4 translocation. Insulin and platelet-derived growth factor (PDGF) (Akt activators) or activation of conventional/novel (c/n) protein kinase C (PKC) and 5' AMP-activated protein kinase (AMPK) all promote a rise in membrane GLUT4 in skeletal muscle and cultured cells. However, the downstream effectors linking these pathways to GLUT4 traffic are unknown. Here we explore the hypothesis that AS160 is a molecular link among diverse signaling cascades converging on GLUT4 translocation. PDGF and insulin increased AS160 phosphorylation in CHO-IR cells. Stimuli that activate c/n PKC or AMPK also elevated AS160 phosphorylation. We therefore examined if these signaling pathways engage AS160 to regulate GLUT4 traffic in muscle cells. Nonphosphorylatable AS160 (4P-AS160) virtually abolished the net surface GLUT4myc gains elicited by insulin, PDGF, K(+) depolarization, or 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside but partly, yet significantly, inhibited the effects of 4-phorbol-12-myristate-13-acetate. However, the hypertonicity or 2,4-dinitrophenol-dependent gains in surface GLUT4myc were unaffected by 4P-AS160. RK-AS160 (GTPase-activating protein [GAP] inactive) or 4PRK-AS160 (GAP inactive, nonphosphorylatable) had no effect on surface GLUT4myc elicited by all stimuli. Collectively, these results indicate that activation of Akt, c/n PKC, or alpha2-AMPK intersect at AS160 to regulate GLUT4 traffic, as well as highlight the potential of AS160 as a therapy target to increase muscle glucose uptake.

Endocytic trafficking of CFTR in health and disease

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl-selective anion channel expressed in epithelial tissues. Mutations in CFTR lead to the genetic disease cystic fibrosis (CF). Within each epithelial cell, CFTR interacts with a large number of transient macromolecular complexes, many of which are involved in the trafficking and targeting of CFTR. Understanding how these complexes regulate the trafficking and fate of CFTR, provides a singular insight not only into the patho-physiology of cystic fibrosis, but also provides potential drug targets to help cure this debilitating disease.

Acute impairment of insulin signalling by dexamethasone in primary cultured rat skeletal myocytes

In this study, we examined the cellular content of the insulin receptor substrate (IRS)-1, the levels of phosphorylated tyrosine (pY) and serine (pS) residues in IRS-1, and the glucose transporters GLUT-1 and GLUT-4 in primary cultured rat skeletal myocytes treated with the glucocorticoid, dexamethasone. Dexamethasone markedly increased basal and insulin-stimulated IRS-1 content 4 to 5-fold (p < 0.01). A similar level of increase was observed for IRS-1 pY content. However, dexamethasone treatment had no effect on IRS-1 pS content. Further, dexamethasone reduced the cellular content of GLUT-1 when insulin and glucose were absent (p < 0.05), but did not significantly affect the expression of GLUT-4 in the presence of insulin (p > 0.05). We conclude that dexamethasone treatment impairs insulin signalling by a mechanism independent of serine-phosphorylation-mediated IRS-1 depletion, or of impairment of GLUT-1 expression. Instead, dexamethasone-induced insulin resistance may be mediated via reduced cellular content of IRS-1 accompanied by parallel reduction in tyrosine phosphorylation in IRS-1.

Regulation of glucose transporters by insulin and extracellular glucose in C2C12 myotubes

It is well established that insulin stimulation of glucose uptake in skeletal muscle cells is mediated through translocation of GLUT4 from intracellular storage sites to the cell surface. However, the established skeletal muscle cell lines, with the exception of L6 myocytes, reportedly show minimal insulin-dependent glucose uptake and GLUT4 translocation. Using C(2)C(12) myocytes expressing exofacial-Myc-GLUT4-enhanced cyan fluorescent protein, we herein show that differentiated C(2)C(12) myotubes are equipped with basic GLUT4 translocation machinery that can be activated by insulin stimulation ( approximately 3-fold increase as assessed by anti-Myc antibody uptake and immunostaining assay). However, this insulin stimulation of GLUT4 translocation was difficult to demonstrate with a conventional 2-deoxyglucose uptake assay because of markedly elevated basal glucose uptake via other glucose transporter(s). Intriguingly, the basal glucose transport activity in C(2)C(12) myotubes appeared to be acutely suppressed within 5 min by preincubation with a pathophysiologically high level of extracellular glucose (25 mM). In contrast, this activity was augmented by acute glucose deprivation via an unidentified mechanism that is independent of GLUT4 translocation but is dependent on phosphatidylinositol 3-kinase activity. Taken together, these findings indicate that regulation of the facilitative glucose transport system in differentiated C(2)C(12) myotubes can be achieved through surprisingly acute glucose-dependent modulation of the activity of glucose transporter(s), which apparently contributes to obscuring the insulin augmentation of glucose uptake elicited by GLUT4 translocation. We herein also describe several methods of monitoring insulin-dependent glucose uptake in C(2)C(12) myotubes and propose this cell line to be a useful model for analyzing GLUT4 translocation in skeletal muscle.

The NHE3 juxtamembrane cytoplasmic domain directly binds ezrin: dual role in NHE3 trafficking and mobility in the brush border

Based on physiological studies, the epithelial brush-border (BB) Na+/H+ antiporter3 (NHE3) seems to associate with the actin cytoskeleton both by binding to and independently of the PDZ domain containing proteins NHERF1 and NHERF2. We now show that NHE3 directly binds ezrin at a site in its C terminus between aa 475-589, which is separate from the PSD95/dlg/zonular occludens-1 (PDZ) interacting domain. This is an area predicted to be alpha-helical, with a positive aa cluster on one side (K516, R520, and R527). Point mutations of these positively charged aa reduced (NHE3 double mutant [R520F, R527F]) or abolished (NHE3 triple mutant [K516Q, R520F, R 527F]) ezrin binding. Functional consequences of these NHE3 point mutants included the following. 1) A marked decrease in surface amount with a greater decrease in NHE3 activity. 2) Decreased surface expression due to decreased rates of exocytosis and plasma membrane delivery of newly synthesized NHE3, with normal total expression levels and slightly reduced endocytosis rates. 3) A longer plasma membrane half-life of mutant NHE3 with normal total half-life. 4) Decreased BB mobile fraction of NHE3 double mutant. These results show that NHE3 binds ezrin directly as well as indirectly and suggest that the former is related to 1) the exocytic trafficking of and plasma membrane delivery of newly synthesized NHE3, which determines the amount of plasma membrane NHE3 and partially determines NHE3 activity, and 2) BB mobility of NHE3, which may increase its delivery from microvilli to the intervillus clefts, perhaps for NHE3-regulated endocytosis.

Insulin-dependent interactions of proteins with GLUT4 revealed through stable isotope labeling by amino acids in cell culture (SILAC)

The insulin-regulated glucose transporter (GLUT4) translocates to the plasma membrane in response to insulin in order to facilitate the postprandial uptake of glucose into fat and muscle cells. While early insulin receptor signaling steps leading to this translocation are well defined, the integration of signaling and regulation of GLUT4 traffic remains elusive. Several lines of evidence suggest an important role for the actin cytoskeleton and for protein-protein interactions in regulating GLUT4 localization by insulin. Here, we applied stable isotope labeling by amino acids in cell culture (SILAC) to identify proteins that interact with GLUT4 in an insulin-regulated manner. Myc-tagged GLUT4 (GLUT4myc) stably expressed in L6 myotubes was immunoprecipitated via the myc epitope from total membranes isolated from basal and insulin-stimulated cells grown in medium containing normal isotopic abundance leucine or deuterated leucine, respectively. Proteins coprecipitating with GLUT4myc were analyzed by liquid chromatography/ tandem mass spectrometry. Of 603 proteins quantified, 36 displayed an insulin-dependent change of their interaction with GLUT4myc of more than 1.5-fold in either direction. Several cytoskeleton-related proteins were elevated in immunoprecipates from insulin-treated cells, whereas components of the ubiquitin-proteasome degradation system were generally reduced. Proteins participating in vesicle traffic also displayed insulin-regulated association. Of cytoskeleton-related proteins, alpha-actinin-4 recovery in GLUT4 immunoprecipitates rose in response to insulin 2.1 +/- 0.5-fold by SILAC and 2.9 +/- 0.8-fold by immunoblotting. Insulin caused GLUT4 and alpha-actinin-4 co-localization as revealed by confocal immunofluorescence microscopy. We conclude that insulin elicits changes in interactions between diverse proteins and GLUT4, and that cytoskeletal proteins, notably alpha-actinin-4, associate with the transporter, potentially to facilitate its routing to the plasma membrane.

GLUT4 Distribution between the Plasma Membrane and the Intracellular Compartments Is Maintained by an Insulin-modulated Bipartite Dynamic Mechanism

The GLUT4 glucose transporter is predominantly retained inside basal fat and muscle cells, and it is rapidly recruited to the plasma membrane with insulin stimulation. There is controversy regarding the mechanism of basal GLUT4 retention. One model is that GLUT4 retention is dynamic, based on slow exocytosis and rapid internalization of the entire pool of GLUT4 (Karylowski, O., Zeigerer, A., Cohen, A., and McGraw, T. E. (2004) Mol. Biol. Cell 15, 870-882). In this model, insulin increases GLUT4 in the plasma membrane by modulating GLUT4 exocytosis and endocytosis. The second model is that GLUT4 retention is static, with approximately 90% of GLUT4 stored in compartments that are not in equilibrium with the cell surface in basal conditions (Govers, R., Coster, A. C., and James, D. E. (2004) Mol. Cell Biol. 24, 6456-6466). In this model, insulin increases GLUT4 in the plasma membrane by releasing it from the static storage compartment. Here we show that under all experimental conditions examined, basal GLUT4 retention is by a bipartite dynamic mechanism involving slow efflux and rapid internalization. To establish that the dynamic model developed in studies of the extreme conditions of >100 nm insulin and no insulin also describes GLUT4 behavior at more physiological insulin concentrations, we characterized GLUT4 trafficking in 0.5 nm insulin. This submaximal insulin concentration promotes an intermediate effect on both GLUT4 exocytosis and endocytosis, resulting in an intermediate degree of redistribution to the plasma membrane. These data establish that changes in the steady-state surface/total distributions of GLUT4 are the result of gradated, insulin-induced changes in GLUT4 exocytosis and endocytosis rates.

Regulation of insulin signalling, glucose uptake and metabolism in rat skeletal muscle cells upon prolonged exposure to resistin

AIMS/HYPOTHESIS

Debate exists regarding the role of resistin in the pathophysiology of insulin resistance. The aim of this study was to directly assess the effects of resistin (0-24 h) on basal and insulin-stimulated glucose uptake and metabolism in skeletal muscle cells and to investigate the mechanisms responsible for the effects of resistin.

METHODS

We used L6 rat skeletal muscle cells and examined [(3)H]2-deoxyglucose uptake, GLUT4 translocation and GLUT protein content. We assessed glucose metabolism by measuring the incorporation of D-[U-(14)C]glucose into glycogen, (14)CO(2) and lactate production, as well as the phosphorylation level and total protein content of insulin signalling proteins, including insulin receptor beta-subunit (IRbeta), insulin receptor substrate (IRS), Akt and glycogen synthase kinase-3beta (GSK-3beta).

RESULTS

Treatment of L6 rat skeletal muscle cells with recombinant resistin (50 nmol/l, 0-24 h) reduced levels of basal and insulin-stimulated 2-deoxyglucose uptake and decreased insulin-stimulated GLUT4myc content at the cell surface, with no alteration in the production of GLUT4 or GLUT1. Resistin also decreased glycogen synthesis and GSK-3beta phosphorylation. Insulin-stimulated oxidation of glucose via the Krebs cycle was reduced by resistin, whereas lactate production was unaltered. Although insulin receptor protein level and phosphorylation were unaltered by resistin, production of IRS-1, but not IRS-2, was downregulated and a decreased tyrosine phosphorylation of IRS-1 was detected. Reduced phosphorylation of Akt on T308 and S473 was observed, while total Akt and Akt1, but not Akt2 or Akt3, production was decreased.

CONCLUSIONS/INTERPRETATION

Our data show that resistin regulates the function of IRS-1 and Akt1 and decreases GLUT4 translocation and glucose uptake in response to insulin. Selective decreases in insulin-stimulated glucose metabolism via oxidation and conversion to glycogen were also induced by resistin. These observations highlight the potential role of resistin in the pathophysiology of type 2 diabetes in obesity.

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.

Proposed mechanisms for the induction of insulin resistance by oxidative stress

In diabetes (type 1 and type 2), increased flux of free fatty acids and glucose is associated with increased mitochondrial reactive oxygen species (ROS) production and, as a consequence, increased oxidative stress. ROS have been shown to activate various cellular stress-sensitive pathways, which can interfere with cellular signaling pathways. Exposure of different cell lines to micromolar concentrations of hydrogen peroxide leads to the activation of stress kinases such as c-Jun N-terminal kinase, p38, I kappaB kinase, and extracellular receptor kinase 1/2. This activation is accompanied by a down-regulation of the cellular response to insulin, leading to a reduced ability of insulin to promote glucose uptake, and glycogen and protein synthesis. The mechanisms leading to this down-regulation in oxidized cells are complicated, involving increased serine/threonine phosphorylation of insulin receptor substrate-1 (IRS1), impaired insulin-stimulated redistribution of IRS1 and phosphatidylinositol-kinase between cytosol and low-density microsomal fraction, followed by a reduced protein kinase-B phosphorylation and GLUT4 translocation to the plasma membrane. In addition, prolonged exposure to ROS affects transcription of glucose transporters: whereas the level of GLUT1 is increased, GLUT4 level is reduced. As can be expected, administration of antioxidants such as lipoic acid in oxidized cells, in animal models of diabetes, and in type 2 diabetes shows improved insulin sensitivity. Thus, oxidative stress is presently accepted as a likely causative factor in the development of insulin resistance.

Full intracellular retention of GLUT4 requires AS160 Rab GTPase activating protein

Insulin controls glucose flux into muscle and fat by regulating the trafficking of GLUT4 between the interior and surface of cells. Here, we show that the AS160 Rab GTPase activating protein (GAP) is a negative regulator of basal GLUT4 exocytosis. AS160 knockdown resulted in a partial redistribution of GLUT4 from intracellular compartments to the plasma membrane, a concomitant increase in basal glucose uptake, and a 3-fold increase in basal GLUT4 exocytosis. Reexpression of wild-type AS160 restored normal GLUT4 behavior to the knockdown adipocytes, whereas reexpression of a GAP domain mutant did not revert the phenotype, providing the first direct evidence that AS160 GAP activity is required for basal GLUT4 retention. AS160 is the first protein identified that is specially required for basal GLUT4 retention. Our findings that AS160 knockdown only partially releases basal GLUT4 retention provides evidence that insulin signals to GLUT4 exocytosis by both AS160-dependent and -independent mechanisms.

Recycling of the Membrane-anchored Chemokine, CX3CL1

CX(3)CL1 (fractalkine) plays an important role in inflammation by acting as both chemoattractant and as an adhesion molecule. As for other chemokines, expression of CX(3)CL1 is known to be regulated at the level of transcription and translation. The unique transmembrane structure of CX(3)CL1 raises the possibility of additional functional regulation by altering its abundance at the cell surface. This could be accomplished in principle by changes in traffic between subcellular compartments. To analyze this possibility we examined the subcellular distribution of CX(3)CL1 in human ECV-304 cells stably expressing untagged or green fluorescent protein-tagged forms of the chemokine. CX(3)CL1 was present in two distinct compartments, diffusely on the plasma membrane and in a punctate juxtanuclear compartment. The latter shared some features with, yet was distinct from the conventional endocytic pathway and may represent a specialized recycling subcompartment. Accordingly, surface CX(3)CL1 was found to be in dynamic equilibrium with the juxtanuclear vesicular compartment. Intracellular CX(3)CL1 co-localized with the SNARE (soluble N-ethylmaleimide factor attachment protein receptor) proteins syntaxin-13 and VAMP-3. Cleavage of VAMP-3 by tetanus toxin or impairment of syntaxin-13 function by expression of a dominant-negative allele inhibited the ability of internalized CX(3)CL1 to traffic back to the plasma membrane. These data demonstrate the existence of a dynamic, SNARE-mediated recycling of CX(3)CL1 from the cell surface to and from an endomembrane storage compartment. The intracellular storage depot may serve as a source of the chemokine that could be rapidly mobilized by stimuli.

Expression of Constitutively Active Akt/Protein Kinase B Signals GLUT4 Translocation in the Absence of an Intact Actin Cytoskeleton

The actin cytoskeleton has been shown to be required for insulin-dependent GLUT4 translocation; however, the role that the actin network plays is unknown. Actin may play a role in formation of an active signaling complex, or actin may be required for movement of vesicles to the plasma membrane surface. To distinguish between these possibilities, we examined the ability of myr-Akt, a constitutively active form of Akt that signals GLUT4 translocation to the plasma membrane in the absence of insulin, to signal translocation of an HA-GLUT4-GFP reporter protein in the presence or absence of an intact cytoskeleton in 3T3-L1 adipocytes. Expression of myr-Akt signaled the redistribution of the GLUT4 reporter protein to the cell surface in the absence or presence of 10 microm latrunculin B, a concentration sufficient to completely inhibit insulin-dependent redistribution of the GLUT4 reporter to the cell surface. These data suggest that the actin network plays a primary role in organization of the insulin-signaling complex. To further support this conclusion, we measured the activation of known signaling proteins using a saturating concentration of insulin in cells pretreated without or with 10 microm latrunculin B. We found that latrunculin treatment did not affect insulin-dependent tyrosine phosphorylation of the insulin receptor beta-subunit and IRS-1 but completely inhibited activation of Akt/PKB enzymatic activity. Phosphorylation of Akt/PKB at Ser-473 and Thr-308 was inhibited by latrunculin B treatment, indicating that the defect in signaling lies prior to Akt/PKB activation. In summary, our data support the hypothesis that the actin network plays a role in organization of the insulin-signaling complex but is not required for vesicle trafficking and/or fusion.

Cellular mechanisms of insulin resistance, lipodystrophy and atherosclerosis induced by HIV protease inhibitors

Accumulating clinical evidence now links HIV protease inhibitors (HPIs) to the pathogenesis of insulin resistance, dyslipidaemia, lipodystrophy and atherosclerosis associated with highly active anti-retroviral therapy. Here we briefly describe the evidence for a distinct causative role for HPIs, and explore the cellular mechanisms proposed to underlie these side-effects. Acute inhibition of GLUT4-mediated glucose transport, and defective insulin signalling induced by chronic exposure to nelfinavir, are described as cellular mechanisms of insulin resistance. Interference with adipogenesis and adipocyte apoptosis and nelfinavir-induced activation of lipolysis are discussed as potential mechanisms of HPI-induced lipodystrophy. HPI-induced free radical production, apoptosis and increased glucose utilization in vascular smooth muscle cells are presented as possible novel mechanisms for atherosclerosis. Common pathways and cause-effect relationships between the various cellular mechanisms presented are then discussed, with emphasis on the role of insulin resistance, free radical production and enhanced lipolysis. Understanding the cellular mechanisms of HPI-induced side-effects will enhance the search for improved anti-retroviral therapy, and may also shed light on the pathogenesis of common forms of insulin resistance, dyslipidaemia and atherosclerosis.

Membrane traffic in skeletal muscle

Skeletal muscle tissue is made up of highly organized multinuclear cells. The internal organization of the muscle cell is dictated by the necessary regular arrangement of repeated units within the protein myofibrils that mediate muscle contraction. Skeletal muscle cells have the usual membrane traffic pathways for partitioning newly synthesized proteins, internalizing cell surface receptors for hormones and nutrients, and mediating membrane repair. However, in muscle, these pathways must be further specialized to deal with targeting to and organizing muscle-specific membrane structures, satisfying the unique metabolic requirements of muscle and meeting the high demand for membrane repair in a tissue that is constantly under mechanical stress. Specialized membrane traffic pathways in muscle also play a role in the formation of muscle through fusion of myoblast membranes and the development of internal muscle-specific membrane structures during myogenesis and regeneration. It has recently become apparent that muscle-specific isoforms of proteins that are known to mediate ubiquitous membrane traffic pathways, as well as novel muscle-specific proteins, are involved in tissue-specific aspects of muscle membrane traffic. Here we describe the specialized membrane structures of skeletal muscle, how these are developed, maintained and repaired by specialized and generic membrane traffic pathways, and how defects in these pathways result in muscle disease.

Protein Kinase C-Independent Effects of Protein Kinase D3 in Glucose Transport in L6 Myotubes

Protein kinase C (PKC) and protein kinase D (PKD) coordinate and regulate many fundamental cellular processes. In this study, we evaluate the role of classic and novel PKC (c/nPKC) and PKD in glucose transport in L6 myotubes. c/nPKC is either activated by short-term phorbol 12-myristate 13-acetate (PMA) treatment or down-regulated by prolonged PMA treatment at a high dose in L6 myotubes. Our results indicate that PMA treatments have little impact on basal and insulin-stimulated glucose uptake and insulin-induced Akt activation. In contrast, the PKC inhibitors Go6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo[2,3-a]pyrrolo[3,4-c] carbazole], Go6983 [2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide], GF 109203X [bisindolylmaleimide I; 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(1H-indol-3-yl)maleimide], and Ro 31-8220 [bisindolylmaleimide IX; 2-{1-[3-(amidinothio)propyl]-1H-indol3-yl}-3-(1-methylindol-3-yl)maleimide] block basal and insulin-stimulated glucose uptake, and their inhibitory effects persist upon down-regulation of c/nPKC by PMA, implying the presence of PKC-independent effectors in mediating their inhibition of glucose uptake. Go6976, the potent cPKC inhibitor that also effectively inhibits PKD, dose-dependently blocks basal glucose uptake in L6 myotubes, whereas Go6983, the nonselective PKC inhibitor that is ineffective for PKD, has little effect on basal glucose uptake, implying the involvement of PKD in this process. Most prominently, adenoviral gene expression of a dominant-negative PKD isoform, PKD3, primarily inhibits basal glucose uptake and, to a lesser extent, insulin-stimulated glucose uptake, whereas overexpression of wild-type PKD3 significantly enhances basal glucose uptake. Moreover, expression of a PKD3-targeted siRNA significantly inhibits basal glucose uptake. Taken together, our results indicate that PKD, specifically PKD3, directly contributes to insulin-independent basal glucose uptake in L6 skeletal muscle cells.

Role of EHD1 and EHBP1 in Perinuclear Sorting and Insulin-regulated GLUT4 Recycling in 3T3-L1 Adipocytes

Insulin stimulates glucose transport in muscle and adipose tissues by recruiting intracellular membrane vesicles containing the glucose transporter GLUT4 to the plasma membrane. The mechanisms involved in the biogenesis of these vesicles and their translocation to the cell surface are poorly understood. Here, we report that an Eps15 homology (EH) domain-containing protein, EHD1, controls the normal perinuclear localization of GLUT4-containing membranes and is required for insulin-stimulated recycling of these membranes in cultured adipocytes. EHD1 is a member of a family of four closely related proteins (EHD1, EHD2, EHD3, and EHD4), which also contain a P-loop near the N terminus and a central coiled-coil domain. Analysis of cultured adipocytes stained with anti-GLUT4, anti-EHD1, and anti-EHD2 antibodies revealed that EHD1, but not EHD2, partially co-localizes with perinuclear GLUT4. Expression of a dominant-negative construct of EHD1 missing the EH domain (DeltaEH-EHD1) markedly enlarged endosomes, dispersed perinuclear GLUT4-containing membranes throughout the cytoplasm, and inhibited GLUT4 translocation to the plasma membranes of 3T3-L1 adipocytes stimulated with insulin. Similarly, small interfering RNA-mediated depletion of endogenous EHD1 protein also markedly dispersed perinuclear GLUT4 in cultured adipocytes. Moreover, EHD1 is shown to interact through its EH domain with the protein EHBP1, which is also required for insulin-stimulated GLUT4 movements and hexose transport. In contrast, disruption of EHD2 function was without effect on GLUT4 localization or translocation to the plasma membrane. Taken together, these results show that EHD1 and EHBP1, but not EHD2, are required for perinuclear localization of GLUT4 and reveal that loss of EHBP1 disrupts insulin-regulated GLUT4 recycling in cultured adipocytes.

Unconventional myosin Myo1c promotes membrane fusion in a regulated exocytic pathway

Glucose homeostasis is controlled in part by regulation of glucose uptake into muscle and adipose tissue. Intracellular membrane vesicles containing the GLUT4 glucose transporter move towards the cell cortex in response to insulin and then fuse with the plasma membrane. Here we show that the fusion step is retarded by the inhibition of phosphatidylinositol (PI) 3-kinase. Treatment of insulin-stimulated 3T3-L1 adipocytes with the PI 3-kinase inhibitor LY294002 causes the accumulation of GLUT4-containing vesicles just beneath the cell surface. This accumulation of GLUT4-containing vesicles near the plasma membrane prior to fusion requires an intact cytoskeletal network and the unconventional myosin motor Myo1c. Remarkably, enhanced Myo1c expression under these conditions causes extensive membrane ruffling and overrides the block in membrane fusion caused by LY294002, restoring the display of GLUT4 on the cell exterior. Ultrafast microscopic analysis revealed that insulin treatment leads to the mobilization of GLUT4-containing vesicles to these regions of Myo1c-induced membrane ruffles. Thus, localized membrane remodeling driven by the Myo1c motor appears to facilitate the fusion of exocytic GLUT4-containing vesicles with the adipocyte plasma membrane.