Vp165 and GLUT4 share similar vesicle pools along their trafficking pathways in rat adipose cells

“Deletion of both Rab-GTPase–activating proteins TBC1D1 and TBC1D4 in mice eliminates insulin- and AICAR-stimulated glucose transport [corrected]

The Rab-GTPase–activating proteins TBC1D1 and TBC1D4 (AS160) were previously shown to regulate GLUT4 translocation in response to activation of AKT and AMP-dependent kinase [corrected]. However, knockout mice lacking either Tbc1d1 or Tbc1d4 displayed only partially impaired insulin-stimulated glucose uptake in fat and muscle tissue. The aim of this study was to determine the impact of the combined inactivation of Tbc1d1 and Tbc1d4 on glucose metabolism in double-deficient (D1/4KO) mice. D1/4KO mice displayed normal fasting glucose concentrations but had reduced tolerance to intraperitoneally administered glucose, insulin, and AICAR. D1/4KO mice showed reduced respiratory quotient, indicating increased use of lipids as fuel. These mice also consistently showed elevated fatty acid oxidation in isolated skeletal muscle, whereas insulin-stimulated glucose uptake in muscle and adipose cells was almost completely abolished. In skeletal muscle and white adipose tissue, the abundance of GLUT4 protein, but not GLUT4 mRNA, was substantially reduced. Cell surface labeling of GLUTs indicated that RabGAP deficiency impairs retention of GLUT4 in intracellular vesicles in the basal state. Our results show that TBC1D1 and TBC1D4 together play essential roles in insulin-stimulated glucose uptake and substrate preference in skeletal muscle and adipose cells.

Sites of glucose transporter-4 vesicle fusion with the plasma membrane correlate spatially with microtubules

In adipocytes, vesicles containing glucose transporter-4 (GLUT4) redistribute from intracellular stores to the cell periphery in response to insulin stimulation. Vesicles then fuse with the plasma membrane, facilitating glucose transport into the cell. To gain insight into the details of microtubule involvement, we examined the spatial organization and dynamics of microtubules in relation to GLUT4 vesicle trafficking in living 3T3-L1 adipocytes using total internal reflection fluorescence (TIRF) microscopy. Insulin stimulated an increase in microtubule density and curvature within the TIRF-illuminated region of the cell. The high degree of curvature and abrupt displacements of microtubules indicate that substantial forces act on microtubules. The time course of the microtubule density increase precedes that of the increase in intensity of fluorescently-tagged GLUT4 in this same region of the cell. In addition, portions of the microtubules are highly curved and are pulled closer to the cell cortex, as confirmed by Parallax microscopy. Microtubule disruption delayed and modestly reduced GLUT4 accumulation at the plasma membrane. Quantitative analysis revealed that fusions of GLUT4-containing vesicles with the plasma membrane, detected using insulin-regulated aminopeptidase with a pH-sensitive GFP tag (pHluorin), preferentially occur near microtubules. Interestingly, long-distance vesicle movement along microtubules visible at the cell surface prior to fusion does not appear to account for this proximity. We conclude that microtubules may be important in providing spatial information for GLUT4 vesicle fusion.

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

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

Dangnyohwan improves glucose utilization and reduces insulin resistance by increasing the adipocyte-specific GLUT4 expression in Otsuka Long-Evans Tokushima Fatty rats

AIM OF THE STUDY

Dangnyohwan (DNH) has been used for treatment of diabetes mellitus. However, the exact cellular and molecular mechanisms underlying the beneficial effects of DNH are not well understood. Therefore, we investigated how DNH improves hyperglycemia and insulin resistance in obese-type diabetes model.

METHODS AND MATERIALS

We examined the effect of DNH on the expression of glucose transporter 4 (GLUT4), GLUT4 translocation, and glucose transport activity in muscle and adipose tissues from Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO) rats.

RESULTS

DNH ameliorated hyperglycemia and impaired glucose tolerance (IGT) observed in 26- and 42-week-old male OLETF rats. The basal and insulin-stimulated [14C]2-Deoxyglucose (2DG) uptake was significantly increased in adipocytes from DNH-treated OLETF rats, as compared with untreated OLETF rats. The expression level of GLUT4 was markedly decreased (by 90-95%) in the adipose tissue of OLETF rats, whereas DNH treatment drastically increased the expression of GLUT4 within 8 weeks. DNH improved GLUT4 recruitment stimulated by insulin in both the 26- and 42-week-old OLETF rat adipocytes.

CONCLUSION

These results suggest that DNH could exert the beneficial effects on hyperglycemia and insulin resistance by increasing the expression and insulin-stimulated translocation of GLUT4 in OLETF rat adipocytes.

Golgin-160 is required for the Golgi membrane sorting of the insulin-responsive glucose transporter GLUT4 in adipocytes

The peripheral Golgi protein golgin-160 is induced during 3T3L1 adipogenesis and is primarily localized to the Golgi cisternae distinct from the trans-Golgi network (TGN) in a general distribution similar to p115. Small interfering RNA (siRNA)-mediated reduction in golgin-160 protein resulted in an increase accumulation of the insulin-responsive amino peptidase (IRAP) and the insulin-regulated glucose transporter (GLUT4) at the plasma membrane concomitant with enhanced glucose uptake in the basal state. The redistribution of GLUT4 was rescued by expression of a siRNA-resistant golgin-160 cDNA. The basal state accumulation of plasma membrane GLUT4 occurred due to an increased rate of exocytosis without any significant effect on the rate of endocytosis. This GLUT4 trafficking to the plasma membrane in the absence of golgin-160 was independent of TGN/Golgi sorting, because it was no longer inhibited by the expression of a dominant-interfering Golgi-localized, gamma-ear-containing ARF-binding protein mutant and displayed reduced binding to the lectin wheat germ agglutinin. Moreover, expression of the amino terminal head domain (amino acids 1-393) had no significant effect on the distribution or insulin-regulated trafficking of GLUT4 or IRAP. In contrast, expression of carboxyl alpha helical region (393-1498) inhibited insulin-stimulated GLUT4 and IRAP translocation, but it had no effect on the sorting of constitutive membrane trafficking proteins, the transferrin receptor, or vesicular stomatitis virus G protein. Together, these data demonstrate that golgin-160 plays an important role in directing insulin-regulated trafficking proteins toward the insulin-responsive compartment in adipocytes.

Interaction of the Akt Substrate, AS160, with the Glucose Transporter 4 Vesicle Marker Protein, Insulin-Regulated Aminopeptidase

Insulin-regulated aminopeptidase (IRAP), a marker of glucose transporter 4 (GLUT4) storage vesicles (GSVs), is the only protein known to traffic with GLUT4. In the basal state, GSVs are sequestered from the constitutively recycling endosomal system to an insulin-responsive, intracellular pool. Insulin induces a rapid translocation of GSVs to the cell surface from this pool, resulting in the incorporation of IRAP and GLUT4 into the plasma membrane. We sought to identify proteins that interact with IRAP to further understand this GSV trafficking process. This study describes our identification of a novel interaction between the amino terminus of IRAP and the Akt substrate, AS160 (Akt substrate of 160 kDa). The validity of this interaction was confirmed by coimmunoprecipitation of both overexpressed and endogenous proteins. Moreover, confocal microscopy demonstrated colocalization of these proteins. In addition, we demonstrate that the IRAP-binding domain of AS160 falls within its second phosphotyrosine-binding domain and the interaction is not regulated by AS160 phosphorylation. We hypothesize that AS160 is localized to GLUT4-containing vesicles via its interaction with IRAP where it inhibits the activity of Rab substrates in its vicinity, effectively tethering the vesicles intracellularly.

Calorie restriction improves whole-body glucose disposal and insulin resistance in association with the increased adipocyte-specific GLUT4 expression in Otsuka Long–Evans Tokushima Fatty rats

Calorie restriction (CR) has been shown to improve peripheral insulin resistance and type 2 diabetes in animal models. However, the exact mechanism of CR on GLUT4 expression and translocation in insulin-sensitive tissues has not been well elucidated. In the present study, we examine the effect of CR on the expression of glucose transporter 4 (GLUT4), GLUT4 translocation, and glucose transport activity in adipose tissue from Otsuka Long-Evans Tokushima Fatty (OLETF) rat and control (LETO) rats. CR (70% of satiated group) ameliorated hyperglycemia and improved impaired glucose tolerance (IGT) in OLETF rats. In skeletal muscle, the expression levels of GLUT4 and GLUT1 were not significantly different between LETO and OLETF rats, and were not affected by CR. By contrast, the expression level of GLUT4 was markedly decreased in the adipose tissue of OLETF rats, but was dramatically increased by CR. The GLUT4 recruitment stimulated by insulin was also improved in OLETF rat adipocytes by CR. The insulin-stimulated 2-deoxyglucose (2DG) uptake was significantly increased in adipocytes from the CR OLETF rats, as compared with the satiated OLETF rats. Taken together, these results suggest that CR improves whole body glucose disposal and insulin resistance in OLETF rats, and that these effects may associate with the increased adipocyte-specific GLUT4 expression.

p115 Interacts with the GLUT4 vesicle protein, IRAP, and plays a critical role in insulin-stimulated GLUT4 translocation

Insulin-regulated aminopeptidase (IRAP) is an abundant cargo protein of Glut4 storage vesicles (GSVs) that traffics to and from the plasma membrane in response to insulin. We used the amino terminus cytoplasmic domain of IRAP, residues 1-109, as an affinity reagent to identify cytosolic proteins that might be involved in GSV trafficking. In this way, we identified p115, a peripheral membrane protein known to be involved in membrane trafficking. In murine adipocytes, we determined that p115 was localized to the perinuclear region by immunofluorescence and throughout the cell by fractionation. By immunofluorescence, p115 partially colocalizes with GLUT4 and IRAP in the perinuclear region of cultured fat cells. The amino terminus of p115 binds to IRAP and overexpression of a N-terminal construct results in its colocalization with GLUT4 throughout the cell. Insulin-stimulated GLUT4 translocation is completely inhibited under these conditions. Overexpression of p115 C-terminus has no significant effect on GLUT4 distribution and translocation. Finally, expression of the p115 N-terminus construct has no effect on the distribution and trafficking of GLUT1. These data suggest that p115 has an important and specific role in insulin-stimulated Glut4 translocation, probably by way of tethering insulin-sensitive Glut4 vesicles at an as yet unknown intracellular site.

Glut4 storage vesicles without Glut4: transcriptional regulation of insulin-dependent vesicular traffic

Two families of transcription factors that play a major role in the development of adipocytes are the CCAAT/enhancer-binding proteins (C/EBPs) and the peroxisome proliferator-activated receptors (PPARs), in particular PPAR gamma. Ectopic expression of either C/EBP alpha or PPAR gamma in NIH 3T3 fibroblasts results in the conversion of these cells to adipocyte-like cells replete with fat droplets. NIH 3T3 cells ectopically expressing C/EBP alpha (NIH-C/EBP alpha) differentiate into adipocytes and exhibit insulin-stimulated glucose uptake, whereas NIH 3T3 cells ectopically expressing PPAR gamma (NIH-PPAR gamma) differentiate but do not exhibit any insulin-stimulated glucose uptake, nor do they express any C/EBP alpha. The reason for the lack of insulin-responsive glucose uptake in the NIH-PPAR gamma cells is their virtual lack of the insulin-responsive glucose transporter, Glut4. The NIH-PPAR gamma cells express functionally active components of the insulin receptor-signaling pathway (the insulin receptor, IRS-1, phosphatidylinositol 3-kinase, and Akt2) at levels comparable to those in responsive cell lines. They also express components of the insulin-sensitive vesicular transport machinery, namely, VAMP2, syntaxin-4, and IRAP, the last of these being the other marker of insulin-regulated vesicular traffic along with Glut4. Interestingly, the NIH-PPAR gamma cells show normal insulin-dependent translocation of IRAP and form an insulin-responsive vesicular compartment as assessed by cell surface biotinylation and sucrose velocity gradient analysis, respectively. Moreover, expression of a Glut4-myc construct in the NIH-PPAR gamma cells results in its insulin-dependent translocation to the plasma membrane as assessed by immunofluorescence and Western blot analysis. Based on these data, we conclude that major role of C/EBP alpha in the context of the NIH-PPAR gamma cells is to regulate Glut4 expression. The differentiated cells possess a large insulin-sensitive vesicular compartment with negligible Glut4, and Glut4 translocation can be reconstituted on expression of this transporter.

Entry of newly synthesized GLUT4 into the insulin-responsive storage compartment is GGA dependent

Following biosynthesis, both GLUT1 and VSV-G proteins appear rapidly (2-3 h) at the plasma membrane, whereas GLUT4 is retained in intracellular membrane compartments and does not display any significant insulin responsiveness until 6-9 h. Surprisingly, the acquisition of insulin responsiveness did not require plasma membrane endocytosis, as expression of a dominant-interfering dynamin mutant (Dyn/K44A) had no effect on the insulin-stimulated GLUT4 translocation. Furthermore, expression of endocytosis-defective GLUT4 mutants or continuous surface labeling with an exofacial specific antibody demonstrated that GLUT4 did not transit the cell surface prior to the acquisition of insulin responsiveness. The expression of a dominant-interfering GGA mutant (VHS-GAT) had no effect on the trafficking of newly synthesized GLUT1 or VSV-G protein to the plasma membrane, but completely blocked the insulin-stimulated translocation of newly synthesized GLUT4. Furthermore, in vitro budding of GLUT4 vesicles but not GLUT1 or the transferrin receptor was inhibited by VHS-GAT. Together, these data demonstrate that following biosynthesis, GLUT4 directly sorts and traffics to the insulin-responsive storage compartment through a specific GGA-sensitive process.

The Formin family protein, formin homolog overexpressed in spleen, interacts with the insulin-responsive aminopeptidase and profilin IIa

Insulin stimulates translocation of glucose transporter isoform type 4 (GLUT4) and the insulin-responsive aminopeptidase (IRAP) from an intracellular storage pool to the plasma membrane in muscle and fat cells. A role for the cytoskeleton in insulin action has been postulated, and the insulin signaling pathway has been well investigated; however, the molecular mechanism by which GLUT4/IRAP-containing vesicles move from an interior location to the cell surface in response to insulin is incompletely understood. Here, we have screened for IRAP-binding proteins using a yeast two-hybrid system and have found that the C-terminal domain of FHOS (formin homolog overexpressed in spleen) interacts with the N-terminal cytoplasmic domain of IRAP. FHOS is a member of the Formin/Diaphanous family of proteins that is expressed most abundantly in skeletal muscle. In addition, there are two novel types of FHOS transcripts generated by alternative mRNA splicing. FHOS78 has a 78-bp insertion and it is expressed mainly in skeletal muscle where it may be the most abundant isoform in humans. The ubiquitously expressed FHOS24 has a 24-bp insertion encoding an in-frame stop codon that results in a truncated polypeptide. It is known that some formin family proteins interact with the actin-binding profilin proteins. Both FHOS and FHOS78 bound to profilin IIa via their formin homology 1 domains, but neither bound profilin I or IIb. Overexpression of FHOS and FHOS78 resulted in enhanced insulin-stimulated glucose uptake in L6 cells to similar levels. However, overexpression of FHOS24, lacking the IRAP-binding domain, did not affect insulin-stimulated glucose uptake. These findings suggest that FHOS mediates an interaction between GLUT4/IRAP-containing vesicles and the cytoskeleton and may participate in exocytosis and/or retention of this membrane compartment.

Fluorometric assay using naphthylamide substrates for assessing novel venom peptidase activities

In the present study we examined the feasibility of using the fluorometry of naphthylamine derivatives for revealing peptidase activities in venoms of the snakes Bothrops jararaca, Bothrops alternatus, Bothrops atrox, Bothrops moojeni, Bothrops insularis, Crotalus durissus terrificus and Bitis arietans, of the scorpions Tityus serrulatus and Tityus bahiensis, and of the spiders Phoneutria nigriventer and Loxosceles intermedia. Neutral aminopeptidase (APN) and prolyl-dipeptidyl aminopeptidase IV (DPP IV) activities were presented in all snake venoms, with the highest levels in B. alternatus. Although all examined peptidase activities showed relatively low levels in arthropod venoms, basic aminopeptidase (APB) activity from P. nigriventer venom was the exception. Compared to the other peptidase activities, relatively high levels of acid aminopeptidase (APA) activity were restricted to B. arietans venom. B. arietans also exhibited a prominent content of APB activity which was lower in other venoms. Relatively low prolyl endopeptidase and proline iminopeptidase activities were, respectively, detectable only in T. bahiensis and B. insularis. Pyroglutamate aminopeptidase activity was undetectable in all venoms. All examined peptidase activities were undetectable in T. serrulatus venom. In this study, the specificities of a diverse array of peptidase activities from representative venoms were demonstrated for the first time, with a description of their distribution which may contribute to guiding further investigations. The expressive difference between snake and arthropod venoms was indicated by APN and DPP IV activities while APA and APB activities distinguished the venom of B. arietans from those of Brazilian snakes. The data reflected the relatively uniform qualitative distribution of the peptidase activities investigated, together with their unequal quantitative distribution, indicating the evolutionary divergence in the processing of peptides in these different venoms and/or the different abilities of the venoms examined to hydrolyze different peptides during envenomation.

GLUT4 retention in adipocytes requires two intracellular insulin-regulated transport steps

Insulin regulates glucose uptake into fat and muscle by modulating the distribution of the GLUT4 glucose transporter between the surface and interior of cells. The GLUT4 trafficking pathway overlaps with the general endocytic recycling pathway, but the degree and functional significance of the overlap are not known. In this study of intact adipocytes, we demonstrate, by using a compartment-specific fluorescence-quenching assay, that GLUT4 is equally distributed between two intracellular pools: the transferrin receptor-containing endosomes and a specialized compartment that excludes the transferrin receptor. These pools of GLUT4 are in dynamic communication with one another and with the cell surface. Insulin-induced redistribution of GLUT4 to the surface requires mobilization of both pools. These data establish a role for the general endosomal system in the specialized, insulin-regulated trafficking of GLUT4. Trafficking through the general endosomal system is regulated by rab11. Herein, we show that rab11 is required for the transport of GLUT4 from endosomes to the specialized compartment and for the insulin-induced translocation to the cell surface, emphasizing the importance of the general endosomal pathway in the specialized trafficking of GLUT4. Based on these findings we propose a two-step model for GLUT4 trafficking in which the general endosomal recycling compartment plays a specialized role in the insulin-regulated traffic of GLUT4. This compartment-based model provides the framework for understanding insulin-regulated trafficking at a molecular level.

Mice deficient in the insulin-regulated membrane aminopeptidase show substantial decreases in glucose transporter GLUT4 levels but maintain normal glucose homeostasis

The insulin-regulated aminopeptidase (IRAP) is a zinc-dependent membrane aminopeptidase. It is the homologue of the human placental leucine aminopeptidase. In fat and muscle cells, IRAP colocalizes with the insulin-responsive glucose transporter GLUT4 in intracellular vesicles and redistributes to the cell surface in response to insulin, as GLUT4 does. To address the question of the physiological function of IRAP, we generated mice with a targeted disruption of the IRAP gene (IRAP-/-). Herein, we describe the characterization of these mice with regard to glucose homeostasis and regulation of GLUT4. Fed and fasted blood glucose and insulin levels in the IRAP-/- mice were normal. Whereas IRAP-/- mice responded to glucose administration like control mice, they exhibited an impaired response to insulin. Basal and insulin-stimulated glucose uptake in extensor digitorum longus muscle, and adipocytes isolated from IRAP-/- mice were decreased by 30-60% but were normal for soleus muscle from male IRAP-/- mice. Total GLUT4 levels were diminished by 40-85% in the IRAP-/- mice in the different muscles and in adipocytes. The relative distribution of GLUT4 in subcellular fractions of basal and insulin-stimulated IRAP-/- adipocytes was the same as in control cells. We conclude that IRAP-/- mice maintain normal glucose homeostasis despite decreased glucose uptake into muscle and fat cells. The absence of IRAP does not affect the subcellular distribution of GLUT4 in adipocytes. However, it leads to substantial decreases in GLUT4 expression.

Adipocytes exhibit abnormal subcellular distribution and translocation of vesicles containing glucose transporter 4 and insulin-regulated aminopeptidase in type 2 diabetes mellitus: implications regarding defects in vesicle trafficking

Insulin resistance in type 2 diabetes is due to impaired stimulation of the glucose transport system in muscle and fat. Different defects are operative in these two target tissues because glucose transporter 4 (GLUT 4) expression is normal in muscle but markedly reduced in fat. In muscle, GLUT 4 is redistributed to a dense membrane compartment, and insulin-mediated translocation to plasma membrane (PM) is impaired. Whether similar trafficking defects are operative in human fat is unknown. Therefore, we studied subcellular localization of GLUT4 and insulin-regulated aminopeptidase (IRAP; also referred to as vp165 or gp160), which is a constituent of GLUT4 vesicles and also translocates to PM in response to insulin. Subcutaneous fat was obtained from eight normoglycemic control subjects (body mass index, 29 +/- 2 kg/m2) and eight type 2 diabetic patients (body mass index, 30 +/- 1 kg/m2; fasting glucose, 14 +/- 1 mM). In adipocytes isolated from diabetics, the basal 3-O-methylglucose transport rate was decreased by 50% compared with controls (7.1 +/- 2.9 vs. 14.1 +/- 3.7 mmol/mm2 surface area/min), and there was no increase in response to maximal insulin (7.9 +/- 2.7 vs. 44.5 +/- 9.2 in controls). In membrane subfractions from controls, insulin led to a marked increase of IRAP in the PM from 0.103 +/- 0.04 to 1.00 +/- 0.33 relative units/mg protein, concomitant with an 18% decrease in low-density microsomes and no change in high-density microsomes (HDM). In type 2 diabetes, IRAP overall expression in adipocytes was similar to that in controls; however, two abnormalities were observed. First, in basal cells, IRAP was redistributed away from low-density microsomes, and more IRAP was recovered in HDM (1.2-fold) and PM (4.4-fold) from diabetics compared with controls. Second, IRAP recruitment to PM by maximal insulin was markedly impaired. GLUT4 was depleted in all membrane subfractions (43-67%) in diabetes, and there was no increase in PM GLUT4 in response to insulin. Type 2 diabetes did not affect the fractionation of marker enzymes. We conclude that in human adipocytes: 1) IRAP is expressed and translocates to PM in response to insulin; 2) GLUT4 depletion involves all membrane subfractions in type 2 diabetes, although cellular levels of IRAP are normal; and 3) in type 2 diabetes, IRAP accumulates in membrane vesicles cofractionating with HDM and PM under basal conditions, and insulin-mediated recruitment to PM is impaired. Therefore, in type 2 diabetes, adipocytes express defects in trafficking of GLUT4/IRAP-containing vesicles similar to those causing insulin resistance in skeletal muscle.

Insulin-regulated release from the endosomal recycling compartment is regulated by budding of specialized vesicles

In several cell types, specific membrane proteins are retained intracellularly and rapidly redistributed to the surface in response to stimulation. In fat and muscle, the GLUT4 glucose transporter is dynamically retained because it is rapidly internalized and slowly recycled to the plasma membrane. Insulin increases the recycling of GLUT4, resulting in a net translocation to the surface. We have shown that fibroblasts also have an insulin-regulated recycling mechanism. Here we show that GLUT4 is retained within the transferrin receptor-containing general endosomal recycling compartment in Chinese hamster ovary (CHO) cells rather than being segregated to a specialized, GLUT4-recycling compartment. With the use of total internal reflection microscopy, we demonstrate that the TR and GLUT4 are transported from the pericentriolar recycling compartment in separate vesicles. These data provide the first functional evidence for the formation of distinct classes of vesicles from the recycling compartment. We propose that GLUT4 is dynamically retained within the endosomal recycling compartment in CHO cells because it is concentrated in vesicles that form more slowly than those that transport TR. In 3T3-L1 adipocytes, cells that naturally express GLUT4, we find that GLUT4 is partially segregated to a separate compartment that is inaccessible to the TR. We present a model for the formation of this specialized compartment in fat cells, based on the general mechanism described in CHO cells, which may explain the increased retention of GLUT4 and its insulin-induced translocation in fat cells.

Insulin-regulated trafficking of dual-labeled glucose transporter 4 in primary rat adipose cells

In isolated rat adipose cells, physiologically relevant insulin target cells, glucose transporter 4 (GLUT4) subcellular trafficking can be assessed by transfection of exofacially HA-tagged GLUT4. To simultaneously visualize the transfected GLUT4, we fused GFP with HA-GLUT4. With the resulting chimeras, GFP-HA-GLUT4 and HA-GLUT4-GFP, we were able to visualize for the first time the cell-surface localization, total expression, and intracellular distribution of GLUT4 in a single cell. Confocal microscopy reveals that the intracellular proportions of both GFP-HA-GLUT4 and HA-GLUT4-GFP are properly targeted to the insulin-responsive aminopeptidase-positive vesicles. Dynamic studies demonstrate close similarities in the trafficking kinetics between the two constructs and with native GLUT4. However, while the basal subcellular distribution of HA-GLUT4-GFP and the response to insulin are indistinguishable from those of HA-GLUT4 and endogenous GLUT4, most of the GFP-HA-GLUT4 is targeted to the plasma membrane with little further insulin response. Thus, HA-GLUT4-GFP will be useful to study GLUT4 trafficking in vivo while GFP on the N-terminus interferes with intracellular retention.

A Glut4-vesicle marker protein, insulin-responsive aminopeptidase, is localized in a novel vesicular compartment in PC12 cells

Glut4-containing vesicles represent a regulated recycling compartment in insulin-sensitive fat and skeletal muscle cells, the nature and origin of which are not fully understood. In addition to Glut4 itself, these vesicles compartmentalize a number of proteins, at least one of which, insulin-responsive aminopeptidase, or IRAP, is completely colocalized with Glut4 in insulin-sensitive tissues. However, unlike Glut4, IRAP is expressed in a variety of other tissues and cell lines. Here, we explored the intracellular localization of IRAP in the rat pheochromocytoma cell line PC12. We found that this protein is present in a distinct population of slowly recycling light vesicles. By gradient centrifugations, immunoadsorption and double immunofluorescent staining, these vesicles are different from transferrin-containing endosomes, small synaptic vesicles and secretory granules and may thus represent a novel compartment in PC12 cells. Glut4-GFP chimera transiently expressed in PC12 cells is targeted to IRAP-containing vesicles indicating that cotargeting of Glut4 and IRAP is not specific for adipocytes and myocytes, but is faithful in a foreign cell type. We suggest that PC12 cells may possess a novel type of a vesicular carrier that may represent the homolog of Glut4-vesicles.

Insulin-responsive compartments containing GLUT4 in 3T3-L1 and CHO cells: regulation by amino acid concentrations

In fat and muscle, insulin stimulates glucose uptake by rapidly mobilizing the GLUT4 glucose transporter from a specialized intracellular compartment to the plasma membrane. We describe a method to quantify the relative proportion of GLUT4 at the plasma membrane, using flow cytometry to measure a ratio of fluorescence intensities corresponding to the cell surface and total amounts of a tagged GLUT4 reporter in individual living cells. Using this assay, we demonstrate that both 3T3-L1 and CHO cells contain intracellular compartments from which GLUT4 is rapidly mobilized by insulin and that the initial magnitude and kinetics of redistribution to the plasma membrane are similar in these two cell types when they are cultured identically. Targeting of GLUT4 to a highly insulin-responsive compartment in CHO cells is modulated by culture conditions. In particular, we find that amino acids regulate distribution of GLUT4 to this kinetically defined compartment through a rapamycin-sensitive pathway. Amino acids also modulate the magnitude of insulin-stimulated translocation in 3T3-L1 adipocytes. Our results indicate a novel link between glucose and amino acid metabolism.

GLUT4 ablation in mice results in redistribution of IRAP to the plasma membrane

Glucose transporter (GLUT) 4 is the insulin responsive glucose transporter in adipose tissue, skeletal muscle, and heart. Insulin elicits increased glucose uptake by recruiting GLUT4 from a specialized intracellular storage site to the cell surface. Expression of various proteins that colocalize with GLUT4 and/or are involved in insulin-stimulated GLUT4 translocation was examined in adipocytes as well as skeletal and cardiac muscles from GLUT4 null mice. Our data demonstrate that expression of insulin-regulated aminopeptidase (IRAP) is divergently regulated in GLUT4 null tissues, e.g., upregulated 1.6-fold in GLUT4 null adipocytes and downregulated in GLUT4 null skeletal muscle (40%) and heart (60%). IRAP exhibited abnormal subcellular distribution and impaired insulin-stimulated translocation in GLUT4-deficient tissues. We propose the compartment containing IRAP and proteins normally associated with GLUT4 vesicle traffics constitutively to the cell surface in GLUT4 null adipocytes and skeletal muscle.

The export of major histocompatibility complex class I molecules from the endoplasmic reticulum of rat brown adipose cells is acutely stimulated by insulin

Major histocompatibility complex class I (MHC-I) molecules have been implicated in several nonimmunological functions including the regulation and intracellular trafficking of the insulin-responsive glucose transporter GLUT4. We have used confocal microscopy to compare the effects of insulin on the intracellular trafficking of MHC-I and GLUT4 in freshly isolated rat brown adipose cells. We also used a recombinant vaccinia virus (rVV) to express influenza virus hemagglutinin (HA) as a generic integral membrane glycoprotein to distinguish global versus specific enhancement of protein export from the endoplasmic reticulum (ER) in response to insulin. In the absence of insulin, MHC-I molecules largely colocalize with the ER-resident protein calnexin and remain distinct from intracellular pools of GLUT4. Surprisingly, insulin induces the rapid export of MHC-I molecules from the ER with a concomitant approximately three-fold increase in their level on the cell surface. This ER export is blocked by brefeldin A and wortmannin but is unaffected by cytochalasin D, indicating that insulin stimulates the rapid transport of MHC-I molecules from the ER to the plasma membrane via the Golgi complex in a phosphatidyl-inositol 3-kinase-dependent and actin-independent manner. We further show that the effect of insulin on MHC-I molecules is selective, because insulin does not affect the intracellular distribution or cell-surface localization of rVV-expressed HA. These results demonstrate that in rat brown adipose cells MHC-I molecule export from the ER is stimulated by insulin and provide the first evidence that the trafficking of MHC-I molecules is acutely regulated by a hormone.

Mutational analysis of the active site of human insulin-regulated aminopeptidase

Insulin-regulated aminopeptidase (IRAP) is a type II integral membrane protein belonging to the gluzincin family of metallopeptidases identified by the characteristic Zn(2+)-coordination sequence element, HEXXH-(18-64X)-E. A second conserved sequence element, the GXMEN motif, positioned 22-32 amino acids N-terminal to the Zn(2+)-coordination sequence element distinguishes the gluzincin aminopeptidases from other gluzincins. To investigate the importance of the G428AMEN and H464ELAH-(18X)-E487 motifs for the activity of IRAP, mutational analysis was carried out. cDNA encoding the full-length transmembrane form of human IRAP was expressed in HEK293 cells and recombinant wild-type IRAP was shown to have biochemical and enzymatic properties similar to those reported for native IRAP and the soluble serum form of IRAP. Mutational analysis using single amino-acid substitutions in the GAMEN motif (G428A, A429G, M430K, M430E, M430I, E431D and E431A) and in the Zn(2+)-binding motif (H464Y, E465D, E465Q, H468Y, E487D and E487Q) resulted in decreased or abolished aminopeptidase activity towards the leucine-para-nitroanilide substrate. The results show that conservation of residues within the GAMEN and Zn(2+)-binding motifs is important for IRAP enzyme activity.

Insulin recruits GLUT4 from specialized VAMP2-carrying vesicles as well as from the dynamic endosomal/trans-Golgi network in rat adipocytes

Insulin treatment of fat cells results in the translocation of the insulin-responsive glucose transporter type 4, GLUT4, from intracellular compartments to the plasma membrane. However, the precise nature of these intracellular GLUT4-carrying compartments is debated. To resolve the nature of these compartments, we have performed an extensive morphological analysis of GLUT4-containing compartments, using a novel immunocytochemical technique enabling high labeling efficiency and 3-D resolution of cytoplasmic rims isolated from rat epididymal adipocytes. In basal cells, GLUT4 was localized to three morphologically distinct intracellular structures: small vesicles, tubules, and vacuoles. In response to insulin the increase of GLUT4 at the cell surface was compensated by a decrease in small vesicles, whereas the amount in tubules and vacuoles was unchanged. Under basal conditions, many small GLUT4 positive vesicles also contained IRAP (88%) and the v-SNARE, VAMP2 (57%) but not markers of sorting endosomes (EEA1), late endosomes, or lysosomes (lgp120). A largely distinct population of GLUT4 vesicles (56%) contained the cation-dependent mannose 6-phosphate receptor (CD-MPR), a marker protein that shuttles between endosomes and the trans-Golgi network (TGN). In response to insulin, GLUT4 was recruited both from VAMP2 and CD-MPR positive vesicles. However, while the concentration of GLUT4 in the remaining VAMP2-positive vesicles was unchanged, the concentration of GLUT4 in CD-MPR-positive vesicles decreased. Taken together, we provide morphological evidence indicating that, in response to insulin, GLUT4 is recruited to the plasma membrane by fusion of preexisting VAMP2-carrying vesicles as well as by sorting from the dynamic endosomal-TGN system.

Demonstration of insulin-responsive trafficking of GLUT4 and vpTR in fibroblasts

Insulin-responsive trafficking of the GLUT4 glucose transporter and the insulin-regulated aminopeptidase (IRAP) in adipose and muscle cells is well established. Insulin regulation of GLUT4 trafficking in these cells underlies the role that adipose tissue and muscle play in the maintenance of whole body glucose homeostasis. GLUT4 is expressed in a very limited number of tissues, most highly in adipose and muscle, while IRAP is expressed in many tissues. IRAP's physiological role in any of the tissues in which it is expressed, however, is unknown. The fact that IRAP, which traffics by the same insulin-regulated pathway as GLUT4, is expressed in ‘non-insulin responsive’ tissues raises the question of whether these other cell types also have a specialized insulin-regulated trafficking pathway. The existence of an insulin-responsive pathway in other cell types would allow regulation of IRAP activity at the plasma membrane as a potentially important physiological function of insulin. To address this question we use reporter molecules for both GLUT4 and IRAP trafficking to measure insulin-stimulated translocation in undifferentiated cells by quantitative fluorescence microscopy. One reporter (vpTR), a chimera between the intracellular domain of IRAP and the extracellular and transmembrane domains of the transferrin receptor, has been previously characterized. The other is a GLUT4 construct with an exofacial HA epitope and a C-terminal GFP. By comparing these reporters to the transferrin receptor, a marker for general endocytic trafficking, we demonstrate the existence of a specialized, insulin-regulated trafficking pathway in two undifferentiated cell types, neither of which normally express GLUT4. The magnitude of translocation in these undifferentiated cells (approximately threefold) is similar to that reported for the translocation of GLUT4 in muscle cells. Thus, undifferentiated cells have the necessary retention and translocation machinery for an insulin response that is large enough to be physiologically important.

Discrimination of GLUT4 vesicle trafficking from fusion using a temperature-sensitive Munc18c mutant

To examine the temporal relationship between pre- and post-docking events, we generated a Munc18c temperature-sensitive mutant (Munc18c/TS) by substitution of arginine 240 with a lysine residue. At the permissive temperature (23 degrees C), overexpression of both the wild type (Munc18c/WT) and the R240K mutant inhibited insulin-stimulated GLUT4/IRAP vesicle translocation. However, at the non-permissive temperature (37 degrees C) only Munc18c/WT inhibited GLUT4/IRAP translocation whereas Munc18c/TS was without effect. Moreover, Munc18c/WT bound to syntaxin 4 at both 23 and 37 degrees C whereas Munc18c/TS bound syntaxin 4 only at 23 degrees C. This was due to a temperature-dependent conformational change in Munc18c/TS, as its ability to bind syntaxin 4 and effects on GLUT4 translocation were rapidly reversible while protein expression levels remained unchanged. Furthermore, insulin stimulation of Munc18c/TS-expressing cells at 23 degrees C followed by temperature shift to 37 degrees C resulted in an increased rate of GLUT4 translocation compared with cells stimulated at 37 degrees C. To date, this is the first demonstration that the rate-limiting step for insulin-stimulated GLUT4 translocation is the trafficking of GLUT4 vesicles and not their fusion with the plasma membrane.

Basic fibroblast growth factor stimulates surface expression and activity of Na(+)/H(+) exchanger NHE3 via mechanism involving phosphatidylinositol 3-kinase

Na(+)/H(+) exchanger NHE3 is a plasma membrane (PM) protein, which contributes to Na(+) absorption in the intestine. Growth factors stimulate NHE3 via phosphatidylinositol 3-kinase (PI3-K), but mechanism of this process is not clear. To examine the hypothesis that growth factors stimulate NHE3 by modulating NHE3 recycling, and that PI3-K participates in this mechanism, we used PS120 fibroblasts expressing a fusion protein of NHE3 and green fluorescent protein. At steady state, approximately 25% of cellular NHE3 content was expressed at PM. Inhibition of PI3-K decreased PM expression of NHE3, which correlated with retention of the exchanger in recycling endosomal compartment. In contrast, basic fibroblast growth factor (bFGF) increased PM expression of NHE3, which was associated with a 2-fold increase in rate constant for exit of the exchanger from the recycling compartment. Qualitatively similar effects of bFGF were observed in cells pretreated with PI3-K inhibitors, but their magnitude was only approximately 50% of that in intact cells. These data suggest that: (i) bFGF stimulates NHE3 by increasing PM expression of the exchanger; (ii) PI3-K mediates PM expression of NHE3 in both basal and bFGF-stimulated conditions, and (iii) not all of the effects of bFGF on NHE3 expression are mediated by PI3-K, suggesting additional regulatory mechanisms.

Insulin-responsive aminopeptidase trafficking in 3T3-L1 adipocytes

The insulin-responsive aminopeptidase (IRAP/VP165/gp160) was identified originally in GLUT4-containing vesicles and shown to translocate in response to insulin, much like the glucose transporter 4 (GLUT4). This study characterizes the trafficking and kinetics of IRAP in exocytosis, endocytosis, and recycling to the membrane in 3T3-L1 adipocytes. After exposure of 3T3-L1 adipocytes to insulin, IRAP translocated to the plasma membrane as assessed by either cell fractionation, surface biotinylation, or the plasma membrane sheet assay. The rate of exocytosis closely paralleled that of GLUT4. In the continuous presence of insulin, IRAP was endocytosed with a half-time of about 3-5 min. IRAP endocytosis is inhibited by cytosol acidification, a property of clathrin-mediated endocytosis, but not by the expression of a constitutively active Akt/PKB. Arrival in an LDM fraction derived via subcellular fractionation exhibited a slower time course than disappearance from the cell surface, suggesting additional endocytic intermediates. As assayed by membrane "sheets," GLUT4 and IRAP showed similar internalization rates that are wortmannin-insensitive and occur with a half-time of roughly 5 min. IRAP remaining on the cell surface 10 min following insulin removal was both biotin- and avidin-accessible, implying the absence of thin-necked invaginations. Finally, endocytosed IRAP quickly recycled back to the plasma membrane in a wortmannin-sensitive process. These results demonstrate rapid endocytosis and recycling of IRAP in the presence of insulin and trafficking that matches GLUT4 in rate.

Munc18c function is required for insulin-stimulated plasma membrane fusion of GLUT4 and insulin-responsive amino peptidase storage vesicles

To examine the functional role of the interaction between Munc18c and syntaxin 4 in the regulation of GLUT4 translocation in 3T3L1 adipocytes, we assessed the effects of introducing three different peptide fragments (20 to 24 amino acids) of Munc18c from evolutionarily conserved regions of the Sec1 protein family predicted to be solvent exposed. One peptide, termed 18c/pep3, inhibited the binding of full-length Munc18c to syntaxin 4, whereas expression of the other two peptides had no effect. In parallel, microinjection of 18c/pep3 but not a control peptide inhibited the insulin-stimulated translocation of endogenous GLUT4 and insulin-responsive amino peptidase (IRAP) to the plasma membrane. In addition, expression of 18c/pep3 prevented the insulin-stimulated fusion of endogenous and enhanced green fluorescent protein epitope-tagged GLUT4- and IRAP-containing vesicles into the plasma membrane, as assessed by intact cell immunofluorescence. However, unlike the pattern of inhibition seen with full-length Munc18c expression, cells expressing 18c/pep3 displayed discrete clusters of GLUT4 abd IRAP storage vesicles at the cell surface which were not contiguous with the plasma membrane. Together, these data suggest that the interaction between Munc18c and syntaxin 4 is required for the integration of GLUT4 and IRAP storage vesicles into the plasma membrane but is not necessary for the insulin-stimulated trafficking to and association with the cell surface.

Separation and partial characterization of three distinct intracellular GLUT4 compartments in rat adipocytes. Subcellular fractionation without homogenization

Insulin recruits GLUT4 from an intracellular location to the plasma membrane in rat adipocytes. The process involves multiple intracellular compartments and multiple protein functions, details of which are largely unknown partly due to our inability to separate individual GLUT4 compartments. Here, by hypotonic lysis, differential centrifugation, and glycerol density gradient sedimentation, we separated intracellular GLUT4 compartments in rat adipocytes into three fractions: plasma membrane-containing fraction T and plasma membrane-free fractions H and L. The GLUT4 contents in fractions T, H, and L were approximately 25, 56, and 18% of total GLUT4, respectively, in basal adipocytes and 55, 42, and 3-4% in insulin-stimulated adipocytes. The plasma membrane GLUT4 contents estimated separately further revealed that intracellular GLUT4 in fraction T amounts to approximately 20% in both basal and insulin-stimulated adipocytes. Organelle-specific marker and membrane traffic-related protein distribution data suggested that intracellular GLUT4 in fraction T represents sorting endosomes, whereas GLUT4 in fractions H and L represents storage endosomes and exocytic vesicles, respectively. The subcellular fractionation without homogenization described here should be useful in identifying the role of the individual GLUT4 compartments and the associated proteins in insulin-induced GLUT4 recruitment in rat adipocytes.

Temporal separation of insulin-stimulated GLUT4/IRAP vesicle plasma membrane docking and fusion in 3T3L1 adipocytes

Examination of the time and temperature dependence of insulin-stimulated GLUT4/IRAP-containing vesicle trafficking demonstrated an approximate 7-fold increase in the half-time for plasma membrane translocation at 23 degrees C (t((1)/(2)) = approximately 30 min) compared with 37 degrees C (t((1)/(2)) = approximately 4 min) without a significant change in the extent of either GLUT4 or IRAP translocation. Localization of the endogenous GLUT4 and expressed GLUT4-enhanced green fluorescent protein fusion protein in intact 3T3L1 adipocytes demonstrated that at 23 degrees C there was a time-dependent accumulation of discrete GLUT4-containing vesicles adjacent to the inner face of the cell surface membrane but that was not contiguous and/or physically incorporated into the plasma membrane. Together, these data demonstrate that the temperature-dependent decrease in the rate of GLUT4 and IRAP translocation results from a reduction in GLUT4/IRAP-containing vesicle fusion and not trafficking or docking to the plasma membrane.

Insulin regulation of protein traffic in rat adipose cells

Rat adipocytes were biotinylated with cell-impermeable reagents, sulfo-N-hydroxysuccinimide-biotin and sulfo-N-hydroxysuccinimide-S-S-biotin in the absence and presence of insulin. Biotinylated and nonbiotinylated populations of the insulin-like growth factor-II/mannose 6-phosphate receptor, the transferrin receptor, and insulin-responsive aminopeptidase were separated by adsorption to streptavidin-agarose to determine the percentage of the biotinylated protein molecules versus their total amount in different subcellular compartments. Results indicate that adipose cells possess at least two distinct cell surface recycling pathways for insulin-like growth factor-II/mannose 6-phosphate receptor (MPR) and transferrin receptor (TfR): one which is mediated by glucose transporter isoform 4(Glut4)-vesicles and another that bypasses this compartment. Under basal conditions, the first pathway is not active, and cell surface recycling of TfR and, to a lesser extent, MPR proceeds via the second pathway. Insulin dramatically stimulates recycling through the first pathway and has little effect on the second. Within the Glut4-containing compartment, insulin has profoundly different effects on intracellular trafficking of insulin-responsive aminopeptidase on one hand and MPR and TfR on the other. After insulin administration, insulin-responsive aminopeptidase is redistributed from Glut4-containing vesicles to the plasma membrane and stays there for at least 30 min with minimal detectable internalization and recycling, whereas MPR and TfR rapidly shuttle between Glut4 vesicles and the plasma membrane in such a way that after 30 min of insulin treatment, virtually every receptor molecule in this compartment completes at least one trafficking cycle to the cell surface. Thus, different recycling proteins, which compose Glut4-containing vesicles, are internalized into this compartment at their own distinctive rates.

Cloning and characterization of a 22 kDa protein from rat adipocytes: a new member of the reticulon family

In the course of our examination of proteins associated with the GLUT4-containing vesicles of rat adipocytes we have identified a new 22 kDa member of the family of endoplasmic reticulum (ER) proteins known as reticulons. The protein, which we refer to as vp20, was purified from a preparation of GLUT4-containing vesicles of rat adipocytes, and tryptic peptides were micro-sequenced. From this information a cDNA encoding a single open reading frame for a protein of 22 kDa was cloned. This protein is homologous to known members of the reticulon protein family. vp20 has two hydrophobic stretches of about 35 amino acids that could be membrane spanning domains and an ER retention motif at its carboxy-terminus. vp20 was most abundant in the high density microsome fraction of adipocytes, which is the fraction most enriched in ER. Only a small fraction of vp20 was present in the GLUT4 vesicle population, and that fraction appears to be due to ER vesicles that were non-specifically bound to the adsorbent. Analysis of tissue distribution of vp20 in rats revealed that it is concentrated in muscle, fat and the brain.

The formation of an insulin-responsive vesicular cargo compartment is an early event in 3T3-L1 adipocyte differentiation

Differentiating 3T3-L1 cells exhibit a dramatic increase in the rate of insulin-stimulated glucose transport during their conversion from proliferating fibroblasts to nonproliferating adipocytes. On day 3 of 3T3-L1 cell differentiation, basal glucose transport and cell surface transferrin binding are markedly diminished. This occurs concomitant with the formation of a distinct insulin-responsive vesicular pool of intracellular glucose transporter 1 (GLUT1) and transferrin receptors as assessed by sucrose velocity gradients. The intracellular distribution of the insulin-responsive aminopeptidase is first readily detectable on day 3, and its gradient profile and response to insulin at this time are identical to that of GLUT1. With further time of differentiation, GLUT4 is expressed and targeted to the same insulin-responsive vesicles as the other three proteins. Our data are consistent with the notion that a distinct insulin-sensitive vesicular cargo compartment forms early during fat call differentiation and its formation precedes GLUT4 expression. The development of this compartment may result from the differentiation-dependent inhibition of constitutive GLUT1 and transferrin receptor trafficking such that there is a large increase in, or the new formation of, a population of postendosomal, insulin-responsive vesicles.

GLUT4 trafficking in insulin-sensitive cells. A morphological review

In recent years, there have been major advances in the understanding of both the cell biology of vesicle trafficking between intracellular compartments and the molecular targeting signals intrinsic to the trafficking proteins themselves. One system to which these advances have been profitably applied is the regulation of the trafficking of a glucose transporter, GLUT4, from intracellular compartment(s) to the cell surface in response to insulin. The unique nature of the trafficking of GLUT4 and its expression in highly differentiated cells makes this a question of considerable interest to cell biologists. Unraveling the tangled web of molecular events coordinating GLUT4 trafficking will eventually lead to a greater understanding of mammalian glucose metabolism, as well as fundamental cell biological principles related to organelle biogenesis and protein trafficking.

Regulation of insulin-stimulated GLUT4 translocation by Munc18c in 3T3L1 adipocytes

Insulin stimulates glucose transporter (GLUT) 4 vesicle translocation from intracellular storage sites to the plasma membrane in 3T3L1 adipocytes through a VAMP2- and syntaxin 4-dependent mechanism. We have observed that Munc18c, a mammalian homolog of the yeast syntaxin-binding protein n-Sec1p, competed for the binding of VAMP2 to syntaxin 4. Consistent with an inhibitory function for Munc18c, expression of Munc18c, but not the related Munc18b isoform, prevented the insulin stimulation of GLUT4 and IRAP/vp165 translocation to the plasma membrane without any significant effect on GLUT1 trafficking. As expected, overexpressed Munc18c was found to co-immunoprecipitate with syntaxin 4 in the basal state. However, these complexes were found to dissociate upon insulin stimulation. Furthermore, endogenous Munc18c was predominantly localized to the plasma membrane and its distribution was not altered by insulin stimulation. Although expression of enhanced green fluorescent protein-Munc18c primarily resulted in a dispersed cytosolic distribution, co-expression with syntaxin 4 resulted in increased localization to the plasma membrane. Together, these data suggest that Munc18c inhibits the docking/fusion of GLUT4-containing vesicles by blocking the binding of VAMP2 to syntaxin 4. Insulin relieves this inhibition by inducing the dissociation of Munc18c from syntaxin 4 and by sequestering Munc18c to an alternative plasma membrane binding site.

Molecular mechanisms involved in GLUT4 translocation in muscle during insulin and contraction stimulation

Studies in mammalian cells have established the existence of numerous intracellular signaling cascades that are critical intermediates in the regulation of various biological functions. Over the past few years considerable research has shown that many of these signaling proteins are expressed in skeletal muscle. However, the detailed mechanisms involved in the regulation of glucose transporter (GLUT4) translocation from intracellular compartments to the cell surface membrane in response to insulin and contractions in skeletal muscle are not well understood. In the present essay we report three different approaches to unravel the GLUT4 translocation mechanism: 1. specific pertubation of the insulin and/or contraction signaling pathways; 2. characterization of the protein composition of GLUT4-containing vesicles with the expectation that knowledge of the constituent proteins of the vesicles may help in understanding their trafficking; 3. degree of co-immunolocalization of the GLUT4 glucose transporters with other membrane marker proteins assessed by immunofluorescense and electron microscopy.

Identification of an insulin-responsive, slow endocytic recycling mechanism in Chinese hamster ovary cells

In adipocytes, the insulin-regulated aminopeptidase (IRAP) is trafficked through the same insulin-regulated recycling pathway as the GLUT4 glucose transporter. We find that a chimera, containing the cytoplasmic domain of IRAP fused to transmembrane and extracellular domains of the transferrin receptor, is slowly recycled and rapidly internalized in Chinese hamster ovary cells. Morphological studies indicate that the chimera is slowly trafficked through the general endosomal recycling compartment rather than being sorted to a specialized recycling pathway. A chimera in which a di-leucine sequence within the cytoplasmic domain of IRAP has been mutated to alanines is rapidly internalized and rapidly recycled, indicating that this di-leucine is required for the slow recycling but not for the rapid internalization. Insulin stimulates a 2-3-fold increase in the recycling of the chimera and only a 1.2-fold increase in the recycling of the transferrin receptor. The effect of insulin on the recycling of the chimera is blocked by wortmannin, a phosphatidylinositol 3'-kinase inhibitor. GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) increases the recycling of the chimera by 50% but has no effect on the recycling of the transferrin receptor. In these studies, we have identified in Chinese hamster ovary cells a novel, slow endocytic recycling mechanism that is regulated by insulin.

Endocytosis of the glucose transporter GLUT4 is mediated by the GTPase dynamin

To study the role of the GTPase dynamin in GLUT4 intracellular recycling, we have overexpressed dynamin-1 wild type and a GTPase-negative mutant (K44A) in primary rat adipose cells. Transfection was accomplished by electroporation using an hemagglutinin (HA)-tagged GLUT4 as a reporter protein. In cells expressing HA-GLUT4 alone, insulin results in an approximately 7-fold increase in cell surface anti-HA antibody binding. Studies with wortmannin indicate that the kinetics of HA-GLUT4-trafficking parallel those of the native GLUT4 and in addition, that newly synthesized HA-GLUT4 goes to the plasma membrane before being sorted into the insulin-responsive compartments. Short term (4 h) coexpression of dynamin-K44A and HA-GLUT4 increases the amount of cell surface HA-GLUT4 in both the basal and insulin-stimulated states. Under conditions of maximal expression of dynamin-K44A (24 h), most or all of the intracellular HA-GLUT4 appears to be present on the cell surface in the basal state, and insulin has no further effect. Measurements of the kinetics of HA-GLUT4 endocytosis show that dynamin-K44A blocks internalization of the glucose transporters. In contrast, expression of dynamin wild type decreases the amount of cell surface HA-GLUT4 in both the basal and insulin-stimulated states. These data demonstrate that the endocytosis of GLUT4 is largely mediated by processes which require dynamin.

Increased intracellular sequestration of the insulin-regulated aminopeptidase upon differentiation of 3T3-L1 cells

In fat and muscle cells, the glucose transporter GLUT4 is sequestered in an intracellular compartment under basal conditions and redistributes markedly to the plasma membrane in response to insulin. Recently, we characterized a membrane aminopeptidase, designated IRAP (insulin-regulated aminopeptidase), that colocalizes with intracellular GLUT4 and similarly redistributes markedly to the plasma membrane in response to insulin in adipocytes. In contrast to GLUT4, IRAP is also expressed in 3T3-L1 fibroblasts, and this finding provided an opportunity to compare its subcellular distribution in fibroblasts and adipocytes. The relative amount of IRAP at the cell surface was measured by a cell surface biotinylation method. The portion of total IRAP at the cell surface in unstimulated adipocytes was 30% of that in unstimulated fibroblasts. Upon insulin treatment the portion of IRAP at the cell surface was the same in fibroblasts and adipocytes, and was increased 1.8-fold in fibroblasts and 8-fold in adipocytes. A similar analysis of the distribution of the transferrin receptor (TfR), the paradigm for recycling plasma membrane receptors, revealed that the portions of the TfR at the cell surface in both the basal and insulin-treated states were almost unchanged upon differentiation, and that insulin caused an increase of about 1. 6-fold in the amount of TfR at the cell surface. These results show that enhanced intracellular sequestration of IRAP occurs during adipogenesis, and that this effect underlies the larger insulin-elicited fold increase of IRAP at the cell surface in adipocytes.

Sortilin is the major 110-kDa protein in GLUT4 vesicles from adipocytes

Vesicles containing the glucose transporter GLUT4 from rat adipocytes contain a major protein of 110 kDa. We have isolated this protein, obtained the sequences of peptides, and cloned a large portion of its cDNA. This revealed that the protein is sortilin, a novel membrane protein that was cloned in another context from a human source while this work was in progress. Subcellular fractionation of rat and 3T3-L1 adipocytes, together with GLUT4 vesicle isolation, showed that sortilin was primarily located in the low density microsomes in vesicles containing GLUT4. Insulin caused a 1.7-fold increase in the amount of sortilin at the plasma membranes of 3T3-L1 adipocytes, as assessed by cell surface biotinylation. The expression of sortilin in 3T3-L1 cells occurred only upon differentiation. Previous characterization of sortilin has led to the suggestion that it functions to sort lumenal proteins from the trans Golgi. The significance of its insulin-stimulated increase at the cell surface and of its expression upon differentiation will require definitive delineation of its function.