Nocodazole inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes via a microtubule-independent mechanism

The destruction of cytoplasmic skeleton leads to the change of nuclear structure and the looseness of lamin A submicroscopic network

The interaction between lamin A and the cytoplasmic skeleton plays a key role in maintaining nuclear mechanical properties. However, the effect of destruction of the cytoplasmic skeleton on the 3D submicroscopic structure of lamin A has not been elucidated. In this study, we developed an image quantization algorithm to quantify changes in the submicroscopic structure of the intact lamin A 3D network within the nucleus. We used blebbistatin or nocodazole to disrupt the fibrillar structure of F-actin or tubulin, respectively, and then quantified changes in the lamin A super-resolution network structure, the morphological and mechanical properties of the nucleus and the spatial distribution of chromosomes. Ultimately, we found for the first time that disruption of the cytoplasmic skeleton changes the lamin A submicroscopic network and nuclear structural characteristics. In summary, this study contributes to understanding the trans-nuclear membrane interaction characteristics of lamin A and the cytoplasmic skeleton.

Precise control of microtubule disassembly in living cells

Microtubules tightly regulate various cellular activities. Our understanding of microtubules is largely based on experiments using microtubule-targeting agents, which, however, are insufficient to dissect the dynamic mechanisms of specific microtubule populations, due to their slow effects on the entire pool of microtubules. To overcome this technological limitation, we have used chemo and optogenetics to disassemble specific microtubule subtypes, including tyrosinated microtubules, primary cilia, mitotic spindles, and intercellular bridges, by rapidly recruiting engineered microtubule-cleaving enzymes onto target microtubules in a reversible manner. Using this approach, we show that acute microtubule disassembly swiftly halts vesicular trafficking and lysosomal dynamics. It also immediately triggers Golgi and ER reorganization and slows the fusion/fission of mitochondria without affecting mitochondrial membrane potential. In addition, cell rigidity is increased after microtubule disruption owing to increased contractile stress fibers. Microtubule disruption furthermore prevents cell division, but does not cause cell death during interphase. Overall, the reported tools facilitate detailed analysis of how microtubules precisely regulate cellular architecture and functions.

Microtubules in insulin action: what's on the tube?

Microtubules (MT) have a role in the intracellular response to insulin stimulation and subsequent glucose transport by glucose transporter 4 (GLUT4), which resides in specialized storage vesicles that travel through the cell. Before GLUT4 is inserted into the plasma membrane for glucose transport, it undergoes complex trafficking through the cell via the integration of cytoskeletal networks. In this review, we highlight the importance of MT elements in insulin action in adipocytes through a summary of MT depolymerization studies, MT-based GLUT4 movement, molecular motor proteins involved in GLUT4 trafficking, as well as MT-related phenomena in response to insulin and links between insulin action and MT-associated proteins.

Dynamic GLUT4 sorting through a syntaxin-6 compartment in muscle cells is derailed by insulin resistance-causing ceramide

GLUT4 constitutively recycles between the plasma membrane and intracellular depots. Insulin shifts this dynamic equilibrium towards the plasma membrane by recruiting GLUT4 to the plasma membrane from insulin-responsive vesicles. Muscle is the primary site for dietary glucose deposition; however, how GLUT4 sorts into insulin-responsive vesicles, and if and how insulin resistance affects this process, is unknown. In L6 myoblasts stably expressing myc-tagged GLUT4, we analyzed the intracellular itinerary of GLUT4 as it internalizes from the cell surface and examined if such sorting is perturbed by C2-ceramide, a lipid metabolite causing insulin resistance. Surface-labeled GLUT4myc that internalized for 30 min accumulated in a Syntaxin-6 (Stx6)- and Stx16-positive perinuclear sub-compartment devoid of furin or internalized transferrin, and displayed insulin-responsive re-exocytosis. C2-ceramide dispersed the Stx6-positive sub-compartment and prevented insulin-responsive re-exocytosis of internalized GLUT4myc, even under conditions not affecting insulin-stimulated signaling towards Akt. Microtubule disruption with nocodazole prevented pre-internalized GLUT4myc from reaching the Stx6-positive perinuclear sub-compartment and from undergoing insulin-responsive exocytosis. Removing nocodazole allowed both parameters to recover, suggesting that the Stx6-positive perinuclear sub-compartment was required for GLUT4 insulin-responsiveness. Accordingly, Stx6 knockdown inhibited by ∼50% the ability of internalized GLUT4myc to undergo insulin-responsive re-exocytosis without altering its overall perinuclear accumulation. We propose that Stx6 defines the insulin-responsive compartment in muscle cells. Our data are consistent with a model where ceramide could cause insulin resistance by altering intracellular GLUT4 sorting.

Microtubule network is required for insulin-induced signal transduction and actin remodeling

Both microtubule and actin are required for insulin-induced glucose uptake. However, the roles of these two cytoskeletons and their relationship in insulin action still remain unclear. In this work, we examined the morphological change of microtubule/actin and their involvement in insulin signal transduction using rat skeletal muscle cells. Insulin rapidly led to microtubule clustering from ventral to dorsal surface of the cell. Microtubule filaments were rearranged to create space where new actin structures formed. Disruption of microtubule prevented insulin-induced actin remodeling and distal insulin signal transduction, with reduction in surface glucose transporter isoform 4 (GLUT4) and glucose uptake. Though microtubule mediated actin remodeling through PKCζ, reorganization of microtubule depended on tyrosine phosphorylation of insulin receptor, the mechanism is different from insulin-induced actin remodeling, which relied on the activity of PI3-kinase and PKCζ. We propose that microtubule network is required for insulin-induced signal transduction and actin remodeling in skeletal muscle 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.

The Axin/TNKS complex interacts with KIF3A and is required for insulin-stimulated GLUT4 translocation

Insulin-stimulated glucose uptake by the glucose transporter GLUT4 plays a central role in whole-body glucose homeostasis, dysregulation of which leads to type 2 diabetes. However, the molecular components and mechanisms regulating insulin-stimulated glucose uptake remain largely unclear. Here, we demonstrate that Axin interacts with the ADP-ribosylase tankyrase 2 (TNKS2) and the kinesin motor protein KIF3A, forming a ternary complex crucial for GLUT4 translocation in response to insulin. Specific knockdown of the individual components of the complex attenuated insulin-stimulated GLUT4 translocation to the plasma membrane. Importantly, TNKS2(-/-) mice exhibit reduced insulin sensitivity and higher blood glucose levels when re-fed after fasting. Mechanistically, we demonstrate that in the absence of insulin, Axin, TNKS and KIF3A are co-localized with GLUT4 on the trans-Golgi network. Insulin treatment suppresses the ADP-ribosylase activity of TNKS, leading to a reduction in ADP ribosylation and ubiquitination of both Axin and TNKS, and a concurrent stabilization of the complex. Inhibition of Akt, the major effector kinase of insulin signaling, abrogates the insulin-mediated complex stabilization. We have thus elucidated a new protein complex that is directly associated with the motor protein kinesin in insulin-stimulated GLUT4 translocation.

Mannose efflux from the cells: a potential source of mannose in blood

All mammals have 50-100 μM mannose in their blood. However, the source of the dynamic pool of mannose in blood is unknown. Most of it is thought to be derived from glucose in the cells. We studied mannose uptake and release by various cell types. Interestingly, our results show that mannose taken up by the cells through transporters is handled differently from the mannose released within the cells due to glycan processing of protein-bound oligosaccharides. Although more than 95% of incoming mannose is catabolized, most of the mannose released by intracellular processing is expelled from the cells as free mannose predominantly via a nocodazole-sensitive sugar transporter. Under physiological conditions, incoming mannose is more accessible to hexokinase, whereas mannose released within the cells is protected from HK and therefore has a different fate. Our data also suggest that generation of free mannose due to the processing of glycoconjugates composed of glucose-derived mannose and its efflux from the cells can account for most of the mannose found in blood and its steady state maintenance.

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.

Akt inhibitors reduce glucose uptake independently of their effects on Akt

The protein kinase Akt is involved in various cellular processes, including cell proliferation, growth and metabolism. Hyperactivation of Akt is commonly observed in human tumours and so this pathway has been the focus of targeted drug discovery. However, Akt also plays an essential role in other physiological processes, such as the insulin-regulated transport of glucose into muscle and fat cells. This process, which is essential for whole-body glucose homoeostasis in mammals, is thought to be mediated via Akt-dependent movement of GLUT4 glucose transporters to the plasma membrane. In the present study, we have investigated the metabolic side effects of non-ATP-competitive allosteric Akt inhibitors. In 3T3-L1 adipocytes, these inhibitors caused a decrease in the Akt signalling pathway concomitant with reduced glucose uptake. Surprisingly, a similar reduction in GLUT4 translocation to the plasma membrane was not observed. Further investigation revealed that the inhibitory effects of these compounds on glucose uptake in 3T3-L1 adipocytes were independent of the Akt signalling pathway. The inhibitors also inhibited glucose transport into other cell types, including human erythrocytes and T-47D breast cancer cells, suggesting that these effects are not specific to GLUT4. We conclude that these drugs may, at least in part, inhibit tumorigenesis through inhibition of tumour cell glucose transport.

Compartmentalization and regulation of insulin signaling to GLUT4 by the cytoskeleton

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

Insulin action on glucose transporters through molecular switches, tracks and tethers

Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 (glucose transporter-4) to the membrane of muscle and fat cells. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

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

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

A pre-docking role for microtubules in insulin-stimulated glucose transporter 4 translocation

Insulin stimulates glucose uptake by inducing translocation of glucose transporter 4 (GLUT4) from intracellular resides to the plasma membrane. How GLUT4 storage vesicles are translocated from the cellular interior to the plasma membrane remains to be elucidated. In the present study, intracellular transport of GLUT4 storage vesicles and the kinetics of their docking at the plasma membrane were comprehensively investigated at single vesicle level in control and microtubule-disrupted 3T3-L1 adipocytes by time-lapse total internal reflection fluorescence microscopy. It is demonstrated that microtubule disruption substantially inhibited insulin-stimulated GLUT4 translocation. Detailed analysis reveals that microtubule disruption blocked the recruitment of GLUT4 storage vesicles to underneath the plasma membrane and abolished the docking of them at the plasma membrane. These data suggest that transport of GLUT4 storage vesicles to the plasma membrane takes place along microtubules and that this transport is obligatory for insulin-stimulated GLUT4 translocation.

The mass action hypothesis: formation of Glut4 storage vesicles, a tissue-specific, regulated exocytic compartment

Insulin stimulates glucose uptake into the target tissues of fat and muscle by recruiting or translocating Glut4 glucose transport proteins to their functional location at the cell surface. In the basal state, Glut4 is sequestered intracellularly in several vesicular compartments, one of which has come to be known as Glut4 storage vesicles (GSVs). The GSVs represent a tissue-specific compartment that is an ultimate target of the insulin signalling cascade. Glut4 translocation has been extensively studied because of its intrinsic scientific importance to cell biology as well as its relevance to the pathology of type 2 diabetes mellitus. I review herein the ontogeny of GSVs and their composition as it relates to a tissue-specific, hormone-sensitive exocytic compartment and propose a mechanism for their formation.

Bi-directional transport of GLUT4 vesicles near the plasma membrane of primary rat adipocytes

Insulin stimulates glucose uptake into adipocytes by mobilizing intracellular membrane vesicles containing GLUT4 proteins to the plasma membrane. Here we applied time-lapse total internal reflection fluorescence microscopy to study moving parameters and characters of exogenously expressed GLUT4 vesicles in basal, insulin and nocodazole treated primary rat adipocytes. Our results showed that microtubules were essential for long-range transport of GLUT4 vesicles but not obligatory for GLUT4 distribution in rat adipocytes. Insulin reduced the mobility of the vesicles, made them tethered/docked to the PM and finally had constitutive exocytosis. Moreover, long-range bi-directional movements of GLUT4 vesicles were visualized for the first time by TIRFM. It is likely that there are interactions between insulin signaling and microtubules, to regulating GLUT4 translocation in rat adipocytes.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Requirement of vimentin filament assembly for beta3-adrenergic receptor activation of ERK MAP kinase and lipolysis

Catecholamine stimulation of beta-adrenergic receptors (betaAR) in adipocytes activates the cAMP-dependent protein kinase to promote liberation of fatty acids as a fuel source. The adipocyte beta3AR also activates extracellular signal-regulated kinases (ERK)-1 and -2 through direct recruitment and activation of Src kinase. This pathway together with cAMP-dependent protein kinase contributes to maximal beta3AR-stimulated lipolysis. In a search for other molecules that might associate with beta3AR upon agonist stimulation, we identified vimentin using a proteomics approach. Immunoprecipitation of beta3AR from adipocytes in the absence or presence of the beta3AR agonist CL316,243, followed by Western blotting for vimentin confirmed this specific interaction. Since vimentin has also been identified on lipid droplets, the functional consequences of blocking the expression or structural integrity of vimentin intermediate filaments on beta3AR regulation of ERK activation and lipolysis was assessed. Following disruption of intermediate filaments with beta,beta'-iminodipropionitrile, as confirmed by confocal microscopy, beta3AR-stimulated ERK activation was blocked, and lipolysis was reduced by more than 40%. Independently, depletion of vimentin by small hairpin RNA (shRNA) completely inhibited beta3AR-mediated ERK activation and significantly reduced lipolysis. By contrast, disruption of actin-containing microfilaments by cytochalasin D or microtubules by nocodazole had no effect on either lipolysis or ERK activation. These results indicate that vimentin plays an essential role in the signal transduction pathway from beta3AR to the activation ERK and its contribution to lipolysis.

Microtubule network is required for insulin signaling through activation of Akt/protein kinase B: evidence that insulin stimulates vesicle docking/fusion but not intracellular mobility

The microtubule network has been shown to be required for insulin-dependent GLUT4 redistribution; however, the precise molecular function has not been elucidated. In this article, we used fluorescence recovery after photobleaching (FRAP) to evaluate the role of microtubules in intracellular GLUT4 vesicle mobility. A comparison of the rate of fluorescence recovery (t((1/2))), and the maximum fluorescence recovered (F(max)) was made between basal and insulin-treated cells with or without nocodazole treatment to disrupt microtubules. We found that intracellular mobility of fluorescently tagged GLUT4 (HA-GLUT4-GFP) was high in basal cells. Mobility was not increased by insulin treatment. Basal mobility was dependent upon an intact microtubule network. Using a constitutively active Akt to signal GLUT4 redistribution, we found that microtubule-based GLUT4 vesicle mobility was not obligatory for GLUT4 plasma membrane insertion. Our findings suggest that microtubules organize the insulin-signaling complex and provide a surface for basal mobility of GLUT4 vesicles. Our data do not support an obligatory requirement for long range microtubule-based movement of GLUT4 vesicles for insulin-mediated GLUT4 redistribution to the cell surface. Taken together, these findings suggest a model in which insulin signaling targets membrane docking and/or fusion rather than GLUT4 trafficking to the cell surface.

How many signals impinge on GLUT4 activation by insulin?

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

Evidence for high-capacity bidirectional glucose transport across the endoplasmic reticulum membrane by genetically encoded fluorescence resonance energy transfer nanosensors

Glucose release from hepatocytes is important for maintenance of blood glucose levels. Glucose-6-phosphate phosphatase, catalyzing the final metabolic step of gluconeogenesis, faces the endoplasmic reticulum (ER) lumen. Thus, glucose produced in the ER has to be either exported from the ER into the cytosol before release into circulation or exported directly by a vesicular pathway. To measure ER transport of glucose, fluorescence resonance energy transfer-based nanosensors were targeted to the cytosol or the ER lumen of HepG2 cells. During perfusion with 5 mM glucose, cytosolic levels were maintained at approximately 80% of the external supply, indicating that plasma membrane transport exceeded the rate of glucose phosphorylation. Glucose levels and kinetics inside the ER were indistinguishable from cytosolic levels, suggesting rapid bidirectional glucose transport across the ER membrane. A dynamic model incorporating rapid bidirectional ER transport yields a very good fit with the observed kinetics. Plasma membrane and ER membrane glucose transport differed regarding sensitivity to cytochalasin B and showed different relative kinetics for galactose uptake and release, suggesting catalysis by distinct activities at the two membranes. The presence of a high-capacity glucose transport system on the ER membrane is consistent with the hypothesis that glucose export from hepatocytes occurs via the cytosol by a yet-to-be-identified set of proteins.

The subcellular fractionation properties and function of insulin receptor substrate-1 (IRS-1) are independent of cytoskeletal integrity

Efficient insulin action requires spatial and temporal coordination of signaling cascades. The prototypical insulin receptor substrate, IRS-1 plays a central role in insulin signaling. By subcellular fractionation IRS-1 is enriched in a particulate fraction, termed the high speed pellet (HSP), and its redistribution from this fraction is associated with signal attenuation and insulin resistance. Anecdotal evidence suggests the cytoskeleton may underpin the localization of IRS-1 to the HSP. In the present study we have taken a systematic approach to examine whether the cytoskeleton contributes to the subcellular fractionation properties and function of IRS-1. By standard microscopy or immunoprecipitation we were unable to detect evidence to support a specific interaction between IRS-1 and the major cytoskeletal components actin (microfilaments), vimentin (intermediate filaments), and tubulin (microtubules) in 3T3-L1 adipocytes or in CHO.IR.IRS-1 cells. Pharmacological disruption of microfilaments and microtubules, individually or in combination, was without effect on the subcellular distribution of IRS-1 or insulin-stimulated tyrosine phosphorylation in either cell type. Phosphorylation of Akt was modestly reduced (20-35%) in 3T3-L1 adipocytes but not in CHO.IR.IRS-1 cells. In cells lacking intermediate filaments (Vim(-/-)) IRS-1 expression, distribution and insulin-stimulated phosphorylation appeared normal. Even after depolymerisation of microfilaments and microtubules, insulin-stimulated phosphorylation of IRS-1 and Akt were maintained in Vim(-/-) cells. Taken together these data indicate that the characteristic subcellular fractionation properties and function of IRS-1 are unlikely to be mediated by cytoskeletal networks and that proximal insulin signaling does not require an intact cytoskeleton.

Disruption of microtubules ablates the specificity of insulin signaling to GLUT4 translocation in 3T3-L1 adipocytes

Although the cytoskeletal network is important for insulin-induced glucose uptake, several studies have assessed the effects of microtubule disruption on glucose transport with divergent results. Here, we investigated the effects of microtubule-depolymerizing reagent, nocodazole and colchicine, on GLUT4 translocation in 3T3-L1 adipocytes. After nocodazole treatment to disrupt microtubules, GLUT4 vesicles were dispersed from the perinuclear region in the basal state, and insulin-induced GLUT4 translocation was partially inhibited by 20-30%, consistent with other reports. We found that platelet-derived growth factor (PDGF), which did not stimulate GLUT4 translocation in intact cells, was surprisingly able to enhance GLUT4 translocation to approximately 50% of the maximal insulin response, in nocodazole-treated cells with disrupted microtubules. This effect of PDGF was blocked by pretreatment with wortmannin and attenuated in cells pretreated with cytochalasin D. Using confocal microscopy, we found an increased co-localization of GLUT4 and F-actin in nocodazole-treated cells upon PDGF stimulation compared with control cells. Furthermore, microinjection of small interfering RNA targeting the actin-based motor Myo1c, but not the microtubule-based motor KIF3, significantly inhibited both insulin- and PDGF-stimulated GLUT4 translocation after nocodazole treatment. In summary, our data suggest that 1) proper perinuclear localization of GLUT4 vesicles is a requirement for insulin-specific stimulation of GLUT4 translocation, and 2) nocodazole treatment disperses GLUT4 vesicles from the perinuclear region allowing them to engage insulin and PDGF-sensitive actin filaments, which can participate in GLUT4 translocation in a phosphatidylinositol 3-kinase-dependent manner.

Akt Activation Is Required at a Late Stage of Insulin-Induced GLUT4 Translocation to the Plasma Membrane

Insulin stimulates the translocation of glucose transporter GLUT4 from intracellular vesicles to the plasma membrane (PM). This involves multiple steps as well as multiple intracellular compartments. The Ser/Thr kinase Akt has been implicated in this process, but its precise role is ill defined. To begin to dissect the role of Akt in these different steps, we employed a low-temperature block. Upon incubation of 3T3-L1 adipocytes at 19 C, GLUT4 accumulated in small peripheral vesicles with a slight increase in PM labeling concomitant with reduced trans-Golgi network labeling. Although insulin-dependent translocation of GLUT4 to the PM was impaired at 19 C, we still observed movement of vesicles toward the surface. Strikingly, insulin-stimulated Akt activity, but not phosphatidylinositol 3 kinase activity, was blocked at 19 C. Consistent with a multistep process in GLUT4 trafficking, insulin-stimulated GLUT4 translocation could be primed by treating cells with insulin at 19 C, whereas this was not the case for Akt activation. These data implicate two insulin-regulated steps in GLUT4 translocation: 1) redistribution of GLUT4 vesicles toward the cell cortex—this process is Akt-independent and is not blocked at 19 C; and 2) docking and/or fusion of GLUT4 vesicles with the PM—this process may be the major Akt-dependent step in the insulin regulation of glucose transport.

Insulin and oleate promote translocation of inosine-5' monophosphate dehydrogenase to lipid bodies

In the present study we identify inosine-5' monophosphate dehydrogenase (IMPDH), a key enzyme in de novo guanine nucleotide biosynthesis, as a novel lipid body-associated protein. To identify new targets of insulin we performed a comprehensive 2-DE analysis of (32)P-labelled proteins isolated from 3T3-L1 adipocytes (Hill et al. J Biol Chem 2000; 275: 24313-24320). IMPDH was identified by liquid chromatography/tandem mass spectrometry as a protein which was phosphorylated in a phosphatidylinositol (PI) 3-kinase-dependent manner upon insulin treatment. Although insulin had no significant effect on IMPDH activity, we observed translocation of IMPDH to lipid bodies following insulin treatment. Induction of lipid body formation with oleic acid promoted dramatic redistribution of IMPDH to lipid bodies, which appeared to be in contact with the endoplasmic reticulum, the site of lipid body synthesis and recycling. Inhibition of PI 3-kinase blocked insulin- and oleate-induced translocation of IMPDH and reduced oleate-induced lipid accumulation. However, we found no evidence of oleate-induced IMPDH phosphorylation, suggesting phosphorylation and translocation may not be coupled events. These data support a role for IMPDH in the dynamic regulation of lipid bodies and fatty acid metabolism and regulation of its activity by subcellular redistribution in response to extracellular factors that modify lipid metabolism.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

Improved glucose homeostasis and enhanced insulin signalling in Grb14-deficient mice

Gene targeting was used to characterize the physiological role of growth factor receptor-bound (Grb)14, an adapter-type signalling protein that associates with the insulin receptor (IR). Adult male Grb14(-/-) mice displayed improved glucose tolerance, lower circulating insulin levels, and increased incorporation of glucose into glycogen in the liver and skeletal muscle. In ex vivo studies, insulin-induced 2-deoxyglucose uptake was enhanced in soleus muscle, but not in epididymal adipose tissue. These metabolic effects correlated with tissue-specific alterations in insulin signalling. In the liver, despite lower IR autophosphorylation, enhanced insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and activation of protein kinase B (PKB) was observed. In skeletal muscle, IR tyrosine phosphorylation was normal, but signalling via IRS-1 and PKB was increased. Finally, no effect of Grb14 ablation was observed on insulin signalling in white adipose tissue. These findings demonstrate that Grb14 functions in vivo as a tissue-specific modulator of insulin action, most likely via repression of IR-mediated IRS-1 tyrosine phosphorylation, and highlight this protein as a potential target for therapeutic intervention.

Insulin promotes formation of polymerized microtubules by a phosphatidylinositol 3-kinase-independent, actin-dependent pathway in 3T3-L1 adipocytes

Direct demonstrations implicating the microtubule cytoskeleton in insulin-mediated adipose/muscle-specific glucose transporter (GLUT4) translocation are beginning to emerge, and one role of the microtubule network appears to be the provision of a solid support for GLUT4 vesicle movement. In the current study we show that insulin treatment increases total polymerized alpha-tubulin in microtubules in a time- and dose-dependent manner that coincides with established insulin-mediated changes in GLUT4 translocation. Insulin stimulates the growth of microtubules through a pathway that requires tyrosine kinase activity, as indicated by inhibition of the effect after treatment with genistein. Insulin-mediated growth was not inhibited by treatment with the MAPK kinase (MEK) inhibitor, PD98059 or by wortmannin, indicating that the effect does not require activation of extracellular signal-regulated kinase 1/2 or phosphatidylinositide 3-kinase. Depolymerization of the actin cytoskeleton with latrunculin B abrogated the effect of insulin on microtubule polymerization, indicating that an intact actin network is a requirement for insulin-dependent modulation of microtubules. Using methods that measure insulin-dependent GLUT4 translocation in populations of adipocytes as opposed to individual cells, we show a statistically significant reduction in translocation (30% inhibition) in the presence of low concentrations of nocodazole (2 mum). This concentration incompletely depolymerizes the microtubule network, revealing that partial depolymerization of microtubules is sufficient to inhibit GLUT4 translocation. It is likely that stabilization of the microtubule network contributes to insulin stimulation of GLUT4 translocation.

GLUT4 is retained by an intracellular cycle of vesicle formation and fusion with endosomes

The intracellularly stored GLUT4 glucose transporter is rapidly translocated to the cell surface upon insulin stimulation. Regulation of GLUT4 distribution is key for the maintenance of whole body glucose homeostasis. We find that GLUT4 is excluded from the plasma membrane of adipocytes by a dynamic retention/retrieval mechanism. Our kinetic studies indicate that GLUT4-containing vesicles continually bud and fuse with endosomes in the absence of insulin and that these GLUT4 vesicles are 5 times as likely to fuse with an endosome as with the plasma membrane. We hypothesize that this intracellular cycle of vesicle budding and fusion is an element of the active mechanism by which GLUT4 is retained. The GLUT4 trafficking pathway does not extensively overlap with that of furin, indicating that the trans-Golgi network, a compartment in which furin accumulates, is not a significant storage reservoir of GLUT4. An intact microtubule cytoskeleton is required for insulin-stimulated recruitment to the cell surface, although it is not required for the basal budding/fusion cycle. Nocodazole disruption of the microtubule cytoskeleton reduces the insulin-stimulated exocytosis of GLUT4, accounting for the reduced insulin-stimulated translocation of GLUT4 to the cell surface.

Localization of low density lipoprotein receptor-related protein 1 to caveolae in 3T3-L1 adipocytes in response to insulin treatment

The insulin-induced translocation of low density lipoprotein receptor-related protein 1 (LRP1) from intracellular membranes to the cell surface in 3T3-L1 adipocytes was differentiation-dependent and did not occur in 3T3-L1 fibroblasts. Prompted by findings that the plasma membrane of 3T3-L1 adipocytes was rich in caveolae, we determined whether LRP1 became caveolae-associated upon insulin stimulation. The caveolae domain was isolated by the well characterized detergent solubilization and sucrose density ultracentrifugation methodology. Under basal conditions, only a trace amount of LRP1 was caveolae-associated despite the markedly elevated caveolin-1 and caveolae after adipocytic cell differentiation. Upon insulin treatment, the amount of LRP1 associated with caveolae was increased by 4-fold within 10 min, which was blocked completely by pretreatment with wortmannin prior to insulin. The caveolar localization of LRP1 in adipocytes was specific to insulin; treatment with platelet-derived growth factor-bb isoform did not promote but rather decreased caveolar localization of LRP1 below basal levels. The insulin-induced caveolar localization of LRP1 was also observed in 3T3-L1 fibroblasts where translocation of LRP1 from intracellular membranes to the cell surface was absent, suggesting that association of LRP1 with caveolae was achieved, at least in part, through lateral transmigration along the plane of plasma membranes. Immunocytochemistry studies revealed partial co-localization of LRP1 (either endogenous LRP1 or an epitope-tagged minireceptor) with caveolin-1 in cells treated with insulin, which was confirmed by co-immunoprecipitation of LRP1 with caveolin-1 in cells treated with insulin but not platelet-derived growth factor-bb. These results suggest that the localization of LRP1 to caveolae responds selectively to extracellular signals.

Interleukin-3-mediated cell survival signals include phosphatidylinositol 3-kinase-dependent translocation of the glucose transporter GLUT1 to the cell surface

Maintenance of glucose uptake is a key component in the response of hematopoietic cells to survival factors. To investigate the mechanism of this response we employed the interleukin-3 (IL-3)-dependent murine mast cell line IC2.9. In these cells, hexose uptake decreased markedly upon withdrawal of IL-3, whereas its readdition led to rapid (t(1/2) approximately 10 min) stimulation of transport, associated with an approximately 4-fold increase in Vmax but no change in Km. Immunocytochemistry and photoaffinity labeling revealed that IL-3 caused translocation of intracellular GLUT1 transporters to the cell surface, whereas a second transporter isoform, GLUT3, remained predominantly intracellular. The inhibitory effects of latrunculin B and jasplakinolide, and of nocodazole and colchicine, respectively, revealed a requirement for both the actin and microtubule cytoskeletons in GLUT1 translocation and transport stimulation. Both IL-3 stimulation of transport and GLUT1 translocation were also prevented by the phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002. The time courses for activation of phosphatidylinositol 3-kinase and its downstream target, protein kinase B, by IL-3 were consistent with a role in IL-3-induced transporter translocation and enhanced glucose uptake. We conclude that one component of the survival mechanisms elicited by IL-3 involves the subcellular redistribution of glucose transporters, thus ensuring the supply of a key metabolic substrate.

Disruption of microtubules in rat skeletal muscle does not inhibit insulin- or contraction-stimulated glucose transport

Insulin and muscle contractions stimulate glucose transport in skeletal muscle through a translocation of intracellular GLUT4 glucose transporters to the cell surface. Judged by immunofluorescence microscopy, part of the GLUT4 storage sites is associated with the extensive microtubule cytoskeleton found in all muscle fibers. Here, we test whether microtubules are required mediators of the effect of insulin and contractions. In three different incubated rat muscles with distinct fiber type composition, depolymerization of microtubules with colchicine for < or =8 h did not inhibit insulin- or contraction-stimulated 2-deoxyglucose transport or force production. On the contrary, colchicine at least partially prevented the approximately 30% decrease in insulin-stimulated transport that specifically developed during 8 h of incubation in soleus muscle but not in flexor digitorum brevis or epitrochlearis muscles. In contrast, nocodazole, another microtubule-disrupting drug, rapidly and dose dependently blocked insulin- and contraction-stimulated glucose transport. A similar discrepancy between colchicine and nocodazole was also found in their ability to block glucose transport in muscle giant "ghost" vesicles. This suggests that the ability of insulin and contractions to stimulate glucose transport in muscle does not require an intact microtubule network and that nocodazole inhibits glucose transport independently of its microtubule-disrupting effect.

Insulin recruits GLUT4 from distinct compartments via distinct traffic pathways with differential microtubule dependence in rat adipocytes

In the present study, we investigated the physiological significance of the microtubules in the subcellular localization and trafficking of GLUT4 in rat primary adipocytes. Morphological and biochemical analyses revealed a dose- and time-dependent disruption of the microtubules by treatment with nocodazole. With nearly complete disruption of the microtubules, the insulin-stimulated glucose transport activity was inhibited by 55%. This inhibition was concomitant with a comparable inhibition of GLUT4 translocation measured by the subcellular fractionation and the cell-surface GLUT4 labeling by trypsin cleavage. In addition, the time-course of insulin stimulation of the glucose transport activity was significantly delayed by microtubule disruption (t(1/2) were 7 and 2.3 min in nocodazole-treated and control cells, respectively), while the rate of GLUT4 endocytosis was little affected. The impaired insulin-stimulated glucose transport activity was not fully restored to the level in control cells by blocking GLUT4 endocytosis, suggesting that the inhibition was due to the existence of a microtubule-dependent subpopulation in the insulin-responsive GLUT4 pool. On the other hand, nocodazole partially inhibited insulin-induced translocation of the insulin-regulated aminopeptidase and the vesicle-associated membrane protein (VAMP)-2 without affecting GLUT1 and VAMP-3. In electrically permeabilized adipocytes, the insulin-stimulated glucose transport was inhibited by 40% by disruption of the microtubules whereas that stimulated with GTP gamma S was not affected. Intriguingly, the two reagents stimulated glucose transport to the comparable level by disruption of the microtubules. These data suggest that insulin recruits GLUT4 to the plasma membrane from at least two distinct intracellular compartments via distinct traffic routes with differential microtubule dependence in rat primary adipocytes.

Threonine deprivation rapidly activates the system A amino acid transporter in primary cultures of rat neurons from the essential amino acid sensor in the anterior piriform cortex

Omnivores show recognition of essential (indispensable) amino acid deficiency by changing their feeding behavior within 20 min, yet the cellular mechanisms of amino acid sensation in eukaryotes are poorly understood. The anterior piriform cortex (APC) of the brain in rats or its analog in birds likely houses the in vivo amino acid chemosensor. Because amino acid transporters adapt rapidly to essential amino acid deficiency in several cell models, we hypothesized that activation of electrogenic amino acid transport in APC neurons might contribute to the function of the amino acid sensor. We evaluated transport systems in primary cultures of neurons from the APC, hippocampus and cerebellum, or glia, incubated in complete or threonine-devoid (deficient) medium. After 10 min in deficient medium, uptake of threonine or a system A-selective substrate, methyl amino-isobutyric acid, was increased 60% in APC neurons only (P < 0.05). These results demonstrated upregulation of system A, an electrogenic amino acid-sodium symporter. This depletion-induced activation required sodium, intact intracellular trafficking, and phosphorylation of signal transduction-related kinases. Efflux studies showed that other transporter types were functional in the APC; they appeared to be altered dynamically in threonine-deficient cells in response to rapid increases in system A activity. The present data provided support for the chemical sensitivity of the APC and its role as the brain area housing the indispensable amino acid chemosensor. They also showed a region-specific, phosphorylation-dependent activation of the system A transporter in the brain in response to threonine deficiency.

Insulin-induced GLUT4 translocation involves protein kinase C-lambda-mediated functional coupling between Rab4 and the motor protein kinesin

Insulin stimulates glucose transport by promoting translocation of GLUT4 proteins from the perinuclear compartment to the cell surface. It has been previously suggested that the microtubule-associated motor protein kinesin, which transports cargo toward the plus end of microtubules, plays a role in translocating GLUT4 vesicles to the cell surface. In this study, we investigated the role of Rab4, a small GTPase-binding protein, and the motor protein KIF3 (kinesin II in mice) in insulin-induced GLUT4 exocytosis in 3T3-L1 adipocytes. Photoaffinity labeling of Rab4 with [gamma-(32)P]GTP-azidoanilide showed that insulin stimulated Rab4 GTP loading and that this insulin effect was inhibited by pretreatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor LY294002 or expression of dominant-negative protein kinase C-lambda (PKC-lambda). Consistent with previous reports, expression of dominant-negative Rab4 (N121I) decreased insulin-induced GLUT4 translocation by 45%. Microinjection of an anti-KIF3 antibody into 3T3-L1 adipocytes decreased insulin-induced GLUT4 exocytosis by 65% but had no effect on endocytosis. Coimmunoprecipitation experiments showed that Rab4, but not Rab5, physically associated with KIF3, and this was confirmed by showing in vitro association using glutathione S-transferase-Rab4. A microtubule capture assay demonstrated that insulin stimulation increased the activity for the binding of KIF3 to microtubules and that this activation was inhibited by pretreatment with the PI3-kinase inhibitor LY294002 or expression of dominant-negative PKC-lambda. Taken together, these data indicate that (i) insulin signaling stimulates Rab4 activity, the association of Rab4 with kinesin, and the interaction of KIF3 with microtubules and (ii) this process is mediated by insulin-induced PI3-kinase-dependent PKC-lambda activation and participates in GLUT4 exocytosis in 3T3-L1 adipocytes.

Conventional kinesin KIF5B mediates insulin-stimulated GLUT4 movements on microtubules

Insulin stimulates glucose uptake in muscle and adipose cells by mobilizing intracellular membrane vesicles containing GLUT4 glucose transporter proteins to the plasma membrane. Here we show in live cultured adipocytes that intracellular membranes containing GLUT4-yellow fluorescent protein (YFP) move along tubulin-cyan fluorescent protein-labeled microtubules in response to insulin by a mechanism that is insensitive to the phosphatidylinositol 3 (PI3)-kinase inhibitor wortmannin. Insulin increased by several fold the observed frequencies, but not velocities, of long-range movements of GLUT4-YFP on microtubules, both away from and towards the perinuclear region. Genomics screens show conventional kinesin KIF5B is highly expressed in adipocytes and this kinesin is partially co-localized with perinuclear GLUT4. Dominant-negative mutants of conventional kinesin light chain blocked outward GLUT4 vesicle movements and translocation of exofacial Myc-tagged GLUT4-green fluorescent protein to the plasma membrane in response to insulin. These data reveal that insulin signaling targets the engagement or initiates the movement of GLUT4-containing membranes on microtubules via conventional kinesin through a PI3-kinase-independent mechanism. This insulin signaling pathway regulating KIF5B function appears to be required for GLUT4 translocation to the plasma membrane.

Evidence for a functional interaction between the ClC-2 chloride channel and the retrograde motor dynein complex

The ClC-2 chloride channel has been implicated in essential physiological functions. Analyses of ClC-2 knock-out mice suggest that ClC-2 expression in retinal pigment epithelia and Sertoli cells normally supports the viability of photoreceptor cells and male germ cells, respectively. Further, other studies suggest that ClC-2 expression in neurons may modify inhibitory synaptic transmission via the gamma-aminobutyric acid, type A receptor. However, complete understanding of the physiological functions of ClC-2 requires elucidation of the molecular basis for its regulation. Using cell imaging and biochemical and electrophysiological techniques, we show that expression of ClC-2 at the cell surface may be regulated via an interaction with the dynein motor complex. Mass spectrometry and Western blot analysis of eluate from a ClC-2 affinity matrix showed that heavy and intermediate chains of dynein bind ClC-2 in vitro. The dynein intermediate chain co-immunoprecipitates with ClC-2 from hippocampal membranes suggesting that they also interact in vivo. Disruption of dynein motor function perturbs ClC-2 localization and increases the functional expression of ClC-2 in the plasma membranes of COS7 cells. Thus, cell surface expression of ClC-2 may be regulated by dynein motor activity. This work is the first to demonstrate an in vivo interaction between an ion channel and the dynein motor complex.

High glucose and insulin promote O-GlcNAc modification of proteins, including alpha-tubulin

Increased flux through the hexosamine biosynthesis pathway has been implicated in the development of glucose-induced insulin resistance and may promote the modification of certain proteins with O-linked N-acetylglucosamine (O-GlcNAc). L6 myotubes (a model of skeletal muscle) were incubated for 18 h in 5 or 25 mM glucose with or without 10 nM insulin. As assessed by immunoblotting with an O-GlcNAc-specific antibody, high glucose and/or insulin enhanced O-GlcNAcylation of numerous proteins, including the transcription factor Sp1, a known substrate for this modification. To identify novel proteins that may be O-GlcNAc modified in a glucose concentration/insulin-responsive manner, total cell membranes were separated by one- or two-dimensional gel electrophoresis. Selected O-GlcNAcylated proteins were identified by mass spectrometry (MS) analysis. MS sequencing of tryptic peptides identified member(s) of the heat shock protein 70 (HSP70) family and rat alpha-tubulin. Immunoprecipitation/immunoblot studies demonstrated several HSP70 isoforms and/or posttranslational modifications, some with selectively enhanced O-GlcNAcylation following exposure to high glucose plus insulin. In conclusion, in L6 myotubes, Sp1, membrane-associated HSP70, and alpha-tubulin are O-GlcNAcylated; the modification is markedly enhanced by sustained increased glucose flux.

Intracellular insulin-responsive glucose transporter (GLUT4) distribution but not insulin-stimulated GLUT4 exocytosis and recycling are microtubule dependent

To investigate the potential role of microtubules in the regulation of insulin-responsive glucose transporter (GLUT4) trafficking in adipocytes, we examined the effects of microtubule depolymerizing and stabilizing agents. In contrast to previous reports, disruption or stabilization of microtubule structures had no significant effect on insulin-stimulated GLUT4 translocation. However, consistent with a more recent study (Molero, J. C., J. P. Whitehead, T. Meerloo, and D. E. James, 2001, J Biol Chem 276:43829-43835) nocodazole did inhibit glucose uptake through a direct interaction with the transporter itself independent of the translocation process. In addition, the initial rate of GLUT4 endocytosis was not significantly affected by microtubule depolymerization. However, these internalized GLUT4 compartments are confined to regions just beneath the plasma membrane and were not exposed to the extracellular space. Furthermore, they were unable to undergo further sorting steps and trafficking to the perinuclear region. Nevertheless, these apparent early endocytic GLUT4 compartments fully responded to a second insulin stimulation with an identical extent of plasma membrane translocation. Together, these data demonstrate that although microtubular organization may play a role in the trafficking of GLUT4 early endocytic vesicles back to the perinuclear region, they do not have a significant role in insulin-stimulated GLUT4 exocytosis, initial endocytosis from the plasma membrane and/or recycling back to the plasma membrane.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.