Chronic treatment with insulin selectively down-regulates cell-surface GLUT4 glucose transporters in 3T3-L1 adipocytes

Silybin restores glucose uptake after tumour necrosis factor-alpha and lipopolysaccharide stimulation in 3T3-L1 adipocytes

Obesity is associated with a low-grade chronic inflammatory process characterized by higher circulating TNFα levels, thus contributing to insulin resistance. This study evaluated the effect of silybin, the main bioactive component of silymarin, which has anti-inflammatory properties, on TNFα levels and its impact on glucose uptake in the adipocyte cell line 3T3-L1 challenged with two different inflammatory stimuli, TNFα or lipopolysaccharide (LPS). Silybin's pre-treatment effect was evaluated in adipocytes pre-incubated with silybin (30 or 80 µM) before challenging with the inflammatory stimuli (TNFα or LPS). For the post-treatment effect, the adipocytes were first challenged with the inflammatory stimuli and then post-treated with silybin. After treatments, TNFα production, glucose uptake, and GLUT4 protein expression were determined. Both inflammatory stimuli increased TNFα secretion, diminished GLUT4 expression, and significantly decreased glucose uptake. Silybin 30 µM only reduced TNFα secretion after the LPS challenge. Silybin 80 µM as post-treatment or pre-treatment decreased TNFα levels, improving glucose uptake. However, glucose uptake enhancement induced by silybin did not depend on GLUT4 protein expression. These results show that silybin importantly reduced TNFα levels and upregulates glucose uptake, independently of GLUT4 protein expression.

Low concentrations of α-lipoic acid reduce palmitic acid-induced alterations in murine hypertrophic adipocytes

Obesity is a metabolic disorder with excessive body fat accumulation, increasing incidence of chronic metabolic diseases. Hypertrophic obesity is associated with local oxidative stress and inflammation. Herein, we evaluated the in vitro activity of micromolar concentrations of α-lipoic acid (ALA) on palmitic acid (PA)-exposed murine hypertrophic 3T3-L1 adipocytes, focussing on the main molecular pathways involved in adipogenesis, inflammation, and insulin resistance. ALA, starting from 1 µM, decreased adipocytes hypertrophy, reducing PA-triggered intracellular lipid accumulation, PPAR-γ levels, and FABP4 gene expression, and counteracted PA-induced intracellular ROS levels and NF-κB activation. ALA reverted PA-induced insulin resistance, restoring PI3K/Akt axis and inducing GLUT-1 and glucose uptake, showing insulin sensitizing properties since it increased their basal levels. In conclusion, this study supports the potential effects of low micromolar ALA against hypertrophy, inflammation, and insulin resistance in adipose tissue, suggesting its important role as pharmacological supplement in the prevention of conditions linked to obesity and metabolic syndrome.

Insulin resistance induced by long-term hyperinsulinemia abolishes the effects of acute insulin exposure on cell-surface nicotinic acetylcholine receptor levels and actin cytoskeleton morphology

Using CHO-K1/A5 cells, a clonal cell line that robustly expresses adult muscle-type nicotinic acetylcholine receptor (nAChR), we explored whether insulin resistance in these mammalian cells affects cell-surface expression of the nAChR, its endocytic internalization, and actin cytoskeleton integrity. Acute nanomolar insulin stimulation resulted in a slow increase in nAChR cell-surface levels, reaching maximum levels at ∼1 h. Long periods of insulin incubation caused CHO-K1/A5 cells to become insulin resistant, as previously observed with several other cell types. Furthermore, long-term insulin treatment abolished the effects of short-term insulin exposure on cell-surface nAChR levels, suggestive of a desensitization phenomenon. It also affected the kinetics of ligand-induced nAChR internalization. Since the integrity of the cortical actin cytoskeleton affects nAChR endocytosis, we also studied the effects of long-term insulin treatment on this meshwork. We found that it significantly affected the cortical actin morphology of CHO-K1/A5 cells and the response of the actin cytoskeleton to a subsequent short-term insulin stimulus. Overall, the present results show for the first time the effects of insulin signaling on cell-surface nAChR expression and actin cytoskeleton-associated internalization.

GLUT4 translocation and dispersal operate in multiple cell types and are negatively correlated with cell size in adipocytes

The regulated translocation of the glucose transporter, GLUT4, to the surface of adipocytes and muscle is a key action of insulin. This is underpinned by the delivery and fusion of GLUT4-containing vesicles with the plasma membrane. Recent studies have revealed that a further action of insulin is to mediate the dispersal of GLUT4 molecules away from the site of GLUT4 vesicle fusion with the plasma membrane. Although shown in adipocytes, whether insulin-stimulated dispersal occurs in other cells and/or is exhibited by other proteins remains a matter of debate. Here we show that insulin stimulates GLUT4 dispersal in the plasma membrane of adipocytes, induced pluripotent stem cell-derived cardiomyocytes and HeLa cells, suggesting that this phenomenon is specific to GLUT4 expressed in all cell types. By contrast, insulin-stimulated dispersal of TfR was not observed in HeLa cells, suggesting that the mechanism may be unique to GLUT4. Consistent with dispersal being an important physiological mechanism, we observed that insulin-stimulated GLUT4 dispersal is reduced under conditions of insulin resistance. Adipocytes of different sizes have been shown to exhibit distinct metabolic properties: larger adipocytes exhibit reduced insulin-stimulated glucose transport compared to smaller cells. Here we show that both GLUT4 delivery to the plasma membrane and GLUT4 dispersal are reduced in larger adipocytes, supporting the hypothesis that larger adipocytes are refractory to insulin challenge compared to their smaller counterparts, even within a supposedly homogeneous population of cells.

In vitro characterization of the effects of chronic insulin stimulation in mouse 3T3-L1 and human SGBS adipocytes

Hyperinsulinemia is the hallmark of the development of insulin resistance and precedes the diagnosis of type 2 diabetes. Here we evaluated the effects of prolonged exposure (≥4 days) to high insulin doses (150 nM) in vitro in two adipose cell types, mouse 3T3-L1 and human SGBS. Chronic insulin treatment significantly decreased lipid droplet size, insulin signalling and insulin-stimulated glucose uptake. 3T3-L1 displayed an increased basal glucose internalization following chronic insulin treatment, which was associated with increased GLUT1 expression. In addition, both cells showed increased basal lipolysis.

In conclusion, we report the effects of prolonged hyperinsulinemia in 3T3-L1 and SGBS, highlighting similarities and discrepancies between the cell types, to be considered when using these cells to model insulin-induced insulin resistance.

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

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

Estimate of reduced glomerular filtration rate by triglyceride-glucose index: insights from a general Chinese population

Objectives: Recent studies have identified triglyceride-glucose index (TyG) as a surrogate of insulin resistance. Since insulin resistance correlates with renal damage, our study aims to investigate the impact of TyG on the risk of reduced eGFR and explore its value to improve the risk stratification and prevention of reduced eGFR. Methods: This cross-sectional study included 6466 participants (mean age:59.57 years, 60.2% females) from rural areas of northeast China between September 2017 to May 2018. TyG was calculated as ln[fasting triglyceride (mg/dL)×fasting plasma glucose (mg/dL)/2]. Reduced eGFR was defined as eGFR<60ml/min per 1.73m². Results: The prevalence of reduced eGFR was 2.94%. After full adjustment, each SD increase of TyG caused 42.6% additional risk for reduced eGFR. When dividing TyG into quartiles, the top quartile had a 1.934 times risk than the bottom quartile. Furthermore, the risk of reduced eGFR increased linearly with the increment of TyG. Subgroup analysis also revealed the association was robust to several risk factors of renal damage. Finally, category-free net reclassification index (0.204, 95% CI: 0.060-0.349, p = 0.005) and integrated discrimination index (0.010, 95% CI: 0.005-0.016, p < 0.001) demonstrated the value of TyG to refine the risk stratification of reduced eGFR. Conclusion: Our work reveals the robust association between TyG and reduced eGFR. Moreover, the present study implicates the potential role of TyG as a risk indicator to optimize the prevention of reduced eGFR. Lastly, the findings also suggest the importance of simultaneous glycemic and lipid control to avoid the development and progression of reduced eGFR.

Modulation of Insulin Resistance and the Adipocyte-Skeletal Muscle Cell Cross-Talk by LCn-3PUFA

The cross-talk between skeletal muscle and adipose tissue is involved in the development of insulin resistance (IR) in skeletal muscle, leading to the decrease in the anabolic effect of insulin. We investigated if the long chain polyunsaturated n-3 fatty acids (LCn-3PUFA), eicosapentaenoic and docosapentaenoic acids (EPA and DPA, respectively) could (1) regulate the development of IR in 3T3-L1 adipocytes and C2C12 muscle cells and (2) inhibit IR in muscle cells exposed to conditioned media (CM) from insulin-resistant adipocytes. Chronic insulin (CI) treatment of adipocytes and palmitic acid (PAL) exposure of myotubes were used to induce IR in the presence, or not, of LCn-3PUFA. EPA (50 µM) and DPA (10 µM) improved PAL-induced IR in myotubes, but had only a partial effect in adipocytes. CM from adipocytes exposed to CI induced IR in C2C12 myotubes. Although DPA increased the mRNA levels of genes involved in fatty acid (FA) beta-oxidation and insulin signaling in adipocytes, it was not sufficient to reduce the secretion of inflammatory cytokines and prevent the induction of IR in myotubes exposed to adipocyte’s CM. Treatment with DPA was able to increase the release of adiponectin by adipocytes into CM. In conclusion, DPA is able to protect myotubes from PAL-induced IR, but not from IR induced by CM from adipocytes.

Glucose transporters: cellular links to hyperglycemia in insulin resistance and diabetes

Abnormal expression and/or function of mammalian hexose transporters contribute to the hallmark hyperglycemia of diabetes. Due to different roles in glucose handling, various organ systems possess specific transporters that may be affected during the diabetic state. Diabetes has been associated with higher rates of intestinal glucose transport, paralleled by increased expression of both active and facilitative transporters and a shift in the location of transporters within the enterocyte, events that occur independent of intestinal hyperplasia and hyperglycemia. Peripheral tissues also exhibit deregulated glucose transport in the diabetic state, most notably defective translocation of transporters to the plasma membrane and reduced capacity to clear glucose from the bloodstream. Expression of renal active and facilitative glucose transporters increases as a result of diabetes, leading to elevated rates of glucose reabsorption. However, this may be a natural response designed to combat elevated blood glucose concentrations and not necessarily a direct effect of insulin deficiency. Functional foods and nutraceuticals, by modulation of glucose transporter activity, represent a potential dietary tool to aid in the management of hyperglycemia and diabetes.

Metabolic, anabolic, and mitogenic insulin responses: A tissue-specific perspective for insulin receptor activators

Insulin acts as the major regulator of the fasting-to-fed metabolic transition by altering substrate metabolism, promoting energy storage, and helping activate protein synthesis. In addition to its glucoregulatory and other metabolic properties, insulin can also act as a growth factor. The metabolic and mitogenic responses to insulin are regulated by divergent post-receptor signaling mechanisms downstream from the activated insulin receptor (IR). However, the anabolic and growth-promoting properties of insulin require tissue-specific inter-relationships between the two pathways, and the nature and scope of insulin-regulated processes vary greatly across tissues. Understanding the nuances of this interplay between metabolic and growth-regulating properties of insulin would have important implications for development of novel insulin and IR modulator therapies that stimulate insulin receptor activation in both pathway- and tissue-specific manners. This review will provide a unique perspective focusing on the roles of "metabolic" and "mitogenic" actions of insulin signaling in various tissues, and how these networks should be considered when evaluating selective pharmacologic approaches to prevent or treat metabolic disease.

Antidiabetic effect of nepodin, a component of Rumex roots, and its modes of action in vitro and in vivo

Many active components derived from edible natural resources such as plant extracts have recently attracted attention for their potential use as functional foods or drugs for preventing and treating metabolic diseases such as diabetes. To obtain a novel modulator of glucose metabolism, we conducted screening of a small compound library in cultured L6 myotubes. We identified nepodin that stimulated glucose uptake dose-dependently in differentiated L6 myotubes. The stimulatory effect of nepodin on glucose uptake was abrogated by a 5'-adenosine monophosphate-activated protein kinase (AMPK) inhibitor. In addition, nepodin stimulated the phosphorylation of AMPK. Nepodin also stimulated the translocation of GLUT4 to the plasma membrane in L6 myoblasts transfected with a Glut4 cDNA-coding vector and in differentiated L6 myotubes. In in vivo study, nepodin suppressed the increases in fasting blood glucose levels and improved the glucose intolerance of C57BL/KsJ-db/db mice, a type 2 diabetic animal model. Nepodin rescued the impaired phosphorylation of AMPK in the skeletal muscle of db/db mice. These results suggest that nepodin has an antidiabetic effect, which is at least partly mediated by stimulation of GLUT4 translocation via AMPK activation by nepodin.

The role of the Niemann-Pick disease, type C1 protein in adipocyte insulin action

The Niemann-Pick disease, type C1 (NPC1) gene encodes a transmembrane protein involved in cholesterol efflux from the lysosome. SNPs within NPC1 have been associated with obesity and type 2 diabetes, and mice heterozygous or null for NPC1 are insulin resistant. However, the molecular mechanism underpinning this association is currently undefined. This study aimed to investigate the effects of inhibiting NPC1 function on insulin action in adipocytes. Both pharmacological and genetic inhibition of NPC1 impaired insulin action. This impairment was evident at the level of insulin signalling and insulin-mediated glucose transport in the short term and decreased GLUT4 expression due to reduced liver X receptor (LXR) transcriptional activity in the long-term. These data show that cholesterol homeostasis through NPC1 plays a crucial role in maintaining insulin action at multiple levels in adipocytes.

Insulin stimulates translocation of human GLUT4 to the membrane in fat bodies of transgenic Drosophila melanogaster

The fruit fly Drosophila melanogaster is an excellent model system for studies of genes controlling development and disease. However, its applicability to physiological systems is less clear because of metabolic differences between insects and mammals. Insulin signaling has been studied in mammals because of relevance to diabetes and other diseases but there are many parallels between mammalian and insect pathways. For example, deletion of Drosophila Insulin-Like Peptides resulted in 'diabetic' flies with elevated circulating sugar levels. Whether this situation reflects failure of sugar uptake into peripheral tissues as seen in mammals is unclear and depends upon whether flies harbor the machinery to mount mammalian-like insulin-dependent sugar uptake responses. Here we asked whether Drosophila fat cells are competent to respond to insulin with mammalian-like regulated trafficking of sugar transporters. Transgenic Drosophila expressing human glucose transporter-4 (GLUT4), the sugar transporter expressed primarily in insulin-responsive tissues, were generated. After expression in fat bodies, GLUT4 intracellular trafficking and localization were monitored by confocal and total internal reflection fluorescence microscopy (TIRFM). We found that fat body cells responded to insulin with increased GLUT4 trafficking and translocation to the plasma membrane. While the amplitude of these responses was relatively weak in animals reared on a standard diet, it was greatly enhanced in animals reared on sugar-restricted diets, suggesting that flies fed standard diets are insulin resistant. Our findings demonstrate that flies are competent to mobilize translocation of sugar transporters to the cell surface in response to insulin. They suggest that Drosophila fat cells are primed for a response to insulin and that these pathways are down-regulated when animals are exposed to constant, high levels of sugar. Finally, these studies are the first to use TIRFM to monitor insulin-signaling pathways in Drosophila, demonstrating the utility of TIRFM of tagged sugar transporters to monitor signaling pathways in insects.

A serum factor induces insulin-independent translocation of GLUT4 to the cell surface which is maintained in insulin resistance

In response to insulin, glucose transporter GLUT4 translocates from intracellular compartments towards the plasma membrane where it enhances cellular glucose uptake. Here, we show that sera from various species contain a factor that dose-dependently induces GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes, human adipocytes, myoblasts and myotubes. Notably, the effect of this factor on GLUT4 is fully maintained in insulin-resistant cells. Our studies demonstrate that the serum-induced increase in cell surface GLUT4 levels is not due to inhibition of its internalization and is not mediated by insulin, PDGF, IGF-1, or HGF. Similarly to insulin, serum also augments cell surface levels of GLUT1 and TfR. Remarkably, the acute effect of serum on GLUT4 is largely additive to that of insulin, while it also sensitizes the cells to insulin. In accordance with these findings, serum does not appear to activate the same repertoire of downstream signaling molecules that are implicated in insulin-induced GLUT4 translocation. We conclude that in addition to insulin, at least one other biological proteinaceous factor exists that contributes to GLUT4 regulation and still functions in insulin resistance. The challenge now is to identify this factor.

The role of adipocyte insulin resistance in the pathogenesis of obesity-related elevations in endocannabinoids

OBJECTIVE

Obesity is associated with an overactive endocannabinoid (EC) system. The mechanisms responsible for increased ECs in obese individuals are poorly understood. Therefore, we examined the role of adipocyte insulin resistance in intracellular EC metabolism.

METHODS

We used 3T3-L1 adipocytes and diet-induced obese (DIO) mice to examine the role of obesity and insulin resistance in the regulation and/or dysregulation of intracellular ECs.

RESULTS

For the first time, we provide evidence that insulin is a major regulator of EC metabolism. Insulin treatment reduced intracellular ECs (2-arachidonylglycerol [2-AG] and anandamide [AEA]) in 3T3-L1 adipocytes. This corresponded with insulin-sensitive expression changes in enzymes of EC metabolism. In insulin-resistant adipocytes, patterns of insulin-induced enzyme expression were disturbed in a manner consistent with elevated EC synthesis and reduced EC degradation. Expression profiling of adipocytes from DIO mice largely recapitulated in vitro changes, suggesting that insulin resistance affects the EC system in vivo. In mice, expression changes of EC synthesis and degradation enzymes were accompanied by increased plasma EC concentrations (2-AG and AEA) and elevated adipose tissue 2-AG.

CONCLUSIONS

Our findings suggest that insulin-resistant adipocytes fail to regulate EC metabolism and decrease intracellular EC levels in response to insulin stimulation. These novel observations offer a mechanism whereby obese insulin-resistant individuals exhibit increased concentrations of ECs.

Peptide rescues GLUT4 recruitment, but not GLUT4 activation, in insulin resistance

Insulin-stimulated GLUT4 recruitment to the plasma membrane is impaired in insulin resistance. We recently reported that a cell permeable phosphoinositide-binding peptide induces GLUT4 recruitment as potently as insulin, but does not activate GLUT4 to initiate glucose uptake. Here we investigated whether the peptide-induced GLUT4 recruitment is intact in insulin resistance. The expression levels of GLUT1 and GLUT4 were unaffected by chronically treating 3T3-L1 adipocytes with insulin. GLUT4 recruitment by acute insulin stimulation after chronic insulin treatment was significantly reduced, but was fully restored by the peptide treatment. However, subsequent acute insulin stimulation to activate GLUT4 failed to increase glucose uptake in peptide-pretreated cells. Insulin-stimulated GLUT1 recruitment was unaffected by the peptide pretreatment. These results suggest that the GLUT4 recruitment signal caused by the peptide is intact in insulin resistance, but GLUT4 activation that occurs subsequent to recruitment is not rescued by the peptide treatment.

Delayed response of insulin-stimulated fluorine-18 deoxyglucose uptake in glucose transporter-4-null mice hearts

OBJECTIVES

We sought to evaluate the time course of insulin-stimulated myocardial glucose uptake (MGU) in mice that had undergone ablation of glucose transporter-4 (GLUT4).

BACKGROUND

The relative importance of GLUT4, the most abundant insulin-responsive glucose transporter, to modulate myocardial glucose metabolism is not well defined.

METHODS

Myocardial glucose uptake was assessed at various time points after glucose (1 mg/g) and insulin (8 mU/g) injection in GLUT4-null (G4N) (n = 48) and wild-type (WT) (n = 48) mice with (18)F-2-deoxy-2-fluoro-d-glucose (FDG) using in vivo positron emission tomography (PET), in vitro gamma-counter biodistribution, and isolated, perfused hearts.

RESULTS

Baseline assessment with PET imaging showed comparable MGU in G4N (0.66 +/- 0.12) and WT (0.67 +/- 0.11, p = 0.70) mice. Early after insulin injection, WT mice demonstrated a 3.5-fold increase in MGU (2.45 +/- 0.45, p = 0.03), whereas G4N mice presented no increase (1.11 +/- 0.24, p = 0.28). At 60 min, MGU was comparable in G4N (3.19 +/- 0.60) and WT (2.66 +/- 0.47, p = 0.28) mice. In vitro gamma-counter biodistribution evaluation confirmed in G4N mice a lack of MGU increase early after insulin, but a slow response over 120 min. The isolated, perfused hearts of G4N mice during short-term (15 min) insulin stimulation displayed no increase in MGU (0.08 +/- 0.01 ml/g/min), whereas WT mice presented a threefold increase (0.22 +/- 0.01 ml/g/min, p < 0.01). With long-term (60 min) insulin stimulation, similar MGU was found in G4N (0.31 +/- 0.02 ml/g/min) and WT (0.33 +/- 0.04 ml/g per min, p = 0.04) mice.

CONCLUSIONS

The G4N mice displayed an increase of MGU in response to insulin similar to that of controls, but with a markedly delayed time response. Our findings underscore the important role of GLUT4 in the rapid adaptive response of myocardial glucose metabolism.

Insulin-like and non-insulin-like selenium actions in 3T3-L1 adipocytes

In insulin-sensitive 3T3-L1 adipocytes, selenium stimulates glucose transport and antilipolysis and these actions of selenium, like insulin actions, are sensitive to wortmanin, an inhibitor of phosphatidylinositol-3-kinase (PI3K). Selenium stimulates PI3K activity that is sustained up to 24 h. Selenium after 5-10 min increases tyrosine phosphorylation of selective cellular proteins, but after 24 h overall tyrosine phosphorylation is increased. Tyrosine phosphorylation of insulin receptor substrate 1 is detected when enriched by immunoprecipitation with anti-PI3K antibody. Selenium, however, does not stimulate insulin receptor tyrosine kinase activity. Selenium also increases phosphorylation of other insulin signaling proteins, including Akt and extracellular signal regulated kinases. Selenium-stimulated glucose transport is accompanied by increases in glucose transporter-1 content in the plasma membrane. These data are consistent with similar selenium action in glucose transport in 3T3-L1 fibroblasts expressing mainly GLUT1. In chronic insulin-induced insulin resistant cells, selenium unlike insulin fully stimulates glucose transport. In summary, selenium stimulates glucose transport and antilipolysis in a PI3K-dependent manner, but independent of insulin receptor activation. Selenium exerts both insulin-like and non-insulin-like actions in cells.

Sustained exposure of L6 myotubes to high glucose and insulin decreases insulin-stimulated GLUT4 translocation but upregulates GLUT4 activity

Hyperglycemia and hyperinsulinemia are cardinal features of acquired insulin resistance. In adipose cell cultures, high glucose and insulin cause insulin resistance of glucose uptake, but because of altered GLUT4 expression and contribution of GLUT1 to glucose uptake, the basis of insulin resistance could not be ascertained. Here we show that GLUT4 determines glucose uptake in L6 myotubes stably overexpressing myc-tagged GLUT4. Preincubation for 24 h with high glucose and insulin (high Glc/Ins) reduced insulin-stimulated GLUT4 translocation by 50%, without affecting GLUT4 expression. Insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation, phosphatidylinositol 3-kinase activation, and Akt phosphorylation also diminished, as did insulin-mediated glucose uptake. However, basal glucose uptake rose by 40% without any gain in surface GLUT4. High Glc/Ins elevated basal p38 mitogen-activated protein kinase (MAPK) phosphorylation and activity, and a short inhibition of p38 MAPK with SB202190 corrected the rise in basal glucose uptake, suggesting that p38 MAPK activity contributes to this rise. We propose that in a cellular model of skeletal muscle, chronic exposure to high Glc/Ins reduced the acute, insulin-elicited GLUT4 translocation. In addition, basal state GLUT4 activity was augmented to partially compensate for the translocation defect, resulting in a more robust glucose uptake than what would be predicted from the amount of cell surface GLUT4 alone.

14-3-3 facilitates insulin-stimulated intracellular trafficking of insulin receptor substrate 1

The appearance of a complex between tyrosine-phosphorylated insulin receptor substrate 1 (IRS-1) and PI3K in a high-speed pellet fraction (HSP) is thought to be a key event in insulin action. Conversely, the disappearance of the IRS-1/PI3K complex from this fraction has been linked to insulin desensitization. The present study examines the role of 14-3-3, a specific phospho-serine binding protein, in mediating the disappearance of IRS-1 from the HSP after insulin treatment. An in vitro pull-down assay using recombinant 14-3-3 revealed that insulin enhances the association of 14-3-3 with IRS-1 in cultured adipocytes and that this is completely inhibited by wortmannin. An association of IRS-1 and 14-3-3 was also observed and was maximal after stimulation by insulin, when endogenous proteins were immunoprecipitated. Epidermal growth factor (EGF), 12-O-tetradecanoylphorbol-13-acetate, and okadaic acid, other agents that cause serine/threonine phosphorylation of IRS-1, also stimulated IRS binding to 14-3-3. The enhancement of IRS-1 binding to 14-3-3 by insulin was accompanied by movement of IRS-1 and the p85 subunit of PI3K from the HSP to the cytosol. In keeping with a key role of 14-3-3 in mediating this redistribution of IRS-1, the complexes of IRS-1 and 14-3-3 were found in the cytosol but not in the HSP of insulin-treated cells. In addition, colocalization of IRS-1 and 14-3-3 was observed in the cytoplasm after insulin treatment by confocal microscopy. Finally, the addition of a phosphorylated 14-3-3 binding peptide to an adipocyte homogenate (to remove 14-3-3 from IRS-1) increased the abundance of IRS-1/PI3K complexes in the HSP and decreased their abundance in the cytosol. These findings strongly suggest that 14-3-3 participates in the intracellular trafficking of IRS-1 by promoting the displacement of serine-phosphorylated IRS-1 from particular structures. They also suggest that 14-3-3 proteins could play an integral role in the process of insulin desensitization.

Inhibition of isoprenoid biosynthesis causes insulin resistance in 3T3-L1 adipocytes

Lovastatin treatment caused down-regulation of the insulin-responsive glucose transporter 4 (Glut4) and up-regulation of Glut1 in 3T3-L1 adipocytes. These changes in protein expression were associated with a marked inhibition of insulin-stimulated glucose transport. Lovastatin had no effect on cell cholesterol levels, but its effects were reversed by mevalonate, demonstrating that inhibition of isoprenoid biosynthesis causes insulin resistance in 3T3-L1 adipocytes. These findings support the notion that whole body insulin resistance may arise as a result of perturbations in general biochemical pathways, rather than primary defects in insulin signalling.

Specific desensitization of glycogen synthase activation by insulin in 3T3-L1 adipocytes. Connection between enzymatic activation and subcellular localization

A protocol was developed in 3T3-L1 adipocytes that resulted in the specific desensitization of glycogen synthase activation by insulin. Cells were pretreated for 15 min with 100 nm insulin, and then recovered for 1.5 h in the absence of hormone. Subsequent basal and insulin-induced phosphorylation of the insulin receptor, IRS-1, MAPK, Akt kinase, and GSK-3 were similar in control and pretreated cells. Additionally, enhanced glucose transport and incorporation into lipid in response to insulin were unaffected. However, pretreatment reduced insulin-stimulated glycogen synthesis by over 50%, due to a nearly complete inhibition of glycogen synthase activation. Removal of extracellular glucose during the recovery period blocked the increase in glycogen levels, and restored insulin-induced glycogen synthase activation. Furthermore, incubation of pretreated 3T3-L1 adipocytes with glycogenolytic agents reversed the desensitization event. Separation of cellular lysates on sucrose gradients revealed that glycogen synthase was primarily located in the dense pellet fraction, with lesser amounts in the lighter fractions. Insulin induced glycogen synthase translocation from the lighter to the denser glycogen-containing fractions. Interestingly, insulin preferentially activated translocated enzyme while having little effect on the majority of glycogen synthase activity in the pellet fraction. In insulin-pretreated cells, glycogen synthase did not return to the lighter fractions during recovery, and thus did not move in response to the second insulin exposure. These results suggest that, in 3T3-L1 adipocytes, the translocation of glycogen synthase may be an important step in the regulation of glycogen synthesis by insulin. Furthermore, intracellular glycogen levels can regulate glycogen synthase activation, potentially through modulation of enzymatic localization.

Glucose deprivation induces Akt-dependent synthesis and incorporation of GLUT1, but not of GLUT4, into the plasma membrane of 3T3-L1 adipocytes

Reduction of the glucose concentration in the culture medium of 3T3-L1 adipose cells below 1.25 mM produces a 4-8-fold stimulation of 2-deoxyglucose uptake which starts after a lag phase of 2 h and is maximal after 10-16 h. In the present study, we employed the 'membrane sheet assay' in order to re-assess the contribution of the transporter isoforms GLUT1 and GLUT4 to this effect. Immunochemical assay of glucose transporters in membranes prepared with the 'sheet assay' revealed that the effect reflected a marked increase of GLUT1 in the plasma membrane with no effect on GLUT4. Glucose deprivation increased the total cellular GLUT1 protein in parallel with the transport activity, whereas GLUT4 was unaltered. The specific PI 3-kinase inhibitor wortmannin inhibited the effect of glucose deprivation on transport activity and also on GLUT1 synthesis. Glucose deprivation produced a moderate, biphasic increase in the activity of the protein kinase Akt/PKB that was inhibitable by wortmannin. When wortmannin was added after stimulation of cells in order to assess the internalization rate of transporters, the effect of insulin was reversed considerably faster (T1/2 = 18 min) than that of glucose deprivation (T1/2 > 60 min). These data are consistent with the conclusion that the effect of glucose deprivation reflects a specific, Akt-dependent de-novo synthesis of GLUT1, and not of GLUT4, and its insertion into a plasma membrane compartment which is distinct from that of the insulin-sensitive GLUT1.

Insulin-mediated cellular insulin resistance decreases osmotic shock-induced glucose transport in 3T3-L1 adipocytes

Similar to insulin, osmotic shock treatment of 3T3-L1 adipocytes causes translocation of GLUT4 protein to the plasma membrane and an increase in glucose transport activity. In our study, we evaluated the effect of chronic insulin treatment on the osmotic shock signaling pathway leading to GLUT4 translocation and glucose uptake. We found that chronic administration of insulin to the adipocytes induced cellular resistance to osmotic shock-stimulated GLUT4 translocation and glucose transport. We found that chronic insulin treatment attenuated shock-induced Gab-1 tyrosine phosphorylation. Furthermore, chronic insulin exposure led to a marked impairment in the ability of Gab-1 to associate with p85 subunit of PI 3-kinase in response to acute shock and insulin stimulation. Cells that were chronically treated with insulin showed a 70% and a 61% decrease in Gab-1 associated PI 3-kinase activity in shock- vs. insulin-treated cells, respectively. In addition, we found that chronic insulin treatment inhibited both insulin- and osmotic shock-induced membrane ruffling, indicating that two PI 3-kinase dependent effects, GLUT4 translocation and membrane ruffling are decreased in chronically insulin-treated cells. The results described above clearly demonstrate that chronic insulin treatment induces a state of cellular resistance to osmotic shock signal transduction.

Development and comparison of two 3T3-L1 adipocyte models of insulin resistance: increased glucose flux vs glucosamine treatment

Insulin resistance can be induced in vivo by intravenous infusion of glucosamine or in cells by incubation with glucosamine. However, a publication (Hresko, R. C., et al. (1998) J. Biol. Chem. 273, 20658-20668) suggests a trivial explanation of glucosamine-induced insulin resistance whereby intracellular ATP pools are depleted presumably due to the phosphorylation of glucosamine to glucosamine 6-phosphate, a hexosamine pathway intermediate. The reduced ATP level impaired insulin receptor (IR) autophosphorylation and tyrosine kinase activity toward substrates. The present work describes the development and comparison of two methods for inducing insulin resistance, by treating 3T3-L1 adipocytes overnight using either 25 mM glucose/5 nM insulin or 2 mM glucosamine. Under these conditions basal glucose transport rates were comparable with controls. Insulin-stimulated 2-deoxyglucose uptake, however, was reduced by approximately 45% in response to both high glucose/insulin and glucosamine treatment, relative to control cells. The total relative amounts of the insulin-responsive glucose transporter, Glut4, remained constant under both treatment conditions. The relative phosphotyrosine (Tyr(P)) contents of the insulin receptor and its substrate 1 (IRS-1) were assessed in whole cell homogenates. With both methods to induce insulin resistance, IR/IRS-1 Tyr(P) levels were virtually indistinguishable from those in control cells. Insulin-stimulated phosphorylation of Akt on Ser(473) was not impaired in insulin-resistant cells. Furthermore, the relative Tyr(P) content of the PDGF receptor was comparable in high glucose/insulin- or glucosamine-treated 3T3-L1 adipocytes upon subsequent challenge with PDGF. Finally, the relative amounts of glutamine:fructose-6-phosphate amidotransferase and O-linked N-acetylglucosamine transferase, two important hexosamine pathway enzymes, were similar in both treatments when compared with controls. Thus, 3T3-L1 adipocytes can be used as a model system for studying insulin resistance induced by increased influx of glucose. Under appropriate experimental conditions, glucosamine treatment can mimic the effects of increased glucose flux without impairment of tyrosine phosphorylation-based signaling.

Combined use of insulin and endothelin-1 causes decrease of protein expression of beta-subunit of insulin receptor, insulin receptor substrate-1, and insulin-stimulated glucose uptake in rat adipocytes

Previously, we reported that insulin-stimulated glucose uptake (ISGU) can be inhibited by endothelin (ET-1). However, the mechanism by which ET-1 impairs ISGU in adipocytes remains unclear. This study investigated the effects of ET-1 on insulin action in rat adipocytes in order to elucidate the molecular mechanism of action of ET-1 on ISGU. The results show that ISGU was increased fivefold after 3-h treatment with 1 nM insulin. Treatment with 100 nM ET-1 had no effect on basal glucose uptake. However, ET-1 inhibited approximately 25% of ISGU and 20% of insulin binding after 3-h treatment in the presence of 1 nM insulin. Expression of the beta-subunit of the insulin receptor (IRbeta) and the insulin receptor substrate-1 (IRS-1) in adipocytes was not significantly affected by 1 nM insulin or by 100 nM ET-1, even after 3-h treatment. However, expressions of IRbeta and IRS-1 were dramatically decreased in a dose- and time-dependent manner when adipocytes were treated with both insulin and ET-1. Approximately 50% of IRbeta and 65% of IRS-1 expression levels were suppressed when adipocytes were simultaneously treated with both 1 nM insulin and 100 nM ET-1 for 3 h. These results suggest that the inhibitory effect of ET-1 on ISGU may be mediated via the insulin receptor and suppression of IRbeta/IRS-1 expression.

Release of insulin receptor substrate proteins from an intracellular complex coincides with the development of insulin resistance

Insulin receptor substrate (IRS) proteins are major substrates of the insulin receptor (IR). IRS-1 associates with an insoluble multiprotein complex, possibly the cytoskeleton, in adipocytes. This localization may facilitate interaction with the IR at the cell surface. In the present study, we examined the hypothesis that the release of IRS proteins from this location may be a mechanism for insulin desensitization. We show that a second IRS protein, IRS-2, is associated with a multiprotein complex in adipocytes with similar characteristics to the IRS-1 complex. Insulin treatment (15-60 min) caused the release of IRS-1 and IRS-2 from this complex (high speed pellet; HSP) into the cytosol, whereas the level of tyrosyl-phosphorylated IRS proteins remained constant. Chronic insulin treatment resulted in a dramatic reduction in IRS-1 and IRS-2 in the HSP, eventually (>2 h) leading to IRS protein degradation and decreased levels of tyrosyl-phosphorylated IRS proteins. Okadaic acid, which rapidly induces insulin resistance in adipocytes independently of IR function, caused an almost quantitative release of IRS-1 into the cytosol commensurate with a significant reduction in tyrosyl-phosphorylated IRS proteins. Platelet-derived growth factor, a factor known to compromise insulin signaling, caused a more moderate release of IRS proteins from the HSP. Collectively, these results suggest that the assembly of IRS-1/IRS-2 into a multiprotein complex facilitates coupling to the IR and that the regulated release from this location may represent a novel mechanism of insulin resistance.

Signaling pathways mediating insulin-stimulated glucose transport

A major action of insulin is to accelerate the rate of uptake of sugar into muscle and adipose cells following a meal. The biochemical mechanism by which this is accomplished has been a subject of intense experimentation, although elucidation of the pathways has remained elusive. In recent years, numerous signaling molecules and cascades modulated by insulin have been identified, although few have been definitively established as important to the metabolic actions of the hormone. An exception to this is the lipid kinase phosphatidylinositide 3'-kinase, which, under many conditions, appears absolutely required for insulin to stimulate hexose uptake into adipocytes. Akt/PKB, a serine/threonine protein kinase activated by insulin in a phosphatidylinositide 3'-kinase-dependent manner, has been implicated as a critical mediator of insulin's actions on metabolism and cell survival. Nonetheless, Akt/PKB's role in many insulin effects, particularly accelerated glucose transport, remains controversial. Interestingly, soluble analogues of ceramide antagonize both insulin's activation of Akt/PKB as well as its stimulation of glucose transport, consistent with a causal relationship between the two.

Mechanism of protein kinase B activation by insulin/insulin-like growth factor-1 revealed by specific inhibitors of phosphoinositide 3-kinase--significance for diabetes and cancer

Protein kinase B (PKB) is a member of the second messenger subfamily of protein kinases. The three isoforms of PKB identified have an amino-terminal pleckstrin homology domain, a central kinase domain, and a carboxy-terminal regulatory domain. PKB is the major downstream target of receptor tyrosine kinases that signal via the phosphoinositide (PI) 3-kinase. The crucial role of lipid second messengers in PKB activation has been dissected through the use of the PI 3-kinase-specific inhibitors wortmannin and LY294002. Receptor-activated PI 3-kinase synthesises the lipid second messenger PI-3,4,5-trisphosphate, leading to the recruitment of PKB to the membrane. Membrane attachment of PKB is mediated by its pleckstrin homology domain binding to PI-3,4,5-trisphosphate or PI-3,4-bisphosphate with high affinity. Activation of PKB alpha and beta is then achieved at the plasma membrane by phosphorylation of Thr308/309 in the A-loop of the kinase domain and Ser473/474 in the carboxy-terminal regulatory region, respectively. The upstream kinase that phosphorylates PKB on Thr308, termed PI-dependent protein kinase-1, has been identified and extensively characterised. A candidate for the Ser473/474 kinase, termed the integrin-linked kinase, has been identified recently. Activated PKB is implicated in glucose metabolism, transcriptional control, and in the regulation of apoptosis in many different cell types. Stimulation of PKB activity protects cells from apoptosis by phosphorylation and inactivation of the pro-apoptotic protein BAD. These results could explain why PKB is overexpressed in some ovarian, breast, and pancreatic carcinomas.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

IGF-I regulates IRS-1 expression in 3T3-L1 adipocytes

IRS-1 (the insulin receptor substrate-1) is required for signaling by both insulin and IGF-I. Chronic treatment of 3T3-L1 adipocytes with insulin at all concentrations results in increased proteolysis of IRS-1. In contrast, treatment with low concentrations of IGF-I (EC50 = 625 pM) for 4 h caused an increase in IRS-1 to 170% of control. Actinomycin D and cycloheximide blocked the IGF-I effect, but not the insulin effect, suggesting that IGF-I stimulated the synthesis of IRS-1. Concentrations of IGF-I high enough to cause significant binding to the insulin receptor resulted in the down-regulation of IRS-1. Phosphatidylinositol 3'-kinase inhibitors blocked both the insulin and IGF-I effects. Chronic IGF-I treatment caused an increase in both acute insulin-stimulated dGlc uptake and acute IGF-I-stimulated dGlc uptake. Chronic insulin treatment caused a decrease in both acute insulin-stimulated dGlc uptake and acute IGF-I-stimulated dGlc uptake.

GLUT4 translocation by insulin in intact muscle cells: detection by a fast and quantitative assay

We report a rapid and sensitive colorimetric approach to quantitate the amount of glucose transporters exposed at the surface of intact cells, using L6 muscle cells expressing GLUT4 containing an exofacial myc epitope. Unstimulated cells exposed to the surface 5 fmol GLUT4myc per mg protein. This value increased to 10 fmol/mg protein in response to insulin as 2-deoxyglucose (10 microM) uptake doubled. The results are substantiated by immunofluorescent detection of GLUT4myc in unpermeabilized cells and by subcellular fractionation. We further show that wortmannin and the cytoskeleton disruptors cytochalasin D and latrunculin B completely blocked these insulin effects. The rapid quantitative assay described here could be of high value to study insulin signals and to screen for potential anti-diabetic drugs.

Development of insulin resistance in 3T3-L1 adipocytes

Insulin resistance is a manifestation of both diabetes mellitus and obesity. However, the mechanism is still not clearly identified. Herein, we describe a procedure that allows us to evaluate the development of insulin resistance in 3T3-L1 adipocytes. Under these conditions, we show that the concentration of insulin required for 50% desensitization of glucose transport activity is 100 pM; maximal desensitization could be achieved with 1 nM. This demonstrates for the first time that 3T3-L1 adipocytes develop insulin resistance in response to physiologically relevant concentrations of insulin. Glucose (or glucosamine), in addition to insulin, was required to establish desensitization. The expression of GLUT4 protein decreased by 50% with exposure to 10 nM insulin. The dose-dependent loss of GLUT4 was similar to the dose dependence for insulin-resistant transport activity. Translocation in the presence of acute insulin was apparent, but the extent of recruitment directly reflected the decrease in GLUT4 protein. GLUT4 mRNA also declined, but the ED50 was approximately 5 nM. Together, these data suggest that the loss of GLUT4 protein likely underlies the cause of desensitization. However, the loss of GLUT4 protein did not correlate with the loss in GLUT4 mRNA suggesting post-translational control of GLUT4 expression.

Expression of a constitutively active Akt Ser/Thr kinase in 3T3-L1 adipocytes stimulates glucose uptake and glucose transporter 4 translocation

Akt is a serine/threonine kinase that requires a functional phosphatidylinositol 3-kinase to be stimulated by insulin and other growth factors. When directed to membranes by the addition of a src myristoylation sequence, Akt becomes constitutively active. In the present studies, the constitutively active Akt and a nonmyristoylated control mutant were expressed in 3T3-L1 cells that can be induced to differentiate into adipocytes. The constitutively active Akt induced glucose uptake into adipocytes in the absence of insulin by stimulating translocation of the insulin-responsive glucose transporter 4 to the plasma membrane. The constitutively active Akt also increased the synthesis of the ubiquitously expressed glucose transporter 1. The increased glucose influx in the 3T3-L1 adipocytes directed lipid but not glycogen synthesis. These results indicate that Akt can regulate glucose uptake and metabolism.

Translocation of GLUT1 does not account for elevated glucose transport in glucose-deprived 3T3-L1 adipocytes

Glucose deprivation increases the rate of glucose transport in 3T3-L1 adipocytes in a protein synthesis-dependent fashion. To determine if translocation of either GLUT1 or GLUT4 is responsible for this phenomenon, we adapted existing fractionation procedures toward isolating 3T3-L1 adipocyte membranes. By Western blot analysis of equal protein, GLUT1 was distributed between plasma membranes, high density "microsomal" membranes, and low density "microsomal" membranes isolated from control cells. GLUT4 comigrated with high density and low density membranes. Glucose deprivation for 12 h did not alter the distribution of either GLUT1 or GLUT4, despite an 8-10-fold increase in glucose transport activity in intact cells. Importantly, increased transport activity was retained in plasma membrane vesicles isolated from glucose-deprived cells. These data show for the first time that the increase in transport activity associated with glucose deprivation does not result from the translocation of either of the glucose transporters known to exist in 3T3-L1 adipocytes. As GLUT4 is excluded from the plasma membrane, these data provide evidence for activation of GLUT1.

Characterization of the insulin-regulated membrane aminopeptidase in 3T3-L1 adipocytes

A novel membrane aminopeptidase has been identified as a major protein in vesicles from rat adipocytes containing the glucose transporter isotype Glut4. In this study we have characterized this aminopeptidase, referred to as vp165, in 3T3-L1 adipocytes. The subcellular distributions of vp165 and Glut4 were determined by immunoisolation of vesicles with antibodies against both proteins, by immunofluorescence, and by subcellular fractionation and immunoblotting. Relative amounts of vp165 at the cell surface in basal and insulin-treated cells were assayed by cell surface biotinylation. These experiments showed that vp165 and Glut4 were entirely colocalized and that vp165 increased markedly at the cell surface in response to insulin, in a way similar to Glut4. When intact cells were assayed with a novel, membrane-impermeant fluorogenic substrate for vp165, we found that insulin stimulated aminopeptidase activity at the cell surface. This observation provides direct evidence for the functional consequence of vp165 translocation.

Alterations in insulin signalling pathway induced by prolonged insulin treatment of 3T3-L1 adipocytes

Insulin-induced glucose transport stimulation, which results from the translocation of glucose transporter 4 (GLUT 4)-containing vesicles, is completely blocked after prolonged insulin treatment of 3T3-L1 adipocytes. Since GLUT 4 expression was reduced by only 30%, we looked at the insulin signaling pathway in this insulin-resistant model. Insulin-induced tyrosine phosphorylation of the major insulin receptor substrate IRS 1 was reduced by 50 +/- 7%, while its expression was decreased by 70 +/- 4%. When cells were treated with worthmannin (a PI3-kinase inhibitor) together with insulin, the expression of IRS 1 diminished to a much lower extent. Associated with the decrease in IRS 1 expression and phosphorylation, the activation by insulin of anti-phosphotyrosine immunoprecipitable PI3-kinase activity and of p44mapk activities was altered. However, the expression of these proteins was normal and p44mapk activity remained responsive to the tumour promoter TPA. Those results indicate that prolonged insulin treatment of 3T3-L1 adipocytes induces an insulin-resistant state with a reduced ability of insulin to stimulate the PI3-kinase and the MAP-kinases and a blockade of glucose transporter translocation.

Insulin-like growth factor I receptor activated by a transmembrane mutation

We constructed mutant receptors by mutating transmembrane Val922 of the human insulin-like growth factor I receptor (IGF-IR). Assays of receptor kinase and autophosphorylation revealed constitutively augmented tyrosine kinase activity of V922E IGF-IR in both transient and stable expression. The constitutively active tyrosine kinase of this mutant was verified by promoted tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) in the absence of IGF-I. In CHO cells stably increasing V922E IGF-IR, both IRS-1 phosphorylation and the IRS-1 associated phosphoinositide 3-kinase activity were stimulated in the absence of IGF-I to the level attained by 1 nM IGF-I stimulation of wild-type IGF-IR, whereas the Ras-mitogen-activated protein kinase pathway was not activated under the same condition. In these CHO cells, V922E IGF-IR significantly stimulated glucose uptake but did not promote mitogenesis in the absence of IGF-I. We thus conclude that the V922E mutation of IGF-IR switches on the intrinsic tyrosine kinase and differentially activates the downstream pathways. This mutant is extremely useful in clarifying the turning-on mechanism of IGF-IR as well as the differential roles of individual downstream pathways of receptor tyrosine kinases.

Different signaling roles of SHPTP2 in insulin-induced GLUT1 expression and GLUT4 translocation

Insulin activates hexose transport via at least two mechanisms: a p21ras-dependent pathway, leading to an increase in the amount of cell surface GLUT1; and a metabolic, p21ras-independent pathway, leading to translocation of the insulin-responsive transporter GLUT4 to the cell surface. Following insulin stimulation, SHPTP2, a non-transmembrane protein-tyrosine phosphatase, associates with insulin receptor substrate 1 via its Src homology 2 (SH2) domains. Microinjection of a glutathione S-transferase fusion protein encoding the N- and C-terminal SH2 domains of SHPTP2 (GST-NC-SH2) or anti-SHPTP2 antibodies into NIH-3T3 fibroblasts overexpressing the insulin receptor blocks insulin-induced DNA synthesis. Microinjection of either GST-NC-SH2 or anti-SHPTP2 antibodies into 3T3-L1 adipocytes inhibited the insulin-stimulated increase in expression of GLUT1. In contrast, translocation of GLUT4 to the cell surface was unaffected by either GST-NC-SH2 or anti-SHPTP2 antibodies. These data confirm a role for SHPTP2 in insulin-stimulated mitogenesis and indicate that whereas SHPTP2 is necessary for insulin-stimulated expression of GLUT1, it is not required for activation of the metabolic pathway leading to GLUT4 translocation.

The effects of insulin on the level and activity of the GLUT4 present in human adipose cells

Human adipose cells are much less responsive to insulin stimulation of glucose transport activity than are rat adipocytes. To assess and characterize this difference, we have determined the rates of 3-O-methyl-D-glucose transport in human adipose cells and have compared these with the levels of glucose transporter 4 (GLUT4) assessed by using the bis-mannose photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propyl-amine, ATB-BMPA. The rates of 3-O-methyl-D-glucose transport and the cell-surface level of GLUT4 are very similar in the human and rat adipocyte in the basal state. The Vmax for 3-O-methyl-D-glucose transport in fully insulin-stimulated human adipose cells is 15-fold lower than in rat adipose cells. Photolabelling of GLUT4 suggests that this low transport activity is associated with a low GLUT4 abundance (39 x 10(4) sites/cell; 19.9 x 10(4) sites at the cell surface). The turnover number for human adipose cell GLUT4 (5.8 x 10(4) min-1) is similar to that observed for GLUT4 in rat adipose cells and the mouse cell line, 3T3L1. Since 50% of the GLUT4 is at the cell surface of both human and rat adipose cells in the fully insulin-stimulated state, an inefficient GLUT4 exocytosis process cannot account for the low transport activity. The intracellular retention process appears to have adapted to release, in the basal state, a greater proportion of the total-cellular pool of GLUT4 to the cell surface of the larger human adipocytes. These cell-surface transporters are presumably necessary to provide the basal metabolic needs of the adipocyte.(ABSTRACT TRUNCATED AT 250 WORDS)

Nutrient control of GLUT1 processing and turnover in 3T3-L1 adipocytes

Metabolic labeling and immunoprecipitation were used to analyze the glucose-dependent regulation of GLUT1 synthesis, processing, and turnover in a murine adipocyte cell line. Metabolically labeled GLUT1 from control cells migrated as a 46-kDa protein, while GLUT1 from cells deprived of glucose for more than 12 h migrated as a 37-kDa protein. On the basis of tunicamycin sensitivity, both GLUT1 species arose from a common protein migrating at 36 kDa. In addition, the rate of synthesis of GLUT1 in control and glucose-deprived cells was similar. In short pulse-chase experiments, we distinguished two species arising from the core GLUT1 protein in control cells; an intermediate and the mature 46-kDa species. In contrast, only one glycoform, the 37-kDa species, arose from the core protein in glucose-deprived cells, which was not further processed in either the presence or absence of glucose. Although 12-18 h of glucose deprivation were required to affect GLUT1 glycosylation, glucose-deprived cells quickly recovered the ability to correctly glycosylate GLUT1 upon the readdition of glucose (t1/2 < 1 h). GLUT1 in control adipocytes exhibited a half-life of approximately 14 h, while that in glucose-deprived adipocytes was greater than 50 h. This effect was readily reversed upon the readdition of glucose. In total, these data show that glucose deprivation alters both the processing (glycosylation) and turnover (degradation) of GLUT1. These results are discussed in light of transport function.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Parallel changes in Glut 4 and Rab4 movements in two insulin-resistant states

Insulin-induced Glut 4 and Rab4 movements were studied in two insulin-resistant states. In adipocytes from streptozotocin diabetic rats, the amount of Glut 4 was decreased by 60%. The remaining Glut 4 molecules were translocated in response to insulin, and in parallel, Rab4 left the intracellular compartment. In contrast, in 3T3-L1 adipocytes rendered insulin-resistant by a prolonged insulin treatment, both Rab4 and Glut 4 remained in the intracellular compartment following an acute insulin stimulation. Those results illustrate a similar behavior of Glut 4 and Rab4 in two situations where insulin resistance results from different mechanisms, and add further support for a role of Rab4 in Glut 4 translocation.