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PubMed2003

Tomosyn interacts with the t-SNAREs syntaxin4 and SNAP23 and plays a role in insulin-stimulated GLUT4 translocation.

The Sec1p-like/Munc18 (SM) protein Munc18a binds to the neuronal t-SNARE Syntaxin1A and inhibits SNARE complex assembly. Tomosyn, a cytosolic Syntaxin1A-binding protein, is thought to regulate the interaction between Syntaxin1A and Munc18a, thus acting as a positive regulator of SNARE assembly. In the present study we have investigated the interaction between b-Tomosyn and the adipocyte SNARE complex involving Syntaxin4/SNAP23/VAMP-2 and the SM protein Munc18c, in vitro, and the potential involvement of Tomosyn in regulating the translocation of GLUT4 containing vesicles, in vivo. Tomosyn formed a high affinity ternary complex with Syntaxin4 and SNAP23 that was competitively inhibited by VAMP-2. Using a yeast two-hybrid assay we demonstrate that the VAMP-2-like domain in Tomosyn facilitates the interaction with Syntaxin4. Overexpression of Tomosyn in 3T3-L1 adipocytes inhibited the translocation of green fluorescent protein-GLUT4 to the plasma membrane. The SM protein Munc18c was shown to interact with the Syntaxin4 monomer, Syntaxin4 containing SNARE complexes, and the Syntaxin4/Tomosyn complex. These data suggest that Tomosyn and Munc18c operate at a similar stage of the Syntaxin4 SNARE assembly cycle, which likely primes Syntaxin4 for entry into the ternary SNARE complex.

3T3 Cells↗
PubMed2004

Nutritional status induces divergent variations of GLUT4 protein content, but not lipoprotein lipase activity, between adipose tissues and muscles in adult cattle.

Metabolic adaptations to variations in food supply are incompletely understood in ruminant animal adipose tissue (AT) and muscle. To explore this, we studied lipid metabolism and glucose transport potential in one internal and one external AT, as well as in one oxidative and one glycolytic muscle from control, 7 d underfed and 21 d refed adult cows. Refeeding increased (+79 to +307 %) the activities of enzymes involved in de novo lipogenesis (fatty acid synthase, malic enzyme, glucose-6-phosphate dehydrogenase) in perirenal and subcutaneous AT; underfeeding did not modify these variables. Underfeeding decreased the activities of lipoprotein lipase (LPL) in perirenal AT (-70 %) and cardiac muscle (-67 %), but did not modify the activities in subcutaneous AT and longissimus thoracis. Refeeding increased LPL activities in all tissues (+40 to +553 %) to levels comparable with (cardiac muscle) or greater than (AT, longissimus thoracis) those observed in control cows. Such variations in perirenal and cardiac muscle LPL activities did not result from variations in LPL mRNA levels, but suggest a post-transcriptional regulation of LPL in these nutritional conditions. Underfeeding did not modify GLUT4 contents in perirenal AT and muscles, while refeeding increased it only in perirenal AT (+250 %). Our present results contrast with previous results in rats, where LPL is regulated in opposite directions in AT and muscles, and GLUT4 is generally increased by fasting and decreased by refeeding in skeletal muscles. The present results highlight the bovine specificity of the response, which probably arises in part from peculiarities of ruminant animals for nutrient digestion and absorption.

Adipose Tissue↗
PubMed2004

Role of calcium and AMP kinase in the regulation of mitochondrial biogenesis and GLUT4 levels in muscle.

Contractile activity induces mitochondrial biogenesis and increases glucose transport capacity in muscle. There has been much research on the mechanisms responsible for these adaptations. The present paper reviews the evidence, which indicates that the decrease in the levels of high-energy phosphates, leading to activation of AMP kinase (AMPK), and the increase in cytosolic Ca(2+), which activates Ca(2+)/calmodulin-dependent protein kinase (CAMK), are signals that initiate these adaptative responses. Although the events downstream of AMPK and CAMK have not been well characterized, these events lead to activation of various transcription factors, including: nuclear respiratory factors (NRF) 1 and 2, which cause increased expression of proteins of the respiratory chain; PPAR-alpha, which up regulates the levels of enzymes of beta oxidation; mitochondrial transcription factor A, which activates expression of the mitochondrial genome; myocyte-enhancing factor 2A, the transcription factor that regulates GLUT4 expression. The well-orchestrated expression of the multitude of proteins involved in these adaptations is mediated by the rapid activation of PPAR gamma co-activator (PGC) 1, a protein that binds to various transcription factors to maximize transcriptional activity. Activating AMPK using 5-aminoimidizole-4-carboxamide-1-beta-D-riboside (AICAR) and increasing cytoplasmic Ca(2+) using caffeine, W7 or ionomycin in L6 myotubes increases the concentration of mitochondrial enzymes and GLUT4 and enhances the binding of NRF-1 and NRF-2 to DNA. AICAR and Ca-releasing agents also increase the levels of PGC-1, mitochondrial transcription factor A and myocyte-enhancing factors 2A and 2D. These results are similar to the responses seen in muscle during the adaptation to endurance exercise and show that L6 myotubes are a suitable model for studying the mechanisms by which exercise causes the adaptive responses in muscle mitochondria and glucose transport.

Adaptation, Physiological↗
PubMed1999

Age-related adipose tissue mRNA expression of ADD1/SREBP1, PPARgamma, lipoprotein lipase, and GLUT4 glucose transporter in rhesus monkeys.

Aging has been shown to have an effect on the capacity to differentiate preadipocytes and on the expression of some genes expressed in adipose tissue. The mRNA levels of adipocyte differentiation-related genes were examined in rhesus monkeys (Macaca Mulatta) ranging in age from 7 to 30 years. The effect of aging on the expression of peroxisome proliferator activated receptor gamma (PPARgamma), adipocyte determination- and differentiation-dependent factor 1/sterol regulatory element binding protein 1 (ADD1/SREBP1), CCAAT/enhancer binding protein alpha (C/EBPalpha), lipoprotein lipase (LPL), GLUT4 glucose transporter, and adipsin were examined by slot blot analysis. Significant inverse correlations were observed between age and the mRNA levels of PPARgamma, ADD1/SREBP1, LPL, and GLUT4. The coordinate downregulation of these genes may be linked to the declining fat mass of senescent animals. There was no correlation between age and the mRNA levels of adipsin. The mRNA levels of these genes were not correlated to body weight orfasting plasma insulin. These findings indicate that aging may have an effect on the adipocyte differentiation program and this effect appears to be gene specific.

Adipocytes↗
PubMed2000

Effect of long-term caloric restriction on GLUT4, phosphatidylinositol-3 kinase p85 subunit, and insulin receptor substrate-1 protein levels in rhesus monkey skeletal muscle.

Caloric restriction (CR), a reduction in calorie intake without malnutrition, improves insulin sensitivity in various species, including mice, rats, rhesus and cynomolgus monkeys, and humans. Skeletal muscle is quantitatively the most important tissue for blood glucose clearance. Therefore, we assessed the effect of 6 years of CR (30% reduction in calorie intake) in male rhesus monkeys (14-20 years old) on muscle expression of several proteins involved in insulin action. Whole body insulin sensitivity (assessed by Modified Minimal Model) was significantly increased in CR relative to Control monkeys. CR did not alter the expression of GLUT4 glucose transporter or phosphatidylinositol-3 kinase p85 subunit (PI3K). Insulin receptor substrate-1(IRS-1) abundance tended to be greater for CR compared to Control monkeys (p = .051), but correlational analysis revealed no association between IRS-1 and insulin sensitivity (r2 = .075, p = .271). These findings indicate that the CR-induced increase in insulin sensitivity in rhesus monkeys is unrelated to alterations in GLUT4, P13K, and IRS-1 abundance.

Aging↗
PubMedDate not supplied

Restriction fragment length polymorphisms at the GLUT4 and GLUT1 gene loci in type 2 diabetes.

Inherited abnormalities of the glucose transporters could explain many of the pathophysiological features of Type 2 diabetes including the strong familial predisposition to the disease. Previous studies have suggested a possible association between an allele of an Xba1 restriction fragment length polymorphism (RFLP) at the GLUT1 gene locus and Type 2 diabetes in Caucasian and Japanese subjects. In order to test this hypothesis further, population association studies were performed at the Xba1/GLUT1 and Kpn1/GLUT4 gene loci employing a group of diabetic patients with a strong family history for the disease. The frequencies of the two alleles at the GLUT1 locus were 0.28 and 0.72 in diabetic patients and 0.31 and 0.69 in control subjects. At the GLUT4 locus, the two alleles had frequencies of 0.24 and 0.76 in diabetic patients and 0.25 and 0.75 in control subjects. These differences were not statistically significant. The present study does not support the hypothesis that genetic variation within the GLUT1 or GLUT4 gene loci may be responsible for familial susceptibility to Type 2 diabetes.

Alleles↗
PubMed2003

A long search for Glut4 activation.

Insulin stimulates glucose transport in its target cells by translocation of the glucose transporter isoform 4 (Glut4) from an intracellular storage pool to the plasma membrane. A large body of evidence indicates that activity of Glut4 at the plasma membrane may vary. Recent findings suggest that p38 MAPK may be involved in regulation of the intrinsic activity of the transporter.

Adipocytes↗
PubMed1999

Protein kinase B/Akt participates in GLUT4 translocation by insulin in L6 myoblasts.

L6 myoblasts stably transfected with a GLUT4 cDNA harboring an exofacial myc epitope tag (L6-GLUT4myc myoblasts) were used to study the role of protein kinase B alpha (PKBalpha)/Akt1 in the insulin-induced translocation of GLUT4 to the cell surface. Surface GLUT4myc was detected by immunofluorescent labeling of the myc epitope in nonpermeabilized cells. Insulin induced a marked translocation of GLUT4myc to the plasma membrane within 20 min. This was prevented by transient transfection of a dominant inhibitory construct of phosphatidylinositol (PI) 3-kinase (Deltap85alpha). Transiently transfected cells were identified by cotransfection of green fluorescent protein. A constitutively active PKBalpha, created by fusion of a viral Gag protein at its N terminus (GagPKB), increased the cell surface density of GLUT4myc compared to that of neighboring nontransfected cells. A kinase-inactive, phosphorylation-deficient PKBalpha/Akt1 construct with the mutations K179A (substitution of alanine for the lysine at position 179), T308A, and S473A (AAA-PKB) behaved as a dominant-negative inhibitor of insulin-dependent activation of cotransfected wild-type hemagglutinin (HA)-tagged PKB. Furthermore, AAA-PKB markedly inhibited the insulin-induced phosphorylation of cotransfected BAD, demonstrating inhibition of the endogenous PKB/Akt. Under the same conditions, AAA-PKB almost entirely blocked the insulin-dependent increase in surface GLUT4myc. PKBalpha with alanine substitutions T308A and S473A (AA-PKB) or K179A (A-PKB) alone was a less potent inhibitor of insulin-dependent activation of wild-type HA-PKB or GLUT4myc translocation than was AAA-PKB. Cotransfection of AAA-PKB with a fourfold DNA excess of HA-PKB rescued insulin-stimulated GLUT4myc translocation. AAA-PKB did not prevent actin bundling (membrane ruffling), though this response was PI 3-kinase dependent. Therefore, it is unlikely that AAA-PKB acted by inhibiting PI 3-kinase signaling. These results outline an important role for PKBalpha/Akt1 in the stimulation of glucose transport by insulin in muscle cells in culture.

Cells, Cultured↗
PubMed2003

Resistin inhibits glucose uptake in L6 cells independently of changes in insulin signaling and GLUT4 translocation.

Elevated levels of resistin have been proposed to cause insulin resistance and therefore may serve as a link between obesity and type 2 diabetes. However, its role in skeletal muscle metabolism is unknown. In this study, we examined the effect of resistin on insulin-stimulated glucose uptake and the upstream insulin-signaling components in L6 rat skeletal muscle cells that were either incubated with recombinant resistin or stably transfected with a vector containing the myc-tagged mouse resistin gene. Transfected clones expressed intracellular resistin, which was released in the medium. Incubation with recombinant resistin resulted in a dose-dependent inhibition of insulin-stimulated 2-deoxyglucose (2-DG) uptake. The inhibitory effect of resistin on insulin-stimulated 2-DG uptake was not the result of impaired GLUT4 translocation to the plasma membrane. Furthermore, resistin did not alter the insulin receptor (IR) content and its phosphorylation, nor did it affect insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation, its association with the p85 subunit of phosphatidylinositol (PI) 3-kinase, or IRS-1-associated PI 3-kinase enzymatic activity. Insulin-stimulated phosphorylation of Akt/protein kinase B-alpha, one of the downstream targets of PI 3-kinase and p38 MAPK phosphorylation, was also not affected by resistin. Expression of resistin also inhibited insulin-stimulated 2-DG uptake when compared with cells expressing the empty vector (L6Neo) without affecting GLUT4 translocation, GLUT1 content, and IRS-1/PI 3-kinase signaling. We conclude that resistin does not alter IR signaling but does affect insulin-stimulated glucose uptake, presumably by decreasing the intrinsic activity of cell surface glucose transporters.

Animals↗
PubMed2003

Aberrant insulin-induced GLUT4 translocation predicts glucose intolerance in the offspring of a diabetic mother.

We examined the long-term effect of in utero exposure to streptozotocin-induced maternal diabetes on the progeny that postnatally received either ad libitum access to milk by being fed by control mothers (CM/DP) or were subjected to relative nutrient restriction by being fed by diabetic mothers (DM/DP) compared with the control progeny fed by control mothers (CM/CP). There was increased food intake, glucose intolerance, and obesity in the CM/DP group and diminished food intake, glucose tolerance, and postnatal growth restriction in the DM/DP group, persisting in the adult. These changes were associated with aberrations in hormonal and metabolic profiles and alterations in hypothalamic neuropeptide Y concentrations. By use of subfractionation and Western blot analysis techniques, the CM/DP group demonstrated a higher skeletal muscle sarcolemma-associated (days 1 and 60) and white adipose tissue plasma membrane-associated (day 60) GLUT4 in the basal state with a lack of insulin-induced translocation. The DM/DP group demonstrated a partial amelioration of this change observed in the CM/DP group. We conclude that the offspring of a diabetic mother with ad libitum postnatal nutrition demonstrates increased food intake and resistance to insulin-induced translocation of GLUT4 in skeletal muscle and white adipose tissue. This in turn leads to glucose intolerance and obesity at a later stage (day 180). Postnatal nutrient restriction results in reversal of this adult phenotype, thereby explaining the phenotypic heterogeneity that exists in this population.

Adipose Tissue↗
PubMed1993

Postnatal development of uncoupling protein, uncoupling protein mRNA, and GLUT4 in adipose tissues of goats.

Adipose tissues have been characterized in newborn goats on the basis of the presence of the 32,000-M(r) mitochondrial uncoupling protein (UCP) diagnostic of brown fat, and early postnatal developmental changes have been determined. Both internal (perirenal, pericardial, and omental) and subcutaneous (hindlimb and neck regions) adipose tissues in newborn goats contained UCP and are therefore brown fat. The insulin-sensitive glucose transporter, GLUT4, was also present in adipose tissues of newborn goats, implying a potential for insulin-stimulated glucose uptake at birth. UCP was still evident in the perirenal fat of goats at 3 wk of age, but the mitochondrial concentration was only 4%, and the amount per cell a mere 0.2%, of that in the newborn. UCP was not detectable, however, in either the omental or subcutaneous adipose tissue at 3 wk of age and had disappeared from the subcutaneous fat before 2 wk. In contrast to UCP, GLUT4 remained throughout the initial 3 wk of postnatal life. The mRNA for UCP was detected in perirenal adipose tissue of newborn goats, indicating that the gene coding for the protein is being expressed at around birth. UCP mRNA was not, however, detectable in the other fat depots; nor was it evident in the perirenal adipose tissue at > or = 2.5 days of age. It is concluded that the adipose tissues of newborn goats (both internal and subcutaneous) represent brown fat and that there is a rapid transition toward white fat over the first weeks of life. This transition occurs at different rates in different depots but is not markedly faster in goats than in other ruminants.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗
PubMed2005

Turning signals on and off: GLUT4 traffic in the insulin-signaling highway.

Insulin stimulation of glucose uptake into skeletal muscle and adipose tissues is achieved by accelerating glucose transporter GLUT4 exocytosis from intracellular compartments to the plasma membrane and minimally reducing its endocytosis. The round trip of GLUT4 is intricately regulated by diverse signaling molecules impinging on specific compartments. Here we highlight the key molecular signals that are turned on and off by insulin to accomplish this task.

Animals↗
PubMed2006

Expression of uncoupling protein 3 and GLUT4 gene in skeletal muscle of preterm newborns: possible control by AMP-activated protein kinase.

We seek to understand the mechanism for the delayed postnatal switch between glycolytic and oxidative metabolism in preterm newborns. Our previous study [Brauner et al. (Pediatr Res 53: 691-697, 2003)] suggested impaired postnatal recruitment of the gene for mitochondrial uncoupling protein 3 (UCP3) by nutritional lipids in skeletal muscle of neonates delivered before approximately 26 wk of gestation. UCP3 is linked to lipid oxidation and may be involved in the defective development of energy metabolism in skeletal muscles of very preterm newborns. In extension of our previous study, autopsy samples of musculus quadriceps femoris from 40 mostly preterm neonates and 5 fetuses were used for quantification of transcripts for UCP3, GLUT4, and their transcriptional regulator, AMP-activated protein kinase (AMPK). The new analysis confirmed the defect in the recruitment of the UCP3 gene expression by lipids in very preterm neonates. It also suggested involvement of AMPK in the control of expression of both metabolic genes, UCP3 and GLUT4, in the skeletal muscle of the newborns. Experiments on adult C57BL/6J mice confirmed the relationships between the transcripts and supported the involvement of AMPK in the control of UCP3 gene expression.

Adult↗
PubMed2000

Calmodulin antagonists inhibit insulin-stimulated GLUT4 (glucose transporter 4) translocation by preventing the formation of phosphatidylinositol 3,4,5-trisphosphate in 3T3L1 adipocytes.

It has been previously reported that calmodulin plays a regulatory role in the insulin stimulation of glucose transport. To examine the basis for this observation, we examined the effect of a panel of calmodulin antagonists that demonstrated a specific inhibition of insulin-stimulated glucose transporter 4 (GLUT4) but not insulin- or platelet-derived growth factor (PDGF)-stimulated GLUT1 translocation in 3T3L1 adipocytes. These treatments had no effect on insulin receptor autophosphorylation or tyrosine phosphorylation of insulin receptor substrate 1 (IRS1). Furthermore, IRS1 or phosphotyrosine antibody immunoprecipitation of phosphatidylinositol (PI) 3-kinase activity was not affected. Despite the marked insulin and PDGF stimulation of PI 3-kinase activity, there was a near complete inhibition of protein kinase B activation. Using a fusion protein of the Grp1 pleckstrin homology (PH) domain with the enhanced green fluorescent protein, we found that the calmodulin antagonists prevented the insulin stimulation of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] formation in vivo. Similarly, although PDGF stimulation increased PI 3-kinase activity in in vitro immunoprecipitation assays, there was also no significant formation of PI(3,4,5)P3 in vivo. These data demonstrate that calmodulin antagonists prevent insulin-stimulated GLUT4 translocation by inhibiting the in vivo production of PI(3,4,5)P3 without directly affecting IRS1- or phosphotyrosine-associated PI 3-kinase activity. This phenomenon is similar to that observed for the PDGF stimulation of 3T3L1 adipocytes.

3T3 Cells↗
PubMed2004

Corosolic acid induces GLUT4 translocation in genetically type 2 diabetic mice.

The effect of corosolic acid (CA) on blood glucose was studied in KK-Ay mice, an animal model of type 2 diabetes. CA (10 mg/kg) reduced the blood glucose (p<0.05) of KK-Ay mice 4 h after single oral administration when compared with the control group. However, CA did not change the plasma insulin. The muscle facilitative glucose transporter isoform 4 (GLUT4) translocation from low-density microsomal membrane to plasma membrane was significantly increased in the orally CA-treated mice when compared with that of the controls (p<0.05). These results suggest that the hypoglycemic effect of CA is derived, at least in part, from an increase in GLUT4 translocation in muscle. Therefore, it may be that CA has beneficial effects on hyperglycemia in type 2 diabetes.

Animals↗
PubMed1998

Comparison of amino acid sequence of the C-terminal domain of insulin-responsive glucose transporter (GLUT4) in livestock mammals.

The nucleotide sequence of cDNA coding the 38-amino acid of the C-terminal domain of insulin-responsive glucose transporter (GLUT4) was determined by the method of reverse transcription-polymerase chain reaction in the sheep, goat and pig, and compared with that of bovine which have been shown to have a unique amino acid conversion of Asn508 to His. The deduced amino acid sequence was completely identical in the three species, and did not have the amino acid conversion at position 508. Western blot analysis confirmed that an antiserum raised against a rat C-terminal peptide cross-reacted efficiently with GLUT4 of these livestock mammals.

Amino Acid Sequence↗
PubMed2002

Thyroid hormone augments GLUT4 expression and insulin-sensitive glucose transport system in differentiating rat brown adipocytes in culture.

The effects of triiodothyronine (T3) on differentiation-dependent expression of GLUT and responses of glucose transport to insulin and norepinephrine (NE) were investigated. Precursor cells of brown adipocytes isolated from the interscapular brown adipose tissue of newborn rats were cultured in the absence or presence of various concentrations of T3. Western bolt analysis revealed that treatment with T3 resulted in an increased expression of GLUT4, in a dose-dependent manner, whereas GLUT1 contents were unchanged. In parallel with the increase in GLUT4 expression, T3 improved insulin sensitivity for glucose transport, being accompanied by an increase in maximal transport rate and a reduction of ED(50). In contrast, T3-treatment of the brown adipocytes during the differentiation process had little effect on NE-regulatable glucose transport system. These results suggest that T3 plays a predominant role in the development of insulin-sensitive glucose transport during differentiation of brown adipocytes.

Adipocytes↗
PubMed1994

Time courses of changes in hepatic and skeletal muscle insulin action and GLUT4 protein in skeletal muscle after STZ injection.

To determine the relative time courses of changes in peripheral and hepatic insulin action and skeletal muscle GLUT4 protein levels after a streptozotocin (STZ) injection in rats, we performed hyperinsulinemic (14-18 nM), euglycemic (7.5 mM) clamps in control (n = 8) and diabetic rats at 1 (n = 7), 3 (n = 8), 7 (n = 8), and 14 (n = 6) days after intraperitoneal STZ (65 mg/kg). Basal plasma glucose concentrations increased from 8.1 +/- 0.2 mM in control rats to 23.5 +/- 1.2 mM 1 day after STZ (P < 0.01) and remained constant thereafter. Basal plasma insulin levels were approximately 35% of control levels in all STZ groups (P < 0.01). Insulin-stimulated whole-body glucose uptake decreased significantly as early as one day after STZ injection (P < 0.01), resulting predominantly from a decrease in whole-body glycolysis. Insulin action to suppress hepatic glucose output was normal on day 1 after STZ but impaired markedly on day 3 and thereafter (P < 0.01). Insulin-stimulated glucose uptake in individual skeletal muscles was not altered until day 7 after STZ, and the magnitudes of decreases in skeletal muscle insulin action on days 7 and 14 were not fully accounted for by the decreases in GLUT4 protein level measured from the same muscles. Our data indicate that there is a temporal hierarchy in the development of insulin resistance in STZ-induced diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗