Divergent mechanisms for the insulin resistant and hyperresponsive glucose transport in adipose cells from fasted and refed rats. Alterations in both glucose transporter number and intrinsic activity

Class II histone deacetylases downregulate GLUT4 transcription in response to increased cAMP signaling in cultured adipocytes and fasting mice

Insulin-mediated glucose uptake is highly sensitive to the levels of the facilitative glucose transporter protein, GLUT4. Repression of GLUT4 expression is correlated with insulin resistance in adipose tissue. We have shown that differentiation-dependent GLUT4 transcription was under control of class II histone deacetylases (HDACs). We hypothesized that HDACs may regulate gene expression in adipocytes as a result of adrenergic activation. To test this hypothesis, we activated cAMP signaling in 3T3-L1 adipocytes and in mice after an overnight fast. Chromatin immunoprecipitation experiments showed the association of HDAC4/5 with the GLUT4 promoter in vivo and in vitro in response to elevated cAMP. Knockdown of HDACs by small interfering RNA in cultured adipocytes prevented the cAMP-dependent decrease in GLUT4 transcription. HDAC4/5 recruitment to the GLUT4 promoter was dependent on the GLUT4 liver X receptor (LXR) binding site. Treatment of cells with an LXR agonist prevented the cAMP-dependent decrease in GLUT4 transcription. A loss of function mutation in the LXR response element was required for cAMP-dependent downregulation of GLUT4 expression in vitro, in fasted mice, and in mice subjected to diet-induced obesity. This suggests that activation of LXR signaling can prevent loss of GLUT4 expression in diabetes and obesity.

Class II histone deacetylases limit GLUT4 gene expression during adipocyte differentiation

Insulin-dependent glucose homeostasis is highly sensitive to the levels of insulin-responsive glucose transporter 4 (GLUT4) expression in adipocytes. The level of GLUT4 protein expression is highly dependent on the rate of GLUT4 gene transcription. GLUT4 gene transcription is decreased in a variety of physiologic states of insulin resistance including type 2 diabetes, obesity, and prolonged fasting. GLUT4 gene expression in adipocytes is differentiation-dependent, with full expression delayed until late in the differentiation program. In this paper, we have tested the hypothesis that differentiation-dependent GLUT4 gene expression in 3T3-L1 adipocytes is dependent on the nuclear concentration of a class II histone deacetylase (HDAC) protein, HDAC5. We have tested this hypothesis by reducing the levels of class II HDACs in the nuclear compartment of 3T3-L1 preadipocytes using two experimental approaches. First, preadipocytes were treated with phenylephrine, an α-adrenergic receptor agonist, to drive HDACS out of the nuclear compartment. Also, the class II HDAC concentrations were reduced using siRNA knockdown. In each case, reduction of nuclear class II HDAC concentration resulted in increased expression of endogenous GLUT4 mRNA in preadipocytes. Together, our data indicate that class II HDAC expression is the major regulatory mechanism for inhibiting GLUT4 expression in the predifferentiated state.

Proteomic analysis of GLUT4 storage vesicles reveals LRP1 to be an important vesicle component and target of insulin signaling

Insulin stimulates the translocation of intracellular GLUT4 to the plasma membrane where it functions in adipose and muscle tissue to clear glucose from circulation. The pathway and regulation of GLUT4 trafficking are complicated and incompletely understood and are likely to be contingent upon the various proteins other than GLUT4 that comprise and interact with GLUT4-containing vesicles. Moreover, not all GLUT4 intracellular pools are insulin-responsive as some represent precursor compartments, thus posing a biochemical challenge to the purification and characterization of their content. To address these issues, we immunodepleted precursor GLUT4-rich vesicles and then immunopurified GLUT4 storage vesicle (GSVs) from primary rat adipocytes and subjected them to semi-quantitative and quantitative proteomic analysis. The purified vesicles translocate to the cell surface almost completely in response to insulin, the expected behavior for bona fide GSVs. In total, over 100 proteins were identified, about 50 of which are novel in this experimental context. LRP1 (low density lipoprotein receptor-related protein 1) was identified as a major constituent of GSVs, and we show it interacts with the lumenal domains of GLUT4 and other GSV constituents. Its cytoplasmic tail interacts with the insulin-signaling pathway target, AS160 (Akt substrate of 160 kDa). Depletion of LRP1 from 3T3-L1 adipocytes reduces GLUT4 expression and correspondingly results in decreased insulin-stimulated 2-[(3)H]deoxyglucose uptake. Furthermore, adipose-specific LRP1 knock-out mice also exhibit decreased GLUT4 expression. These findings suggest LRP1 is an important component of GSVs, and its expression is needed for the formation of fully functional GSVs.

Caveolin-1 loss of function accelerates glucose transporter 4 and insulin receptor degradation in 3T3-L1 adipocytes

Caveolae are a specialized type of lipid rafts that are stabilized by oligomers of caveolin protein. Caveolae are particularly enriched in adipocytes. Here we analyzed the effects of caveolin-1 knockdown and caveolae ablation on adipocyte function. To this end, we obtained several multiclonal mouse 3T3-L1 cell lines with a reduced expression of caveolin-1 (95% reduction) by a small interfering RNA approach using lentiviral vectors. Control cell lines were obtained by lentiviral infection with lentiviral vectors encoding appropriate scrambled RNAs. Caveolin-1 knockdown adipocytes showed a drastic reduction in the number of caveolae (95% decrease) and cholera toxin labeling was reorganized in dynamic plasma membrane microdomains. Caveolin-1 depletion caused a specific decrease in glucose transporter 4 (GLUT4) and insulin receptor protein levels. This reduction was not the result of a generalized defect in adipocyte differentiation or altered gene expression but was explained by faster degradation of these proteins. Caveolin-1 knockdown adipocytes showed reductions in insulin-stimulated glucose transport, insulin-triggered GLUT4 recruitment to the cell surface, and insulin receptor activation. In all, our data indicate that caveolin-1 loss of function reduces maximal insulin response through lowered stability and diminished expression of insulin receptors and GLUT4. We propose that caveolin-1/caveolae control insulin action in adipose cells.

GLUT4 enhancer factor (GEF) interacts with MEF2A and HDAC5 to regulate the GLUT4 promoter in adipocytes

The insulin-responsive glucose transporter, GLUT4, is regulated in various physiologic states at the transcriptional level. When expressed in transgenic mice, the human GLUT4 promoter is governed by two cis-acting sequences: an MEF2 binding domain and Domain I, that function both as positive and negative regulators depending on the physiologic state. MEF2 proteins and GLUT4 enhancer factor (GEF) are known ligands for these cis-acting elements, but their mechanism of action is unclear. To begin to understand this important process, we have characterized GEF structural domains and its interactions with the MEF2A isoform. We find that the C terminus of GEF comprises its DNA-binding domain, but does not contribute to GEF homo-oligomerization. We also have found that GEF dimerizes with increased affinity to a hypophosphorylated form of MEF2A. Furthermore, we demonstrated that MEF2A binding to its cognate binding site can increase the DNA binding activity of GEF to Domain I, suggesting a novel mechanism for MEF2A transcriptional activation. Finally, we have demonstrated that the transcriptional co-repressor HDAC5 can interact with GEF in the absence of MEF2 proteins and specifically inhibit GLUT4 promoter activity. These findings lead to the hypothesis that GEF and the MEF2 proteins form a complex on the GLUT4 promoter that allows for recruitment of transcriptional co-regulators (repressors and/or activators) to control GLUT4 promoter activity.

Sennidin stimulates glucose incorporation in rat adipocytes

A novel small molecule compound which exerts insulin mimetic is desirable. Dozens of natural products that have quinone, naphthoquinone, or anthraquinone structure, were tested by a glucose incorporation assay. We found that sennidin A, anthraquinone derivative, stimulated glucose incorporation to near level of maximal insulin-stimulated and sennidin B, a stereoisomer of sennidin A, also stimulated, but the activity of sennidin B was lower than sennidin A. Sennidin A-stimulated glucose incorporation was completely inhibited by wortmannin. Sennidin A did not induce tyrosine phosphorylation of insulin receptor (IR) and insulin receptor substrate-1 (IRS-1), but induced phosphorylation of Akt and glucose transporter 4 (GLUT4) translocation. Our results suggest that in rat adipocytes, sennidin A stimulates glucose incorporation in the phosphatidylinositol 3-kinase (PI3K)- and Akt-dependent, but in the IR/IRS1-independent manner.

Insulin regulates the membrane arrival, fusion, and C-terminal unmasking of glucose transporter-4 via distinct phosphoinositides

Insulin increases glucose uptake into muscle via glucose transporter-4 (GLUT4) translocation to the cell membrane, but the regulated events in GLUT4 traffic are unknown. Here we focus on the role of class IA phosphatidylinositol (PI) 3-kinase and specific phosphoinositides in the steps of GLUT4 arrival and fusion with the membrane, using L6 muscle cells expressing GLUT4myc. To this end, we detected the availability of the myc epitope at the cell surface or intravesicular spaces and of the cytosol-facing C-terminal epitope, in cells and membrane lawns derived from them. We observed the following: (a) Wortmannin and LY294002 at concentrations that inhibit class IA PI 3-kinase reduced but did not abate the C terminus gain, yet the myc epitope was unavailable for detection unless lawns or cells were permeabilized, suggesting the presence of GLUT4myc in docked, unfused vesicles. Accordingly, GLUT4myc-containing vesicles were detected by immunoelectron microscopy of membranes from cells pretreated with wortmannin and insulin, but not insulin or wortmannin alone. (b) Insulin caused greater immunological availability of the C terminus than myc epitopes, suggesting that C terminus unmasking had occurred. Delivering phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) to intact cells significantly increased lawn-associated myc signal without C terminus gain. Conversely, phosphatidylinositol 3-phosphate (PI3P) increased the detection of C terminus epitope without any myc gain. We propose that insulin regulates GLUT4 membrane arrival, fusion, and C terminus unmasking, through distinct phosphoinositides. PI(3,4,5)P(3) causes arrival and fusion without unmasking, whereas PI3P causes arrival and unmasking without fusion.

GLUT4 overexpression or deficiency in adipocytes of transgenic mice alters the composition of GLUT4 vesicles and the subcellular localization of GLUT4 and insulin-responsive aminopeptidase

The majority of GLUT4 is sequestered in unique intracellular vesicles in the absence of insulin. Upon insulin stimulation GLUT4 vesicles translocate to, and fuse with, the plasma membrane. To determine the effect of GLUT4 content on the distribution and subcellular trafficking of GLUT4 and other vesicle proteins, adipocytes of adipose-specific, GLUT4-deficient (aP2-GLUT4-/-) mice and adipose-specific, GLUT4-overexpressing (aP2-GLUT4-Tg) mice were studied. GLUT4 amount was reduced by 80-95% in aP2-GLUT4-/- adipocytes and increased approximately 10-fold in aP2-GLUT4-Tg adipocytes compared with controls. Insulin-responsive aminopeptidase (IRAP) protein amount was decreased 35% in aP2-GLUT4-/- adipocytes and increased 45% in aP2-GLUT4-Tg adipocytes. VAMP2 protein was also decreased by 60% in aP2-GLUT4-/- adipocytes and increased 2-fold in aP2-GLUT4-Tg adipocytes. IRAP and VAMP2 mRNA levels were unaffected in aP2-GLUT4-Tg, suggesting that overexpression of GLUT4 affects IRAP and VAMP2 protein stability. The amount and subcellular distribution of syntaxin4, SNAP23, Munc-18c, and GLUT1 were unchanged in either aP2-GLUT4-/- or aP2-GLUT4-Tg adipocytes, but transferrin receptor was partially redistributed to the plasma membrane in aP2-GLUT4-Tg adipocytes. Immunogold electron microscopy revealed that overexpression of GLUT4 in adipocytes increased the number of GLUT4 molecules per vesicle nearly 2-fold and the number of GLUT4 and IRAP-containing vesicles per cell 3-fold. In addition, the proportion of cellular GLUT4 and IRAP at the plasma membrane in unstimulated aP2-GLUT4-Tg adipocytes was increased 4- and 2-fold, respectively, suggesting that sequestration of GLUT4 and IRAP is saturable. Our results show that GLUT4 overexpression or deficiency affects the amount of other GLUT4-vesicle proteins including IRAP and VAMP2 and that GLUT4 sequestration is saturable.

Acute and chronic leptin treatment mediate contrasting effects on signaling, glucose uptake, and GLUT4 translocation in L6-GLUT4myc myotubes

We have previously shown that in L6-GLUT4myc rat skeletal muscle cells, acute treatment with leptin reduced insulin-stimulated glucose uptake without altering insulin-stimulated GLUT4 translocation. In contrast, we show here that the ability of leptin to increase phosphorylation of its receptor and to reduce insulin-stimulated glucose uptake was lost in cells that were continuously exposed to leptin for 24 h. This desensitization correlated with an increase in expression of suppressor of cytokine signaling-3 (SOCS-3). Time course analysis demonstrated that the transition from acute to chronic effects of leptin occurs after 2 h. The desensitization of leptin action at 24 h was not reversed by 30 min washout period prior to re-exposing cells to leptin. However, despite insulin-stimulated glucose uptake being unaffected upon 24 h preincubation with leptin, a small but significant decrease (37%) in insulin-stimulated GLUT4 translocation and phosphorylation of Akt on T308 was detected. Insulin-stimulated phosphorylation of Akt on S473 or of p38 MAPK were unaffected. These results suggest that the chronic leptin treatment leads to desensitization of leptin signaling yet can simultaneously decrease the ability of insulin to phosphorylate Akt on T308 and translocate GLUT4. However, this does not manifest as a reduction in total glucose uptake into L6 myotubes.

Maturation of the regulation of GLUT4 activity by p38 MAPK during L6 cell myogenesis

Insulin stimulates glucose uptake in skeletal muscle cells and fat cells by promoting the rapid translocation of GLUT4 glucose transporters to the plasma membrane. Recent work from our laboratory supports the concept that insulin also stimulates the intrinsic activity of GLUT4 through a signaling pathway that includes p38 MAPK. Here we show that regulation of GLUT4 activity by insulin develops during maturation of skeletal muscle cells into myotubes in concert with the ability of insulin to stimulate p38 MAPK. In L6 myotubes expressing GLUT4 that carries an exofacial myc-epitope (L6-GLUT4myc), insulin-stimulated GLUT4myc translocation equals in magnitude the glucose uptake response. Inhibition of p38 MAPK with SB203580 reduces insulin-stimulated glucose uptake without affecting GLUT4myc translocation. In contrast, in myoblasts, the magnitude of insulin-stimulated glucose uptake is significantly lower than that of GLUT4myc translocation and is insensitive to SB203580. Activation of p38 MAPK by insulin is considerably higher in myotubes than in myoblasts, as is the activation of upstream kinases MKK3/MKK6. In contrast, the activation of all three Akt isoforms and GLUT4 translocation are similar in myoblasts and myotubes. Furthermore, GLUT4myc translocation and phosphorylation of regulatory sites on Akt in L6-GLUT4myc myotubes are equally sensitive to insulin, whereas glucose uptake and phosphorylation of regulatory sites on p38 MAPK show lower sensitivity to the hormone. These observations draw additional parallels between Akt and GLUT4 translocation and between p38 MAPK and GLUT4 activation. Regulation of GLUT4 activity by insulin develops upon muscle cell differentiation and correlates with p38 MAPK activation by insulin.

Practical aspects of overexpressing bacterial secondary membrane transporters for structural studies

Membrane transporter proteins play critical physiological roles in the cell and constitute 5-10% of prokaryotic and eukaryotic genomes. High-resolution structural information is essential for understanding the functional mechanism of these proteins. A prerequisite for structural study is to overexpress such proteins in large quantities. In the last few years, over 20 bacterial membrane transporters were overexpressed at a level of 1 mg/l of culture or higher, most often in Escherichia coli. In this review, we analyzed those factors that affect the quantity and quality of the protein produced, and summarized recent progress in overexpression of membrane transporters from bacterial inner membrane. Rapid progress in genome sequencing provides opportunities for expressing several homologues and orthologues of the target protein simultaneously, while the availability of various expression vectors allows flexible experimental design. Careful optimization of cell culture conditions can drastically improve the expression level and homogeneity of the target protein. New sample preparation techniques for mass spectrometry of membrane proteins have enabled one to identity the rigid protein core, which can be subsequently overexpressed. Size-exclusion chromatography on HPLC has proven to be an efficient method in screening detergent, pH an other conditions required for maintaining the stability and monodispersity of the protein. Such high-quality preparations of membrane transporter proteins will probably lead to successful crystallization and structure determination of these proteins in the next few years.

Regulation of insulin signal transduction pathway by a small-molecule insulin receptor activator

Insulin regulates cellular metabolism and growth through activation of insulin receptors (IRs). We recently identified a non-peptide small-molecule IR activator (compound 2), which induced human IR tyrosine kinase activity in Chinese-hamster ovary cells expressing human IR [Qureshi, Ding, Li, Szalkowski, Biazzo-Ashnault, Xie, Saperstein, Brady, Huskey, Shen et al. (2000) J. Biol. Chem. 275, 36590-36595]. Oral treatment with this compound resulted in correction of hyperglycaemia, hypertriacylglycerolaemia and hyperinsulinaemia in several rodent models of diabetes. In the present study, we have found that this compound increased tyrosine phosphorylation of the IR beta-subunit and IR substrate 1 in primary rat adipocytes as well as induced phosphorylation of Akt, the 70 kDa ribosomal protein S6 kinase and glycogen synthase-3 (deactivation) in Chinese-hamster ovary cells expressing human IR. Similar to insulin, compound 2 stimulated glucose uptake, glycogen synthesis and inhibited isoprenaline-stimulated lipolysis in adipocytes. A structurally related analogue (compound 3) was devoid of the above activities suggesting that the activity of compound 2 is specifically mediated by targeted IR activation. The effects of compound 2 on stimulation of glucose uptake, glycogen synthesis and inhibition of lipolysis were blocked by wortmannin, consistent with the involvement of a phosphoinositide 3-kinase-dependent pathway. In addition, compound 2, but not compound 3, exhibited additive or synergistic effects with sub-maximal concentrations of insulin in rat adipocytes. Thus the IR activator was capable of activating insulin-mediated signalling and metabolic pathways in primary adipocytes. These results demonstrate that IR activators have implications for the future development of new therapeutic approaches to Type I and Type II diabetes.

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.

GLUT4 translocation precedes the stimulation of glucose uptake by insulin in muscle cells: potential activation of GLUT4 via p38 mitogen-activated protein kinase

We previously reported that SB203580, an inhibitor of p38 mitogen-activated protein kinase (p38 MAPK), attenuates insulin-stimulated glucose uptake without altering GLUT4 translocation. These results suggested that insulin might activate GLUT4 via a p38 MAPK-dependent pathway. Here we explore this hypothesis by temporal and kinetic analyses of the stimulation of GLUT4 translocation, glucose uptake and activation of p38 MAPK isoforms by insulin. In L6 myotubes stably expressing GLUT4 with an exofacial Myc epitope, we found that GLUT4 translocation (t(1/2)=2.5 min) preceded the stimulation of 2-deoxyglucose uptake (t(1/2)=6 min). This segregation of glucose uptake from GLUT4 translocation became more apparent when the two parameters were measured at 22 degrees C. Preincubation with the p38 MAPK inhibitors SB202190 and SB203580 reduced insulin-stimulated transport of either 2-deoxyglucose or 3-O-methylglucose by 40-60%. Pretreatment with SB203580 lowered the apparent transport V(max) of insulin-mediated 2-deoxyglucose and 3-O-methylglucose without any significant change in the apparent K(m) for either hexose. The IC(50) values for the partial inhibition of 2-deoxyglucose uptake by SB202190 and SB203580 were 1 and 2 microM respectively, and correlated with the IC(50) for full inhibition of p38 MAPK by the two inhibitors in myotubes (2 and 1.4 microM, respectively). Insulin caused a dose- (EC(50)=15 nM) and time- (t(1/2)=3 min) dependent increase in p38 MAPK phosphorylation, which peaked at 10 min (2.3+/-0.3-fold). In vitro kinase assay of immunoprecipitates from insulin-stimulated myotubes showed activation of p38 alpha (2.6+/-0.3-fold) and p38 beta (2.3+/-0.2-fold) MAPK. These results suggest that activation of GLUT4 follows GLUT4 translocation and that both mechanisms contribute to the full stimulation of glucose uptake by insulin. Furthermore, activation of GLUT4 may occur via an SB203580-sensitive pathway, possibly involving p38 MAPK.

Sustained hyperglycemia and insulin resistance induced by dietary restriction

This investigation considered whether defects in insulin secretion and insulin action may result in the development of sustained hyperglycemia induced by refeeding standard chow pellets. Hyperglycemia, sustained for 36 h was induced, in mice that ate standard chow pellets ad libitum after 48 h fasting, but not 24 h fasting. In 48 h-fasted mice, serum insulin levels were remarkably low and the ability of insulin secretion to respond to glucose was decreased, although insulin-stimulated glucose disposal was not impaired. However, hyperinsulinemia was observed after refeeding for 12 h. The 12 h-refed mice had impaired glucose tolerance and were remarkably insulin resistant. These results suggest that hyperglycemia induced by the fasting-refeeding was caused by hyperphagia and the failure of insulin secretion, and maintained the resulting induced insulin resistance.

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

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

Discovery of a small molecule insulin mimetic with antidiabetic activity in mice

Insulin elicits a spectrum of biological responses by binding to its cell surface receptor. In a screen for small molecules that activate the human insulin receptor tyrosine kinase, a nonpeptidyl fungal metabolite (L-783,281) was identified that acted as an insulin mimetic in several biochemical and cellular assays. The compound was selective for insulin receptor versus insulin-like growth factor I (IGFI) receptor and other receptor tyrosine kinases. Oral administration of L-783,281 to two mouse models of diabetes resulted in significant lowering in blood glucose levels. These results demonstrate the feasibility of discovering novel insulin receptor activators that may lead to new therapies for diabetes.

Time-dependent and tissue-specific effects of circulating glucose on fetal ovine glucose transporters

To determine the cellular adaptations to fetal hyperglycemia and hypoglycemia, we examined the time-dependent effects on basal (GLUT-1 and GLUT-3) and insulin-responsive (GLUT-4) glucose transporter proteins by quantitative Western blot analysis in fetal ovine insulin-insensitive (brain and liver) and insulin-sensitive (myocardium, skeletal muscle, and adipose) tissues. Maternal glucose infusions causing fetal hyperglycemia resulted in a transient 30% increase in brain GLUT-1 but not GLUT-3 levels and a decline in liver and adipose GLUT-1 and myocardial and skeletal muscle GLUT-1 and GLUT-4 levels compared with gestational age-matched controls. Maternal insulin infusions leading to fetal hypoglycemia caused a decline in brain GLUT-3, an increase in brain GLUT-1, and a subsequent decline in liver GLUT-1, with no significant change in insulin-sensitive myocardium, skeletal muscle, and adipose tissue GLUT-1 or GLUT-4 concentrations, compared with gestational age-matched sham controls. We conclude that fetal glucose transporters are subject to a time-dependent and tissue- and isoform-specific differential regulation in response to altered circulating glucose and/or insulin concentrations. These cellular adaptations in GLUT-1 (and GLUT-3) are geared toward protecting the conceptus from perturbations in substrate availability, and the adaptations in GLUT-4 are geared toward development of fetal insulin resistance.

Short-term exercise enhances insulin-stimulated GLUT-4 translocation and glucose transport in adipose cells

This investigation examined the effects of short-term exercise training on insulin-stimulated GLUT-4 glucose transporter translocation and glucose transport activity in rat adipose cells. Male Wistar rats were randomly assigned to a sedentary (Sed) or swim training group (Sw, 4 days; final 3 days: 2 x 3 h/day). Adipose cell size decreased significantly but minimally (approximately 20%), whereas total GLUT-4 increased by 30% in Sw vs. Sed rats. Basal 3-O-methyl-D-[14C]glucose transport was reduced by 62%, whereas maximally insulin-stimulated (MIS) glucose transport was increased by 36% in Sw vs. Sed rats. MIS cell surface GLUT-4 photolabeling was 44% higher in the Sw vs. Sed animals, similar to the increases observed in MIS glucose transport activity and total GLUT-4. These results suggest that increases in total GLUT-4 and GLUT-4 translocation to the cell surface contribute to the increase in MIS glucose transport with short-term exercise training. In addition, the results suggest that the exercise training-induced adaptations in glucose transport occur more rapidly than previously thought and with minimal changes in adipose cell size.

Lipoic acid reduces glycemia and increases muscle GLUT4 content in streptozotocin-diabetic rats

Alpha lipoic acid (lipoate [LA]), a cofactor of alpha-ketodehydrogenase, exhibits unique antioxidant properties. Recent studies suggest a direct effect of LA on glucose metabolism in both human and experimental diabetes. This study examines the possibility that LA positively affects glucose homeostasis in streptozotocin (STZ)-induced diabetic rats by altering skeletal muscle glucose utilization. Blood glucose concentration in STZ-diabetic rats following 10 days of intraperitoneal (i.p.) injection of LA 30 mg/kg was reduced compared with that in vehicle-treated diabetic rats (495 +/- 131 v 641 +/- 125 mg/dL in fed state, P = .003, and 189 +/- 48 v 341 +/- 36 mg/dL after 12-hour fast, P = .001). No effect of LA on plasma insulin was observed. Gastrocnemius muscle crude membrane GLUT4 protein was elevated both in control and in diabetic rats treated with LA by 1.5- and 2.8-fold, respectively, without significant changes in GLUT4 mRNA levels. Gastrocnemius lactic acid was increased in diabetic rats (19.9 +/- 5.5 v 10.4 +/- 2.8 mumol/g muscle, P < .05 v nondiabetic rats), and was normal in LA-treated diabetic rats (9.1 +/- 5.0 mumol/g muscle). Insulin-stimulated 2-deoxyglucose (2 DG) uptake into isolated soleus muscle was reduced in diabetic rats compared with the control group (474 +/- 15 v 568 +/- 52 pmol/mg muscle 30 min, respectively, P = .05). LA treatment prevented this reduction, resulting in insulin-stimulated glucose uptake comparable to that of nondiabetic animals. These results suggest that daily LA treatment may reduce blood glucose concentrations in STZ-diabetic rats by enhancing muscle GLUT4 protein content and by increasing muscle glucose utilization.

Plasma glucose and cortisol responses to exogenous insulin in fasted donkeys

Susceptibility to equine hyperlipaemia is increased by poor food intake. To assess the contribution of changes in insulin sensitivity, plasma glucose and cortisol responses to an intravenous insulin challenge (0.4 IU kg-1 bodyweight) were compared with those observed after saline administration in six donkeys fasted either overnight or for three days. Three days of fasting decreased both the rate of insulin-induced hypoglycemia and the maximal hypoglycemic response. A transitory increase in plasma cortisol which peaked within one to four hours of insulin administration was observed in three of the six overnight-fasted donkeys and in all of the three-day fasted donkeys; inter-animal variation appeared to exist in the responsiveness of the hypothalamic-pituitary-adrenocortical (HPA) axis to stimulation by insulin-induced hypoglycemia. Fasting is likely to present a risk of equine hyperlipaemia, at least on part, by the reduction in tissue sensitivity to the glucoregulatory action of insulin.

The IRS-2 gene on murine chromosome 8 encodes a unique signaling adapter for insulin and cytokine action

Signal transduction by insulin and IGF-1, several interleukins (IL-2, IL-4, IL-9, IL-13), interferons, GH, and other cytokines involves IRS proteins, which link the receptors for these factors to signaling molecules with Src homology-2 domains (SH2-proteins). We recently reported the amino acid sequence of murine IRS-2; in order to examine a potential genetic role for this molecule in disease, we isolated the murine IRS-2 gene and compared the expression pattern of IRS-2 against IRS-1. Like IRS-1, IRS-2 is encoded by a single exon. Whereas IRS-1 is located on murine chromosome 1, IRS-2 is located on murine chromosome 8 near the insulin receptor. IRS-2 is expressed together with IRS-1 in many cells and tissues; however, IRS-2 predominates in murine hematopoietic cells where it may be essential for cytokine signaling; IRS-1 predominates in adipocytes and differentiated 3T3-L1 cells where it contributes to the normal insulin response. In 32D cells, IRS-1 and IRS-2 undergo differential tyrosine phosphorylation during insulin or IL-4 stimulation, as assessed indirectly by interaction with various recombinant SH2 domains. Thus, signaling specificity through the IRS proteins may be accomplished by specific expression patterns and distinct phosphorylation patterns during interaction with various activated receptors.

Diverse effects of Glut 4 ablation on glucose uptake and glycogen synthesis in red and white skeletal muscle

The ability of muscles from Glut 4-null mice to take up and metabolize glucose has been studied in the isolated white EDL and red soleus muscles. In EDL muscles from male or female Glut 4-null mice, basal deoxyglucose uptake was lower than in control muscles and was not stimulated by insulin. In parallel, glycogen synthesis and content were decreased. Soleus muscles from male Glut 4-null mice took up twice more deoxyglucose in the absence of insulin than control muscles, but did not respond to insulin. In females, soleus deoxyglucose uptake measured in the absence of hormone was similar in Glut 4-null mice and in control mice. This uptake was stimulated twofold in Glut 4-null mice and threefold in control mice. Basal glycogen synthesis was increased by 4- and 2.2-fold in male and female null mice, respectively, compared to controls, and insulin had no or small (20% stimulation over basal) effect. These results indicate that while EDL muscles behaved as expected, soleus muscles were able to take up a large amount of glucose in the absence (males) or the presence of insulin (females). Whether this is due to a change in Glut 1 intrinsic activity or targeting and/or to the appearance of another glucose transporter remains to be determined.

Islet transplantation under the kidney capsule fully corrects the impaired skeletal muscle glucose transport system of streptozocin diabetic rats

Chronic insulin therapy improves but does not restore impaired insulin-mediated muscle glucose uptake in human diabetes or muscle glucose uptake, transport, and transporter translocation in streptozocin diabetic rats. To determine whether this inability is due to inadequate insulin replacement, we studied fasted streptozocin-induced diabetic Lewis rats either untreated or after islet transplantation under the kidney capsule. Plasma glucose was increased in untreated diabetics and normalized by the islet transplantation (110 +/- 5, 452 +/- 9, and 102 +/- 3 mg/dl in controls, untreated diabetics, and transplanted diabetics, respectively). Plasma membrane and intracellular microsomal membrane vesicles were prepared from hindlimb skeletal muscle of basal and maximally insulin-stimulated rats. Islet transplantation normalized plasma membrane carrier-mediated glucose transport Vmax, plasma membrane glucose transporter content, and insulin-induced transporter translocation. There were no differences in transporter intrinsic activity (Vmax/Ro) among the three groups. Microsomal membrane GLUT4 content was reduced by 30% in untreated diabetic rats and normal in transplanted diabetics, whereas the insulin-induced changes in microsomal membrane GLUT4 content were quantitatively similar in the three groups. There were no differences in plasma membrane GLUT1 among the groups and between basal and insulin stimulated states. Microsomal membrane GLUT1 content was increased 60% in untreated diabetics and normalized by the transplantation. In conclusion, an adequate insulin delivery in the peripheral circulation, obtained by islet transplantation, fully restores the muscle glucose transport system to normal in streptozocin diabetic rats.

Dexamethasone-induced impairment in skeletal muscle glucose transport is not reversed by inhibition of free fatty acid oxidation

Our previous studies suggested a possible role for the glucose-free fatty acid (FFA) cycle, ie, preferential utilization of FFA by muscle at the expense of glucose, in dexamethasone (DEX)-induced insulin resistance. To determine whether this resistance could be reversed by inhibiting FFA utilization, we used etomoxir, a potent inhibitor of mitochondrial FFA oxidation. Male Sprague-Dawley rats were injected subcutaneously with 1 mg/kg DEX or the vehicle every other day for 10 days, and half of each group was administered 10 mg/kg etomoxir by gavage once per day and 1 hour before the experiment. As expected, etomoxir treatment increased serum FFA levels and inhibited FFA oxidation by diaphragm in vitro. Administration of etomoxir decreased serum glucose and insulin concentrations under basal conditions in both control and DEX-treated animals, implying enhanced insulin sensitivity. DEX treatment significantly increased endogenous glucose production and decreased whole-body glucose disposal, as well as 2-deoxyglucose (2-DG) uptake by skeletal muscle during euglycemic-hyperinsulinemic clamps. Administration of etomoxir led to small but significant increases in glucose disposal rates of both control (14%) and DEX (23%) groups, but had no effect on residual endogenous glucose production. Thus, DEX-induced insulin resistance was marginally ameliorated but not completely reversed by etomoxir. Depressed 2-DG uptake by individual muscle tissues observed in the present study in conjunction with the absence of free intracellular glucose in muscle tissue following glucose-insulin infusion strongly suggests that the primary defect in glucose metabolism is at the level of transport. Neither overall abundance of the insulin-sensitive glucose transporter (GLUT-4) in skeletal muscle nor its distribution between intracellular stores and plasma membrane were modified by DEX treatment, either, under basal conditions or in response to acute insulin stimulus. These results suggest a defect(s) in the inherent activity of plasma membrane-bound GLUT-4 as the likely mechanism for DEX-induced insulin resistance.

Expression of ob mRNA and its encoded protein in rodents. Impact of nutrition and obesity

The mutant gene responsible for obesity in the ob/ob mouse was recently identified by positional cloning (Zhang Y., R. Proenca, M. Maffel, M. Barone, L. Leopold, and J.M. Friedman. 1994. Nature (Lond.) 372:425). The encoded protein and to represent and "adipostat" signal reflecting the state of energy stores. We confirm that the adipocyte is the source of ob mRNA and that the predicted 16-kD ob protein is present in rodent serum as detected by Western blot. To evaluate the hypothesis that it might represent an adipostat, we assessed serum levels of ob protein and expression of ob mRNA in adipose cells and tissue of rodents in response to a variety of perturbations which effect body fat mass. Both ob protein and ob mRNA expression are markedly increased in obesity. The levels of ob protein are approximately 5-10-fold elevated in serum of db/db mice, in mice with hypothalamic lesions caused by neonatal administration of monosodium glutamate (MSG), and in mice with toxigene induced brown fat ablation, (UCP-DTA). Very parallel changes are observed in adipocyte ob mRNA expression in these models and in ob/ob mice. As predicted however, no serum ob protein could be detected in the ob/ob mice. By contrast to obesity, starvation of normal rats and mice for 1-3 d markedly suppresses ob mRNA abundance, and this is reversed with refeeding. Similarly, ob protein concentration in normal mice falls to undetectable levels with starvation. In the ob/ob, UCP-DTA and MSG models, overexpression of ob mRNA is reversed by caloric restriction. These data support the hypothesis that expression of ob mRNA and protein are regulated as a function of energy stores, and that ob serves as a circulating feedback signal to sites involved in regulation of energy homeostasis.

Mechanisms and time course of impaired skeletal muscle glucose transport activity in streptozocin diabetic rats

Skeletal muscle glucose transport is altered in diabetes in humans, as well as in rats. To investigate the mechanisms of this abnormality, we measured glucose transport Vmax, the total transporter number, their average intrinsic activity, GLUT4 and GLUT1 contents in skeletal muscle plasma membrane vesicles from basal or insulin-stimulated streptozocin diabetic rats with different duration of diabetes, treated or not with phlorizin. The glucose transport Vmax progressively decreased with the duration of diabetes. In the basal state, this decrease was primarily associated with the reduction of transporter intrinsic activity, which appeared earlier than any change in transporter number or GLUT4 and GLUT1 content. In the insulin-stimulated state, the decrease of transport was mainly associated with severe defects in transporter translocation. Phlorizin treatment partially increased the insulin-stimulated glucose transport by improving the transporter translocation defects. In conclusion, in streptozocin diabetes (a) reduction of intrinsic activity plays a major and early role in the impairment of basal glucose transport; (b) a defect in transporter translocation is the mechanism responsible for the decrease in insulin-stimulated glucose transport; and (c) hyperglycemia per se affects the insulin-stimulated glucose transport by altering the transporter translocation.

GLUT-4 degradation rate: reduction in rat adipocytes in fasting and streptozotocin-induced diabetes

With the use of [3H]leucine pulse-chase and immunoprecipitation methods, we measured the rate of GLUT-4 degradation in rat adipocytes in the steady state at 37 degrees C. We also studied the relationship of the reduced GLUT-4 levels observed in fasted and streptozotocin (STZ)-induced diabetic rats on degradation. GLUT-4 degradation was a simple, first-order decay process. The decay was describable by a single, first-order rate constant (k). A k value of 0.061/h was estimated in control rat adipocytes. In the adipocytes of fasted and STZ-induced diabetic rats, cellular GLUT-4 contents were reduced to 36 and 43% of the control, respectively. The rates of GLUT-4 degradation were also reduced significantly, with kappa values of 0.038 and 0.041/h, respectively. These changes were reversible; the decreased values returned to control values when GLUT-4 contents were normalized by refeeding and insulin injection. These findings demonstrate the presence of a posttranslational mechanism in rat adipocytes that reduces the GLUT-4 degradation rate constant when the cellular GLUT-4 level is reduced by a pretranslational defect.

Insulin down-regulates expression of the insulin-responsive glucose transporter (GLUT4) gene: effects on transcription and mRNA turnover

Insulin rapidly represses expression of the gene encoding the insulin-responsive glucose transporter (GLUT4) in 3T3-L1 mouse adipocytes. Upon exposure to the hormone the cellular level of GLUT4 mRNA falls (t1/2 approximately 2.5 hr) to 20-30% of its initial level within 10 hr. This is followed by a similar decrease in the level of GLUT4 protein. Down-regulation of GLUT4 mRNA is a result of both rapid repression of transcription of the GLUT4 gene and an increased rate of turnover of the GLUT4 message. As a consequence of prolonged exposure to insulin, 3T3-L1 adipocytes lose their capacity for acute stimulation of hexose uptake by insulin. These findings provide an explanation for the resistance of glucose uptake to insulin in adipose tissue observed in non-insulin-dependent (type 2) diabetes mellitus, particularly that associated with hyperinsulinemia and obesity.

Changes in insulin-receptor mRNA levels in skeletal muscle and brown adipose tissue of weanling rats during fasting and refeeding

Tissue-specific alterations in insulin sensitivity occur in response to fasting and refeeding, as part of the integrated adaptive mechanisms employed to adjust to major changes in nutritional status. In the present study the effects of fasting and refeeding on insulin-receptor, actin and myosin mRNA levels in skeletal muscle, and insulin-receptor and uncoupling-protein mRNA in brown adipose tissue of rats have been examined. Insulin-receptor mRNA levels increased markedly in both skeletal muscle and brown adipose tissue after a 40 h fast, the increase being greater in brown fat (8-fold) than in muscle (2-fold). On refeeding for 4 h, the insulin-receptor mRNA level in both tissues declined rapidly to control levels. An increase in insulin-receptor mRNA level was also observed in brown adipose tissue after a 16 h fast, although not in skeletal muscle. In contrast to the insulin-receptor mRNA, the level of the mRNA for the mitochondrial uncoupling protein declined markedly in brown adipose tissue during a 40 h fast. These results indicate that insulin-receptor mRNA levels are modulated in response to the alterations in nutritional status that occur during fasting and refeeding; this may reflect a nutritional influence on transcription of the receptor-protein gene.

Insulin resistance in obese Zucker rat (fa/fa) skeletal muscle is associated with a failure of glucose transporter translocation

The genetically obese Zucker rat (fa/fa) is characterized by a severe resistance to the action of insulin to stimulate skeletal muscle glucose transport. The goal of the present study was to identify whether the defect associated with this insulin resistance involves an alteration of transporter translocation and/or transporter activity. Various components of the muscle glucose transport system were investigated in plasma membranes isolated from basal or maximally insulin-treated skeletal muscle of lean and obese Zucker rats. Measurements of D- and L-glucose uptake by membrane vesicles under equilibrium exchange conditions indicated that insulin treatment resulted in a four-fold increase in the Vmax for carrier-mediated transport for lean animals [from 4.5 to 17.5 nmol/(mg.s)] but only a 2.5-fold increase for obese rats [from 3.6 to 9.1 nmol/(mg.s)]. In the lean animals, this increase in glucose transport function was associated with a 1.8-fold increase in the transporter number as indicated by cytochalasin B binding, a 1.4-fold increase in plasma membrane GLUT4 protein, and a doubling of the average carrier turnover number (intrinsic activity). In the obese animals, there was no change in plasma membrane transporter number measured by cytochalasin B binding, or in GLUT4 or GLUT1 protein. However, there was an increase in carrier turnover number similar to that seen in the lean litter mates. Measurements of GLUT4 mRNA in red gastrocnemius muscle showed no difference between lean and obese rats. We conclude that the insulin resistance of the obese rats involves the failure of translocation of transporters, while the action of insulin to increase the average carrier turnover number is normal.

Facilitative glucose transporters: regulatory mechanisms and dysregulation in diabetes

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Effect of diabetes and fasting on GLUT-4 (muscle/fat) glucose-transporter expression in insulin-sensitive tissues. Heterogeneous response in heart, red and white muscle

1. GLUT-4 glucose-transporter protein and mRNA levels were assessed in heart, red muscle and white muscle, as well as in brown and white adipose tissue from 7-day streptozotocin-induced diabetic and 48 h-fasted rats. 2. In agreement with previous data, white adipose tissue showed a substantial decrease in GLUT-4 mRNA and protein levels in response to both diabetes and fasting. Similarly, GLUT-4 mRNA and protein markedly decreased in brown adipose tissue in both insulinopenic conditions. 3. Under control conditions, the level of expression of GLUT-4 protein content differed substantially in heart, red and white skeletal muscle. Thus GLUT-4 protein was maximal in heart, and red muscle had a greater GLUT-4 content compared with white muscle. In spite of the large differences in GLUT-4 protein content, GLUT-4 mRNA levels were equivalent in heart and red skeletal muscle. 4. In heart, GLUT-4 mRNA decreased to a greater extent than GLUT-4 protein in response to diabetes and fasting. In contrast, red muscle showed a greater decrease in GLUT-4 protein than in mRNA in response to diabetes or fasting, and in fact no decrease in GLUT-4 mRNA content was detectable in fasting. On the other hand, preparations of white skeletal muscle showed a substantial increase in GLUT-4 mRNA under both insulinopenic conditions, and that was concomitant to either a modest decrease in GLUT-4 protein in diabetes or to no change in fasting. 5. These results indicate that (a) the effects of diabetes and fasting are almost identical and lead to changes in GLUT-4 expression that are tissue-specific, (b) white adipose tissue, brown adipose tissue and heart respond similarly to insulin deficiency by decreasing GLUT-4 mRNA to a larger extent than GLUT-4 protein, and (c) red and white skeletal muscle respond to insulinopenic conditions in a heterogeneous manner which is characterized by enhanced GLUT-4 mRNA/protein ratios.

Increased gene expression of lipogenic enzymes and glucose transporter in white adipose tissue of suckling and weaned obese Zucker rats

Previous experiments have shown that insulin-induced glucose utilization is increased in white adipose tissue of young obese Zucker rats. We have investigated the possible role of over-expression of the muscle/fat glucose transporter (Glut 4) and key lipogenic enzymes in this increased insulin-responsiveness. The amount or activity and the mRNA concentrations of Glut 4, fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC) were measured before and after weaning in white adipose tissue of obese and lean Zucker rats. Comparison of the levels of Glut 4 and lipogenic-enzyme expression in 15-day-old suckling and 30-day-old weaned rats on a high-carbohydrate diet shows a marked increase in the latter group. The increase was, in lean and obese rats respectively, 6- and 7-fold for the amount of Glut 4 and 2- and 3-fold for its mRNA concentrations, 40- and 100-fold for the activity of lipogenic enzymes (FAS and ACC) and 30- and 10-fold for their mRNA concentrations. Furthermore, all these parameters, except the amount of Glut 4, were 2-5-fold higher in obese rats, both before and after weaning. Changes at weaning were largely blunted when rats were weaned on to a high-fat diet, although the differences between lean and obese rats persisted, and even became significant for the amount of Glut 4. Whatever the experimental conditions, plasma insulin levels were significantly higher in obese than in lean rats. These results indicate the existence of an enhanced expression of Glut 4, FAS and ACC in white adipose tissue of young obese fa/fa rats which could be related to the increased plasma insulin levels.

Whole body and hepatic insulin action in normal, starved, and diabetic rats

In normal (N), 3-days starved (S), and streptozotocin-treated (65 mg/kg) 3-days diabetic (D) rats we examined the in vivo dose-response relationship between plasma insulin levels vs. whole body glucose uptake (BGU) and inhibition of hepatic glucose production (HGP) in conscious rats, as determined with the four-step sequential hyperinsulinemic euglycemic clamp technique, combined with [3-3H]glucose infusion. Twelve-hour fasting (basal) HGP was 3.0 +/- 0.2, 2.1 +/- 0.2, and 5.4 +/- 0.5 mg/min in N, S, and D rats, respectively. Next, all rats were clamped at matched glycemia (6 mM). Lowering plasma glucose in D rats from +/- 20 to 6.0 mM did not increase plasma norepinephrine, epinephrine, glucagon, and corticosterone levels. For BGU, insulin sensitivity was increased (70 +/- 11 microU/ml) in S and unchanged (113 +/- 21 microU/ml) in D compared with N rats (105 +/- 10 microU/ml). Insulin responsiveness was unchanged (12.4 +/- 0.8 mg/min) in S and decreased (8.5 +/- 0.8 mg/min) in D compared with N rats (12.3 +/- 0.7 mg/min). For HGP, insulin sensitivity was unchanged (68 +/- 10 microU/ml) in S and decreased (157 +/- 21 microU/ml) in D compared with N rats (71 +/- 5 microU/ml). Insulin responsiveness was identical among N, S, and D rats (complete suppression of HGP). In summary, 1) insulin resistance in D rats is caused by hepatic insensitivity and by a reduction in BGU responsiveness. 2) S rats show normal hepatic insulin action, but insulin sensitivity for BGU is increased. Therefore, S and D rats both suffering from a comparable catabolic state (10-15% body wt loss in 3 days) show opposite effects on in vivo insulin action. This indicates that in vivo insulin resistance in D rats is not caused by the catabolic state per se.

A mathematical model and computer simulation study of insulin sensitive glucose transporter regulation

A mathematical model of insulin sensitive glucose transporter regulation is developed. Model structure is based on experimental evidence from adipocytes and myocytes. Model parameters correspond with known cellular processes. As an example, computer simulation results are compared with data from rat adipocytes. Cellular processes explicitly represented in the model include state-dependent glucose transporter synthesis and degradation rates, insulin sensitive glucose transporter translocation rates, and a glucose transporter endocytosis rate. Most of these processes are represented as first-order events. Using more complex representations of the model structure (e.g. higher order rate constants or saturable pathways) or alternative structures did not result in qualitatively better results. The model is able to accurately simulate the insulin sensitive, insulin concentration dependent, reversible translocation of glucose transporters observed in normal adipocytes. The model is also able to accurately simulate the changes in regulation of glucose transporter translocation observed with increases in cell surface area. Finally, the model can simulate pathogenic states which induce impairment of glucose transporter regulation (e.g. altered glucose transporter regulation in adipocytes from rats on high fat diets, rats with streptozotocin induced diabetes, and fasted rats). Since the structure of our model is sufficient to explain glucose transporter regulation in both normal and pathological states, it may aid in understanding the post-receptor components of insulin resistance (decreased sensitivity or responsiveness to insulin) seen in pathological states such as obesity and diabetes mellitus.

The effect of fasting on the activation in vivo of the insulin receptor kinase

Fasting causes insulin resistance in liver and fat, and increases insulin sensitivity in muscle. We studied the response in vitro and in vivo to insulin of the insulin receptor tyrosine kinase in muscle and liver from 72 h fasted and control rats. Insulin was injected intraperitoneally together with glucose, and blood and tissue samples were obtained 0, 5, 15 and 30 min later. Basal serum glucose and insulin levels were significantly higher in control than in fasting rats. Serum glucose rose to approximately 300 mg/dl at 5 min and then progressively declined without hypoglycaemia. Receptors were prepared from whole tissue by wheat germ lectin affinity chromatography. 125I-insulin binding to purified receptors was increased by fasting in both muscle (18%) and liver (50%). In untreated fasting and control animals, muscle and liver insulin receptor tyrosine kinase activity was stimulated to similar levels by insulin added in vitro. With only insulin treatment in vivo, muscle receptor tyrosine kinase behaved similarly in fasting and control animals with maximal activation at 15 min post injection. In liver, insulin in vivo stimulated receptor tyrosine kinase activity maximally at 5 min post injection in both fasting and control, but in fasting animals the treatment in vivo caused a significantly larger and more prolonged activation of the enzymic activity, possibly due to a decrease in the rate of dephosphorylation and deactivation of the beta subunits.

Some aspects of metabolic adaptations in lipid metabolism during starvation are mimicked by epinephrine in rat adipocytes

1. The effects of fasting on the neutral lipid synthesis to insulin and/or epinephrine in isolated fat cells have been examined using [1-14C]glucose. 2. The ability of adipocytes from starved rats to synthesize fatty acids from both labeled substrates was markedly diminished compared to adipocytes from control rats. 3. The response of lipogenic stimulation to insulin at all concentrations tested was greatly diminished in adipocytes from 24 hr starved rats. 4. [1-14C]glucose utilization rates in the absence or in the presence of insulin were not significantly different in adipocytes from 24 hr starved rats as compared with control adipocytes, although basal and insulin stimulated glyceride-glycerol synthesis were significantly higher in starved adipocytes. 5. Epinephrine acutely inhibited [1-14C]acetate incorporation into fatty acids for insulin-stimulated lipogenesis in control adipocytes, in contrast, this lipolytic agent strongly increased [1-14C]glucose conversion to triacylglycerols. 6. In both cases, the differences in lipid synthesis capacities found in both nutritional states were abolished by epinephrine.

Facilitative glucose transporters: an expanding family

The uptake of glucose into most eukaryotic cells is accomplished by a carrier-mediated transport system, facilitative diffusion, which transports glucose down its chemical gradient in a stereospecific manner. Recent studies have shown that facilitative transport of glucose across the plasma membrane is mediated by a family of structurally related proteins. This review summarizes the structural and functional features of the family of facilitative glucose transporters.

Regulation of glucose transporter messenger RNA in insulin-deficient states

Recent studies have indicated that a family of structurally related proteins with distinct but overlapping tissue distributions are responsible for facilitative glucose transport in mammalian tissues. Insulin primarily stimulates glucose transport by inducing the redistribution of a unique glucose transporter protein from an intracellular pool to the plasma membrane. This 509-amino-acid integral membrane protein, termed GLUT-4, is the main insulin-responsive glucose transporter in adipose and muscle tissues. We have observed a dramatic decrease (tenfold) in the steady-state levels of GLUT-4 messenger RNA in adipose tissue from fasted rats or rats made insulin deficient with streptozotocin. Insulin treatment of the streptozotocin-diabetic rats or refeeding the fasted animals causes a rapid recovery of the GLUT-4 mRNA to levels significantly above those observed in untreated control animals. By contrast, the levels of the erythrocyte/HepG2/rat brain-type glucose transporter mRNA remain essentially unchanged under these conditions. These data suggest that the in vivo expression of GLUT-4 mRNA in rat adipose tissue is regulated by insulin.

Decreased expression of the insulin-responsive glucose transporter in diabetes and fasting

Cellular resistance to insulin caused by a reduction in insulin-mediated glucose uptake can be produced in rats by chemically inducing diabetes with streptozotocin and by fasting. Two glucose transporter isoforms are expressed in fat cells: (1) the insulin-responsive species which is found only in fat and muscle, and (2) a species corresponding to the erythrocyte/Hep G2/rat brain transporter. We show here that fat cells isolated from streptozotocin diabetic rats and from fasted rats show a significant (60-80%) decrease in the amount of immunologically detectable insulin-sensitive glucose transporter and no change in the level of the Hep G2/rat brain transporter. Administration of insulin and refeeding, respectively, result in a return of the insulin-sensitive glucose transporter to levels that are normal or slightly above normal. Thus, peripheral tissue insulin resistance could be due to the specific reduction in the amount of insulin-sensitive glucose transporter.

Regulation of glucose transporter-specific mRNA levels in rat adipose cells with fasting and refeeding. Implications for in vivo control of glucose transporter number

Fasting in the rat is associated with a rapid and progressive decrease in insulin-stimulated glucose transport activity in adipose cells, which is not only restored to normal, but increased transiently to supranormal levels by refeeding. The mechanisms for these changes in glucose transport activity appear to involve alterations in both glucose transporter number and intrinsic activity (glucose turnover number). In this study, we use the human hepatoma Hep G2 glucose transporter complementary DNA clone to examine the molecular basis for these alterations. Extractable RNA per adipose cell is decreased 35% with 3 d of fasting and increased to 182% of control with 6 d of refeeding after 2 d of fasting. This parallels changes in adipose cell intracellular water, so that total RNA/water space remains relatively constant. When the changes in total RNA/cell are taken into account, Northern and slot blot analyses with quantitative densitometry reveal a 36% decrease in specific glucose transporter mRNA level in cells from the fasted rats. The mRNA level in cells from the fasted/refed rats is restored to normal. These observations correlate closely with previous measurements of glucose transporter number in adipose cell membrane fractions using cytochalasin B binding and Western blotting. The levels of specific mRNAs for tubulin and actin on a per cell basis show similar but more dramatic changes and mRNAs encoding several differentiation-dependent adipose cell proteins are also significantly affected. Thus, the levels of mRNA for multiple adipose cell genes are affected by fasting and refeeding. In particular, this demonstrates that in vivo metabolic alterations can influence the level of a glucose transporter mRNA in adipose cells. This may have implications for the regulation of glucose transporter number and glucose transport activity.