Dissociation of insulin-stimulated glucose transport from the translocation of glucose carriers in rat adipose cells

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Insulin receptor substrates-1 and -2 are both depleted but via different mechanisms after down-regulation of glucose transport in rat adipocytes

Alterations in muscle and adipose tissue insulin receptor substrate (IRS)-1 and IRS-2 are associated with, and commonly believed to contribute to, development of insulin resistance. In this study, we investigated the mechanisms behind previously observed reductions in IRS levels due to high concentrations of glucose and insulin and their significance in the impairment of glucose uptake capacity in primary rat adipocytes. Semiquantitative RT-PCR analysis showed that insulin (10(4) microU/ml) alone or in combination with glucose (15 mm) markedly suppressed IRS-2 gene expression, whereas IRS-1 mRNA was unaffected by the culture conditions. The negative effect of a high glucose/high insulin setting on IRS-1 protein level was still exerted when protein synthesis was inhibited with cycloheximide. Impairment of glucose uptake capacity after treatment with high glucose and insulin was most pronounced after 3 h, whereas IRS-1 and IRS-2 protein levels were unaffected up to 6 h but were reduced after 16 h. Moreover, impaired glucose uptake capacity could only partially be reversed by subsequent incubation at physiological conditions. These novel results suggest that: 1) in a high glucose/high insulin setting depletion of IRS-1 and IRS-2 protein, respectively, occurs via different mechanisms, and IRS-2 gene expression is suppressed, whereas IRS-1 depletion is due to posttranslational mechanisms; 2) IRS-1 and IRS-2 protein depletion is a secondary event in the development of insulin resistance in this model of hyperglycemia/hyperinsulinemia; and 3) depletion of cellular IRS in adipose tissue may be a consequence rather than a cause of insulin resistance and hyperinsulinemia in type 2 diabetes.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Insulin regulates alternative splicing of protein kinase C beta II through a phosphatidylinositol 3-kinase-dependent pathway involving the nuclear serine/arginine-rich splicing factor, SRp40, in skeletal muscle cells

Insulin regulates the inclusion of the exon encoding protein kinase C (PKC) betaII mRNA. In this report, we show that insulin regulates this exon inclusion (alternative splicing) via the phosphatidylinositol 3-kinase (PI 3-kinase) signaling pathway through the phosphorylation state of SRp40, a factor required for insulin-regulated splice site selection for PKCbetaII mRNA. By taking advantage of a well known inhibitor of PI 3-kinase, LY294002, we demonstrated that pretreatment of L6 myotubes with LY294002 blocked insulin-induced PKCbetaII exon inclusion as well as phosphorylation of SRp40. In the absence of LY294002, overexpression of SRp40 in L6 cells mimicked insulin-induced exon inclusion. When antisense oligonucleotides targeted to a putative SRp40-binding sequence in the betaII-betaI intron were transfected into L6 cells, insulin effects on splicing and glucose uptake were blocked. Taken together, these results demonstrate a role for SRp40 in insulin-mediated alternative splicing independent of changes in SRp40 concentration but dependent on serine phosphorylation of SRp40 via a PI 3-kinase signaling pathway. This switch in PKC isozyme expression is important for increases in the glucose transport effect of insulin. Significantly, insulin regulation of PKCbetaII exon inclusion occurred in the absence of cell growth and differentiation demonstrating that insulin-induced alternative splicing of PKCbetaII mRNA in L6 cells occurs in response to a metabolic change.

Adenosine and adenosine triphosphate modulate the substrate binding affinity of glucose transporter GLUT1 in vitro

Evidence indicates that a large portion of the facilitative glucose transporter isoform GLUT1 in certain animal cells is kept inactive and activated in response to acute metabolic stresses. A reversible interaction of a certain inhibitor molecule with GLUT1 protein has been implicated in this process. In an effort to identify this putative GLUT1 inhibitor molecule, we studied here the effects of adenosine and adenosine triphosphate (ATP) on the binding of D-glucose to GLUT1 by assessing their abilities to displace cytochalasin B (CB), using purified GLUT1 in vesicles. At pH 7.4, adenosine competitively inhibited CB binding to GLUT1 and also reduced the substrate binding affinity by more than an order of magnitude, both with an apparent dissociation constant (K(D)) of 3.0 mM. ATP had no effect on CB and D-glucose binding to GLUT1, but reduced adenosine binding affinity to GLUT1 by 2-fold with a K(D) of 30 mM. At pH 3.6, however, ATP inhibited the CB binding nearly competitively, and increased the substrate binding affinity by 4--5-fold, both with an apparent K(D) of 1.22 mM. These findings clearly demonstrate that adenosine and ATP interact with GLUT1 in vitro and modulate its substrate binding affinity. They also suggest that adenosine and ATP may regulate GLUT1 intrinsic activity in certain cells where adenosine reduces the substrate-binding affinity while ATP increases the substrate-binding affinity by interfering with the adenosine effect and/or by enhancing the substrate-binding affinity at an acidic compartment.

Prevention of dexamethasone-induced insulin resistance by metformin

This study investigates the effect of the antidiabetic drug metformin on dexamethasone-induced hyperglycaemia and insulin resistance in mice. Normal mice were treated with dexamethasone (2.5 mg/kg/day p.o.) plus metformin (250 mg/kg/day p.o.) and pair-fed to those receiving dexamethasone alone. Metformin reduced the extent of dexamethasone-induced hyperglycaemia and decreased insulin resistance as indicated by an improved insulin-hypoglycaemia test. Metformin-treated mice also showed increased basal glucose uptake into isolated diaphragm (by 38%), soleus (by 19%) and deep (red) quadriceps (by 31%). Measurements in the quadriceps showed that the increase in glucose uptake occurred without increasing either the mRNA levels or total cellular membrane abundance of the GLUT1 or GLUT4 glucose transporter isoforms. Thus metformin can ameliorate dexamethasone-induced hyperglycaemia and insulin resistance in part by increasing glucose disposal into skeletal muscle. Since this was achieved in quadriceps muscle without increasing mRNA or total membrane abundance of GLUT1 or GLUT4, it is possible that metformin might influence the intrinsic activity of glucose transporters, as well as altering their intracellular translocation.

Effect of triiodothyronine on glucose transport in rat adipocytes

The in vitro effect of thyroid hormones on glucose transport in insulin-stimulated muscle cells or adipocytes is still unclear. The objective of the present study was to assess the direct effect of 3,3',5-triiodothyronine (T3) on glucose transport and on the translocation of insulin-regulatable glucose transporter (GLUT4) in insulin-stimulated rat adipocytes. This evaluation was performed using an in vitro assay to avoid the well-known systemic effects of this hormone ( e.g.: hyperinsulinemia). Adipocytes were isolated from epididymal adipose tissue of Sprague-Dawley rats. Glucose transport assay and immunoblot analysis of GLUT4 were carried out in insulin-stimulated and unstimulated adipocytes after treating with or without T3. The results were as follows; 1) T3 inhibited the glucose transport in insulin-stimulated and unstimulated adipocytes in a dose-dependent manner. 2) T3 decreased the maximal response level (Vmax) but did not alter the sensitivity (Km) of glucose transport to insulin. 3) T3 did not affect the translocation of GLUT4 from the intracellular pool to the plasma membrane. We concluded that T3 inhibits the glucose transport in insulin-stimulated adipocytes in a post-receptor level without affecting the translocation of GLUT4 from the intracellular pool to the plasma membrane. This suggests that T3 acts by decreasing the intrinsic activity or the accessibility of GLUT4 in the plasma membrane.

Stimulation of glucose transport in Clone 9 cells by insulin and thyroid hormone: role of GLUT-1 activation

Thyroid hormone (T3) and insulin are both shown to stimulate glucose transport in Clone 9 cells, a rat liver cell line in which the utilization of glucose is limited by transport rate and in which only the GLUT-1 transporter isoform is expressed. Pre-treatment of these cells with T3 moreover substantially enhances the stimulatory effect of insulin such that at maximally effective hormone concentrations the effects of T3 and insulin on glucose transport are more than additive and indeed nearly multiplicative, suggesting that the mechanisms mediating the enhancement of glucose transport differ between the two hormones. Cell surface biotinylation followed by Western-blot analysis of plasma membrane fractions showed that the stimulatory effects of T3 and insulin on glucose transport, whether acting singly or in combination, exceed the attendant increases in the abundance of GLUT-1 in the plasma membrane. It is suggested that activation of GLUT-1 molecules pre-existing in the plasma membrane plays a major role in mediating the stimulatory effects of T3 and insulin on glucose transport in this cell line.

Comparison of isoproterenol with BRL37344 in activation of beta 3-adrenoceptors to inhibit the uptake of [14C]deoxy-D-glucose and translocation of glucose transporter (GLUT4) to membrane fraction in rat adipocytes

In an attempt to understand the role of beta 3-adrenoceptors in the regulation of glucose uptake, the effect of isoproterenol was compared with BRL37344 in isolated white adipocytes of the rat using [14C]deoxy-D-glucose as an indicator. In the presence of BRL37344, the specific agonist of beta 3-adrenoceptors, spontaneous uptake of [14C]deoxy-D-glucose (glucose uptake) into adipocytes was markedly attenuated. Similar concentration-dependent inhibition of glucose uptake was also observed in the samples treated with isoproterenol, an agonist for all kinds of beta-adrenoceptors. Action of BRL37344 was blocked by propranolol at concentrations sufficient to abolish the activity of isoproterenol. Pindolol reversed BRL37344-induced inhibition more effectively than propranolol. Moreover, unlike the action of isoproterenol, BRL37344 did not modify the insulin-stimulated glucose uptake. Translocation of glucose transporter (GLUT4) from cytosol to membrane stimulated with insulin was reduced by isoproterenol but not by BRL37344. Combination with the findings that isoproterenol prolonged the time for insulin to reach maximum stimulation of glucose uptake, leads to the conclusion that delay of insulin action by isoproterenol can be considered as one of the mechanisms for this inhibition. The results obtained suggest that BRL37344 decreased the spontaneous uptake of glucose via an activation of beta 3-adrenoceptors while the insulin stimulated glucose uptake was inhibited by isoproterenol but not by BRL37344.

Both acute and chronic near-normoglycaemia are required to improve insulin resistance in type 1 (insulin-dependent) diabetes mellitus

To determine the impact of both short- and long-term "near-normoglycaemia" on insulin resistance in Type 1 (insulin-dependent) diabetes hepatic glucose production (mg.kg-1.min-1) and peripheral glucose utilisation ("M-value", mg.kg-1.min-1) were estimated during an euglycaemic hyperinsulinaemic clamp (10 mU.kg.min) in patients with either good (HbA1c < 5.8%, groups A and B) or poor (HbA1c > 7.5%, groups C and D) long-term metabolic control (time > 12 months) and in healthy subjects (HbA1c: 5.08 +/- 0.20%; n = 8). To this end blood glucose was stabilized at 6.7 mmol/l by overnight (t = 12 h) i.v. regular insulin in groups (n = 8 each) A (HbA1c: 5.49 +/- 0.46%) and C (HbA1c: 8.83 +/- 1.20%), while groups B (HbA1c: 5.55 +/- 0.19%) and D (HbA1c: 8.51 +/- 1.09%) were kept overnight on long-acting insulin without feed-back control of blood glucose before euglycaemic clamping. Thereby, pre-equilibration of blood glucose at 6.7 mmol/l was shown to normalize basal hepatic glucose production (A: 2.27 +/- 0.48; C 2.50 +/- 0.57 mg.kg-1.min-1) despite different HbA1c values, whereas basal hepatic glucose production stayed elevated in groups B (3.09 +/- 0.38 mg.kg-1.min-1) and D (3.21 +/- 0.58 mg.kg-1.min-1) with poor actual glycaemia (B: 10.9 +/- 4.6; D: 12.1 +/- 4.6 mmol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine administration stimulates GLUT4 translocation but reduces glucose transport in muscle

Epinephrine opposes glucose transport in muscle. Therefore, we investigated the effects of epinephrine administration (25 micrograms/100g body weight) on glucose transport and glucose transporters in rat muscle. Ninety minutes after epinephrine injection 3-O-methyl glucose transport was reduced (approximately 47%) in perfused muscles of the rat hindlimb. Translocation of the insulin-regulatable glucose transporter (GLUT4) in the epinephrine-injected animals was confirmed by the marked increments in the GLUT-4 in the plasma membranes and their concomitant reduction in the intracellular membranes. We speculate a) that it is epinephrine which translocated GLUT4 via a cAMP-linked pathway, and b) that the intrinsic activity reductions are caused either by the glycation of the transporter by the persistent hyperglycemia and/or by epinephrine via the phosphorylation of the GLUT4 transporter protein in muscle.

Inhibitors of protein synthesis cause increased hexose transport in cultured human fibroblasts by a mechanism other than transporter translocation

We have investigated the effect of various inhibitors of protein synthesis on hexose transport in human skin fibroblasts using 2-deoxy-D-glucose (2-DG) and 3-0-methyl-D-glucose (3-OMG) to measure hexose transport. Exposure of glucose-fed, serum-free cultures to cycloheximide (CHX) (50 micrograms/ml) for 6 h resulted in increased 2-DG transport (3.81 +/- .53 vs. 6.62 +/- .88 nmoles/mg protein/2 min; n = 9) and 3-OMG transport (1.36 +/- .66 vs. 3.18 +/- .83 nmoles/mg protein/30 sec; n = 4) in the CHX exposed group. Under these conditions inhibition of protein synthesis was greater than 90%. This CHX induced transport increase was time dependent (approaching maximum within 1 h of exposure to CHX) and related to an increase in the Vmax of hexose transport in the CHX exposed group (18.4 +/- 2.4 vs. 4.8 +/- 1.1 nmoles 2-DG/mg protein/min) with no difference in the transport Km (1.55 +/- .63 vs. 2.92 +/- .59 mM). Further, the CHX induced increase in hexose transport was reversible. Exposure of human fibroblasts to inhibitors of protein synthesis with different mechanisms of action (e.g., puromycin, pactamycin, or CHX) all generated hexose transport increases in a concentration-dependent fashion correlating with their increasing inhibitory effects on protein synthesis. Nucleotidase enriched (i.e., plasma membrane) fractions of control and CHX-exposed cells showed no differences in D-glucose inhibitable cytochalasin B binding activity. Further, quantitative Western analysis of nucleotidase enriched fractions indicated CHX exposure resulted in no significant increase in glucose transporter mass compared with control plasma membrane fractions. Glucose deprived cells, however, which exhibited increased sugar transport comparable to the CHX-exposed group, did show increased glucose transporter mass in the plasma membrane fraction. The data indicate that inhibitors of protein synthesis can cause a significant elevation in hexose transport and that the hexose transporter mass in the isolated plasma membrane fractions did not reflect the whole cell transport change. It is suggested that a mechanism other than glucose transporter translocation to the plasma membrane may be involved in causing this sugar transport increase.

Lack of beta 2-adrenoceptor induced long-acting effect on glucose tolerance in type 2 diabetic patients

The long-acting effect of a 10-min pulse infusion of the beta 2-adrenergic agonist fenoterol on oral glucose tolerance tests in controls and in normotensive patients with type 2 diabetes mellitus on diet was compared. During an oral glucose load starting 2 h after fenoterol control persons showed hyperglycemia (area: 25,950 +/- 467 vs. 22,650 +/- 410, P less than 0.01), hyperinsulinemia (area: 13,980 +/- 1050 vs. 8160 +/- 405, P less than 0.02) and a pronounced fall of serum potassium (area: 775 +/- 26 vs. 748 +/- 25, P less than 0.02). The patient group showed no late response to fenoterol: plasma glucose (area: 51,000 +/- 382 vs. 51,300 +/- 413, n.s.), serum insulin (area: 7215 +/- 233 vs. 8280 +/- 410, n.s.), serum potassium (area: 748 +/- 26 vs. 750 +/- 24, n.s.). The data show that there is a defect of the beta 2-adrenergic long-acting effect on glucose metabolism and on insulin release in type 2 diabetes mellitus.

Acute and chronic effects of hyperglycaemia on glucose metabolism

In normal man, several hormonal and metabolic adjustments allow the maintenance of the blood glucose concentration within narrow limits. Hyperglycaemia participates in this regulation via stimulation of glucose disposal and inhibition of glucose production. The effects are mediated, in addition to changes in insulin and glucagon secretion, by the mass-action effect of glucose. In both Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetic patients, hyperglycaemia, by mass-action abnormally elevates the basal glucose utilization rate but compensates for reduced postprandial insulin-stimulated glucose disposal. When exposed to chronic hyperglycaemia, the body tissues seem to protect themselves, at least partly, against excessive glucose utilization. These protective mechanisms include both a reduction in insulin stimulated glucose disposal and insulin secretion. Chronic hyperglycaemia may also reduce non-insulin-dependent glucose utilization, at least in rats. In Type 1 diabetic patients with normal peripheral insulin concentrations, chronic hyperglycaemia per se could be a major cause of insulin resistance. In Type 2 diabetic patients, insulin resistance is often already present before the development of overt fasting hyperglycaemia. At the diabetic stage, hyperglycaemia could, however, maintain a self-perpetuating cycle, where the deleterious effects of high glucose concentrations on insulin action and secretion cause further deterioration of glycaemic control. The biochemical basis for hyperglycaemia-induced insulin resistance is still far from clear, but could involve changes in the glucose transporter number and gene expression.

Interleukin-1 stimulates glucose transport in rat adipose cells. Evidence for receptor discrimination between IL-1 beta and IL-1 alpha

The effect of interleukin 1 (IL-1) on glucose transport activity in isolated rat adipose cells was examined. IL-1 beta stimulated 3-O-methylglucose (3OMG) transport in a time and dose dependent manner. This effect appears to be due to increased maximal transport velocity (Vmax) of the carrier. Addition of insulin and IL-1 beta resulted in an additive stimulation of transport, suggesting different mechanisms. IL-1 alpha had no effect on glucose transport. Glu-4, a relatively inactive IL-1 beta analogue in most cells, stimulated glucose uptake in a time and dose dependent manner with kinetics indistinguishable from those of IL-1 beta.

Insulin-responsive glucose transporters are concentrated in a cell surface-derived membrane fraction of 3T3-L1 adipocytes

The recently proposed mechanistic concept of a receptor-regulated entrance compartment for hexose transport formed by microvilli on 3T3-L1 adipocytes predicted a preferential localization of glucose transporters in these structures. The cytochalasin B-binding technique was used to determine in basal and insulin-stimulated cells the distribution of glucose transporters between plasma membranes, low density microsomes (LDM) and two cell surface-derived membrane fractions prepared by a hydrodynamic shearing technique. The shearing procedure applied prior to homogenization yielded a low density surface-derived vesicle (LDSV) fraction which contained nearly 60% of the cellular glucose transporters and the total insulin-sensitive transporter pool. The rest of the glucose transporter population was localized within the plasma membrane (5%) and the LDM fraction (37%). Pretreatment of the cells with insulin (20 mU/ml for 10 min) reduced the transporter content of the LDSV fraction by 40% and increased that of the plasma membrane fraction 4-fold. The transporter containing LDSV fraction was clearly differentiated from the LDM fraction by its low specific galactosyltransferase activity and its insulin-sensitivity. Scanning electron microscopy revealed that the LDSV fraction contained a rather uniform population of spherical vesicles of 100-200 nm in diameter.

Effect of phenylarsine oxide on fluid phase endocytosis: further evidence for activation of the glucose transporter

We have shown previously that insulin stimulates fluid phase endocytosis in 3T3-L1 adipocytes (Gibbs et al., 1986). Using [14C]sucrose as an endocytotic marker, we show here that phenylarsine oxide, a trivalent arsenical which binds neighboring dithiols, blocked not only insulin-stimulated fluid phase endocytosis, but basal endocytosis as well. The Ki for this process was 6 microM in the presence or absence of insulin and the time required for inhibition was less than 2.5 min, the limit of detection in our assay system. These results can be compared with the inhibitory effect of phenylarsine oxide on insulin-stimulated glucose transport. Although the Ki for insulin-stimulated transport (7 microM) was similar to that for inhibition of endocytosis, basal glucose transport was not affected by the inhibitor. Further, when cells were prestimulated with insulin causing maximal stimulation of the glucose transport rate, phenylarsine oxide induced a time-dependent reduction to the basal rate (t 1/2 of 10 min), despite the fact that endocytosis was blocked immediately. This observation suggests that if the transporter is recycled by an exocytotic/endocytotic mechanism, it is distinct from fluid-phase endocytosis/exocytosis, which is a vesicle-mediated process, and provides further evidence that the transporter may undergo intrinsic activation/inactivation which does not require vesicle movement.

Changes in glucose transporters in muscle in response to exercise

The mechanism underlying the increase in glucose uptake in response to muscular contraction is not known, although it has been established that the change does not require insulin. It is our hypothesis that exercise, like insulin, stimulates translocation of glucose transporters to the plasma membrane. To test this hypothesis an experiment was performed to determine whether glucose transporters are translocated from an intracellular membrane to the plasma membrane during exercise. Untrained male rats weighing approximately 250 g were exercised by treadmill running for 2 h at 25 m/min. They were killed immediately after completion of exercise, and the gastrocnemius and quadriceps muscles were quickly removed. Sedentary animals were treated in the same way. Plasma and intracellular membranes were isolated by sucrose density gradient centrifugation and cytochalasin B binding assays were performed. Exercise resulted in a redistribution of glucose transporters from the intracellular membrane to the plasma membrane. The ratio of cytochalasin B binding sites in the membrane fractions (intracellular/plasma membrane) was 3.2 +/- 0.6 in rested animals and 1.3 +/- 0.3 after exercise. The concentration of glucose transporters was increased in the plasma membrane (from 19.8 +/- 1.8 to 30.4 +/- 3.9 pmol/mg protein) and decreased in the intracellular membrane (from 20.7 +/- 3.0 to 10.8 +/- 1.1 pmol/mg protein) in response to exercise. These results suggest that at least part of the increase in glucose uptake that occurs during exercise is the result of a redistribution of glucose transporters to the plasma membrane.

Mechanism for increased insulin-stimulated glucose metabolism in adipocytes from 13-week-old obese Zucker rats

A study was made on the mechanism of increased glucose metabolism in enlarged adipocytes from 13-week-old obese Zucker rats showing hyperinsulinaemia and hyperglycaemia. Glucose metabolism was assessed by measuring CO2 production from glucose and the concentration of glucose transporters was estimated by immunoblotting. In comparing adipocytes from obese rats with those from lean rats, the basal rates of glucose oxidation at 0.02 mmol/l glucose increased 2.6-fold per unit cellular surface area and the transporters in the plasma membrane increased 1.4-fold per protein, while that in low-density microsome was 67% of the value in lean rats. The increase of glucose oxidation rates observed in basal cells from obese rats could be partly explained by translocation of the transporters from the intracellular site to the plasma membrane. In the presence of insulin, as the basal rates of glucose oxidation increased in obese rats, maximally insulin-stimulated oxidation increased 4-fold in lean rats and 1.7-fold in obese rats. Thus, the rates of insulin-stimulated oxidation on a per unit cellular surface area as well as the transporters on a per protein basis in the plasma membrane became almost similar in cells from both groups of rats. Since protein content per cell increased with cell enlargement, increased glucose metabolism per cell which was observed in adipocytes from the obese rats was mainly due to an increase of glucose transporters accompanied by a similar degree of cellular protein increase.

Regulation of hexose transport in rat myoblasts during growth and differentiation

We report here the effects of growth conditions and myogenic differentiation on rat myoblast hexose transport activities. We have previously shown that in undifferentiated myoblasts the preferred substrates for the high (HAHT)- and low (LAHT)-affinity hexose transport systems are 2-deoxyglucose (2-DG) and 3-O-methyl-D-glucose (3-OMG), respectively. The present study shows that at cell density higher than 4.4 x 10(4) cells/cm2, the activities of both transport processes decrease with increasing cell densities of the undifferentiated myoblasts. Since the transport affinities are not altered, the observed decrease is compatible with the notion that the number of functional hexose transporters may be decreased in the plasma membrane. Myogenic differentiation is found to alter the 2-DG, but not the 3-OMG, transport affinity. The Km values of 2-DG uptake are elevated upon the onset of fusion and are directly proportional to the extent of fusion. This relationship between myogenesis and hexose transport is further explored by using cultures impaired in myogenesis. Treatment of cells with 5-bromo-2'-deoxyuridine abolishes not only myogenesis but also the myogenesis-induced change in 2-DG transport affinity. Similarly, alteration in 2-DG transport affinity cannot be observed in a myogenesis-defective mutant, D1. However, under myogenesis-permissive condition, the myogenesis of this mutant is also accompanied by changes in its 2-DG transport affinity. The myotube 2-DG transport system also differs from its myoblast counterpart in its response to sulfhydryl reagents and in its turnover rate. It may be surmised from the above observations that myogenesis results in the alteration of the turnover rate or in the modification of the 2-DG transport system. Although glucose starvation has no effect on myogenesis, it is found to alter the substrate specificity and transport capacity of HAHT. In conclusion, the present study shows that hexose transport in rat myoblasts is very sensitive to the growth conditions and the stages of differentiation of the cultures. This may explain why different hexose transport properties have been observed with myoblasts grown under different conditions.

Exercise and insulin stimulate skeletal muscle glucose transport through different mechanisms

This study was designed to examine the effects of acute exercise, insulin stimulation, and their combination on the kinetics of glucose transport in rat skeletal muscle. Sarcolemmal (SL) membranes were isolated from control (C), acute exercise (E), insulin-stimulated (I), and combined (E + I) rats. Michaelis-Menten kinetics indicated that the Vmax for glucose transport was increased after each perturbation compared with C but were not different from each other (E, 4,334 +/- 377; I, 4,424 +/- 668; E + I, 4,338 +/- 602; and C, 1,366 +/- 124 pmol.mg protein-1.s-1). The apparent Km was unchanged. Scatchard plots of cytochalasin B binding sites indicated that both I and E + I increased the number of binding sites compared both E and C (9.4 +/- 0.5 and 7.8 +/- 0.5 vs. 5.1 +/- 0.2 and 5.5 +/- 0.3 pmol/mg protein) without altering the dissociation constant. The increase in Vmax was greater than the increase in cytochalasin B binding sites indicating that both I and E + I caused an increase in the turnover rate of transport molecules as well as an increase in the total number of transport molecules. Because there was no change in the Km for glucose transport and no increase in cytochalasin B binding sites after exercise, the increase in Vmax was due solely to an increased turnover rate of existing transport molecules.

Phenylarsine oxide stimulates hexose transport in 3T3-L1 adipocytes by a mechanism other than an increase in surface transporters

Phenylarsine oxide (PAO) has been shown to exert a biphasic effect on glucose transport in 3T3-L1 adipocytes. At 10 microM, PAO activates transport threefold, but at higher concentrations an inhibition of transport is observed. In this paper we report a procedure for the subcellular fractionation of these cells which we use to examine the distribution of glucose transporters following PAO challenge. Quantitative immunoblotting showed that the glucose transporter content of the plasma membrane fraction increased with increasing PAO concentrations; a parallel increase in another insulin-responsive protein, the transferrin receptor, also occurred. However, cell-surface labeling procedures for the glucose transporter and transferrin receptor showed that PAO actually decreased the cell-surface concentrations of these proteins; the basis of this discrepancy may be that in the presence of PAO, intracellular vesicles containing these proteins associate with the plasma membrane, but do not fuse with it. The possibility that PAO modulated transport by direct interaction with the glucose transporter was investigated by examining the effects of PAO on transport in both erythrocytes and a reconstituted system of purified erythrocyte transporter in lipid vesicles. PAO was without effect on the rate of transport in these systems. The hypothesis that the stimulatory effect of PAO on transport might be due to the activation of the insulin receptor kinase activity was examined by assessing the phosphotyrosine content of the receptor and other proteins using anti-phosphotyrosine antibodies. PAO alone caused no detectable increase in receptor phosphotyrosine content. However, the combination of PAO and insulin led to the tyrosine phosphorylation of two proteins of Mr 68,000 and 57,000 which were not detected in cells treated with either PAO or insulin, and an increased phosphotyrosine content of proteins of Mr 95,000 and 165,000 when compared to cells treated with insulin alone.

Mechanism of O2- (-) and H2O2-induced stimulation of sugar transport in mouse fibroblast BALB/3T3 cells

Xanthine/xanthine oxidase and H2O2 stimulated sugar transport. Application of superoxide dismutase and catalase to the cells showed an inhibitory effect on these agent-stimulated sugar transports. Addition of amiloride and 4-acetamide-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), which abolish the cytoplasmic alkalinization, inhibited the stimulation of sugar transport by xanthine/xanthine oxidase in the presence of catalase. The calmodulin antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and trifluoperazine inhibited H2O2-stimulated sugar transport. These results suggest that O2- stimulates sugar transport in an intracellular pH-dependent manner and that H2O2 stimulates sugar transport in a calcium-calmodulin-dependent manner. These mechanisms may be involved in sugar-transport stimulation in mouse fibroblast BALB/3T3 cells by the tumor-promoting phorbol ester phorbol-12,13-dibutyrate and insulin, since the stimulatory effects of these agents were inhibited by scavengers of oxygen radicals.

Insulin stimulation of glucose uptake can be mediated by diacylglycerol in adipocytes

An early effect of insulin in adipocytes is to stimulate glucose uptake. The increased uptake appears to be due to mobilization of glucose transporters from an intracellular location to the plasma membrane and to enhanced intrinsic activity of the transporters. Little is known about the insulin-generated signals causing these changes. Phorbol esters have been shown to mimic the insulin effect, but phosphorylation of the transporter does not seem to be involved. A phospho-oligosaccharide was recently shown to mimic the effects of insulin on protein phosphorylation, suggesting that it could be a mediator for some intracellular metabolic effects of the hormone, but it did not affect glucose uptake. A diacyglycerol is produced in the plasma membrane in conjunction with the generation of the phospho-oligosaccharide. Here I show that added 1,2-diacylglycerols potently increase glucose transporter-mediated uptake of glucose in rat adipocytes, but without activation of protein kinase C.

Mechanism of insulin action on glucose transport in rat skeletal muscle

This study was designed to examine the effect of insulin stimulation on glucose transport in rat skeletal muscle. Sarcolemmal vesicles (SL) were isolated from the gastrocnemius-plantaris and quadriceps muscles from insulin-stimulated and control groups. The insulin-stimulated group received an intravenous insulin injection (1 U/kg) 10 min before isolation. The early time course of specific D-glucose transport was linear through 2 s. Michaelis-Menten kinetics at 1.5 s indicated that the Vmax for glucose transport was increased after insulin stimulation compared with controls (4,424 +/- 668 vs. 1,366 +/- 124 pmol.mg protein -1.s-1), whereas the Km remained unchanged (19.4 +/- 0.6 vs. 21.6 +/- 3.1 mM). Scatchard plots for the D-glucose-inhibitable class of cytochalasin B binding sites indicated that insulin stimulation increased the number of binding sites in the SL vesicles (9.3 +/- 0.6 vs. 5.5 +/- 0.3 pmol/mg protein) without altering the Kd (48 +/- 3 vs. 46 +/- 3 nM). That the increase in Vmax was greater than the increase in cytochalasin B binding sites indicates that insulin stimulation caused an increase in the turnover rate of existing transport molecules as well as an increase in the total number of SL glucose transport molecules.

Sedimentation characteristics of vesicles associated with insulin-sensitive intracellular glucose transporter from rat adipocytes

The sedimentation characteristics of vesicles associated with the insulin-sensitive intracellular glucose transporter from rat adipocytes were studied. The method used was sucrose density gradient centrifugation, which was carried out under non-equilibrium and equilibrium (isopycnic) conditions. The glucose transport activity was determined by the reconstitution method. As reported previously, the sedimentation velocity of the intracellular glucose-transport activity was considerably slower than that of the counterpart in the plasma membrane. It was found, however, that the specific gravity of the slow-sedimenting glucose-transport activity was almost identical to that of the activity in the plasma membrane (d = 1.118-1.122). It is concluded that the intracellular glucose transport activity is associated not with low-density microsomal vesicles, but with unidentified slow-sedimenting vesicles that have a specific gravity similar to that of the plasma membrane.

Insulin stimulation of adipocyte membrane glucose transport. A graded biologic response insensitive to bilayer lipid disordering

Aspects of the mechanism by which insulin stimulates the membrane glucose transport system were examined by assessing the influence of the bilayer lipid structure on transport stimulation characteristics, and considering the form of the insulin dose-response curve. We tested the effects of membrane lipid perturbation on the insulin stimulation process. Benzyl alcohol, at concentrations (25 mM) that grossly fluidize lipids forming the adipocyte membrane bilayer matrix, caused 50% inhibition of intrinsic transporter activity. However, this membrane perturbation had no significant effect on either the insulin dose-response curve (conducted at 37 degrees) or the time-course of the insulin stimulation of hexose transport (conducted at 32 degrees). These data are difficult to rationalize in terms of a model in which transport stimulation involves interaction of transporters and hormone-bound receptors that is limited by lateral diffusion of these proteins in the fluid lipid bilayer. Curve-fitting experimental insulin dose-response data for stimulation of 2-deoxy-D-glucose and D-glucose uptake provided an estimate of an insulin "association constant" for transport regulation that may be compared with recent insulin receptor binding data. Similar magnitude constants were obtained whether estimated directly from plots of transport velocity versus arithmetic hormone dose, or by extrapolation from linear segments of sigmoidal velocity versus log dose plots, or from inverse (Lineweaver-Burk-type) plots of the insulin dose-response data. Insulin apparently regulates transport by associating with a binding site, having an apparent dissociation constant which is determinable through kinetic measurements of hexose uptake (KDapp approx. 17-40 pM). This is in good agreement with the dissociation constant, KD, determined from Scatchard plots of recent binding data to adipocytes, for a class of receptors representing the "high affinity" binding sites for insulin. Insulin dose-response curve simulations also indicated that the stimulation process may be classified in pharmacologic terms as a typical graded biologic response and may involve insulin association with a site that regulates transport rates in a manner kinetically analogous to allosteric modulation of a V-series enzyme by a noncompetitive ligand. From the results we suggest that a relatively close association occurs between transport and receptor proteins in the membrane, where the relative activation of transport depends on the fractional occupancy of functional high affinity receptors by insulin, and the insulin stimulation of transport involves regions of the membrane that are not influenced significantly by