Leptin inhibits glycogen synthesis in the isolated soleus muscle of obese (ob/ob) mice

The ob gene product, leptin, causes significant and dose-dependent inhibition of basal and insulin-stimulated glycogen synthesis in isolated soleus muscle from ob/ob mice, and a smaller, non-significant inhibition in muscle from wild-type mice. Leptin had no inhibitory effect on glycogen synthesis in soleus muscle from the diabetic (db/db) mice, which lack the functional leptin receptor. The full-length leptin receptor (Ob-Rb), is expressed in soleus muscle of both ob/ob and wild-type mice, however with no detectable differences in expression level. These results suggest that hyperleptinaemia may attenuate insulin action on glucose storage in skeletal muscle.     

Leptin increases glucose transport and utilization in skeletal muscle in vitro

1. The present study examines the effect of leptin on glucose transport and metabolism in incubated soleus muscle from male lean albino rats. 2. Insulin (100 microU/ml) increased glucose uptake by twofold while the leptin group (100 nmol/l) reached 75% of the insulin response after 1 hr of incubation. However, leptin did not potentiate the insulin effect on glucose uptake in soleus muscle. 3. Leptin elicited a significant increase (27.7%) in total lactate production, accompanied by a three-fold increment in glycogen synthesis from [U-14C]D-glucose. 4. Insulin raised glycogen synthesis by sixfold. The leptin plus insulin group increased glycogen synthesis by eightfold, which is equivalent to the sum of the separated leptin and insulin groups. 5. Leptin per se exerts an insulin-like effect stimulating glucose uptake, glycogen synthesis, and lactate formation and also seems to potentiate the effect of insulin on glucose incorporation into glycogen in incubated soleus muscle.     

Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice

Hypersecretion of insulin from the pancreas is among the earliest detectable metabolic alterations in some genetically obese animals including the ob/ob mouse and in some obesity-prone humans. Since the primary cause of obesity in the ob/ob mouse is a lack of leptin due to a mutation in the ob gene, we tested the hypothesis that leptin targets a regulatory pathway in pancreatic islets to prevent hypersecretion of insulin. Insulin secretion is regulated by changes in blood glucose, as well as by peptides from the gastrointestinal tract and neurotransmitters that activate the pancreatic islet adenylyl cyclase (e.g., glucagon-like peptide-1) and phospholipase C (PLC) (e.g., acetylcholine) signaling pathways to further potentiate glucose-induced insulin secretion. Effects of leptin on each of these regulatory pathways were thus examined. Leptin did not influence glucose or glucagon-like peptide-1-induced insulin secretion from islets of either ob/ob or lean mice, consistent with earlier findings that these regulatory pathways do not contribute to the early-onset hypersecretion of insulin from islets of ob/ob mice. However, leptin did constrain the enhanced PLC- mediated insulin secretion characteristic of islets from ob/ob mice, without influencing release from islets of lean mice. A specific enhancement in PLC-mediated insulin secretion is the earliest reported developmental alteration in insulin secretion from islets of ob/ob mice, and thus a logical target for leptin action. This action of leptin on PLC-mediated insulin secretion was dose-dependent, rapid-onset (i.e., within 3 min), and reversible. Leptin was equally effective in constraining the enhanced insulin release from islets of ob/ob mice caused by protein kinase C (PKC) activation, a downstream mediator of the PLC signal pathway. One function of leptin in control of body composition is thus to target a PKC-regulated component of the PLC-PKC signaling system within islets to prevent hypersecretion of insulin.     

Effects of leptin adipose tissue lipoprotein lipase in the obese ob/ob mouse

OBJECTIVE: To characterize the adaptations of lipid metabolism, with special emphasis on tissue lipoprotein lipase, to negative energy balance brought by chronic treatment of obese ob/ob mice with leptin. DESIGN: According to a 2 x 2 factorial analysis, lean and obese C57BL/6J mice were subcutaneously infused with leptin (100 micrograms.kg-1.day-1) or vehicle (phosphate-buffered saline) during seven days. RESULTS: Cumulative food intake and final body weight of vehicle-infused obese mice were twofold higher than in lean controls. Leptin decreased cumulative food intake and body weight of obese, but not lean mice. Lipoprotein lipase (LPL) activity in white inguinal and epididymal and brown interscapular adipose tissues of control obese mice was at least twofold higher than in lean mice, but comparable in the vastus lateralis muscle. Leptin treatment of obese mice significantly lowered LPL activity to that of lean mice in all tissues examined. Vehicle-infused obese mice had higher liver triglyceride content and were hypertriglyceridemic compared to lean mice, and triglyceride concentrations in plasma and liver were decreased proportionally after leptin treatment. Leptin lowered glycemia and insulinemia of obese mice to lean levels and decreased plasma corticosterone. Leptin infusion had no notable effect on tissue lipoprotein lipase nor plasma variables of lean mice. CONCLUSIONS: Leptin infusion abolished hyperinsulinemia in the ob/ob mouse, an effect that was probably responsible for the concomitant normalization of adipose LPL activity. This study shows that decreased LPL activity, plasma triglyceride concentrations and hepatic triglyceride production constitute some of the adaptive peripheral adaptations of lipid metabolism, which accompany the reduction in fat mass accretion brought by leptin treatment of the obese ob/ob mouse.     

T-cell specific immunosuppression by prodigiosin isolated from Serratia marcescens

One of the characteristics of obesity-associated diabetes is an elevated fasting plasma insulin concentration with a weak insulin secretory response to subsequent glucose stimulation. Evidence suggests that hyperglycemia and hyperlipidemia may contribute to the initiation and progression of this disordered islet glucose sensing. It has been proposed that reducing hyperglycemia and hyperlipidemia per se may improve islet glucose sensing. Here we studied glucose-dependent insulin release in islets isolated from ob/ob mice treated with dopamine agonists (bromocriptine and SKF38393, BC/SKF) which significantly reduced circulating glucose and lipid levels of ob/ob mice. Islets from BC/SKF-treated mice showed a marked decrease of the elevated basal insulin release to levels similar to lean mice. Such treatment also induced a higher secretory response to glucose stimulation compared with that in ob/ob mice with sustained hyperglycemia and hyperlipidemia. Similarly, when islets from untreated ob/ob mice were cultured for 7 days in 11 mM glucose in the absence of free fatty acid, the basal insulin release was significantly decreased and high glucose stimulated insulin release increased compared with that from islets cultured in medium containing 30 mM glucose and 2 mM oleate. The BC/SKF-induced reduction of elevated basal insulin release was associated with decreased hexokinase activity and basal cyclic AMP content in islet tissue. Our results demonstrate that dopamine agonist treatment improves basal insulin release in ob/ob mice and this effect may be mediated, in part, by a reduction of hyperglycemia and hyperlipidemia.     

Modifications of glucose and lipid metabolism in cold-acclimated lean and genetically obese rats

Glucose turnover rate, 2-deoxy-D-[3H]glucose (2-DG) uptake, lipid synthesis in liver, white adipose tissue, and brown adipose tissue (BAT) were measured in lean FA/FA and genetically obese fa/fa rats either kept at 21 degrees C or acclimated to a cold environment (4 degrees C). After 10 days at 4 degrees C, lean rats increased their glucose turnover rate; 2-DG uptake as well as lipid synthesis in BAT were markedly stimulated. After cold acclimation, obese rats also increased glucose turnover; however, BAT glucose utilization was only slightly stimulated. Basal hyperinsulinemia and muscle insulin resistance of the obese group (as assessed by reduced 2-DG uptake in the soleus muscle) were present at room temperature and persisted at 4 degrees C. Total BAT lipid synthesis was increased to the same extent as in lean rats. Obese rat liver lipid synthesis, already much higher than normal at 21 degrees C, was further increased by cold exposure. We conclude that obese cold-acclimated fa/fa rats do not improve their muscle insulin resistance and barely improve BAT glucose utilization. We further suggest that an additional activation of hepatic lipid synthesis and oxidation thereof could participate in the heat production needed by the cold-acclimated obese rats.     

Leptin inhibits insulin-stimulated incorporation of glucose into lipids and stimulates glucose decarboxylation in isolated rat adipocytes

Leptin is an adipocyte hormone involved in the regulation of energy homeostasis. Generally accepted biological effects of leptin are inhibition of food intake and stimulation of metabolic rate in ob/ob mice, that are defective in the leptin gene. In contrast to these centrally mediated effects of leptin, we are reporting here on leptin effects on glucose incorporation into lipids and glucose decarboxylation in adipocytes isolated from male lean albino rats. Adipocytes previously cultivated (15 h) in the presence of leptin presented a 25% (P < 0.05) reduction of the insulin stimulated incorporation of glucose into lipids. Concurrently, the basal conversion of (U-14C)D-glucose into 14CO2 increased (110%) in the leptin cultivated adipocytes and reached values (1.54 nmol/10(5) cells) similar to the insulin stimulated group (not cultivated with leptin) (1.40 nmol/10(5) cells). In addition, in the presence of insulin, the leptin cultivated adipocytes elicited a 162% (P < 0.05) increase in 14CO2 production that was significantly higher than the increase observed for the not-leptin-cultivated insulin group (92%). We conclude that leptin: 1) directly inhibits the insulin stimulated glucose incorporation into lipids; 2) stimulates glucose decarboxylation, and also potentiates the effect of insulin on glucose decarboxylation in isolated adipocytes. Leptin per se does not alter glucose incorporation into lipids.     

Multihormonal control of ob gene expression and leptin secretion from cultured human visceral adipose tissue: increased responsiveness to glucocorticoids in obesity

The direct role of hormones on leptin synthesis has not yet been studied in cultured adipose cells or tissue from lean and obese subjects. Moreover, this hormonal regulation has never been addressed in human visceral fat, although this site plays a determinant role in obesity-linked disorders. In this study, we investigated the hormonal control of ob expression and leptin production in cultured visceral adipose tissue from lean and obese subjects. We more particularly focused on the interactions between glucocorticoids and insulin. We also briefly tackled the role of cAMP, which is still unknown in man. Visceral (and subcutaneous) adipose tissues from eight obese (body mass index, 41 +/- 2 kg/m2) and nine nonobese (24 +/- 1 kg/m2) subjects were sampled during elective abdominal surgery, and explants were cultured for up to 48 h in MEM. The addition of dexamethasone to the medium increased ob gene expression and leptin secretion in a time-dependent manner. Forty-eight hours after dexamethasone (50 nmol/L) addition, the cumulative integrated ob messenger ribonucleic acid (mRNA) and leptin responses were, respectively, approximately 5- and 4-fold higher in obese than in lean subjects. These responses closely correlated with the body mass index. The stimulatory effect of the glucocorticoid was also concentration dependent (EC50 = approximately 10 nmol/L). Although the maximal response was higher in obese than in lean subjects, the EC50 values were roughly similar in both groups. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion. Singularly, insulin even inhibited the dexamethasone-induced rise in ob mRNA and leptin release. This inhibition was observed in both lean and obese subjects, whereas the expected stimulation of insulin on glucose metabolism and the accumulation of mRNA species for the insulin-sensitive transporter GLUT4 and glyceraldehyde-3-phosphate dehydrogenase occurred in lean patients only. This inhibitory effect was already detectable at 10 nmol/L insulin and was also observed in subcutaneous fat. Although a lowering of intracellular cAMP concentrations is involved in some of the effects of insulin on adipose tissue, this cannot account for the present finding, because the addition of cAMP to the medium also decreased ob mRNA and leptin secretion (regardless of whether dexamethasone was present). In conclusion, glucocorticoids, at physiological concentrations, stimulated leptin secretion by enhancing the pretranslational machinery in human visceral fat. This effect was more pronounced in obese subjects due to a greater responsiveness of the ob gene and could contribute to the metabolic abnormalities associated with central obesity by para/endocrine actions of hyperleptinemia on adipocytes and liver. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion, and even prevented the positive response to dexamethasone by a cAMP-independent mechanism that remained functional despite insulin resistance.     

Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity

Mutations in the obese (ob) gene lead to obesity. This gene has been recently cloned, but the factors regulating its expression have not been elucidated. To address the regulation of the ob gene with regard to body weight and nutritional factors, Northern blot analysis was used to assess ob mRNA in adipose tissue from mice [lean, obese due to diet, or genetically (yellow agouti) obese] under different nutritional conditions. ob mRNA was elevated in both forms of obesity, compared to lean controls, correlated with elevations in plasma insulin and body weight, but not plasma glucose. In lean C57BL/6J mice, but not in mice with diet-induced obesity, ob mRNA decreased after a 48-hr fast. Similarly, in lean C57BL/6J controls, but not in obese yellow mice, i.p. glucose injection significantly increased ob mRNA. For up to 30 min after glucose injection, ob mRNA in lean mice significantly correlated with plasma glucose, but not with plasma insulin. In a separate study with only lean mice, ob mRNA was inhibited >90% by fasting, and elevated approximately 2-fold 30 min after i.p. injection of either glucose or insulin. These results suggest that in lean animals glucose and insulin enhance ob gene expression. In contrast to our results in lean mice, in obese animals ob mRNA is elevated and relatively insensitive to nutritional state, possibly due to chronic exposure to elevated plasma insulin and/or glucose.     

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.     

Regulation of skeletal muscle hexokinase II by insulin in nondiabetic and NIDDM subjects

Impaired muscle glucose phosphorylation to glucose-6-phosphate by hexokinases (HKs)-I and -II may contribute to insulin resistance in NIDDM and obesity. HK-II expression is regulated by insulin. We tested the hypothesis that basal and insulin-stimulated expression of HK-II is decreased in NIDDM and obese subjects. Skeletal muscle HK-I and HK-II activities were measured in seven lean and six obese normal subjects and eight patients with NIDDM before and at 3 and 5 h of a hyperinsulinemic (80 mU x m(-2) x min(-1)) euglycemic clamp. To assess whether changes in HK-II expression seen during a glucose clamp are likely to be physiologically relevant, we also measured HK-I and HK-II activity in 10 lean normal subjects before and after a high-carbohydrate meal. After an overnight fast, total HK, HK-I, and HK-II activities were similar in lean and obese control subjects; but HK-II was lower in NIDDM patients than in lean subjects (1.42 +/- 0.16 [SE] vs. 2.33 +/- 0.24 nmol x min(-1) x mg(-1) molecular weight, P < 0.05) and accounted for a lower proportion of total HK (33 +/- 3 vs. 47 +/- 3%, P < 0.025). HK-II (but not HK-I) activity increased during the clamp in lean and obese subjects by 34 and 36% after 3 h and by 14 and 22% after 5 h of hyperinsulinemia; no increase was found in the NIDDM patients. In the lean subjects, muscle HK-II activity also increased by 15% 4 h after the meal, from 2.47 +/- 0.19 basally to 2.86 +/- 0.28 nmol x min(-1) x mg(-1) protein (P < 0.05). During the clamps, muscle HK-II activity correlated with muscle citrate synthase activity in the normal subjects (r = 0.58, P < 0.05) but not in the NIDDM patients. A weak relationship was noted between muscle HK-II activity and glucose disposal rate at the end of the clamp when all three groups were combined (r = 0.49, P < 0.05). In summary, NIDDM patients have lower muscle HK-II activity basally and do not increase the activity of this enzyme in response to a 5-h insulin stimulus. This defect may contribute to their insulin resistance. In nondiabetic obese subjects, muscle HK-II expression and its regulation by insulin are normal.     

Limitations to exercise- and maximal insulin-stimulated muscle glucose uptake

The hypothesis of this investigation was that insulin and muscle contraction, by increasing the rate of skeletal muscle glucose transport, would bias control so that glucose delivery to the sarcolemma (and t tubule) and phosphorylation of glucose intracellularly would exert more influence over glucose uptake. Because of the substantial increases in blood flow (and hence glucose delivery) that accompany exercise, we predicted that glucose phosphorylation would become more rate determining during exercise. The transsarcolemmal glucose gradient (TSGG; the glucose concentration difference across the membrane) is inversely related to the degree to which glucose transport determines the rate of glucose uptake. The TSGG was determined by using isotopic methods in conscious rats during euglycemic hyperinsulinemia [Ins; 20 mU/(kg. min); n = 7], during treadmill exercise (Ex, n = 6), and in sedentary, saline-infused rats (Bas, n = 13). Rats received primed, constant intravenous infusions of trace 3-O-[3H]methyl-D-glucose and [U-14C]mannitol. Then 2-deoxy-[3H]glucose was infused for the calculation of a glucose metabolic index (Rg). At the end of experiments, rats were anesthetized, and soleus muscles were excised. Total soleus glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-[3H]methyl-D-glucose (which distributes on the basis of the TSGG) were used to calculate ranges of possible glucose concentrations ([G]) at the inner and outer sarcolemmal surfaces ([G]im and [G]om, respectively). Soleus Rg was increased in Ins and further increased in Ex. In Ins, total soleus glucose, [G]om, and the TSGG were decreased compared with Bas, while [G]im remained near 0. In Ex, total soleus glucose and [G]im were increased compared with Bas, and there was not a decrease in [G]om as was observed in Ins. In addition, accumulation of intracellular free 2-deoxy-[3H]glucose occurred in soleus in both Ex and Ins. Taken together, these data indicate that, in Ex, glucose phosphorylation becomes an important limitation to soleus glucose uptake. In Ins, both glucose delivery and glucose phosphorylation influence the rate of soleus glucose uptake more than under basal conditions.     

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.     

The conserved amino-terminal domain of hSRP1 alpha is essential for nuclear protein import

Transgenic mice that overexpressed IGFBP-1 are hyperinsulinemic in the first week of life and gradually develop fasting hyperglycemia. In adult transgenic mice, the hypoglycemic response to IGF-I but not insulin or des (1-3) IGF-I was attenuated (P < 0.05) compared with wild-type mice. Furthermore, in isolated adipocytes from transgenic mice, the stimulatory effect of IGF-I but not insulin on 2-deoxy-[3H]-glucose uptake was reduced (P < 0.02). In contrast, in isolated soleus muscle, the effects of both IGF-I and insulin on 2-deoxy-3H-glucose uptake and on [3H]-glucose incorporation into glycogen were significantly reduced compared to wild-type mice. The decline in specific activity of the 2-deoxy-3H-glucose, a measure of glucose appearance in the circulation, was more marked in transgenic animals (P < 0.05). In addition, tissue uptake of glucose was significantly higher in diaphragm, heart, intestine, liver, soleus muscle, and adipose tissue from fasting transgenic mice. Plasma concentrations of alanine, lysine, and methionine were also elevated in transgenic mice. These data suggest that overexpression of IGFBP-1 attenuates the hypoglycemic effect of endogenous IGF-I, which is initially compensated for by enhanced pancreatic insulin production. However, in adult mice pancreatic insulin content is reduced, insulin resistance is demonstrable in skeletal muscle and fasting hyperglycemia develops.     

Insulin regulation of vascular smooth muscle glucose transport in insulin-sensitive and resistant rats

Previous studies from this laboratory demonstrate insulin regulation of vascular smooth muscle intracellular calcium transport and of vascular tone. Accordingly, we measured insulin regulated glucose transport and GLUT4 protein in aortic tissue from insulin-resistant and sensitive rats in order to document classical insulin action in this tissue. We assessed (via in vitro muscle-bath and perfusion) basal and insulin-stimulated [3H]-2-deoxyglucose transport in aorta from lean and obese Zucker rats. In the muscle-bath system, insulin significantly stimulated aortic 2-deoxyglucose transport by 30%, but there was no difference in either baseline or insulin-stimulated glucose transport in lean versus obese rats. However, in the perfusion system, in which only the luminal surface of the vessel was exposed, insulin significantly (p < 0.04) stimulated 2-deoxyglucose transport in the aorta of only the lean animals. Western blot analysis demonstrated that aorta from lean and obese Zucker rats contained substantial concentrations of GLUT4 protein. Surprisingly, in the muscle-bath system 2-deoxyglucose transport was markedly higher in the aorta than in the soleus (p < 0.0002). These data demonstrate that vascular smooth muscle is an insulin-sensitive tissue which may be partially impaired in insulin resistant animals.     

Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice

Correction of the obese state induced by genetic leptin deficiency reduces elevated levels of both blood glucose and hypothalamic neuropeptide Y (NPY) mRNA in ob/ob mice. To determine whether these responses are due to a specific action of leptin or to the reversal of the obese state, we investigated the specificity of the effect of systemic leptin administration to ob/ob mice (n = 8) on levels of plasma glucose and insulin and on hypothalamic expression of NPY mRNA. Saline-treated controls were either fed ad libitum (n = 8) or pair-fed to the intake of the leptin-treated group (n = 8) to control for changes of food intake induced by leptin. The specificity of the effect of leptin was further assessed by 1) measuring NPY gene expression in db/db mice (n = 6) that are resistant to leptin, 2) measuring NPY gene expression in brain areas outside the hypothalamus, and 3) measuring the effect of leptin administration on hypothalamic expression of corticotropin-releasing hormone (CRH) mRNA. Five daily intraperitoneal injections of recombinant mouse leptin (150 micrograms) in ob/ob mice lowered food intake by 56% (P < 0.05), body weight by 4.1% (P < 0.05), and levels of NPY mRNA in the hypothalamic arcuate nucleus by 42.3% (P < 0.05) as compared with saline-treated controls. Pair-feeding of ob/ob mice to the intake of leptin-treated animals produced equivalent weight loss, but did not alter expression of NPY mRNA in the arcuate nucleus. Leptin administration was also without effect on food intake, body weight, or NPY mRNA levels in the arcuate nucleus of db/db mice. In ob/ob mice, leptin did not alter NPY mRNA levels in cerebral cortex or hippocampus or the expression of CRH mRNA in the hypothalamic paraventricular nucleus (PVN). Leptin administration to ob/ob mice also markedly reduced serum glucose (8.3 +/- 1.2 vs. 24.5 +/- 3.8 mmol/l; P < 0.01) and insulin levels (7,263 +/- 1,309 vs. 3,150 +/- 780 pmol/l), but was ineffective in db/db mice. Pair-fed mice experienced reductions of glucose and insulin levels that were < 60% of the reduction induced by leptin. The results suggest that in ob/ob mice, systemic administration of leptin inhibits NPY gene overexpression through a specific action in the arcuate nucleus and exerts a hypoglycemic action that is partly independent of its weight-reducing effects. Furthermore, both effects occur before reversal of the obesity syndrome. Defective leptin signaling due to either leptin deficiency (in ob/ob mice) or leptin resistance (in db/db mice) therefore leads directly to hyperglycemia and the overexpression of hypothalamic NPY that is implicated in the pathogenesis of the obesity syndrome.     

Secretion, purification, and characterisation of barley alpha-amylase produced by heterologous gene expression in Aspergillus niger

The role of the actin cytoskeleton and/or GTPases of the Rho/Rac-family in glucose transport regulation was investigated in 3T3-L1 cells with clostridial toxins which depolymerize actin by inactivation of Rho/Rac (Clostridium difficile toxin B and Clostiridium sordellii lethal toxin (LT)) or by direct ADP-ribosylation (Clostridium botulinum C2 toxin). Toxin B and C2 reduced insulin-stimulated, but not basal, 2-deoxyglucose (2-DOG) uptake rates in 3T3-L1 fibroblasts. In parallel, the toxins produced morphological alterations of the cells reflecting disruption of the actin cytoskeleton. Both toxins reduced the maximum response to insulin but failed to alter the half-maximally stimulating concentrations of insulin. In 3T3-L1 adipocytes, the lethal toxin reduced the effect of insulin on 2-DOG uptake, whereas toxin B and C2 failed to affect glucose transport or cell morphology. When cells were exposed to the toxins after treatment with insulin, both toxin B and the lethal toxin, in contrast to the phosphatidylinositol (PI) 3-kinase inhibitor wortmannin, failed to reduce the 2-DOG uptake rates. Thus, both translocation to the plasma membrane and internalization of glucose transporters were inhibited by the toxins, whereas the PI 3-kinase inhibitor selectively affects translocation. The data suggest that the effects of the clostridial toxins on trafficking of glucose transporters are mediated by the depolymerization of the actin cytoskeleton and are an indirect consequence of Rho or Rac inactivation. It is suggested that pathways signalling through Rac or Rho may play a modulatory role in glucose transport regulation through their effects on the actin network.     

Non-insulin and non-exercise related increase of glucose utilization in rats and mice

The effects of high-energy phosphate contents in muscles on glucose tolerance and glucose uptake into tissues were studied in rats and mice. Enhanced glucose tolerance associated with depleted high-energy phosphates and elevated glycogen content in muscles and liver was observed in animals fed creatine analogue beta-guanidinopropionic acid (beta-GPA). Distribution of infused 2-[1-14C]deoxy-D-glucose in tissues especially in the soleus muscle, kidney, and brain was greater in mice fed beta-GPA than controls. The glucose uptake was decreased when the contents of ATP and glycogen were normalized following creatine supplementation. Plasma insulin in animals at rest was lower and its concentration after intraperitoneal glucose infusion tended to be less in animals fed beta-GPA than controls (p > 0.05), although the pattern of insulin response to glucose loading was similar to the control. The daily voluntary activity in beta-GPA fed mice was also less than controls. These results suggest that improved glucose tolerance is not related to elevated insulin concentration and/or decreased glycogen following exercise. Such improvement may be due to an increased mitochondrial energy metabolism caused by depletion of high-energy phosphates.