bluuebunny/crossref_metadata_2025 huggingface dataset:
Tissue-Specific Regulation of Mitochondrial and Cytoplasmic Protein Synthesis Rates by Insulin
Authors: Boirie Yves , Short Kevin R. , Ahlman Bo , Charlton Michael , Nair K. Sreekumaran
Published: March 2007

In vivo studies have reported conflicting effects of insulin on mixed tissue protein synthesis rates. To test the hypothesis that insulin has differential effects on synthesis rates of various protein fractions in different organs, we infused miniature swine (n = 8 per group) with saline, insulin alone (at 0.7 mU/kg−1 · min−1), or insulin plus an amino acid mixture for 8 h. Fractional synthesis rate (FSR) of mitochondrial and cytoplasmic proteins in liver, heart, and skeletal muscle, as well as myosin heavy chain (MHC) in muscle, were measured using l-[1-13C]leucine as a tracer. The FSR of mitochondrial and cytoplasmic proteins were highest in liver, followed by heart and then muscle. Mitochondrial FSR in muscle was higher during insulin and insulin plus amino acid infusions than during saline. Insulin had no significant effect on FSR of MHC in muscle. In contrast, FSR of both mitochondrial and cytoplasmic proteins were not stimulated by insulin in liver. Insulin also did not increase FSR of mitochondrial in heart, whereas insulin and amino acid stimulated FSR of cytoplasmic protein. In conclusion, insulin stimulates the synthesis of muscle mitochondrial proteins, with no significant stimulatory effect on synthesis of sarcoplasmic and MHC. These results demonstrate that insulin has different effects on synthesis rates of specific protein fractions in the liver, heart, and skeletal muscle.
Effects of Streptozocin Diabetes and Diabetes Treatment by Islet Transplantation on In Vivo Insulin Signaling in Rat Heart
Authors: Laviola Luigi , Belsanti Gaetana , Davalli Alberto M. , Napoli Raffaele , Perrini Sebastio , Weir Gordon C. , Giorgino Riccardo , Giorgino Francesco
Published: March 2007

The insulin signaling cascade was investigated in rat myocardium in vivo in the presence of streptozocin (STZ)-induced diabetes and after diabetes treatment by islet transplantation under the kidney capsule. The levels of insulin-stimulated tyrosine phosphorylation of the insulin receptor β-subunit, insulin receptor substrate (IRS)-2, and p52Shc were increased in diabetic compared with control heart, whereas tyrosine phosphorylation of IRS-1 was unchanged. The amount of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) and the level of PI 3-kinase activity associated with IRS-2 were also elevated in diabetes, whereas no changes in IRS-1–associated PI 3-kinase were observed. Insulin-induced phosphorylation of Akt on Thr-308 was increased fivefold in diabetic heart, whereas Akt phosphorylation on Ser-473 was normal. In contrast with Akt phosphorylation, insulin-induced phosphorylation of glycogen synthase kinase (GSK)-3, a major cellular substrate of Akt, was markedly reduced in diabetes. In islet-transplanted rats, the majority of the alterations in insulin-signaling proteins found in diabetic rats were normalized, but insulin stimulation of IRS-2 tyrosine phosphorylation and association with PI 3-kinase was blunted. In conclusion, in the diabetic heart, 1) IRS-1, IRS-2, and p52Shc are differently altered, 2) the levels of Akt phosphorylation on Ser-473 and Thr-308, respectively, are not coordinately regulated, and 3) the increased activity of proximal-signaling proteins (i.e., IRS-2 and PI 3-kinase) is not propagated distally to GSK-3. Islet transplantation under the kidney capsule is a potentially effective therapy to correct several diabetes-induced abnormalities of insulin signaling in cardiac muscle but does not restore the responsiveness of all signaling reactions to insulin.
Transgenic Overexpression of Hepatocyte Growth Factor in the β-Cell Markedly Improves Islet Function and Islet Transplant Outcomes in Mice
Authors: García-Ocaña Adolfo , Vasavada Rupangi C. , Cebrian Ana , Reddy Vasumathi , Takane Karen K. , López-Talavera Juan-Carlos , Stewart Andrew F.
Published: March 2007

Recent advances in human islet transplantation have highlighted the need for expanding the pool of β-cells available for transplantation. We have developed three transgenic models in which growth factors (hepatocyte growth factor [HGF], placental lactogen, or parathyroid hormone-related protein) have been targeted to the β-cell using rat insulin promoter (RIP). Each displays an increase in islet size and islet number, and each displays insulin-mediated hypoglycemia. Of these three models, the RIP-HGF mouse displays the least impressive phenotype under basal conditions. In this study, we show that this mild basal phenotype is misleading and that RIP-HGF mice have a unique and salutary phenotype. Compared with normal islets, RIP-HGF islets contain more insulin per β-cell (50 ± 5 vs. 78 ± 9 ng/islet equivalent [IE] in normal vs. RIP-HGF islets, P < 0.025), secrete more insulin in response to glucose in vivo (0.66 ± 0.06 vs. 0.91 ± 0.10 ng/ml in normal vs. RIP-HGF mice, P < 0.05) and in vitro (at 22.2 mmol/l glucose: 640 ± 120.1 vs. 1,615 ± 196.9 pg · μg protein−1 · 30 min−1 in normal vs. RIP-HGF islets, P < 0.01), have two- to threefold higher GLUT2 and glucokinase steady-state mRNA levels, take up and metabolize glucose more effectively, and most importantly, function at least twice as effectively after transplantation. These findings indicate that HGF has surprisingly positive effects on β-cell mitogenesis, glucose sensing, β-cell markers of differentiation, and transplant survival. It appears to have a unique and unanticipated effective profile as an islet mass- and function-enhancing agent in vivo.
Acute 2DG-Induced Glucoprivation or Dexamethasone Abolishes 2DG-Induced Glucoregulatory Responses to Subsequent Glucoprivation
Authors: Sanders Nicole M. , Ritter Sue
Published: March 2007

Behavioral, neuroendocrine, and autonomic responses to glucoprivation are impaired after a glucoprivic episode. A life-threatening manifestation of this effect, known as hypoglycemia-associated autonomic failure (HAAF), occurs in diabetic patients as a result of prior inadvertent hypoglycemia resulting from insulin therapy. Glucocorticoids, which are elevated by glucoprivation, have been implicated in the pathogenesis of HAAF. The goal of the present study was to examine the effect of glucocorticoids on glucoregulatory responses in a rat model of HAAF. 2-deoxy-d-glucose (2DG; 200 mg/kg) was used to induce glucoprivation. Rats were injected with saline, 2DG, or the synthetic glucocorticoid, dexamethasone (DEX; 250 μg/rat) in the morning. Then 6 h later, rats were injected with 2DG, and their feeding and hyperglycemic responses were measured. Both 2DG and DEX in the morning eliminated glucoprivic feeding and hyperglycemic responses in the afternoon test. Epinephrine (0.3 mg/kg) administration in the afternoon elicited marked hyperglycemia in animals given 2DG that morning, demonstrating that glycogen depletion from morning glucoprivation was not responsible for the absence of the hyperglycemic response in the afternoon test. The effects of prior saline or 2DG treatment on subsequent glucoprivic feeding were also examined in adrenalectomized rats in which the source of endogenous glucocorticoids was removed. In these animals, prior glucoprivation did not attenuate 2DG-induced feeding in the afternoon test. These findings demonstrate that a single glucoprivic episode is sufficient to cause impairment in glucoregulatory responses to a second glucoprivic episode in the same day. In addition, these results strongly implicate glucocorticoids in the pathogenesis of HAAF.
Uncoupling Protein 3 Content Is Decreased in Skeletal Muscle of Patients With Type 2 Diabetes
Authors: Schrauwen Patrick , Hesselink Matthijs K.C. , Blaak Ellen E. , Borghouts Lars B. , Schaart Gert , Saris Wim H.M. , Keizer Hans A.
Published: March 2007

Recently, a role for uncoupling protein-3 (UCP3) in carbohydrate metabolism and in type 2 diabetes has been suggested. Mice overexpressing UCP3 in skeletal muscle showed reduced fasting plasma glucose levels, improved glucose tolerance after an oral glucose load, and reduced fasting plasma insulin levels. However, data regarding the expression of UCP3 in patients with type 2 diabetes is inconsistent, and so far, there have been no reports of UCP3 protein content. Here we compared, for the first time, the protein levels of UCP3 in vastus lateralis muscle in 14 male type 2 diabetic patients (age 49.8 ± 2.1 years; BMI 27.2 ± 1.2 kg/m2; mean ± SE) with 16 male control subjects (age 48.0 ± 1.9 years; BMI 23.4 ± 0.6 kg/m2). We found that UCP3 protein levels were twice as low in patients with type 2 diabetes compared with control subjects (117 ± 16 vs. 58 ± 12 AU; P = 0.007). There was no correlation between UCP3 content and BMI. In conclusion, UCP3 content is lower in type 2 diabetic patients compared with healthy control subjects. These results are consistent with a role for UCP3 in glucose homeostasis and suggest a role for UCP3 in type 2 diabetes.
The Discovery of Type 1 Diabetes
Authors: Gale Edwin A.M.
Published: March 2007

The etiological heterogeneity of idiopathic diabetes has been recognized for 25 years, and subdivision into type 1 and type 2 diabetes is fundamental to the way we think about the disease. Review of the literature suggests that the concept of type 1 diabetes as an immunemediated disease emerged rapidly over the period from 1974 to 1976 and showed many of the features of a classic paradigm shift. A few key observations triggered recognition and acceptance of the new paradigm, but the necessary context was provided by scientific developments in areas mainly unrelated to diabetes. The disease paradigm established by 1976 is still widely accepted, and its essential features have been modified only in detail by the revolution in molecular biology that has occurred over the intervening period. Notwithstanding, some of the underlying assumptions remain imprecise, unchallenged, or unconfirmed. Appreciation of the historical origin and subsequent evolution of these fundamental concepts could stimulate critical analysis and help prepare the way for a new paradigm.
Protection Against Oxidative Stress—Induced Insulin Resistance in Rat L6 Muscle Cells by Micromolar Concentrations of α-Lipoic Acid
Authors: Maddux Betty A. , See Wendy , Lawrence John C. , Goldfine Amy L. , Goldfine Ira D. , Evans Joseph L.
Published: March 2007

In diabetic patients, α-lipoic acid (LA) improves skeletal muscle glucose transport, resulting in increased glucose disposal; however, the molecular mechanism of action of LA is presently unknown. We studied the effects of LA on basal and insulin-stimulated glucose transport in cultured rat L6 muscle cells that overexpress GLUT4. When 2-deoxy-D-glucose uptake was measured in these cells, they were more sensitive and responsive to insulin than wild-type L6 cells. LA, at concentrations ≤1 mmol/l, had only small effects on glucose transport in cells not exposed to oxidative stress. When cells were exposed to glucose oxidase and glucose to generate H2O2 and cause oxidative stress, there was a marked decrease in insulin-stimulated glucose transport. Pretreatment with LA over the concentration range of 10-1,000 μmol/l protected the insulin effect from inhibition by H2O2. Both the R and S isomers of LA were equally effective. In addition, oxidative stress caused a significant decrease (∼50%) in reduced glutathione concentration, along with the rapid activation of the stress-sensitive p38 mitogen-activated protein kinase. Pretreatment with LA prevented both of these events, coincident with protecting insulin action. These studies indicate that in muscle, the major site of insulin-stimulated glucose disposal, one important effect of LA on the insulin-signaling cascade is to protect cells from oxidative stress-induced insulin resistance.
Peroxisome Proliferator—Activated Receptor (PPAR)-αActivation Lowers Muscle Lipids and Improves Insulin Sensitivity in High Fat—Fed Rats
Authors: Ye Ji-Ming , Doyle Patrick J. , Iglesias Miguel A. , Watson David G. , Cooney Gregory J. , Kraegen Edward W.
Published: March 2007

Peroxisome proliferator—activated receptor (PPAR)-α agonists lower circulating lipids, but the consequences for muscle lipid metabolism and insulin sensitivity are not clear. We investigated whether PPAR-αactivation improves insulin sensitivity in insulin-resistant rats and compared the effects with PPAR-γ activation. Three-week high fat-fed male Wistar rats were untreated or treated with the specific PPAR-α agonist WY14643 or the PPAR-γ agonist pioglitazone (both 3 mg · kg-1· day-1) for the last 2 weeks of high-fat feeding. Like pioglitazone, WY14643 lowered basal plasma levels of glucose, triglycerides(-16% vs. untreated), and leptin (-52%), and also muscle triglyceride (-34%)and total long-chain acyl-CoAs (LCACoAs) (-41%) (P < 0.05). In contrast to pioglitazone, WY14643 substantially reduced visceral fat weight and total liver triglyceride content (P < 0.01) without increasing body weight gain. WY14643 and pioglitazone similarly enhanced wholebody insulin sensitivity (clamp glucose infusion rate increased 35 and 37% and glucose disposal 22 and 15%, respectively, vs. untreated). Both agents enhanced insulin-mediated muscle glucose metabolic index (Rg') and reduced muscle triglyceride and LCACoA accumulation (P < 0.05). Although pioglitazone had more potent effects than WY14643 on muscle insulin sensitization, this was associated with its greater effect to reduce muscle LCACoA accumulation. Overall insulin-mediated muscle Rg' was inversely correlated with the content of LCACoAs (r = -0.74, P =0.001) and with plasma triglyceride levels (r = -0.77, P< 0.001). We conclude that even though WY14643 and pioglitazone,representing PPAR-α and PPAR-γ activation, respectively, may alter muscle lipid supply by different mechanisms, both significantly improve muscle insulin action in the high fat—fed rat model of insulin resistance, and this effect is proportional to the degree to which they reduce muscle lipid accumulation.
Beta-cell adaptation and decompensation during the progression of diabetes.
Authors: Weir G C , Laybutt D R , Kaneto H , Bonner-Weir S , Sharma A
Published: March 2007

Inadequate beta-cell function is an essential component of all forms of diabetes. The most obvious problem is a failure to maintain sufficient beta-cell mass and function to cope with whatever insulin resistance is present. The most striking functional defect is a loss of acute glucose-induced insulin secretion (GIIS). This review discusses the ways in which beta-cells successfully adapt to increased demand and then decompensate as diabetes develops. Successful adaptation is achieved through increased beta-cell mass and increased insulin secretion. The hypothesis is explored that beta-cells exposed to the diabetic milieu lose their differentiation, which leads to loss of specialized functions such as GIIS. This concept has been strengthened by the finding of dedifferentiation of beta-cells in a rat model of partial pancreatectomy that includes a reduction of insulin gene expression, which may further contribute to decreased insulin production. Another finding was increased expression of c-Myc, which probably contributes to an increase in the expression of lactate dehydrogenase and the development of beta-cell hypertrophy. Arguments are developed that the beta-cell changes found in diabetes are better correlated with increased glucose levels than with non-esterified fatty acid levels, thus supporting the importance of glucose toxicity.
A model for glucose control of insulin secretion during 24 h of free living.
Authors: Mari A , Camastra S , Toschi E , Giancaterini A , Gastaldelli A , Mingrone G , Ferrannini E
Published: March 2007

The aim of this work was to develop a mathematical model describing the functional dependence of insulin secretion on plasma glucose concentrations during 24 h of free living. We obtained hourly central venous blood samples from a group of healthy volunteers who spent 24 h in a calorimetric chamber, where they consumed standardized meals. Insulin secretory rates were reconstructed from plasma C-peptide concentrations by deconvolution. The relationship between insulin release and plasma glucose concentrations was modeled as the sum of three components: a static component (describing the dependence on plasma glucose concentration itself, with an embedded circadian oscillation), a dynamic component (modeling the dependence on glucose rate of change), and a residual component (including the fraction of insulin secretion not explained by glucose levels). The model fit of the individual 24-h secretion profiles was satisfactory (within the assigned experimental error of glucose and C-peptide concentrations). The static component yielded a dose-response function in which insulin release increased quasi-linearly (from 40 to 400 pmol/min on average) over the range of 4-9 mmol/l glucose. The dynamic component was significantly different from zero in coincidence with meal-related glucose excursions. The circadian oscillation and the residual component accounted for the day/night difference in the ability of glucose to stimulate insulin release. Over 24 h, total insulin release averaged 257+/-58 nmol (or 43+/-10 U). The static and dynamic component together accounted for approximately 80% of total insulin release. The model proposed here provides a detailed robust description of glucose-related insulin release during free-living conditions. In nondiabetic subjects, non-glucose-dependent insulin release is a small fraction of total insulin secretion.
Endocrine pancreas plasticity under physiological and pathological conditions.
Authors: Bernard-Kargar C , Ktorza A
Published: March 2007

Endocrine pancreas plasticity may be defined as the ability of the organ to adapt the beta-cell mass to the variations in insulin demand. For example, during late pregnancy and obesity, the increase of the beta-cell mass, in association with beta-cell hyperactivity, contributes to insulin oversecretion in response to insulin resistance. There is increasing evidence that the ability of the beta-cell mass to expand in adult mammals is much higher than previously thought. During pregnancy, placental hormones, especially placental lactogens, are mainly responsible for the changes in beta-cell mass. The factors involved in beta-cell growth in obesity are far from clear, although increased free fatty acids seem to be the main candidate. Many data suggest that the impairment of insulin secretion in type 2 diabetes is partly related to reduction of beta-cell mass, at least relative to prevailing insulin demand. This defect may originate from genetic predisposition, but the situation is likely worsened by environmental factors such as hyperglycemia (glucotoxicity) and hyperlipidemia (lipotoxicity). Better understanding of beta-cell growth and regeneration mechanisms may allow new strategies in the treatment of type 2 diabetes based on early limitation of beta-cell damage and/or restoration of a functional beta-cell mass.
Role of apoptosis in pancreatic beta-cell death in diabetes.
Authors: Chandra J , Zhivotovsky B , Zaitsev S , Juntti-Berggren L , Berggren P O , Orrenius S
Published: March 2007

Apoptosis is a physiological form of cell death that occurs during normal development, and critical mediators of this process include caspases, reactive oxygen species, and Ca2+. Excessive apoptosis of the pancreatic beta-cell has been associated with diabetes. Consequently, apoptosis research has focused on how infiltrating macrophages or cytotoxic T-cells might kill pancreatic beta-cells using cytokines or death receptor triggering. Meanwhile, the intracellular events in the target beta-cell have been largely ignored. Elucidation of such targets might help develop improved treatment strategies for diabetes. This article will outline recent developments in apoptosis research, with emphasis on mechanisms that may be relevant to beta-cell death in type 1 and type 2 diabetes. Several of the models proposed in beta-cell killing converge on Ca2+ signaling, indicating that the pancreatic beta-cell may be an ideal system in which to carefully dissect the role of Ca2+ during apoptosis.
Developmental biology of the pancreas.
Authors: Edlund H
Published: March 2007

All pancreatic cell types (endocrine, exocrine, and ductal) are derived from the same endodermal dorsal and ventral anlage, which grow together to form the definitive pancreas. Golosow and Grobstein were pioneers in the field of pancreatic developmental research, as were Wessells and Cohen, who already in the 1960s performed classic embryological experiments describing the morphogenesis of the pancreas and the epithelio-mesenchymal interactions that are instrumental for proper pancreas development. Recent findings suggest that follistatin and fibroblast growth factors represent some of these key mesenchymal factors that actively promote at least pancreatic exocrine development. The true endodermal origin of the pancreatic endocrine cells became evident by experiments performed by the groups of LeDouarin and Rutter in the 1970s. The newly acquired insights regarding the specification of pancreatic endocrine cells as controlled by the notch signaling pathway (i.e., similar to the mechanisms by which neurons are specified during neurogenesis) have provided a novel understanding of the long acknowledged similarities between neurons and the pancreatic endocrine cells. Last, the identification of a number of distinct transcription factors operating at various levels of pancreatic development and in different cell types has provided useful information both on pancreas development and on various pancreatic disorders such as diabetes. Interestingly, four of the hitherto defined five different maturity-onset diabetes of the young (MODY) genes correspond to transcription factors, and, in addition, several transcription factors have also been linked to type 2 diabetes.
Role of pancreatic beta-cells in the process of beta-cell death.
Authors: Pipeleers D , Hoorens A , Marichal-Pipeleers M , Van de Casteele M , Bouwens L , Ling Z
Published: March 2007

Studies on the pathogenesis of type 1 diabetes have mainly focused on the role of the immune system in the destruction of pancreatic beta-cells. Lack of data on the cellular and molecular events at the beta-cell level is caused by the inaccessibility of these cells during development of the disease. Indirect information has been collected from isolated rodent and human islet cell preparations that were exposed to cytotoxic conditions. This article reviews in vitro experiments that investigated the role of beta-cells in the process of beta-cell death. beta-Cells rapidly die in necrosis because of toxic levels of oxidizing radicals or of nitric oxide; they progressively become apoptotic after prolonged culture at low glucose or with proinflammatory cytokines. Their susceptibility to necrosis or apoptosis varies with their functional state and thus with the environmental conditions. A change in cellular phenotype can alter its recognition of potentially cytotoxic agents and its defense mechanisms against cell death. These observations support the view that beta-cells are not necessarily passive victims of a cytotoxic process but can actively participate in a process of beta-cell death. Their role will be influenced by neighboring non-beta-cells, which can make the islet internal milieu more protective or toxic for the beta-cells. We consider duct cells as potentially important contributors to this local process.
Defective Function of Fas in Patients With Type 1 Diabetes Associated With Other Autoimmune Diseases
Authors: DeFranco Simona , Bonissoni Sara , Cerutti Franco , Bona Gianni , Bottarel Flavia , Cadario Francesco , Brusco Alfredo , Loffredo Giuseppe , Rabbone Ivana , Corrias Andrea , Pignata Claudio , Ramenghi Ugo , Dianzani Umberto
Published: March 2007

Fas (CD95) triggers programmed cell death and is involved in cell-mediated cytotoxicity and in shutting off the immune response. Inherited loss-of-function mutations hitting the Fas system cause the autoimmune/lymphoproliferative syndrome (ALPS). We have recently shown that ALPS patients’ families display increased frequency of common autoimmune diseases, including type 1 diabetes. This work evaluates Fas function in type 1 diabetic patients without typical ALPS. Cell death induced by anti-Fas monoclonal antibody was investigated in T-cells from 13 patients with type 1 diabetes alone and 19 patients with type 1 diabetes plus other autoimmune diseases (IDDM-P). Moreover, we analyzed 19 patients with thyroiditis alone (TYR), because most IDDM-P patients displayed thyroiditis. Frequency of resistance to Fas-induced cell death was significantly higher in patients with IDDM-P (73%) than in type 1 diabetic (23%) or TYR (16%) patients or in normal control subjects (3%). The defect was specific because resistance to methyl-prednisolone–induced cell death was not significantly increased in any group. Fas was always expressed at normal levels, and no Fas mutations were detected in four Fas-resistant IDDM-P patients. Analysis of the families of two Fas-resistant patients showing that several members were Fas-resistant suggests that the defect has a genetic component. Moreover, somatic fusion of T-cells from Fas-resistant subjects and the Fas-sensitive HUT78 cell line generates Fas-resistant hybrid cells, which suggests that the Fas resistance is due to molecules exerting a dominant-negative effect on a normal Fas system. These data suggest that Fas defects may be a genetic factor involved in the development of polyreactive type 1 diabetes.
Insulin Resistance With Enhanced Insulin Signaling in High-Salt Diet–Fed Rats
Authors: Ogihara Takehide , Asano Tomoichiro , Ando Katsuyuki , Chiba Yuko , Sekine Nobuo , Sakoda Hideyuki , Anai Motonobu , Onishi Yukiko , Fujishiro Midori , Ono Hiraku , Shojima Nobuhiro , Inukai Kouichi , Fukushima Yasushi , Kikuchi Masatoshi , Fujita Toshiro
Published: March 2007

Previous clinical studies showed an apparent correlation between hypertension and insulin resistance, and patients with diabetes are known to have increased blood pressure responsiveness to salt loading. To investigate the effect of high salt intake on insulin sensitivity and the insulin signaling pathway, a high-salt diet (8% NaCl) or a normal diet was given to 7-week-old SD rats for 2 weeks. High salt–fed rats developed slightly but significantly higher systolic blood pressure than controls (133 ± 2 vs. 117 ± 2 mmHg, P < 0.001), with no change in food intake or body weight. High salt–fed rats were slightly hyperglycemic (108.5 ± 2.8 vs. 97.8 ± 2.5 mg/dl, P = 0.01) and slightly hyperinsulinemic (0.86 ± 0.07 vs. 0.61 ± 0.06 ng/ml, P = 0.026) in the fasting condition, as compared with controls. Hyperinsulinemic-euglycemic clamp study revealed a 52.7% decrease in the glucose infusion rate and a 196% increase in hepatic glucose production in high salt–fed rats, which also showed a 66.4% decrease in 2-deoxyglucose uptake into isolated skeletal muscle and a 44.5% decrease in insulin-induced glycogen synthase activation in liver, as compared with controls. Interestingly, despite the presence of insulin resistance, high salt–fed rats showed enhanced insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1, IRS-2 (liver and muscle), and IRS-3 (liver only). Phosphatidylinositol (PI) 3-kinase activities associated with IRS and phosphotyrosine in the insulin-stimulated condition increased 2.1- to 4.1-fold, as compared with controls. Insulin-induced phosphorylation of Ser-473 of Akt and Ser-21 of glycogen synthase kinase-3 also increased 2.9- and 2-fold, respectively, in the liver of the high salt–fed rats. Therefore, in both the liver and muscle of high salt–fed rats, intracellular insulin signaling leading to PI 3-kinase activation is enhanced and insulin action is attenuated. The hyperinsulinemic-euglycemic clamp study showed that decreased insulin sensitivity induced with a high-salt diet was not reversed by administration of pioglitazone. The following can be concluded: 1) a high-salt diet may be a factor promoting insulin resistance, 2) the insulin-signaling step impaired by high salt intake is likely to be downstream from PI 3-kinase or Akt activation, and 3) this unique insulin resistance mechanism may contribute to the development of diabetes in patients with hypertension.
Preferential Channeling of Energy Fuels Toward Fat Rather Than Muscle During High Free Fatty Acid Availability in Rats
Authors: Fabris Roberto , Nisoli Enzo , Lombardi Anna Maria , Tonello Cristina , Serra Roberto , Granzotto Marnie , Cusin Isabelle , Rohner-Jeanrenaud Françoise , Federspil Giovanni , Carruba Michele O. , Vettor Roberto
Published: March 2007

The preferential channeling of different fuels to fat and changes in the transcription profile of adipose tissue and skeletal muscle are poorly understood processes involved in the pathogenesis of obesity and insulin resistance. Carbohydrate and lipid metabolism may play relevant roles in this context. Freely moving lean Zucker rats received 3- and 24-h infusions of Intralipid (Pharmacia and Upjohn, Milan, Italy) plus heparin, or saline plus heparin, to evaluate how an increase in free fatty acids (nonesterified fatty acid [NEFA]) modulates fat tissue and skeletal muscle gene expression and thus influences fuel partitioning. Glucose uptake was determined in various tissues at the end of the infusion period by means of the 2-deoxy-[1-3H]-d-glucose technique after a euglycemic-hyperinsulinemic clamp: high NEFA levels markedly decreased insulin-mediated glucose uptake in red fiber–type muscles but enhanced glucose utilization in visceral fat. Using reverse transcriptase–polymerase chain reaction and Northern blotting analyses, the mRNA expression of fatty acid translocase (FAT)/CD36, GLUT4, tumor necrosis factor (TNF)-α, peroxisome proliferator–activated receptor (PPAR)-γ, leptin, uncoupling protein (UCP)-2, and UCP-3 was investigated in different fat depots and skeletal muscles before and after the study infusions. GLUT4 mRNA levels significantly decreased (by ∼25%) in red fiber–type muscle (soleus) and increased (by ∼45%) in visceral adipose tissue. Furthermore, there were marked increases in FAT/CD36, TNF-α, PPAR-γ, leptin, UCP2, and UCP3 mRNA levels in the visceral fat and muscle of the treated animals in comparison with those measured in the saline-treated animals. These data suggest that the in vivo gene expression of FAT/CD36, GLUT4, TNF-α, PPAR-γ, leptin, UCP2, and UCP3 in visceral fat and red fiber–type muscle are differently regulated by circulating lipids and that selective insulin resistance seems to favor, at least in part, a prevention of fat accumulation in tissues not primarily destined for fat storage, thus contributing to increased adiposity and the development of a prediabetic syndrome.
Reduced Postprandial Concentrations of Intact Biologically Active Glucagon-Like Peptide 1 in Type 2 Diabetic Patients
Authors: Vilsbøll Tina , Krarup Thure , Deacon Carolyn F. , Madsbad Sten , Holst Jens J.
Published: March 2007

Incretin hormones importantly enhance postprandial insulin secretion but are rapidly degraded to inactive metabolites by ubiquitous dipeptidyl peptidase IV. The concentrations of the intact biologically active hormones remain largely unknown. Using newly developed assays for intact glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP), we measured plasma concentrations after a mixed breakfast meal (566 kcal) in 12 type 2 diabetic patients (age 57 years [range 49–67], BMI 31 kg/m2 [27–38], and HbA1c 9.2% [7.0–12.5]) and 12 matched healthy subjects. The patients had fasting hyperglycemia (10.7 mmol/l [8.0–14.8]) increasing to 14.6 mmol/l (11.5–21.5) 75 min after meal ingestion. Fasting levels of insulin and C-peptide were similar to those of the healthy subjects, but the postprandial responses were reduced and delayed. Fasting levels and meal responses were similar between patients and healthy subjects for total GIP (intact + metabolite) as well as intact GIP, except for a small decrease in the patients at 120 min; integrated areas for intact hormone (area under the curve [AUC]INT) averaged 52 ± 4% (for patients) versus 56 ± 3% (for control subjects) of total hormone AUC (AUCTOT). AUCINT for GLP-1 averaged 48 ± 2% (for patients) versus 51 ± 5% (for control subjects) of AUCTOT. AUCTOT for GLP-1 as well as AUCINT tended to be reduced in the patients (P = 0.2 and 0.07, respectively); but the profile of the intact GLP-1 response was characterized by a small early rise (30–45 min) and a significantly reduced late phase (75–150 min) (P < 0.02). The measurement of intact incretin hormones revealed that total as well as intact GIP responses were minimally decreased in patients with type 2 diabetes, whereas the late intact GLP-1 response was strongly reduced, supporting the hypothesis that an impaired function of GLP-1 as a transmitter in the enteroinsular axis contributes to the inappropriate insulin secretion in type 2 diabetes.
Studies With Apolipoprotein A-II Transgenic Mice Indicate a Role for HDLs in Adiposity and Insulin Resistance
Authors: Castellani Lawrence W. , Goto Aimie M. , Lusis Aldons J.
Published: March 2007

Apolipoprotein A-II (apoA-II) is the second most abundant protein in HDLs. Genetic studies in humans have provided evidence of linkage of the apoA-II gene locus to plasma free fatty acid (FFA) levels and to type 2 diabetes, and transgenic mice overexpressing mouse apoA-II have elevated levels of both FFA and triglycerides. We now show that apoA-II promotes insulin resistance and has diverse effects on fat homeostasis. ApoA-II transgenic mice have increased adipose mass and higher plasma leptin levels than C57BL/6J control mice. Fasting glucose levels were similar between apoA-II transgenic and control mice, but plasma insulin levels were elevated approximately twofold in the apoA-II transgenic mice. Compared with control mice, apoA-II transgenic mice exhibited a delay in plasma clearance of a glucose bolus. Adipose tissue isolated from fasted apoA-II transgenic mice exhibited a 50% decrease in triglyceride hydrolysis compared with adipose tissue from control mice. This is consistent with a normal response of adipose tissue to the increased insulin levels in the apoA-II transgenic mice and may partially explain the increased fat deposition. Skeletal muscle isolated from fasted apoA-II transgenic mice exhibited reduced uptake of 2-deoxyglucose compared with muscles isolated from control mice. Our observations indicate that a primary disturbance in lipoprotein metabolism can result in several traits associated with insulin resistance, consistent with the hypothesis that insulin resistance and type 2 diabetes can, under certain circumstances, be related primarily to altered lipid metabolism rather than glucose metabolism.
Assessment of Postabsorptive Renal Glucose Metabolism in Humans With Multiple Glucose Tracers
Authors: Moller Niels , Rizza Robert A. , Ford G. Charles , Nair K. Sreekumaran
Published: March 2007

The contribution of the kidneys to postabsorptive endogenous glucose production is a matter of controversy. To assess whether this could relate to the use of various isotopical methods with different analytical performance capabilities, we measured glucose kinetics in 12 healthy subjects. Blood samples were taken from the femoral artery and the renal vein after 4 h of [6,6-2H2]glucose infusion (for gas chromatography [GC]/mass spectrometry [MS] analysis), and renal plasma flow was determined with paraaminohippurate. In addition, six subjects received uniformly labeled [13C]glucose (for GC/combustion/isotope ratio MS [IRMS]) and [3-3H]glucose (for counting of radioactive disintegrations). Arterial glucose concentrations (means ± SD) were 4.2 ± 0.1 mmol/l, and endogenous glucose production rates using [2H2]glucose were 2.2 ± 0.1 mg · kg–1 · min–1 or 818 ± 50 μmol/min. Dilution of [2H2]glucose across the kidney was 0.79 ± 1.32%, and renal glucose production (RGP) rates were 27 ± 72 μmol/min. In the six subjects receiving additional tracers, dilutions across the kidney were 2.83 ± 0.72 and 0.54 ± 1.20 (for [U-13C]glucose and [3-3H]glucose, respectively, the dilution with [U-13C] being higher than that with [2H2] (P = 0.007). Corresponding RGP values were 144 ± 39 and 43 ± 76 μmol/min for [U-13C] and [3-3H], respectively. In conclusion, we found that the highly sensitive [U-13C] GC/Combustion/IRMS technique showed consistent dilution of label across the kidney, whereas the less sensitive techniques gave some negative values and smaller RGP rates. Thus, depending on which technique is being used, a fivefold difference in calculated RGP values may be encountered. The methodological variability of our data suggests that extrapolation from regional renal measurements to the whole-body level should be perfumed with caution.
A Ser311Cys Mutation in the Human Dopamine Receptor D2 Gene Is Associated With Reduced Energy Expenditure
Authors: Tataranni P. Antonio , Baier Leslie , Jenkinson Chris , Harper Inge , Del Parigi Angelo , Bogardus Clifton
Published: March 2007

Brain dopaminergic pathways play a major role in the control of movement. Absence of the murine dopamine D2 receptor gene (drd2) produces bradykinesia and hypothermia. A Ser311Cys mutation of the human DRD2 produces a marked functional impairment of the receptor and is associated with higher BMI in some populations. We hypothesized that the Ser311Cys mutation of DRD2 may inhibit energy expenditure. Here we report that total energy expenditure (doubly labeled water) measured in 89 nondiabetic Pima Indians was 244 kcal/day lower in homozygotes for the Cys311-encoding allele when compared with those heterozygous and homozygous for the Ser311-encoding allele (P = 0.056). The 24-h resting energy expenditure (respiratory chamber) measured in 320 nondiabetic Pimas was also 87 kcal/day lower in homozygotes for the Cys311-encoding allele when compared with those heterozygous and homozygous for the Ser311-encoding allele (P = 0.026). These findings are the first evidence that a genetic mutation is associated with reduced energy expenditure in humans. Because the impact of this mutation on human obesity is small, we suggest that either the energy deficit induced is not large enough to significantly influence body weight in this population and/or that the Cys311-encoding allele is also associated with reduced energy intake.
Ca2+-Dependent Exocytosis ofl-Glutamate by αTC6, Clonal Mouse Pancreatic α-Cells
Authors: Yamada Hiroshi , Otsuka Masato , Hayashi Mitsuko , Nakatsuka Shuuichi , Hamaguchi Kazuyuki , Yamamoto Akitsugu , Moriyama Yoshinori
Published: March 2007

Pancreatic islet cells express receptors and transporters for l-glutamate and are thus believed to use l-glutamate as an intercellular signaling molecule. However, the mechanism by which l-glutamate appears in the islets is unknown. In the present study, we investigated whether l-glutamate is secreted through exocytosis by αTC6 cells (clonal mouse pancreatic α-cells). An appreciable amount of l-glutamate was released from cultured cells after the addition of KCl or A23187 in the presence of Ca2+ and 10 mmol/l glucose in the medium. The KCl-induced glutamate release was significantly reduced when assayed in the absence of Ca2+ or when the cells were pretreated with EGTA-AM. The KCl-induced Ca2+-dependent glutamate release was inhibited ∼40% by voltage-gated Ca2+ channel blockers, such as nifedipine at 20 μmol/l. The degree of KCl-induced Ca2+-dependent glutamate release was correlated with an increase in intracellular [Ca2+], as monitored by fura-2 fluorescence. Botulinum neurotoxin type E inhibited 55% of the KCl-induced Ca2+-dependent glutamate release, followed by specific cleavage of 25 kDa synaptosomal-associated protein. Furthermore, bafilomycin A1, a specific inhibitor of vacuolar H+-ATPase, inhibited 40% of the KCl-induced Ca2+-dependent glutamate release. Immunoelectronmicroscopy with antibodies against synaptophysin, a marker for neuronal synaptic vesicles and endocrine synaptic-like microvesicles, revealed a large number of synaptophysin-positive clear vesicles in cells. Digitonin-permeabilized cells took up l-glutamate only in the presence of MgATP, which is sensitive to bafilomycin A1 or 3,5-di-tert-butyl-4-hydroxybenzylidene-malononitrile (a proton conductor) but insensitive to either oligomycin or vanadate. From these results, it was concluded that αTC6 cells accumulate l-glutamate in the synaptophysin-containing vesicles in an ATP-dependent manner and secrete it through a Ca2+-dependent exocytic mechanism. The Ca2+-dependent glutamate release was also triggered when cells were transferred in the medium containing 1 mmol/l glucose, suggesting that low glucose treatment stimulates the release of glutamate. Our results are consistent with the idea that l-glutamate is secreted by α-cells through Ca2+-dependent regulated exocytosis.
Effect of Acute and Antecedent Hypoglycemia on Sympathetic Neural Activity and Catecholamine Responsiveness in Normal Rats
Authors: Sivitz William I. , Herlein Judith A. , Morgan Donald A. , Fink Brian D. , Phillips Bradley G. , Haynes William G.
Published: March 2007

Adrenergic responsiveness to acute hypoglycemia is impaired after prior episodes of hypoglycemia. Although circulating epinephrine responses are blunted, associated alterations in adrenal sympathetic nerve activity (SNA) have not been reported. We examined adrenal nerve traffic in normal conscious rats exposed to acute insulin-induced hypoglycemia compared with insulin with (clamped) euglycemia. We also examined adrenal SNA and catecholamine responses to insulin-induced hypoglycemia in normal conscious rats after two antecedent episodes of hypoglycemia (days −2 and −1) compared with prior episodes of sham treatment. Acute insulin-induced hypoglycemia increased adrenal sympathetic nerve traffic compared with insulin administration with clamped euglycemia (165 ± 12 vs. 118 ± 21 spikes/s [P < 0.05]; or to 138 ± 8 vs. 114 ± 10% of baseline [P < 0.05]). In additional experiments, 2 days of antecedent hypoglycemia (days –2 and –1) compared with sham treatment significantly enhanced baseline adrenal SNA measured immediately before subsequent acute hypoglycemia on day 0 (180 ± 11 vs. 130 ± 12 spikes/s, respectively; P < 0.005) and during subsequent acute hypoglycemia (229 ± 17 vs. 171 ± 16 spikes/s; P < 0.05). However, antecedent hypoglycemia resulted in a nonsignificant reduction in hypoglycemic responsiveness of adrenal SNA when expressed as percent increase over baseline (127 ± 5% vs. 140 ± 14% of baseline). Antecedent hypoglycemia, compared with sham treatment, resulted in diminished epinephrine responsiveness to subsequent hypoglycemia. Norepinephrine responses to hypoglycemia were not significantly altered by antecedent hypoglycemia. In summary, prior hypoglycemia in normal rats increased adrenal sympathetic tone, but impaired epinephrine responsiveness to acute hypoglycemia. Hence, these data raise the intriguing possibility that increased sympathetic tone resulting from antecedent hypoglycemia downregulates subsequent epinephrine responsiveness to hypoglycemia. Alternatively, it is possible that the decrease in epinephrine responsiveness after antecedent hypoglycemia could be the result of reduced adrenal sympathetic nerve responsiveness.
Circulating Concentrations of the Adipocyte Protein Adiponectin Are Decreased in Parallel With Reduced Insulin Sensitivity During the Progression to Type 2 Diabetes in Rhesus Monkeys
Authors: Hotta Kikuko , Funahashi Tohru , Bodkin Noni L. , Ortmeyer Heidi K. , Arita Yukio , Hansen Barbara C. , Matsuzawa Yuji
Published: March 2007

Adiponectin is an adipose-specific plasma protein whose plasma concentrations are decreased in obese subjects and type 2 diabetic patients. This protein possesses putative antiatherogenic and anti-inflammatory properties. In the current study, we have analyzed the relationship between adiponectin and insulin resistance in rhesus monkeys (Macaca mulatta), which spontaneously develop obesity and which subsequently frequently progress to overt type 2 diabetes. The plasma levels of adiponectin were decreased in obese and diabetic monkeys as in humans. Prospective longitudinal studies revealed that the plasma levels of adiponectin declined at an early phase of obesity and remained decreased after the development of type 2 diabetes. Hyperinsulinemic-euglycemic clamp studies revealed that the obese monkeys with lower plasma adiponectin showed significantly lower insulin-stimulated peripheral glucose uptake (M rate). The plasma levels of adiponectin were significantly correlated to M rate (r = 0.66, P < 0.001). Longitudinally, the plasma adiponectin decreased in parallel to the progression of insulin resistance. No clear association was found between the plasma levels of adiponectin and its mRNA levels in adipose tissue. These results suggest that reduction in circulating adiponectin may be related to the development of insulin resistance.
Troglitazone Treatment Increases Plasma Vascular Endothelial Growth Factor in Diabetic Patients and Its mRNA in 3T3-L1 Adipocytes
Authors: Emoto Masahiro , Anno Takatoshi , Sato Yutaka , Tanabe Katsuya , Okuya Shigeru , Tanizawa Yukio , Matsutani Akira , Oka Yoshitomo
Published: March 2007

Troglitazone is one of the thiazolidinediones, a new class of oral antidiabetic compounds that are ligands of peroxisome proliferator–activated receptor-γ. This study on vascular endothelial growth factor (VEGF), also known as vascular permeability factor, was prompted by our clinical observation that the characteristics of troglitazone-induced edema were very similar to those caused by vascular hyperpermeability. When Japanese diabetic patients were screened for plasma VEGF, we found levels to be significantly (P < 0.001) increased in troglitazone-treated subjects (120.1 ± 135.0 pg/ml, n = 30) compared with those treated with diet alone (29.2 ± 36.1 pg/ml, n = 10), sulfonylurea (25.8 ± 22.2 pg/ml, n = 10), or insulin (24.6 ± 19.0 pg/ml, n = 10). Involvement of troglitazone in increased VEGF levels was further supported by the plasma VEGF levels in five patients before treatment (20.2 ± 7.0 pg/ml), after 3 months of troglitazone treatment (83.6 ± 65.9 pg/ml), and 3 months after discontinuation (28.0 ± 11.6 pg/ml). We further demonstrated that troglitazone, as well as rosiglitazone, at the plasma concentrations observed in patients, increased VEGF mRNA levels in 3T3-L1 adipocytes. VEGF is an angiogenic and mitogenic factor and is currently considered the most likely cause of neovascularization and hyperpermeability in diabetic proliferative retinopathy. Although increased VEGF may be beneficial for subjects with macroangiopathy and troglitazone is currently not available for clinical use, vascular complications, especially diabetic retinopathy, must be followed with great caution in subjects treated with thiazolidinediones.