bluuebunny/crossref_metadata_2025 huggingface dataset:
Hepatic Lipase mRNA, Protein, and Plasma Enzyme Activity Is Increased in the Insulin-Resistant, Fructose-Fed Syrian Golden Hamster and Is Partially Normalized by the Insulin Sensitizer Rosiglitazone
Authors: Lewis Gary F. , Murdoch Susan , Uffelman Kristine , Naples Mark , Szeto Linda , Albers Alegria , Adeli Khosrow , Brunzell John D.
Published: March 2007

Postheparin plasma hepatic lipase (HL) activity has been shown to correlate with features of the metabolic syndrome and type 2 diabetes in humans. We examined HL postheparin plasma enzyme activity, hepatocyte mRNA, and protein mass in the insulin-resistant, fructose-fed Syrian golden hamster, and the response of the insulin-sensitizing peroxisome proliferator–activated receptor-γ agonist rosiglitazone. Male Syrian golden hamsters were treated for 5 weeks with 1) normal diet (DIET group), 2) 60% fructose diet (FRUC group), or 3) 60% fructose and rosiglitazone (20 mmol · kg−1 · day−1) (FRUC+RSG group). Hepatocyte HL mRNA, protein mass, and postheparin plasma HL activity were increased in FRUC compared with DIET hamsters. FRUC+RSG hamsters had partial normalization of HL mRNA, mass, and activity. There was a shift in the size of LDL particles from large to small in FRUC animals and a shift back to large LDL size in FRUC+RSG. This is the first demonstration that HL hepatocyte mRNA, mass, and plasma enzymatic activity increase concomitantly with induction of an insulin-resistant state and can be partially normalized by treatment with an insulin sensitizer. The increase in HL in insulin-resistant states may play an important role in the typical dyslipidemia of these conditions, and reduction of HL could explain some of the beneficial effects of insulin sensitizers on the plasma lipid profile.
Leukocyte-Derived Myeloperoxidase Amplifies High-Glucose—Induced Endothelial Dysfunction Through Interaction With High-Glucose—Stimulated, Vascular Non—Leukocyte-Derived Reactive Oxygen Species
Authors: Zhang Chunxiang , Yang Jian , Jennings Lisa K.
Published: March 2007

Vascular non–leukocyte-derived reactive oxygen species (ROS), such as superoxide and hydrogen peroxide (H2O2), have emerged as important molecules in diabetic endothelial dysfunction. In addition, leukocyte-derived myeloperoxidase (MPO) has been implicated in vascular injury, and its injury response is H2O2 dependent. It is well known that MPO can use leukocyte-derived H2O2; however, it is unknown whether the vascular-bound MPO can use high-glucose–stimulated, vascular non–leukocyte-derived H2O2 to induce diabetic endothelial dysfunction. In the present study, we demonstrated that MPO activity is increased in vessels from diabetic rats. In high-glucose–incubated rat aortas and in carotid arteries from rats with acute hyperglycemia, vascular-bound MPO utilized high-glucose–stimulated H2O2 to amplify the ROS-induced impairment of endothelium-dependent relaxation via reduction of nitric oxide bioavailability. Hypochlorous acid (HOCL)-modified LDL, a specific biomarker for the MPO/HOCL/chlorinating species pathway, was detected in LDL- and MPO-bound vessels with high-glucose–stimulated H2O2. The results suggest that vascular-bound MPO could use high-glucose–stimulated H2O2 to amplify high-glucose–induced injury in the vascular wall. MPO/H2O2/HOCL/chlorinating species may represent an important pathway in diabetes complications and a new mechanism in phagocyte- and systemic infection–induced exacerbation of diabetic vascular diseases.
Genetics of the APM1 Locus and Its Contribution to Type 2 Diabetes Susceptibility in French Caucasians
Authors: Gibson Fernando , Froguel Philippe
Published: March 2007

We have carried out a detailed reexamination of the genetics of the APM1 locus and its contribution to the genetic basis of type 2 diabetes susceptibility in the French Caucasian population. The G allele of single nucleotide polymorphism −11426 in the APM1 promoter showed modest association with type 2 diabetes (odds ratio 1.44 [95% CI 1.04–1.98]; P = 0.03), providing corroborative evidence that single nucleotide polymorphisms in the APM1 promoter region contribute to the genetic risk of type 2 diabetes. A “sliding window” analysis identified haplotypes 1-1-1, 1-1-1-1, and 1-1-1-1-1 as being strongly protective against type 2 diabetes (P ≤ 0.0001). Evidence is presented that the APM1 gene is a locus of low linkage disequilibrium, high haplotype diversity, and high recombination. We were unable to obtain data to support the hypothesis that genetic variation in the APM1 gene is a major contributor to the type 2 diabetes linkage result at chromosome 3q27. Finally, in families with early-onset type 2 diabetes, we obtained suggestive evidence of a linkage peak for serum adiponectin levels (logarithm of odds = 2.1) that closely matched the position of the type 2 diabetes linkage peak. This result indicated that the type 2 diabetes susceptibility locus at 3q27 influences both genetic predisposition to type 2 diabetes and serum adiponectin levels in patients with type 2 diabetes.
Association of Protein Tyrosine Phosphatase 1B Gene Polymorphisms With Type 2 Diabetes
Authors: Bento Jennifer L. , Palmer Nicholette D. , Mychaleckyj Josyf C. , Lange Leslie A. , Langefeld Carl D. , Rich Stephen S. , Freedman Barry I. , Bowden Donald W.
Published: March 2007

The PTPN1 gene codes for protein tyrosine phosphatase 1B (PTP1B) (EC 3.1.3.48), which negatively regulates insulin signaling by dephosphorylating the phosphotyrosine residues of the insulin receptor kinase activation segment. PTPN1 is located in 20q13, a genomic region linked to type 2 diabetes in multiple genetic studies. Surveys of the gene have previously identified only a few uncommon coding single nucleotide polymorphisms (SNPs). We have carried out a detailed association analysis of 23 noncoding SNPs spanning the 161-kb genomic region, which includes the PTPN1 gene. These SNPs have been assessed for association with type 2 diabetes in two independently ascertained collections of Caucasian subjects with type 2 diabetes and two control groups. Association is observed between multiple SNPs and type 2 diabetes. The most consistent evidence for association occurred with SNPs spanning the 3′ end of intron 1 of PTPN1 through intron 8 (P values ranging from 0.043 to 0.004 in one case-control set and 0.038–0.002 in a second case-control set). Analysis of the combined case-control data increased the evidence of SNP association with type 2 diabetes (P = 0.005–0.0016). All of the associated SNPs lie in a single 100-kb haplotype block that encompasses the PTPN1 gene. Analysis of haplotypes indicates a significant difference between haplotype frequencies in type 2 diabetes case and control subjects (P = 0.0035–0.0056), with one common haplotype (36%) contributing strongly to the evidence for association with type 2 diabetes. Odds ratios calculated from single SNP or haplotype data are in the proximity of 1.3. Haplotype-based calculation of population-attributable risk (PAR) results in an estimated PAR of 17–20% based on different models and assumptions. These results suggest that PTPN1 is a significant contributor to type 2 diabetes susceptibility in the Caucasian population. This risk is likely due to noncoding polymorphisms.
Replication of an Association Between the Lymphoid Tyrosine Phosphatase Locus (LYP/PTPN22) With Type 1 Diabetes, and Evidence for Its Role as a General Autoimmunity Locus
Authors: Smyth Deborah , Cooper Jason D. , Collins Joanne E. , Heward Joanne M. , Franklyn Jayne A. , Howson Joanna M.M. , Vella Adrian , Nutland Sarah , Rance Helen E. , Maier Lisa , Barratt Bryan J. , Guja Cristian , Ionescu-Tı̂rgovişte Constantin , Savage David A. , Dunger David B. , Widmer Barry , Strachan David P. , Ring Susan M. , Walker Neil , Clayton David G. , Twells Rebecca C.J. , Gough Stephen C.L. , Todd John A.
Published: March 2007

In the genetic analysis of common, multifactorial diseases, such as type 1 diabetes, true positive irrefutable linkage and association results have been rare to date. Recently, it has been reported that a single nucleotide polymorphism (SNP), 1858C>T, in the gene PTPN22, encoding Arg620Trp in the lymphoid protein tyrosine phosphatase (LYP), which has been shown to be a negative regulator of T-cell activation, is associated with an increased risk of type 1 diabetes. Here, we have replicated these findings in 1,388 type 1 diabetic families and in a collection of 1,599 case and 1,718 control subjects, confirming the association of the PTPN22 locus with type 1 diabetes (family-based relative risk (RR) 1.67 [95% CI 1.46–1.91], and case-control odds ratio (OR) 1.78 [95% CI 1.54–2.06]; overall P = 6.02 × 10−27). We also report evidence for an association of Trp620 with another autoimmune disorder, Graves’ disease, in 1,734 case and control subjects (P = 6.24 × 10−4; OR 1.43 [95% CI 1.17–1.76]). Taken together, these results indicate a more general association of the PTPN22 locus with autoimmune disease.
Alterations in Postprandial Hepatic Glycogen Metabolism in Type 2 Diabetes
Authors: Krssak Martin , Brehm Attila , Bernroider Elisabeth , Anderwald Christian , Nowotny Peter , Man Chiara Dalla , Cobelli Claudio , Cline Gary W. , Shulman Gerald I. , Waldhäusl Werner , Roden Michael
Published: March 2007

Decreased skeletal muscle glucose disposal and increased endogenous glucose production (EGP) contribute to postprandial hyperglycemia in type 2 diabetes, but the contribution of hepatic glycogen metabolism remains uncertain. Hepatic glycogen metabolism and EGP were monitored in type 2 diabetic patients and nondiabetic volunteer control subjects (CON) after mixed meal ingestion and during hyperglycemic-hyperinsulinemic-somatostatin clamps applying 13C nuclear magnetic resonance spectroscopy (NMRS) and variable infusion dual-tracer technique. Hepatocellular lipid (HCL) content was quantified by 1H NMRS. Before dinner, hepatic glycogen was lower in type 2 diabetic patients (227 ± 6 vs. CON: 275 ± 10 mmol/l liver, P < 0.001). After meal ingestion, net synthetic rates were 0.76 ± 0.16 (type 2 diabetic patients) and 1.36 ± 0.15 mg · kg−1 · min−1 (CON, P < 0.02), resulting in peak concentrations of 283 ± 15 and 360 ± 11 mmol/l liver. Postprandial rates of EGP were ∼0.3 mg · kg−1 · min−1 (30–170 min; P < 0.05 vs. CON) higher in type 2 diabetic patients. Under clamp conditions, type 2 diabetic patients featured ∼54% lower (P < 0.03) net hepatic glycogen synthesis and ∼0.5 mg · kg−1 · min−1 higher (P < 0.02) EGP. Hepatic glucose storage negatively correlated with HCL content (R = −0.602, P < 0.05). Type 2 diabetic patients exhibit 1) reduction of postprandial hepatic glycogen synthesis, 2) temporarily impaired suppression of EGP, and 3) no normalization of these defects by controlled hyperglycemic hyperinsulinemia. Thus, impaired insulin sensitivity and/or chronic glucolipotoxicity in addition to the effects of an altered insulin-to-glucagon ratio or increased free fatty acids accounts for defective hepatic glycogen metabolism in type 2 diabetic patients.
Hepatic PTP-1B Expression Regulates the Assembly and Secretion of Apolipoprotein B–Containing Lipoproteins
Authors: Qiu Wei , Avramoglu Rita Kohen , Dubé Nadia , Chong Taryne M. , Naples Mark , Au Crystal , Sidiropoulos Konstantinos G. , Lewis Gary F. , Cohn Jeffrey S. , Tremblay Michel L. , Adeli Khosrow
Published: March 2007

Protein tyrosine phosphatase-1B (PTP-1B) plays an important role in regulation of insulin signal transduction, and modulation of PTP-1B expression seems to have a profound effect on insulin sensitivity and diet-induced weight gain. The molecular link between PTP-1B expression and metabolic dyslipidemia, a major complication of insulin resistance, was investigated in the present study using PTP-1B knockout mice as well as overexpression and suppression of PTP-1B. Chronic fructose feeding resulted in a significant increase in plasma VLDL in wild-type mice but not in PTP-1B knockout mice. Lipoprotein profile analysis of plasma from PTP-1B knockout mice revealed a significant reduction in apolipoprotein B (apoB100) lipoproteins, associated with reduced hepatic apoB100 secretion from isolated primary hepatocytes. In addition, treatment of cultured hepatoma cells with PTP-1B siRNA reduced PTP-1B mass by an average of 41% and was associated with a 53% decrease in secretion of metabolically labeled apoB100. Conversely, adenoviral-mediated overexpression of PTP-1B in HepG2 cells downregulated the phosphorylation of insulin receptor and insulin receptor substrate-1 and caused increases in cellular and secreted apoB100 as a result of increased intracellular apoB100 stability. Collectively, these findings suggest that PTP-1B expression level is a key determinant of hepatic lipoprotein secretion, and its overexpression in the liver can be sufficient to induce VLDL overproduction and the transition to a metabolic dyslipidemic state.
LRb-STAT3 Signaling Is Required for the Neuroendocrine Regulation of Energy Expenditure by Leptin
Authors: Bates Sarah H. , Dundon Trevor A. , Seifert Matthew , Carlson Michael , Maratos-Flier Eleftheria , Myers Martin G.
Published: March 2007

Secretion of leptin from adipose tissue communicates body energy status to the neuroendocrine system by activating the long form of the leptin receptor (LRb). Lack of leptin or LRb (as in db/db mice) results in obesity that stems from the combined effects of hyperphagia and decreased energy expenditure. We have previously generated mice in which LRb is replaced with a mutant LRb (LRbS1138) that specifically disrupts LRb→STAT3 (signal transducer and activator of transcription-3) signaling; mice homozygous for this mutant (s/s) display increased feeding and are obese. We have now examined energy expenditure in s/s and db/db mice. Consistent with the increased lean body mass of s/s animals, locomotor activity and acute cold tolerance (partly a measure of shivering thermogenesis) in s/s mice were modestly but significantly improved compared with db/db mice, although they were decreased compared with wild-type mice. Total and resting metabolic rates were similarly depressed in s/s and db/db mice, however. Indeed, s/s and db/db mice display similar reductions in thyroid function and brown adipose tissue expression of uncoupling protein-1, which is regulated by sympathetic nervous system (SNS) tone. Thus, the LRb→STAT3 signal is central to both the control of energy expenditure by leptin and the neuroendocrine regulation of the SNS and the thyroid axis.
Fibroblast Growth Factor 1
Authors: Hutley Louise , Shurety Wenda , Newell Felicity , McGeary Ross , Pelton Nicole , Grant Jennifer , Herington Adrian , Cameron Donald , Whitehead Jon , Prins Johannes
Published: March 2007

Obesity, with its related problems, is recognized as the fastest growing disease epidemic facing the world, yet we still have limited insight into the regulation of adipose tissue mass in humans. We have previously shown that adipose-derived microvascular endothelial cells (MVECs) secrete a factor(s) that increases proliferation of human preadipocytes. We now demonstrate that coculture of human preadipocytes with MVECs significantly increases preadipocyte differentiation, evidenced by dramatically increased triacylglycerol accumulation and glycerol-3-phosphate dehydrogenase activity compared with controls. Subsequent analysis identified fibroblast growth factor (FGF)-1 as an adipogenic factor produced by MVECs. Expression of FGF-1 was demonstrated in MVECs but not in preadipocytes, while preadipocytes were shown to express FGF receptors 1–4. The proliferative effect of MVECs on human preadipocytes was blocked using a neutralizing antibody specific for FGF-1. Pharmacological inhibition of FGF-1 signaling at multiple steps inhibits preadipocyte replication and differentiation, supporting the key adipogenic role of FGF-1. We also show that 3T3-L1 cells, a highly efficient murine model of adipogenesis, express FGF-1 and, unlike human preadipocytes, display no increased differentiation potential in response to exogenous FGF-1. Conversely, FGF-1–treated human preadipocytes proliferate rapidly and differentiate with high efficiency in a manner characteristic of 3T3-L1 cells. We therefore suggest that FGF-1 is a key human adipogenic factor, and these data expand our understanding of human fat tissue growth and have significant potential for development of novel therapeutic strategies in the prevention and management of human obesity.
Endogenous Ghrelin in Pancreatic Islets Restricts Insulin Release by Attenuating Ca2+ Signaling in β-Cells
Authors: Dezaki Katsuya , Hosoda Hiroshi , Kakei Masafumi , Hashiguchi Suzuko , Watanabe Masatomo , Kangawa Kenji , Yada Toshihiko
Published: March 2007

Ghrelin, isolated from the human and rat stomach, is the endogenous ligand for the growth hormone (GH) secretagogue receptor, which is expressed in a variety of tissues, including the pancreatic islets. It has been shown that low plasma ghrelin levels correlates with elevated fasting insulin levels and type 2 diabetes. Here we show a physiological role of endogenous ghrelin in the regulation of insulin release and blood glucose in rodents. Acylated ghrelin, the active form of the peptide, was detected in the pancreatic islets. Counteraction of endogenous ghrelin by intraperitoneal injection of specific GH secretagogue receptor antagonists markedly lowered fasting glucose concentrations, attenuated plasma glucose elevation, and enhanced insulin responses during the glucose tolerance test (GTT). Conversely, intraperitoneal exogenous ghrelin GH-independently elevated fasting glucose concentrations, enhanced plasma glucose elevation, and attenuated insulin responses during GTT. Neither GH secretagogue receptor antagonist nor ghrelin affected the profiles of the insulin tolerance test. In isolated islets, GH secretagogue receptor blockade and antiserum against acylated ghrelin markedly enhanced glucose-induced increases in insulin release and intracellular Ca2+ concentration ([Ca2+]i), whereas ghrelin at a relatively high concentration (10 nmol/l) suppressed insulin release. In single β-cells, ghrelin attenuated glucose-induced first-phase and oscillatory [Ca2+]i increases via the GH secretagogue receptor and in a pertussis toxin-sensitive manner. Ghrelin also increased tetraethylammonium-sensitive delayed outward K+ currents in single β-cells. These findings reveal that endogenous ghrelin in islets acts on β-cells to restrict glucose-induced insulin release at least partly via attenuation of Ca2+ signaling, and that this insulinostatic action may be implicated in the upward control of blood glucose. This function of ghrelin, together with inducing GH release and feeding, suggests that ghrelin underlies the integrative regulation of energy homeostasis.
Diet-Induced Glucose Intolerance in Mice With Decreased β-Cell ATP-Sensitive K+ Channels
Authors: Remedi Maria S. , Koster Joseph. C. , Markova Kamelia , Seino Susumu , Miki Takashi , Patton Brian L. , McDaniel Michael L. , Nichols Colin G.
Published: March 2007

ATP-sensitive K+ channels (KATP channels) control electrical activity in β-cells and therefore are key players in excitation-secretion coupling. Partial suppression of β-cell KATP channels in transgenic (AAA) mice causes hypersecretion of insulin and enhanced glucose tolerance, whereas complete suppression of these channels in Kir6.2 knockout (KO) mice leads to hyperexcitability, but mild glucose intolerance. To test the interplay of hyperexcitability and dietary stress, we subjected AAA and KO mice to a high-fat diet. After 3 months on the diet, both AAA and KO mice converted to an undersecreting and markedly glucose-intolerant phenotype. Although Kir6.2 is expressed in multiple tissues, its primary functional consequence in both AAA and KO mice is enhanced β-cell electrical activity. The results of our study provide evidence that, when combined with dietary stress, this hyperexcitability is a causal diabetic factor. We propose an “inverse U” model for the response to enhanced β-cell excitability: the expected initial hypersecretion can progress to undersecretion and glucose-intolerance, either spontaneously or in response to dietary stress.
High Glucose Is Necessary for Complete Maturation of Pdx1-VP16–Expressing Hepatic Cells into Functional Insulin-Producing Cells
Authors: Cao Li-Zhen , Tang Dong-Qi , Horb Marko E. , Li Shi-Wu , Yang Li-Jun
Published: March 2007

Pdx1 has been shown to convert hepatocytes into both exocrine and endocrine pancreatic cells in mice, but it fails to selectively convert hepatocytes into pure insulin-producing cells (IPCs). The molecular mechanisms underlying the transdifferentiation remain unclear. In this study, we generated a stably transfected rat hepatic cell line named WB-1 that expresses an active form of Pdx1 along with a reporter gene, RIP-eGFP. Our results demonstrate that Pdx1 induces the expression of multiple genes related to endocrine pancreas development and islet function in these liver cells. We do not however find any expression of the late-stage genes (Pax4, Pax6, Isl-1, and MafA) related to β-cell development, and the cells do not secrete insulin upon the glucose challenge. Yet when WB-1 cells are transplanted into diabetic NOD-scid mice, these genes become activated and hyperglycemia is completely reversed. Detailed comparison of gene expression profiles between pre- and posttransplanted WB-1 cells demonstrates that the WB-1 cells have similar properties as that seen in pancreatic β-cells. In addition, in vitro culture in high-glucose medium is sufficient to induce complete maturation of WB-1 cells into functional IPCs. In summary, we find that Pdx1-VP16 is able to selectively convert hepatic cells into pancreatic endocrine precursor cells. However, complete transdifferentiation into functional IPCs requires additional external factors, including high glucose or hyperglycemia. Thus, transdifferentiation of hepatocytes into functional IPCs may serve as a viable therapeutic option for patients with type 1 diabetes.
Insulin Stimulates and Diabetes Inhibits O-Linked N-Acetylglucosamine Transferase and O-Glycosylation of Sp1
Authors: Majumdar Gipsy , Wright Jeremiah , Markowitz Paul , Martinez-Hernandez Antonio , Raghow Rajendra , Solomon Solomon S.
Published: March 2007

Insulin stimulates both the biosynthesis of transcription factor Sp1 and its O-linked N-acetylglucosaminylation (O-GlcNAcylation), which promotes nuclear localization of Sp1 and its ability to transactivate calmodulin (CaM) gene transcription. To investigate this further, we incubated H-411E liver cells with insulin (10,000 μU/ml) and quantified the subcellular distribution of O-GlcNAc transferase (OGT) and O-GlcNAc–modified Sp1. We also examined the phosphorylation of Sp1 using both Western blot and incorporation of 32P into Sp1. The results demonstrate that insulin, but not glucagon, stimulates OGT synthesis and enhances cytosolic staining of OGT (histochemical). Insulin increases O-GlcNAc-Sp1, which peaks at 30 min, followed by decline at 4 h. In contrast, insulin initiates phosphorylation of Sp1 early, followed by a continued increase in phosphorylated Sp1 (PO4-Sp1) at 4 h. A reciprocal relationship between O-GlcNAc-Sp1 and PO4-Sp1 was observed. To explore the pathophysiological relevance, we localized OGT in liver sections from streptozotocin (STZ)-induced diabetic rats. We observed that staining of OGT in STZ-induced diabetic rat liver is clearly diminished, but it was substantially restored after 6 days of insulin treatment. We conclude that insulin stimulates CaM gene transcription via a dynamic interplay between O-glycosylation and phosphorylation of Sp1 that modulates stability, mobility, subcellular compartmentalization, and activity.
Effect of Long-Term Administration of α-Lipoic Acid on Retinal Capillary Cell Death and the Development of Retinopathy in Diabetic Rats
Authors: Kowluru Renu A. , Odenbach Sarah
Published: March 2007

Oxidative stress is increased in the retina in diabetes, and it is considered to play an important role in the development of retinopathy. α-Lipoic acid, a thiol antioxidant, has been shown to have beneficial effects on polyneuropathy and on the parameters of oxidative stress in various tissues, including nerve, kidney, and retina. The purpose of this study was to examine the effect of α-lipoic acid on retinal capillary cell apoptosis and the development of pathology in diabetes. Retina was used from streptozotocin-induced diabetic rats receiving diets supplemented with or without α-lipoic acid (400 mg/kg) for 11 months of diabetes. Capillary cell apoptosis (by terminal transferase-mediated dUTP nick-end labeling) and formation of acellular capillaries were investigated in the trypsin-digested retinal microvessels. The effect of α-lipoic acid administration on retinal 8-hydroxy-2′deoxyguanosine (8-OHdG) and nitrotyrosine levels was determined by enzyme-linked immunosorbent assay. α-Lipoic acid administration for the entire duration of diabetes inhibited capillary cell apoptosis and the number of acellular capillaries in the retina, despite similar severity of hyperglycemia in the two diabetic groups (with and without α-lipoic acid). Retinal 8-OHdG and nitrotyrosine levels were increased by over twofold and 70%, respectively, in diabetes, and α-lipoic acid administration inhibited these increases. Our results demonstrate that the long-term administration of α-lipoic acid has beneficial effects on the development of diabetic retinopathy via inhibition of accumulation of oxidatively modified DNA and nitrotyrosine in the retina. α-Lipoic acid supplementation represents an achievable adjunct therapy to help prevent vision loss in diabetic patients.
Metformin Prevents the Development of Acute Lipid-Induced Insulin Resistance in the Rat Through Altered Hepatic Signaling Mechanisms
Authors: Cleasby Mark E. , Dzamko Nicolas , Hegarty Bronwyn D. , Cooney Gregory J. , Kraegen Edward W. , Ye Ji-Ming
Published: March 2007

Metformin reduces the incidence of progression to type 2 diabetes in humans with obesity or impaired glucose tolerance. We used an animal model to investigate whether metformin could prevent acute lipid-induced insulin resistance and the mechanisms involved. Metformin or vehicle was administered to rats daily for 1 week. Rats were studied basally, after 3.75 h of intralipid-heparin or glycerol infusion, or after 5 h of infusion with a hyperinsulinemic-euglycemic clamp between 3 and 5 h. Metformin had no effect on plasma triacylglycerol or nonesterified fatty acid concentrations and did not alter glucose turnover or gluconeogenic enzyme mRNA after lipid infusion. However, metformin normalized hepatic glucose output and increased liver glycogen during lipid infusion and clamp. Basal liver (but not muscle or fat) AMP-activated protein kinase activity was increased by metformin (by 310%; P < 0.01), associated with increased phosphorylation of acetyl CoA carboxylase. Postclamp liver but not muscle phosphorylated/total Akt protein was increased, whereas basal c-Jun NH2-terminal kinase-1 and -2 protein expression were reduced (by 39 and 53%, respectively; P < 0.05). Metformin also increased hepatic basal IκBα levels (by 260%; P < 0.001) but had no effect on tyrosine phosphorylation or expression of insulin receptor substrate-1 (IRS-1). In summary, metformin opposes the development of acute lipid-induced insulin resistance in the liver through alterations in multiple signaling pathways.
Leptin Impairs Insulin Signaling in Rat Adipocytes
Authors: Pérez Coralia , Fernández-Galaz Carmen , Fernández-Agulló Teresa , Arribas Carmen , Andrés Antonio , Ros Manuel , Carrascosa José M.
Published: March 2007

Leptin modulates glucose homeostasis by acting as an insulin-sensitizing factor in most insulin target tissues. Nevertheless, insulin-dependent glucose uptake in white adipose tissue decreases after in vivo treatment with leptin. Moreover, elevated leptin concentrations inhibit insulin metabolic effects in adipocytes. Here we studied both, direct and centrally mediated effects of leptin on insulin signaling in rat adipocytes. Adipocyte incubation with low leptin concentrations did not modify the insulin stimulation of mitogen-activated protein kinase (MAPK). However, at elevated concentrations, leptin impaired insulin-stimulated MAPK activity, glycogen synthase kinase (GSK)3β phosphorylation, and insulin receptor tyrosine phosphorylation without altering vanadate stimulation. An increase of suppressor of cytokine signaling-3 protein was also observed. Central administration of leptin decreased insulin effects on adipocyte MAPK and GSK3β phosphorylation. In insulin-resistant aged rats with hyperleptinemia and central leptin resistance, insulin poorly stimulated MAPK and central leptin infusion did not further deteriorate adipocyte insulin responsiveness. Food restriction increased MAPK stimulation by insulin and restored the ability of centrally infused leptin to attenuate adipocyte insulin signaling in aged rats. We conclude that leptin can modulate, in an inhibitory manner, adipocyte insulin signaling by two different ways: as an autocrine signal and, indirectly, through neuroendocrine pathways. These mechanisms may be of relevance in situations of hyperleptinemia, such as aging and/or obesity.
Exercise Induces Hypoglycemia in Rats With Islet Transplantation
Authors: Omer Abdulkadir , Duvivier-Kali Valérie F. , Aschenbach William , Tchipashvili Vaja , Goodyear Laurie J. , Weir Gordon C.
Published: March 2007

Recently, islet transplantation in patients with type 1 diabetes has had greater success than in the past, but the important question of whether the kinetics of islet secretion are able to accommodate the metabolic demands of special conditions such as exercise remains unanswered. Syngeneic rat islets (4,000 islet equivalents/rat) were transplanted into the liver, kidney, and peritoneal cavity (encapsulated or nonencapsulated) of rats with streptozocin-induced diabetes. Normoglycemic transplanted rats and age-matched controls were subjected to 30 min of moderate exercise on a treadmill 5 weeks after transplantation. Although control rats maintained near normoglycemia during and after exercise, the rats with islet transplants had significantly lower blood glucose levels. For the rats with islets in the liver, increased C-peptide levels were found at 30 min (790 ± 125 and 1,450 ± 250 pmol/l at 0 and 30 min, respectively; P < 0.01), whereas a decrease was found in controls and in rats with islets transplanted into the peritoneal cavity or under the kidney capsule. Moreover, increased glucagon levels were found after exercise in the rats with islets transplanted into the liver (62 ± 6, 165 ± 29, 155 ± 27, and 97 ± 13 pg/ml at 0, 30, 60, and 90 min, respectively; P < 0.05), whereas no changes in glucagon levels were observed in controls. In conclusion, moderate exercise caused hypoglycemia in rats with islet transplants in different sites including liver, kidney, and peritoneal cavity. C-peptide and glucagon responses to exercise were very different in rats with transplanted islets compared with controls. This islet dysfunction led to exercise-induced hypoglycemia.
Gene Expression Profiles Define a Key Checkpoint for Type 1 Diabetes in NOD Mice
Authors: Eckenrode Sarah E. , Ruan Qingguo , Yang Ping , Zheng Weipeng , McIndoe Richard A. , She Jin-Xiong
Published: March 2007

cDNA microarrays with >11,000 cDNA clones from an NOD spleen cDNA library were used to identify temporal gene expression changes in NOD mice (1–10 weeks), which spontaneously develop type 1 diabetes, and changes between NOD and NOD congenic mice (NOD.Idd3/Idd10 and NOD.B10Sn-H2b), which have near zero incidence of insulitis and diabetes. The expression profiles identified two distinct groups of mice corresponding to an immature (1–4 weeks) and mature (6–10 weeks) state. The rapid switch of gene expression occurring around 5 weeks of age defines a key immunological checkpoint. Sixty-two known genes are upregulated, and 18 are downregulated at this checkpoint in the NOD. The expression profiles are consistent with increased antibody production, antigen presentation, and cell proliferation associated with an active autoimmune response. Seven of these genes map to confirmed diabetes susceptibility regions. Of these seven, three are excellent candidate genes not previously implicated in type 1 diabetes. Ten genes are differentially expressed between the NOD and congenic NOD at the immature stage (Hspa8, Hif1a, and several involved in cellular functions), while the other 70 genes exhibit expression differences during the mature (6−10 week) stage, suggesting that the expression differences of a small number of genes before onset of insulitis determine the disease progression.
Mixed Hematopoietic Chimerism Allows Cure of Autoimmune Diabetes Through Allogeneic Tolerance and Reversal of Autoimmunity
Authors: Nikolic Boris , Takeuchi Yasuo , Leykin Igor , Fudaba Yasuhiro , Smith R. Neal , Sykes Megan
Published: March 2007

Bone marrow transplantation from diabetes-resistant strains with complete replacement of the recipient immune system by the allogeneic donor has led to tolerance to donor islets and cure of diabetes in a mouse model of type 1 diabetes. However, the ability to tolerize host T-cells of diabetic NOD mice is unknown. We demonstrate that nonmyeloablative conditioning achieves mixed hematopoietic chimerism across major histocompatibility complex (MHC) barriers in spontaneously diabetic NOD mice. This conditioning preserves alloreactive and autoreactive diabetogenic host NOD T-cells, but when mixed chimerism was established, diabetic NOD mice accepted donor-type allogeneic islet grafts and were cured of diabetes, despite a significant recipient T-cell contribution. Furthermore, induction of mixed chimerism permitted acceptance of NOD islet grafts, demonstrating reversal of autoimmunity. Allogeneic bone marrow transplantation was critical for tolerization of diabetogenic and alloreactive host T-cells. Thus, mixed hematopoietic chimerism induces tolerance to donor islets and reverses established autoimmunity in diabetic NOD mice.
Reduction in Glucagon Receptor Expression by an Antisense Oligonucleotide Ameliorates Diabetic Syndrome indb/dbMice
Authors: Liang Yin , Osborne Melville C. , Monia Brett P. , Bhanot Sanjay , Gaarde William A. , Reed Chantal , She Pengxiang , Jetton Thomas L. , Demarest Keith T.
Published: March 2007

Excess glucagon levels contribute to the hyperglycemia associated with type 2 diabetes. Reducing glucagon receptor expression may thus ameliorate the consequences of hyperglucagonemia and improve blood glucose control in diabetic patients. This study describes the antidiabetic effects of a specific glucagon receptor antisense oligonucleotide (GR-ASO) in db/db mice. The ability of GR-ASOs to inhibit glucagon receptor mRNA expression was demonstrated in primary mouse hepatocytes by quantitative real-time RT-PCR. Intraperitoneal administration of GR-ASO at a dosage of 25 mg/kg twice a week in db/db mice for 3 weeks resulted in 1) decreased glucagon receptor mRNA expression in liver; 2) decreased glucagon-stimulated cAMP production in hepatocytes isolated from GR-ASO–treated db/db mice; 3) significantly reduced blood levels of glucose, triglyceride, and free fatty acids; 4) improved glucose tolerance; and 5) a diminished hyperglycemic response to glucagon challenge. Neither lean nor db/db mice treated with GR-ASO exhibited hypoglycemia. Suppression of GR expression was also associated with increased (∼10-fold) levels of plasma glucagon. No changes were observed in pancreatic islet cytoarchitecture, islet size, or α-cell number. However, α-cell glucagon levels were increased significantly. Our studies support the concept that antagonism of glucagon receptors could be an effective approach for controlling blood glucose in diabetes.
Examination of Candidate Chromosomal Regions for Type 2 Diabetes Reveals a Susceptibility Locus on Human Chromosome 8p23.1
Authors: Pezzolesi Marcus G. , Nam Moonsuk , Nagase Terumasa , Klupa Tomasz , Dunn Jonathon S. , Mlynarski Wojciech M. , Rich Stephen S. , Warram James H. , Krolewski Andrzej S.
Published: March 2007

In a panel of large Caucasian pedigrees, we genotyped markers in eight chromosomal regions previously reported as supporting linkage with type 2 diabetes. We previously reported significant linkage on chromosome 20q (maximum logarithm of odds score [MLS] = 2.79) in this panel. In the present analysis, candidate regions on 1q, 2q, 3q, 5q, 9q, and 10q yielded little evidence for linkage; a region on 2p (MLS = 1.64, P = 0.01 at position 9.0 cM) gave suggestive evidence of linkage; and a region on 8p (MLS = 3.67, P = 2.8 × 10−5, at position 7.6 cM) gave significant evidence of linkage. Conditional analyses were performed for both 2p and 8p regions and the region reported on 20q. The MLS for 2p increased from 1.64 to 1.79 (empirical P = 0.142) when conditioned for heterogeneity on 20q. The case was similar for 8p, where the MLS increased from 3.67 to 4.51 (empirical P = 0.023) when conditioned on families without evidence of linkage at 20q. In conclusion, our data support a type 2 diabetes susceptibility locus on chromosome 8p that appears to be independent from other susceptibility loci. Although we were able to replicate linkage in our pedigrees on chromosome 2p, we did not find evidence of linkage for regions on 1q, 2q, 3q, 5q, 9q, or 10q.
Leptin Effects on Pancreatic β-Cell Gene Expression and Function
Authors: Seufert Jochen
Published: March 2007

The hormone leptin is secreted from white adipocytes, and serum levels of leptin correlate with adipose tissue mass. Leptin was first described to act on the satiety center in the hypothalamus through specific receptors (leptin receptor [ObR]) to restrict food intake and enhance energy expenditure. Important peripheral actions of leptin involve inhibition of insulin biosynthesis and secretion in pancreatic β-cells. In turn, insulin stimulates leptin secretion from adipose tissue, establishing a hormonal regulatory feedback loop—the so-called “adipo-insular axis.” Multiple signal transduction pathways are involved in leptin signaling in pancreatic β-cells. We have identified the proinsulin gene and protein phosphatase 1 gene as leptin repressed genes and the gene for the suppressor of cytokine signaling 3 protein as a leptin-induced gene in pancreatic β-cells. The molecular effects of leptin culminate to restrict insulin secretion and biosynthesis to adapt glucose homeostasis to the amount of body fat. In most overweight individuals, however, physiological regulation of body weight by leptin seems to be disturbed, representing “leptin resistance.” This leptin resistance at the level of the pancreatic β-cell may contribute to dysregulation of the adipo-insular axis and promote the development of hyperinsulinemia and manifest type 2 diabetes in overweight patients.
Acute ω-3 Fatty Acid Enrichment Selectively Reverses High-Saturated Fat Feeding-Induced Insulin Hypersecretion But Does Not Improve Peripheral Insulin Resistance
Authors: Holness Mark J. , Smith Nicholas D. , Greenwood Gemma K. , Sugden Mary C.
Published: March 2007

In rats fed a high-saturated fat diet, replacement of a small percentage of total fatty acids with long-chain ω-3 fatty acids from fish oil for the duration of high-fat feeding prevents the development of insulin resistance. We investigated the effect of acute (24-h) modulation of dietary fat composition on glucose-stimulated insulin secretion (GSIS) in rats made insulin resistant by high-saturated fat feeding for 4 weeks. Insulin secretion after an intravenous glucose challenge was greatly increased by high-saturated fat feeding. Glucose tolerance was minimally perturbed, demonstrating insulin hypersecretion compensated for insulin resistance. The effect of high-saturated fat feeding to enhance GSIS was retained in perifused islets, such that glucose stimulus-secretion coupling was potentiated. Acute replacement of 7% of dietary fatty acids with long-chain ω-3 fatty acids reversed insulin hypersecretion in vivo, and the effect of long-term high-saturated fat feeding to enhance insulin secretion by perifused islets was also completely reversed. Although a hyperbolic relationship existed between insulin secretion and action in the high-saturated fat and control groups, lowered insulin secretion in the acute fish oil-supplemented group was not accompanied by improved insulin action, and glucose tolerance was adversely affected. Our studies are important because they demonstrate that hyperinsulinemia can be rapidly reversed via the dietary provision of small amounts of long-chain ω-3 fatty acids. However, this “insulin sparing” action of acute dietary long-chain ω-3 fatty acids occurs in the absence of an acute improvement in insulin sensitivity and therefore at the expense of maintenance of glucose tolerance.
Islet Complex Lipids
Authors: Ramanadham Sasanka , Song Haowei , Bao Shunzhong , Hsu Fong-Fu , Zhang Sheng , Ma Zhongmin , Jin Chun , Turk John
Published: March 2007

The β-isoform of group VIA calcium-independent phospholipase A2 (iPLA2β) does not require calcium for activation, is stimulated by ATP, and is sensitive to inhibition by a bromoenol lactone suicide substrate. Several potential functions have been proposed for iPLA2β. Our studies indicate that iPLA2β is expressed in β-cells and participates in glucose-stimulated insulin secretion but is not involved in membrane phospholipid remodeling. If iPLA2β plays a signaling role in glucose-stimulated insulin secretion, then conditions that impair iPLA2β functions might contribute to the diminished capacity of β-cells to secrete insulin in response to glucose, which is a prominent characteristic of type 2 diabetes. Our recent studies suggest that iPLA2β might also participate in β-cell proliferation and apoptosis and that various phospholipid-derived mediators are involved in these processes. Detailed characterization of the iPLA2β protein level reveals that β-cells express multiple isoforms of the enzyme, and our studies involve the hypothesis that different isoforms have different functions.
Putative Metabolic Effects of the Liver X Receptor (LXR)
Authors: Steffensen Knut R. , Gustafsson Jan-Åke
Published: March 2007

The nuclear receptors liver X receptor (LXR)α and LXRβ are sensors of cholesterol metabolism and lipid biosynthesis. They have recently been found to be regulators of inflammatory cytokines, suppressors of hepatic glucose production, and involved in different cell-signaling pathways. LXRα is a target gene of the peroxisome proliferator-activated receptor-γ, a target of drugs used in treating elevated levels of glucose seen in diabetes. Furthermore, insulin induces LXRα in hepatocytes, resulting in increased expression of lipogenic enzymes and suppression of key enzymes in gluconeogenesis, including PEPCK. LXR seems to have an important role in the regulation of glucocorticoid action and a role in the overall energy homeostasis suggested by its putative regulatory effect on leptin and uncoupling protein 1. The physiological roles of LXR indicate that it is an interesting potential target for drug treatment of diabetes.
Aged Transgenic Mice With Increased Glucocorticoid Sensitivity in Pancreatic β-Cells Develop Diabetes
Authors: Davani Behrous , Portwood Neil , Bryzgalova Galina , Reimer Martina Kvist , Heiden Thomas , Östenson Claes-Göran , Okret Sam , Ahren Bo , Efendic Suad , Khan Akhtar
Published: March 2007

Glucocorticoids are diabetogenic hormones because they decrease glucose uptake, increase hepatic glucose production, and inhibit insulin release. To study the long-term effects of increased glucocorticoid sensitivity in β-cells, we studied transgenic mice overexpressing the rat glucocorticoid receptor targeted to the β-cells using the rat insulin I promoter. Here we report that these mice developed hyperglycemia both in the fed and the overnight-fasted states at 12–15 months of age. Progression from impaired glucose tolerance, previously observed in the same colony at the age of 3 months, to manifest diabetes was not associated with morphological changes or increased apoptosis in the β-cells. Instead, our current results suggest that the development of diabetes is due to augmented inhibition of insulin secretion through α2-adrenergic receptors (α2-ARs). Thus, we found a significantly higher density of α2-ARs in the islets of transgenic mice compared with controls, based on binding studies with the α2-AR agonist UK 14304. Furthermore, incubation of islets with benextramine, a selective antagonist of the α2-AR, restored insulin secretion in response to glucose in isolated islets from transgenic mice, whereas it had no effect on control islets. These results indicate that the chronic enhancement of glucocorticoid signaling in pancreatic β-cells results in hyperglycemia and impaired glucose tolerance. This effect may involve signaling pathways that participate in the regulation of insulin secretion via the α2-AR.
Nutrients Induce Different Ca2+ Signals in Cytosol and Nucleus in Pancreatic β-Cells
Authors: Quesada Ivan , Martin Franz , Roche Enrique , Soria Bernat
Published: March 2007

Specific activation of Ca2+-dependent functions is achieved by the particular dynamics and local restriction of Ca2+ signals. It has been shown that changes in amplitude, duration, or frequency of Ca2+ signals modulate gene transcription. Thus, Ca2+ variations should be finely controlled within the nucleus. Although a variety of mechanisms in the nuclear membrane have been demonstrated to regulate nuclear Ca2+, the existence of an autonomous Ca2+ homeostasis within the nucleus is still questioned. In the pancreatic β-cell, besides their effect on insulin secretion, Ca2+ messages generated by nutrients also exert their action on gene expression. However, the dynamics of these Ca2+ signals in relation to nuclear function have been explored little in islet cells. In the current study, Ca2+ changes both in the nucleoplasm and in the cytosol of INS-1 and pancreatic β-cells were monitored using spot confocal microscopy. We show that nutrients trigger Ca2+ signals of higher amplitude in the nucleus than in the cytosol. These amplitude-modulated Ca2+ signals transmitted to the nucleus might play an important role in the control of gene expression in the pancreatic β-cell.
Systemic and Forearm Triglyceride Metabolism
Authors: Miles John M. , Park Yongsoon S. , Walewicz Dorota , Russell-Lopez Crystal , Windsor Sheryl , Isley William L. , Coppack Simon W. , Harris William S.
Published: March 2007

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with 3H on both the glycerol backbone and the fatty acid portion of the molecule; 14C glycerol and 14C oleate were coinfused to quantify the systemic and forearm release of 3H glycerol and 3H oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 ± 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 ± 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 ± 4 and 59 ± 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 ± 2 vs. 36 ± 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.