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Gao J, Li H, Xu H, Liu Y, Cai M, Shi Y, Zhang J, Wang H. High glucose-induced glucagon resistance and membrane distribution of GCGR revealed by super-resolution imaging. iScience 2023; 26:105967. [PMID: 36824278 PMCID: PMC9941209 DOI: 10.1016/j.isci.2023.105967] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2022] [Revised: 12/29/2022] [Accepted: 01/10/2023] [Indexed: 01/15/2023] Open
Abstract
The glucagon receptor (GCGR) is a member of the class B G protein-coupled receptor family. Many research works have been carried out on GCGR structure, glucagon signaling pathway, and GCGR antagonists. However, the expression and fine distribution of GCGR proteins in response to glucagon under high glucose remain unclear. Using direct stochastic optical reconstruction microscopy (dSTORM) imaging, nanoscale GCGR clusters were observed on HepG2 cell membranes, and high glucose promoted GCGR expression and the formation of more and larger clusters. Moreover, glucagon stimulation under high glucose did not inhibit GCGR levels as significantly as that under low glucose and did not increase the downstream cyclic 3,5'-adenosine monophosphate-protein kinase A (cAMP-PKA) signal, and there were still large-size clusters on the membranes, indicating that high glucose induced glucagon resistance. In addition, high glucose induced stronger glucagon resistance in hepatoma cells compared with hepatic cells. Our work will pave a way to further our understanding of the pathogenesis of diabetes and develop more effective drugs targeting GCGR.
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Affiliation(s)
- Jing Gao
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China,Corresponding author
| | - Hongru Li
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China,University of Science and Technology of China, Hefei, Anhui 230027, China
| | - Haijiao Xu
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China
| | - Yong Liu
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China
| | - Mingjun Cai
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China
| | - Yan Shi
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China
| | - Jingrui Zhang
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China
| | - Hongda Wang
- State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Research Center of Biomembranomics, Changchun, Jilin 130022, China,University of Science and Technology of China, Hefei, Anhui 230027, China,Laboratory for Marine Biology and Biotechnology, Qing dao National Laboratory for Marine Science and Technology, Wenhai Road, Aoshanwei, Jimo, Qingdao, Shandong 266237, China,Corresponding author
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2
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The Bioavailability, Extraction, Biosynthesis and Distribution of Natural Dihydrochalcone: Phloridzin. Int J Mol Sci 2021; 22:ijms22020962. [PMID: 33478062 PMCID: PMC7835879 DOI: 10.3390/ijms22020962] [Citation(s) in RCA: 30] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/24/2020] [Revised: 01/15/2021] [Accepted: 01/17/2021] [Indexed: 12/17/2022] Open
Abstract
Phloridzin is an important phytochemical which was first isolated from the bark of apple trees. It is a member of the dihydrochalcones and mainly distributed in the plants of the Malus genus, therefore, the extraction method of phloridzin was similar to those of other phenolic substances. High-speed countercurrent chromatography (HSCCC), resin adsorption technology and preparative high-performance liquid chromatography (HPLC) were used to separate and purify phloridzin. Many studies showed that phloridzin had multiple pharmacological effects, such as antidiabetic, anti-inflammatory, antihyperglycaemic, anticancer and antibacterial activities. Besides, the physiological activities of phloridzin are cardioprotective, neuroprotective, hepatoprotective, immunomodulatory, antiobesity, antioxidant and so on. The present review summarizes the biosynthesis, distribution, extraction and bioavailability of the natural compound phloridzin and discusses its applications in food and medicine.
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Shokry E, Marchioro L, Uhl O, Bermúdez MG, García-Santos JA, Segura MT, Campoy C, Koletzko B. Investigation of the impact of birth by cesarean section on fetal and maternal metabolism. Arch Gynecol Obstet 2019; 300:589-600. [PMID: 31201538 DOI: 10.1007/s00404-019-05213-w] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2018] [Accepted: 06/07/2019] [Indexed: 12/11/2022]
Abstract
PURPOSE Elective cesarean section (CS) was related to long-term adverse health effects in the offspring, but little is known about underlying mechanisms. Our study investigates the metabolic changes in both maternal and cord blood associated with CS in comparison to vaginal delivery (VD) to explore potential causal pathways. METHODS Samples obtained from PREOBE study participants were subjected to LC-MS/MS-targeted metabolomics comprising > 200 metabolites. RESULTS Elective CS showed an impact on both maternal and cord blood metabolomes. In maternal blood, the CS group showed lower levels of phospholipids (PL), principally ether-linked phosphatidylcholines (aaPC), pyruvic acid, branched chain keto-acids (BCKA), and other gluconeogenic substrates, but since the CS group showed different HDL levels in comparison to the VD group, we could not exclude contribution of the latter in the findings. In cord blood, the most remarkable finding in the CS group was the high levels of Cys; conversely, the lower levels of non-esterified fatty acids (NEFA), some tricarboxylic acid (TCA) cycle metabolites, gluconeogenic substrates, markers of β-oxidation, and the sum of hexoses were lower in CS-born babies in addition to tendentially lower levels of PL. CONCLUSIONS We speculate that lower levels of maternal and fetal corticosteroids in CS, due to less stressful condition, cause metabolic perturbations at birth initiating future negative health outcomes. This further supports the early programming hypothesis.
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Affiliation(s)
- Engy Shokry
- Division of Metabolic and Nutritional Medicine, Dr. von Hauner Children's Hospital, LMU Medical Center, LMU-Ludwig-Maximilians-Universität Munich, Campus Innenstadt, Lindwurmstr. 4, 80337, Munich, Germany
| | - Linda Marchioro
- Division of Metabolic and Nutritional Medicine, Dr. von Hauner Children's Hospital, LMU Medical Center, LMU-Ludwig-Maximilians-Universität Munich, Campus Innenstadt, Lindwurmstr. 4, 80337, Munich, Germany
| | - Olaf Uhl
- Division of Metabolic and Nutritional Medicine, Dr. von Hauner Children's Hospital, LMU Medical Center, LMU-Ludwig-Maximilians-Universität Munich, Campus Innenstadt, Lindwurmstr. 4, 80337, Munich, Germany
| | - Mercedes G Bermúdez
- Department of Paediatrics, School of Medicine, EURISTIKOS Excellence Centre for Paediatric Research, University of Granada, Avenida de la Investigación 11, 18016, Granada, Spain
| | - Jose Antonio García-Santos
- Department of Paediatrics, School of Medicine, EURISTIKOS Excellence Centre for Paediatric Research, University of Granada, Avenida de la Investigación 11, 18016, Granada, Spain
| | - Mª Teresa Segura
- Department of Paediatrics, School of Medicine, EURISTIKOS Excellence Centre for Paediatric Research, University of Granada, Avenida de la Investigación 11, 18016, Granada, Spain
| | - Cristina Campoy
- Department of Paediatrics, School of Medicine, EURISTIKOS Excellence Centre for Paediatric Research, University of Granada, Avenida de la Investigación 11, 18016, Granada, Spain
| | - Berthold Koletzko
- Division of Metabolic and Nutritional Medicine, Dr. von Hauner Children's Hospital, LMU Medical Center, LMU-Ludwig-Maximilians-Universität Munich, Campus Innenstadt, Lindwurmstr. 4, 80337, Munich, Germany.
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4
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Talukder JR, Griffin A, Jaima A, Boyd B, Wright J. Lactoferrin ameliorates prostaglandin E2-mediated inhibition of Na+-glucose cotransport in enterocytes. Can J Physiol Pharmacol 2014; 92:9-20. [DOI: 10.1139/cjpp-2013-0211] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Various immunoinflammatory cytokines are produced during chronic intestinal inflammation, which inhibits Na+-glucose cotransport (SGLT1) in villus cells. Lactoferrin (Lf), abundantly present in colostrum, is a multifunctional glycoprotein that is absorbed by receptor-mediated transcytosis in humans and animals and has been shown to exert anti-inflammatory effects. Therefore, this study aimed to examine whether Lf would prevent PGE2 effect on SGLT1 for glucose absorption in enterocytes. Intestinal epithelial cells (IEC-6) were grown on transwell plates, treated with phlorizin, PGE2, AH6809, and Lf, and 3-O-methyl d-glucopyranose (OMG) uptake was measured in 10 days postconfluent. Na+-dependent OMG uptake, phlorizin, and immunoblotting studies established the activity and apical membrane localization of SGLT1 in IEC-6 cells. PGE2 inhibited SGLT1 in a concentration- and time-dependent manner with an inhibitory constant (Ki) of 50.0 nmol/L and that was antagonized by prostanoid receptor inhibitor, AH6809. PGE2 did not alter Na+/K+-ATPase activity. In contrast, quantitative real-time polymerase chain reaction and Western blot analyses revealed that SGLT1-specific transcripts and protein expression level were decreased 3-fold by PGE2. Furthermore, PGE2 treatment increased intracellular cyclic adenosine monophosphate (cAMP) and Ca2+ concentrations and decreased SGLT1 expression on the apical membrane, and these effects were ameliorated by Lf. Therefore, we conclude that Lf ameliorates the PGE2 inhibition of SGLT1 most likely via the Ca2+- and cAMP-signaling pathways.
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Affiliation(s)
- Jamilur R. Talukder
- Department of Biology, LeMoyne-Owen College, 807 Walker Avenue, Memphis, TN 38126, USA
| | - Ashley Griffin
- Department of Biology, LeMoyne-Owen College, 807 Walker Avenue, Memphis, TN 38126, USA
| | - Antara Jaima
- Department of Biology, LeMoyne-Owen College, 807 Walker Avenue, Memphis, TN 38126, USA
| | - Brittney Boyd
- Department of Biology, LeMoyne-Owen College, 807 Walker Avenue, Memphis, TN 38126, USA
| | - Jaleesa Wright
- Department of Biology, LeMoyne-Owen College, 807 Walker Avenue, Memphis, TN 38126, USA
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5
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Cho YM, Merchant CE, Kieffer TJ. Targeting the glucagon receptor family for diabetes and obesity therapy. Pharmacol Ther 2012; 135:247-78. [DOI: 10.1016/j.pharmthera.2012.05.009] [Citation(s) in RCA: 93] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2012] [Accepted: 05/15/2012] [Indexed: 12/11/2022]
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6
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Marroquí L, Batista TM, Gonzalez A, Vieira E, Rafacho A, Colleta SJ, Taboga SR, Boschero AC, Nadal A, Carneiro EM, Quesada I. Functional and structural adaptations in the pancreatic α-cell and changes in glucagon signaling during protein malnutrition. Endocrinology 2012; 153:1663-72. [PMID: 22334714 DOI: 10.1210/en.2011-1623] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Chronic malnutrition leads to multiple changes in β-cell function and peripheral insulin actions to adapt glucose homeostasis to these restricted conditions. However, despite glucose homeostasis also depends on glucagon effects, the role of α-cells in malnutrition is largely unknown. Here, we studied α-cell function and hepatic glucagon signaling in mice fed with low-protein (LP) or normal-protein diet for 8 wk after weaning. Using confocal microscopy, we found that inhibition of Ca²⁺ signaling by glucose was impaired in α-cells of LP mice. Consistent with these findings, the ability of glucose to inhibit glucagon release in isolated islets was also diminished in LP mice. This altered secretion was not related with changes in either glucagon gene expression or glucagon content. A morphometric analysis showed that α-cell mass was significantly increased in malnourished animals, aspect that was probably related with their enhanced plasma glucagon levels. When we analyzed the hepatic function, we observed that the phosphorylation of protein kinase A and cAMP response-binding element protein in response to fasting or exogenous glucagon was impaired in LP mice. Additionally, the up-regulated gene expression in response to fasting observed in the hepatic glucagon receptor as well as several key hepatic enzymes, such as peroxisome proliferator-activated receptor γ, glucose-6-phosphatase, and phosphoenolpyruvate carboxykinase, was altered in malnourished animals. Finally, liver glycogen mobilization in response to fasting and the ability of exogenous glucagon to raise plasma glucose levels were lower in LP mice. Therefore, chronic protein malnutrition leads to several alterations in both the α-cell function and hepatic glucagon signaling.
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Affiliation(s)
- Laura Marroquí
- Instituto de Bioingeniería, Universidad Miguel Hernández, 03202 Elche, Spain
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Ke J, Zhang C, Harikumar KG, Zylstra-Diegel CR, Wang L, Mowry LE, Miller LJ, Williams BO, Xu HE. Modulation of β-catenin signaling by glucagon receptor activation. PLoS One 2012; 7:e33676. [PMID: 22438981 PMCID: PMC3306284 DOI: 10.1371/journal.pone.0033676] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2011] [Accepted: 02/14/2012] [Indexed: 11/18/2022] Open
Abstract
The glucagon receptor (GCGR) is a member of the class B G protein–coupled receptor family. Activation of GCGR by glucagon leads to increased glucose production by the liver. Thus, glucagon is a key component of glucose homeostasis by counteracting the effect of insulin. In this report, we found that in addition to activation of the classic cAMP/protein kinase A (PKA) pathway, activation of GCGR also induced β-catenin stabilization and activated β-catenin–mediated transcription. Activation of β-catenin signaling was PKA-dependent, consistent with previous reports on the parathyroid hormone receptor type 1 (PTH1R) and glucagon-like peptide 1 (GLP-1R) receptors. Since low-density-lipoprotein receptor–related protein 5 (Lrp5) is an essential co-receptor required for Wnt protein mediated β-catenin signaling, we examined the role of Lrp5 in glucagon-induced β-catenin signaling. Cotransfection with Lrp5 enhanced the glucagon-induced β-catenin stabilization and TCF promoter–mediated transcription. Inhibiting Lrp5/6 function using Dickkopf-1(DKK1) or by expression of the Lrp5 extracellular domain blocked glucagon-induced β-catenin signaling. Furthermore, we showed that Lrp5 physically interacted with GCGR by immunoprecipitation and bioluminescence resonance energy transfer assays. Together, these results reveal an unexpected crosstalk between glucagon and β-catenin signaling, and may help to explain the metabolic phenotypes of Lrp5/6 mutations.
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Affiliation(s)
- Jiyuan Ke
- Laboratory of Structural Sciences, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
- * E-mail: (JK); (HEX)
| | - Chenghai Zhang
- Laboratory of Structural Sciences, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
| | - Kaleeckal G. Harikumar
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona, United States of America
| | - Cassandra R. Zylstra-Diegel
- Laboratory of Cell Signaling and Carcinogenesis, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
| | - Liren Wang
- Laboratory of Structural Sciences, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
| | - Laura E. Mowry
- Laboratory of Cell Signaling and Carcinogenesis, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
| | - Laurence J. Miller
- Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Scottsdale, Arizona, United States of America
| | - Bart O. Williams
- Laboratory of Cell Signaling and Carcinogenesis, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
| | - H. Eric Xu
- Laboratory of Structural Sciences, Van Andel Research Institute, Grand Rapids, Michigan, United States of America
- VARI/SIMM Center, Center for Structure and Function of Drug Targets, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, People's Republic of China
- * E-mail: (JK); (HEX)
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Iizuka K, Tomita R, Takeda J, Horikawa Y. Rat glucagon receptor mRNA is directly regulated by glucose through transactivation of the carbohydrate response element binding protein. Biochem Biophys Res Commun 2012; 417:1107-12. [DOI: 10.1016/j.bbrc.2011.12.042] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2011] [Accepted: 12/09/2011] [Indexed: 12/11/2022]
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Abstract
Recent investigations have demonstrated that melatonin influences carbohydrate metabolism mediated by insulin-inhibiting effects on pancreatic β-cells. This study evaluated whether melatonin has also an effect on pancreatic α-cells and glucagon expression as well as the glucagon secretion in vitro and in vivo. Glucagon-producing pancreatic α-cell line αTC1 clone 9 (αTC1.9) was used, which was characterized as an appropriate model with glucose responsiveness and expression of the melatonin receptors MT1 and MT2. The results demonstrate that melatonin incubation significantly enhanced the expression as well as the secretion of glucagon. These effects appeared to be more pronounced under hyperglycemic conditions compared to basal glucose concentrations. Notably, in vivo studies demonstrated that long-term oral melatonin administration led to significantly elevated plasma glucagon concentrations in Wistar rats. In contrast, plasma glucagon levels were found to be slightly decreased in type 2 diabetic Goto-Kakizaki rats. Moreover, investigations measuring the relative glucagon receptor mRNA expression showed marked differences in the liver of melatonin-substituted rats as well as in melatonin receptor knockout mice. In conclusion, these findings revealed evidence that melatonin influences pancreatic glucagon expression and secretion as well as the peripheral glucagon action.
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Affiliation(s)
- Ina Bähr
- Institute of Anatomy and Cell Biology, Martin Luther University Halle-Wittenberg, Halle/Saale, Germany
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10
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Delivery by Caesarean section, rather than vaginal delivery, promotes hepatic steatosis in piglets. Clin Sci (Lond) 2009; 118:47-59. [PMID: 19445654 DOI: 10.1042/cs20090169] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
There has been a marked increase in the number of babies born by elective CS (Caesarean section). Following CS, the lack of normal stimuli that occur at birth alters the thermogeneic response, but any effects on hepatic metabolism have not been identified. In the present study, we compared the effect of delivery on hepatic metabolism in piglets, born either by CS or VD (vaginal delivery) and fed by TPN (total parenteral nutrition), by measuring lipid metabolism and enzyme activity coupled with metabolomic and genomic approaches. Hepatic lipid in the CS piglets at 7 days post-partum was in excess of 5 mg/g of liver consistent with hepatic steatosis, whereas in the VD piglets the amount of lipid was markedly lower (3 mg/g of liver) and below the threshold for a diagnosis of steatosis. Metabolomic analysis indicated that CS resulted in higher hepatic glycerol and lower glycerol phosphate dehydrogenase activity, suggesting that CS causes a decrease in hepatic gluconeogenesis from glycerol. CS also resulted in altered cholesterol handling and gene expression, despite the same dietary intake for 7 days post-partum. Furthermore, the CS piglets had a lower expression of interferon-responsive genes, but a higher expression of markers of immature hepatocytes. In conclusion, the results suggest that VD promotes normal liver maturation and hepatic metabolism, thereby reducing the accumulation of hepatic lipid.
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11
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Keller MP, Choi Y, Wang P, Davis DB, Rabaglia ME, Oler AT, Stapleton DS, Argmann C, Schueler KL, Edwards S, Steinberg HA, Chaibub Neto E, Kleinhanz R, Turner S, Hellerstein MK, Schadt EE, Yandell BS, Kendziorski C, Attie AD. A gene expression network model of type 2 diabetes links cell cycle regulation in islets with diabetes susceptibility. Genome Res 2008; 18:706-16. [PMID: 18347327 DOI: 10.1101/gr.074914.107] [Citation(s) in RCA: 280] [Impact Index Per Article: 17.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Insulin resistance is necessary but not sufficient for the development of type 2 diabetes. Diabetes results when pancreatic beta-cells fail to compensate for insulin resistance by increasing insulin production through an expansion of beta-cell mass or increased insulin secretion. Communication between insulin target tissues and beta-cells may initiate this compensatory response. Correlated changes in gene expression between tissues can provide evidence for such intercellular communication. We profiled gene expression in six tissues of mice from an obesity-induced diabetes-resistant and a diabetes-susceptible strain before and after the onset of diabetes. We studied the correlation structure of mRNA abundance and identified 105 co-expression gene modules. We provide an interactive gene network model showing the correlation structure between the expression modules within and among the six tissues. This resource also provides a searchable database of gene expression profiles for all genes in six tissues in lean and obese diabetes-resistant and diabetes-susceptible mice, at 4 and 10 wk of age. A cell cycle regulatory module in islets predicts diabetes susceptibility. The module predicts islet replication; we found a strong correlation between (2)H(2)O incorporation into islet DNA in vivo and the expression pattern of the cell cycle module. This pattern is highly correlated with that of several individual genes in insulin target tissues, including Igf2, which has been shown to promote beta-cell proliferation, suggesting that these genes may provide a link between insulin resistance and beta-cell proliferation.
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Affiliation(s)
- Mark P Keller
- Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53076, USA
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12
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Polakof S, Míguez JM, Soengas JL. In vitro evidences for glucosensing capacity and mechanisms in hypothalamus, hindbrain, and Brockmann bodies of rainbow trout. Am J Physiol Regul Integr Comp Physiol 2007; 293:R1410-20. [PMID: 17567722 DOI: 10.1152/ajpregu.00283.2007] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
We aimed to support in vitro the glucosensing capacity observed in vivo in rainbow trout hypothalamus, hindbrain, and Brockmann bodies (BB) and to obtain preliminary evidence of the mechanisms involved. The response of parameters involved in the glucosensing capacity [hexokinase, hexokinase IV (glucokinase), and pyruvate kinase activities and glucose and glycogen levels] was assessed in these tissues incubated for 1 h with 2, 4, or 8 mM d-glucose alone (control) or with specific agonists/inhibitors of the steps involved in glucosensing capacity in mammals. These agents were a competitor for glucose phosphorylation (15 mM mannose), sulfonylurea receptor-1 effectors (500 μM tolbutamide or diazoxide), glycolytic intermediates (15 mM glycerol, lactate, or pyruvate), and inhibitors of glucose transport (10 μM cytochalasin B), glycolysis [20 mM 2-deoxy-d-glucose (2-DG)], and L-type calcium channel (1 μM nifedipine). Control incubations of the three tissues displayed increased glucose and glycogen levels and glucokinase activities in response to increased medium glucose, thus supporting our previous in vivo studies. Furthermore, critical components of the glucosensing mammalian machinery are apparently functioning in the three tissues. The responses in brain regions to all substances tested (except 2-DG and nifedipine) were similar to those observed in mammals, suggesting a similar glucosensing machinery. In contrast, in BB, only the effects of 2-DG, lactate, pyruvate, diazoxide, and nifedipine were similar to those of mammalian β-cells, suggesting that some of the components of the piscine glucosensing model are different than those of mammals. Such differences may relate to the importance of amino acids rather than glucose signaling in the trout BB.
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Affiliation(s)
- Sergio Polakof
- Laboratorio de Fisioloxía Animal, Facultade de Bioloxía, Edificio de Ciencias Experimentais, Universidade de Vigo, E-36310 Vigo, Spain
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13
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Mortensen OH, Dichmann DS, Abrahamsen N, Grunnet N, Nishimura E. Identification of a novel human glucagon receptor promoter: regulation by cAMP and PGC-1alpha. Gene 2007; 393:127-36. [PMID: 17374560 DOI: 10.1016/j.gene.2007.01.023] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2006] [Revised: 01/26/2007] [Accepted: 01/30/2007] [Indexed: 11/25/2022]
Abstract
Previously we have demonstrated that glucagon receptor mRNA expression in cultured rat hepatocytes and pancreatic islets can be regulated by various factors, including cAMP; however, the regulation of the human glucagon receptor gene has not been well-defined. Here we have characterized the promoter regions of the human glucagon receptor gene. Primer extension studies yielded multiple products in both liver and pancreas, corresponding to transcription start sites situated at -166 and -477 relative to the start of translation, indicating two putative promoters. Both transcription start sites were found to be active, when sequence immediately upstream of the start sites were cloned into luciferase reporter constructs. The transcriptional activity of the proximal promoter, but not the distal promoter, could be inhibited approximately 50% by cAMP, indicating that the previously observed inhibitory effects of cAMP on glucagon receptor mRNA expression is mediated at the level of gene transcription. The cAMP-mediated downregulation of the proximal promoter was examined by deletion analysis in the human hepatoma cell line HepG2 and the cAMP responsiveness was found to be located in a region between 1051 and 1016 base pairs upstream of the transcription start site, which contains several putative cAMP responsive elements. Expression of peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha), known to be upregulated in the liver by fasting, was found to abolish the cAMP-dependent downregulation of glucagon receptor mRNA expression in vitro, whereas overexpression of PGC-1beta had no effect.
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Affiliation(s)
- Ole Hartvig Mortensen
- Department of Medical Biochemistry and Genetics, University of Copenhagen, Copenhagen, Denmark.
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14
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Sloop KW, Michael MD, Moyers JS. Glucagon as a target for the treatment of Type 2 diabetes. Expert Opin Ther Targets 2007; 9:593-600. [PMID: 15948676 DOI: 10.1517/14728222.9.3.593] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Glucagon is the key counter-regulatory hormone that opposes the action of insulin. In states of relative hypoglycaemia, glucagon acts to increase blood glucose by stimulating hepatic glycogen breakdown and gluconeogenesis to achieve euglycaemia. Type 2 diabetes is characterised by inappropriate regulation of hepatic glucose production, which is due, at least in part, to an imbalance in the bihormonal relationship between plasma levels of glucagon and insulin. The glucose-lowering effects of glucagon peptide antagonists and antiglucagon neutralising antibodies first demonstrated the potential of glucagon receptor (GCGR) antagonism as a treatment for hyperglycaemia. In recent years, the development of GCGR antisense oligonucleotides and small molecular weight GCGR antagonists have been pursued as possible therapeutic agents to target glucagon action as a treatment for Type 2 diabetes.
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Affiliation(s)
- Kyle W Sloop
- Endocrine Discovery, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46285, USA.
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Charbonneau A, Unson CG, Lavoie JM. High-fat diet-induced hepatic steatosis reduces glucagon receptor content in rat hepatocytes: potential interaction with acute exercise. J Physiol 2006; 579:255-67. [PMID: 17053032 PMCID: PMC2075374 DOI: 10.1113/jphysiol.2006.121954] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Studies have revealed that high-fat (HF) diets promote hyperglycaemia, whole-body insulin resistance and non-alcoholic fatty liver disease (NAFLD). Recently, hepatic glucagon resistance has been shown to occur in rats fed a HF diet. More precisely, diet-induced obesity (DIO) reduces the number of hepatic plasma membrane glucagon receptors (GR), which results in a diminished response to glucagon during a hyperglucagonaemic clamp. The present study was undertaken to test the hypothesis that a HF-DIO is associated with a desensitization and destruction of the hepatic GR. We also hypothesized that a single bout of endurance exercise would modify the GR cellular distribution under our DIO model. Male rats were either fed a standard (SD) or a HF diet for two weeks. Each group was subdivided into a non-exercised (Rest) and an acute exercised (EX) group. The HF diet resulted in a reduction of total hepatic GR (55%) and hepatic plasma membrane GR protein content (20%). These changes were accompanied by a significant increase in endosomal and lysosomal GR content with the feeding of a HF diet. The reduction of GR plasma membrane as well as the increase in endosomal GR was strongly correlated with an increase of PKC-alpha, suggesting a role of PKC-alpha in GR desensitization. EX increased significantly PKC-alpha protein content in both diets, suggesting a role of PKC-alpha in EX-induced GR desensitization. The present results suggest that liver lipid infiltration plays a role in reducing glucagon action in the liver through a reduction in total cellular and plasma membrane GR content. Furthermore, the GR desensitization observed in our in vivo model of HF diet-induced hepatic steatosis and in EX individuals may be regulated by PKC-alpha.
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16
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Charbonneau A, Melancon A, Lavoie C, Lavoie JM. Alterations in hepatic glucagon receptor density and in Gsalpha and Gialpha2 protein content with diet-induced hepatic steatosis: effects of acute exercise. Am J Physiol Endocrinol Metab 2005; 289:E8-14. [PMID: 15687107 DOI: 10.1152/ajpendo.00570.2004] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The present study was undertaken to test the hypothesis that a high-fat diet-induced liver lipid infiltration is associated with a reduction of hepatic glucagon receptor density (B(max)) and affinity (K(d)), and with a decrease in stimulatory G protein (G(s)alpha) content while enhancing inhibitory G protein (G(i)alpha(2)) expression. We also hypothesized that, under this dietary condition, a single bout of endurance exercise would restore hepatic glucagon receptor parameters and G protein expression to standard levels. Female Sprague-Dawley rats were fed either a standard (SD) or a high-fat diet (HF; 40% kcal) for 2 wk (n = 20 rats/group). Each dietary group was thereafter subdivided into a nonexercised (Rest) and an acute-exercised group (Ac-Ex). The acute exercise consisted of a single bout of endurance exercise on a treadmill (30 min, 26 m/min, and 0% slope) immediately before being killed. The HF compared with the SD diet was associated with significantly (P < 0.05) higher values in hepatic triglyceride concentrations (123%), fat pad weight, and plasma free fatty acid (FFA) concentrations. The HF diet also resulted in significantly (P < 0.05) lower hepatic glucagon receptor density (45%) and G(s)alpha protein content (75%), as well as higher (P < 0.05) G(i)alpha(2) protein content (27%), with no significant effects on glucagon receptor affinity. Comparisons of all individual liver triglyceride and B(max) values revealed that liver triglycerides were highly (P < 0.003) predictive of the decreased glucagon receptor density (R = -0.512). Although the 30-min exercise bout resulted in some typical exercise effects (P < 0.05), such as an increase in FFA (SD diet), a decrease in insulin levels, and an increase in plasma glucagon concentrations (SD diet), it did not change any of the responses related to liver glucagon receptors and G proteins, with the exception of a significant (P < 0.05) decrease in G(i)alpha(2) protein content under the HF diet. The present results indicate that the feeding of an HF diet is associated with a reduction in plasma membrane hepatic glucagon receptor density and G(s)alpha protein content, which is not attenuated by a 30-min exercise bout. It is suggested that liver lipid infiltration plays a role in reducing glucagon action in the liver through a reduction in glucagon receptor density and glucagon-mediated signal transduction.
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Affiliation(s)
- Alexandre Charbonneau
- Département de Kinésiologie, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montreal, QC, Canada, H3C 3J7
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17
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Abstract
One paradox of hormonal regulation during exercise is the maintenance of glucose homeostasis after endurance training despite a lower increase in plasma glucagon. One explanation could be that liver sensitivity to glucagon is increased by endurance training. Glucagon exerts its effect through a 62 KDa glycoprotein receptor, member of the G protein-coupled receptor. To determine whether changes with exercise in glucagon sensitivity occurred at the level of the glucagon receptor (GR), binding characteristics of hepatic glucagon receptors were ascertained in rat purified plasma membranes. Saturation kinetics indicated no difference in the dissociation constant or affinity of glucagon receptor, but a significantly higher glucagon receptor binding density in liver in endurance trained compared to untrained animals. Along with endurance training, it appears that fasting also changes GR binding characteristics. In animals fasting 24 hrs, a significant increase in glucagon receptor density was also reported. Although the exact mechanism remains unknown, there is no doubt that the liver can adapt to physiological stress through modulation of GR binding characteristics to enhance the hepatic glucose production responsiveness to glucagon. Key words: glucagon sensitivity, liver, endurance training, rats
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Affiliation(s)
- Carole Lavoie
- Département des sciences de l'activité physique, Université du Québec à Trois-Rivieres, Case Postale 500, Trois-Rivieres, Québec, Canada
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Abstract
The dihydrochalcone phlorizin is a natural product and dietary constituent found in a number of fruit trees. It has been used as a pharmaceutical and tool for physiology research for over 150 years. Phlorizin's principal pharmacological action is to produce renal glycosuria and block intestinal glucose absorption through inhibition of the sodium-glucose symporters located in the proximal renal tubule and mucosa of the small intestine. This review covers the role phlorizin has played in the history of diabetes mellitus and its use as an agent to understand fundamental concepts in renal physiology as well as summarizes the physiology of cellular glucose transport and the pathophysiology of renal glycosuria. It reviews the biology and pathobiology of glucose transporters and discusses the medical botany of phlorizin and the potential effects of plant flavonoids, such as phlorizin, on human metabolism. Lastly, it describes the clinical pharmacology and toxicology of phlorizin, including investigational uses of phlorizin and phlorizin analogs in the treatment of diabetes, obesity, and stress hyperglycemia.
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Affiliation(s)
- Joel R L Ehrenkranz
- Department of Medicine, University of Colorado Health Sciences Center, Denver, Colorado, USA.
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19
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Zhao H, Yakar S, Gavrilova O, Sun H, Zhang Y, Kim H, Setser J, Jou W, LeRoith D. Phloridzin improves hyperglycemia but not hepatic insulin resistance in a transgenic mouse model of type 2 diabetes. Diabetes 2004; 53:2901-9. [PMID: 15504971 DOI: 10.2337/diabetes.53.11.2901] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
The chronic hyperglycemia that occurs in type 2 diabetes may cause deterioration of beta-cell function and insulin resistance in peripheral tissues. Mice that express a dominant-negative IGF-1 receptor, specifically in skeletal muscle (MKR mice), exhibit severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyper-glycemia. To determine the role of hyperglycemia in the worsening of the diabetes state in these animals, MKR mice were treated with phloridzin (PHZ), which inhibits intestinal glucose uptake and renal glucose reabsorption. Blood glucose levels were decreased and urine glucose levels were increased in response to PHZ treatment in MKR mice. PHZ treatment also increased food intake in MKR mice; however, the fat mass was decreased and lean body mass did not change. Serum insulin, fatty acid, and triglyceride levels were not affected by PHZ treatment in MKR mice. Hyperinsulinemic-euglycemic clamp analysis demonstrated that glucose uptake in white adipose tissue was significantly increased in response to PHZ treatment. Despite the reduction in blood glucose following PHZ treatment, there was no improvement in insulin-stimulated whole-body glucose uptake in MKR mice and neither was there suppression of endogenous glucose production by insulin. These results suggest that glucotoxicity plays little or no role in the worsening of insulin resistance that occurs in the MKR mouse model of type 2 diabetes.
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Affiliation(s)
- Hong Zhao
- Diabetes Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-1758, USA
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20
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Gorovits N, Cui L, Busik JV, Ranalletta M, Hauguel de-Mouzon S, Charron MJ. Regulation of hepatic GLUT8 expression in normal and diabetic models. Endocrinology 2003; 144:1703-11. [PMID: 12697674 DOI: 10.1210/en.2002-220968] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
GLUT8 is a novel glucose transporter protein that is widely distributed in tissues including liver, a central organ of regulation of glucose homeostasis. The purpose of the current study was to investigate expression and regulation of hepatic GLUT8 mRNA and protein. Therefore, Northern and immunoblot analysis, semiquantitative RT-PCR, and immunofluorescence microscopy were performed using mouse livers at different stages of embryonic and postnatal development and in type 1 (streprozotocin treated) and type 2 (GLUT4 heterozygous) diabetes. GLUT8 mRNA and protein expression in embryonic liver was differentially regulated depending on the prenatal and postnatal developmental stage of the mice. Immunofluorescence microscopy of liver from wild-type mice demonstrated the highest levels of GLUT8 protein in perivenous hepatocytes pointing to its role in regulation of glycolytic flux. In diabetic scenarios, GLUT8 mRNA levels were correlated with circulating insulin; specifically, GLUT8 mRNA decreased in a type 1 diabetes model and increased in a type 2 diabetes model, suggesting a regulatory role for insulin in GLUT8 mRNA expression. While up-regulation of GLUT8 protein occurred in both models of diabetes, only in streptozotocin diabetic livers was GLUT8 zonation altered. These data demonstrate that GLUT8 mRNA and protein are differentially regulated in liver in response to physiologic and pathologic (diabetes) milieu and suggests that GLUT8 is intimately linked to glucose homeostasis.
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MESH Headings
- Animals
- Animals, Newborn/metabolism
- Diabetes Mellitus, Experimental/embryology
- Diabetes Mellitus, Experimental/metabolism
- Diabetes Mellitus, Type 1/embryology
- Diabetes Mellitus, Type 1/metabolism
- Diabetes Mellitus, Type 2/embryology
- Diabetes Mellitus, Type 2/genetics
- Diabetes Mellitus, Type 2/metabolism
- Female
- Fetus/metabolism
- Gene Deletion
- Glucose Transport Proteins, Facilitative
- Glucose Transporter Type 4
- Heterozygote
- Immunologic Techniques
- Liver/metabolism
- Mice
- Mice, Inbred CBA
- Monosaccharide Transport Proteins/genetics
- Monosaccharide Transport Proteins/metabolism
- Muscle Proteins
- RNA, Messenger/metabolism
- Reference Values
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Affiliation(s)
- Naira Gorovits
- Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, USA
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Smih F, Rouet P, Lucas S, Mairal A, Sengenes C, Lafontan M, Vaulont S, Casado M, Langin D. Transcriptional regulation of adipocyte hormone-sensitive lipase by glucose. Diabetes 2002; 51:293-300. [PMID: 11812735 DOI: 10.2337/diabetes.51.2.293] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Hormone-sensitive lipase (HSL) catalyzes the rate-limiting step in the mobilization of fatty acids from adipose tissue, thus determining the supply of energy substrates in the body. HSL mRNA was positively regulated by glucose in human adipocytes. Pools of stably transfected 3T3-F442A adipocytes were generated with human adipocyte HSL promoter fragments from -2,400/+38 to -31/+38 bp linked to the luciferase gene. A glucose-responsive region was mapped within the proximal promoter (-137 bp). Electromobility shift assays showed that upstream stimulatory factor (USF)-1 and USF2 and Sp1 and Sp3 bound to a consensus E-box and two GC-boxes in the -137-bp region. Cotransfection of the -137/+38 construct with USF1 and USF2 expression vectors produced enhanced luciferase activity. Moreover, HSL mRNA levels were decreased in USF1- and USF2-deficient mice. Site-directed mutagenesis of the HSL promoter showed that the GC-boxes, although contributing to basal promoter activity, were dispensable for glucose responsiveness. Mutation of the E-box led to decreased promoter activity and suppression of the glucose response. Analogs and metabolites were used to determine the signal metabolite of the glucose response. The signal is generated downstream of glucose-6-phosphate in the glycolytic pathway before the triose phosphate step.
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Affiliation(s)
- Fatima Smih
- INSERM Unité 317, Institut Louis Bugnard, Centre Hospitalier Universitaire de Rangueil, Université Paul Sabatier, Toulouse, France
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22
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Exton JH. Glucagon Signal‐Transduction Mechanisms. Compr Physiol 2001. [DOI: 10.1002/cphy.cp070213] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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Portois L, Tastenoy M, Svoboda M. Mutational analysis of the glucose regulatory element in the promoter of the glucagon receptor gene. Ann N Y Acad Sci 2001; 921:148-56. [PMID: 11193818 DOI: 10.1111/j.1749-6632.2000.tb06961.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Recently, we identified a glucose regulatory element in the promoter of the rat glucagon receptor gene. The effect of glucose is centered on a highly palindromic sequence of 19 nucleotides that we called the G box (Portois et al., 1999, J. Biol. Chem. 274: 8181-8190). This sequence contains two E boxes. Recently, we investigated the role of each individual E box, as well as the contribution of the sequences located upstream and downstream from this G box. (1) Mutation of nucleotides "CA" to "GT" in the first E box (position -543 to -542) suppressed the activation of the CAT reporter gene by glucose. In contrast, mutation of the nucleotides "CA" to "GT" in the second E box (position -534 to -533) had no effect on this glucose activation. (2) Deletion of a sequence upstream from the G box (nucleotides -579 to -555) suppressed the activation by glucose, whereas deletion of a sequence located downstream from the G box (nucleotides -501 to -443) had no effect on this parameter. (3) Subcloning of a small promoter fragment of only 49 nucleotides (-560 to -512) into the pCat5 plasmid conferred to transfected cells sensitivity to glucose in terms of CAT activity. Consequently, all transactivation factors required for this glucose effect must act via this short gene fragment.
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Affiliation(s)
- L Portois
- Laboratory of Biochemistry and Nutrition, Medical School, Université Libre de Bruxelles, Brussels, Belgium
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Geiger A, Salazar G, Le Cam A, Kervran A. Characterization of an enhancer element in the proximal promoter of the mouse glucagon receptor gene. BIOCHIMICA ET BIOPHYSICA ACTA 2001; 1517:236-42. [PMID: 11342104 DOI: 10.1016/s0167-4781(00)00279-7] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
The 5'-flanking region of the mouse glucagon receptor has been previously cloned and two promoter regions were characterized. Functional analysis of the proximal promoter was now performed to characterize cis-acting element(s) regulating basal gene expression. Promoter analysis using deletion constructs in a rat cell line (CA-77) expressing the glucagon receptor, showed that the region from -64 to +127 relative to the proximal transcription start site was sufficient for maximal proximal promoter activity. A DNA sequence spanning the -28 to -16 region organized as an imperfect palindrome was demonstrated to be functional as a cis-acting enhancer. Constructs including several copies of this motif strongly increased activity of the heterologous thymidine kinase promoter. Gel mobility shift assays performed with different DNA fragments spanning this region confirmed that it specifically bound nuclear protein(s) from CA-77 cells, mouse MIN-6 cells or mouse liver. Mutations in the core sequence of this site impaired both reporter gene activity and nuclear protein binding. The palindrome is a novel DNA sequence with no homology to existing transcription factor binding site database. This is the first characterization of a functional cis-acting sequence into the proximal promoter of the mouse glucagon receptor that may support constitutive expression of the gene.
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Affiliation(s)
- A Geiger
- Bâtiment INSERM U376, CHU Arnaud de Villeneuve, 371, rue Doyen G. Giraud, 34295 Montpellier Cedex 5, France
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Légaré A, Drouin R, Milot M, Massicotte D, Péronnet F, Massicotte G, Lavoie C. Increased density of glucagon receptors in liver from endurance-trained rats. Am J Physiol Endocrinol Metab 2001; 280:E193-6. [PMID: 11120674 DOI: 10.1152/ajpendo.2001.280.1.e193] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The binding properties of glucagon receptors were determined in plasma membranes isolated from liver of untrained (n = 6) and swimming endurance-trained Sprague-Dawley male rats (n = 7; 3 h/day, 5 days/wk, for 8 wk). Plasma membranes were purified from liver by aqueous two-phase affinity partitioning, and saturation kinetics were obtained by incubation of plasma membranes (10 microg of proteins/150 microl) with (125)I-labeled glucagon at concentrations ranging from 0.15 to 3.0 nM for 30 min at 30 degrees C. Saturating curve analysis indicated no difference in the affinity of glucagon receptors (0.57 +/- 0.06 and 0.77 +/- 0.09 nM in untrained and trained groups, respectively) but a significant higher glucagon receptor density in liver from untrained vs. trained rats (3.09 +/- 0.12 vs. 4.28 +/- 0.19 pmol/mg proteins). These results suggest that the reported increase in liver glucagon sensitivity in endurance-trained subjects (Drouin R, Lavoie C, Bourque J, Ducros F, Poisson D, and Chiasson J-L. Am J Physiol Endocrinol Metab 274: E23-E28, 1998) could be partly due to an increased glucagon receptor density in response to training.
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Affiliation(s)
- A Légaré
- Département de chimie-biologie, Université du Québec à Trois-Rivières, Trois-Rivières, Quebec G9A 5H7, Canada
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26
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Burcelin R, del Carmen Muñoz M, Guillam MT, Thorens B. Liver hyperplasia and paradoxical regulation of glycogen metabolism and glucose-sensitive gene expression in GLUT2-null hepatocytes. Further evidence for the existence of a membrane-based glucose release pathway. J Biol Chem 2000; 275:10930-6. [PMID: 10753892 DOI: 10.1074/jbc.275.15.10930] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We investigated the impact of GLUT2 gene inactivation on the regulation of hepatic glucose metabolism during the fed to fast transition. In control and GLUT2-null mice, fasting was accompanied by a approximately 10-fold increase in plasma glucagon to insulin ratio, a similar activation of liver glycogen phosphorylase and inhibition of glycogen synthase and the same elevation in phosphoenolpyruvate carboxykinase and glucose-6-phosphatase mRNAs. In GLUT2-null mice, mobilization of glycogen stores was, however, strongly impaired. This was correlated with glucose-6-phosphate (G6P) levels, which remained at the fed values, indicating an important allosteric stimulation of glycogen synthase by G6P. These G6P levels were also accompanied by a paradoxical elevation of the mRNAs for L-pyruvate kinase. Re-expression of GLUT2 in liver corrected the abnormal regulation of glycogen and L-pyruvate kinase gene expression. Interestingly, GLUT2-null livers were hyperplasic, as revealed by a 40% increase in liver mass and 30% increase in liver DNA content. Together, these data indicate that in the absence of GLUT2, the G6P levels cannot decrease during a fasting period. This may be due to neosynthesized glucose entering the cytosol, being unable to diffuse into the extracellular space, and being phosphorylated back to G6P. Because hepatic glucose production is nevertheless quantitatively normal, glucose produced in the endoplasmic reticulum may also be exported out of the cell through an alternative, membrane traffic-based pathway, as previously reported (Guillam, M.-T., Burcelin, R., and Thorens, B. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12317-12321). Therefore, in fasting, GLUT2 is not required for quantitative normal glucose output but is necessary to equilibrate cytosolic glucose with the extracellular space. In the absence of this equilibration, the control of hepatic glucose metabolism by G6P is dominant over that by plasma hormone concentrations.
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Affiliation(s)
- R Burcelin
- Institute of Pharmacology and Toxicology, University of Lausanne, 27, rue du Bugnon, CH-1005 Lausanne, Switzerland
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27
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Abstract
The glucagon receptor gene is a member of a gene family, the expression of which is strongly upregulated by glucose. This review deals with the structure of both the glucagon receptor gene and its promoter. Attention is focused on the glucose regulatory element that we discovered in the promoter of this gene. Regulation by glucose of genes implicated in glucose homeostasis represents one mechanism contributing to the control of fuel utilization. Its deficiency or imbalance could potentially lead to or participate in pathological situations such as diabetes mellitus. On the other hand, the regulatory element of the glucagon receptor gene promoter could be used as a tool for the glucose-regulated expression of other genes. Indeed, an analysis of the glucagon receptor gene promoter demonstrated that only a short fragment of the genomic DNA, easy to subclone, contains all required elements for activation by glucose. Its potential use for gene therapy is also considered, therefore, in this report.
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Affiliation(s)
- M Svoboda
- Laboratory of Biochemistry and Nutrition, Université Libre de Bruxelles, Brussels, B-1070, Belgium. msvobod@.ulb.ac.be
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Portois L, Maget B, Tastenoy M, Perret J, Svoboda M. Identification of a glucose response element in the promoter of the rat glucagon receptor gene. J Biol Chem 1999; 274:8181-90. [PMID: 10075722 DOI: 10.1074/jbc.274.12.8181] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We cloned the 5' upstream region of the rat glucagon receptor gene, demonstrating that the 5' noncoding domain of the glucagon receptor mRNA contained two untranslated exons of 131 and 166 nucleotides (nt), respectively, separated by two introns of 0.6 and 3.2 kilobase pairs. We also observed an alternative splicing involving the 166-base pair exon. Cloning of up to 2 kilobase pairs of the newly identified genomic domain and transfection of various constructs driving a reporter gene, in pancreatic islet cell line INS-1, uncovered a strong glucose regulation of the promoter activity of plasmids containing up to nucleotide -868, or more, upstream from the transcriptional start point. This promoter activity displayed threshold-like behavior, with low activity of the promoter below 5 mM glucose, and maximal activation as of 10 mM glucose. This glucose regulation was mapped to a highly palindromic 19-nucleotide region between nt -545 and -527. Indeed, deletion or mutation of this sequence abolished the glucose regulation. This domain contained two palindromic "E-boxes" CACGTG and CAGCTG separated by 3 nt, a feature similar to the "L4 box" found in the pyruvate kinase L gene promoter. This is the first description of a G protein-coupled receptor gene promoter regulated by glucose.
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Affiliation(s)
- L Portois
- Department of Biochemistry and Nutrition, Medical School, Université Libre de Bruxelles, Building G/E, CP 611, 808 Route de Lennik, B-1070 Brussels, Belgium
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