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Zhang Z, Ji G, Li M. Glucokinase regulatory protein: a balancing act between glucose and lipid metabolism in NAFLD. Front Endocrinol (Lausanne) 2023; 14:1247611. [PMID: 37711901 PMCID: PMC10497960 DOI: 10.3389/fendo.2023.1247611] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/26/2023] [Accepted: 08/14/2023] [Indexed: 09/16/2023] Open
Abstract
Non-alcoholic fatty liver disease (NAFLD) is a common liver disease worldwide, affected by both genetics and environment. Type 2 diabetes (T2D) stands as an independent environmental risk factor that precipitates the onset of hepatic steatosis and accelerates its progression to severe stages of liver damage. Furthermore, the coexistence of T2D and NAFLD magnifies the risk of cardiovascular disease synergistically. However, the association between genetic susceptibility and metabolic risk factors in NAFLD remains incompletely understood. The glucokinase regulator gene (GCKR), responsible for encoding the glucokinase regulatory protein (GKRP), acts as a regulator and protector of the glucose-metabolizing enzyme glucokinase (GK) in the liver. Two common variants (rs1260326 and rs780094) within the GCKR gene have been associated with a lower risk for T2D but a higher risk for NAFLD. Recent studies underscore that T2D presence significantly amplifies the effect of the GCKR gene, thereby increasing the risk of NASH and fibrosis in NAFLD patients. In this review, we focus on the critical roles of GKRP in T2D and NAFLD, drawing upon insights from genetic and biological studies. Notably, prior attempts at drug development targeting GK with glucokinase activators (GKAs) have shown potential risks of augmented plasma triglycerides or NAFLD. Conversely, overexpression of GKRP in diabetic rats improved glucose tolerance without causing NAFLD, suggesting the crucial regulatory role of GKRP in maintaining hepatic glucose and lipid metabolism balance. Collectively, this review sheds new light on the complex interaction between genes and environment in NAFLD, focusing on the GCKR gene. By integrating evidence from genetics, biology, and drug development, we reassess the therapeutic potential of targeting GK or GKRP for metabolic disease treatment. Emerging evidence suggests that selectively activating GK or enhancing GK-GKRP binding may represent a holistic strategy for restoring glucose and lipid metabolic balance.
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Affiliation(s)
| | | | - Meng Li
- Institute of Digestive Diseases, Longhua Hospital, Shanghai University of Traditional Chinese Medicine, Shanghai, China
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Sreelekshmi M, Raghu KG. Vanillic acid mitigates the impairments in glucose metabolism in HepG2 cells through BAD-GK interaction during hyperinsulinemia. J Biochem Mol Toxicol 2021; 35:1-8. [PMID: 33651899 DOI: 10.1002/jbt.22750] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2020] [Revised: 12/02/2020] [Accepted: 02/18/2021] [Indexed: 11/09/2022]
Abstract
Glucokinase (GK), a key regulator of hepatic glucose metabolism in the liver and glucose sensor and mediator in the secretion of insulin in the pancreas, is not studied in detail for its therapeutic application in diabetes. Herein, we study the alteration in GK activity during hyperinsulinemia-induced insulin resistance in HepG2 cells. We also investigated the link between GK and Bcl-2-associated death receptor (BAD) during hyperinsulinemia. There are emerging demands for GK activators from natural resources, and we selected vanillic acid (VA) to evaluate its potential as GK activators during hyperinsulinemia in HepG2 cells. VA is a phenolic compound and a commonly used food additive in many food industries. We found that VA safeguarded GK inhibition during hyperinsulinemia significantly in HepG2 cells. VA also prevented the depletion of glycogen synthesis during hyperinsulinemia, which is evident from protein expression studies of phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, glycogen synthase, and glycogen synthase kinase-3β. This was associated with activation of BAD activity, which was also confirmed by Western blotting. Molecular docking revealed strong binding between GK active site and VA, supporting their strong interaction. These are the first in vitro data to indicate the beneficial properties of VA with respect to insulin resistance induced by hyperinsulinemia by GK activation. Since it is activated via BAD, the hypoglycemia associated with general GK activation is not expected here and therefore has significant implications for future therapies against diabetes.
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Affiliation(s)
- Mohan Sreelekshmi
- Biochemistry and Molecular Mechanism Laboratory, Agro-processing and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala, India.,Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
| | - Kozhiparambil Gopalan Raghu
- Biochemistry and Molecular Mechanism Laboratory, Agro-processing and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala, India.,Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
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3
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Salgado M, Ordenes P, Villagra M, Uribe E, García-Robles MDLA, Tarifeño-Saldivia E. When a Little Bit More Makes the Difference: Expression Levels of GKRP Determines the Subcellular Localization of GK in Tanycytes. Front Neurosci 2019; 13:275. [PMID: 30983961 PMCID: PMC6449865 DOI: 10.3389/fnins.2019.00275] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2019] [Accepted: 03/08/2019] [Indexed: 01/01/2023] Open
Abstract
Glucose homeostasis is performed by specialized cells types that detect and respond to changes in systemic glucose concentration. Hepatocytes, β-cells and hypothalamic tanycytes are part of the glucosensor cell types, which express several proteins involved in the glucose sensing mechanism such as GLUT2, Glucokinase (GK) and Glucokinase regulatory protein (GKRP). GK catalyzes the phosphorylation of glucose to glucose-6-phosphate (G-6P), and its activity and subcellular localization are regulated by GKRP. In liver, when glucose concentration is low, GKRP binds to GK holding it in the nucleus, while the rise in glucose concentration induces a rapid export of GK from the nucleus to the cytoplasm. In contrast, hypothalamic tanycytes display inverse compartmentalization dynamic in response to glucose: a rise in the glucose concentration drives nuclear compartmentalization of GK. The underlying mechanism responsible for differential GK subcellular localization in tanycytes has not been described yet. However, it has been suggested that relative expression between GK and GKRP might play a role. To study the effects of GKRP expression levels in the subcellular localization of GK, we used insulinoma 832/13 cells and hypothalamic tanycytes to overexpress the tanycytic sequences of Gckr. By immunocytochemistry and Western blot analysis, we observed that overexpression of GKRP, independently of the cellular context, turns GK localization to a liver-like fashion, as GK is mainly localized in the nucleus in response to low glucose. Evaluating the expression levels of GKRP in relation to GK through RT-qPCR, suggest that excess of GKRP might influence the pattern of GK subcellular localization. In this sense, we propose that the low expression of GKRP (in relation to GK) observed in tanycytes is responsible, at least in part, for the compartmentalization pattern observed in this cell type. Since GKRP behaves as a GK inhibitor, the regulation of GKRP expression levels or activity in tanycytes could be used as a therapeutic target to regulate the glucosensing activity of these cells and consequently to regulate feeding behavior.
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Affiliation(s)
- Magdiel Salgado
- Department of Cellular Biology, Faculty of Biological Sciences, University of Concepción, Concepción, Chile
| | - Patricio Ordenes
- Department of Cellular Biology, Faculty of Biological Sciences, University of Concepción, Concepción, Chile
| | - Marcos Villagra
- Department of Cellular Biology, Faculty of Biological Sciences, University of Concepción, Concepción, Chile
| | - Elena Uribe
- Department of Biochemistry and Molecular Biology, Faculty of Biological Sciences, University of Concepción, Concepción, Chile
| | | | - Estefanía Tarifeño-Saldivia
- Department of Biochemistry and Molecular Biology, Faculty of Biological Sciences, University of Concepción, Concepción, Chile
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4
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Johansson BB, Fjeld K, Solheim MH, Shirakawa J, Zhang E, Keindl M, Hu J, Lindqvist A, Døskeland A, Mellgren G, Flatmark T, Njølstad PR, Kulkarni RN, Wierup N, Aukrust I, Bjørkhaug L. Nuclear import of glucokinase in pancreatic beta-cells is mediated by a nuclear localization signal and modulated by SUMOylation. Mol Cell Endocrinol 2017. [PMID: 28648619 DOI: 10.1016/j.mce.2017.06.020] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
The localization of glucokinase in pancreatic beta-cell nuclei is a controversial issue. Although previous reports suggest such a localization, the mechanism for its import has so far not been identified. Using immunofluorescence, subcellular fractionation and mass spectrometry, we present evidence in support of glucokinase localization in beta-cell nuclei of human and mouse pancreatic sections, as well as in human and mouse isolated islets, and murine MIN6 cells. We have identified a conserved, seven-residue nuclear localization signal (30LKKVMRR36) in the human enzyme. Substituting the residues KK31,32 and RR35,36 with AA led to a loss of its nuclear localization in transfected cells. Furthermore, our data indicates that SUMOylation of glucokinase modulates its nuclear import, while high glucose concentrations do not significantly alter the enzyme nuclear/cytosolic ratio. Thus, for the first time, we provide data in support of a nuclear import of glucokinase mediated by a redundant mechanism, involving a nuclear localization signal, and which is modulated by its SUMOylation. These findings add new knowledge to the functional role of glucokinase in the pancreatic beta-cell.
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Affiliation(s)
- Bente Berg Johansson
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Center for Medical Genetics and Molecular Medicine, Haukeland University Hospital, Bergen, Norway; Department of Pediatrics, Haukeland University Hospital, Bergen, Norway
| | - Karianne Fjeld
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Center for Medical Genetics and Molecular Medicine, Haukeland University Hospital, Bergen, Norway
| | - Marie Holm Solheim
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Center for Medical Genetics and Molecular Medicine, Haukeland University Hospital, Bergen, Norway; Section on Integrative Physiology and Metabolism, Joslin Diabetes Center and Harvard Medical School, Boston, MA, USA
| | - Jun Shirakawa
- Section on Islet Cell and Regenerative Biology, Joslin Diabetes Center, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School and Harvard Stem Cell Institute, Boston, MA, USA; Department of Endocrinology and Metabolism, Yokohama City University, Yokohama, Japan
| | | | - Magdalena Keindl
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Hormone Laboratory, Haukeland University Hospital, Bergen, Norway
| | - Jiang Hu
- Section on Islet Cell and Regenerative Biology, Joslin Diabetes Center, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School and Harvard Stem Cell Institute, Boston, MA, USA
| | | | - Anne Døskeland
- Proteomics Unit (PROBE), Department of Biomedicine, University of Bergen, Norway
| | - Gunnar Mellgren
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Hormone Laboratory, Haukeland University Hospital, Bergen, Norway; Department of Clinical Science, University of Bergen, Bergen, Norway
| | | | - Pål Rasmus Njølstad
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Department of Pediatrics, Haukeland University Hospital, Bergen, Norway
| | - Rohit N Kulkarni
- Section on Islet Cell and Regenerative Biology, Joslin Diabetes Center, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School and Harvard Stem Cell Institute, Boston, MA, USA
| | - Nils Wierup
- Lund University Diabetes Centre, Malmö, Sweden
| | - Ingvild Aukrust
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Center for Medical Genetics and Molecular Medicine, Haukeland University Hospital, Bergen, Norway
| | - Lise Bjørkhaug
- KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, Norway; Department of Biomedicine, University of Bergen, Bergen, Norway; Department of Biomedical Laboratory Sciences and Chemical Engineering, Western Norway University of Applied Sciences, Bergen, Norway.
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Affiliation(s)
- Loranne Agius
- Institutes of Cellular Medicine and Ageing and Health, Medical School, Newcastle University, Newcastle upon Tyne, NE2 4HH United Kingdom;
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Jin L, Guo T, Li Z, Lei Z, Li H, Mao Y, Wang X, Zhou N, Zhang Y, Hu R, Zhang X, Niu G, Irwin DM, Tan H. Role of glucokinase in the subcellular localization of glucokinase regulatory protein. Int J Mol Sci 2015; 16:7377-93. [PMID: 25849650 PMCID: PMC4425023 DOI: 10.3390/ijms16047377] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2015] [Revised: 03/03/2015] [Accepted: 03/18/2015] [Indexed: 11/16/2022] Open
Abstract
Glucokinase (GCK) is the rate-limiting enzyme of liver glucose metabolism. Through protein-protein interactions, glucokinase regulatory protein (GCKR) post-transcriptionally regulates GCK function in the liver, and causes its nuclear localization. However the role of GCK in regulating GCKR localization is unknown. In the present study, using in vitro and in vivo models, we examined the levels of GCK and GCKR, and their subcellular localization. We found that total cellular levels of GCKR did not vary in the in vivo models, but its subcellular localization did. In animals with normal levels of GCK, GCKR is mainly localized to the nuclei of hepatocytes. In seven-day old rats and liver-specific Gck gene knockout mice (animals that lack or have reduced levels of GCK protein), GCKR was found primarily in the cytoplasm. The interaction of GCK with GCKR was further examined using in vitro models where we varied the levels of GCK and GCKR. Varying the level of GCK protein had no effect on total cellular GCKR protein levels. Taken together, our results indicate that GCK is important for the localization of GCKR to the nucleus and raises the possibility that GCKR may have functions in addition to those regulating GCK activity in the cytoplasm.
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Affiliation(s)
- Ling Jin
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Tingting Guo
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Zhixin Li
- Department of Integrated Traditional Chinese and Western Medicine, Peking University, Health Science Center, Beijing 100191, China.
| | - Zhen Lei
- Department of Pharmacology, Ningxia Medical University, Yinchuan 750004, China.
| | - Hui Li
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Yiqing Mao
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Xi Wang
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Na Zhou
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Yizhuang Zhang
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Ruobi Hu
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Xuehui Zhang
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
| | - Gang Niu
- Beijing N&N Genetech Company, Beijing 100082, China.
| | - David M Irwin
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON M5S 1A8, Canada.
| | - Huanran Tan
- Department of Pharmacology, Peking University, Health Science Center, Beijing 100191, China.
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7
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Imaging liver and brain glycogen metabolism at the nanometer scale. NANOMEDICINE-NANOTECHNOLOGY BIOLOGY AND MEDICINE 2014; 11:239-45. [PMID: 25262580 DOI: 10.1016/j.nano.2014.09.007] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/21/2014] [Revised: 08/04/2014] [Accepted: 09/12/2014] [Indexed: 11/20/2022]
Abstract
In mammals, glycogen synthesis and degradation are dynamic processes regulating blood and cerebral glucose-levels within a well-defined physiological range. Despite the essential role of glycogen in hepatic and cerebral metabolism, its spatiotemporal distribution at the molecular and cellular level is unclear. By correlating electron microscopy and ultra-high resolution ion microprobe (NanoSIMS) imaging of tissue from fasted mice injected with (13)C-labeled glucose, we demonstrate that liver glycogenesis initiates in the hepatocyte perinuclear region before spreading toward the cell membrane. In the mouse brain, we observe that (13)C is inhomogeneously incorporated into astrocytic glycogen at a rate ~25 times slower than in the liver, in agreement with prior bulk studies. This experiment, using temporally resolved, nanometer-scale imaging of glycogen synthesis and degradation, provides greater insight into glucose metabolism in mammalian organs and shows how this technique can be used to explore biochemical pathways in healthy and diseased states.
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8
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Perreault L, Færch K, Kerege AA, Bacon SD, Bergman BC. Hepatic glucose sensing is impaired, but can be normalized, in people with impaired fasting glucose. J Clin Endocrinol Metab 2014; 99:E1154-62. [PMID: 24731008 PMCID: PMC4079303 DOI: 10.1210/jc.2013-3248] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
OBJECTIVE Abnormal endogenous glucose production (EGP) is a characteristic feature in people with impaired fasting glucose (IFG). We sought to determine whether impaired hepatic glucose sensing contributes to abnormal EGP in IFG and whether it could be experimentally restored. METHODS Glucose production (rate of appearance; Ra) and flux (glucose cycling) were assessed during a hyperglycemic-euinsulinemic somatostatin clamp with an infusion of [6,6-(2)H2-]glucose and [2-(2)H]glucose before and after enhanced hepatic glucokinase activity via an infusion of low-dose fructose in people with IFG and normal glucose tolerance (NGT). RESULTS During euglycemia, neither endogenous glucose production [(6,6-(2)H2)-glucose Ra; P = 0.53] or total glucose output (TGO; [2-(2)H]-glucose Ra; P = .12) was different between groups, but glucose cycling ([2-(2)H]glucose Ra to [6,6-(2)H2-]glucose Ra; a surrogate measure of hepatic glucokinase activity in the postabsorptive state) was lower in IFG than NGT (P = .04). Hyperglycemia suppressed EGP more in NGT than IFG (P < .01 for absolute or relative suppression, NGT vs IFG), whereas TGO decreased similarly in both groups (P = .77). The addition of fructose completely suppressed EGP in IFG (P < .01) and tended to do the same to TGO (P = .01; no such changes in NGT, P = .39-.55). Glucose cycling (which reflects glucose-6-phosphatase activity during glucose infusion) was similar in IFG and NGT (P = .51) during hyperglycemia and was unchanged and comparable between groups with the addition of fructose (P = .24). CONCLUSIONS In summary, glucose sensing is impaired in IFG but can be experimentally restored with low-dose fructose. Glucokinase activation may prove to be a novel strategy for the prevention of diabetes in this high-risk group.
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Affiliation(s)
- Leigh Perreault
- Division of Endocrinology, Metabolism, and Diabetes (L.P., A.A.K., S.D.B., B.C.B.), University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045; and Steno Diabetes Center (K.F.), DK-2820 Gentofte, Denmark
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Cullen KS, Al-Oanzi ZH, O'Harte FPM, Agius L, Arden C. Glucagon induces translocation of glucokinase from the cytoplasm to the nucleus of hepatocytes by transfer between 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase-2 and the glucokinase regulatory protein. BIOCHIMICA ET BIOPHYSICA ACTA 2014; 1843:1123-34. [PMID: 24566088 PMCID: PMC4024195 DOI: 10.1016/j.bbamcr.2014.02.006] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 10/29/2013] [Revised: 02/07/2014] [Accepted: 02/12/2014] [Indexed: 01/28/2023]
Abstract
Glucokinase activity is a major determinant of hepatic glucose metabolism and blood glucose homeostasis. Liver glucokinase activity is regulated acutely by adaptive translocation between the nucleus and the cytoplasm through binding and dissociation from its regulatory protein (GKRP) in the nucleus. Whilst the effect of glucose on this mechanism is well established, the role of hormones in regulating glucokinase location and its interaction with binding proteins remains unsettled. Here we show that treatment of rat hepatocytes with 25mM glucose caused decreased binding of glucokinase to GKRP, translocation from the nucleus and increased binding to 6-phosphofructo 2-kinase/fructose 2,6 bisphosphatase-2 (PFK2/FBPase2) in the cytoplasm. Glucagon caused dissociation of glucokinase from PFK2/FBPase2, concomitant with phosphorylation of PFK2/FBPase2 on Ser-32, uptake of glucokinase into the nucleus and increased interaction with GKRP. Two novel glucagon receptor antagonists attenuated the action of glucagon. This establishes an unequivocal role for hormonal control of glucokinase translocation. Given that glucagon excess contributes to the pathogenesis of diabetes, glucagon may play a role in the defect in glucokinase translocation and activity evident in animal models and human diabetes.
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Affiliation(s)
- Kirsty S Cullen
- Institute of Cellular Medicine, Newcastle University, Newcastle Upon Tyne, UK
| | - Ziad H Al-Oanzi
- Institute of Cellular Medicine, Newcastle University, Newcastle Upon Tyne, UK; Department of Laboratory Medicine, Al-Jouf University, Sakaka, Saudi Arabia
| | - Finbarr P M O'Harte
- The Saad Centre for Pharmacy & Diabetes, School of Biomedical Sciences, University of Ulster, Coleraine, UK
| | - Loranne Agius
- Institute of Cellular Medicine, Newcastle University, Newcastle Upon Tyne, UK
| | - Catherine Arden
- Institute of Cellular Medicine, Newcastle University, Newcastle Upon Tyne, UK.
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10
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Salgado M, Tarifeño-Saldivia E, Ordenes P, Millán C, Yañez MJ, Llanos P, Villagra M, Elizondo-Vega R, Martínez F, Nualart F, Uribe E, de los Angeles García-Robles M. Dynamic localization of glucokinase and its regulatory protein in hypothalamic tanycytes. PLoS One 2014; 9:e94035. [PMID: 24739934 PMCID: PMC3989220 DOI: 10.1371/journal.pone.0094035] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2013] [Accepted: 03/11/2014] [Indexed: 11/28/2022] Open
Abstract
Glucokinase (GK), the hexokinase involved in glucose sensing in pancreatic β cells, is also expressed in hypothalamic tanycytes, which cover the ventricular walls of the basal hypothalamus and are implicated in an indirect control of neuronal activity by glucose. Previously, we demonstrated that GK was preferentially localized in tanycyte nuclei in euglycemic rats, which has been reported in hepatocytes and is suggestive of the presence of the GK regulatory protein, GKRP. In the present study, GK intracellular localization in hypothalamic and hepatic tissues of the same rats under several glycemic conditions was compared using confocal microscopy and Western blot analysis. In the hypothalamus, increased GK nuclear localization was observed in hyperglycemic conditions; however, it was primarily localized in the cytoplasm in hepatic tissue under the same conditions. Both GK and GKRP were next cloned from primary cultures of tanycytes. Expression of GK by Escherichia coli revealed a functional cooperative protein with a S0.5 of 10 mM. GKRP, expressed in Saccharomyces cerevisiae, inhibited GK activity in vitro with a Ki 0.2 µM. We also demonstrated increased nuclear reactivity of both GK and GKRP in response to high glucose concentrations in tanycyte cultures. These data were confirmed using Western blot analysis of nuclear extracts. Results indicate that GK undergoes short-term regulation by nuclear compartmentalization. Thus, in tanycytes, GK can act as a molecular switch to arrest cellular responses to increased glucose.
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Affiliation(s)
- Magdiel Salgado
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Estefanía Tarifeño-Saldivia
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Patricio Ordenes
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Carola Millán
- Facultad de Artes Liberales, Universidad Adolfo Ibañez, Viña del Mar, Chile
| | - María José Yañez
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Paula Llanos
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Marcos Villagra
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Roberto Elizondo-Vega
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Fernando Martínez
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Francisco Nualart
- Departamento de Biología Celular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
| | - Elena Uribe
- Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias Biológicas, Universidad de Concepción, Concepción, Chile
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11
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Kaminski MT, Schultz J, Waterstradt R, Tiedge M, Lenzen S, Baltrusch S. Glucose-induced dissociation of glucokinase from its regulatory protein in the nucleus of hepatocytes prior to nuclear export. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2014; 1843:554-64. [DOI: 10.1016/j.bbamcr.2013.12.002] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2013] [Revised: 11/18/2013] [Accepted: 12/04/2013] [Indexed: 12/12/2022]
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12
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Bisphenol A impairs hepatic glucose sensing in C57BL/6 male mice. PLoS One 2013; 8:e69991. [PMID: 23922885 PMCID: PMC3726717 DOI: 10.1371/journal.pone.0069991] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2013] [Accepted: 06/18/2013] [Indexed: 01/23/2023] Open
Abstract
AIMS/HYPOTHESIS Glucose sensing (eg. glucokinase activity) becomes impaired in the development of type 2 diabetes, the etiology of which is unclear. Estrogen can stimulate glucokinase activity, whereas the pervasive environmental pollutant bisphenol A (BPA) can inhibit estrogen action, hence we aimed to determine the effect of BPA on glucokinase activity directly. METHODS To evaluate a potential acute effect on hepatic glucokinase activity, BPA in water (n = 5) vs. water alone (n = 5) was administered at the EPA's purported "safe dose" (50 µg/kg) by gavage to lean 6-month old male C57BL/6 mice. Two hours later, animals were euthanized and hepatic glucokinase activity measured over glucose levels from 1-20 mmol/l in liver homogenate. To determine the effect of chronic BPA exposure on hepatic glucokinase activity, lean 6-month old male C57BL/6 mice were provided with water (n = 15) or water with 1.75 mM BPA (∼50 µg/kg/day; n = 14) for 2 weeks. Following the 2-week exposure, animals were euthanized and glucokinase activity measured as above. RESULTS Hepatic glucokinase activity was signficantly suppressed after 2 hours in animals given an oral BPA bolus compared to those who received only water (p = 0.002-0.029 at glucose 5-20 mmol/l; overall treatment effect p<0.001). Exposure to BPA over 2 weeks also suppressed hepatic glucokinase activity in exposed vs. unexposed mice (overall treatment effect, p = 0.003). In both experiments, the Hill coefficient was higher and Vmax lower in mice treated with BPA. CONCLUSIONS/INTERPRETATION Both acute and chronic exposure to BPA significantly impair hepatic glucokinase activity and function. These findings identify a potential mechanism for how BPA may increase risk for diabetes.
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Rees MG, Wincovitch S, Schultz J, Waterstradt R, Beer NL, Baltrusch S, Collins FS, Gloyn AL. Cellular characterisation of the GCKR P446L variant associated with type 2 diabetes risk. Diabetologia 2012; 55:114-22. [PMID: 22038520 PMCID: PMC3276843 DOI: 10.1007/s00125-011-2348-5] [Citation(s) in RCA: 86] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/26/2011] [Accepted: 09/28/2011] [Indexed: 11/25/2022]
Abstract
AIMS/HYPOTHESIS Translation of genetic association signals into molecular mechanisms for diabetes has been slow. The glucokinase regulatory protein (GKRP; gene symbol GCKR) P446L variant, associated with inverse modulation of glucose- and lipid-related traits, has been shown to alter the kinetics of glucokinase (GCK) inhibition. As GCK inhibition is associated with nuclear sequestration, we aimed to determine whether this variant also alters the direct interaction between GKRP and GCK and their intracellular localisation. METHODS Fluorescently tagged rat and human wild-type (WT)- or P446L-GCKR and GCK were transiently transfected into HeLa cells and mouse primary hepatocytes. Whole-cell and nuclear fluorescence was quantified in individual cells exposed to low- or high-glucose conditions (5.5 or 25 mmol/l glucose, respectively). Interaction between GCK and GKRP was measured by sensitised emission-based fluorescence resonance energy transfer (FRET) efficiency. RESULTS P446L-GKRP had a decreased degree of nuclear localisation, ability to sequester GCK and direct interaction with GCK as measured by FRET compared with WT-GKRP. Decreased interaction was observed between WT-GKRP and GCK at high compared with low glucose, but not between P446L-GKRP and GCK. Rat WT-GKRP and P446L-GKRP behaved quite differently: both variants responded to high glucose by diminished sequestration of GCK but showed no effect of the P446L variant on nuclear localisation or GCK sequestration. CONCLUSIONS/INTERPRETATION Our study suggests the common human P446L-GKRP variant protein results in elevated hepatic glucose uptake and disposal by increasing active cytosolic GCK. This would increase hepatic lipid biosynthesis but decrease fasting plasma glucose concentrations and provides a potential mechanism for the protective effect of this allele on type 2 diabetes risk.
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Affiliation(s)
- M. G. Rees
- Oxford Centre for Diabetes Endocrinology & Metabolism, University of Oxford, Churchill Hospital, Headington, Oxford, OX3 7LJ UK
- National Human Genome Research Institute, National Institutes of Health, Bethesda, MD USA
| | - S. Wincovitch
- National Human Genome Research Institute, National Institutes of Health, Bethesda, MD USA
| | - J. Schultz
- Institute for Medical Biochemistry & Molecular Biology, University of Rostock, Rostock, Germany
| | - R. Waterstradt
- Institute for Medical Biochemistry & Molecular Biology, University of Rostock, Rostock, Germany
| | - N. L. Beer
- Oxford Centre for Diabetes Endocrinology & Metabolism, University of Oxford, Churchill Hospital, Headington, Oxford, OX3 7LJ UK
| | - S. Baltrusch
- Institute for Medical Biochemistry & Molecular Biology, University of Rostock, Rostock, Germany
| | - F. S. Collins
- National Human Genome Research Institute, National Institutes of Health, Bethesda, MD USA
| | - A. L. Gloyn
- Oxford Centre for Diabetes Endocrinology & Metabolism, University of Oxford, Churchill Hospital, Headington, Oxford, OX3 7LJ UK
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14
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Massa ML, Gagliardino JJ, Francini F. Liver glucokinase: An overview on the regulatory mechanisms of its activity. IUBMB Life 2011; 63:1-6. [PMID: 21280170 DOI: 10.1002/iub.411] [Citation(s) in RCA: 77] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2010] [Accepted: 11/29/2010] [Indexed: 11/09/2022]
Abstract
Blood glucose is the primary cellular substrate and in vivo must be tightly maintained. The liver plays a key role in glucose homeostasis increasing or decreasing glucose output and uptake during fasting and feeding. Glucokinase (GCK) is central to this process. Its activity is modulated in a coordinated manner via a complex set of mechanisms: in the postprandial period, the simultaneous rise in glucose and insulin increases GCK activity by enhanced gene expression, changes in cellular location, and interaction with regulatory proteins. Conversely, in the fasting state, the combined decrease in glucose and insulin concentrations and increase in glucagon concentrations, halt GCK activity. Herein we summarize the current knowledge regarding the regulation of hepatic GCK activity.
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Affiliation(s)
- María L Massa
- CENEXA, Centro de Endocrinología Experimental y Aplicada (UNLP-CONICET LA PLATA, Centro Colaborador OPS/OMS), Facultad de Ciencias Médicas, 60 y 120, 1900 La Plata, Argentina
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15
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Watanabe F, Furuya E. Quantitative image analysis reveals that phosphorylation of liver-type isozyme of fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase does not affect nuclear translocation of glucokinase in rat primary hepatocytes. J Biochem 2010; 148:713-9. [PMID: 20843823 DOI: 10.1093/jb/mvq107] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
We have developed a new quantification method to measure translocation of glucokinase between nucleus and cytoplasm in primary hepatocytes. The method is robust, reliable and sensitive with the use of a high content fluorescence microscope, which can analyse more than 20,000 hepatocytes under each experimental condition. Frequency distributions of the nuclear and cytoplasmic contents of glucokinase did not exhibit a Gaussian distribution. Moreover, the distributions have large standard deviation values compared with their average values. These results indicate that a large number of cells must be analysed for the accurate quantification. Glucose and sorbitol promoted the translocation of glucokinase from nucleus to cytoplasm. These results show good agreement with previous reports. However, glucagon did not affect the localization of glucokinase. Under the same conditions, liver-type isozyme of fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase (F6P2K), whose dephosphorylated form has been proposed as a cytoplasmic binding protein with glucokinase, was completely phosphorylated. These results indicate that the phosphorylation and dephosphorylation of F6P2K does not have any appreciable effect on the intracellular localization of glucokinase.
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Affiliation(s)
- Fusao Watanabe
- Department of Chemistry, Osaka Medical College, 2-7 Daigaku-machi, Takatsuki, Osaka, Japan.
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16
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Abstract
The glucokinase (GCK) gene was one of the first candidate genes to be identified as a human “diabetes gene". Subsequently, important advances were made in understanding the impact of GCK in the regulation of glucose metabolism. Structure elucidation by crystallography provided insight into the kinetic properties of GCK. Protein interaction partners of GCK were discovered. Gene expression studies revealed new facets of the tissue distribution of GCK, including in the brain, and its regulation by insulin in the liver. Metabolic control analysis coupled to gene overexpression and knockout experiments highlighted the unique impact of GCK as a regulator of glucose metabolism. Human GCK mutants were studied biochemically to understand disease mechanisms. Drug development programs identified small molecule activators of GCK as potential antidiabetics. These advances are summarized here, with the aim of offering an integrated view of the role of GCK in the molecular physiology and medicine of glucose homeostasis.
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Affiliation(s)
- P B Iynedjian
- Department of Cell Physiolgy and Metabolism, University of Geneva School of Medicine, CMU 1 Rue Michel-Servet, 1211 Geneva 4, Switzerland.
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17
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Abstract
Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.
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18
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Mukhtar MH, Payne VA, Arden C, Harbottle A, Khan S, Lange AJ, Agius L. Inhibition of glucokinase translocation by AMP-activated protein kinase is associated with phosphorylation of both GKRP and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Am J Physiol Regul Integr Comp Physiol 2008; 294:R766-74. [PMID: 18199594 DOI: 10.1152/ajpregu.00593.2007] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The rate of glucose phosphorylation in hepatocytes is determined by the subcellular location of glucokinase and by its association with its regulatory protein (GKRP) in the nucleus. Elevated glucose concentrations and precursors of fructose 1-phosphate (e.g., sorbitol) cause dissociation of glucokinase from GKRP and translocation to the cytoplasm. In this study, we investigated the counter-regulation of substrate-induced translocation by AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside), which is metabolized by hepatocytes to an AMP analog, and causes activation of AMP-activated protein kinase (AMPK) and depletion of ATP. During incubation of hepatocytes with 25 mM glucose, AICAR concentrations below 200 microM activated AMPK without depleting ATP and inhibited glucose phosphorylation and glucokinase translocation with half-maximal effect at 100-140 microM. Glucose phosphorylation and glucokinase translocation correlated inversely with AMPK activity. AICAR also counteracted translocation induced by a glucokinase activator and partially counteracted translocation by sorbitol. However, AICAR did not block the reversal of translocation (from cytoplasm to nucleus) after substrate withdrawal. Inhibition of glucose-induced translocation by AICAR was greater than inhibition by glucagon and was associated with phosphorylation of both GKRP and the cytoplasmic glucokinase binding protein, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) on ser-32. Expression of a kinase-active PFK2 variant lacking ser-32 partially reversed the inhibition of translocation by AICAR. Phosphorylation of GKRP by AMPK partially counteracted its inhibitory effect on glucokinase activity, suggesting altered interaction of glucokinase and GKRP. In summary, mechanisms downstream of AMPK activation, involving phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and GKRP are involved in the ATP-independent inhibition of glucose-induced glucokinase translocation by AICAR in hepatocytes.
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Affiliation(s)
- Mohammed H Mukhtar
- Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, NE2 4HH, UK
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19
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Payne VA, Arden C, Lange AJ, Agius L. Contributions of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 to the elevated glycolysis in hepatocytes from Zucker fa/fa rats. Am J Physiol Regul Integr Comp Physiol 2007; 293:R618-25. [PMID: 17553851 DOI: 10.1152/ajpregu.00061.2007] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The insulin-resistant Zucker fa/fa rat has elevated hepatic glycolysis and activities of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 (PFK2). The latter catalyzes the formation and degradation of fructose-2,6-bisphosphate (fructose-2,6-P2) and is a glucokinase-binding protein. The contributions of glucokinase and PFK2 to the elevated glycolysis in fa/fa hepatocytes were determined by overexpressing these enzymes individually or in combination. Metabolic control analysis was used to determine enzyme coefficients on glycolysis and metabolite concentrations. Glucokinase had a high control coefficient on glycolysis in all hormonal conditions tested, whereas PFK2 had significant control only in the presence of glucagon, which phosphorylates PFK2 and suppresses glycolysis. Despite the high control strength of glucokinase, the elevated glycolysis in fa/fa hepatocytes could not be explained by the elevated glucokinase activity alone. In hepatocytes from fa/fa rats, glucokinase translocation between the nucleus and the cytoplasm was refractory to glucose but responsive to glucagon. Expression of a kinase-active PFK2 variant reversed the glucagon effect on glucokinase translocation and glucose phosphorylation, confirming the role for PFK2 in sequestering glucokinase in the cytoplasm. Glucokinase had a high control on glucose-6-phosphate content; however, like PFK2, it had a relative modest effect on the fructose-2,6-P2 content. However, combined overexpression of glucokinase and PFK2 had a synergistic effect on fructose-2,6-P2 levels, suggesting that interaction of these enzymes may be a prerequisite for formation of fructose-2,6-P2. Cumulatively, this study provides support for coordinate roles for glucokinase and PFK2 in the elevated hepatic glycolysis in fa/fa rats.
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Affiliation(s)
- Victoria A Payne
- Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, UK
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20
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Arden C, Trainer A, de la Iglesia N, Scougall KT, Gloyn AL, Lange AJ, Shaw JAM, Matschinsky FM, Agius L. Cell biology assessment of glucokinase mutations V62M and G72R in pancreatic beta-cells: evidence for cellular instability of catalytic activity. Diabetes 2007; 56:1773-82. [PMID: 17389332 DOI: 10.2337/db06-1151] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Mutations in the glucokinase (GK) gene cause defects in blood glucose homeostasis. In some cases (V62M and G72R), the phenotype cannot be explained by altered enzyme kinetics or protein instability. We used transient and stable expression of green fluorescent protein (GFP) GK chimaeras in MIN6 beta-cells to study the phenotype defect of V62M and G72R. GK activity in lysates of MIN6 cell lines stably expressing wild-type or mutant GFP GK showed the expected affinity for glucose and response to pharmacological activators, indicating the expression of catalytically active enzymes. MIN6 cells stably expressing GFP V62M or GFP G72R had a lower GK activity-to-GK immunoreactivity ratio and GK activity-to-GK mRNA ratio but not GK immunoreactivity-to-GK mRNA ratio than wild-type GFP GK. Heterologous expression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FDP2) in cell lines increased GK activity for wild-type GK and V62M but not for G72R, whereas expression of liver GK regulatory protein (GKRP) increased GK activity for wild type but not V62M or G72R. Lack of interaction of these mutants with GKRP was also evident in hepatocyte transfections from the lack of nuclear accumulation. These results suggest that cellular loss of GK catalytic activity rather than impaired translation or enhanced protein degradation may account for the hyperglycemia in subjects with V62M and G72R mutations.
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Affiliation(s)
- Catherine Arden
- Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, UK
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21
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Futamura M, Hosaka H, Kadotani A, Shimazaki H, Sasaki K, Ohyama S, Nishimura T, Eiki JI, Nagata Y. An allosteric activator of glucokinase impairs the interaction of glucokinase and glucokinase regulatory protein and regulates glucose metabolism. J Biol Chem 2006; 281:37668-74. [PMID: 17028192 DOI: 10.1074/jbc.m605186200] [Citation(s) in RCA: 81] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Glucokinase (GK) plays a key role in the control of blood glucose homeostasis. We identified a small molecule GK activator, compound A, that increased the glucose affinity and maximal velocity (V(max)) of GK. Compound A augmented insulin secretion from isolated rat islets and enhanced glucose utilization in primary cultured rat hepatocytes. In rat oral glucose tolerance tests, orally administrated compound A lowered plasma glucose elevation with a concomitant increase in plasma insulin and hepatic glycogen. In liver, GK activity is acutely controlled by its association to the glucokinase regulatory protein (GKRP). In order to decipher the molecular aspects of how GK activator affects the shuttling of GK between nucleus and cytoplasm, the effect of compound A on GK-GKRP interaction was further investigated. Compound A increased the level of cytoplasmic GK in both isolated rat primary hepatocytes and the liver tissues from rats. Experiments in a cell-free system revealed that compound A interacted with glucose-bound free GK, thereby impairing the association of GK and GKRP. On the other hand, compound A did not bind to glucose-unbound GK or GKRP-associated GK. Furthermore, we found that glucose-dependent GK-GKRP interaction also required ATP. Given the combined prominent role of GK on insulin secretion and hepatic glucose metabolism where the GK-GKRP mechanism is involved, activation of GK has a new therapeutic potential in the treatment of type 2 diabetes.
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Affiliation(s)
- Mayumi Futamura
- Tsukuba Research Institute, Banyu Pharmaceutical Co., Ltd., Tsukuba, Ibaraki 300-2611, Japan
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22
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Arden C, Baltrusch S, Agius L. Glucokinase regulatory protein is associated with mitochondria in hepatocytes. FEBS Lett 2006; 580:2065-70. [PMID: 16542652 DOI: 10.1016/j.febslet.2006.03.009] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2005] [Revised: 03/01/2006] [Accepted: 03/02/2006] [Indexed: 11/16/2022]
Abstract
The association of glucokinase with liver mitochondria has been reported [Danial et al. (2003) BAD and glucokinase reside in a mitochondrial complex that integrates glycolysis and apoptosis. Nature 424, 952-956]. We confirmed association of glucokinase immunoreactivity with rat liver mitochondria using Percoll gradient centrifugation and demonstrated its association with the 68 kDa regulatory protein (GKRP) but not with the binding protein phosphofructokinase-2/fructose bisphosphatase-2. Substrates and glucagon induced adaptive changes in the mitochondrial glucokinase/GKRP ratio suggesting a regulatory role for GKRP. Combined with previous observations that GKRP overexpression partially inhibits glycolysis [de la Iglesia et al. (2000) The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte. J. Biol. Chem. 275, 10597-10603] these findings suggest that there may be distinct glycolytic pools of glucokinase.
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Affiliation(s)
- Catherine Arden
- School of Clinical Medical Sciences-Diabetes, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, NE2 4HH, UK
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23
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Kumar S, Saradhi M, Chaturvedi NK, Tyagi RK. Intracellular localization and nucleocytoplasmic trafficking of steroid receptors: an overview. Mol Cell Endocrinol 2006; 246:147-56. [PMID: 16388893 DOI: 10.1016/j.mce.2005.11.028] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Subcellular compartmentalization and dynamic movements of steroid receptors are major steps in executing their transcription regulatory function. Though significant progress has been made in understanding the mechanisms underlying nuclear import of NLS-bearing proteins, our general and mechanistic understanding about the nuclear export processes has begun to emerge only recently. The discovery of most commonly utilized CRM1/exportin1 dependent nuclear export pathway is attributed to a potent nuclear export inhibitor leptomycin B that helped dissecting this and other nuclear export pathways. Simultaneously, utilization of green fluorescent protein (GFP)-tagged intracellular steroid receptors has contributed to not only resolving controversial issue of subcellular localization of unliganded hormone receptors but also provided further insight into finer details of receptor dynamics in living cells. With judicious use of leptomycin B and expression of GFP-tagged receptors in living cells, existence of exportin1/CRM1 independent pathway(s), nuclear export signals and receptors for bi-directional translocation that are unique to steroid receptor trafficking have been specified. Currently, we appear to be arriving at a consensus that steroid/nuclear receptors follow dynamic nucleocytoplasmic processes that deviate from the ones commonly utilized by majority of other proteins.
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Affiliation(s)
- Sanjay Kumar
- Special Centre for Molecular Medicine, Jawaharlal Nehru University, New Delhi 110067, India
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24
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Baltrusch S, Francini F, Lenzen S, Tiedge M. Interaction of glucokinase with the liver regulatory protein is conferred by leucine-asparagine motifs of the enzyme. Diabetes 2005; 54:2829-37. [PMID: 16186382 DOI: 10.2337/diabetes.54.10.2829] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
The glucokinase regulatory protein (GRP) plays a pivotal role in the regulation of metabolic flux in liver by the glucose-phosphorylating enzyme glucokinase. Random peptide phage display library screening for binding partners of GRP allowed the identification of an asparagine-leucine consensus motif. Asparagine-leucine motifs of glucokinase located in the hinge region, as well as in the large domain, were changed by site-directed mutagenesis. The L58R/N204Y and the L309R/N313Y glucokinase mutants showed a significantly reduced interaction with GRP. The L355R/N350Y mutant had a fivefold-higher binding affinity for GRP than wild-type glucokinase. Imaging of glucokinase and GRP fluorescence fusion proteins revealed that the L58R/N204Y glucokinase mutant lacked glucose-dependent translocation by GRP, whereas the L355R/N350Y glucokinase mutant was trapped in the nucleus due to high affinity for GRP. The results indicate that the L58/N204 motif in the hinge region confers binding to GRP, while the L355/N350 motif may modulate the binding affinity for GRP. This latter motif is part of the alpha10 helix of glucokinase and accessible to GRP in the free and complex conformation.
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Affiliation(s)
- Simone Baltrusch
- Institute of Clinical Biochemistry, Hannover Medical School, D-30623 Hannover, Germany.
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25
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Abstract
The liver is an important site of postprandial glucose disposal, accounting for the removal of up to 30% of an oral glucose load. The liver is also centrally involved in dietary lipid and amino acid uptake, and the presence of either or both of these nutrients can influence hepatic glucose uptake. The composition of ingested carbohydrate also influences hepatic glucose metabolism. For example, fructose can increase hepatic glucose uptake. In addition, fructose extraction by the liver is exceedingly high, approaching 50% to 70% of fructose delivery. The selective hepatic metabolism of fructose, and the ability of fructose to increase hepatic glucose uptake can, under appropriate conditions (eg, diets enriched in sucrose or fructose, high fructose concentrations), provoke major adaptations in hepatic metabolism. Potential adaptations that can arise in response to these conditions and putative mechanisms driving these adaptations are the subject of this review.
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Affiliation(s)
- Michael E Bizeau
- Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, CO 80523, USA
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Payne VA, Arden C, Wu C, Lange AJ, Agius L. Dual role of phosphofructokinase-2/fructose bisphosphatase-2 in regulating the compartmentation and expression of glucokinase in hepatocytes. Diabetes 2005; 54:1949-57. [PMID: 15983194 DOI: 10.2337/diabetes.54.7.1949] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Hepatic glucokinase is regulated by a 68-kDa regulatory protein (GKRP) that is both an inhibitor and nuclear receptor for glucokinase. We tested the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) in regulating glucokinase compartmentation in hepatocytes. PFK2 catalyzes formation or degradation of the regulator of glycolysis fructose 2,6-bisphosphate (fructose 2,6-P2), depending on its phosphorylation state (ser-32), and is also a glucokinase-binding protein. Incubation of hepatocytes at 25 mmol/l glucose causes translocation of glucokinase from the nucleus to the cytoplasm and an increase in fructose 2,6-P2. Glucagon caused phosphorylation of PFK2-ser-32, lowered the fructose 2,6-P2 concentration, and inhibited glucose-induced translocation of glucokinase. These effects of glucagon were reversed by expression of a kinase-active PFK2 mutant (S32A/H258A) that overrides the suppression of fructose 2,6-P2 but not by overexpression of wild-type PFK2. Overexpression of PFK2 potentiated glucokinase expression in hepatocytes transduced with an adenoviral vector-encoding glucokinase by a mechanism that does not involve stabilization of glucokinase protein from degradation. It is concluded that PFK2 has a dual role in regulating glucokinase in hepatocytes: it potentiates glucokinase protein expression by posttranscriptional mechanisms and favors its cytoplasmic compartmentation. Thus, it acts in a complementary mechanism to GKRP, which also regulates glucokinase protein expression and compartmentation.
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Affiliation(s)
- Victoria A Payne
- School of Clinical Medical Sciences-Diabetes, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, NE2 4HH, UK
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27
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Chu CA, Fujimoto Y, Igawa K, Grimsby J, Grippo JF, Magnuson MA, Cherrington AD, Shiota M. Rapid translocation of hepatic glucokinase in response to intraduodenal glucose infusion and changes in plasma glucose and insulin in conscious rats. Am J Physiol Gastrointest Liver Physiol 2004; 286:G627-34. [PMID: 14656711 DOI: 10.1152/ajpgi.00218.2003] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The rate of liver glucokinase (GK) translocation from the nucleus to the cytoplasm in response to intraduodenal glucose infusion and the effect of physiological rises of plasma glucose and/or insulin on GK translocation were examined in 6-h-fasted conscious rats. Intraduodenal glucose infusion (28 mg.kg(-1).min(-1) after a priming dose at 500 mg/kg) elevated blood glucose levels (mg/dl) in the artery and portal vein from 90 +/- 3 and 87 +/- 3 to 154 +/- 4 and 185 +/- 4, respectively, at 10 min. At 120 min, the levels had decreased to 133 +/- 6 and 156 +/- 5, respectively. Plasma insulin levels (ng/ml) in the artery and the portal vein rose from 0.7 +/- 0.1 and 1.8 +/- 0.3 to 11.8 +/- 1.5 and 20.2 +/- 2.0 at 10 min, respectively, and 12.4 +/- 3.1 and 18.0 +/- 4.8 at 30 min, respectively. GK was rapidly exported from the nucleus as determined by measuring the ratio of the nuclear to the cytoplasmic immunofluorescence (N/C) of GK (2.9 +/- 0.3 at 0 min to 1.7 +/- 0.2 at 10 min, 1.5 +/- 0.1 at 20 min, 1.3 +/- 0.1 at 30 min, and 1.3 +/- 0.1 at 120 min). When plasma glucose (arterial; mg/dl) and insulin (arterial; ng/ml) levels were clamped for 30 min at 93 +/- 7 and 0.7 +/- 0.1, 81 +/- 5 and 8.9 +/- 1.3, 175 +/- 5 and 0.7 +/- 0.1, or 162 +/- 5 and 9.2 +/- 1.5, the N/C of GK was 3.0 +/- 0.5, 1.8 +/- 0.1, 1.5 +/- 0.1, and 1.2 +/- 0.1, respectively. The N/C of GK regulatory protein (GKRP) did not change in response to the intraduodenal glucose infusion or the rise in plasma glucose and/or insulin levels. The results suggest that GK but not GKRP translocates rapidly in a manner that corresponds with changes in the hepatic glucose balance in response to glucose ingestion in vivo. Additionally, the translocation of GK is induced by the postprandial rise in plasma glucose and insulin.
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Affiliation(s)
- Chang An Chu
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615, USA
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Brocklehurst KJ, Payne VA, Davies RA, Carroll D, Vertigan HL, Wightman HJ, Aiston S, Waddell ID, Leighton B, Coghlan MP, Agius L. Stimulation of hepatocyte glucose metabolism by novel small molecule glucokinase activators. Diabetes 2004; 53:535-41. [PMID: 14988235 DOI: 10.2337/diabetes.53.3.535] [Citation(s) in RCA: 118] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Glucokinase (GK) has a major role in the control of blood glucose homeostasis and is a strong potential target for the pharmacological treatment of type 2 diabetes. We report here the mechanism of action of two novel and potent direct activators of GK: 6-[(3-isobutoxy-5-isopropoxybenzoyl)amino]nicotinic acid(GKA1) and 5-([3-isopropoxy-5-[2-(3-thienyl)ethoxy]benzoyl]amino)-1,3,4-thiadiazole-2-carboxylic acid(GKA2), which increase the affinity of GK for glucose by 4- and 11-fold, respectively. GKA1 increased the affinity of GK for the competitive inhibitor mannoheptulose but did not affect the affinity for the inhibitors palmitoyl-CoA and the endogenous 68-kDa regulator (GK regulatory protein [GKRP]), which bind to allosteric sites or to N-acetylglucosamine, which binds to the catalytic site. In hepatocytes, GKA1 and GKA2 stimulated glucose phosphorylation, glycolysis, and glycogen synthesis to a similar extent as sorbitol, a precursor of fructose 1-phosphate, which indirectly activates GK through promoting its dissociation from GKRP. Consistent with their effects on isolated GK, these compounds also increased the affinity of hepatocyte metabolism for glucose. GKA1 and GKA2 caused translocation of GK from the nucleus to the cytoplasm. This effect was additive with the effect of sorbitol and is best explained by a "glucose-like" effect of the GK activators in translocating GK to the cytoplasm. In conclusion, GK activators are potential antihyperglycemic agents for the treatment of type 2 diabetes through the stimulation of hepatic glucose metabolism by a mechanism independent of GKRP.
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Affiliation(s)
- Katy J Brocklehurst
- Cardiovascular and Gastrointestinal Department, AstraZeneca, Macclesfield, Cheshire, U.K
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Ferrer JC, Favre C, Gomis RR, Fernández-Novell JM, García-Rocha M, de la Iglesia N, Cid E, Guinovart JJ. Control of glycogen deposition. FEBS Lett 2003; 546:127-32. [PMID: 12829248 DOI: 10.1016/s0014-5793(03)00565-9] [Citation(s) in RCA: 153] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Traditionally, glycogen synthase (GS) has been considered to catalyze the key step of glycogen synthesis and to exercise most of the control over this metabolic pathway. However, recent advances have shown that other factors must be considered. Moreover, the control of glycogen deposition does not follow identical mechanisms in muscle and liver. Glucose must be phosphorylated to promote activation of GS. Glucose-6-phosphate (Glc-6-P) binds to GS, causing the allosteric activation of the enzyme probably through a conformational rearrangement that simultaneously converts it into a better substrate for protein phosphatases, which can then lead to the covalent activation of GS. The potency of Glc-6-P for activation of liver GS is determined by its source, since Glc-6-P arising from the catalytic action of glucokinase (GK) is much more effective in mediating the activation of the enzyme than the same metabolite produced by hexokinase I (HK I). As a result, hepatic glycogen deposition from glucose is subject to a system of control in which the 'controller', GS, is in turn controlled by GK. In contrast, in skeletal muscle, the control of glycogen synthesis is shared between glucose transport and GS. The characteristics of the two pairs of isoenzymes, liver GS/GK and muscle GS/HK I, and the relationships that they establish are tailored to suit specific metabolic roles of the tissues in which they are expressed. The key enzymes in glycogen metabolism change their intracellular localization in response to glucose. The changes in the intracellular distribution of liver GS and GK triggered by glucose correlate with stimulation of glycogen synthesis. The translocation of GS, which constitutes an additional mechanism of control, causes the orderly deposition of hepatic glycogen and probably represents a functional advantage in the metabolism of the polysaccharide.
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Affiliation(s)
- Juan C Ferrer
- Departament de Bioquímica i Biologia Molecular, Universitat de Barcelona, 08028 Barcelona, Spain
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Abstract
Transdifferentiation of pancreas to liver is a well-recognized phenomenon and has been described in animal experiments and human pathology. We recently produced an in vitro model for the transdifferentiation (or conversion) of the pancreatic cell line AR42J-B13 to hepatocytes based on culture with dexamethasone (Dex). To determine whether the hepatocytes express markers of hepatic intermediary metabolism and detoxification, we investigated the patterns of expression of glucokinase, cytochrome P450s CYP3A1 and CYP2B1/2, testosterone/4-nitrophenol uridine diphosphate glucuronosyltransferase (UDPGT), and aryl sulfotransferase. All were expressed. We also determined the expression of 2 enzymes involved in ammonia detoxification: carbamoylphosphate synthetase I (CPS I) and glutamine synthetase (GS). These enzymes are normally strictly compartmentalized in liver in a wide periportal pattern and the last downstream perivenous hepatocytes, respectively. Following culture with Dex, CPS I and GS are expressed in 2 different cell populations, suggesting that both periportal and perivenous hepatocytes are induced. We also produced a reporter assay based on the activation of green fluorescent protein (GFP) by the transthyretin (TTR) promoter or glucose-6-phosphatase (G6Pase) promoter. After culture with Dex, transfected cells begin to express GFP, showing that hepatic promoters are activated in concert with the induction of the hepatocyte phenotype. Lastly, we examined the stability of the hepatic phenotype and found that some cells still express liver markers (transferrin or albumin) up to 14 days after removal of Dex. In conclusion, these results suggest that pancreatic hepatocytes produced by this method may offer an alternative model to primary cultures of hepatocytes for the study of liver function.
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Affiliation(s)
- David Tosh
- Developmental Biology Programme, Department of Biology and Biochemistry, University of Bath, Bath, England.
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31
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Commerford SR, Ferniza JB, Bizeau ME, Thresher JS, Willis WT, Pagliassotti MJ. Diets enriched in sucrose or fat increase gluconeogenesis and G-6-Pase but not basal glucose production in rats. Am J Physiol Endocrinol Metab 2002; 283:E545-55. [PMID: 12169448 DOI: 10.1152/ajpendo.00120.2002] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
High-fat (HFD) and high-sucrose diets (HSD) reduce insulin suppression of glucose production in vivo, increase the capacity for gluconeogenesis in vitro, and increase glucose-6-phosphatase (G-6-Pase) activity in whole cell homogenates. The present study examined the effects of HSD and HFD on in vivo gluconeogenesis, the catalytic and glucose-6-phosphate translocase subunits of G-6-Pase, glucokinase (GK) translocation, and glucose cycling. Rats were fed a high-starch control diet (STD; 68% cornstarch), HSD (68% sucrose), or HFD (45% fat) for 7-13 days. The ratio of 3H in C6:C2 of glucose after 3H2O injection into 6- to 8-h-fasted rats was significantly increased in HSD (0.68 +/- 0.07) and HFD (0.71 +/- 0.08) vs. STD (0.40 +/- 0.10). G-6-Pase activity was significantly higher in HSD and HFD vs. STD in both intact and disrupted liver microsomes. HSD and HFD significantly increased the amount of the p36 catalytic subunit protein, whereas the p46 glucose-6-phosphate translocase protein was increased in HSD only. Despite increased nonglycerol gluconeogenesis and increased G-6-Pase, basal glucose and insulin levels as well as glucose production were not significantly different among groups. Hepatocyte cell suspensions were used to ascertain whether diet-induced adaptations in glucose phosphorylation and GK might serve to compensate for upregulation of G-6-Pase. Tracer-estimated glucose phosphorylation and glucose cycling (glucose <--> glucose 6-phosphate) were significantly higher in cells isolated from HSD only. After incubation with either 5 or 20 mM glucose and no insulin, GK activity (nmol. mg protein(-1). min(-1)) in digitonin-treated eluates (translocated GK) was significantly higher in HSD (32 +/- 4 and 146 +/- 6) vs. HFD (4 +/- 1 and 83 +/- 10) and STD (9 +/- 2 and 87 +/- 9). Thus short-term, chronic exposure to HSD and HFD increase in vivo gluconeogenesis and the G-6-Pase catalytic subunit. Exposure to HSD diet also leads to adaptations in glucose phosphorylation and GK translocation.
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Affiliation(s)
- S Renee Commerford
- Exercise Science Research Institute, Arizona State University, Tempe, Arizona 85287, USA
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Gomis RR, Cid E, García-Rocha M, Ferrer JC, Guinovart JJ. Liver glycogen synthase but not the muscle isoform differentiates between glucose 6-phosphate produced by glucokinase or hexokinase. J Biol Chem 2002; 277:23246-52. [PMID: 11882651 DOI: 10.1074/jbc.m111208200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Using adenovirus-mediated gene transfer into FTO-2B cells, a rat hepatoma cell line, we have overexpressed hexokinase I (HK I), glucokinase (GK), liver glycogen synthase (LGS), muscle glycogen synthase (MGS), and combinations of each of the two glucose-phosphorylating enzymes with each one of the GS isoforms. FTO-2B cells do not synthesize glycogen even when incubated with high doses of glucose. Adenovirus-induced overexpression of HK I and/or LGS, two enzymes endogenously expressed by these cells, did not produce a significant increase in the levels of active GS and the total glycogen content. In contrast, GK overexpression led to the glucose-dependent activation of endogenous or overexpressed LGS and to the accumulation of glycogen. Similarly overexpressed MGS was efficiently activated by the glucose-6-phosphate (Glc-6-P) produced by either endogenous or overexpressed HK I and by overexpressed GK. These results indicate the existence of at least two pools of Glc-6-P in the cell, one of them is accessible to both isoforms of GS and is replenished by the action of GK, whereas LGS is excluded from the cellular compartment where the Glc-6-P produced by HK I is directed. These findings are interpreted in terms of the metabolic role that the two pairs of enzymes, HK I-MGS in the muscle and GK-LGS in the hepatocyte, perform in their respective tissues.
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Affiliation(s)
- Roger R Gomis
- Department of Biochemistry and Molecular Biology and Barcelona Science Park, Universitat de Barcelona, E-08028 Barcelona, Spain
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Slosberg ED, Desai UJ, Fanelli B, St Denny I, Connelly S, Kaleko M, Boettcher BR, Caplan SL. Treatment of type 2 diabetes by adenoviral-mediated overexpression of the glucokinase regulatory protein. Diabetes 2001; 50:1813-20. [PMID: 11473043 DOI: 10.2337/diabetes.50.8.1813] [Citation(s) in RCA: 62] [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
The enzyme glucokinase (GK) plays a central role in glucose homeostasis. Hepatic GK activity is acutely controlled by the action of the GK regulatory protein (GKRP). In vitro evidence suggests that GKRP reversibly binds to GK and inhibits its activity; however, less is known about the in vivo function of GKRP. To further explore the physiological role of GKRP in vivo, we used an E1/E2a/E3-deficient adenoviral vector containing the cDNA encoding human GKRP (Av3hGKRP). High fat diet-induced diabetic mice were administered Av3hGKRP or a control vector lacking a transgene (Av3Null). Surprisingly, the Av3hGKRP-treated mice showed a significant improvement in glucose tolerance and had lower fasting blood glucose levels than Av3Null-treated mice. A coincident decrease in insulin levels indicated that the Av3hGKRP-treated mice had sharply improved insulin sensitivity. These mice also exhibited lower leptin levels, reduced body weight, and decreased liver GK activity. In vitro experiments indicated that GKRP was able to increase both GK protein and enzymatic activity levels, suggesting that another role for GKRP is to stabilize and/or protect GK. These data are the first to indicate the ability of GKRP to treat type 2 diabetes and therefore have significant implications for future therapies of this disease.
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Affiliation(s)
- E D Slosberg
- Novartis Institute for Biomedical Research, Metabolic and Cardiovascular Diseases, Summit, New Jersey 07901, USA.
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Caseras A, Metón I, Fernández F, Baanante IV. Glucokinase gene expression is nutritionally regulated in liver of gilthead sea bream (Sparus aurata). BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1493:135-41. [PMID: 10978515 DOI: 10.1016/s0167-4781(00)00173-1] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Glucose intolerance in carnivorous fish has been attributed to the lack of hepatic glucokinase (GK) activity. Transcription/translation assay and transient transfection of COS-7 cells with a cDNA encoding Sparus aurata liver GK showed the functionality of the enzyme in vitro. The endogenous fish hepatic GK had lower affinity for glucose than the rat enzyme. The GK activity values in fed fish were similar to those reported for starved and diabetic rats. In this study, we also addressed the nutritional regulation of GK gene expression in fish liver. Starvation and energy restriction decreased S. aurata hepatic GK mRNA and activity levels, as previously reported in rats. In contrast, the fish enzyme expression exhibited a delayed onset during the daily feeding rhythm. These findings demonstrate for the first time the presence and the nutritional modulation of a functional GK activity in fish liver and contribute to explain the low ability of carnivorous fish to metabolize carbohydrates.
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Affiliation(s)
- A Caseras
- Department de Bioquimica i Biologia Molecular, Facultat de Farmácia, Universitat de Barcelona, Avda, Diagonal 643, 08028 Barcelona, Spain
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