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Rodríguez-Rivera NS, Barrera-Oviedo D. Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance. Int J Mol Sci 2024; 25:4079. [PMID: 38612888 PMCID: PMC11012456 DOI: 10.3390/ijms25074079] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2024] [Revised: 03/15/2024] [Accepted: 03/29/2024] [Indexed: 04/14/2024] Open
Abstract
Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.
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Affiliation(s)
- Nidia Samara Rodríguez-Rivera
- Laboratorio de Farmacología y Bioquímica Clínica, Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Mexico City 04510, Mexico;
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Antipsychotics impair regulation of glucose metabolism by central glucose. Mol Psychiatry 2022; 27:4741-4753. [PMID: 36241692 DOI: 10.1038/s41380-022-01798-y] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/26/2022] [Revised: 09/06/2022] [Accepted: 09/09/2022] [Indexed: 12/14/2022]
Abstract
Hypothalamic detection of elevated circulating glucose triggers suppression of endogenous glucose production (EGP) to maintain glucose homeostasis. Antipsychotics alleviate symptoms associated with schizophrenia but also increase the risk for impaired glucose metabolism. In the current study, we examined whether two acutely administered antipsychotics from different drug classes, haloperidol (first generation antipsychotic) and olanzapine (second generation antipsychotic), affect the ability of intracerebroventricular (ICV) glucose infusion approximating postprandial levels to suppress EGP. The experimental protocol consisted of a pancreatic euglycemic clamp, followed by kinomic and RNA-seq analyses of hypothalamic samples to determine changes in serine/threonine kinase activity and gene expression, respectively. Both antipsychotics inhibited ICV glucose-mediated increases in glucose infusion rate during the clamp, a measure of whole-body glucose metabolism. Similarly, olanzapine and haloperidol blocked central glucose-induced suppression of EGP. ICV glucose stimulated the vascular endothelial growth factor (VEGF) pathway, phosphatidylinositol 3-kinase (PI3K) pathway, and kinases capable of activating KATP channels in the hypothalamus. These effects were inhibited by both antipsychotics. In conclusion, olanzapine and haloperidol impair central glucose sensing. Although results of hypothalamic analyses in our study do not prove causality, they are novel and provide the basis for a multitude of future studies.
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Ježek P, Holendová B, Jabůrek M, Dlasková A, Plecitá-Hlavatá L. Contribution of Mitochondria to Insulin Secretion by Various Secretagogues. Antioxid Redox Signal 2022; 36:920-952. [PMID: 34180254 PMCID: PMC9125579 DOI: 10.1089/ars.2021.0113] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Significance: Mitochondria determine glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells by elevating ATP synthesis. As the metabolic and redox hub, mitochondria provide numerous links to the plasma membrane channels, insulin granule vesicles (IGVs), cell redox, NADH, NADPH, and Ca2+ homeostasis, all affecting insulin secretion. Recent Advances: Mitochondrial redox signaling was implicated in several modes of insulin secretion (branched-chain ketoacid [BCKA]-, fatty acid [FA]-stimulated). Mitochondrial Ca2+ influx was found to enhance GSIS, reflecting cytosolic Ca2+ oscillations induced by action potential spikes (intermittent opening of voltage-dependent Ca2+ and K+ channels) or the superimposed Ca2+ release from the endoplasmic reticulum (ER). The ATPase inhibitory factor 1 (IF1) was reported to tune the glucose sensitivity range for GSIS. Mitochondrial protein kinase A was implicated in preventing the IF1-mediated inhibition of the ATP synthase. Critical Issues: It is unknown how the redox signal spreads up to the plasma membrane and what its targets are, what the differences in metabolic, redox, NADH/NADPH, and Ca2+ signaling, and homeostasis are between the first and second GSIS phase, and whether mitochondria can replace ER in the amplification of IGV exocytosis. Future Directions: Metabolomics studies performed to distinguish between the mitochondrial matrix and cytosolic metabolites will elucidate further details. Identifying the targets of cell signaling into mitochondria and of mitochondrial retrograde metabolic and redox signals to the cell will uncover further molecular mechanisms for insulin secretion stimulated by glucose, BCKAs, and FAs, and the amplification of secretion by glucagon-like peptide (GLP-1) and metabotropic receptors. They will identify the distinction between the hub β-cells and their followers in intact and diabetic states. Antioxid. Redox Signal. 36, 920-952.
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Affiliation(s)
- Petr Ježek
- Department of Mitochondrial Physiology, Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic
| | - Blanka Holendová
- Department of Mitochondrial Physiology, Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic
| | - Martin Jabůrek
- Department of Mitochondrial Physiology, Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic
| | - Andrea Dlasková
- Department of Mitochondrial Physiology, Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic
| | - Lydie Plecitá-Hlavatá
- Department of Mitochondrial Physiology, Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic
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Corkey BE, Deeney JT, Merrins MJ. What Regulates Basal Insulin Secretion and Causes Hyperinsulinemia? Diabetes 2021; 70:2174-2182. [PMID: 34593535 PMCID: PMC8576498 DOI: 10.2337/dbi21-0009] [Citation(s) in RCA: 26] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/15/2021] [Accepted: 07/28/2021] [Indexed: 12/12/2022]
Abstract
We hypothesize that basal hyperinsulinemia is synergistically mediated by an interplay between increased oxidative stress and excess lipid in the form of reactive oxygen species (ROS) and long-chain acyl-CoA esters (LC-CoA). In addition, ROS production may increase in response to inflammatory cytokines and certain exogenous environmental toxins that mislead β-cells into perceiving nutrient excess when none exists. Thus, basal hyperinsulinemia is envisioned as an adaptation to sustained real or perceived nutrient excess that only manifests as a disease when the excess demand can no longer be met by an overworked β-cell. In this article we will present a testable hypothetical mechanism to explain the role of lipids and ROS in basal hyperinsulinemia and how they differ from glucose-stimulated insulin secretion (GSIS). The model centers on redox regulation, via ROS, and S-acylation-mediated trafficking via LC-CoA. These pathways are well established in neural systems but not β-cells. During GSIS, these signals rise and fall in an oscillatory pattern, together with the other well-established signals derived from glucose metabolism; however, their precise roles have not been defined. We propose that failure to either increase or decrease ROS or LC-CoA appropriately will disturb β-cell function.
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Affiliation(s)
- Barbara E Corkey
- Department of Medicine, Boston University School of Medicine, Boston, MA
| | - Jude T Deeney
- Department of Medicine, Boston University School of Medicine, Boston, MA
| | - Matthew J Merrins
- Department of Biomolecular Chemistry and Section of Endocrinology, Diabetes and Metabolism, Department of Medicine, University of Wisconsin-Madison School of Medicine and Public Health, Madison, WI
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Ježek P, Holendová B, Jabůrek M, Tauber J, Dlasková A, Plecitá-Hlavatá L. The Pancreatic β-Cell: The Perfect Redox System. Antioxidants (Basel) 2021; 10:antiox10020197. [PMID: 33572903 PMCID: PMC7912581 DOI: 10.3390/antiox10020197] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2020] [Revised: 01/20/2021] [Accepted: 01/25/2021] [Indexed: 12/12/2022] Open
Abstract
Pancreatic β-cell insulin secretion, which responds to various secretagogues and hormonal regulations, is reviewed here, emphasizing the fundamental redox signaling by NADPH oxidase 4- (NOX4-) mediated H2O2 production for glucose-stimulated insulin secretion (GSIS). There is a logical summation that integrates both metabolic plus redox homeostasis because the ATP-sensitive K+ channel (KATP) can only be closed when both ATP and H2O2 are elevated. Otherwise ATP would block KATP, while H2O2 would activate any of the redox-sensitive nonspecific calcium channels (NSCCs), such as TRPM2. Notably, a 100%-closed KATP ensemble is insufficient to reach the -50 mV threshold plasma membrane depolarization required for the activation of voltage-dependent Ca2+ channels. Open synergic NSCCs or Cl- channels have to act simultaneously to reach this threshold. The resulting intermittent cytosolic Ca2+-increases lead to the pulsatile exocytosis of insulin granule vesicles (IGVs). The incretin (e.g., GLP-1) amplification of GSIS stems from receptor signaling leading to activating the phosphorylation of TRPM channels and effects on other channels to intensify integral Ca2+-influx (fortified by endoplasmic reticulum Ca2+). ATP plus H2O2 are also required for branched-chain ketoacids (BCKAs); and partly for fatty acids (FAs) to secrete insulin, while BCKA or FA β-oxidation provide redox signaling from mitochondria, which proceeds by H2O2 diffusion or hypothetical SH relay via peroxiredoxin "redox kiss" to target proteins.
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Affiliation(s)
- Vincent Poitout
- Centre de Recherche du Centre hospitalier de l'Université de Montréal, Montreal Diabetes Research Center, and Department of Medicine, University of Montreal, Montreal, Canada
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Rorsman P, Ashcroft FM. Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men. Physiol Rev 2018; 98:117-214. [PMID: 29212789 PMCID: PMC5866358 DOI: 10.1152/physrev.00008.2017] [Citation(s) in RCA: 456] [Impact Index Per Article: 76.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2017] [Revised: 05/30/2017] [Accepted: 06/18/2017] [Indexed: 12/14/2022] Open
Abstract
The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.
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Affiliation(s)
- Patrik Rorsman
- Oxford Centre for Diabetes, Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, United Kingdom; Department of Neuroscience and Physiology, Metabolic Research Unit, Göteborg, Sweden; and Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
| | - Frances M Ashcroft
- Oxford Centre for Diabetes, Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, United Kingdom; Department of Neuroscience and Physiology, Metabolic Research Unit, Göteborg, Sweden; and Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
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Proks P, Puljung MC, Vedovato N, Sachse G, Mulvaney R, Ashcroft FM. Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels. Philos Trans R Soc Lond B Biol Sci 2017; 371:rstb.2015.0426. [PMID: 27377720 PMCID: PMC4938026 DOI: 10.1098/rstb.2015.0426] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/17/2016] [Indexed: 11/29/2022] Open
Abstract
KATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P2 and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing KATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles. This article is part of the themed issue ‘Evolution brings Ca2+ and ATP together to control life and death’.
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Affiliation(s)
- Peter Proks
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Michael C Puljung
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Natascia Vedovato
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Gregor Sachse
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Rachel Mulvaney
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
| | - Frances M Ashcroft
- Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
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Doliba NM, Liu Q, Li C, Chen P, Liu C, Naji A, Matschinsky FM. Inhibition of cholinergic potentiation of insulin secretion from pancreatic islets by chronic elevation of glucose and fatty acids: Protection by casein kinase 2 inhibitor. Mol Metab 2017; 6:1240-1253. [PMID: 29031723 PMCID: PMC5641685 DOI: 10.1016/j.molmet.2017.07.017] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/05/2017] [Revised: 07/24/2017] [Accepted: 07/31/2017] [Indexed: 01/24/2023] Open
Abstract
Objectives Chronic hyperlipidemia and hyperglycemia are characteristic features of type 2 diabetes (T2DM) that are thought to cause or contribute to β-cell dysfunction by “glucolipotoxicity.” Previously we have shown that acute treatment of pancreatic islets with fatty acids (FA) decreases acetylcholine-potentiated insulin secretion. This acetylcholine response is mediated by M3 muscarinic receptors, which play a key role in regulating β-cell function. Here we examine whether chronic FA exposure also inhibits acetylcholine-potentiated insulin secretion using mouse and human islets. Methods Islets were cultured for 3 or 4 days at different glucose concentration with 0.5 mM palmitic acid (PA) or a 2:1 mixture of PA and oleic acid (OA) at 1% albumin (PA/BSA molar ratio 3.3). Afterwards, the response to glucose and acetylcholine were studied in perifusion experiments. Results FA-induced impairment of insulin secretion and Ca2+ signaling depended strongly on the glucose concentrations of the culture medium. PA and OA in combination reduced acetylcholine potentiation of insulin secretion more than PA alone, both in mouse and human islets, with no evidence of a protective role of OA. In contrast, lipotoxicity was not observed with islets cultured for 3 days in medium containing less than 1 mM glucose and a mixture of glutamine and leucine (7 mM each). High glucose and FAs reduced endoplasmic reticulum (ER) Ca2+ storage capacity; however, preserving ER Ca2+ by blocking the IP3 receptor with xestospongin C did not protect islets from glucolipotoxic effects on insulin secretion. In contrast, an inhibitor of casein kinase 2 (CK2) protected the glucose dependent acetylcholine potentiation of insulin secretion in mouse and human islets against glucolipotoxicity. Conclusions These results show that chronic FA treatment decreases acetylcholine potentiation of insulin secretion and that this effect is strictly glucose dependent and might involve CK2 phosphorylation of β-cell M3 muscarinic receptors. Glucolipotoxicity impairs acetylcholine-potentiation of insulin secretion. Glucose amplification of insulin secretion rather than triggering is damaged by FA. Inhibitor of casein kinase 2 preserved islet function against glucolipotoxicity.
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Affiliation(s)
- Nicolai M Doliba
- Department of Biochemistry and Biophysics, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA; Institute for Diabetes, Obesity and Metabolism, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA.
| | - Qin Liu
- Department of Biochemistry and Biophysics, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA
| | - Changhong Li
- Institute for Diabetes, Obesity and Metabolism, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA; Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA
| | - Pan Chen
- Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA
| | - Chengyang Liu
- Department of Surgery, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA; Institute for Diabetes, Obesity and Metabolism, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA
| | - Ali Naji
- Department of Surgery, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA; Institute for Diabetes, Obesity and Metabolism, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA
| | - Franz M Matschinsky
- Department of Biochemistry and Biophysics, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA; Institute for Diabetes, Obesity and Metabolism, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA
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Fogle KJ, Hertzler JI, Shon JH, Palladino MJ. The ATP-sensitive K channel is seizure protective and required for effective dietary therapy in a model of mitochondrial encephalomyopathy. J Neurogenet 2016; 30:247-258. [PMID: 27868454 DOI: 10.1080/01677063.2016.1252765] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Effective therapies are lacking for mitochondrial encephalomyopathies (MEs). MEs are devastating diseases that predominantly affect the energy-demanding tissues of the nervous system and muscle, causing symptoms such as seizures, cardiomyopathy, and neuro- and muscular degeneration. Even common anti-epileptic drugs which are frequently successful in ameliorating seizures in other diseases tend to have a lower success rate in ME, highlighting the need for novel drug targets, especially those that may couple metabolic sensitivity to neuronal excitability. Furthermore, alternative epilepsy therapies such as dietary modification are gaining in clinical popularity but have not been thoroughly studied in ME. Using the Drosophila ATP61 model of ME, we have studied dietary therapy throughout disease progression and found that it is highly effective against the seizures of ME, especially a high fat/ketogenic diet, and that the benefits are dependent upon a functional KATP channel complex. Further experiments with KATP show that it is seizure-protective in this model, and that pharmacological promotion of its open state also ameliorates seizures. These studies represent important steps forward in the development of novel therapies for a class of diseases that is notoriously difficult to treat, and lay the foundation for mechanistic studies of currently existing therapies in the context of metabolic disease.
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Affiliation(s)
- Keri J Fogle
- a Department of Pharmacology & Chemical Biology , University of Pittsburgh School of Medicine , Pittsburgh , PA , USA.,b Pittsburgh Institute for Neurodegenerative Diseases (PIND), University of Pittsburgh School of Medicine , Pittsburgh , PA , USA
| | - J Ian Hertzler
- a Department of Pharmacology & Chemical Biology , University of Pittsburgh School of Medicine , Pittsburgh , PA , USA.,b Pittsburgh Institute for Neurodegenerative Diseases (PIND), University of Pittsburgh School of Medicine , Pittsburgh , PA , USA
| | - Joy H Shon
- a Department of Pharmacology & Chemical Biology , University of Pittsburgh School of Medicine , Pittsburgh , PA , USA.,b Pittsburgh Institute for Neurodegenerative Diseases (PIND), University of Pittsburgh School of Medicine , Pittsburgh , PA , USA
| | - Michael J Palladino
- a Department of Pharmacology & Chemical Biology , University of Pittsburgh School of Medicine , Pittsburgh , PA , USA.,b Pittsburgh Institute for Neurodegenerative Diseases (PIND), University of Pittsburgh School of Medicine , Pittsburgh , PA , USA
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Velasco M, Díaz-García CM, Larqué C, Hiriart M. Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells. Mol Pharmacol 2016; 90:341-57. [PMID: 27436126 DOI: 10.1124/mol.116.103861] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2016] [Accepted: 07/18/2016] [Indexed: 12/11/2022] Open
Abstract
Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly regulated. In this review, we focus on the principal ionic channels (KATP, Nav, and Cav channels) involved in GSIS and how classic and new proteins, hormones, and drugs regulate it. Moreover, we also discuss advances on how metabolic disorders such as metabolic syndrome and diabetes mellitus change channel activity leading to changes in insulin secretion.
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Affiliation(s)
- Myrian Velasco
- Department of Neurodevelopment and Physiology, Neuroscience Division, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Carlos Manlio Díaz-García
- Department of Neurodevelopment and Physiology, Neuroscience Division, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Carlos Larqué
- Department of Neurodevelopment and Physiology, Neuroscience Division, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Marcia Hiriart
- Department of Neurodevelopment and Physiology, Neuroscience Division, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
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Inhibition of Monoacylglycerol Lipase Activity Decreases Glucose-Stimulated Insulin Secretion in INS-1 (832/13) Cells and Rat Islets. PLoS One 2016; 11:e0149008. [PMID: 26867016 PMCID: PMC4750965 DOI: 10.1371/journal.pone.0149008] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2015] [Accepted: 01/26/2016] [Indexed: 01/06/2023] Open
Abstract
Lipid signals derived from lipolysis and membrane phospholipids play an important role in glucose-stimulated insulin secretion (GSIS), though the exact secondary signals remain unclear. Previous reports have documented a stimulatory role of exogenously added mono-acyl-glycerol (MAG) on insulin secretion from cultured β-cells and islets. In this report we have determined effects of increasing intracellular MAG in the β-cell by inhibiting mono-acyl-glycerol lipase (MGL) activity, which catalyzes the final step in triacylglycerol breakdown, namely the hydrolysis of MAG to glycerol and free fatty acid (FA). To determine the role of MGL in GSIS, we used three different pharmacological agents (JZL184, MJN110 and URB602). All three inhibited GSIS and depolarization-induced insulin secretion in INS-1 (832/13). JZL184 significantly inhibited both GSIS and depolarization-induced insulin secretion in rat islets. JZL184 significantly decreased lipolysis and increased both mono- and diacyglycerol species in INS-1 cells. Analysis of the kinetics of GSIS showed that inhibition was greater during the sustained phase of secretion. A similar pattern was observed in the response of Ca2+ to glucose and depolarization but to a lesser degree suggesting that altered Ca2+ handling alone could not explain the reduction in insulin secretion. In addition, a significant reduction in long chain-CoA (LC-CoA) was observed in INS-1 cells at both basal and stimulatory glucose following inhibition of MGL. Our data implicate an important role for MGL in insulin secretion.
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Stark R, Reichenbach A, Andrews ZB. Hypothalamic carnitine metabolism integrates nutrient and hormonal feedback to regulate energy homeostasis. Mol Cell Endocrinol 2015; 418 Pt 1:9-16. [PMID: 26261054 DOI: 10.1016/j.mce.2015.08.002] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/23/2015] [Revised: 07/31/2015] [Accepted: 08/03/2015] [Indexed: 01/11/2023]
Abstract
The maintenance of energy homeostasis requires the hypothalamic integration of nutrient feedback cues, such as glucose, fatty acids, amino acids, and metabolic hormones such as insulin, leptin and ghrelin. Although hypothalamic neurons are critical to maintain energy homeostasis research efforts have focused on feedback mechanisms in isolation, such as glucose alone, fatty acids alone or single hormones. However this seems rather too simplistic considering the range of nutrient and endocrine changes associated with different metabolic states, such as starvation (negative energy balance) or diet-induced obesity (positive energy balance). In order to understand how neurons integrate multiple nutrient or hormonal signals, we need to identify and examine potential intracellular convergence points or common molecular targets that have the ability to sense glucose, fatty acids, amino acids and hormones. In this review, we focus on the role of carnitine metabolism in neurons regulating energy homeostasis. Hypothalamic carnitine metabolism represents a novel means for neurons to facilitate and control both nutrient and hormonal feedback. In terms of nutrient regulation, carnitine metabolism regulates hypothalamic fatty acid sensing through the actions of CPT1 and has an underappreciated role in glucose sensing since carnitine metabolism also buffers mitochondrial matrix levels of acetyl-CoA, an allosteric inhibitor of pyruvate dehydrogenase and hence glucose metabolism. Studies also show that hypothalamic CPT1 activity also controls hormonal feedback. We hypothesis that hypothalamic carnitine metabolism represents a key molecular target that can concurrently integrate nutrient and hormonal information, which is critical to maintain energy homeostasis. We also suggest this is relevant to broader neuroendocrine research as it predicts that hormonal signaling in the brain varies depending on current nutrient status. Indeed, the metabolic action of ghrelin, leptin or insulin at POMC or NPY neurons may depend on appropriate nutrient-sensing in these neurons and we hypothesize carnitine metabolism is critical in the integrative processing. Future research is required to examine the neuron-specific effects of carnitine metabolism on concurrent nutrient- and hormonal-sensing in AgRP and POMC neurons.
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Affiliation(s)
- Romana Stark
- Department of Physiology, Monash University, Clayton, Victoria 3183, Australia
| | - Alex Reichenbach
- Department of Physiology, Monash University, Clayton, Victoria 3183, Australia
| | - Zane B Andrews
- Department of Physiology, Monash University, Clayton, Victoria 3183, Australia.
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Hansen SB. Lipid agonism: The PIP2 paradigm of ligand-gated ion channels. BIOCHIMICA ET BIOPHYSICA ACTA 2015; 1851:620-8. [PMID: 25633344 PMCID: PMC4540326 DOI: 10.1016/j.bbalip.2015.01.011] [Citation(s) in RCA: 107] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/28/2014] [Revised: 01/05/2015] [Accepted: 01/17/2015] [Indexed: 01/08/2023]
Abstract
The past decade, membrane signaling lipids emerged as major regulators of ion channel function. However, the molecular nature of lipid binding to ion channels remained poorly described due to a lack of structural information and assays to quantify and measure lipid binding in a membrane. How does a lipid-ligand bind to a membrane protein in the plasma membrane, and what does it mean for a lipid to activate or regulate an ion channel? How does lipid binding compare to activation by soluble neurotransmitter? And how does the cell control lipid agonism? This review focuses on lipids and their interactions with membrane proteins, in particular, ion channels. I discuss the intersection of membrane lipid biology and ion channel biophysics. A picture emerges of membrane lipids as bona fide agonists of ligand-gated ion channels. These freely diffusing signals reside in the plasma membrane, bind to the transmembrane domain of protein, and cause a conformational change that allosterically gates an ion channel. The system employs a catalog of diverse signaling lipids ultimately controlled by lipid enzymes and raft localization. I draw upon pharmacology, recent protein structure, and electrophysiological data to understand lipid regulation and define inward rectifying potassium channels (Kir) as a new class of PIP2 lipid-gated ion channels.
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Affiliation(s)
- Scott B Hansen
- Department of Molecular Therapeutics, The Scripps Research Institute, Jupiter FL 33458, USA; Department of Neuroscience, The Scripps Research Institute, Jupiter FL 33458, USA.
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15
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Neess D, Bek S, Engelsby H, Gallego SF, Færgeman NJ. Long-chain acyl-CoA esters in metabolism and signaling: Role of acyl-CoA binding proteins. Prog Lipid Res 2015; 59:1-25. [PMID: 25898985 DOI: 10.1016/j.plipres.2015.04.001] [Citation(s) in RCA: 115] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2015] [Revised: 03/11/2015] [Accepted: 04/09/2015] [Indexed: 02/03/2023]
Abstract
Long-chain fatty acyl-CoA esters are key intermediates in numerous lipid metabolic pathways, and recognized as important cellular signaling molecules. The intracellular flux and regulatory properties of acyl-CoA esters have been proposed to be coordinated by acyl-CoA-binding domain containing proteins (ACBDs). The ACBDs, which comprise a highly conserved multigene family of intracellular lipid-binding proteins, are found in all eukaryotes and ubiquitously expressed in all metazoan tissues, with distinct expression patterns for individual ACBDs. The ACBDs are involved in numerous intracellular processes including fatty acid-, glycerolipid- and glycerophospholipid biosynthesis, β-oxidation, cellular differentiation and proliferation as well as in the regulation of numerous enzyme activities. Little is known about the specific roles of the ACBDs in the regulation of these processes, however, recent studies have gained further insights into their in vivo functions and provided further evidence for ACBD-specific functions in cellular signaling and lipid metabolic pathways. This review summarizes the structural and functional properties of the various ACBDs, with special emphasis on the function of ACBD1, commonly known as ACBP.
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Affiliation(s)
- Ditte Neess
- Villum Center for Bioanalytical Sciences, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark
| | - Signe Bek
- Villum Center for Bioanalytical Sciences, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark
| | - Hanne Engelsby
- Villum Center for Bioanalytical Sciences, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark
| | - Sandra F Gallego
- Villum Center for Bioanalytical Sciences, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark
| | - Nils J Færgeman
- Villum Center for Bioanalytical Sciences, Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark.
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16
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Abstract
The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.
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17
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Fatty acid transporter CD36 mediates hypothalamic effect of fatty acids on food intake in rats. PLoS One 2013; 8:e74021. [PMID: 24040150 PMCID: PMC3765350 DOI: 10.1371/journal.pone.0074021] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2013] [Accepted: 07/26/2013] [Indexed: 12/05/2022] Open
Abstract
Variations in plasma fatty acid (FA) concentrations are detected by FA sensing neurons in specific brain areas such as the hypothalamus. These neurons play a physiological role in the control of food intake and the regulation of hepatic glucose production. Le Foll et al. previously showed in vitro that at least 50% of the FA sensing in ventromedial hypothalamic (VMH) neurons is attributable to the interaction of long chain FA with FA translocase/CD36 (CD36). The present work assessed whether in vivo effects of hypothalamic FA sensing might be partly mediated by CD36 or intracellular events such as acylCoA synthesis or β-oxidation. To that end, a catheter was implanted in the carotid artery toward the brain in male Wistar rats. After 1 wk recovery, animals were food-deprived for 5 h, then 10 min infusions of triglyceride emulsion, Intralipid +/− heparin (IL, ILH, respectively) or saline/heparin (SH) were carried out and food intake was assessed over the next 5 h. Experimental groups included: 1) Rats previously injected in ventromedian nucleus (VMN) with shRNA against CD36 or scrambled RNA; 2) Etomoxir (CPT1 inhibitor) or saline co-infused with ILH/SH; and 3) Triacsin C (acylCoA synthase inhibitor) or saline co-infused with ILH/SH. ILH significantly lowered food intake during refeeding compared to SH (p<0.001). Five hours after refeeding, etomoxir did not affect this inhibitory effect of ILH on food intake while VMN CD36 depletion totally prevented it. Triacsin C also prevented ILH effects on food intake. In conclusion, the effect of FA to inhibit food intake is dependent on VMN CD36 and acylCoA synthesis but does not required FA oxidation.
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Abstract
The pancreatic islet β cell senses circulating levels of calorigenic nutrients to secrete insulin according to the needs of the organism. Altered insulin secretion is linked to various disorders such as diabetes, hypoglycemic states, and cardiometabolic diseases. Fuel stimuli, including glucose, free fatty acids, and amino acids, promote insulin granule exocytosis primarily via their metabolism in β cells and the production of key signaling metabolites. This paper reviews our current knowledge of the pathways involved in both positive and negative metabolic signaling for insulin secretion and assesses the role of established and candidate metabolic coupling factors, keeping recent developments in focus.
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Affiliation(s)
- Marc Prentki
- Molecular Nutrition Unit, Centre de Recherche du Centre Hospitalier de l'Université de Montréal, QC, Canada.
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19
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Bushman JD, Zhou Q, Shyng SL. A Kir6.2 pore mutation causes inactivation of ATP-sensitive potassium channels by disrupting PIP2-dependent gating. PLoS One 2013; 8:e63733. [PMID: 23700433 PMCID: PMC3659044 DOI: 10.1371/journal.pone.0063733] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2013] [Accepted: 04/05/2013] [Indexed: 11/18/2022] Open
Abstract
In the absence of intracellular nucleotides, ATP-sensitive potassium (KATP) channels exhibit spontaneous activity via a phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent gating process. Previous studies show that stability of this activity requires subunit-subunit interactions in the cytoplasmic domain of Kir6.2; selective mutagenesis and disease mutations at the subunit interface result in time-dependent channel inactivation. Here, we report that mutation of the central glycine in the pore-lining second transmembrane segment (TM2) to proline in Kir6.2 causes KATP channel inactivation. Unlike C-type inactivation, a consequence of selectivity filter closure, in many K(+) channels, the rate of inactivation in G156P channels was insensitive to changes in extracellular ion concentrations or ion species fluxing through the pore. Instead, the rate of G156P inactivation decreased with exogenous application of PIP2 and increased when PIP2-channel interaction was inhibited with neomycin or poly-L-lysine. These findings indicate the G156P mutation reduces the ability of PIP2 to stabilize the open state of KATP channels, similar to mutations in the cytoplasmic domain that produce inactivation. Consistent with this notion, when PIP2-dependent open state stability was substantially increased by addition of a second gain-of-function mutation, G156P inactivation was abolished. Importantly, bath application and removal of Mg(2+)-free ATP or a nonhydrolyzable analog of ATP, which binds to the cytoplasmic domain of Kir6.2 and causes channel closure, recover G156P channel from inactivation, indicating crosstalk between cytoplasmic and transmembrane domains. The G156P mutation provides mechanistic insight into the structural and functional interactions between the pore and cytoplasmic domains of Kir6.2 during gating.
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Affiliation(s)
- Jeremy D. Bushman
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States of America
| | - Qing Zhou
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States of America
| | - Show-Ling Shyng
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States of America
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20
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Ortiz D, Gossack L, Quast U, Bryan J. Reinterpreting the action of ATP analogs on K(ATP) channels. J Biol Chem 2013; 288:18894-902. [PMID: 23665564 DOI: 10.1074/jbc.m113.476887] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Neuroendocrine-type K(ATP) channels, (SUR1/Kir6.2)4, couple the transmembrane flux of K(+), and thus membrane potential, with cellular metabolism in various cell types including insulin-secreting β-cells. Mutant channels with reduced activity are a cause of congenital hyperinsulinism, whereas hyperactive channels are a cause of neonatal diabetes. A current regulatory model proposes that ATP hydrolysis is required to switch SUR1 into post-hydrolytic conformations able to antagonize the inhibitory action of nucleotide binding at the Kir6.2 pore, thus coupling enzymatic and channel activities. Alterations in SUR1 ATPase activity are proposed to contribute to neonatal diabetes and type 2 diabetes risk. The regulatory model is partly based on the reduced ability of ATP analogs such as adenosine 5'-(β,γ-imino)triphosphate (AMP-PNP) and adenosine 5'-O-(thiotriphosphate) (ATPγS) to stimulate channel activity, presumably by reducing hydrolysis. This study uses a substitution at the catalytic glutamate, SUR(1E1507Q), with a significantly increased affinity for ATP, to probe the action of these ATP analogs on conformational switching. ATPγS, a slowly hydrolyzable analog, switches SUR1 conformations, albeit with reduced affinity. Nonhydrolyzable AMP-PNP and adenosine 5'-(β,γ-methylenetriphosphate) (AMP-PCP) alone fail to switch SUR1, but do reverse ATP-induced switching. AMP-PCP displaces 8-azido-[(32)P]ATP from the noncanonical NBD1 of SUR1. This is consistent with structural data on an asymmetric bacterial ABC protein that shows that AMP-PNP binds selectively to the noncanonical NBD to prevent conformational switching. The results imply that MgAMP-PNP and MgAMP-PCP (AMP-PxP) fail to activate K(ATP) channels because they do not support NBD dimerization and conformational switching, rather than by limiting enzymatic activity.
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Affiliation(s)
- David Ortiz
- Pacific Northwest Diabetes Research Institute, Seattle, Washington 98122, USA
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21
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Meloni AR, DeYoung MB, Lowe C, Parkes DG. GLP-1 receptor activated insulin secretion from pancreatic β-cells: mechanism and glucose dependence. Diabetes Obes Metab 2013; 15:15-27. [PMID: 22776039 PMCID: PMC3556522 DOI: 10.1111/j.1463-1326.2012.01663.x] [Citation(s) in RCA: 225] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/30/2012] [Revised: 06/19/2012] [Accepted: 07/05/2012] [Indexed: 12/18/2022]
Abstract
The major goal in the treatment of type 2 diabetes mellitus is to control the hyperglycaemia characteristic of the disease. However, treatment with common therapies such as insulin or insulinotrophic sulphonylureas (SU), while effective in reducing hyperglycaemia, may impose a greater risk of hypoglycaemia, as neither therapy is self-regulated by ambient blood glucose concentrations. Hypoglycaemia has been associated with adverse physical and psychological outcomes and may contribute to negative cardiovascular events; hence minimization of hypoglycaemia risk is clinically advantageous. Stimulation of insulin secretion from pancreatic β-cells by glucagon-like peptide 1 receptor (GLP-1R) agonists is known to be glucose-dependent. GLP-1R agonists potentiate glucose-stimulated insulin secretion and have little or no activity on insulin secretion in the absence of elevated blood glucose concentrations. This 'glucose-regulated' activity of GLP-1R agonists makes them useful and potentially safer therapeutics for overall glucose control compared to non-regulated therapies; hyperglycaemia can be reduced with minimal hypoglycaemia. While the inherent mechanism of action of GLP-1R agonists mediates their glucose dependence, studies in rats suggest that SUs may uncouple this dependence. This hypothesis is supported by clinical studies showing that the majority of events of hypoglycaemia in patients treated with GLP-1R agonists occur in patients treated with a concomitant SU. This review aims to discuss the current understanding of the mechanisms by which GLP-1R signalling promotes insulin secretion from pancreatic β-cells via a glucose-dependent process.
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Affiliation(s)
- A R Meloni
- Amylin Pharmaceuticals, Inc., San Diego, CA 92121, USA.
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22
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Reactive oxygen species stimulate insulin secretion in rat pancreatic islets: studies using mono-oleoyl-glycerol. PLoS One 2012; 7:e30200. [PMID: 22272304 PMCID: PMC3260220 DOI: 10.1371/journal.pone.0030200] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2011] [Accepted: 12/12/2011] [Indexed: 11/25/2022] Open
Abstract
Chronic exposure (24–72 hrs) of pancreatic islets to elevated glucose and fatty acid leads to glucolipoxicity characterized by basal insulin hypersecretion and impaired glucose-stimulated insulin secretion (GSIS). Our aim was to determine the mechanism for basal hypersecretion of insulin. We used mono-oleoyl-glycerol (MOG) as a tool to rapidly increase lipids in isolated rat pancreatic ß-cells and in the clonal pancreatic ß-cell line INS-1 832/13. MOG (25–400 µM) stimulated basal insulin secretion from ß-cells in a concentration dependent manner without increasing intracellular Ca2+ or O2 consumption. Like GSIS, MOG increased NAD(P)H and reactive oxygen species (ROS). The mitochondrial reductant ß-hydroxybutyrate (ß-OHB) also increased the redox state and ROS production, while ROS scavengers abrogated secretion. Diazoxide (0.4 mM) did not prevent the stimulatory effect of MOG, confirming that the effect was independent of the KATP-dependent pathway of secretion. MOG was metabolized to glycerol and long-chain acyl-CoA (LC-CoA), whereas, acute oleate did not similarly increase LC-CoA. Inhibition of diacylglycerol kinase (DGK) did not mimic the effect of MOG on insulin secretion, indicating that MOG did not act primarily by inhibiting DGK. Inhibition of acyl-CoA synthetase (ACS) reduced the stimulatory effect of MOG on basal insulin secretion by 30% indicating a role for LC-CoA. These data suggest that basal insulin secretion is stimulated by increased ROS production, due to an increase in the mitochondrial redox state independent of the established components of GSIS.
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23
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Abstract
Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] are required for the activity of many different ion channels. This chapter will highlight various aspects of this paradigm, by discussing current knowledge on four different ion channel families: inwardly rectifying K(+) (Kir) channels, KCNQ voltage gated K(+) channels, voltage gated Ca(2+) (VGCC) channels and Transient Receptor Potential (TRP) channels. Our main focus is to discuss functional aspects of this regulation, i.e. how changes in the concentration of PtdIns(4,5)P(2) in the plasma membrane upon phospholipase C activation may modulate the activity of ion channels, and what are the major determinants of this regulation. We also discuss how channels act as coincidence detectors sensing phosphoinositide levels and other signalling molecules. We also briefly discuss the available methods to study phosphoinositide regulation of ion channels, and structural aspects of interaction of ion channel proteins with these phospholipids. Finally, in several cases the effect of PtdIns(4,5)P(2) is more complex than a simple dependence of ion channel activity on the lipid, and we will discuss some these complexities.
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Affiliation(s)
- Nikita Gamper
- Institute of Membrane and Systems Biology, Faculty of Biological Sciences, University of Leeds, LS2 9JT, Leeds, UK,
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24
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Marfori M, Kobe B, Forwood JK. Ligand-induced conformational changes within a hexameric Acyl-CoA thioesterase. J Biol Chem 2011; 286:35643-35649. [PMID: 21849495 PMCID: PMC3195577 DOI: 10.1074/jbc.m111.225953] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2011] [Revised: 06/12/2011] [Indexed: 11/06/2022] Open
Abstract
Acyl-coenzyme A (acyl-CoA) thioesterases play a crucial role in the metabolism of activated fatty acids, coenzyme A, and other metabolic precursor molecules including arachidonic acid and palmitic acid. These enzymes hydrolyze coenzyme A from acyl-CoA esters to mediate a range of cellular functions including β-oxidation, lipid biosynthesis, and signal transduction. Here, we present the crystal structure of a hexameric hot-dog domain-containing acyl-CoA thioesterase from Bacillus halodurans in the apo-form and provide structural and comparative analyses to the coenzyme A-bound form to identify key conformational changes induced upon ligand binding. We observed dramatic ligand-induced changes at both the hot-dog dimer and the trimer-of-dimer interfaces; the dimer interfaces in the apo-structure differ by over 20% and decrease to about half the size in the ligand-bound state. We also assessed the specificity of the enzyme against a range of fatty acyl-CoA substrates and have identified a preference for short-chain fatty acyl-CoAs. Coenzyme A was shown both to negatively regulate enzyme activity, representing a direct inhibitory feedback, and consistent with the structural data, to destabilize the quaternary structure of the enzyme. Coenzyme A-induced conformational changes in the C-terminal helices of enzyme were assessed through mutational analysis and shown to play a role in regulating enzyme activity. The conformational changes are likely to be conserved from bacteria through to humans and provide a greater understanding, particularly at a structural level, of thioesterase function and regulation.
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Affiliation(s)
- Mary Marfori
- School of Chemistry and Molecular Biosciences and Australian Infectious Disease Research Centre, University of Queensland, Brisbane, Queensland 4072
| | - Bostjan Kobe
- School of Chemistry and Molecular Biosciences and Australian Infectious Disease Research Centre, University of Queensland, Brisbane, Queensland 4072; Division of Chemistry and Structural Biology, Institute for Molecular Bioscience, University of Queensland, Brisbane, Queensland 4072.
| | - Jade K Forwood
- School of Biomedical Sciences, Charles Sturt University, Wagga Wagga, New South Wales 2650, Australia.
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25
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Soriano S, Ripoll C, Fuentes E, Gonzalez A, Alonso-Magdalena P, Ropero AB, Quesada I, Nadal A. Regulation of K(ATP) channel by 17β-estradiol in pancreatic β-cells. Steroids 2011; 76:856-60. [PMID: 21470558 DOI: 10.1016/j.steroids.2011.03.017] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/14/2010] [Revised: 02/17/2011] [Accepted: 03/25/2011] [Indexed: 01/05/2023]
Abstract
ATP-sensitive potassium channels (K(ATP)) regulate electrical activity and insulin secretion in pancreatic β-cells. When glucose concentration increases, the [ATP]/[ADP] ratio rises closing K(ATP) channels, and the membrane potential depolarizes, triggering insulin secretion. This pivotal role of K(ATP) channels is used not only by glucose but also by neurotransmitters, hormones and other physiological agents to modulate electrical and secretory β-cell response. In recent years, it has been demonstrated that estrogens and estrogen receptors are involved in glucose homeostasis, and that they can modulate the electrical activity and insulin secretion of pancreatic β-cells. The hormone 17β-estradiol (E2), at physiological levels, is implicated in maintaining normal insulin sensitivity for β-cell function. Long term exposure to E2 increases insulin content, insulin gene expression and insulin release via the estrogen receptor α (ERα), while rapid responses to E2 can regulate K(ATP) channels increasing cGMP levels through the estrogen receptor β (ERβ) and type A guanylate cyclase receptor (GC-A). This review summarizes the main actions of 17β-estradiol on K(ATP) channels and the subsequent insulin release in pancreatic β-cells.
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Affiliation(s)
- Sergi Soriano
- Instituto de Bioingeniería and CIBER de Diabetes y Enfermedades Metabólicas Asociadas (CIBERDEM), Universidad Miguel Hernández de Elche, 03202 Elche, Alicante, Spain.
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26
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Abstract
Unique subpopulations of specialized metabolic sensing neurons reside in a distributed network throughout the brain and respond to alterations in ambient levels of various metabolic substrates by altering their activity. Variations in local brain substrate levels reflect their transport across the blood- and cerebrospinal-brain barriers as well as local production by astrocytes. There are a number of mechanisms by which such metabolic sensing neurons alter their activity in response to changes in substrate levels, but it is clear that these neurons cannot be considered in isolation. They are heavily dependent on astrocyte and probably tanycyte metabolism and function but also respond to hormones (e.g. leptin and insulin) and cytokines that cross the blood-brain barrier from the periphery as well as hard-wired neural inputs from metabolic sensors in peripheral sites such as the hepatic portal vein, gastrointestinal tract, and carotid body. Thus, these specialized neurons are capable of monitoring and integrating multiple signals from the periphery as a means of regulating peripheral energy homeostasis.
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Affiliation(s)
- Barry E Levin
- Neurology Service (127C), Veterans Affairs Medical Center, 385 Tremont Avenue, East Orange, New Jersey 07018, USA.
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27
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Landrock D, Atshaves BP, McIntosh AL, Landrock KK, Schroeder F, Kier AB. Acyl-CoA binding protein gene ablation induces pre-implantation embryonic lethality in mice. Lipids 2010; 45:567-80. [PMID: 20559753 PMCID: PMC2997683 DOI: 10.1007/s11745-010-3437-9] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2010] [Accepted: 05/29/2010] [Indexed: 11/26/2022]
Abstract
Unique among the intracellular lipid binding proteins, acyl-CoA binding protein (ACBP) exclusively binds long-chain fatty acyl-CoAs (LCFA-CoAs). To test if ACBP is an essential protein in mammals, the ACBP gene was ablated by homologous recombination in mice. While ACBP heterozygotes appeared phenotypically normal, intercrossing of the heterozygotes did not produce any live homozygous deficient (null) ACBP((-/-)) pups. Heterozygous and wild type embryos were detected at all post-implantation stages, but no homozygous ACBP-null embryos were obtained-suggesting that an embryonic lethality occurred at a pre-implantation stage of development, or that embryos never formed. While ACBP-null embryos were not detected at any blastocyst stage, ACBP-null embryos were detected at the morula (8-cell), cleavage (2-cell), and zygote (1-cell) pre-implantation stages. Two other LCFA-CoA binding proteins, sterol carrier protein-2 (SCP-2) and sterol carrier protein-x (SCP-x) were significantly upregulated at these stages. These findings demonstrate for the first time that ACBP is an essential protein required for embryonic development and its loss of function may be initially compensated by concomitant upregulation of two other LCFA-CoA binding proteins, but only at the earliest pre-implantation stages. The fact that ACBP is the first known intracellular lipid binding protein whose deletion results in embryonic lethality suggests its vital importance in mammals.
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Affiliation(s)
- Danilo Landrock
- Department of Pathobiology, Texas A&M University, TAMU 4467, College Station, TX 77843-4467, USA
| | - Barbara P. Atshaves
- Department of Physiology and Pharmacology, Texas A&M University, TAMU 4466, College Station, TX 77843-4466, USA
| | - Avery L. McIntosh
- Department of Physiology and Pharmacology, Texas A&M University, TAMU 4466, College Station, TX 77843-4466, USA
| | - Kerstin K. Landrock
- Department of Physiology and Pharmacology, Texas A&M University, TAMU 4466, College Station, TX 77843-4466, USA
| | - Friedhelm Schroeder
- Department of Physiology and Pharmacology, Texas A&M University, TAMU 4466, College Station, TX 77843-4466, USA
| | - Ann B. Kier
- Department of Pathobiology, Texas A&M University, TAMU 4467, College Station, TX 77843-4467, USA
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28
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Lim A, Park SH, Sohn JW, Jeon JH, Park JH, Song DK, Lee SH, Ho WK. Glucose deprivation regulates KATP channel trafficking via AMP-activated protein kinase in pancreatic beta-cells. Diabetes 2009; 58:2813-9. [PMID: 19720793 PMCID: PMC2780861 DOI: 10.2337/db09-0600] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
OBJECTIVE AMP-activated protein kinase (AMPK) and the ATP-sensitive K(+) (K(ATP)) channel are metabolic sensors that become activated during metabolic stress. AMPK is an important regulator of metabolism, whereas the K(ATP) channel is a regulator of cellular excitability. Cross talk between these systems is poorly understood. RESEARCH DESIGN AND METHODS Rat pancreatic beta-cells or INS-1 cells were pretreated for 2 h at various concentrations of glucose. Maximum K(ATP) conductance (G(max)) was monitored by whole-cell measurements after intracellular ATP washout using ATP-free internal solutions. K(ATP) channel activity (NPo) was monitored by inside-out patch recordings in the presence of diazoxide. Distributions of K(ATP) channel proteins (Kir6.2 and SUR1) were examined using immunofluorescence imaging and surface biotinylation studies. Insulin secretion from rat pancreatic islets was measured using an enzyme immunoassay. RESULTS G(max) and NPo in cells pretreated with glucose-free or 3 mmol/l glucose solutions were significantly higher than in cells pretreated in 11.1 mmol/l glucose solutions. Immunofluorescence imaging and biotinylation studies revealed that glucose deprivation induced an increase in the surface level of Kir6.2 without affecting the total cellular amount. Increases in G(max) and the surface level of Kir6.2 were inhibited by compound C, an AMPK inhibitor, and siAMPK transfection. The effects of glucose deprivation on K(ATP) channels were mimicked by an AMPK activator. Glucose deprivation reduced insulin secretion, but this response was attenuated by compound C. CONCLUSIONS K(ATP) channel trafficking is regulated by energy status via AMPK, and this mechanism may play a key role in inhibiting insulin secretion under low energy status.
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Affiliation(s)
- Ajin Lim
- National Research Laboratory for Cell Physiology, Seoul National University College of Medicine, Seoul, Korea
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
| | - Sun-Hyun Park
- National Research Laboratory for Cell Physiology, Seoul National University College of Medicine, Seoul, Korea
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
| | - Jong-Woo Sohn
- National Research Laboratory for Cell Physiology, Seoul National University College of Medicine, Seoul, Korea
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
| | - Ju-Hong Jeon
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
| | - Jae-Hyung Park
- Department of Physiology and Chronic Disease Research Center, Keimyung University School of Medicine, Daegu, Korea
| | - Dae-Kyu Song
- Department of Physiology and Chronic Disease Research Center, Keimyung University School of Medicine, Daegu, Korea
| | - Suk-Ho Lee
- National Research Laboratory for Cell Physiology, Seoul National University College of Medicine, Seoul, Korea
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
| | - Won-Kyung Ho
- National Research Laboratory for Cell Physiology, Seoul National University College of Medicine, Seoul, Korea
- Department of Physiology, Seoul National University College of Medicine, Seoul, Korea
- Corresponding author: Won-Kyung Ho,
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29
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Remedi MS, Koster JC. K(ATP) channelopathies in the pancreas. Pflugers Arch 2009; 460:307-20. [PMID: 19921246 DOI: 10.1007/s00424-009-0756-x] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2009] [Revised: 10/20/2009] [Accepted: 10/23/2009] [Indexed: 12/14/2022]
Abstract
Adenosine-triphosphate-sensitive potassium channels (KATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic beta-cell. The critical involvement of KATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in KATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in KATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which KATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.
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Affiliation(s)
- Maria S Remedi
- Department of Cell Biology and Physiology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, USA.
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Craig TJ, Shimomura K, Holl RW, Flanagan SE, Ellard S, Ashcroft FM. An in-frame deletion in Kir6.2 (KCNJ11) causing neonatal diabetes reveals a site of interaction between Kir6.2 and SUR1. J Clin Endocrinol Metab 2009; 94:2551-7. [PMID: 19351728 PMCID: PMC7611921 DOI: 10.1210/jc.2009-0159] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
CONTEXT Activating mutations in genes encoding the Kir6.2 (KCNJ11) and SUR1 (ABCC8) subunits of the pancreatic ATP-sensitive K(+) channel are a common cause of permanent neonatal diabetes (PNDM). All Kir6.2 mutations identified to date are missense mutations. We describe here a novel in-frame deletion (residues 28-32) in Kir6.2 in a heterozygous patient with PNDM without neurological problems that are detectable by standard evaluation. OBJECTIVE The aim of the study was to identify the mutation responsible for neonatal diabetes in this patient and characterize its functional effects. DESIGN Wild-type and mutant Kir6.2/SUR1 channels were examined by heterologous expression in Xenopus oocytes. RESULTS The Kir6.2-28Delta32 mutation produced a significant decrease in ATP inhibition and an increase in whole-cell K(ATP) currents, explaining the diabetes of the patient. Tolbutamide block was only slightly reduced in the simulated heterozygous state, suggesting that the patient should respond to sulfonylurea therapy. The mutation decreased ATP inhibition indirectly, by increasing the intrinsic (unliganded) channel open probability. Neither effect was observed when Kir6.2 was expressed in the absence of SUR1, suggesting that the mutation impairs coupling between SUR1 and Kir6.2. Coimmunoprecipitation studies further revealed that the mutation disrupted a physical interaction between Kir6.2 and residues 1-288 (but not residues 1-196) of SUR1. CONCLUSIONS We report a novel KCNJ11 mutation causing PNDM. Our results show that residues 28-32 in the N terminus of Kir6.2 interact both physically and functionally with SUR1 and suggest that residues 196-288 of SUR1 are important in this interaction.
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MESH Headings
- ATP-Binding Cassette Transporters/metabolism
- Animals
- Binding Sites/genetics
- Diabetes Mellitus, Type 2/congenital
- Diabetes Mellitus, Type 2/genetics
- Diabetes Mellitus, Type 2/metabolism
- Gene Deletion
- Humans
- Infant
- Infant, Newborn
- Infant, Newborn, Diseases/genetics
- Male
- Models, Biological
- Open Reading Frames/genetics
- Potassium Channels, Inwardly Rectifying/chemistry
- Potassium Channels, Inwardly Rectifying/genetics
- Potassium Channels, Inwardly Rectifying/metabolism
- Potassium Channels, Inwardly Rectifying/physiology
- Protein Binding/genetics
- Receptors, Drug/metabolism
- Sulfonylurea Receptors
- Xenopus
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Affiliation(s)
- Tim J Craig
- Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, United Kingdom
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Jo YH, Su Y, Gutierrez-Juarez R, Chua S. Oleic acid directly regulates POMC neuron excitability in the hypothalamus. J Neurophysiol 2009; 101:2305-16. [PMID: 19261705 DOI: 10.1152/jn.91294.2008] [Citation(s) in RCA: 83] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The mammalian CNS relies on a constant supply of external glucose for its undisturbed operation. However, neurons can readily switch to using fatty acids and ketones as alternative fuels. Here, we show that oleic acid (OA) excites pro-opiomelanocortin (POMC) neurons by inhibition of ATP-activated potassium (K(ATP)) channels. The involvement of K(ATP) channels is further supported by experiments in SUR1 KO animals. Inhibition of beta-oxidation using carnitine palmitoyltransferase-1 inhibitors blocks OA-induced depolarization. The depolarizing effect of OA is specific because it is not mimicked by octanoic acid. Furthermore, OA does not regulate the excitability of agouti-related peptide neurons. High-fat feeding alters POMC neuron excitability, but not its response to OA. Thus beta-oxidation in POMC neurons may mediate the appetite-suppressing (anorexigenic) effects of OA.
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Affiliation(s)
- Young-Hwan Jo
- Albert Einstein College of Medicine, Department of Medicine, Division of Endocrinology, 1300 Morris Park Ave, Bronx, NY 10467, USA.
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32
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Aittoniemi J, Fotinou C, Craig TJ, de Wet H, Proks P, Ashcroft FM. Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator. Philos Trans R Soc Lond B Biol Sci 2009; 364:257-67. [PMID: 18990670 DOI: 10.1098/rstb.2008.0142] [Citation(s) in RCA: 110] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.
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Affiliation(s)
- Jussi Aittoniemi
- Department of Physiology, Henry Wellcome Centre for Gene Function, University of Oxford, Parks Road, Oxford, UK
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33
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Tammaro P, Ashcroft FM. A cytosolic factor that inhibits KATP channels expressed in Xenopus oocytes by impairing Mg-nucleotide activation by SUR1. J Physiol 2009; 587:1649-56. [PMID: 19237428 PMCID: PMC2683953 DOI: 10.1113/jphysiol.2008.165126] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
ATP-sensitive K(+) (K(ATP)) channels couple cell metabolism to cell electrical activity. Wild-type (Kir6.2/SUR1) K(ATP) channels heterologously expressed in Xenopus oocytes give rise to very small inward currents in cell-attached patches. A large increase in the current is observed on patch excision into zero ATP solution. This is presumably due to loss of intracellular ATP leading to unblock of K(ATP) channels. In contrast, channels containing Kir6.2 mutations associated with reduced ATP-sensitivity display non-zero cell-attached currents. Unexpectedly, these cell-attached currents are significantly smaller (by approximately 40%) than those observed when excised patches are exposed to physiological ATP concentrations (1-10 mm). Cramming the patch back into the oocyte cytoplasm restores mutant K(ATP) current amplitude to that measured in the cell-attached mode. This implies that the magnitude of the cell-attached current is regulated not only by intracellular ATP but also by another cytoplasmic factor/s. This factor seems to require the nucleotide-binding domains of SUR1 to be effective. Thus a mutant Kir6.2 (Kir6.2DeltaC-I296L) expressed in the absence of SUR1 exhibited currents of similar magnitude in cell-attached patches as in inside-out patches exposed to 10 mm MgATP. Similar results were found when Kir6.2-I296L was coexpressed with an SUR1 mutant that is insensitive to MgADP or MgATP activation. This suggests the oocyte contains a cytoplasmic factor that reduces nucleotide binding/hydrolysis at the NBDs of SUR1. In conclusion, our results reveal a novel regulatory mechanism for the K(ATP) channel. This was not evident for wild-type channels because of their high sensitivity to block by ATP.
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Affiliation(s)
- Paolo Tammaro
- Department of Physiology, Anatomy and Genetics, Parks Road, Oxford OX1 3PT, UK
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Abstract
Beta-cells in pancreatic islets form complex syncytia. Sufficient cell-to-cell electrical coupling seems to ensure coordinated depolarization pattern and insulin release that can be further modulated by rich innervation. The complex structure and coordinated action develop after birth during fast proliferation of the endocrine tissue. These emergent properties can be lost due to various reasons later in life and can lead to glucose intolerance and diabetes mellitus. Pancreas slice is a novel method of choice to study the physiology of beta-cells still embedded in their normal cellulo-social context. I present major advantages, list drawbacks and provide an overview on recent advances in our understanding of the physiology of beta-cells using the pancreas slice approach.
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Affiliation(s)
- M Rupnik
- Faculty of Medicine, Institute of Physiology, University of Maribor, Maribor, Slovenia.
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35
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Modeling K(ATP) channel gating and its regulation. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2008; 99:7-19. [PMID: 18983870 DOI: 10.1016/j.pbiomolbio.2008.10.002] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.
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Abstract
ATP-sensitive potassium (KATP) channels are composed of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits. Binding of ATP to Kir6.2 leads to inhibition of channel activity. Because there are four subunits and thus four ATP-binding sites, four binding events are possible. ATP binds to both the open and closed states of the channel and produces a decrease in the mean open time, a reduction in the mean burst duration, and an increase in the frequency and duration of the interburst closed states. Here, we investigate the mechanism of interaction of ATP with the open state of the channel by analyzing the single-channel kinetics of concatenated Kir6.2 tetramers containing from zero to four mutated Kir6.2 subunits that possess an impaired ATP-binding site. We show that the ATP-dependent decrease in the mean burst duration is well described by a Monod-Wyman-Changeux model in which channel closing is produced by all four subunits acting in a single concerted step. The data are inconsistent with a Hodgkin-Huxley model (four independent steps) or a dimer model (two independent dimers). When the channel is open, ATP binds to a single ATP-binding site with a dissociation constant of 300 μM.
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Affiliation(s)
- Tim J Craig
- Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
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Webster NJ, Searle GJ, Lam PPL, Huang YC, Riedel MJ, Harb G, Gaisano HY, Holt A, Light PE. Elevation in intracellular long-chain acyl-coenzyme A esters lead to reduced beta-cell excitability via activation of adenosine 5'-triphosphate-sensitive potassium channels. Endocrinology 2008; 149:3679-87. [PMID: 18372336 DOI: 10.1210/en.2007-1138] [Citation(s) in RCA: 12] [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
Closure of pancreatic beta-cell ATP-sensitive potassium (K(ATP)) channels links glucose metabolism to electrical activity and insulin secretion. It is now known that saturated, but not polyunsaturated, long-chain acyl-coenyzme A esters (acyl-CoAs) can potently activate K(ATP) channels when superfused directly across excised membrane patches, suggesting a plausible mechanism to account for reduced beta-cell excitability and insulin secretion observed in obesity and type 2 diabetes. However, reduced beta-cell excitability due to elevation of endogenous saturated acyl-CoAs has not been confirmed in intact pancreatic beta-cells. To test this notion directly, endogenous acyl-CoA levels were elevated within primary mouse beta-cells using virally delivered overexpression of long-chain acyl-CoA synthetase-1 (AdACSL-1), and the effects on beta-cell K(ATP) channel activity and cell excitability was assessed using the perforated whole-cell and cell-attached patch-clamp technique. Data indicated a significant increase in K(ATP) channel activity in AdACSL-1-infected beta-cells cultured in medium supplemented with palmitate/oleate but not with the polyunsaturated fat linoleate. No changes in the ATP/ADP ratio were observed in any of the groups. Furthermore, AdACSL-1-infected beta-cells (with palmitate/oleate) showed a significant decrease in electrical responsiveness to glucose and tolbutamide and a hyperpolarized resting membrane potential at 5 mm glucose. These results suggest a direct link between intracellular fatty ester accumulation and K(ATP) channel activation, which may contribute to beta-cell dysfunction in type 2 diabetes.
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Affiliation(s)
- Nicola J Webster
- Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada
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38
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Tarasov AI, Nicolson TJ, Riveline JP, Taneja TK, Baldwin SA, Baldwin JM, Charpentier G, Gautier JF, Froguel P, Vaxillaire M, Rutter GA. A rare mutation in ABCC8/SUR1 leading to altered ATP-sensitive K+ channel activity and beta-cell glucose sensing is associated with type 2 diabetes in adults. Diabetes 2008; 57:1595-604. [PMID: 18346985 PMCID: PMC6101196 DOI: 10.2337/db07-1547] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
OBJECTIVE ATP-sensitive K(+) channels (K(ATP) channels) link glucose metabolism to the electrical activity of the pancreatic beta-cell to regulate insulin secretion. Mutations in either the Kir6.2 or sulfonylurea receptor (SUR) 1 subunit of the channel have previously been shown to cause neonatal diabetes. We describe here an activating mutation in the ABCC8 gene, encoding SUR1, that is associated with the development of type 2 diabetes only in adults. RESEARCH DESIGN AND METHODS Recombinant K(ATP) channel subunits were expressed using pIRES2-based vectors in human embryonic kidney (HEK) 293 or INS1(832/13) cells and the subcellular distribution of c-myc-tagged SUR1 channels analyzed by confocal microscopy. K(ATP) channel activity was measured in inside-out patches and plasma membrane potential in perforated whole-cell patches. Cytoplasmic [Ca(2+)] was imaged using Fura-Red. RESULTS A mutation in ABCC8/SUR1, leading to a Y356C substitution in the seventh membrane-spanning alpha-helix, was observed in a patient diagnosed with hyperglycemia at age 39 years and in two adult offspring with impaired insulin secretion. Single K(ATP) channels incorporating SUR1-Y356C displayed lower sensitivity to MgATP (IC(50) = 24 and 95 micromol/l for wild-type and mutant channels, respectively). Similar effects were observed in the absence of Mg(2+), suggesting an allosteric effect via associated Kir6.2 subunits. Overexpression of SUR1-Y356C in INS1(832/13) cells impaired glucose-induced cell depolarization and increased in intracellular free Ca(2+) concentration, albeit more weakly than neonatal diabetes-associated SUR1 mutants. CONCLUSIONS An ABCC8/SUR1 mutation with relatively minor effects on K(ATP) channel activity and beta-cell glucose sensing causes diabetes in adulthood. These data suggest a close correlation between altered SUR1 properties and clinical phenotype.
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Affiliation(s)
- Andrei I Tarasov
- Section of Cell Biology, Division of Medicine, Imperial College London, London, UK
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Abstract
An explosion of work over the last decade has produced insight into the multiple hereditary causes of a nonimmunological form of diabetes diagnosed most frequently within the first 6 months of life. These studies are providing increased understanding of genes involved in the entire chain of steps that control glucose homeostasis. Neonatal diabetes is now understood to arise from mutations in genes that play critical roles in the development of the pancreas, of beta-cell apoptosis and insulin processing, as well as the regulation of insulin release. For the basic researcher, this work is providing novel tools to explore fundamental molecular and cellular processes. For the clinician, these studies underscore the need to identify the genetic cause underlying each case. It is increasingly clear that the prognosis, therapeutic approach, and genetic counseling a physician provides must be tailored to a specific gene in order to provide the best medical care.
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Affiliation(s)
- Lydia Aguilar-Bryan
- Pacific Northwest Diabetes Research Institute, 720 Broadway, Seattle, Washington 98122, USA.
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Tucker SJ, Baukrowitz T. How highly charged anionic lipids bind and regulate ion channels. ACTA ACUST UNITED AC 2008; 131:431-8. [PMID: 18411329 PMCID: PMC2346576 DOI: 10.1085/jgp.200709936] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Stephen J Tucker
- Oxford Centre for Gene Function, Department of Physiology, Anatomy, and Genetics, University of Oxford, Oxford, UK
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Wolfgang MJ, Lane MD. Hypothalamic malonyl-coenzyme A and the control of energy balance. Mol Endocrinol 2008; 22:2012-20. [PMID: 18356287 DOI: 10.1210/me.2007-0538] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
An intermediate in the fatty acid biosynthetic pathway, malonyl-coenzyme A (CoA), has emerged as a major regulator of energy homeostasis not only in peripheral metabolic tissues but also in regions of the central nervous system that control satiety and energy expenditure. Fluctuations in hypothalamic malonyl-CoA lead to changes in food intake and peripheral energy expenditure in a manner consistent with an anorexigenic signaling intermediate. Hypothalamic malonyl-CoA is regulated by nutritional and endocrine cues including glucose and leptin, respectively. That malonyl-CoA is an essential component in the energy homeostatic signaling system of the hypothalamus is supported by convergence of physiological, pharmacological, and genetic evidence. This review will focus on evidence implicating malonyl-CoA as a central player in the control of body weight and adiposity as well as clues to the molecular mechanism by which carbon flux through the fatty acid biosynthetic pathway is linked to the neural control of energy balance.
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Affiliation(s)
- Michael J Wolfgang
- Department of Biological Chemistry, Johns Hopkins University School of Medicine, 512 WBSB, 725 North Wolfe Street, Baltimore, Maryland 21205, USA
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42
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Schulze DU, Düfer M, Wieringa B, Krippeit-Drews P, Drews G. An adenylate kinase is involved in KATP channel regulation of mouse pancreatic beta cells. Diabetologia 2007; 50:2126-34. [PMID: 17704905 DOI: 10.1007/s00125-007-0742-9] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/24/2007] [Accepted: 05/25/2007] [Indexed: 10/22/2022]
Abstract
AIMS/HYPOTHESIS In a previous study, we demonstrated that a creatine kinase (CK) modulates K(ATP) channel activity in pancreatic beta cells. To explore phosphotransfer signalling pathways in more detail, we examined whether K(ATP) channel regulation in beta cells is determined by a metabolic interaction between adenylate kinase (AK) and CK. METHODS Single channel activity was measured with the patch-clamp technique in the inside-out (i/o) and open-cell attached (oca) configuration. RESULTS The ATP sensitivity of K(ATP) channels was higher in i/o patches than in permeabilised beta cells (oca). One reason for this observation could be that the local ATP:ADP ratio in the proximity of the channels is determined by factors not active in i/o patches. AMP (0.1 mmol/l) clearly increased open channel probability in the presence of ATP (0.125 mmol/l) in permeabilised cells but not in excised patches. This suggests that AK-catalysed ADP production in the vicinity of the channels is involved in K(ATP) channel regulation. The observation that the stimulatory effect of AMP on K(ATP) channels was prevented by the AK inhibitor P (1),P (5)-di(adenosine-5')pentaphosphate (Ap(5)A; 20 micromol/l) and abolished in the presence of the non-metabolisable ATP analogue adenosine 5'-(beta,gamma-imido)triphosphate tetralithium salt (AMP-PNP; 0.12 mmol/l) strengthens this idea. In beta cells from AK1 knockout mice, the effect of AMP was less pronounced, though not completely suppressed. The increase in K(ATP) channel activity induced by AMP in the presence of ATP was outweighed by phosphocreatine (1 mmol/l). We suggest that this is due to an elevation of the ATP concentration by CK. CONCLUSIONS/INTERPRETATION We propose that phosphotransfer events mediated by AK and CK play an important role in determining the effective concentrations of ATP and ADP in the microenvironment of pancreatic beta cell K(ATP) channels. Thus, these enzymes determine the open probability of K(ATP) channels and eventually the actual rate of insulin secretion.
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Affiliation(s)
- D U Schulze
- Institute of Pharmacy, Department of Pharmacology, University of Tübingen, Auf der Morgenstelle 8, 72076, Tübingen, Germany
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Ashcroft FM. The Walter B. Cannon Physiology in Perspective Lecture, 2007. ATP-sensitive K+ channels and disease: from molecule to malady. Am J Physiol Endocrinol Metab 2007; 293:E880-9. [PMID: 17652156 DOI: 10.1152/ajpendo.00348.2007] [Citation(s) in RCA: 88] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
This essay is based on a lecture given to the American Physiological Society in honor of Walter B. Cannon, an advocate of homeostasis. It focuses on the role of the ATP-sensitive potassium K(+) (K(ATP)) channel in glucose homeostasis and, in particular, on its role in insulin secretion from pancreatic beta-cells. The beta-cell K(ATP) channel comprises pore-forming Kir6.2 and regulatory SUR1 subunits, and mutations in either type of subunit can result in too little or too much insulin release. Here, I review the latest information on the relationship between K(ATP) channel structure and function, and consider how mutations in the K(ATP) channel genes lead to neonatal diabetes or congenital hyperinsulinism.
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Affiliation(s)
- Frances M Ashcroft
- Henry Wellcome Centre for Gene Function, Dept. of Physiology, Anatomy and Genetics, Univ. of Oxford, Parks Road, Oxford OX1 3PT, UK.
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Smith AJ, Taneja TK, Mankouri J, Sivaprasadarao A. Molecular cell biology of KATPchannels: implications for neonatal diabetes. Expert Rev Mol Med 2007; 9:1-17. [PMID: 17666135 DOI: 10.1017/s1462399407000403] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractATP-sensitive potassium (KATP) channels play a key role in the regulation of insulin secretion by coupling glucose metabolism to the electrical activity of pancreatic β-cells. To generate an electric signal of suitable magnitude, the plasma membrane of the β-cell must contain an appropriate number of channels. An inadequate number of channels can lead to congenital hyperinsulinism, whereas an excess of channels can result in the opposite condition, neonatal diabetes. KATPchannels are made up of four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1), encoded by the genesKCNJ11andABCC8, respectively. Following synthesis, the subunits must assemble into an octameric complex to be able to exit the endoplasmic reticulum and reach the plasma membrane. While this biosynthetic pathway ensures supply of channels to the cell surface, an opposite pathway, involving clathrin-mediated endocytosis, removes channels back into the cell. The balance between these two processes, perhaps in conjunction with endocytic recycling, would dictate the channel density at the cell membrane. In this review, we discuss the molecular signals that contribute to this balance, and how an imbalance could lead to a disease state such as neonatal diabetes.
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Affiliation(s)
- Andrew J Smith
- Institute of Membrane and Systems Biology, University of Leeds, Leeds, UK
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Bränström R, Leibiger IB, Leibiger B, Klement G, Nilsson J, Arhem P, Aspinwall CA, Corkey BE, Larsson O, Berggren PO. Single residue (K332A) substitution in Kir6.2 abolishes the stimulatory effect of long-chain acyl-CoA esters: indications for a long-chain acyl-CoA ester binding motif. Diabetologia 2007; 50:1670-7. [PMID: 17522836 DOI: 10.1007/s00125-007-0697-x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/20/2007] [Accepted: 04/03/2007] [Indexed: 01/09/2023]
Abstract
AIMS/HYPOTHESIS The pancreatic beta cell ATP-sensitive potassium (K(ATP)) channel, composed of the pore-forming alpha subunit Kir6.2, a member of the inward rectifier K+channel family, and the regulatory beta subunit sulfonylurea receptor 1 (SUR1), a member of the ATP-binding cassette superfamily, couples the metabolic state of the cell to electrical activity. Several endogenous compounds are known to modulate K(ATP) channel activity, including ATP, ADP, phosphatidylinositol diphosphates and long-chain acyl coenzyme A (LC-CoA) esters. LC-CoA esters have been shown to interact with Kir6.2, but the mechanism and binding site(s) have yet to be identified. MATERIALS AND METHODS Using multiple sequence alignment of known acyl-CoA ester interacting proteins, we were able to identify four conserved amino acid residues that could potentially serve as an acyl-CoA ester-binding motif. The motif was also recognised in the C-terminal region of Kir6.2 (R311-332) but not in SUR1. RESULTS Oocytes expressing Kir6.2DeltaC26 K332A repeatedly generated K(+)currents in inside-out membrane patches that were sensitive to ATP, but were only weakly activated by 1 mumol/l palmitoyl-CoA ester. Compared with the control channel (Kir6.2DeltaC26), Kir6.2DeltaC26 K332A displayed unaltered ATP sensitivity but significantly decreased sensitivity to palmitoyl-CoA esters. Coexpression of Kir6.2DeltaC26 K332A and SUR1 revealed slightly increased activation by palmitoyl-CoA ester but significantly decreased activation by the acyl-CoA esters compared with the wild-type K(ATP) channel and Kir6.2DeltaC26+SUR1. Computational modelling, using the crystal structure of KirBac1.1, suggested that K332 is located on the intracellular domain of Kir6.2 and is accessible to intracellular modulators such as LC-CoA esters. CONCLUSIONS/INTERPRETATION These results verify that LC-CoA esters interact at the pore-forming subunit Kir6.2, and on the basis of these data we propose an acyl-CoA ester binding motif located in the C-terminal region.
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Affiliation(s)
- R Bränström
- Rolf Luft Research Center for Diabetes and Endocrinology, Karolinska Institutet, Karolinska University Hospital, Stockholm, Sweden.
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Olofsson CS, Collins S, Bengtsson M, Eliasson L, Salehi A, Shimomura K, Tarasov A, Holm C, Ashcroft F, Rorsman P. Long-term exposure to glucose and lipids inhibits glucose-induced insulin secretion downstream of granule fusion with plasma membrane. Diabetes 2007; 56:1888-97. [PMID: 17456851 DOI: 10.2337/db06-1150] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Mouse beta-cells cultured at 15 mmol/l glucose for 72 h had reduced ATP-sensitive K+ (K(ATP)) channel activity (-30%), increased voltage-gated Ca2+ currents, higher intracellular free Ca2+ concentration ([Ca2+]i; +160%), more exocytosis (monitored by capacitance measurements, +100%), and greater insulin content (+230%) than those cultured at 4.5 mmol/l glucose. However, they released 20% less insulin when challenged with 20 mmol/l glucose. Glucose-induced (20 mmol/l) insulin secretion was reduced by 60-90% in islets cocultured at 4.5 or 15 mmol/l glucose and either oleate or palmitate (0.5 mmol/l). Free fatty acid (FFA)-induced inhibition of secretion was not associated with any major changes in [Ca2+]i or islet ATP content. Palmitate stimulated exocytosis by twofold or more but reduced K+-induced secretion by up to 60%. Basal (1 mmol/l glucose) K(ATP) channel activity was 40% lower in islets cultured at 4.5 mmol/l glucose plus palmitate and 60% lower in islets cultured at 15 mmol/l glucose plus either of the FFAs. Insulin content decreased by 75% in islets exposed to FFAs in the presence of high (15 mmol/l), but not low (4.5 mmol/l), glucose concentrations, but the number of secretory granules was unchanged. FFA-induced inhibition of insulin secretion was not associated with increased transcript levels of the apoptosis markers Bax (BclII-associated X protein) and caspase-3. We conclude that glucose and FFAs reduce insulin secretion by interference with the exit of insulin via the fusion pore.
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Affiliation(s)
- Charlotta S Olofsson
- Department of Clinical Sciences, Clinical Research Centre, Lund University, Malmö, Sweden
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Doliba NM, Wehrli SL, Vatamaniuk MZ, Qin W, Buettger CW, Collins HW, Matschinsky FM. Metabolic and ionic coupling factors in amino acid-stimulated insulin release in pancreatic beta-HC9 cells. Am J Physiol Endocrinol Metab 2007; 292:E1507-19. [PMID: 17264232 DOI: 10.1152/ajpendo.00282.2006] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Fuel stimulation of insulin secretion from pancreatic beta-cells is thought to be mediated by metabolic coupling factors that are generated by energized mitochondria, including protons, adenine nucleotides, and perhaps certain amino acids (AA), as for instance aspartate, glutamate, or glutamine (Q). The goal of the present study was to evaluate the role of such factors when insulin release (IR) is stimulated by glucose or AA, alone or combined, using (31)P, (23)Na and (1)H NMR technology, respirometry, and biochemical analysis to study the metabolic events that occur in continuously superfused mouse beta-HC9 cells contained in agarose beads and enhanced by the phosphodiesterase inhibitor IBMX. Exposing beta-HC9 cells to high glucose or 3.5 mM of a physiological mixture of 18 AA (AAM) plus 2 mM glutamine caused a marked stimulation of insulin secretion associated with increased oxygen consumption, cAMP release, and phosphorylation potential as evidenced by higher phosphocreatine and lower P(i) peak areas of (31)P NMR spectra. Diazoxide blocked stimulation of IR completely, suggesting involvement of ATP-dependent potassium (K(ATP)) channels in this process. However, levels of MgATP and MgADP concentrations, which regulate channel activity, changed only slowly and little, whereas the rate of insulin release increased fast and very markedly. The involvement of other candidate coupling factors was therefore considered. High glucose or AAM + Q increased pH(i). The availability of temporal pH profiles allowed the precise computation of the phosphate potential (ATP/P(i) x ADP) in fuel-stimulated IR. Intracellular Na+ levels were greatly elevated by AAM + Q. However, glutamine alone or together with 2-amino-2-norbornanecarboxylic acid (which activates glutamate dehydrogenase) decreased beta-cell Na levels. Stimulation of beta-cells by glucose in the presence of AAM + Q (0.5 mM) was associated with rising cellular concentrations of glutamate and glutamine and strikingly lower aspartate levels. Methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the glucose enhancement of AMM + Q-induced IR and associated changes in glutamine and aspartate but did not prevent the accumulation of glutamate. The results of this study demonstrate again that an increased phosphate potential and a functional K(ATP) channel are essential for metabolic coupling during fuel-stimulated insulin release but illustrate that determining the identity and relative importance of all participating coupling factors and second messengers remains a challenge largely unmet.
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Affiliation(s)
- Nicolai M Doliba
- Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, PA, USA.
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de Barros Reis MA, Arantes VC, Cunha DA, Latorraca MQ, Toyama MH, Carneiro EM, Boschero AC. Increased L-CPT-1 activity and altered gene expression in pancreatic islets of malnourished adult rats: a possible relationship between elevated free fatty acid levels and impaired insulin secretion. J Nutr Biochem 2007; 19:85-90. [PMID: 17531461 DOI: 10.1016/j.jnutbio.2007.01.005] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2006] [Revised: 12/29/2006] [Accepted: 01/11/2007] [Indexed: 10/23/2022]
Abstract
Intrauterine growth restriction is associated with chronically elevated levels of serum fatty acids and reduced glucose-stimulated insulin secretion. Lipid metabolism in pancreatic beta cells is critical for the regulation of insulin secretion, and the chronic exposure to fatty acids results in higher palmitate oxidation rates and an altered insulin response to glucose. Using a rat model of isocaloric protein restriction, we examined whether pre- and postnatal protein malnutrition influences the properties of pancreatic islet carnitine palmitoyltransferase-1 (liver isoform, L-CPT-1), a rate-limiting enzyme that regulates fatty acid oxidation in mitochondria. The activity of L-CPT-1 in pancreatic islets increased in the low protein (LP), although the L-CPT-1 mRNA levels were unaffected by malnutrition. The susceptibility of enzyme to inhibition by malonyl-CoA was unaltered and the content of malonyl-CoA was reduced in LP cells. Because the mitochondrial oxidation of fatty acids is related to the altered expression of a number of genes encoding proteins involved in insulin secretion, the levels of expression of insulin and GLUT-2 mRNA were assessed. A reduced expression of both genes was observed in malnourished rats. These results provide further evidence that increased L-CPT-1 activity and changes in gene expression in pancreatic islets may be involved in the reduced insulin secretion seen in malnourished rats.
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Freeman H, Shimomura K, Cox RD, Ashcroft FM. Nicotinamide nucleotide transhydrogenase: a link between insulin secretion, glucose metabolism and oxidative stress. Biochem Soc Trans 2007; 34:806-10. [PMID: 17052203 DOI: 10.1042/bst0340806] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
This paper reviews recent studies on the role of Nnt (nicotinamide nucleotide transhydrogenase) in insulin secretion and detoxification of ROS (reactive oxygen species). Glucose-stimulated insulin release from pancreatic beta-cells is mediated by increased metabolism. This elevates intracellular [ATP], thereby closing KATP channels (ATP-sensitive potassium channels) and producing membrane depolarization, activation of voltage-gated Ca2+ channels, Ca2+ influx and, consequently, insulin secretion. The C57BL/6J mouse displays glucose intolerance and reduced insulin secretion, which results from a naturally occurring deletion in the Nnt gene. Transgenic expression of the wild-type Nnt gene in C57BL/6J mice rescues the phenotype. Knockdown of Nnt in the insulin-secreting cell line MIN6 with small interfering RNA dramatically reduced Ca2+ influx and insulin secretion. Similarly, mice carrying ENU (N-ethyl-N-nitrosourea)-induced loss-of-function mutations in Nnt were glucose intolerant and secreted less insulin during a glucose tolerance test. Islets isolated from these mice showed impaired insulin secretion in response to glucose, but not to the KATP channel blocker tolbutamide. This is explained by the fact that glucose failed to elevate ATP in Nnt mutant islets. Nnt is a nuclear-encoded mitochondrial protein involved in detoxification of ROS. beta-Cells isolated from Nnt mutant mice showed increased ROS production on glucose stimulation. We hypothesize that Nnt mutations enhance glucose-dependent ROS production and thereby impair beta-cell mitochondrial metabolism, possibly via activation of uncoupling proteins. This reduces ATP production and lowers KATP channel activity. Consequently, glucose-dependent electrical activity and insulin secretion are impaired.
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Affiliation(s)
- H Freeman
- University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK
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50
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Mountjoy PD, Bailey SJ, Rutter GA. Inhibition by glucose or leptin of hypothalamic neurons expressing neuropeptide Y requires changes in AMP-activated protein kinase activity. Diabetologia 2007; 50:168-77. [PMID: 17093945 DOI: 10.1007/s00125-006-0473-3] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/10/2006] [Accepted: 09/04/2006] [Indexed: 12/31/2022]
Abstract
AIMS/HYPOTHESIS Changes in the activity of glucose-excited and glucose-inhibited neurons within the basomedial hypothalamus are key to the central regulation of satiety. However, the molecular mechanisms through which these cells respond to extracellular stimuli remain poorly understood. Here, we investigate the role of 5'-AMP-activated protein kinase (AMPK), a trimeric complex encoded by seven distinct genes of the PRKA family, in the responses to glucose and leptin of each cell type. METHODS The activity of isolated rat basomedial hypothalamic neurons was assessed by: (1) recording cellular voltage responses under current clamp; (2) measuring intracellular free Ca(2+) with fluo-3 or fura-2; and (3) developing a neuropeptide Y (NPY) promoter-driven adenovirally produced ratiometric 'pericam' (a green fluorescent protein-based Ca(2+) sensor) to monitor [Ca(2+)] changes selectively in NPY-positive neurons. RESULTS The stimulatory effects of decreased (0 or 1.0 vs 15 mmol/l) glucose on glucose-inhibited neurons were mimicked by the AMPK activator, 5-amino-imidazole-4-carboxamide riboside (AICAR) and blocked by the inhibitor Compound C. Similarly, AICAR reversed the inhibitory effects of leptin in the majority of glucose-inhibited neurons. The responses to glucose of Npy-expressing cells, which represented approximately 40 % of all glucose-inhibited neurons, were also sensitive to Compound C or AICAR. Forced changes in AMPK activity had no effect on glucose-excited and non-glucose-responsive neurons. CONCLUSIONS/INTERPRETATION Changes in AMPK activity are involved in the responses of glucose-inhibited neurons to large fluctuations in glucose concentration, and possibly also to leptin. This mechanism may contribute to the acute reduction of electrical activity and Ca(2+) oscillation frequency in these, but not other neurons, in the basomedial hypothalamus.
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Affiliation(s)
- P D Mountjoy
- Henry Wellcome Laboratories of Integrated Cell Signalling and Department of Biochemistry, School of Medical Sciences, University of Bristol, Bristol, UK
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