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Glycine -methyltransferase deletion in mice diverts carbon flux from gluconeogenesis to pathways that utilize excess methionine cycle intermediates.
Hughey CC, Trefts E, Bracy DP, James FD, Donahue EP, Wasserman DH
(2018) J Biol Chem 293: 11944-11954
MeSH Terms: Animals, Carbon, Citric Acid Cycle, Energy Metabolism, Fatty Liver, Gene Deletion, Gluconeogenesis, Glucose, Glycine N-Methyltransferase, Liver, Male, Metabolic Flux Analysis, Methionine, Mice, Mice, Knockout, S-Adenosylmethionine
Show Abstract · Added March 26, 2019
Glycine -methyltransferase (GNMT) is the most abundant liver methyltransferase regulating the availability of the biological methyl donor, -adenosylmethionine (SAM). Moreover, GNMT has been identified to be down-regulated in hepatocellular carcinoma (HCC). Despite its role in regulating SAM levels and association of its down-regulation with liver tumorigenesis, the impact of reduced GNMT on metabolic reprogramming before the manifestation of HCC has not been investigated in detail. Herein, we used H/C metabolic flux analysis in conscious, unrestrained mice to test the hypothesis that the absence of GNMT causes metabolic reprogramming. GNMT-null (KO) mice displayed a reduction in blood glucose that was associated with a decline in both hepatic glycogenolysis and gluconeogenesis. The reduced gluconeogenesis was due to a decrease in liver gluconeogenic precursors, citric acid cycle fluxes, and anaplerosis and cataplerosis. A concurrent elevation in both hepatic SAM and metabolites of SAM utilization pathways was observed in the KO mice. Specifically, the increase in metabolites of SAM utilization pathways indicated that hepatic polyamine synthesis and catabolism, transsulfuration, and lipogenesis pathways were increased in the KO mice. Of note, these pathways utilize substrates that could otherwise be used for gluconeogenesis. Also, this metabolic reprogramming occurs before the well-documented appearance of HCC in GNMT-null mice. Together, these results indicate that GNMT deletion promotes a metabolic shift whereby nutrients are channeled away from glucose formation toward pathways that utilize the elevated SAM.
© 2018 Hughey et al.
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MeSH Terms
Cytosolic phosphoenolpyruvate carboxykinase as a cataplerotic pathway in the small intestine.
Potts A, Uchida A, Deja S, Berglund ED, Kucejova B, Duarte JA, Fu X, Browning JD, Magnuson MA, Burgess SC
(2018) Am J Physiol Gastrointest Liver Physiol 315: G249-G258
MeSH Terms: Amino Acids, Animals, Blood Glucose, Cytosol, Energy Metabolism, Gluconeogenesis, Glucose, Intestine, Small, Lipid Metabolism, Mice, Phosphoenolpyruvate Carboxykinase (ATP)
Show Abstract · Added May 1, 2018
Cytosolic phosphoenolpyruvate carboxykinase (PEPCK) is a gluconeogenic enzyme that is highly expressed in the liver and kidney but is also expressed at lower levels in a variety of other tissues where it may play adjunct roles in fatty acid esterification, amino acid metabolism, and/or TCA cycle function. PEPCK is expressed in the enterocytes of the small intestine, but it is unclear whether it supports a gluconeogenic rate sufficient to affect glucose homeostasis. To examine potential roles of intestinal PEPCK, we generated an intestinal PEPCK knockout mouse. Deletion of intestinal PEPCK ablated ex vivo gluconeogenesis but did not significantly affect glycemia in chow, high-fat diet, or streptozotocin-treated mice. In contrast, postprandial triglyceride secretion from the intestine was attenuated in vivo, consistent with a role in fatty acid esterification. Intestinal amino acid profiles and C tracer appearance into these pools were significantly altered, indicating abnormal amino acid trafficking through the enterocyte. The data suggest that the predominant role of PEPCK in the small intestine of mice is not gluconeogenesis but rather to support nutrient processing, particularly with regard to lipids and amino acids. NEW & NOTEWORTHY The small intestine expresses gluconeogenic enzymes for unknown reasons. In addition to glucose synthesis, the nascent steps of this pathway can be used to support amino acid and lipid metabolisms. When phosphoenolpyruvate carboxykinase, an essential gluconeogenic enzyme, is knocked out of the small intestine of mice, glycemia is unaffected, but mice inefficiently absorb dietary lipid, have abnormal amino acid profiles, and inefficiently catabolize glutamine. Therefore, the initial steps of intestinal gluconeogenesis are used for processing dietary triglycerides and metabolizing amino acids but are not essential for maintaining blood glucose levels.
3 Communities
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11 MeSH Terms
Hepatic Gi signaling regulates whole-body glucose homeostasis.
Rossi M, Zhu L, McMillin SM, Pydi SP, Jain S, Wang L, Cui Y, Lee RJ, Cohen AH, Kaneto H, Birnbaum MJ, Ma Y, Rotman Y, Liu J, Cyphert TJ, Finkel T, McGuinness OP, Wess J
(2018) J Clin Invest 128: 746-759
MeSH Terms: Animals, Blood Glucose, Extracellular Signal-Regulated MAP Kinases, Female, GTP-Binding Protein alpha Subunits, Gi-Go, Gene Expression Profiling, Glucagon, Gluconeogenesis, Glucose, Hepatocytes, Homeostasis, Humans, Liver, Male, Mice, Mice, Inbred C57BL, Mice, Knockout, Oxygen, Phosphatidylinositol 3-Kinases, Phosphorylation, Reactive Oxygen Species, Receptors, Glucagon, Signal Transduction
Show Abstract · Added March 14, 2018
An increase in hepatic glucose production (HGP) is a key feature of type 2 diabetes. Excessive signaling through hepatic Gs-linked glucagon receptors critically contributes to pathologically elevated HGP. Here, we tested the hypothesis that this metabolic impairment can be counteracted by enhancing hepatic Gi signaling. Specifically, we used a chemogenetic approach to selectively activate Gi-type G proteins in mouse hepatocytes in vivo. Unexpectedly, activation of hepatic Gi signaling triggered a pronounced increase in HGP and severely impaired glucose homeostasis. Moreover, increased Gi signaling stimulated glucose release in human hepatocytes. A lack of functional Gi-type G proteins in hepatocytes reduced blood glucose levels and protected mice against the metabolic deficits caused by the consumption of a high-fat diet. Additionally, we delineated a signaling cascade that links hepatic Gi signaling to ROS production, JNK activation, and a subsequent increase in HGP. Taken together, our data support the concept that drugs able to block hepatic Gi-coupled GPCRs may prove beneficial as antidiabetic drugs.
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23 MeSH Terms
Loss of hepatic AMP-activated protein kinase impedes the rate of glycogenolysis but not gluconeogenic fluxes in exercising mice.
Hughey CC, James FD, Bracy DP, Donahue EP, Young JD, Viollet B, Foretz M, Wasserman DH
(2017) J Biol Chem 292: 20125-20140
MeSH Terms: AMP-Activated Protein Kinases, Animals, Energy Metabolism, Gluconeogenesis, Glucose, Glycogenolysis, Homeostasis, Isotope Labeling, Liver, Mice, Mice, Knockout, Physical Conditioning, Animal
Show Abstract · Added March 14, 2018
Pathologies including diabetes and conditions such as exercise place an unusual demand on liver energy metabolism, and this demand induces a state of energy discharge. Hepatic AMP-activated protein kinase (AMPK) has been proposed to inhibit anabolic processes such as gluconeogenesis in response to cellular energy stress. However, both AMPK activation and glucose release from the liver are increased during exercise. Here, we sought to test the role of hepatic AMPK in the regulation of glucose-producing and citric acid cycle-related fluxes during an acute bout of muscular work. We used H/C metabolic flux analysis to quantify intermediary metabolism fluxes in both sedentary and treadmill-running mice. Additionally, liver-specific AMPK α1 and α2 subunit KO and WT mice were utilized. Exercise caused an increase in endogenous glucose production, glycogenolysis, and gluconeogenesis from phosphoenolpyruvate. Citric acid cycle fluxes, pyruvate cycling, anaplerosis, and cataplerosis were also elevated during this exercise. Sedentary nutrient fluxes in the postabsorptive state were comparable for the WT and KO mice. However, the increment in the endogenous rate of glucose appearance during exercise was blunted in the KO mice because of a diminished glycogenolytic flux. This lower rate of glycogenolysis was associated with lower hepatic glycogen content before the onset of exercise and prompted a reduction in arterial glucose during exercise. These results indicate that liver AMPKα1α2 is required for maintaining glucose homeostasis during an acute bout of exercise.
© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.
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12 MeSH Terms
Glucose autoregulation is the dominant component of the hormone-independent counterregulatory response to hypoglycemia in the conscious dog.
Gregory JM, Rivera N, Kraft G, Winnick JJ, Farmer B, Allen EJ, Donahue EP, Smith MS, Edgerton DS, Williams PE, Cherrington AD
(2017) Am J Physiol Endocrinol Metab 313: E273-E283
MeSH Terms: Adipose Tissue, Adrenalectomy, Animals, Blood Glucose, Dogs, Gluconeogenesis, Glucose, Glucose Clamp Technique, Homeostasis, Hypoglycemia, Hypoglycemic Agents, Infusions, Intravenous, Insulin, Liver, Liver Glycogen, Muscle, Skeletal, Norepinephrine, Portal Vein, Sympathetic Nervous System
Show Abstract · Added April 23, 2018
The contribution of hormone-independent counterregulatory signals in defense of insulin-induced hypoglycemia was determined in adrenalectomized, overnight-fasted conscious dogs receiving hepatic portal vein insulin infusions at a rate 20-fold basal. Either euglycemia was maintained () or hypoglycemia (≈45 mg/dl) was allowed to occur. There were three hypoglycemic groups: one in which hepatic autoregulation against hypoglycemia occurred in the absence of sympathetic nervous system input (), one in which autoregulation occurred in the presence of norepinephrine (NE) signaling to fat and muscle (), and one in which autoregulation occurred in the presence of NE signaling to fat, muscle, and liver (). Average net hepatic glucose balance (NHGB) during the last hour for was -0.7 ± 0.1, 0.3 ± 0.1 ( < 0.01 vs. ), 0.7 ± 0.1 ( = 0.01 vs. ), and 0.8 ± 0.1 ( = 0.7 vs. ) mg·kg·min, respectively. Hypoglycemia per se () increased NHGB by causing an inhibition of net hepatic glycogen synthesis. NE signaling to fat and muscle () increased NHGB further by mobilizing gluconeogenic precursors resulting in a rise in gluconeogenesis. Lowering glucose per se decreased nonhepatic glucose uptake by 8.9 mg·kg·min, and the addition of increased neural efferent signaling to muscle and fat blocked glucose uptake further by 3.2 mg·kg·min The addition of increased neural efferent input to liver did not affect NHGB or nonhepatic glucose uptake significantly. In conclusion, even in the absence of increases in counterregulatory hormones, the body can defend itself against hypoglycemia using glucose autoregulation and increased neural efferent signaling, both of which stimulate hepatic glucose production and limit glucose utilization.
Copyright © 2017 the American Physiological Society.
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Metabolic responses to exogenous ghrelin in obesity and early after Roux-en-Y gastric bypass in humans.
Tamboli RA, Antoun J, Sidani RM, Clements A, Harmata EE, Marks-Shulman P, Gaylinn BD, Williams B, Clements RH, Albaugh VL, Abumrad NN
(2017) Diabetes Obes Metab 19: 1267-1275
MeSH Terms: Acylation, Anti-Obesity Agents, Cohort Studies, Combined Modality Therapy, Cross-Over Studies, Energy Metabolism, Gastric Bypass, Ghrelin, Gluconeogenesis, Glucose Clamp Technique, Human Growth Hormone, Humans, Infusions, Intravenous, Insulin Resistance, Liver, Muscle, Skeletal, Obesity, Morbid, Pancreatic Polypeptide, Pancreatic Polypeptide-Secreting Cells, Pituitary Gland, Anterior, Postoperative Care, Preoperative Care, Protein Precursors, Single-Blind Method
Show Abstract · Added April 3, 2017
AIMS - Ghrelin is a gastric-derived hormone that stimulates growth hormone (GH) secretion and has a multi-faceted role in the regulation of energy homeostasis, including glucose metabolism. Circulating ghrelin concentrations are modulated in response to nutritional status, but responses to ghrelin in altered metabolic states are poorly understood. We investigated the metabolic effects of ghrelin in obesity and early after Roux-en-Y gastric bypass (RYGB).
MATERIALS AND METHODS - We assessed central and peripheral metabolic responses to acyl ghrelin infusion (1 pmol kg  min ) in healthy, lean subjects (n = 9) and non-diabetic, obese subjects (n = 9) before and 2 weeks after RYGB. Central responses were assessed by GH and pancreatic polypeptide (surrogate for vagal activity) secretion. Peripheral responses were assessed by hepatic and skeletal muscle insulin sensitivity during a hyperinsulinaemic-euglycaemic clamp.
RESULTS - Ghrelin-stimulated GH secretion was attenuated in obese subjects, but was restored by RYGB to a response similar to that of lean subjects. The heightened pancreatic polypeptide response to ghrelin infusion in the obese was attenuated after RYGB. Hepatic glucose production and hepatic insulin sensitivity were not altered by ghrelin infusion in RYGB subjects. Skeletal muscle insulin sensitivity was impaired to a similar degree in lean, obese and post-RYGB individuals in response to ghrelin infusion.
CONCLUSIONS - These data suggest that obesity is characterized by abnormal central, but not peripheral, responsiveness to ghrelin that can be restored early after RYGB before significant weight loss. Further work is necessary to fully elucidate the role of ghrelin in the metabolic changes that occur in obesity and following RYGB.
© 2017 John Wiley & Sons Ltd.
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24 MeSH Terms
Liver AMP-Activated Protein Kinase Is Unnecessary for Gluconeogenesis but Protects Energy State during Nutrient Deprivation.
Hasenour CM, Ridley DE, James FD, Hughey CC, Donahue EP, Viollet B, Foretz M, Young JD, Wasserman DH
(2017) PLoS One 12: e0170382
MeSH Terms: AMP-Activated Protein Kinases, Adenosine Triphosphate, Animals, Energy Metabolism, Gluconeogenesis, Liver, Male, Mice, Mice, Inbred C57BL, Mice, Knockout
Show Abstract · Added April 27, 2017
AMPK is an energy sensor that protects cellular energy state by attenuating anabolic and promoting catabolic processes. AMPK signaling is purported to regulate hepatic gluconeogenesis and substrate oxidation; coordination of these processes is vital during nutrient deprivation or pathogenic during overnutrition. Here we directly test hepatic AMPK function in regulating metabolic fluxes that converge to produce glucose and energy in vivo. Flux analysis was applied in mice with a liver-specific deletion of AMPK (L-KO) or floxed control littermates to assess rates of hepatic glucose producing and citric acid cycle (CAC) fluxes. Fluxes were assessed in short and long term fasted mice; the latter condition is a nutrient stressor that increases liver AMP/ATP. The flux circuit connecting anaplerosis with gluconeogenesis from the CAC was unaffected by hepatic AMPK deletion in short and long term fasting. Nevertheless, depletion of hepatic ATP was exacerbated in L-KO mice, corresponding to a relative elevation in citrate synthase flux and accumulation of branched-chain amino acid-related metabolites. L-KO mice also had a physiological reduction in flux from glycogen to G6P. These results demonstrate AMPK is unnecessary for maintaining gluconeogenic flux from the CAC yet is critical for stabilizing liver energy state during nutrient deprivation.
1 Communities
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10 MeSH Terms
Decreased Consumption of Branched-Chain Amino Acids Improves Metabolic Health.
Fontana L, Cummings NE, Arriola Apelo SI, Neuman JC, Kasza I, Schmidt BA, Cava E, Spelta F, Tosti V, Syed FA, Baar EL, Veronese N, Cottrell SE, Fenske RJ, Bertozzi B, Brar HK, Pietka T, Bullock AD, Figenshau RS, Andriole GL, Merrins MJ, Alexander CM, Kimple ME, Lamming DW
(2016) Cell Rep 16: 520-530
MeSH Terms: Adipose Tissue, White, Amino Acids, Branched-Chain, Animals, Blood Glucose, Dietary Proteins, Fibroblast Growth Factors, Gluconeogenesis, Glucose Intolerance, Humans, Insulin-Secreting Cells, Male, Mice, Inbred C57BL, Middle Aged, Obesity, Organ Size, Stress, Physiological
Show Abstract · Added August 2, 2016
Protein-restricted (PR), high-carbohydrate diets improve metabolic health in rodents, yet the precise dietary components that are responsible for these effects have not been identified. Furthermore, the applicability of these studies to humans is unclear. Here, we demonstrate in a randomized controlled trial that a moderate PR diet also improves markers of metabolic health in humans. Intriguingly, we find that feeding mice a diet specifically reduced in branched-chain amino acids (BCAAs) is sufficient to improve glucose tolerance and body composition equivalently to a PR diet via metabolically distinct pathways. Our results highlight a critical role for dietary quality at the level of amino acids in the maintenance of metabolic health and suggest that diets specifically reduced in BCAAs, or pharmacological interventions in this pathway, may offer a translatable way to achieve many of the metabolic benefits of a PR diet.
Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.
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16 MeSH Terms
A novel experimental strategy to assess the metabolic effects of selective activation of a G(q)-coupled receptor in hepatocytes in vivo.
Li JH, Jain S, McMillin SM, Cui Y, Gautam D, Sakamoto W, Lu H, Jou W, McGuinness OP, Gavrilova O, Wess J
(2013) Endocrinology 154: 3539-51
MeSH Terms: Animals, Antidiuretic Hormone Receptor Antagonists, Cells, Cultured, Diabetes Mellitus, Type 2, Enzyme Activators, Female, G-Protein-Coupled Receptor Kinases, GTP-Binding Protein alpha Subunits, Gq-G11, Gluconeogenesis, Glycogenolysis, Hepatocytes, Humans, Hypoglycemic Agents, Male, Mice, Mice, Obese, Mice, Transgenic, Protein Engineering, Protein Interaction Domains and Motifs, Receptor, Muscarinic M3, Receptors, Vasopressin, Recombinant Fusion Proteins, Specific Pathogen-Free Organisms
Show Abstract · Added July 21, 2014
Increased hepatic glucose production is a key pathophysiological feature of type 2 diabetes. Like all other cell types, hepatocytes express many G protein-coupled receptors (GPCRs) that are linked to different functional classes of heterotrimeric G proteins. The important physiological functions mediated by G(s)-coupled hepatic glucagon receptors are well-documented. In contrast, little is known about the in vivo physiological roles of hepatocyte GPCRs that are linked to G proteins of the G(q) family. To address this issue, we established a transgenic mouse line (Hep-Rq mice) that expressed a G(q)-linked designer receptor (Rq) in a hepatocyte-selective fashion. Importantly, Rq could no longer bind endogenous ligands but could be selectively activated by a synthetic drug, clozapine-N-oxide. Clozapine-N-oxide treatment of Hep-Rq mice enabled us to determine the metabolic consequences caused by selective activation of a G(q)-coupled GPCR in hepatocytes in vivo. We found that acute Rq activation in vivo led to pronounced increases in blood glucose levels, resulting from increased rates of glycogen breakdown and gluconeogenesis. We also demonstrated that the expression of the V(1b) vasopressin receptor, a G(q)-coupled receptor expressed by hepatocytes, was drastically increased in livers of ob/ob mice, a mouse model of diabetes. Strikingly, treatment of ob/ob mice with a selective V(1b) receptor antagonist led to reduced glucose excursions in a pyruvate challenge test. Taken together, these findings underscore the importance of G(q)-coupled receptors in regulating hepatic glucose fluxes and suggest novel receptor targets for the treatment of type 2 diabetes.
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23 MeSH Terms
Dissecting the actions of widely used diabetes drugs.
Unger RH, Berglund ED, Habener JF, Cherrington AD
(2013) Nat Med 19: 272-3
MeSH Terms: Animals, Cyclic AMP, Diabetes Mellitus, Glucagon, Gluconeogenesis, Humans, Hypoglycemic Agents, Liver, Metformin, Phenformin
Added February 13, 2015
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10 MeSH Terms