Metabolism
Abstract
The increased formation of methylglyoxal (MG) under hyperglycemia is associated with the development of microvascular complications in patients with diabetes mellitus; however, the effects of elevated MG levels in vivo are poorly understood. In zebrafish, a transient knockdown of glyoxalase 1, the main MG detoxifying system, led to the elevation of endogenous MG levels and blood vessel alterations. To evaluate effects of a permanent knockout of glyoxalase 1 in vivo, glo1–/– zebrafish mutants were generated using CRISPR/Cas9. In addition, a diet-induced–obesity zebrafish model was used to analyze glo1–/– zebrafish under high nutrient intake. Glo1–/– zebrafish survived until adulthood without growth deficit and showed increased tissue MG concentrations. Impaired glucose tolerance developed in adult glo1–/– zebrafish and was indicated by increased postprandial blood glucose levels and postprandial S6 kinase activation. Challenged by an overfeeding period, fasting blood glucose levels in glo1–/– zebrafish were increased which translated into retinal blood vessel alterations. Thus, the data have identified a defective MG detoxification as a metabolic prerequisite and glyoxalase 1 alterations as a genetic susceptibility to the development of type 2 diabetes mellitus under high nutrition intake.
Authors
Elisabeth Lodd, Lucas M. Wiggenhauser, Jakob Morgenstern, Thomas H. Fleming, Gernot Poschet, Michael Büttner, Christoph T. Tabler, David P. Wohlfart, Peter P. Nawroth, Jens Kroll
Abstract
BACKGROUND Metabolic syndrome (MetS) is highly correlated with obesity and cardiovascular risk, but the importance of dietary carbohydrate independent of weight loss in MetS treatment remains controversial. Here, we test the theory that dietary carbohydrate intolerance (i.e., the inability to process carbohydrate in a healthy manner) rather than obesity per se is a fundamental feature of MetS.METHODS Individuals who were obese with a diagnosis of MetS were fed three 4-week weight-maintenance diets that were low, moderate, and high in carbohydrate. Protein was constant and fat was exchanged isocalorically for carbohydrate across all diets.RESULTS Despite maintaining body mass, low-carbohydrate (LC) intake enhanced fat oxidation and was more effective in reversing MetS, especially high triglycerides, low HDL-C, and the small LDL subclass phenotype. Carbohydrate restriction also improved abnormal fatty acid composition, an emerging MetS feature. Despite containing 2.5 times more saturated fat than the high-carbohydrate diet, an LC diet decreased plasma total saturated fat and palmitoleate and increased arachidonate.CONCLUSION Consistent with the perspective that MetS is a pathologic state that manifests as dietary carbohydrate intolerance, these results show that compared with eucaloric high-carbohydrate intake, LC/high-fat diets benefit MetS independent of whole-body or fat mass.TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT02918422.FUNDING Dairy Management Inc. and the Dutch Dairy Association.
Authors
Parker N. Hyde, Teryn N. Sapper, Christopher D. Crabtree, Richard A. LaFountain, Madison L. Bowling, Alex Buga, Brandon Fell, Fionn T. McSwiney, Ryan M. Dickerson, Vincent J. Miller, Debbie Scandling, Orlando P. Simonetti, Stephen D. Phinney, William J. Kraemer, Sarah A. King, Ronald M. Krauss, Jeff S. Volek
Abstract
Mutations in BSCL2 gene underlie human type 2 Berardinelli-Seip Congenital Lipodystrophy (BSCL2) disease. Global Bscl2−/− mice recapitulate human BSCL2 lipodystrophy and develop insulin resistance and hypertrophic cardiomyopathy. The pathological mechanisms underlying the development of lipodystrophy and cardiomyopathy in BSCL2 are controversial. Here we report that Bscl2−/− mice develop cardiac hypertrophy due to increased basal IGF1 receptor (IGF1R)-mediated PI3K/AKT signaling. Bscl2−/− hearts exhibited increased adipose triglyceride lipase (ATGL) protein stability and expression causing drastic reduction of glycerolipids. Excessive fatty acid oxidation was overt in Bscl2−/− hearts, partially attributing to the hyperacetylation of cardiac mitochondrial proteins. Intriguingly, pharmacological inhibition or genetic inactivation of ATGL could rescue adipocyte differentiation and lipodystrophy in Bscl2−/− cells and mice. Restoring a small portion of fat mass by ATGL partial deletion in Bscl2−/− mice not only reversed the systemic insulin resistance, but also ameliorated cardiac protein hyperacetylation, normalized cardiac substrate metabolism and improved contractile function. Collectively, our study uncovers novel pathways underlying lipodystrophy-induced cardiac hypertrophy and metabolic remodeling and pinpoints ATGL as a downstream target of BSCL2 in regulating the development of lipodystrophy and its associated cardiomyopathy.
Authors
Hongyi Zhou, Xinnuo Lei, Yun Yan, Todd Lydic, Jie Li, Neal L. Weintraub, Huabo Su, Weiqin Chen
Abstract
The evolutionary conserved Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex is one of the crucial multiprotein complexes that facilitates endosomal recycling of transmembrane proteins. Defects in WASH components have been associated with inherited developmental and neurological disorders in humans. Here, we show that hepatic ablation of the WASH component Washc1 in chow-fed mice increases plasma concentrations of cholesterol in both LDLs and HDLs, without affecting hepatic cholesterol content, hepatic cholesterol synthesis, biliary cholesterol excretion, or hepatic bile acid metabolism. Elevated plasma LDL cholesterol was related to reduced hepatocytic surface levels of the LDL receptor (LDLR) and the LDLR-related protein LRP1. Hepatic WASH ablation also reduced the surface levels of scavenger receptor class B type I and, concomitantly, selective uptake of HDL cholesterol into the liver. Furthermore, we found that WASHC1 deficiency increases LDLR proteolysis by the inducible degrader of LDLR, but does not affect proprotein convertase subtilisin/kexin type 9–mediated LDLR degradation. Remarkably, however, loss of hepatic WASHC1 may sensitize LDLR for proprotein convertase subtilisin/kexin type 9–induced degradation. Altogether, these findings identify the WASH complex as a regulator of LDL as well as HDL metabolism and provide in vivo evidence for endosomal trafficking of scavenger receptor class B type I in hepatocytes.
Authors
Melinde Wijers, Paolo Zanoni, Nalan Liv, Dyonne Y. Vos, Michelle Y. Jäckstein, Marieke Smit, Sanne Wilbrink, Justina C. Wolters, Ydwine T. van der Veen, Nicolette Huijkman, Daphne Dekker, Niels Kloosterhuis, Theo H. van Dijk, Daniel D. Billadeau, Folkert Kuipers, Judith Klumperman, Arnold von Eckardstein, Jan Albert Kuivenhoven, Bart van de Sluis
Abstract
BACKGROUND. Dietary changes have led to a growing prevalence of Type 2 diabetes and non-alcoholic fatty liver disease. A hallmark of both disorders is hepatic lipid accumulation, derived in part from increased de novo lipogenesis. Despite high protein diets being popular for weight loss to tackle these metabolic disorders, the effect of dietary protein on de novo lipogenesis is poorly studied. We aimed to characterise the effect of dietary protein on de novo lipid synthesis. METHODS. Herein, we use a 3-way crossover interventional study in healthy males to determine the effect of high protein feeding on de novo lipogenesis as well as in vitro models to determine the effects of specific amino acids on fatty acid synthesis. The primary outcome was change in de novo lipogenesis-associated triglycerides in response to protein feeding. RESULTS. We demonstrate that high protein feeding, rich in glutamate, increases de novo lipogenesis-associated triglycerides in plasma (2-fold compared to Control; p < 0.0001) and liver-derived very low-density lipoprotein particles (1.8 fold; p < 0.0001) in samples from human subjects (n = 9 per group). In hepatocytes, we show that glutamate derived carbon is incorporated into palmitate and subsequently into triglycerides. In addition, supplementation with glutamate, glutamine and leucine, but not lysine increases synthesised triglyceride content in cells and decreases glucose uptake. Glutamate, glutamine and leucine increase activation of protein kinase B, suggesting that these amino acids induce de novo lipogenesis via the insulin signalling cascade. CONCLUSION. These findings provide mechanistic insight into how select amino acids may induce de novo lipogenesis and insulin resistance, suggesting that high protein feeding to tackle diabetes and obesity requires greater consideration.
Authors
Evelina Charidemou, Tom Ashmore, Xuefei Li, Ben D. McNally, James A. West, Sonia Liggi, Matthew Harvey, Elise Orford, Julian L. Griffin
Abstract
The Mitochondrial Pyruvate Carrier (MPC) occupies a central metabolic node by transporting cytosolic pyruvate into the mitochondrial matrix and linking glycolysis with mitochondrial metabolism. Two reported human MPC1 mutations cause developmental abnormalities, neurological problems, metabolic deficits, and for one patient, early death. We aimed to understand biochemical mechanisms by which the human patient C289T and T236A MPC1 alleles disrupt MPC function. MPC1 C289T encodes two protein variants, a mis-spliced, truncation mutant (A58G) and a full length point mutant (R97W). MPC1 T236A encodes a full length point mutant (L79H). Using human patient fibroblasts and complementation of CRISPR-deleted, MPC1 null mouse C2C12 cells, we investigated how MPC1 mutations cause MPC deficiency. Truncated MPC1 A58G protein was intrinsically unstable and failed to form MPC complexes. The MPC1 R97W protein was less stable but when overexpressed formed complexes with MPC2 that retained pyruvate transport activity. Conversely, MPC1 L79H protein formed stable complexes with MPC2, but these complexes failed to transport pyruvate. These findings inform MPC structure-function relationships and delineate three distinct biochemical pathologies resulting from two human patient MPC1 mutations. They also illustrate an efficient gene pass-through system for mechanistically investigating human inborn errors in pyruvate metabolism.
Authors
Lalita Oonthonpan, Adam J. Rauckhorst, Lawrence R. Gray, Audrey C. Boutron, Eric B. Taylor
Abstract
Diabetic β cell failure is associated with β cell dedifferentiation. To identify effector genes of dedifferentiation, we integrated analyses of histone methylation as a surrogate of gene activation status and RNA expression in β cells sorted from mice with multiparity-induced diabetes. Interestingly, only a narrow subset of genes demonstrated concordant changes to histone methylation and RNA levels in dedifferentiating β cells. Notable among them was the α cell signature gene Gc, encoding a vitamin D-binding protein. While diabetes was associated with Gc induction, Gc-deficient islets did not induce β cell dedifferentiation markers and maintained normal ex vivo insulin secretion in the face of metabolic challenge. Moreover, Gc-deficient mice exhibited a more robust insulin secretory response than normal controls during hyperglycemic clamps. The data are consistent with a functional role of Gc activation in β cell dysfunction, and indicate that multiparity-induced diabetes is associated with altered β cell fate.
Authors
Taiyi Kuo, Manashree Damle, Bryan J. González, Dietrich Egli, Mitchell A. Lazar, Domenico Accili
Abstract
Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent, and potentially morbid, disease that affects one-third of the U.S. population. Normal liver safely accommodates lipid excess during fasting or carbohydrate restriction by increasing their oxidation to acetyl-CoA and ketones, yet lipid excess during NAFLD leads to hyperglycemia and, in some, steatohepatitis. To examine potential mechanisms, flux through pathways of hepatic oxidative metabolism and gluconeogenesis were studied using five simultaneous stable isotope tracers in ketotic (24-hour fast) individuals with a wide range of hepatic triglyceride contents (0-52%). Ketogenesis was progressively impaired as hepatic steatosis and glycemia worsened. Conversely, the alternative pathway for acetyl-CoA metabolism, oxidation in the tricarboxylic (TCA) cycle, was upregulated in NAFLD as ketone production diminished and positively correlated with rates of gluconeogenesis and plasma glucose concentrations. Increased respiration and energy generation that occurred in liver when β-oxidation and TCA cycle activity were coupled may explain these findings, inasmuch as oxygen consumption was higher during fatty liver and highly correlated with gluconeogenesis. These findings demonstrate that increased glucose production and hyperglycemia in NAFLD is not a consequence of acetyl-CoA production per se, but how acetyl-CoA is further metabolized in liver.
Authors
Justin A. Fletcher, Stanislaw Deja, Santhosh Satapati, Xiaorong Fu, Shawn C. Burgess, Jeffrey D. Browning
Abstract
Glucagon, a hormone released from pancreatic alpha-cells, plays a key role in maintaining proper glucose homeostasis and has been implicated in the pathophysiology of diabetes. In vitro studies suggest that intra-islet glucagon can modulate the function of pancreatic beta-cells. However, because of the lack of suitable experimental tools, the in vivo physiological role of this intra-islet cross-talk has remained elusive. To address this issue, we generated a novel mouse model that selectively expressed an inhibitory designer G protein-coupled receptor (Gi DREADD) in α-cells only. Drug-induced activation of this inhibitory designer receptor almost completely shut off glucagon secretion in vivo, resulting in significantly impaired insulin secretion, hyperglycemia, and glucose intolerance. Additional studies with mouse and human islets indicated that intra-islet glucagon stimulates insulin release primarily by activating β-cell GLP-1 receptors. These new findings strongly suggest that intra-islet glucagon signaling is essential for maintaining proper glucose homeostasis in vivo. Our work may pave the way toward the development of novel classes of antidiabetic drugs that act by modulating intra-islet cross-talk between α- and β-cells.
Authors
Lu Zhu, Diptadip Dattaroy, Jonathan Pham, Lingdi Wang, Luiz F. Barella, Yinghong Cui, Kenneth J. Wilkins, Bryan L. Roth, Ute Hochgeschwender, Franz M. Matschinsky, Klaus H. Kaestner, Nicolai M. Doliba, Jürgen Wess
Abstract
Myelomonocytic cells are critically involved in iron turnover as aged RBC recyclers. Human monocytes are divided in 3 subpopulations of classical, intermediate, and nonclassical cells, differing in inflammatory and migratory phenotype. Their functions in iron homeostasis are, however, unclear. Here, we asked whether the functional diversity of monocyte subsets translates into differences in handling physiological and pathological iron species. By microarray data analysis and flow cytometry we identified a set of iron-related genes and proteins upregulated in classical and, in part, intermediate monocytes. These included the iron exporter ferroportin (FPN1), ferritin, transferrin receptor, putative transporters of non-transferrin-bound iron (NTBI), and receptors for damaged erythrocytes. Consequently, classical monocytes displayed superior scavenging capabilities of potentially toxic NTBI, which were augmented by blocking iron export via hepcidin. The same subset and, to a lesser extent, the intermediate population, efficiently cleared damaged erythrocytes in vitro and mediated erythrophagocytosis in vivo in healthy volunteers and patients having received blood transfusions. To summarize, our data underline the physiologically important function of the classical and intermediate subset in clearing NTBI and damaged RBCs. As such, these cells may play a nonnegligible role in iron homeostasis and limit iron toxicity in iron overload conditions.
Authors
David Haschka, Verena Petzer, Florian Kocher, Christoph Tschurtschenthaler, Benedikt Schaefer, Markus Seifert, Sieghart Sopper, Thomas Sonnweber, Clemens Feistritzer, Tara L. Arvedson, Heinz Zoller, Reinhard Stauder, Igor Theurl, Guenter Weiss, Piotr Tymoszuk
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