Metabolic Regulation by Leucine of Translation Initiation Through the mTOR-Signaling Pathway by Pancreatic β-Cells

• Published in: Diabetes (New York, N.Y.) Vol. 50; no. 2; pp. 353 - 360
• Main Authors: Xu, G, Kwon, G, Cruz, W S, Marshall, C A, McDaniel, M L
• Format: Journal Article
• Language: English
• Published: Alexandria, VA American Diabetes Association 01.02.2001
• Subjects: Allosteric Regulation; Amino acid metabolism; Amino acids; Amino Acids, Cyclic - pharmacology; Biological and medical sciences; Cell Line; Decarboxylation; Dehydrogenases; Diabetes; Endocrine pancreas; Enzyme Activation; Fundamental and applied biological sciences. Psychology; Glutamate Dehydrogenase - metabolism; Glutamic Acid - physiology; Insulin; Islets of Langerhans - cytology; Islets of Langerhans - physiology; Isoleucine - pharmacology; Kinases; Leucine; Leucine - metabolism; Leucine - pharmacology; Leucine - physiology; Metabolism; Mitochondria; Mitochondria - physiology; Models, Biological; Morphology. Functional localizations; Oxidation-Reduction; Phosphorylation; Phosphotransferases (Alcohol Group Acceptor) - physiology; Physiological aspects; Physiology; Protein Biosynthesis - physiology; Protein Kinases; Protein synthesis; Proteins; Ribosomal Protein S6 Kinases - metabolism; Signal Transduction - physiology; TOR Serine-Threonine Kinases; Valine - pharmacology; Vertebrates: endocrinology; United States; Signal transduction; Mitochondria; Cell line; B-Cell; Aminoacid; Langerhans islet; Translation initiation; Leucine; Metabolism; In vitro; Endocrine pancreas
• ISSN: 0012-1797; 1939-327X
• DOI: 10.2337/diabetes.50.2.353

Metabolic Regulation by Leucine of Translation Initiation Through the mTOR-Signaling Pathway by Pancreatic β-Cells Guang Xu , Guim Kwon , Wilhelm S. Cruz , Connie A. Marshall and Michael L. McDaniel From the Department of Pathology and Immunology (G.X., G.K., W.C., C.A.M., M.L.M.), Washington University School of Medicine, St. Louis, Missouri. Address correspondence and reprint requests to Michael L. McDaniel, PhD, Department of Pathology and Immunology, Washington University School of Medicine, Box 8118, 660 South Euclid Ave., St. Louis, MO 63110. E-mail: mcdaniel{at}pathology.wustl.edu . Abstract Recent findings have demonstrated that the branched-chain amino acid leucine can activate the translational regulators, phosphorylated heat- and acid-stable protein regulated by insulin (PHAS-I) and p70 S6 kinase (p70 s6k ), in an insulin-independent and rapamycin-sensitive manner through mammalian target of rapamycin (mTOR), although the mechanism for this activation is undefined. It has been previously established that leucine-induced insulin secretion by β-cells involves increased mitochondrial metabolism by oxidative decarboxylation and allosteric activation of glutamate dehydrogenase (GDH). We now show that these same intramitochondrial events that generate signals for leucine-induced insulin exocytosis are required to activate the mTOR mitogenic signaling pathway by β-cells. Thus, a minimal model consisting of leucine and glutamine as substrates for oxidative decarboxylation and an activator of GDH, respectively, confirmed the requirement for these two metabolic components and mimicked closely the synergistic interactions achieved by a complete complement of amino acids to activate p70 s6k in a rapamycin-sensitive manner. Studies using various leucine analogs also confirmed the close association of mitochondrial metabolism and the ability of leucine analogs to activate p70 s6k . Furthermore, selective inhibitors of mitochondrial function blocked this activation in a reversible manner, which was not associated with a global reduction in ATP levels. These findings indicate that leucine at physiological concentrations stimulates p70 s6k phosphorylation via the mTOR pathway, in part, by serving both as a mitochondrial fuel and an allosteric activator of GDH. Leucine-mediated activation of protein translation through mTOR may contribute to enhanced β-cell function by stimulating growth-related protein synthesis and proliferation associated with the maintenance of β-cell mass. Footnotes Ac-Leu-NH 2 , N -acetyl leucine amide; Ac-Leu-NHMe, N -acetyl leucine N -methyl-amide; AOAA, aminooxyacetic acid; BCH, b(±) 2-amino-bicyclo [2,2,1] heptane-2-carboxylic acid; ECL, enhanced chemiluminescence; GDH, glutamate dehydrogenase; H-Leu-NH 2 HCl, leucine-amide hydrochloride; KIC, α-ketoisocaproic acid; KRBB, Krebs-Ringer bicarbonate buffer; MEM, minimal essential medium; mTOR, mammalian target of rapamycin; MTS, [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; p70 s6k , p70 S6 kinase; PHAS-I, phosphorylated heat- and acid-stable protein regulated by insulin; PI 3-K, phosphoinositide 3-kinase. Accepted October 11, 2000. Received July 27, 2000. by the American Diabetes Association, Inc.