Signaling Elements Involved in the Metabolic Regulation of mTOR by Nutrients, Incretins, and Growth Factors in Islets

  1. Guim Kwon1,
  2. Connie A. Marshall1,
  3. Kirk L. Pappan1,
  4. Maria S. Remedi2 and
  5. Michael L. McDaniel1
  1. 1Departments of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri
  2. 2Departments of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri
  1. Address correspondence and reprint requests to Michael L. McDaniel, Washington University School of Medicine, Campus Box 8118, 660 South Euclid Ave., St. Louis, MO 63110. E-mail: mcdaniel{at}pathology.wustl.edu

Abstract

Mammalian target of rapamycin (mTOR) is a protein kinase that integrates signals from mitogens and the nutrients, glucose and amino acids, to regulate cellular growth and proliferation. Previous findings demonstrated that glucose robustly activates mTOR in an amino acid-dependent manner in rodent and human islets. Furthermore, activation of mTOR by glucose significantly increases rodent islet DNA synthesis that is abolished by rapamycin. Glucagon-like peptide-1 (GLP-1) agonists, through the production of cAMP, have been shown to enhance glucose-dependent proinsulin biosynthesis and secretion and to stimulate cellular growth and proliferation. The objective of this study was to determine if the glucose-dependent and cAMP-mediated mechanism by which GLP-1 agonists enhance β-cell growth and proliferation is mediated, in part, through mTOR. Our studies demonstrated that forskolin-generated cAMP resulted in activation of mTOR at basal glucose concentrations as assessed by phosphorylation of S6K1, a downstream effector of mTOR. Conversely, an adenylyl cyclase inhibitor partially blocked glucose-induced S6K1 phosphorylation. Furthermore, the GLP-1 receptor agonist, Exenatide, dose-dependently enhanced phosphorylation of S6K1 at an intermediate glucose concentration (8 mmol/l) in a rapamycin-sensitive manner. To determine the mechanism responsible for this potentiation of mTOR, the effects of intra- and extracellular Ca2+ were examined. Glyburide, an inhibitor of ATP-sensitive K+ channels (KATP channels), provided partial activation of mTOR at basal glucose concentrations due to the influx of extracellular Ca2+, and diazoxide, an activator of KATP channels, resulted in partial inhibition of S6K1 phosphorylation by 20 mmol/l glucose. Furthermore, Exenatide or forskolin reversed the inhibition by diazoxide, probably through mobilization of intracellular Ca2+ stores by cAMP. BAPTA, a chelator of intracellular Ca2+, resulted in inhibition of glucose-stimulated S6K1 phosphorylation due to a reduction in cytosolic Ca2+ concentrations. Selective blockade of glucose-stimulated Ca2+ influx unmasked a protein kinase A (PKA)-sensitive component involved in the mobilization of intracellular Ca2+ stores, as revealed with the PKA inhibitor H-89. Overall, these studies support our hypothesis that incretin-derived cAMP participates in the metabolic activation of mTOR by mobilizing intracellular Ca2+ stores that upregulate mitochondrial dehydrogenases and result in enhanced ATP production. ATP can then modulate KATP channels, serve as a substrate for adenylyl cyclase, and possibly directly regulate mTOR activation.

Footnotes

  • This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.

    • Accepted April 27, 2004.
    • Received February 28, 2004.
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