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

Signaling Elements Involved in the Metabolic Regulation of mTOR by Nutrients, Incretins, and Growth Factors in Islets Guim Kwon 1 , Connie A. Marshall 1 , Kirk L. Pappan 1 , Maria S. Remedi 2 and Michael L. McDaniel 1 1 Departments of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri 2 Departments of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 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 Ca 2+ were examined. Glyburide, an inhibitor of ATP-sensitive K + channels (K ATP channels), provided partial activation of mTOR at basal glucose concentrations due to the influx of extracellular Ca 2+ , and diazoxide, an activator of K ATP channels, resulted in partial inhibition of S6K1 phosphorylation by 20 mmol/l glucose. Furthermore, Exenatide or forskolin reversed the inhibition by diazoxide, probably throu