1. mTORC1 Signaling: A Double-Edged Sword in Diabetic β Cells
Amin Ardestani, Blaz Lupse, Yoshiaki Kido, Gil Leibowitz, Kathrin Maedler 
Cell Metabolism 
Volume 27, Issue 2, Pages (February 2018)
DOI: /j.cmet
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2. Figure 1
A Simplified Presentation of mTOR Signaling under Physiological Conditions
The mTOR pathway integrates nutrient and growth factor signaling to regulate cellular metabolism, growth, and survival. In response to cellular stimulation by mitogens (such as insulin or IGF1), PI3K generates PIP3, which promotes the activation of mTORC2 and AKT. AKT phosphorylates and inhibits the upstream inhibitor of mTORC1, known as TSC1/2, leading to GTPase Rheb stimulation and mTORC1 activation. Also, nutrients such as amino acids and glucose function through the Rag family of GTPases to activate mTORC1 by localizing mTORC1 to the lysosome. Under high-energy conditions (high ATP:ADP ratio), AMPK is inhibited, and thus mTORC1 is activated through loss of both phosphorylation-dependent activation of TSC2 and direct inactivation of mTORC1. mTORC1 promotes anabolic growth through stimulation of proteins, nucleotides, and lipid biosynthesis as well as inhibition of catabolic processes such as autophagy (through inhibition of ULK1 and TFEB). mTORC1 promotes protein translation through phosphorylation of its downstream substrates S6K1 and 4E-BP1 (leading to activation of S6K and inhibition of 4EBP). mTORC2 is mainly activated by extracellular stimuli such as growth factors; together with AKT, it promotes cell survival and proliferation through phosphorylation of key downstream substrates (activation of AKT, SGK1, and PKC and inhibition of MST1). Phosphorylation of AKT by mTORC2 enhances its activity, and mTORC2 activation is at least partially mediated by PI3K-dependent PIP3 production. 4EBP, eIF4E-binding protein; AMPK, AMP-activated protein kinase; AKT/PKB, protein kinase B; IRS1/2, insulin receptor substrate 1 and 2; MST1, mammalian STE20-like kinase 1; mTORC1/2, mechanistic target of rapamycin complex 1 and 2; PDK1, phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3-kinase; PIP3, phosphotidylinositol (3,4,5)-trisphosphate; PKC, Protein kinase C; Rag, Ras-related GTPase; Rheb, Ras homolog enriched in brain; S6K1, ribosomal S6 kinase 1; SGK1, Serum/Glucocorticoid Regulated Kinase 1; TFEB, transcription factor EB; TSC1/2, tuberous sclerosis protein complex 1 and 2; ULK1, UNC-5 like autophagy activating kinase 1.
Cell Metabolism  , DOI: ( /j.cmet )
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3. Figure 2 Kinetic Model of mTORC1 Signaling in β cells
(A) Illustration of the hypothesized relationship between mTORC1 activation and metabolic output. Short term and transient mTORC1 activation (green line) correlates with nutrient or growth factor stimulation of β cell replication, anabolic growth, and insulin secretion under physiological conditions. Chronic hyperactivation of mTORC1 (red line) triggered by continued overnutrition correlates with defective insulin secretion, ER stress, and impaired β cell survival under diabetic conditions.
(B) The effect of mTORC1 hyperactivation on pancreatic β cell mass and metabolic parameters in β-TSC2-KO mice (with constitutive activation of mTORC1) over time. Mice show increased β cell mass, hyperinsulinemia, and improved glucose homeostasis in the early phase of their life, but they become hyperglycemic and severely hypoinsulinemic due to a dramatic loss of β cells at an older age. Representative pancreatic sections from 6- and 40-week-old β-TSC2-KO mice immunostained with insulin (red) and glucagon (green) are shown.
Scale bars, 50 μm. Reproduced from Shigeyama et al. (2008) with permission. ER, endoplasmic reticulum; β-TSC2-KO, β cell-specific TSC2 knockout; TSC1/2, tuberous sclerosis protein complex 1 and 2.
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4. Figure 3 mTORC1-S6K-Mediated Negative Feedback Loops
(A–D) mTORC1 controls several negative feedback loops that regulate RTK-IRS1/2-PI3K and mTORC2 signaling via (A) direct inhibition of IRS1/2, (B) Grb10-dependent inhibition of IRS1/2, (C) S6K1-mediated inhibition of mTORC2 through Rictor, and (D) S6K1-mediated inhibition of mTORC2 through Sin1.
(E) mTORC1-S6K1-mediated negative loops constitute intracellular regulatory mechanisms controlling cell growth and homeostasis through restraining excessive actions of insulin and mTORC2 signaling under physiological conditions.
(F) Chronic mTORC1-S6K1 hyperactivation triggered by obesity and nutrient overload (elevated glucose, FFAs and amino acids as well as hyperinsulinemia) initiates feedback mechanisms which—in the long run—attenuate insulin and mTORC2 signaling that then could result in diminished functional β cell mass and development of T2D.
AKT/PKB, protein kinase B; FFA, free fatty acids; Grb10, growth-factor-bound protein 10; IRS1/2, insulin receptor substrate 1 and 2; mTORC1/2, mechanistic target of rapamycin complex 1 and 2; PI3K, phosphoinositide 3-kinase; Rag, Ras-related GTPase; Raptor, regulatory-associated protein of mTOR; Rictor, rapamycin-insensitive companion of mTOR; RTK, receptor tyrosine kinase; S6K1, ribosomal S6 kinase 1; Sin1-MAPKAP1, mitogen-activated protein kinase-associated protein; T2D, type 2 diabetes.
Cell Metabolism  , DOI: ( /j.cmet )
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5. Figure 4 mTORC1-Dependent Regulation of Autophagy
In the healthy β cell, autophagy is tightly regulated by AMPK-mTORC1 signaling pathways in response to nutrient stimuli (e.g., glucose, amino acids), ensuring proper insulin secretion and glucose homeostasis. In T2D, dysregulation of AMPK-mTORC1 signaling likely contributes to the β cell failure. Prolonged overabundance of nutrients leads to chronic AMPK suppression as well as mTORC1 activation, consequently leading to progressive deterioration of the protective autophagy mechanism in β cells, which will disrupt insulin secretion and energy homeostasis. AMPK, AMP-activated protein kinase; mTORC1/2, mechanistic target of rapamycin complex 1 and 2; T2D, type 2 diabetes.
Cell Metabolism  , DOI: ( /j.cmet )
Copyright © 2017 Elsevier Inc. Terms and Conditions