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      The Emerging Role of Branched-Chain Amino Acids in Insulin Resistance and Metabolism

      review-article
      Nutrients
      MDPI
      branched-chain amino acids (BCAAs), insulin resistance, mammalian target of rapamycin complex 1 (mTORC1), metabolism

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          Abstract

          Insulin is required for maintenance of glucose homeostasis. Despite the importance of insulin sensitivity to metabolic health, the mechanisms that induce insulin resistance remain unclear. Branched-chain amino acids (BCAAs) belong to the essential amino acids, which are both direct and indirect nutrient signals. Even though BCAAs have been reported to improve metabolic health, an increased BCAA plasma level is associated with a high risk of metabolic disorder and future insulin resistance, or type 2 diabetes mellitus (T2DM). The activation of mammalian target of rapamycin complex 1 (mTORC1) by BCAAs has been suggested to cause insulin resistance. In addition, defective BCAA oxidative metabolism might occur in obesity, leading to a further accumulation of BCAAs and toxic intermediates. This review provides the current understanding of the mechanism of BCAA-induced mTORC1 activation, as well as the effect of mTOR activation on metabolic health in terms of insulin sensitivity. Furthermore, the effects of impaired BCAA metabolism will be discussed in detail.

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          Most cited references59

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          Sestrin2 is a leucine sensor for the mTORC1 pathway.

          Leucine is a proteogenic amino acid that also regulates many aspects of mammalian physiology, in large part by activating the mTOR complex 1 (mTORC1) protein kinase, a master growth controller. Amino acids signal to mTORC1 through the Rag guanosine triphosphatases (GTPases). Several factors regulate the Rags, including GATOR1, aGTPase-activating protein; GATOR2, a positive regulator of unknown function; and Sestrin2, a GATOR2-interacting protein that inhibits mTORC1 signaling. We find that leucine, but not arginine, disrupts the Sestrin2-GATOR2 interaction by binding to Sestrin2 with a dissociation constant of 20 micromolar, which is the leucine concentration that half-maximally activates mTORC1. The leucine-binding capacity of Sestrin2 is required for leucine to activate mTORC1 in cells. These results indicate that Sestrin2 is a leucine sensor for the mTORC1 pathway.
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            Growing roles for the mTOR pathway.

            The mammalian TOR (mTOR) pathway is a key regulator of cell growth and proliferation and increasing evidence suggests that its deregulation is associated with human diseases, including cancer and diabetes. The mTOR pathway integrates signals from nutrients, energy status and growth factors to regulate many processes, including autophagy, ribosome biogenesis and metabolism. Recent work identifying two structurally and functionally distinct mTOR-containing multiprotein complexes and TSC1/2, rheb, and AMPK as upstream regulators of mTOR is beginning to reveal how mTOR can sense diverse signals and produce a myriad of responses.
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              Regulation of mTORC1 by amino acids.

              The mechanistic target of rapamycin complex I (mTORC1) is a central regulator of cellular and organismal growth, and hyperactivation of this pathway is implicated in the pathogenesis of many human diseases including cancer and diabetes. mTORC1 promotes growth in response to the availability of nutrients, such as amino acids, which drive mTORC1 to the lysosomal surface, its site of activation. How amino acid levels are communicated to mTORC1 is only recently coming to light by the discovery of a lysosome-based signaling system composed of Rags (Ras-related GTPases) and Ragulator v-ATPase, GATOR (GAP activity towards Rags), and folliculin (FLCN) complexes. Increased understanding of this pathway will not only provide insight into growth control but also into the human pathologies triggered by its deregulation. Copyright © 2014 Elsevier Ltd. All rights reserved.
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                Author and article information

                Journal
                Nutrients
                Nutrients
                nutrients
                Nutrients
                MDPI
                2072-6643
                01 July 2016
                July 2016
                : 8
                : 7
                Affiliations
                Department of Molecular Medicine, School of Medicine, Gachon University, Incheon 406-840, Korea; msyoon@ 123456gachon.ac.kr ; Tel.: +82-32-899-6067; Fax: +82-32-899-6039
                Article
                nutrients-08-00405
                10.3390/nu8070405
                4963881
                27376324
                5f0e441e-6cfe-4321-ad59-43b9e10e84f6
                © 2016 by the author; licensee MDPI, Basel, Switzerland.

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license ( http://creativecommons.org/licenses/by/4.0/).

                Categories
                Review

                Nutrition & Dietetics
                branched-chain amino acids (bcaas),insulin resistance,mammalian target of rapamycin complex 1 (mtorc1),metabolism

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