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      Development of betulinic acid as an agonist of TGR5 receptor using a new in vitro assay

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          Abstract

          Background

          G-protein-coupled bile acid receptor 1, also known as TGR5 is known to be involved in glucose homeostasis. In animal models, treatment with a TGR5 agonist induces incretin secretion to reduce hyperglycemia. Betulinic acid, a triterpenoid present in the leaves of white birch, has been introduced as a selective TGR5 agonist. However, direct activation of TGR5 by betulinic acid has not yet been reported.

          Methods

          Transfection of TGR5 into cultured Chinese hamster ovary (CHO-K1) cells was performed to establish the presence of TGR5. Additionally, TGR5-specific small interfering RNA was employed to silence TGR5 in cells (NCI-H716 cells) that secreted incretins. Uptake of glucose by CHO-K1 cells was evaluated using a fluorescent indicator. Amounts of cyclic adenosine monophosphate and glucagon-like peptide were quantified using enzyme-linked immunosorbent assay kits.

          Results

          Betulinic acid dose-dependently increases glucose uptake by CHO-K1 cells transfected with TGR5 only, which can be considered an alternative method instead of radioligand binding assay. Additionally, signals coupled to TGR5 activation are also increased by betulinic acid in cells transfected with TGR5. In NCI-H716 cells, which endogenously express TGR5, betulinic acid induces glucagon-like peptide secretion via increasing calcium levels. However, the actions of betulinic acid were markedly reduced in NCI-H716 cells that received TGR5-silencing treatment. Therefore, the present study demonstrates the activation of TGR5 by betulinic acid for the first time.

          Conclusion

          Similar to the positive control lithocholic acid, which is the established agonist of TGR5, betulinic acid has been characterized as a useful agonist of TGR5 and can be used to activate TGR5 in the future.

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          Most cited references 20

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          The biology of incretin hormones.

          Gut peptides, exemplified by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted in a nutrient-dependent manner and stimulate glucose-dependent insulin secretion. Both GIP and GLP-1 also promote beta cell proliferation and inhibit apoptosis, leading to expansion of beta cell mass. GLP-1, but not GIP, controls glycemia via additional actions on glucose sensors, inhibition of gastric emptying, food intake and glucagon secretion. Furthermore, GLP-1, unlike GIP, potently stimulates insulin secretion and reduces blood glucose in human subjects with type 2 diabetes. This article summarizes current concepts of incretin action and highlights the potential therapeutic utility of GLP-1 receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors for the treatment of type 2 diabetes.
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            Pleiotropic roles of bile acids in metabolism.

            Enzymatic oxidation of cholesterol generates numerous distinct bile acids that function both as detergents that facilitate digestion and absorption of dietary lipids, and as hormones that activate four distinct receptors. Activation of these receptors alters gene expression in multiple tissues, leading to changes not only in bile acid metabolism but also in glucose homeostasis, lipid and lipoprotein metabolism, energy expenditure, intestinal motility and bacterial growth, inflammation, liver regeneration, and hepatocarcinogenesis. This review covers the roles of specific bile acids, synthetic agonists, and their cognate receptors in controlling these diverse functions, as well as their current use in treating human diseases. Copyright © 2013 Elsevier Inc. All rights reserved.
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              The bile acid membrane receptor TGR5 as an emerging target in metabolism and inflammation.

              Bile acids (BAs) are amphipathic molecules that facilitate the uptake of lipids, and their levels fluctuate in the intestine as well as in the blood circulation depending on food intake. Besides their role in dietary lipid absorption, bile acids function as signaling molecules capable to activate specific receptors. These BA receptors are not only important in the regulation of bile acid synthesis and their metabolism, but also regulate glucose homeostasis, lipid metabolism, and energy expenditure. These processes are important in diabetes and other facets of the metabolic syndrome, which represents a considerable increasing health burden. In addition to the function of the nuclear receptor FXRα in regulating local effects in the organs of the enterohepatic axis, increasing evidence points to a crucial role of the G-protein coupled receptor (GPCR) TGR5 in mediating systemic actions of BAs. Here we discuss the current knowledge on BA receptors, with a strong focus on the cell membrane receptor TGR5, which emerges as a valuable target for intervention in metabolic diseases. Copyright © 2011 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.
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                Author and article information

                Journal
                Drug Des Devel Ther
                Drug Des Devel Ther
                Drug Design, Development and Therapy
                Drug Design, Development and Therapy
                Dove Medical Press
                1177-8881
                2016
                22 August 2016
                : 10
                : 2669-2676
                Affiliations
                [1 ]Division of Cardiology, Department of Internal Medicine, Zhongxing Branch of Taipei City Hospital
                [2 ]Department of History and Geography, University of Taipei, Taipei, Taiwan
                [3 ]Department of Psychosomatic Internal Medicine, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan
                [4 ]Department of Medical Research
                [5 ]Department of Neurosurgery, Chi-Mei Medical Center, Yong Kang
                [6 ]Institute of Medical Science, College of Health Science, Chang Jung Christian University, Tainan
                [7 ]Department of Surgery, Pingtung Hospital
                [8 ]Division of Neurosurgery, Department of Surgery, Kaohsiung Medical University Chung-Ho Memorial Hospital, School of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
                Author notes
                Correspondence: Juei-Tang Cheng, Institute of Medical Sciences, Chang Jung, Christian University, 1 Changzon Road, Gueiren, Tainan, 71101, Taiwan, Tel +886 6 251 7864, Fax +886 6 283 2639, Email jtcheng@ 123456mail.cjcu.edu.tw
                Kung-Shing Lee, Department of Surgery, Kaohsiung, Medical University, 100 Shih-Chuan First Road, Kaohsiung, 80708, Taiwan, Tel +886 7 331 8839, Fax +886 7 736 5822, Email leekungshing@ 123456yahoo.com.tw
                Article
                dddt-10-2669
                10.2147/DDDT.S113197
                5001664
                © 2016 Lo et al. This work is published and licensed by Dove Medical Press Limited

                The full terms of this license are available at https://www.dovepress.com/terms.php and incorporate the Creative Commons Attribution – Non Commercial (unported, v3.0) License ( http://creativecommons.org/licenses/by-nc/3.0/). By accessing the work you hereby accept the Terms. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed.

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                Original Research

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