Institute of Pharmacy, Department of Pharmacology, Eberhard Karls University of Tübingen, Tübingen, Germany.
Institute of Pharmaceutical and Medicinal Chemistry, Department of Pharmacology, University of Münster, Münster, Germany.
Diabetes. 2019 Feb;68(2):324-336. doi: 10.2337/db18-0315. Epub 2018 Nov 8.
The Takeda-G-protein-receptor-5 (TGR5) mediates physiological actions of bile acids. Since it was shown that TGR5 is expressed in pancreatic tissue, a direct TGR5 activation in β-cells is currently postulated and discussed. The current study reveals that oleanolic acid (OLA) affects murine β-cell function by TGR5 activation. Both a G inhibitor and an inhibitor of adenylyl cyclase (AC) prevented stimulating effects of OLA. Accordingly, OLA augmented the intracellular cAMP concentration. OLA and two well-established TGR5 agonists, RG239 and tauroursodeoxycholic acid (TUDCA), acutely promoted stimulus-secretion coupling (SSC). OLA reduced K current and elevated current through Ca channels. Accordingly, in mouse and human β-cells, TGR5 ligands increased the cytosolic Ca concentration by stimulating Ca influx. Higher OLA concentrations evoked a dual reaction, probably due to activation of a counterregulating pathway. Protein kinase A (PKA) was identified as a downstream target of TGR5 activation. In contrast, inhibition of phospholipase C and phosphoinositide 3-kinase did not prevent stimulating effects of OLA. Involvement of exchange protein directly activated by cAMP 2 (Epac2) or farnesoid X receptor (FXR2) was ruled out by experiments with knockout mice. The proposed pathway was not influenced by local glucagon-like peptide 1 (GLP-1) secretion from α-cells, shown by experiments with MIN6 cells, and a GLP-1 receptor antagonist. In summary, these data clearly demonstrate that activation of TGR5 in β-cells stimulates insulin secretion via an AC/cAMP/PKA-dependent pathway, which is supposed to interfere with SSC by affecting K and Ca currents and thus membrane potential.
甘氨熊脱氧胆酸（TGR5）介导胆酸的生理作用。由于已经表明 TGR5 在胰腺组织中表达，因此目前推测并讨论了 TGR5 在β细胞中的直接激活作用。本研究表明，齐墩果酸（OLA）通过 TGR5 激活来影响小鼠β细胞功能。G 蛋白抑制剂和腺苷酸环化酶（AC）抑制剂均可阻止 OLA 的刺激作用。因此，OLA 增加了细胞内 cAMP 浓度。OLA 和两种成熟的 TGR5 激动剂 RG239 和牛磺熊脱氧胆酸（TUDCA）均可急性促进刺激-分泌偶联（SSC）。OLA 减少了 K 电流并增加了钙通道电流。因此，在小鼠和人β细胞中，TGR5 配体通过刺激 Ca 内流增加了细胞质 Ca 浓度。较高的 OLA 浓度引起双重反应，可能是由于激活了反向调节途径。蛋白激酶 A（PKA）被鉴定为 TGR5 激活的下游靶标。相反，抑制磷脂酶 C 和磷酸肌醇 3-激酶并不能阻止 OLA 的刺激作用。通过敲除小鼠的实验排除了 cAMP 直接激活交换蛋白 2（Epac2）或法尼醇 X 受体（FXR2）的参与。提出的途径不受来自α细胞的局部胰高血糖素样肽 1（GLP-1）分泌的影响，MIN6 细胞和 GLP-1 受体拮抗剂的实验表明了这一点。总之，这些数据清楚地表明，β细胞中 TGR5 的激活通过 AC/cAMP/PKA 依赖性途径刺激胰岛素分泌，该途径通过影响 K 和 Ca 电流以及膜电位来干扰 SSC。
