Department of Molecular and Cellular Biology, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan.
Laboratory of Epigenetics and Metabolism, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan.
Endocr J. 2022 May 30;69(5):487-493. doi: 10.1507/endocrj.EJ21-0661. Epub 2021 Nov 19.
T1R3 is a class C G protein-coupled receptor family member that forms heterodimeric umami and sweet taste receptors with T1R1 and T1R2, respectively, in the taste cells of taste buds. T1R3 is expressed in 3T3-L1 cells in homomeric form and negatively regulates adipogenesis in a Gαs-dependent but cAMP-independent manner. Although T1R3 expression is markedly upregulated during adipogenesis, its physiological role in mature adipocytes remains obscure. Here, we show that stimulation of T1R3 with sucralose or saccharin induces microtubule disassembly in differentiated 3T3-L1 adipocytes. The effect was reproduced by treatment with cholera toxin or isoproterenol but not with forskolin. Treatment with sucralose or saccharin for 3 h inhibited insulin-stimulated glucose uptake by 32% and 45% in differentiated adipocytes, respectively, similar to the inhibitory effect of nocodazole (by 33%). Isoproterenol treatment inhibited insulin-stimulated glucose transport by 26%, whereas sucralose did not affect the intrinsic activity of the glucose transporter, indicating that it inhibited insulin-induced GLUT4 translocation to the plasma membrane. Immunostaining analysis showed that insulin-stimulated GLUT4 accumulation on the plasma membrane was abrogated in sucralose-treated cells, in association with depolymerization of microtubules. Sucralose-mediated inhibition of GLUT4 translocation was reversed by the overexpression of dominant-negative Gαs (Gαs-G226A) or knockdown of Gαs. Additionally, membrane fractionation analysis showed that sucralose treatment reduced GLUT4 levels in the plasma membrane fraction from insulin-stimulated adipocytes. We have identified a novel non-gustatory role for homomeric T1R3 in adipocytes, and activation of the T1R3 receptor negatively regulates insulin action of glucose transport via Gαs-dependent microtubule disassembly.
T1R3 是 C 类 G 蛋白偶联受体家族成员，它分别与 T1R1 和 T1R2 形成鲜味和甜味味觉感受器，存在于味蕾的味觉细胞中。T1R3 以同源二聚体的形式在 3T3-L1 细胞中表达，并以 Gαs 依赖但 cAMP 独立的方式负调控脂肪生成。尽管 T1R3 在脂肪生成过程中表达明显上调，但它在成熟脂肪细胞中的生理作用仍不清楚。在这里，我们显示蔗糖素或糖精刺激 T1R3 可诱导分化的 3T3-L1 脂肪细胞中的微管解聚。该作用可通过霍乱毒素或异丙肾上腺素处理再现，但不能通过福司可林处理再现。用蔗糖素或糖精处理 3 小时可分别抑制分化的脂肪细胞中胰岛素刺激的葡萄糖摄取 32%和 45%，类似于诺考达唑（抑制 33%）的抑制作用。异丙肾上腺素处理抑制胰岛素刺激的葡萄糖转运 26%，而蔗糖素不影响葡萄糖转运体的固有活性，表明它抑制胰岛素诱导的 GLUT4 向质膜转位。免疫染色分析显示，在蔗糖素处理的细胞中，胰岛素刺激的 GLUT4 在质膜上的积累被消除，同时微管解聚。蔗糖素介导的 GLUT4 易位抑制作用可通过过表达显性失活的 Gαs（Gαs-G226A）或敲低 Gαs 逆转。此外，膜分馏分析显示，蔗糖素处理降低了胰岛素刺激的脂肪细胞中质膜部分的 GLUT4 水平。我们已经确定了同源 T1R3 在脂肪细胞中的一个新的非味觉作用，并且 T1R3 受体的激活通过 Gαs 依赖的微管解聚负调控胰岛素刺激的葡萄糖转运。
