Section of Endocrinology and Metabolism, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8020, USA.
J Biol Chem. 2013 Jul 12;288(28):20135-50. doi: 10.1074/jbc.M113.458075. Epub 2013 Jun 6.
Insulin stimulates glucose uptake in 3T3-L1 adipocytes in part by causing endoproteolytic cleavage of TUG (tether containing a ubiquitin regulatory X (UBX) domain for glucose transporter 4 (GLUT4)). Cleavage liberates intracellularly sequestered GLUT4 glucose transporters for translocation to the cell surface. To test the role of this regulation in muscle, we used mice with muscle-specific transgenic expression of a truncated TUG fragment, UBX-Cter. This fragment causes GLUT4 translocation in unstimulated 3T3-L1 adipocytes. We predicted that transgenic mice would have GLUT4 translocation in muscle during fasting. UBX-Cter expression caused depletion of PIST (PDZ domain protein interacting specifically with TC10), which transmits an insulin signal to TUG. Whereas insulin stimulated TUG proteolysis in control muscles, proteolysis was constitutive in transgenic muscles. Fasting transgenic mice had decreased plasma glucose and insulin concentrations compared with controls. Whole-body glucose turnover was increased during fasting but not during hyperinsulinemic clamp studies. In muscles with the greatest UBX-Cter expression, 2-deoxyglucose uptake during fasting was similar to that in control muscles during hyperinsulinemic clamp studies. Fasting transgenic mice had increased muscle glycogen, and GLUT4 targeting to T-tubule fractions was increased 5.7-fold. Whole-body oxygen consumption (VO2), carbon dioxide production (VCO2), and energy expenditure were increased by 12-13%. After 3 weeks on a high fat diet, the decreased fasting plasma glucose in transgenic mice compared with controls was more marked, and increased glucose turnover was not observed; the transgenic mice continued to have an increased metabolic rate. We conclude that insulin stimulates TUG proteolysis to translocate GLUT4 in muscle, that this pathway impacts systemic glucose homeostasis and energy metabolism, and that the effects of activating this pathway are maintained during high fat diet-induced insulin resistance in mice.
胰岛素刺激 3T3-L1 脂肪细胞摄取葡萄糖的部分原因是通过导致 TUG（含有葡萄糖转运蛋白 4（GLUT4）泛素调节 X（UBX）结构域的连接蛋白）的内蛋白水解裂解。裂解使细胞内隔离的 GLUT4 葡萄糖转运蛋白释放出来，以便转运到细胞表面。为了测试这种调节在肌肉中的作用，我们使用了肌肉特异性过表达截断的 TUG 片段 UBX-Cter 的转基因小鼠。该片段在未刺激的 3T3-L1 脂肪细胞中引起 GLUT4 易位。我们预测，在禁食期间，转基因小鼠的肌肉中会出现 GLUT4 易位。UBX-Cter 表达导致 PDZ 结构域蛋白相互作用特定位点与 TC10（PIST）的消耗，该蛋白将胰岛素信号传递给 TUG。虽然胰岛素刺激对照肌肉中的 TUG 蛋白水解，但在转基因肌肉中，蛋白水解是组成型的。与对照相比，禁食转基因小鼠的血浆葡萄糖和胰岛素浓度降低。与对照相比，禁食期间整个身体的葡萄糖周转率增加，但在高胰岛素钳夹研究中没有增加。在具有最大 UBX-Cter 表达的肌肉中，禁食期间的 2-脱氧葡萄糖摄取与对照肌肉在高胰岛素钳夹研究期间的摄取相似。禁食转基因小鼠的肌肉糖原增加，GLUT4 靶向 T 小管分数增加了 5.7 倍。整个身体的耗氧量（VO2）、二氧化碳产生（VCO2）和能量消耗增加了 12-13%。高脂饮食 3 周后，与对照组相比，转基因小鼠的空腹血浆葡萄糖降低更为明显，并且没有观察到葡萄糖周转率增加；转基因小鼠继续保持较高的代谢率。我们得出的结论是，胰岛素刺激 TUG 蛋白水解以在肌肉中易位 GLUT4，该途径影响全身葡萄糖稳态和能量代谢，并且在高脂肪饮食诱导的胰岛素抵抗期间，激活该途径的效果在小鼠中得以维持。
