Anthony Joshua C, Reiter Ali K, Anthony Tracy G, Crozier Stephen J, Lang Charles H, MacLean David A, Kimball Scot R, Jefferson Leonard S
Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
Diabetes. 2002 Apr;51(4):928-36. doi: 10.2337/diabetes.51.4.928.
In this study, food-deprived (18 h) control rats and rats with alloxan-induced diabetes were orally administered saline or the amino acid leucine to assess whether it regulates protein synthesis independently of a change in serum insulin concentrations. Immediately after leucine administration, diabetic rats were infused with insulin (0.0, 4.0, or 20 pmol small middle dot min(-1) small middle dot kg(-1)) for 1 h to examine the role of the hormone in the protein synthetic response to leucine. In control rats, leucine stimulated protein synthesis by 58% and increased phosphorylation of the translational repressor, eukaryotic initiation factor (eIF) 4E-binding protein (BP)-1, 4E-BP1, fivefold. Consequently, association of the mRNA cap-binding protein eukaryotic initiation factor (eIF)4E with 4E-BP1 was reduced to 50% of control values, and eIF4G*eIF4E complex assembly was increased 80%. Furthermore, leucine increased the phosphorylation of the 70-kDa ribosomal protein S6 (rp S6) and the ribosomal protein S6 kinase (S6K1). Diabetes attenuated protein synthesis compared with control rats. Nonetheless, in diabetic rats, leucine increased protein synthesis by 53% without concomitant changes in the phosphorylation of 4E-BP1 or S6K1. Skeletal muscle protein synthesis was stimulated in diabetic rats infused with insulin, but rates of synthesis remained less than values in nondiabetic controls that were administered leucine. Phosphorylation of 4E-BP1 and S6K1 was increased in diabetic rats infused with insulin in a dose-dependent manner, and the response was enhanced by leucine. The results suggest that leucine enhances protein synthesis in skeletal muscle through both insulin-dependent and -independent mechanisms. The insulin-dependent mechanism is associated with increased phosphorylation of 4E-BP1 and S6K1. In contrast, the insulin-independent effect on protein synthesis is mediated by an unknown mechanism.
在本研究中，对禁食（18小时）的对照大鼠和用四氧嘧啶诱导糖尿病的大鼠口服给予生理盐水或氨基酸亮氨酸，以评估其是否独立于血清胰岛素浓度变化来调节蛋白质合成。在给予亮氨酸后立即对糖尿病大鼠输注胰岛素（0.0、4.0或20 pmol·min⁻¹·kg⁻¹）1小时，以研究该激素在亮氨酸诱导的蛋白质合成反应中的作用。在对照大鼠中，亮氨酸使蛋白质合成增加58%，并使翻译抑制因子真核起始因子（eIF）4E结合蛋白（BP）-1（4E-BP1）的磷酸化增加了五倍。因此，mRNA帽结合蛋白真核起始因子（eIF）4E与4E-BP1的结合减少至对照值的50%，并且eIF4G·eIF4E复合物组装增加了80%。此外，亮氨酸增加了70 kDa核糖体蛋白S6（rp S6）和核糖体蛋白S6激酶（S6K1）的磷酸化。与对照大鼠相比，糖尿病使蛋白质合成减弱。尽管如此，在糖尿病大鼠中，亮氨酸使蛋白质合成增加53%，而4E-BP1或S6K1的磷酸化没有相应变化。输注胰岛素的糖尿病大鼠的骨骼肌蛋白质合成受到刺激，但合成速率仍低于给予亮氨酸的非糖尿病对照大鼠的值。输注胰岛素的糖尿病大鼠中4E-BP1和S6K1的磷酸化呈剂量依赖性增加，并且亮氨酸增强了该反应。结果表明，亮氨酸通过胰岛素依赖和非依赖机制增强骨骼肌中的蛋白质合成。胰岛素依赖机制与4E-BP1和S6K1磷酸化增加有关。相反，对蛋白质合成的胰岛素非依赖作用是由未知机制介导的。
