AMPK在胰岛素信号转导通路中的作用

The Role of AMPK in Regulation of the Insulin Signal Transduction Pathway

单磷酸腺苷活化蛋白激酶(AMP-activated potein kinase,AMPK)作为一种细胞能量调节器,当细胞经历代谢应激反应时,伴随着细胞内AMP水平或AMP与ATP的比例升高,AMPK被AMP激活,其活化的结果导致脂肪酸氧化的增加以产生更多ATP;同时,抑制ATP消耗,综合效应是帮助细胞度过急性损伤,暂时保障细胞的存活。因为一些治疗2型糖尿病的药物通过激活AMPK而发挥作用,故AMPK被认为是各种潜在的和有效的抗糖尿病药物的靶效应器。5-氨基-4-氨甲酰咪唑核苷(5-amino-4-imidazolecarboxamide riboside,AICAR),进入细胞后被磷酸化变成ZMP,后者类似AMP也能够激活AMPK。因此,我们采用AICAR激活AMPK,观察活化的AMPK对脂肪细胞能量代谢及胰岛素信号途径的作用。结果显示,脂肪细胞中的AMPK被激活后,丙酰辅酶A(malonyl-CoA,一种脂肪酸氧化作用的抑制剂及脂肪酸合成的前体中间产物)浓度下降80%;在已分化的3T3-F442a脂肪细胞中,AICAR通过激活AMPK,增强胰岛素对Akt/PKB的激活和GSK3的磷酸化。相反,在AICAR预处理的细胞中,胰岛素对mTOR的激活能力被降低;同时,mTOR下游效应器(如p70S6K、S6及4E-BP1)的磷酸化也降低。结果还显示,AMPK可在体外直接磷酸化mTOR,并抑制其自身的磷酸化活性。与此相应,2-双脱氧葡萄糖诱导的AMPK活化可导致TSC2缺乏的MEF细胞中mTOR的抑制。以上研究结果表明,AMPK在多个方面调节mTOR,并首次揭示AMPK直接磷酸化mTOR,导致其酶活性的改变。总之,AMPK全面参与了调节脂肪细胞的能量代谢:抑制脂肪酸及蛋白质的合成、刺激脂肪酸的氧化作用。

The AMP-activated protein kinase （AMPK） serves as a fuel gauge activated under metabolic stresses which is usually accompanied by increasing intracellular levels of AMP or the ration of AMP to ATE As a result, AMPK stimulates fatty acid oxidation to generate more ATP and inhibits ATP-consuming processes in coping with stresses and assuring acute cell surviving programs. Since several clinically used anti-diabetic drugs acti- vate AMPK, it is regarded as a promising and effective drug target for type 2 diabetes. Using 5-aminoimidazole-4- carboxamide l-D-ribonucleoside （AICAR）, a cell permeable agent that is phosphorylated inside cells and converted to ZMP, an analog of AMP and a canonical pharmacological AMPK activator, we explored the effect of AMPK on energy metabolism and the insulin signaling pathway in adipocytes. Our results showed that when AMPK was activated, the concentration of malonyl-CoA, an inhibitor of fatty acid oxidation and an intermediate in fatty acid synthesis, diminished by 80%. They also showed that AICAR activated AMPK in differentiated 3T3-F442a adipocytes and enhanced insulin stimulated Akt/PKB activation as well as GSK3]3 phosphorylation. In contrast, the ability of insulin to activate mTOR, a critical kinase for protein synthesis, was impaired by preincubation of cells with AICAR. This led to a decrease in the phosphorylation of downstream effectors of mTOR, including p70S6K, S6 and 4E-BP1. Furthermore, our results revealed that AMPK directly phosphorylated mTOR and inhibited its auto-kinase activity in vitro and that AMPK activation by 2-deoxide glucose caused an inhibition of mTOR in MEF cells deficient in TSC2. Thus, our results for the first time demonstrate that AMPK phosphorylates mTOR which leads to the inhibition of its kinase activity and indicate that AMPK regulates mTOR at multiple levels. Collectively, these studies suggest that AMPK exerts a global effect on energy metabolism in adipocytes in which it inhibits both fatty acid and protein synthesis and stimulates fatty acid oxidation.