Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
Proc Natl Acad Sci U S A. 2012 Oct 2;109(40):16184-9. doi: 10.1073/pnas.1213343109. Epub 2012 Aug 27.
Insulin activates sterol regulatory element-binding protein-1c (SREBP-1c) in liver, thereby increasing fatty acid and triglyceride synthesis. We created a line of transgenic rats that produce epitope-tagged human SREBP-1c in liver under control of the constitutive apolipoprotein E promoter/enhancer. This system allows us to dissect the pathway by which insulin stimulates SREBP-1c processing without interference by the insulin-mediated increase in SREBP-1c mRNA. Rats are used because freshly isolated rat hepatocytes respond much more robustly to insulin than do mouse hepatocytes. The data reveal that insulin-mediated stimulation of SREBP-1c processing requires the mechanistic target of rapamycin complex 1 (mTORC1), which also is required for insulin-mediated SREBP-1c mRNA induction. However, in contrast to mRNA induction, insulin stimulation of SREBP-1c processing is blocked by an inhibitor of p70 S6-kinase. The data indicate that the pathways for insulin enhancement of SREBP-1c mRNA and proteolytic processing diverge after mTORC1. Stimulation of processing requires the mTORC1 target p70 S6-kinase, whereas induction of mRNA bypasses this enzyme. Insulin stimulation of both processes is blocked by glucagon. The transgenic rat system will be useful in further defining the molecular mechanism for insulin stimulation of lipid synthesis in liver in normal and diabetic states.
胰岛素在肝脏中激活固醇调节元件结合蛋白-1c（SREBP-1c），从而增加脂肪酸和甘油三酯的合成。我们创建了一种转基因大鼠系，该大鼠在载脂蛋白 E 启动子/增强子的控制下在肝脏中产生表位标记的人 SREBP-1c。该系统使我们能够在不受胰岛素介导的 SREBP-1c mRNA 增加干扰的情况下，剖析胰岛素刺激 SREBP-1c 加工的途径。之所以选择大鼠，是因为新鲜分离的大鼠肝细胞对胰岛素的反应比小鼠肝细胞要强得多。数据表明，胰岛素介导的 SREBP-1c 加工刺激需要雷帕霉素复合物 1（mTORC1），mTORC1 也需要胰岛素介导的 SREBP-1c mRNA 诱导。然而，与 mRNA 诱导相反，p70 S6-激酶抑制剂可阻断胰岛素刺激的 SREBP-1c 加工。这些数据表明，胰岛素增强 SREBP-1c mRNA 和蛋白水解加工的途径在 mTORC1 之后出现分歧。加工的刺激需要 mTORC1 靶标 p70 S6-激酶，而 mRNA 的诱导则绕过该酶。两种过程都被胰高血糖素阻断。转基因大鼠系统将有助于进一步确定胰岛素在正常和糖尿病状态下刺激肝脏脂质合成的分子机制。
