Cagen Lauren M, Deng Xiong, Wilcox Henry G, Park Edwards A, Raghow Rajendra, Elam Marshall B
Department of Pharmacology, The University of Tennessee Health Science Center, 874 Union Avenue, Memphis, TN 38163, USA.
Biochem J. 2005 Jan 1;385(Pt 1):207-16. doi: 10.1042/BJ20040162.
The enhanced synthesis of fatty acids in the liver and adipose tissue in response to insulin is critically dependent on the transcription factor SREBP-1c (sterol-regulatory-element-binding protein 1c). Insulin increases the expression of the SREBP-1c gene in intact liver and in hepatocytes cultured in vitro. To learn the mechanism of this stimulation, we analysed the activation of the rat SREBP-1c promoter and its truncated or mutated congeners driving a luciferase reporter gene in transiently transfected rat hepatocytes. The rat SREBP-1c promoter contains binding sites for LXR (liver X receptor), Sp1, NF-Y (nuclear factor-Y) and SREBP itself. We have found that each of these sites is required for the full stimulatory response of the SREBP-1c promoter to insulin. Mutation of either the putative LXREs (LXR response elements) or the SRE (sterol response element) in the proximal SREBP-1c promoter reduced the stimulatory effect of insulin by about 50%. Insulin and the LXR agonist TO901317 increased the association of SREBP-1 with the SREBP-1c promoter. Ectopic expression of LXRalpha or SREBP-1c increased activity of the SREBP-1c promoter, and this effect is further enhanced by insulin. The Sp1 and NF-Y sites adjacent to the SRE are also required for full activation of the SREBP-1c promoter by insulin. We propose that the combined actions of the SRE, LXREs, Sp1 and NF-Y elements constitute an insulin-responsive cis-acting unit of the SREBP-1c gene in the liver.
肝脏和脂肪组织中脂肪酸合成增强以响应胰岛素，这一过程严重依赖转录因子SREBP - 1c（固醇调节元件结合蛋白1c）。胰岛素可增加完整肝脏及体外培养肝细胞中SREBP - 1c基因的表达。为了解这种刺激的机制，我们分析了大鼠SREBP - 1c启动子及其截短或突变的同源物在瞬时转染大鼠肝细胞中驱动荧光素酶报告基因的激活情况。大鼠SREBP - 1c启动子含有肝X受体（LXR）、Sp1、核因子Y（NF - Y）及SREBP自身的结合位点。我们发现这些位点中的每一个对于SREBP - 1c启动子对胰岛素的完全刺激反应都是必需的。近端SREBP - 1c启动子中假定的LXR反应元件（LXREs）或固醇反应元件（SRE）发生突变，会使胰岛素的刺激作用降低约50%。胰岛素和LXR激动剂TO901317增加了SREBP - 1与SREBP - 1c启动子的结合。异位表达LXRα或SREBP - 1c可增加SREBP - 1c启动子的活性，且胰岛素可进一步增强这种作用。SRE附近的Sp1和NF - Y位点对于胰岛素完全激活SREBP - 1c启动子也是必需的。我们提出，SRE、LXREs、Sp1和NF - Y元件的共同作用构成了肝脏中SREBP - 1c基因的胰岛素反应性顺式作用单元。
