Insulin inhibition of transcription stimulated by the forkhead protein Foxo1 is not solely due to nuclear exclusion
Insulin inhibition of transcription stimulated by the forkhead protein Foxo1 is not solely due to nuclear exclusion
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DOI：
10.1210/en.2003-0481
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发表时间：
2003-12-01
期刊：
影响因子：
4.8
通讯作者：
Rechler, MM
中科院分区：
文献类型：
--
作者：
Tsai, WC;Bhattacharyya, N;Rechler, MM
FOXO家族叉头转录因子刺激参与许多基本细胞过程的靶基因的转录，包括细胞存活、细胞周期进展、DNA修复和胰岛素敏感性。FOXO蛋白的活性主要通过胰岛素和其它细胞因子激活蛋白激酶B（PKB）/Akt来调节。PKB/Akt磷酸化FOXO蛋白中的三个共有位点，导致它们从细胞核输出并抑制FOXO刺激的转录。人们普遍认为，转录的降低是由于细胞核中FOXO蛋白丰度的降低。在本研究中，我们将Foxo 1（小鼠FOXO 1）核输出信号中的Leu（375）突变为丙氨酸，使其在胰岛素治疗后仍留在H4 IIE大鼠肝癌细胞的细胞核中，并确定胰岛素是否仍能抑制Foxo 1突变体刺激的转录。尽管保留在细胞核中的Foxo 1突变体，胰岛素抑制L375 A-Foxo 1刺激的转录到相同程度的转录刺激野生型Foxo 1。使用含有大鼠IGF-结合蛋白-1启动子或最小启动子的报告质粒获得了类似的结果，所述最小启动子具有与FOXO蛋白结合的胰岛素应答元件的三个拷贝。我们的结论是，胰岛素可以抑制Foxo 1刺激的转录，即使当Foxo 1的核出口被阻止，表明胰岛素抑制可以发生不依赖于改变转录因子的亚细胞分布的直接机制。
The FOXO family of forkhead transcription factors stimulates the transcription of target genes involved in many fundamental cell processes, including cell survival, cell cycle progression, DNA repair, and insulin sensitivity. The activity of FOXO proteins is principally regulated by activation of protein kinase B (PKB)/Akt by insulin and other cytokines. PKB/Akt phosphorylates three consensus sites in FOXO proteins, leading to their export from the nucleus and the inhibition of FOXO-stimulated transcription. It has been widely accepted that the decreased transcription results from reduced abundance of FOXO proteins in the nucleus. In the present study we mutated Leu(375) to alanine in the nuclear export signal of Foxo1 (mouse FOXO1), so that it would remain in the nucleus of H4IIE rat hepatoma cells after insulin treatment, and determined whether insulin could still inhibit transcription stimulated by the Foxo1 mutant. Despite the retention of the Foxo1 mutant in the nucleus, insulin inhibited L375A-Foxo1-stimulated transcription to the same extent as transcription stimulated by wild-type Foxo1. Similar results were obtained using reporter plasmids containing the rat IGF-binding protein-1 promoter or a minimal promoter with three copies of the insulin response element to which FOXO proteins bind. We conclude that insulin can inhibit Foxo1-stimulated transcription even when nuclear export of Foxo1 is prevented, indicating that insulin inhibition can occur by direct mechanisms that do not depend on altering the subcellular distribution of the transcription factor.