University of Washington, Seattle, USA.
Diabetes. 2011 Jan;60(1):127-37. doi: 10.2337/db09-1806. Epub 2010 Oct 7.
Insulin receptor (IR) translocates to the nucleus, but its recruitment to gene loci has not been demonstrated. Here, we tested the hypothesis that IR and its downstream mitogenic transducers are corecruited to two prototypic insulin-inducible genes: early growth response 1 (egr-1), involved in mitogenic response, and glucokinase (Gck), encoding a key metabolic enzyme.
We used RNA and chromatin from insulin-treated rat hepatic tumor cell line expressing human insulin receptor (HTC-IR) and livers from lean and insulin-resistant ob/ob glucose-fed mice in quantitative RT-PCR and chromatin immunoprecipitation studies to determine gene expression levels and associated recruitment of RNA polymerase II (Pol II), insulin receptor, and cognate signaling proteins to gene loci, respectively.
Insulin-induced egr-1 mRNA in HTC-IR cells was associated with corecruitment of IR signaling cascade (IR, SOS, Grb2, B-Raf, MEK, and ERK) to this gene. Recruitment profiles of phosphorylated IR, B-Raf, MEK, and Erk along egr-1 transcribed region were similar to those of elongating Pol II. Glucose-feeding increased Gck mRNA expression in livers of lean but not ob/ob mice. In lean mice, there was glucose feeding-induced recruitment of IR and its transducers to Gck gene synchronized with elongating Pol II. In sharp contrast, in glucose-fed ob/ob mice, the Gck recruitment patterns of active MEK/Erk, IR, and Pol II were asynchronous.
IR and its signal transducers recruited to genes coupled to elongating Pol II may play a role in maintaining productive mRNA synthesis of target genes. These studies suggest a possibility that impaired Pol II processivity along genes bearing aberrant levels of IR/signal transducers is a previously unrecognized facet of insulin resistance.
胰岛素受体（IR）易位到细胞核，但尚未证明其募集到基因位点。在这里，我们检验了这样一个假设，即 IR 及其下游有丝分裂原信号转导器被共同募集到两个典型的胰岛素诱导基因：早期生长反应 1（egr-1），参与有丝分裂反应，和葡糖激酶（Gck），编码关键的代谢酶。
我们使用 RNA 和染色质来自表达人胰岛素受体（HTC-IR）的胰岛素处理的大鼠肝细胞瘤细胞系和来自瘦型和胰岛素抵抗的 ob/ob 葡萄糖喂养小鼠的肝脏，在定量 RT-PCR 和染色质免疫沉淀研究中，分别确定基因表达水平和相关的 RNA 聚合酶 II（Pol II）、胰岛素受体和同源信号蛋白募集到基因位点。
胰岛素诱导 HTC-IR 细胞中的 egr-1 mRNA 与 IR 信号级联（IR、SOS、Grb2、B-Raf、MEK 和 ERK）共同募集到该基因有关。磷酸化的 IR、B-Raf、MEK 和 Erk 在 egr-1 转录区的募集谱与延伸中的 Pol II 相似。在瘦型小鼠中，葡萄糖喂养增加了肝脏中 Gck mRNA 的表达，但在 ob/ob 小鼠中则没有。在瘦型小鼠中，胰岛素诱导的 IR 和其转导物募集到与延伸中的 Pol II 同步的 Gck 基因。与此形成鲜明对比的是，在葡萄糖喂养的 ob/ob 小鼠中，活跃的 MEK/Erk、IR 和 Pol II 的 Gck 募集模式是不同步的。
与延伸中的 Pol II 偶联的基因募集的 IR 和其信号转导器可能在维持靶基因的有效 mRNA 合成中发挥作用。这些研究表明，IR/信号转导器在基因上的水平异常导致 Pol II 沿基因的进程性降低是胰岛素抵抗的一个以前未被认识的方面。
