Iynedjian P B, Pilot P R, Nouspikel T, Milburn J L, Quaade C, Hughes S, Ucla C, Newgard C B
Institut de Biochimie Clinique, University of Geneva School of Medicine, Switzerland.
Proc Natl Acad Sci U S A. 1989 Oct;86(20):7838-42. doi: 10.1073/pnas.86.20.7838.
Glucokinase, a key regulatory enzyme of glucose metabolism in mammals, provides an interesting model of tissue-specific gene expression. The single-copy gene is expressed principally in liver, where it gives rise to a 2.4-kilobase mRNA. The islets of Langerhans of the pancreas also contain glucokinase. Using a cDNA complementary to rat liver glucokinase mRNA, we show that normal pancreatic islets and tumoral islet cells contain a glucokinase mRNA species approximately 400 nucleotides longer than hepatic mRNA. Hybridization with synthetic oligonucleotides and primer-extension analysis show that the liver and islet glucokinase mRNAs differ in the 5' region. Glucokinase mRNA is absent from the livers of fasted rats and is strongly induced within hours by an oral glucose load. In contrast, islet glucokinase mRNA is expressed at a constant level during the fasting-refeeding cycle. The level of glucokinase protein in islets measured by immunoblotting is unaffected by fasting and refeeding, whereas a 3-fold increase in the amount of enzyme occurs in liver during the transition from fasting to refeeding. From these data, we conclude (i) that alternative splicing and/or the use of distinct tissue-specific promoters generate structurally distinct mRNA species in liver and islets of Langerhans and (ii) that tissue-specific transcription mechanisms result in inducible expression of the glucokinase gene in liver but not in islets during the fasting-refeeding transition.
葡萄糖激酶是哺乳动物葡萄糖代谢的关键调节酶，它提供了一个有趣的组织特异性基因表达模型。这个单拷贝基因主要在肝脏中表达，产生一种2.4千碱基的信使核糖核酸。胰腺的胰岛也含有葡萄糖激酶。利用与大鼠肝脏葡萄糖激酶信使核糖核酸互补的互补脱氧核糖核酸，我们发现正常胰岛和肿瘤胰岛细胞中含有的葡萄糖激酶信使核糖核酸种类比肝脏信使核糖核酸长约400个核苷酸。与合成寡核苷酸杂交和引物延伸分析表明，肝脏和胰岛的葡萄糖激酶信使核糖核酸在5'区域存在差异。禁食大鼠的肝脏中没有葡萄糖激酶信使核糖核酸，口服葡萄糖负荷后数小时内会强烈诱导其产生。相比之下，在禁食-再喂食周期中，胰岛葡萄糖激酶信使核糖核酸以恒定水平表达。通过免疫印迹法测定，胰岛中葡萄糖激酶蛋白的水平不受禁食和再喂食的影响，而在从禁食到再喂食的转变过程中，肝脏中该酶的量增加了3倍。从这些数据中，我们得出结论：（i）选择性剪接和/或使用不同的组织特异性启动子在肝脏和胰岛中产生结构不同的信使核糖核酸种类；（ii）组织特异性转录机制导致在禁食-再喂食转变过程中，肝脏中葡萄糖激酶基因可诱导表达，而胰岛中则不然。
