Ross S A, Keller S R, Lienhard G E
Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755, USA.
Biochem J. 1998 Mar 1;330 ( Pt 2)(Pt 2):1003-8. doi: 10.1042/bj3301003.
In fat and muscle cells, the glucose transporter GLUT4 is sequestered in an intracellular compartment under basal conditions and redistributes markedly to the plasma membrane in response to insulin. Recently, we characterized a membrane aminopeptidase, designated IRAP (insulin-regulated aminopeptidase), that colocalizes with intracellular GLUT4 and similarly redistributes markedly to the plasma membrane in response to insulin in adipocytes. In contrast to GLUT4, IRAP is also expressed in 3T3-L1 fibroblasts, and this finding provided an opportunity to compare its subcellular distribution in fibroblasts and adipocytes. The relative amount of IRAP at the cell surface was measured by a cell surface biotinylation method. The portion of total IRAP at the cell surface in unstimulated adipocytes was 30% of that in unstimulated fibroblasts. Upon insulin treatment the portion of IRAP at the cell surface was the same in fibroblasts and adipocytes, and was increased 1.8-fold in fibroblasts and 8-fold in adipocytes. A similar analysis of the distribution of the transferrin receptor (TfR), the paradigm for recycling plasma membrane receptors, revealed that the portions of the TfR at the cell surface in both the basal and insulin-treated states were almost unchanged upon differentiation, and that insulin caused an increase of about 1. 6-fold in the amount of TfR at the cell surface. These results show that enhanced intracellular sequestration of IRAP occurs during adipogenesis, and that this effect underlies the larger insulin-elicited fold increase of IRAP at the cell surface in adipocytes.
在脂肪细胞和肌肉细胞中，葡萄糖转运蛋白GLUT4在基础条件下被隔离在细胞内区室中，并在胰岛素作用下显著重新分布到质膜上。最近，我们鉴定了一种膜氨基肽酶，命名为IRAP（胰岛素调节氨基肽酶），它与细胞内的GLUT4共定位，并且在脂肪细胞中同样会在胰岛素作用下显著重新分布到质膜上。与GLUT4不同，IRAP在3T3-L1成纤维细胞中也有表达，这一发现为比较其在成纤维细胞和脂肪细胞中的亚细胞分布提供了机会。通过细胞表面生物素化方法测量细胞表面IRAP的相对含量。未刺激的脂肪细胞中细胞表面总IRAP的比例是未刺激的成纤维细胞中的30%。胰岛素处理后，成纤维细胞和脂肪细胞中细胞表面IRAP的比例相同，成纤维细胞中增加了1.8倍，脂肪细胞中增加了8倍。对循环质膜受体的典型代表转铁蛋白受体（TfR）分布的类似分析表明，在基础状态和胰岛素处理状态下，细胞表面TfR的比例在分化后几乎没有变化，并且胰岛素使细胞表面TfR的量增加了约1.6倍。这些结果表明，在脂肪生成过程中，IRAP的细胞内隔离增强，并且这种效应是脂肪细胞中胰岛素诱导的细胞表面IRAP更大倍数增加的基础。
