Pearce D, Verrey F, Chen S Y, Mastroberardino L, Meijer O C, Wang J, Bhargava A
Department of Medicine, and Department of Cellular and Molecular Pharmacology, University of California, San Francisco 94143, USA.
Kidney Int. 2000 Apr;57(4):1283-9. doi: 10.1046/j.1523-1755.2000.00963.x.
Mineralocorticoids stimulate electrogenic Na+ transport in tight epithelia by altering the transcription of specific genes. Although the earliest mineralocorticoid effect is to increase the activity of the epithelial sodium channel (ENaC), ENaC mRNA and protein levels do not change. Instead, physiologic observations suggest that a mineralocorticoid target gene(s) encodes an ENaC regulator(s). To begin to identify and characterize mineralocorticoid-regulated target genes, we used suppression-subtractive hybridization to generate a cDNA library from A6 cells, a stable cell line of Xenopus laevis of distal nephron origin. A serine-threonine kinase, SGK, was identified from this screen. Sequence comparison revealed that frog, rat, and human SGK are 92% identical and 96% similar at the amino acid level. SGK mRNA was confirmed by Northern blot to be strongly and rapidly corticosteroid stimulated in A6 cells. In situ hybridization revealed that SGK was strongly stimulated by aldosterone in rat collecting duct but not proximal tubule cells. Low levels of SGK were present in rat glomeruli, but SGK was unregulated in this structure. Finally, SGK stimulated ENaC activity approximately sevenfold when coexpressed in Xenopus laevis oocytes. These data suggest that SGK is an important mediator of aldosterone effects on Na+ transport in tight epithelia. In view of the existence of SGK homologues in invertebrates, it is interesting to speculate that SGK is an ancient kinase that was adapted to the control of epithelial Na+ transport by early vertebrates as they made the transition from a marine to a freshwater environment.
盐皮质激素通过改变特定基因的转录来刺激紧密上皮细胞中的电中性钠转运。尽管最早的盐皮质激素效应是增加上皮钠通道（ENaC）的活性，但ENaC的mRNA和蛋白质水平并未改变。相反，生理学观察表明，一个盐皮质激素靶基因编码一种ENaC调节因子。为了开始鉴定和表征盐皮质激素调节的靶基因，我们使用抑制性消减杂交技术从源自非洲爪蟾远端肾单位的稳定细胞系A6细胞中构建了一个cDNA文库。从该筛选中鉴定出一种丝氨酸 - 苏氨酸激酶，即SGK。序列比较显示，蛙、大鼠和人类的SGK在氨基酸水平上有92%的同一性和96%的相似性。通过Northern印迹法证实，SGK mRNA在A6细胞中受到皮质类固醇的强烈且快速刺激。原位杂交显示，醛固酮在大鼠集合管中强烈刺激SGK，但在近端小管细胞中则不然。大鼠肾小球中存在低水平的SGK，但该结构中的SGK不受调节。最后，当在非洲爪蟾卵母细胞中共表达时，SGK刺激ENaC活性约7倍。这些数据表明，SGK是醛固酮对紧密上皮细胞中钠转运作用的重要介导因子。鉴于无脊椎动物中存在SGK同源物，有趣的是推测SGK是一种古老的激酶，早期脊椎动物在从海洋环境向淡水环境转变时，它被用于控制上皮钠转运。
