Roos S, Kanai Y, Prasad P D, Powell T L, Jansson T
Perinatal Center, Department of Physiology, Institute of Neuroscience and Physiology, University of Gothenburg, PO Box 432, Gothenburg SE-405 30, Sweden.
Am J Physiol Cell Physiol. 2009 Jan;296(1):C142-50. doi: 10.1152/ajpcell.00330.2008. Epub 2008 Nov 5.
The activity of placental amino acid transporters is decreased in intrauterine growth restriction (IUGR), but the underlying regulatory mechanisms have not been established. Inhibition of the mammalian target of rapamycin (mTOR) signaling pathway has been shown to decrease the activity of the system L amino acid transporter in human placental villous fragments, and placental mTOR activity is decreased in IUGR. In the present study, we used cultured primary trophoblast cells to study mTOR regulation of placental amino acid transporters in more detail and to test the hypothesis that mTOR alters amino acid transport activity by changes in transporter expression. Inhibition of mTOR by rapamycin significantly reduced the activity of system A (-17%), system L (-28%), and taurine (-40%) amino acid transporters. mRNA expression of isoforms of the three amino acid transporter systems in response to mTOR inhibition was measured using quantitative real-time PCR. mRNA expression of l-type amino acid transporter 1 (LAT1; a system L isoform) and taurine transporter was reduced by 13% and 50%, respectively; however, mTOR inhibition did not alter the mRNA expression of system A isoforms (sodium-coupled neutral amino acid transporter-1, -2, and -4), LAT2, or 4F2hc. Rapamycin treatment did not significantly affect the protein expression of any of the transporter isoforms. We conclude that mTOR signaling regulates the activity of key placental amino acid transporters and that this effect is not due to a decrease in total protein expression. These data suggest that mTOR regulates placental amino acid transporters by posttranslational modifications or by affecting transporter translocation to the plasma membrane.
宫内生长受限（IUGR）时胎盘氨基酸转运体的活性降低，但其潜在的调节机制尚未明确。已有研究表明，抑制雷帕霉素靶蛋白（mTOR）信号通路可降低人胎盘绒毛片段中L型氨基酸转运系统的活性，且IUGR时胎盘mTOR活性降低。在本研究中，我们使用原代培养的滋养层细胞更详细地研究mTOR对胎盘氨基酸转运体的调节作用，并检验mTOR通过改变转运体表达来改变氨基酸转运活性这一假说。雷帕霉素抑制mTOR可显著降低A系统（-17%）、L系统（-28%）和牛磺酸（-40%）氨基酸转运体的活性。使用定量实时PCR检测了三种氨基酸转运系统亚型在mTOR抑制后的mRNA表达。L型氨基酸转运体1（LAT1；一种L系统亚型）和牛磺酸转运体的mRNA表达分别降低了13%和50%；然而，mTOR抑制并未改变A系统亚型（钠偶联中性氨基酸转运体-1、-2和-4）、LAT2或4F2hc的mRNA表达。雷帕霉素处理对任何一种转运体亚型的蛋白表达均无显著影响。我们得出结论，mTOR信号通路调节胎盘关键氨基酸转运体的活性，且这种作用并非由于总蛋白表达降低所致。这些数据表明，mTOR通过翻译后修饰或影响转运体向质膜的转位来调节胎盘氨基酸转运体。
