Mammalian target of rapamycin signalling modulates amino acid uptake by regulating transporter cell surface abundance in primary human trophoblast cells

Abstract

Key points Inadequate nutrient supply during fetal life results in intrauterine growth restriction (IUGR), which may lead to obesity, diabetes, and cardiovascular disease later in life. A decreased placental amino acid transporter activity has been implicated in the pathophysiology of IUGR; however, the mechanisms regulating placental amino acid transporters in the human are largely unknown. We show that inhibition of mammalian target of rapamycin complex 1 or 2 markedly decreases the activity of key placental amino acid transporters in cultured primary human placental cells, mediated by modulating the movement of specific transporter isoforms between the cell interior and the plasma membrane. Because mTOR signalling is inhibited in the IUGR placenta, these findings identify one possible mechanism by which fetal nutrient supply is reduced in this pregnancy complication. Our data may help us better understand the regulation of amino acid transporters and the molecular mechanisms underlying IUGR. Abstract Abnormal fetal growth increases the risk for perinatal complications and predisposes for the development of obesity, diabetes and cardiovascular disease later in life. Emerging evidence suggests that changes in placental amino acid transport directly contribute to altered fetal growth. However, the molecular mechanisms regulating placental amino acid transport are largely unknown. Here we combined small interfering (si) RNA‐mediated silencing approaches with protein expression/localization and functional studies in cultured primary human trophoblast cells to test the hypothesis that mammalian target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) regulate amino acid transporters by post‐translational mechanisms. Silencing raptor (inhibits mTORC1) or rictor (inhibits mTORC2) markedly decreased basal System A and System L amino acid transport activity but had no effect on growth factor‐stimulated amino acid uptake. Simultaneous inhibition of mTORC1 and 2 completely inhibited both basal and growth factor‐stimulated amino acid transport activity. In contrast, mTOR inhibition had no effect on serotonin transport. mTORC1 or mTORC2 silencing markedly decreased the plasma membrane expression of specific System A (SNAT2, SLC38A2) and System L (LAT1, SLC7A5) transporter isoforms without affecting global protein expression. In conclusion, mTORC1 and mTORC2 regulate human trophoblast amino acid transporters by modulating the cell surface abundance of specific transporter isoforms. This is the first report showing regulation of amino acid transport by mTORC2. Because placental mTOR activity and amino acid transport are decreased in human intrauterine growth restriction our data are consistent with the possibility that dysregulation of placental mTOR plays an important role in the development of abnormal fetal growth.