Amino acid transporters: éminences grises of nutrient signalling mechanisms?

Nutrient signalling by the mTOR (mammalian target of rapamycin) pathway involves upstream sensing of free AA (amino acid) concentrations. Several AA-regulated kinases have recently been identified as putative intracellular AA sensors. Their activity will reflect the balance between AA flows through underlying mechanisms which together determine the size of the intracellular free AA pool. For indispensable AAs, these mechanisms are primarily (i) AA transport across the cell membrane, and (ii) protein synthesis/breakdown. The System L AA transporter is the primary conduit for cellular entry of indispensable neutral AAs (including leucine and phenylalanine) and potentially a key modulator of AA-sensitive mTOR signalling. Coupling of substrate flows through System L and other AA transporters (e.g. System A) may extend the scope for sensing nutrient abundance. Factors influencing AA transporter activity (e.g. hormones) may affect intracellular AA concentrations and hence indirectly mTOR pathway activity. Several AA transporters are themselves regulated by AA availability through ‘adaptive regulation’, which may help to adjust the gain of AA sensing. The substrate-binding sites of AA transporters are potentially direct sensors of AA availability at both faces of the cell surface, and there is growing evidence that AA transporters of the SNAT (sodium-coupled neutral AA transporter) and PAT (proton-assisted AA transporter) families may operate, at least under some circumstances, as transporter-like sensors (or ‘transceptors’) upstream of mTOR.

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Regulation of placental amino acid transporter activity by mammalian target of rapamycin

AJP Cell Physiology ◽

10.1152/ajpcell.00330.2008 ◽

2009 ◽

Vol 296 (1) ◽

pp. C142-C150 ◽

Cited By ~ 98

Author(s):

S. Roos ◽

Y. Kanai ◽

P. D. Prasad ◽

T. L. Powell ◽

T. Jansson

Keyword(s):

Amino Acid ◽

Protein Expression ◽

Mrna Expression ◽

Mammalian Target Of Rapamycin ◽

Amino Acid Transporter ◽

Amino Acid Transporters ◽

Mtor Inhibition ◽

System A ◽

System L ◽

Acid Transporter

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.

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Placental mTOR links maternal nutrient availability to fetal growth

Biochemical Society Transactions ◽

10.1042/bst0370295 ◽

2009 ◽

Vol 37 (1) ◽

pp. 295-298 ◽

Cited By ~ 92

Author(s):

Sara Roos ◽

Theresa L. Powell ◽

Thomas Jansson

Keyword(s):

Amino Acid ◽

Fetal Growth ◽

Intrauterine Growth Restriction ◽

Fat Body ◽

Mammalian Target Of Rapamycin ◽

Amino Acid Transporters ◽

Acid Transport ◽

Intrauterine Growth ◽

Mtor Activity ◽

Mtor Signalling

The mTOR (mammalian target of rapamycin) signalling pathway functions as a nutrient sensor, both in individual cells and, more globally, in organs such as the fat body in Drosophila and the hypothalamus in the rat. The activity of placental amino acid transporters is decreased in IUGR (intrauterine growth restriction), and recent experimental evidence suggests that these changes contribute directly to the restricted fetal growth. We have shown that mTOR regulates the activity of the placental L-type amino acid transporter system and that placental mTOR activity is decreased in IUGR. The present review summarizes the emerging evidence implicating placental mTOR signalling as a key mechanism linking maternal nutrient and growth factor concentrations to amino acid transport in the human placenta. Since fetal growth is critically dependent on placental nutrient transport, placental mTOR signalling plays an important role in the regulation of fetal growth.

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Regulation of amino acid transporters by glucose and growth factors in cultured primary human trophoblast cells is mediated by mTOR signaling

AJP Cell Physiology ◽

10.1152/ajpcell.00191.2009 ◽

2009 ◽

Vol 297 (3) ◽

pp. C723-C731 ◽

Cited By ~ 100

Author(s):

S. Roos ◽

O. Lagerlöf ◽

M. Wennergren ◽

T. L. Powell ◽

T. Jansson

Keyword(s):

Amino Acid ◽

Glucose Deprivation ◽

Mtor Signaling ◽

Amino Acid Transporters ◽

Trophoblast Cells ◽

Human Trophoblast ◽

System A ◽

System L ◽

Igf I ◽

Human Trophoblast Cells

Inhibition of mammalian target of rapamycin (mTOR) signaling in cultured human primary trophoblast cells reduces the activity of key placental amino acid transporters. However, the upstream regulators of placental mTOR are unknown. We hypothesized that glucose, insulin, and IGF-I regulate placental amino acid transporters by inducing changes in mTOR signaling. Primary human trophoblast cells were cultured for 24 h with media containing various glucose concentrations, insulin, or IGF-I, with or without the mTOR inhibitor rapamycin, and, subsequently, the activity of system A, system L, and taurine (TAUT) transporters was measured. Glucose deprivation (0.5 mM glucose) did not significantly affect Thr172-AMP-activated protein kinase phosphorylation or REDD1 expression but decreased S6 kinase 1 phosphorylation at Thr389. The activity of system L decreased in a dose-dependent manner in response to decreasing glucose concentrations. This effect was abolished in the presence of rapamycin. Glucose deprivation had two opposing effects on system A activity: 1) an “adaptive” upregulation mediated by an mTOR-independent mechanism and 2) downregulation by an mTOR-dependent mechanism. TAUT activity was increased after incubating cells with glucose-deprived media, and this effect was largely independent of mTOR signaling. Insulin and IGF-I increased system A activity and insulin stimulated system L activity, effects that were abolished by rapamycin. We conclude that the mTOR pathway represents an important intracellular regulatory link between nutrient and growth factor concentrations and amino acid transport in the human placenta.

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Differential Expression of System L Amino Acid Transporters during Wound Healing Process in the Skin of Young and Old Rats

Biological and Pharmaceutical Bulletin ◽

10.1248/bpb.31.395 ◽

2008 ◽

Vol 31 (3) ◽

pp. 395-399

Author(s):

Moon-Jin Jeong ◽

Chun Sung Kim ◽

Joo-Cheol Park ◽

Heung-Joong Kim ◽

Yeong Mu Ko ◽

...

Keyword(s):

Wound Healing ◽

Amino Acid ◽

Differential Expression ◽

Healing Process ◽

Wound Healing Process ◽

Amino Acid Transporters ◽

System L ◽

Old Rats

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l-Leucine availability regulates phosphatidylinositol 3-kinase, p70 S6 kinase and glycogen synthase kinase-3 activity in L6 muscle cells: evidence for the involvement of the mammalian target of rapamycin (mTOR) pathway in the l-leucine-induced up-regulation of System A amino acid transport

Biochemical Journal ◽

10.1042/0264-6021:3500361 ◽

2000 ◽

Vol 350 (2) ◽

pp. 361 ◽

Cited By ~ 38

Author(s):

Karine PEYROLLIER ◽

Eric HAJDUCH ◽

Anne S. BLAIR ◽

Russell HYDE ◽

Harinder S. HUNDAL

Keyword(s):

Amino Acid ◽

Glycogen Synthase ◽

Mammalian Target Of Rapamycin ◽

Glycogen Synthase Kinase 3 ◽

Acid Transport ◽

Phosphatidylinositol 3 Kinase ◽

S6 Kinase ◽

P70 S6 Kinase ◽

System A ◽

L6 Muscle Cells

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A rapid method for the functional reconstitution of amino acid transport systems from rat liver plasma membranes. Partial purification of System A

Biochemical Journal ◽

10.1042/bj2550963 ◽

1988 ◽

Vol 255 (3) ◽

pp. 963-969 ◽

Cited By ~ 8

Author(s):

A R Quesada ◽

J D McGivan

Keyword(s):

Amino Acid ◽

Rapid Method ◽

Amino Acid Transport ◽

Plasma Membranes ◽

Transport Systems ◽

Partial Purification ◽

Acid Transport ◽

Transport Activity ◽

System A ◽

System L

A rapid method for the functional reconstruction of amino acid transport from liver plasma-membrane vesicles using the neutral detergent decanoyl-N-glucamide (‘MEGA-10’) is described. The method is a modification of that previously employed in this laboratory for reconstitution of amino acid transport systems from kidney brush-border membranes [Lynch & McGivan (1987) Biochem. J. 244, 503-508]. The transport activities termed ‘System A’, ‘System N’, and ‘System L’ are all reconstituted. The reconstitution procedure is rapid and efficient and is suitable as an assay for transport activity in studies involving membrane fractionation. By using this reconstitution procedure, System A transport activity was partially purified by lectin-affinity chromatography.

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Characterization of amino acid transport in human endothelial cells

AJP Cell Physiology ◽

10.1152/ajpcell.1993.265.4.c1006 ◽

1993 ◽

Vol 265 (4) ◽

pp. C1006-C1014 ◽

Cited By ~ 33

Author(s):

O. Bussolati ◽

R. Sala ◽

A. Astorri ◽

B. M. Rotoli ◽

V. Dall'Asta ◽

...

Keyword(s):

Amino Acids ◽

Endothelial Cells ◽

Amino Acid ◽

Umbilical Vein ◽

Specific System ◽

Human Umbilical Vein ◽

System A ◽

System L ◽

Intracellular Amino Acids ◽

Umbilical Vein Endothelial Cells

The transport of amino acids has been studied in human umbilical vein endothelial cells. Neutral amino acids enter human umbilical vein endothelial cells through three distinct agencies endowed with the characteristics of systems A, ASC, and L. Each system has been studied by evaluating the influx of preferential substrates. The influx of L-proline and 2-methylaminoisobutyric acid occurs through an Na(+)-dependent adaptively regulated trans-inhibited agency identifiable with system A. L-Threonine influx occurs mainly through a distinct Na(+)-dependent trans-stimulated pathway corresponding to system ASC. System L accounts for Na(+)-independent influx of L-leucine. These systems cooperate for the transport of L-glutamine, which is due mainly to system ASC, whereas the component due to the operation of system A increases upon amino acid starvation. No clear evidence was found for a glutamine-specific system ("system N"). Two systems, one Na+ dependent (system XAG-) and the other Na+ independent (system xc-), transport anionic amino acids. L-Arginine influx exhibits a poor dependence on extracellular Na+, whereas it is sensitive to conditions known to change membrane potential and to trans-stimulation by intracellular amino acids. These features are consistent with a process mediated by system y+ and may be of significance for the regulation of the intracellular concentration of L-arginine.

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Downregulation of Placental Amino Acid Transporter Expression and mTORC1 Signaling Activity Contributes to Fetal Growth Retardation in Diabetic Rats

International Journal of Molecular Sciences ◽

10.3390/ijms21051849 ◽

2020 ◽

Vol 21 (5) ◽

pp. 1849

Author(s):

Jie Xu ◽

Jiao Wang ◽

Yang Cao ◽

Xiaotong Jia ◽

Yujia Huang ◽

...

Keyword(s):

Amino Acid ◽

Rat Model ◽

Amino Acid Transporter ◽

Amino Acid Transporters ◽

Amino Acid Transport System ◽

Intrauterine Growth ◽

System L ◽

Acid Transporter ◽

Transporter Expression

Alterations in placental transport may contribute to abnormal fetal intrauterine growth in pregnancies complicated by diabetes, but it is not clear whether the placental amino acid transport system is altered in diabetic pregnancies. We therefore studied the changes in the expressions of placental amino acid transporters in a rat model of diabetes induced by streptozotocin, and tested the effects of hyperglycemia on trophoblast amino acid transporter in vitro. Our results showed that the expressions for key isoforms of system L amino acid transporters were significantly reduced in the placentas of streptozotocin-induced diabetic pregnant rats, which was associated with the decreased birthweight in the rats. A decreased placental efficiency and decreased placental mammalian target of rapamycin (mTOR) complex 1 (mTORC1) activity were also found in the rat model. In addition, hyperglycemia in vitro could inhibit amino acid transporter expression and mTORC1 activity in human trophoblast. Inhibition of mTORC1 activity led to reduced amino acid transporter expression in placental trophoblast. We concluded that reduced placental mTORC1 activity during pregnancy resulted in decreased placental amino acid transporter expression and, subsequently, contributed to fetal intrauterine growth restriction in pregnancies complicated with diabetes.

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