Distinct Sensor Pathways in the Hierarchical Control of SNAT2, a Putative Amino Acid Transceptor, by Amino Acid Availability

Mammalian nutrient sensors are novel targets for therapeutic intervention in disease states such as insulin resistance and muscle wasting; however, the proteins responsible for this important task are largely uncharacterized. To address this issue we have dissected an amino acid (AA) sensor/effector regulon that controls the expression of the System A amino acid transporter SNAT2 in mammalian cells, a paradigm nutrient-responsive process, and found evidence for the convergence of at least two sensor/effector pathways. During AA withdrawal, JNK is activated and induces the expression of SNAT2 in L6 myotubes by stimulating an intronic nutrient-sensitive domain. A sensor for large neutral AA (e.g. Tyr, Gln) inhibits JNK activation and SNAT2 up-regulation. Additionally, shRNA and transporter chimeras demonstrate that SNAT2 provides a repressive signal for gene transcription during AA sufficiency, thus echoing AA sensing by transceptor (transporter-receptor) orthologues in yeast (Gap1/Ssy1) and Drosophila (PATH). Furthermore, the SNAT2 protein is stabilized during AA withdrawal. Mammalian nutrient sensors are novel targets for therapeutic intervention in disease states such as insulin resistance and muscle wasting; however, the proteins responsible for this important task are largely uncharacterized. To address this issue we have dissected an amino acid (AA) sensor/effector regulon that controls the expression of the System A amino acid transporter SNAT2 in mammalian cells, a paradigm nutrient-responsive process, and found evidence for the convergence of at least two sensor/effector pathways. During AA withdrawal, JNK is activated and induces the expression of SNAT2 in L6 myotubes by stimulating an intronic nutrient-sensitive domain. A sensor for large neutral AA (e.g. Tyr, Gln) inhibits JNK activation and SNAT2 up-regulation. Additionally, shRNA and transporter chimeras demonstrate that SNAT2 provides a repressive signal for gene transcription during AA sufficiency, thus echoing AA sensing by transceptor (transporter-receptor) orthologues in yeast (Gap1/Ssy1) and Drosophila (PATH). Furthermore, the SNAT2 protein is stabilized during AA withdrawal. Amino acids (AAs) 2The abbreviations used are: AA, amino acid; AS, asparagine synthetase; CaR, extracellular Ca2+ receptor; CMV, cytomegalovirus (promoter); ERK, extracellular signal-regulated kinase; IRES, internal ribosome entry segment; JNK, Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; Me-AIB, α-methylaminoisobutyric acid; PI3K, phosphinositide-3-kinase; SNAT2, sodium-dependent neutral amino acid transporter 2; S6K, ribosomal protein S6 kinase; UTR, untranslated region; PBS, phosphate-buffered saline. 2The abbreviations used are: AA, amino acid; AS, asparagine synthetase; CaR, extracellular Ca2+ receptor; CMV, cytomegalovirus (promoter); ERK, extracellular signal-regulated kinase; IRES, internal ribosome entry segment; JNK, Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; Me-AIB, α-methylaminoisobutyric acid; PI3K, phosphinositide-3-kinase; SNAT2, sodium-dependent neutral amino acid transporter 2; S6K, ribosomal protein S6 kinase; UTR, untranslated region; PBS, phosphate-buffered saline. act through the coordination of several signaling pathways to modulate distinct, albeit inter-related, processes (1Hyde R. Taylor P.M. Hundal H.S. Biochem. J. 2003; 373: 1-18Crossref PubMed Scopus (282) Google Scholar, 2Kilberg M.S. Pan Y.X. Chen H. Leung-Pineda V. Annu. Rev. Nutr. 2005; 25: 59-85Crossref PubMed Scopus (217) Google Scholar, 3Holsbeeks I. Lagatie O. Van Nuland A. d. Van V. Thevelein J.M. Trends Biochem. Sci. 2004; 29: 556-564Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar), including protein synthesis and turnover, hormone action and release, and the synthesis, transport, and metabolism of AAs. The nutrient sensors that influence these processes are poorly defined in higher eukaryotes, so, in an attempt to clarify AA sensing in mammalian cells, the molecular regulation of a paradigm AA-sensitive process (the System A amino acid transporter) has been investigated. Sodium-dependent neutral amino acid transporter 2 (SNAT2, encoded by the gene SLC38A2) exhibits functional and regulatory properties of the classically defined System A transporter (4Mackenzie B. Erickson J.D. Pflugers Arch. 2004; 447: 784-795Crossref PubMed Scopus (404) Google Scholar). Hence, stimulation of System A by AA deprivation (aka adaptive regulation) or insulin occurs concurrent with increased SNAT2 expression (5Hyde R. Christie G.R. Litherland G.J. Hajduch E. Taylor P.M. Hundal H.S. Biochem. J. 2001; 355: 563-568Crossref PubMed Scopus (72) Google Scholar) or the plasma membrane recruitment of SNAT2 (6Hyde R. Peyrollier K. Hundal H.S. J. Biol. Chem. 2002; 277: 13628-13634Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar), respectively. Palii et al. (7Palii S.S. Chen H. Kilberg M.S. J. Biol. Chem. 2004; 279: 3463-3471Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar) isolated a tripartite AA responsive domain in the first intron of SLC38A2 that is required for increasing SLC38A2 mRNA during nutrient stress. Pharmacological and genetic interventions have implicated the classical extracellular signal-regulated kinase (ERK) and stress-activated Jun N-terminal kinase (JNK) mitogen-activated protein kinases (MAPK) in the adaptive regulation of System A (8Franchi-Gazzola R. Visigalli R. Bussolati O. Dall'Asta V. Gazzola G.C. J. Biol. Chem. 1999; 274: 28922-28928Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 9Lopez-Fontanals M. Rodriguez-Mulero S. Casado F.J. Derijard B. Pastor-Anglada M. J. Gen. Physiol. 2003; 122: 5-16Crossref PubMed Scopus (30) Google Scholar) in certain cell types, although nutrient signaling loci upstream of MAPKs have not been identified to date. System A substrates, including the synthetic AA analogue α-methylaminoisobutyric acid (Me-AIB), suppress the increase in SNAT2 expression that occurs in AA-starved cells (9Lopez-Fontanals M. Rodriguez-Mulero S. Casado F.J. Derijard B. Pastor-Anglada M. J. Gen. Physiol. 2003; 122: 5-16Crossref PubMed Scopus (30) Google Scholar, 10Ling R. Bridges C.C. Sugawara M. Fujita T. Leibach F.H. Prasad P.D. Ganapathy V. Biochim. Biophys. Acta. 2001; 1512: 15-21Crossref PubMed Scopus (89) Google Scholar, 11Gazzola R.F. Sala R. Bussolati O. Visigalli R. Dall'Asta V. Ganapathy V. Gazzola G.C. FEBS Lett. 2001; 490: 11-14Crossref PubMed Scopus (73) Google Scholar). Hence, the concept that SNAT2 activity may regulate transporter expression has been and and of sensing have been (1Hyde R. Taylor P.M. Hundal H.S. Biochem. J. 2003; 373: 1-18Crossref PubMed Scopus (282) Google Scholar). SNAT2 may as a during the may to signaling such AA are in I. Lagatie O. Van M. J. Thevelein J.M. 2003; PubMed Scopus Google Scholar, B. K. R.F. E. J. Biol. PubMed Scopus Google Scholar) have to in mammalian this transceptor for the of System that are not for this transporter Tyr, System A J. Biol. Chem. Full Text PDF PubMed Google Scholar). AA deprivation is to functional expression of SNAT2 in L6 myotubes and cells through and protein The of the process is by a in the of of JNK adaptive regulation and the to regulate SNAT2 expression are of JNK with a shRNA is that SNAT2 as a mammalian AA in an gene expression and and and and SNAT2 B. H. H. H. Erickson J.D. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and T. R. T. Sugawara M. Leibach F.H. Prasad P.D. Ganapathy V. J. Physiol. Physiol. 2001; PubMed Google Scholar) in to a The in and The SNAT2 L6 and SNAT2 cells and to the of myotubes as (6Hyde R. Peyrollier K. Hundal H.S. J. Biol. Chem. 2002; 277: 13628-13634Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar). and cells in cells in with an AA at (5Hyde R. Christie G.R. Litherland G.J. Hajduch E. Taylor P.M. Hundal H.S. Biochem. J. 2001; 355: 563-568Crossref PubMed Scopus (72) Google Scholar) for and in the for cell to or cell membrane E. Hundal H.S. PubMed Scopus Google Scholar). cells with in with and cells with or as E. Hundal H.S. PubMed Scopus Google Scholar), with L6 cells with to in for at with and in with by in the with as L6 myotubes in for and to 2 AA for as protein or is to a of cell protein by as E. Hundal H.S. PubMed Scopus Google Scholar). with signaling the of the or in of cell the System to the in a activity in to cells and with A or the with and with in for with and in for in in the or of for 2 with of with in with PBS, and with in The and by of and and at the in and 2 the and are the to the activity and and and the increase in System A activity during AA the of System A activity by an of the of SNAT2 the of of adaptive regulation of System the of extracellular System A and adaptive and a of the of to and System A are Amino of System A of L6 myotubes induces a increase in System A activity R. Christie G.R. Litherland G.J. Hajduch E. Taylor P.M. Hundal H.S. Biochem. J. 2001; 355: 563-568Crossref PubMed Scopus (72) Google Scholar), and through a process protein synthesis in in the of the The that regulate System A activity by 2 to AA-starved myotubes and with the that the System A transporter a repressive signal during the to the for SNAT2 Sci. S. A. PubMed Scopus Google Scholar) the with the repressive although found to System A used to demonstrate that although 2 System A not with System A at this and with SNAT2 and the not SNAT2 in L6 myotubes (5Hyde R. Christie G.R. Litherland G.J. Hajduch E. Taylor P.M. Hundal H.S. Biochem. J. 2001; 355: 563-568Crossref PubMed Scopus (72) Google Scholar). Hence, the of the SNAT2 protein expression a in the N-terminal domain of SNAT2 and to two and that increase in to AA the of these proteins we that the higher is SNAT2, and the is a of A is however, this protein is to not Tyr, and the of SNAT2 protein and a repressive of with Amino acids that stimulation Tyr, with cells for a repressive SNAT2 a large System A transport, the repressive of System A activity may largely to of plasma membrane SNAT2 by Kilberg M.S. J. Biol. Chem. Full Text PDF PubMed Google Scholar). To a repressive SNAT2 gene of the Amino SNAT2 and SLC38A2 AA responsive domain isolated by Palii et al. a an amino acid in and a in of to SNAT2 expression during AA (7Palii S.S. Chen H. Kilberg M.S. J. Biol. Chem. 2004; 279: 3463-3471Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar). The SNAT2 and two of this the first intron of the gene and the first intron and nutrient responsive upstream of in L6 The of of upstream in a of transcription to as with upstream of the SNAT2 (7Palii S.S. Chen H. Kilberg M.S. J. Biol. Chem. 2004; 279: 3463-3471Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar). The deprivation the of intron activity not by AA The activation by the nutrient responsive domain of SNAT2 intron in the or of AA is the of AA we used or in is to for SNAT2 intron (7Palii S.S. Chen H. Kilberg M.S. J. Biol. Chem. 2004; 279: 3463-3471Abstract Full Text Full Text PDF PubMed Scopus (78) Google for SNAT2 intron C.C. E. R. Gazzola G.C. Bussolati O. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). a a by the cytomegalovirus by nutrient the of the repressive Tyr, 2 activity to a AA, not expression of this that nutrient-responsive controls to a for SNAT2 we the that of to transporter C.C. E. R. Gazzola G.C. Bussolati O. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) has found an internal ribosome entry in the of the SLC38A2 however, in certain an has been (e.g. J. I. M. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google the SNAT2 not transporter expression during nutrient a may that SNAT2 protein may stabilized during nutrient and to this we a SNAT2 a in cells, these are L6 cells and in found to System A in a in of cells with a for the SNAT2 The signal cells been with not cells with shRNA thus that the protein are a of AA deprivation of the cells for to a increase in an increased of protein in these cells in L6 cells that the used not to and to the of this increase in transporter the gene and in to the an of nutrient deprivation not Additionally, that the two an to in of these is that during SNAT2 protein is during A of the that SNAT2 is stabilized during nutrient deprivation is that a of the SNAT2 may to such through The of SNAT2 are the for with the however, the of SNAT2 the cell membrane is poorly have been in the of B. H. H. H. Erickson J.D. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, Sci. S. A. PubMed Scopus Google Scholar, M. T. Ganapathy Leibach F.H. Ganapathy V. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), as in a large the by Sugawara et al. M. T. Ganapathy Leibach F.H. Ganapathy V. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) the of et al. B. H. H. H. Erickson J.D. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) and et al. Sci. S. A. PubMed Scopus Google Scholar) in the at an To the two in cells and the cells to by the cells with not with to an signal to in Hence, in the is the extracellular the of proteins is the and the and the large extracellular and is that regulatory are the To the of SNAT2 a SNAT2 and a that to the first extracellular of the two proteins the SNAT2 in this the influence of the in of cells, protein expression to that of and increased AA deprivation Hence, by the SNAT2 we have the of the protein to AA System A controls a of signaling pathways. To pathways to System A L6 myotubes with signal during to of the the or not System A of and and JNK System A inhibits H. J. Biochem. PubMed Scopus Google Scholar) and has been to in AA-starved mammalian not System A a for in the regulation of System A. AA is to increase the of and JNK in L6 cells with (8Franchi-Gazzola R. Visigalli R. Bussolati O. Dall'Asta V. Gazzola G.C. J. Biol. Chem. 1999; 274: 28922-28928Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 9Lopez-Fontanals M. Rodriguez-Mulero S. Casado F.J. Derijard B. Pastor-Anglada M. J. Gen. Physiol. 2003; 122: 5-16Crossref PubMed Scopus (30) Google Scholar), JNK and signaling activation and in L6 cells, 2 and of nutrient During AA kinase upstream of the kinase and kinase of The protein kinase has been to the during amino acid J. A. V. S. Biochem. Biophys. 2004; PubMed Scopus Google however, although increased with AA not this in L6 of in L6 myotubes J. I. M. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). of JNK a AA sensor responsible for SNAT2 that sensor signaling through the pathways that SNAT2 and (9Lopez-Fontanals M. Rodriguez-Mulero S. Casado F.J. Derijard B. Pastor-Anglada M. J. Gen. Physiol. 2003; 122: 5-16Crossref PubMed Scopus (30) Google Scholar) JNK in this to the and AA signaling JNK is poorly To address we the of to the JNK activation in AA-starved cells the and JNK and Tyr, and SNAT2 expression and and System A and not JNK by to have to that of JNK, the of the by is the A AA sensor may regulate the two sensor processes may The extracellular as a sensor for large neutral and the Ca2+ of Sci. S. A. PubMed Scopus Google Scholar). that regulate JNK and are the for CaR, we to System A by L6 cells in the or in the of Ca2+ a of AA not to System A activity the of a to System A not extracellular Furthermore, Ca2+ not the repressive of extracellular and the and the sensor at the plasma is poorly and an of acid not the repressive of has The of by or to the that this sensor is a transporter for amino have not found that JNK, not System A. of JNK signaling for the of System repressive not with for inhibits the stimulation of SNAT2 expression in L6 cells and has been to the of SNAT2 mRNA by (9Lopez-Fontanals M. Rodriguez-Mulero S. Casado F.J. Derijard B. Pastor-Anglada M. J. Gen. Physiol. 2003; 122: 5-16Crossref PubMed Scopus (30) Google Scholar, 10Ling R. Bridges C.C. Sugawara M. Fujita T. Leibach F.H. Prasad P.D. Ganapathy V. Biochim. Biophys. Acta. 2001; 1512: 15-21Crossref PubMed Scopus (89) Google Scholar, 11Gazzola R.F. Sala R. Bussolati O. Visigalli R. Dall'Asta V. Ganapathy V. Gazzola G.C. FEBS Lett. 2001; 490: 11-14Crossref PubMed Scopus (73) Google Scholar). that not JNK we that a sensor with distinct, although AA in with the AA sensor to System A SNAT2 SNAT2 as an Amino the that regulate System A expression and that as System A have been and Gazzola et al. J. Biol. Chem. Full Text PDF PubMed Google Scholar, G.C. Dall'Asta V. J. Biol. Chem. Full Text PDF PubMed Google Scholar) to that AA by System A a signal that gene expression of the System A this we that the expression of SNAT2 nutrient sensing by the cells with the and with the used in of SNAT2 in these cells in a in activity in with this SNAT2 expression shRNA to an activity of the in the shRNA the SNAT2 the of a that SNAT2 to to the of during AA is that the increase in activity AA is the activation of signaling and the of repressive signaling in may required to adaptive of SNAT2 for such convergence is in and SNAT2 to the signaling pathways. SNAT2 as an Amino Amino of transceptor is that the of SNAT2 for AA the of by the the of JNK signaling the of the AA for the that the two sensor/effector pathways are to in the sensor for AA has the that the first of repressive SNAT2 expression as the of that AA is of the SNAT2 to JNK and may SNAT2 expression through a of SNAT2 and the to SNAT2 regulation may dissected through a first to a for of the SNAT2 although not JNK at the is for these the System A is a of a of the System A Me-AIB, and is to the of the System A in an with AA Kilberg M.S. J. Biol. Chem. Full Text PDF PubMed Google Scholar). is for the by the functional activity of the System A repressive that may to the found to System A activity an of Furthermore, a of the JNK used as a for AA signaling in this is the of this for SNAT2 the of repressive of the of this to the of 2 to System A with the The of JNK and Hence, to have a higher for the for used to for of the adaptive and of System A activity by and The for by thus the of to regulate JNK to with with not are in with that SNAT2 expression in with a sensor with AA we to that the of SNAT2 gene expression with the of SNAT2 for Me-AIB, that of the regulatory of this AA that the To of with Me-AIB, an to SNAT2 gene Hence, L6 cells the for in the of of to and cell for activity the of the L6 System A The of the L6 System A for of to for SNAT2 (4Mackenzie B. Erickson J.D. Pflugers Arch. 2004; 447: 784-795Crossref PubMed Scopus (404) Google The cells and by for the The thus for of the by is to the for the of for transporter and the regulation of gene expression is the to a a of in have identified two nutrient sensing and signaling to the repressive of the adaptive regulation of the System A A activation of the kinase JNK by nutrient to several not of are SNAT2 and A for SNAT2 is for not suppress JNK activation (e.g. The by these the adaptive is to the for at the extracellular of SNAT2, of a transceptor for Amino that System A with the J. Biol. Chem. Full Text PDF PubMed Google Scholar) has been used as evidence the that the System A has a in AA sensing G.C. Dall'Asta V. J. Biol. Chem. Full Text PDF PubMed Google Scholar), an for this by an amino acid sensor upstream of JNK and in L6 AA sensor is may have a in nutrient regulation of processes AA AA deprivation through an S. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar) and is in the AA of signaling in and the regulation of Annu. Rev. Nutr. PubMed Google Scholar). The paradigm JNK is to the SNAT2 S.S. Pan Y.X. Kilberg M.S. Biochem. J. PubMed Scopus Google Scholar). The of the not adaptive regulation in L6 cells, activation of by AA withdrawal, may that a of the is to in L6 cells or that are The signaling is for of the of SNAT2 C.C. E. R. Gazzola G.C. Bussolati O. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, S.S. Pan Y.X. Kilberg M.S. Biochem. J. PubMed Scopus Google Scholar). cells, the of the transcription J. A. V. S. Biochem. Biophys. 2004; PubMed Scopus Google Scholar), and the of such as asparagine to the SNAT2 during deprivation S.S. Pan Y.X. Kilberg M.S. Biochem. J. PubMed Scopus Google Scholar, Y.X. Chen H. Kilberg M.S. Biochem. J. Scopus Google Scholar), and System A may C.C. E. R. Gazzola G.C. Bussolati O. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) in cells of the are the not to an important of the SNAT2 adaptive regulatory in L6 muscle cells, at least during AA withdrawal, for several in L6 cells is of a to SNAT2 A activation is of System activity process of in cell (e.g. C.C. I. K. M. Biochem. J. PubMed Scopus Google Scholar) as by deprivation signaling R. Chen H. Palii S.S. Kilberg M.S. J. Nutr. 2002; PubMed Scopus Google for not an increase in System A activity in L6 myotubes not of SNAT2 mRNA by AA deprivation occurs and is to the protein synthesis of mRNA is by and is to a AA deprivation with a for R. Chen H. Palii S.S. Kilberg M.S. J. Nutr. 2002; PubMed Scopus Google Scholar) and synthetic AA and AA are of SNAT2 although are to the of for the The SNAT2 of or poorly of although System A of these with the System A and as such we that SNAT2 is an AA to with System A transporter expression J. E. Biol. PubMed Scopus (30) Google Scholar) and Kilberg M.S. J. Biol. Chem. Full Text PDF PubMed Google and System A in L6 cells in The of SNAT2 to gene expression a shRNA and The that the repressive of SNAT2 with with the that shRNA the expression of a SNAT2 to that SNAT2 expression through a signal at least responsive to the of the we the that a SNAT2 such as has the that AA sensing by SNAT2 occurs at the cell membrane is by several that a in and by SNAT2, not the for AA Ganapathy V. Hundal H.S. Taylor P.M. Biochem. J. PubMed Scopus Google extracellular SNAT2 activity and amino acid in L6 cells A. J. H. S. K. K. J. J. 2002; PubMed Scopus Google Scholar), not a SNAT2 adaptive regulatory or the to amino acid deprivation SNAT2 activity not SNAT2 extracellular AA to a A. J. H. S. K. K. J. J. 2002; PubMed Scopus Google Scholar), the SNAT2 adaptive and B. K. R.F. E. J. Biol. PubMed Scopus Google Scholar) have a for sensors to in nutrient to SNAT2, signaling and the of the the of the SNAT2 sensor to a for regulation of SNAT2 gene transcription in muscle cells, the JNK activated repressive and adaptive regulation JNK not may in the signaling of the SNAT2 AA a have been in yeast and Drosophila I. Lagatie O. Van M. J. Thevelein J.M. 2003; PubMed Scopus Google Scholar, 2005; PubMed Scopus Google Scholar, I. S. E. A. B. Biol. 1999; PubMed Google Scholar). in the of a signaling the expression of a of AA I. S. E. A. B. Biol. 1999; PubMed Google Scholar). in Drosophila is a AA sensor controls signaling 2005; PubMed Scopus Google Scholar). The sensor is of a gene is to the System gene and is that SNAT2 is a mammalian of this the we SNAT2 to and not this at the although and the System A that a transporter has signaling in the of System A SNAT2 expression through a transporter through SNAT2, that the mammalian and with SNAT2 and M. H. B. Pflugers Arch. 2004; 447: PubMed Scopus (89) Google Scholar). that in we activity in L6 cells, identified as by not with certain SNAT2 to to System A and not with SNAT2, have a adaptive important SNAT2 and is that the System A transporter to AA and or transporter A may with a yeast AA signaling I. Lagatie O. Van M. J. Thevelein J.M. 2003; PubMed Scopus Google Scholar, M. Biol. PubMed Scopus Google Scholar) and an to membrane transport, with extracellular AA the of the signaling and in metabolism and expression of I. Lagatie O. Van M. J. Thevelein J.M. 2003; PubMed Scopus Google Scholar). and of that SNAT2 is at the of protein to the of a protein to at the of and at several of the in R. Taylor P.M. Hundal H.S. Biochem. J. 2003; 373: 1-18Crossref PubMed Scopus (282) Google and Kilberg M.S. J. Biol. Chem. Full Text PDF PubMed Google for a of The that of L6 myotubes for in a of System A activity in not is with the that SNAT2 protein is stabilized in an A domain the of SNAT2 to for this have been identified in the of and that signaling I. Lagatie O. Van M. J. Thevelein J.M. 2003; PubMed Scopus Google Scholar) and may in the of is evidence T. M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) that of SNAT2 by the a process proteins as targets for may to the of SNAT2 at the cell is that of this process to the of SNAT2 during AA withdrawal, that extracellular AA of yeast the plasma to for M. Biol. PubMed Scopus Google Scholar). to the of System A the in the of and nutrient an in the is that a AA SNAT2 expression (7Palii S.S. Chen H. Kilberg M.S. J. Biol. Chem. 2004; 279: 3463-3471Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar), AA deprivation SNAT2 expression in a that by AAs. SNAT2 the are to in the of a of and this an not to an in transporter or The stimulation of SNAT2 in cells S.S. Pan Y.X. Kilberg M.S. Biochem. J. PubMed Scopus Google Scholar, Y.X. Chen H. Kilberg M.S. Biochem. J. Scopus Google Scholar) may the stimulation of SNAT2 expression in the of AA may largely to signaling of SNAT2 or a of to these pathways AA such as and activity to a with the cells in and as in AA may have large SNAT2 for the Amino of System have with to a for the regulation of System A by nutrient repressive signaling pathways of at least two AA sensing of these the SNAT2 transporter to as an amino acid SNAT2 may a to nutrient signaling pathways (e.g. the and to AA or as a of The of SNAT2 in AA not that is of (4Mackenzie B. Erickson J.D. Pflugers Arch. 2004; 447: 784-795Crossref PubMed Scopus (404) Google Scholar), the neutral AA and as AA is a transporter a of has been that SNAT2 the of A substrates, through that for extracellular AA, such as by System Pflugers Arch. 2003; PubMed Scopus Google for extracellular SNAT2 and and AA sensing to to regulation of the expression of SNAT2, activity to to AA and nutrient SNAT2 mRNA is in muscle (4Mackenzie B. Erickson J.D. Pflugers Arch. 2004; 447: 784-795Crossref PubMed Scopus (404) Google Scholar) and in have that muscle System A is activated by insulin and AA deprivation A. M. Biochem. J. PubMed Scopus (64) Google Scholar). Additionally, has System A in muscle K. M. K. S. K. J. J. 2002; 29: PubMed Scopus Google Scholar). the the of muscle System A a of System A to H.S. J. Physiol. PubMed Scopus Google Scholar). may to of SNAT2 by the by the is that a of SNAT2 in muscle may as a nutrient as to a Erickson for the SNAT2 and Ganapathy of for the