SLC1A5 (solute carrier family 1 (neutral amino acid transporter), member 5)

2014-02-01   Cesare Indiveri , Lorena Pochini , Michele Galluccio , Mariafrancesca Scalise 

Department DiBEST (Biologia, Ecologia, Scienze della Terra) Unit of Biochemistry, Molecular Biotechnology, University of Calabria, 87036 Arcavacata di Rende, Italy

Identity

HGNC
LOCATION
19q13.32
LOCUSID
ALIAS
AAAT,ASCT2,ATBO,M7V1,M7VS1,R16,RDRC
FUSION GENES

Abstract

Review on human SLC1A5, with data on DNA\/RNA, on the protein encoded and pathological and physiological implications.

DNA/RNA

Atlas Image
Figure 1. Isoforms of SLC1A5 gene. The three isoforms are present in the minus strand of the chromosome 19 in position 19q13.3. NM_005628: isoform one, encodes for the longest peptide and is constituted by 8 exons; NM_001145144: isoform two, due to alternative splicing is characterized by only four exons; NM_ 001145145: isoform three presents seven exons. The nucleotide sequence is depicted as black lines. Coding nucleotides and untranslated (UTR) regions are indicated by red and white boxes, respectively. Exons are indicated by roman numbers.

Description

The SLC1A5 gene, located at 19q13.3, counts 28692 nucleotides with 8 exons. It has been found in 56 different organisms (NCBI). The gene encodes a protein involved in sodium-dependent neutral amino acid transport (Kekuda et al., 1996; Pingitore et al., 2013).

Transcription

Three isoforms (transcripts) are reported either on NCBI and Ensembl databases for SLC1A5 human gene, deriving from different translation start. They differ in length, particularly at 5 extremity. The first variant NM_005628 represents the longest transcript, constituted by 2873 nucleotides and 8 exons. This transcript encodes a peptide of 541 amino acids. The second variant NM_001145144 is constituted by 1737 nucleotides and differs in the 5 UTR from the variant NM_005628. In NM_001145144 the translation starts downstream the third exon generating a shorter peptide of 313 aa. The third isoform NM_ 001145145 has 1927 nucleotides and lacks the first exon. It presents a different translation start at 5, coding a peptide of 339 amino acids. A longer transcript, XM_005259167, is reported only in NCBI database. It has been identified by automated computational analysis. More than 400 SNP(s), both in coding and non-coding regions of the SLC1A5 gene, are reported in dbSNP database (dbSNP). More than 40 are responsible of amino acid substitutions with unknown significance. Only the variant SLC1A5-P17A (rs3027956) is associated with breast cancer (Savas et al., 2006). A region constituted by 907 bp upstream of the ASCT2 gene possesses promoter activity (Bungard and McGivan, 2004). In this region the following putative elements have been identified: an amino acid-regulatory element, a consensus site for binding of the transcription factor activator protein 1 (AP1) and a consensus binding sites for nuclear and hepatocyte nuclear factors.

Pseudogene

The gene is virtually present in all vertebrates. The better known orthologous of the human gene are those from rat, mouse and rabbit. Identity between the human and rat, mouse, rabbit are 79%, 82% and 85%, respectively.

Proteins

Atlas Image
Figure 2. Homology structural model of hASCT2. Ribbon diagram viewing of the transporter from the lateral side. The model was built using the glutamate transporter Glpth from Pyrococcus horikoshii crystal structure (1XFH) as the template by Modeller V9.13. The homology model was represented using SpdbViewer 4.01. Asn 163 and 212, predicted as glycosilation sites, are highlighted in blue; Ser 183, 261 and Thr 206, 207, 329, predicted as phosphorilation sites are highlighted in red and orange, respectively. Prediction according to Scan Prosite.

Description

541 amino acids; molecular mass 56598,34 Da.
Human SLC1A5 is a permease (membrane transporter). The 3D structure is not available. Homology modeling highlights a structure similar to that of the glutamate transporter of P. horikoshii (1XFH). N- and C-terminal ends are intracellular. Potential site of N-glycosylation and phosphorilation are predicted. In the structural model, at least one glycosylation site is extracellular and the phosphorilation sites are intracellular (Fig. 2).

Expression

Human SLC1A5 has been originally named ASCT2 from AlaSerCysTransporter2 or ATB0. The acronym ASCT2 is the most frequently used to designate this transport system. It is expressed in many tissues, including brain, (Bröer and Brookes, 2001; Deitmer et al., 2003; Gliddon et al., 2009). There is functional evidence of the expression of ASCT2 in kidney and intestine (Bode, 2001). Besides Caco-2 cells, apparently, also the HT-29 intestinal cell line functionally expresses ASCT2 (Kekuda et al., 1996; Kekuda et al., 1997). Poly(A)1 RNA isolated from several tissues of human origin revealed expression in placenta, lung, skeletal muscle, kidney, and pancreas (Kekuda et al., 1996).

Localisation

The protein is localized in the plasma membrane.

Function

Transport mediated by the human ASCT2 has been originally studied in intact cell systems over-expressing the transport protein (Kekuda et al., 1996; Kekuda et al., 1997). Recently, hASCT2 was over-expressed in the yeast P. pastoris, purified and reconstituted in artificial phospholipid vesicles (proteoliposomes), in absence of other interfering transporters. All experimental systems concur in demonstrating that hASCT2 is an obligate exchanger of neutral amino acid. This antiport requires the presence of extracellular Na+ which cannot be substituted by Li+ or K+. The Na+ ex:amino acidex stoichiometry of the human transporter is likely to be 1:1. Competition studies on 3H-glutamine, 3H-threonine or 3H-alanine transport performed in cells indicated that other potential substrates of hASCT2 are valine, leucine, serine, cysteine, asparagine, methionine, isoleucine, tryptophan, histidine, phenylalanine. While glutamate, lysine, arginine along with MeAIB [α-(methylamino)isobutyric acid] and BCH [2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid] are neither transported nor inhibit hASCT2. Experiments with radioactive compounds confirmed the competition data (Torres-Zamorano et al., 1998). In proteoliposomes, inhibition has been confirmed for most but not for all of the amino acids. Moreover, proteoliposome studies highlighted an asymmetric specificity for amino acids allowing to distinguish the amino acids inwardly transported (alanine, cysteine, valine, methionine) from those bi-directionally transported (glutamine, serine, asparagine, and threonine). The functional asymmetry was also confirmed by the kinetic analysis of [3H]glutamine/glutamine antiport: different Km values were measured on the external and internal sides of proteoliposomes, 0,097 and 1,8 mM, respectively. The SH reagents HgCl2, mersalyl and pOHMB potently inhibited hASCT2 mediated transport (Pingitore et al., 2013).
The physiological role of hASCT2 consists in providing cells with some neutral amino acids exporting others on the basis of the metabolic need of cells consistently with the intra and extracellular amino acid concentrations. In brain, particularly, hASCT2 contributes to glutamine homeostasis of neurons and astrocytes. On the basis of experiments performed with animal models, it was hypothesized that hASCT2 mediates efflux of glutamine from astrocytes, a process that is critical for the functioning of the glutamate-glutamine cycle to recover synaptically released glutamate in exchange with glutamine efflux (Bröer et al., 1999). The glutamine-glutamate cycle has been shown also in placenta. Glutamine crosses the placenta and enters the fetal liver where it is deamidated to glutamate. About 90% of glutamate generated by the liver is taken up by the placenta and used in the metabolism. The glutamine-glutamate cycle between the placenta and the fetal liver is obligatory for the generation of NADPH in the placenta (Torres-Zamorano et al., 1998). Among other functions reported for hASCT2 there is the regulation of mTOR pathway, translation and autophagy. The transporter regulates an increase in the intracellular concentration of glutamine which is then used by another plasma membrane transporter, named LAT1 (SLC7A5) (Galluccio et al., 2013) as efflux substrate to regulate the uptake of extracellular leucine with subsequent activation of mTORC1 (Nicklin et al., 2009). Moreover, it has been proposed that a group of retroviruses specifically uses the hASCT2 as a common cell surface receptor following a co-evolution phenomenon. The orthologous murine transporter mASCT2 is inactive as a viral receptor (Marin et al., 2003).

Implicated in

Entity name
Molecular basis of cancerogenesis
Note
Tumor cells acquire altered metabolism. Due to these changes, the expression of membrane transporters involved in providing nutrients is altered. The plasma membrane transporter for glutamine ASCT2 has been clearly associated to cancer development and progression, together with another amino acid membrane transporter, LAT1 specific for glutamine and other neutral amino acids (Fuchs and Bode, 2005). The energetic needs of cancer cells are different from normal ones due to the Warburg effect. According to this phenomenon ATP derives from anaerobic glycolisis bypassing mitochondrial function (Ganapathy et al., 2009). In this scenario glutamine provided by means of ASCT2 and LAT1 transport function sustains tumor growth and signaling through mTOR pathway (Nicklin et al., 2009). The importance of ASCT2 in this network is revealed by induction of apoptosis when silencing its gene in human hepatoma cells (Fuchs et al., 2004).
In the following paragraphs specific examples of human cancers are reported.
Entity name
Prostate cancer
Note
Tissue microarray technology (TMA) has been used for studying ASCT2 in normal prostatic tissue, in benign prostatic hyperplasia and in prostate adenocarcinoma. In particular, a negative prognosis and a shorter time of recurrence for adenocarcinoma were associated to hASCT2 expression. Moreover, a more aggressive behavior of adenocarcinoma is described (Li et al., 2003).
Entity name
Colorectal carcinoma
Note
The expression of ASCT2 in colorectal carcinoma is normally associated to a decrease of percentage in patient survival (Witte et al., 2002).
Entity name
Neuroblastoma and glioma
Note
Neuroblastoma are childhood tumors very often benign. In some cases, however, neuroblastoma became malignant. One of the biological marker of this second category is the increased uptake of glutamine and other neutral aminoacids via ASCT2 (Wasa et al., 2002). Human glioma C6 cells have been demonstrated to mediate uptake of glutamine via ASCT2 (Dolinska et al., 2003).
Entity name
Hepatoma
Note
Hepatocell carcinoma (HCC) is the most common malignant tumor of liver and one of the main cause of death. A study reported that higher rate of glutamine uptake via ASCT2 is a common feature of six examined hepatoma cell line (Bode et al., 2002; Fuchs et al., 2004).
Entity name
Lung cancer
Note
ASCT2 has been found over expressed in lung cancer by proteomic approach and then confirmed at molecular level. Pharmacologic and genetic targeting of ASCT2 decreased cell growth and viability in lung cancer cells, an effect mediated in part by mTOR signaling (Hassanein et al., 2013).
Entity name
Breast cancer
Note
In breast cancer ASCT2 has been found over expressed together with other proteins related to glutamine metabolism like glutamminase and glutamate dehydrogenase (Kim et al., 2012). The study revealed that this metabolism is essential for sustaining breast cancer development and that the protein levels are different according to different subtypes of cancer. The subtype HER2 showed the highest level of glutamine related proteins and that the basal-like breast cancers are more dependent on glutamine compared to luminal-likeones.
Entity name
Other diseases
Note
Due to importance of glutamine in cell metabolism and the chromosomal localization of SLC1A5 gene, several association studies have been conducted to ascertain the involvement of hASCT2 in pathologies like cystinuria, cystic fibrosis, schizophrenia, Hartnup disorder and pre-eclampsia. However, no genetic associations have been revealed.

Bibliography

Pubmed IDLast YearTitleAuthors
123815192002Molecular and functional analysis of glutamine uptake in human hepatoma and liver-derived cells.Bode BP et al
115332962001Recent molecular advances in mammalian glutamine transport.Bode BP et al
105370791999The astroglial ASCT2 amino acid transporter as a mediator of glutamine efflux.Bröer A et al
106986972000Neutral amino acid transporter ASCT2 displays substrate-induced Na+ exchange and a substrate-gated anion conductance.Bröer A et al
113314002001Transfer of glutamine between astrocytes and neurons.Bröer S et al
151750062004Glutamine availability up-regulates expression of the amino acid transporter protein ASCT2 in HepG2 cells and stimulates the ASCT2 promoter.Bungard CI et al
129692602003Glutamine efflux from astrocytes is mediated by multiple pathways.Deitmer JW et al
127420972003Glutamine transport in C6 glioma cells shows ASCT2 system characteristics.Dolińska M et al
159169032005Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime?Fuchs BC et al
145636742004Inducible antisense RNA targeting amino acid transporter ATB0/ASCT2 elicits apoptosis in human hepatoma cells.Fuchs BC et al
239122402013Cloning, large scale over-expression in E. coli and purification of the components of the human LAT 1 (SLC7A5) amino acid transporter.Galluccio M et al
189927692009Nutrient transporters in cancer: relevance to Warburg hypothesis and beyond.Ganapathy V et al
190127492009Cellular distribution of the neutral amino acid transporter subtype ASCT2 in mouse brain.Gliddon CM et al
232130572013SLC1A5 mediates glutamine transport required for lung cancer cell growth and survival.Hassanein M et al
87025191996Cloning of the sodium-dependent, broad-scope, neutral amino acid transporter Bo from a human placental choriocarcinoma cell line.Kekuda R et al
92274831997Molecular and functional characterization of intestinal Na(+)-dependent neutral amino acid transporter B0.Kekuda R et al
235077042013Expression of glutamine metabolism-related proteins according to molecular subtype of breast cancer.Kim S et al
129260822003Expression of neutral amino acid transporter ASCT2 in human prostate.Li R et al
125843182003N-linked glycosylation and sequence changes in a critical negative control region of the ASCT1 and ASCT2 neutral amino acid transporters determine their retroviral receptor functions.Marin M et al
192035852009Bidirectional transport of amino acids regulates mTOR and autophagy.Nicklin P et al
237567782013Large scale production of the active human ASCT2 (SLC1A5) transporter in Pichia pastoris--functional and kinetic asymmetry revealed in proteoliposomes.Pingitore P et al
165950732006Functional nsSNPs from carcinogenesis-related genes expressed in breast tissue: potential breast cancer risk alleles and their distribution across human populations.Savas S et al
95881991998Sodium-dependent homo- and hetero-exchange of neutral amino acids mediated by the amino acid transporter ATB degree.Torres-Zamorano V et al
119972382002Characterization of L-glutamine transport by a human neuroblastoma cell line.Wasa M et al
125299632002Overexpression of the neutral amino acid transporter ASCT2 in human colorectal adenocarcinoma.Witte D et al

Other Information

Locus ID:

NCBI: 6510
MIM: 109190
HGNC: 10943
Ensembl: ENSG00000105281

Variants:

dbSNP: 6510
ClinVar: 6510
TCGA: ENSG00000105281
COSMIC: SLC1A5

RNA/Proteins

Gene IDTranscript IDUniprot
ENSG00000105281ENST00000412532Q15758
ENSG00000105281ENST00000434726Q15758
ENSG00000105281ENST00000542575Q15758
ENSG00000105281ENST00000593713M0QX44
ENSG00000105281ENST00000594991M0QXM4
ENSG00000105281ENST00000598022M0R144

Expression (GTEx)

0
50
100
150
200
250
300

Pathways

PathwaySourceExternal ID
Protein digestion and absorptionKEGGko04974
Protein digestion and absorptionKEGGhsa04974
Central carbon metabolism in cancerKEGGhsa05230
Central carbon metabolism in cancerKEGGko05230
Transmembrane transport of small moleculesREACTOMER-HSA-382551
SLC-mediated transmembrane transportREACTOMER-HSA-425407
Transport of inorganic cations/anions and amino acids/oligopeptidesREACTOMER-HSA-425393
Amino acid transport across the plasma membraneREACTOMER-HSA-352230

Protein levels (Protein atlas)

Not detected
Low
Medium
High

References

Pubmed IDYearTitleCitations
264553252016ASCT2/SLC1A5 controls glutamine uptake and tumour growth in triple-negative basal-like breast cancer.116
232130572013SLC1A5 mediates glutamine transport required for lung cancer cell growth and survival.86
256938382015Targeting ASCT2-mediated glutamine uptake blocks prostate cancer growth and tumour development.84
245319842014Targeting glutamine transport to suppress melanoma cell growth.72
120503562002The envelope glycoprotein of human endogenous retrovirus type W uses a divergent family of amino acid transporters/cell surface receptors.53
251420202015ATF4 and N-Myc coordinate glutamine metabolism in MYCN-amplified neuroblastoma cells through ASCT2 activation.45
271292762016Deletion of Amino Acid Transporter ASCT2 (SLC1A5) Reveals an Essential Role for Transporters SNAT1 (SLC38A1) and SNAT2 (SLC38A2) to Sustain Glutaminolysis in Cancer Cells.44
173294002007ASCT2 silencing regulates mammalian target-of-rapamycin growth and survival signaling in human hepatoma cells.40
246033032014ASC amino-acid transporter 2 (ASCT2) as a novel prognostic marker in non-small cell lung cancer.39
293343722018Pharmacological blockade of ASCT2-dependent glutamine transport leads to antitumor efficacy in preclinical models.36

Citation

Cesare Indiveri ; Lorena Pochini ; Michele Galluccio ; Mariafrancesca Scalise

SLC1A5 (solute carrier family 1 (neutral amino acid transporter), member 5)

Atlas Genet Cytogenet Oncol Haematol. 2014-02-01

Online version: http://atlasgeneticsoncology.org/gene/42313/slc1a5