Transport of D-serine via the amino acid transporter ATB(0,+) expressed in the colon

D-Serine, synthesized endogenously in the brain, is an important modulator of glutamatergic neurotransmission. Since colonic bacteria produce D-serine, we asked the question whether there are transport mechanisms in the colon that might make this exogenously produced D-serine available to the host. Here we identify for the first time an amino acid transporter in the intestine for high-affinity active transport of D-serine. This transporter, called ATB(0,+), is a Na(+)- and Cl(-)-coupled transporter for L-enantiomers of neutral and cationic amino acids. Here we demonstrate that ATB(0,+) is also capable of mediating the Na(+)- and Cl(-)-coupled transport of D-serine. The affinity of ATB(0,+) for L-serine and D-serine is similar, the K(t) value for the two enantiomers being approximately 150 microM. In addition to D-serine, ATB(0,+) transports D-alanine, D-methionine, D-leucine, and D-tryptophan. However, several other neutral and cationic amino acids that are transportable substrates for ATB(0,+) as L-enantiomers are not transported when presented as D-enantiomers. ATB(0,+) is expressed in the intestinal tract, interestingly not in the proximal intestine but in the distal intestine. Expression is most predominant in the colon where the transporter is localized to the luminal membrane of colonocytes, making this transporter uniquely suitable for absorption of bacteria-derived D-serine.     

Na(+)-dependent neutral amino acid transporter ATB(0) is a rabbit epithelial cell brush-border protein

System B⁰ activity accounts for the majority ofintestinal and kidney luminal neutral amino acid absorption. An aminoacid transport system, called ATB⁰ (also known as ASCT2),with functional characteristics similar to those of systemB⁰, has been recently cloned. We generated polyclonalantibodies to human and rabbit ATB⁰ COOH-terminal peptidesand used Western blot analysis to detect ATB⁰ protein inrabbit tissues, rabbit ileal brush-border membrane vesicles (BBMV), andHeLa cells transfected with plasmids containing ATB⁰ cDNAs.Immunohistochemistry was used to localize ATB⁰ in rabbitkidney and intestine. In Western blots of rabbit tissues, ATB⁰ was a broad smear of 78- to 85-kDa proteins. Intransfected HeLa cells, ATB⁰ appeared as a smear consistingof 57- to 65-kDa proteins. The highest expression was found in thekidney. ATB⁰ was enriched in rabbit ileal BBMV and in HeLacells transfected with ATB⁰ cDNAs. In the kidney and in theintestine, ATB⁰ was confined to the brush-border membrane(BBM) of the proximal tubular cell and of the enterocyte, respectively.Tissue and intracellular distribution of ATB⁰ proteinparallels that of system B⁰ activity. ATB⁰protein could be the transporter responsible for system B⁰in the BBM of epithelial cells.

     

Regulation of amino acid/carnitine transporter B 0,+ (ATB 0,+) in astrocytes by protein kinase C: independent effects on raft and non-raft transporter subpopulations

Neutral and basic amino acid transporter B(0,+) belongs to a Na,Cl-dependent superfamily of proteins transporting neurotransmitters, amino acids and osmolytes, known to be regulated by protein kinase C (PKC). The present study demonstrates an increased phosphorylation of B(0,+) on serine moiety after treatment of rat astrocytes with phorbol 12-myristate 13-acetate, a process correlated with an augmented activity of l-leucine transport and an enhanced presence of the transporter at the cell surface. After solubilization with Triton X-100 and sucrose gradient centrifugation, B(0,+) was detected in non-raft as well as in detergent-resistant raft fractions under control conditions, while phorbol 12-myristate 13-acetate treatment resulted in a complete disappearance of the transporter from the raft fraction. B(0,+) was observed to interact with caveolin-1 and flotillin-1 (reggie-2) proteins, the markers of detergent-resistant microdomains of plasma membrane. As verified in immunocytochemistry and immunoprecipitation experiments, modification of PKC activity did not affect these interactions. It is proposed that PKC reveals different effects on raft and non-raft subpopulations of B(0,+). Phorbol ester treatment results in trafficking of the transporter from the intracellular pool to non-raft microdomains and increased activity, while B(0,+) present in raft microdomains undergoes either internalization or is transferred laterally to non-raft domains.     

A polarized localization of amino acid/carnitine transporter B(0,+) (ATB(0,+)) in the blood-brain barrier

Brain capillary endothelial cells control the uptake and efflux from the brain of many hydrophilic compounds due to highly specialized transporters often localized in a polarized way. Localization of Na⁺- and Cl⁻-dependent amino acid and carnitine transporter B^0,+ (ATB^0,+) was studied in a co-culture of bovine brain capillary endothelial cells (BBCEC) grown on filters above astrocytes (an in vitro blood-brain barrier model). Immunoblotting and three-dimensional immunocytochemistry analysis with anti-B^0,+antibodies demonstrated the presence of this transporter and its prevalent co-localization with P-glycoprotein i.e. at the apical side. The sensitivity of leucine uptake through the apical membrane to 2-aminobicyclo-[2.2.1]-heptane-2-carboxylic acid (BCH), d-serine as well as sodium and chloride replacement confirm the functioning of ATB^0,+ and suggests an important physiological role of ATB^0,+ in controlling the delivery of amino acids and carnitine to the brain.     

Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+)

L-carnitine is absorbed in the intestinal tract via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+). Loss-of-function mutations in OCTN2 may be associated with inflammatory bowel disease (IBD), suggesting a role for carnitine in intestinal/colonic health. In contrast, ATB(0,+) is upregulated in bowel inflammation. Butyrate, a bacterial fermentation product, is beneficial for prevention/treatment of ulcerative colitis. Butyryl-L-carnitine (BC), a butyrate ester of carnitine, may have potential for treatment of gut inflammation, since BC would supply both butyrate and carnitine. We examined the transport of BC via ATB(0,+) to determine if this transporter could serve as a delivery system for BC. We also examined the transport of BC via OCTN2. Studies were done with cloned ATB(0,+) and OCTN2 in heterologous expression systems. BC inhibited ATB(0,+)-mediated glycine transport in mammalian cells (IC(50), 4.6 +/- 0.7 mM). In Xenopus laevis oocytes expressing human ATB(0,+), BC induced Na(+) -dependent inward currents under voltage-clamp conditions. The currents were saturable with a K(0.5) of 1.4 +/- 0.1 mM. Na(+) activation kinetics of BC-induced currents suggested involvement of two Na(+) per transport cycle. BC also inhibited OCTN2-mediated carnitine uptake (IC(50), 1.5 +/- 0.3 microM). Transport of BC via OCTN2 is electrogenic, as evidenced from BC-induced inward currents. These currents were Na(+) dependent and saturable (K(0.5), 0.40 +/- 0.02 microM). We conclude that ATB(0,+) is a low-affinity/high-capacity transporter for BC, whereas OCTN2 is a high-affinity/low-capacity transporter. ATB(0,+) may mediate intestinal absorption of BC when OCTN2 is defective.     

Functional characterization and cloning of amino acid transporter B(0,+) (ATB(0,+)) in primary cultured rat pneumocytes

Cationic amino acid transport in primary cultured rat pneumocytes exhibiting characteristics of alveolar epithelial type I-like cells are described. Asymmetry and activator ion dependency of (3)H-L-arginine uptake were characterized from the apical or basolateral fluid of pneumocytes grown on permeable support. Substrate specificity of transport was evaluated as a function of (3)H-L-arginine uptake inhibition in the presence of other amino acids. Transepithelial transport studies estimated (3)H-L-arginine flux in the apical-to-basolateral and basolateral-to-apical directions. Full length cDNA of rat amino acid transporter B(0,+) (rATB(0,+)) was cloned and its relative expression level studied. Results indicate that uptake of (3)H-L-arginine from apical fluid is dependent on Na(+) and Cl(-). Zwitterionic and cationic amino acids (excluding L-proline and anionic amino acids) inhibited uptake of (3)H-L-arginine from apical, but not basolateral incubation fluid. Apical-to-basolateral transepithelial flux of (3)H-L-arginine was 20x higher than basolateral-to-apical transport. Kinetic studies of (3)H-L-arginine uptake from apical fluid revealed maximal velocity (V(max)) and Michaelis-Menten constants (K(t)) of 33.32 +/- 2.12 pmol/mg protein/15 min and 0.50 +/- 0.11 mM, respectively, in a cooperative process having a coupling ratio of 1.18 +/- 0.16 with Na(+) and 1.11 +/- 0.13 with Cl(-). Expression of rATB(0,+) mRNA was identified by RT-PCR and Northern analysis. Corresponding cloned 3.2 kb rATB(0,+) cDNA sequence exhibits pronounced homology in deduced amino acid sequence to mouse (95% identity and 97% similarity) and human (89% identity and 95% similarity) ATB(0,+) homologues. We conclude that rat pneumocytes express ATB(0,+), which may partly contribute towards recovering cationic and neutral amino acids from alveolar luminal fluid.     

Upregulation of the amino acid transporter ATB0,+ (SLC6A14) in colorectal cancer and metastasis in humans

ATB(0,+) (SLC6A14) is a Na(+)/Cl(-)-coupled arginine transporter expressed at low levels in normal colon. Arginine is an essential amino acid for tumor cells. Arginine is also the substrate for nitric oxide synthases (NOSs). Since arginine and arginine-derived nitric oxide (NO) play a critical role in cancer, we examined the expression of ATB(0,+) in colorectal cancer. Paired normal and cancer tissues from colectomy specimens of 10 patients with colorectal cancer and from the liver tissue of one patient with hepatic metastasis from a colonic primary were used for the analysis of the levels of ATB(0,+) mRNA, inducible NOS (iNOS) mRNA and the corresponding proteins. Tissues samples from the colon, liver, and lymph nodes of an additional patient with metastatic colon cancer were analyzed for ATB(0,+) protein alone. We also examined the levels of nitrotyrosylated proteins. The ATB(0,+) mRNA increased 22.9+/-3.0-fold in colorectal cancer compared to normal tissue and the increase was evident in each of the 10 cases examined. iNOS mRNA increased 5.2+/-1.1-fold in cancer specimens. The changes in mRNA levels were associated with an increase in ATB(0,+), iNOS, and nitrotyrosylated proteins. The increased expression of ATB(0,+) and iNOS was also demonstrated in liver and lymph node specimens with metastases from colonic primaries. This study strongly suggests that the upregulation of ATB(0,+) may have a pathogenic role in colorectal cancer. Since ATB(0,+) is a versatile transporter not only for arginine but also for several drugs including NOS inhibitors, these findings have significant clinical and therapeutic relevance.     

Expression of the mammalian Na+-independent L system amino acid transporter in Xenopus laevis oocytes

System L is primarily responsible for the Na+-independent transport of neutral amino acids, those with bulky chains such as leucine, isoleucine, phenylalanine, etc., into mammalian cells. mRNA from rat kidney and human lymphoid cells, when microinjected into Xenopus laevis oocytes, induced expression of this transport system. The expressed transport exhibits characteristics similar to those reported for the System L amino acid transporter from a variety of mammalian cells. Injection of size-fractionated mRNA indicates that the System L transporter in both the rat kidney and human lymphoid cells is encoded by mRNA of about 3 to 4 kb.     

Expression of the mammalian system A neutral amino acid transporter in Xenopus oocytes

In this report, we demonstrate the expression of the mammalian System A neutral amino acid transporter in Xenopus laevis oocytes following microinjection of mRNA from rat liver, Chinese hamster ovary (CHO) cells, and human placenta. Stage 6 oocytes were injected with poly(A+) mRNA from one of these three sources and incubated for 24 h prior to assaying Na(+)-dependent 2-aminoisobutyric acid transport to monitor the increase in System A activity. The endogenous 2-aminoisobutyric acid uptake rates in oocytes were sufficiently slow so as to provide a low background value that was subtracted to obtain transport rates for the mammalian carrier alone. The degree of expression of the mammalian System A activity in Xenopus oocytes corresponded to the known transport rates in the tissue from which the mRNA was prepared. For example, hepatic mRNA from glucagon-treated rats produced greater System A activity than mRNA from control animals, and the mRNA from the CHO transport mutant cell line alar4-H3.9, which overproduces System A, resulted in higher transport rates than mRNA from the parental cell line (CHO-K1). Fractionation of total mRNA poly(A+) by nondenaturing agarose gel electrophoresis revealed transport activity associated with a 2.0-2.5-kilobase mRNA fraction common to each of the three tissues tested.     

Differential influence of the 4F2 heavy chain and the protein related to b(0,+) amino acid transport on substrate affinity of the heteromeric b(0,+) amino acid transporter

We provide evidence here that b(0,+) amino acid transporter (b(0, +)AT) interacts with 4F2 heavy chain (4F2hc) as well as with the protein related to b(0,+) amino acid transporter (rBAT) to constitute functionally competent b(0,+)-like amino acid transport systems. This evidence has been obtained by co-expression of b(0, +)AT and 4F2hc or b(0,+)AT and rBAT in human retinal pigment epithelial cells and in COS-1 cells. The ability to interact with 4F2hc and rBAT is demonstrable with mouse b(0,+)AT as well as with human b(0,+)AT. Even though both the 4F2hc x b(0,+)AT complex and the rBAT x b(0,+)AT complex exhibit substrate specificity that is characteristic of system b(0,+), these two complexes differ significantly in substrate affinity. The 4F2hc x b(0,+)AT complex has higher substrate affinity than the rBAT x b(0,+)AT complex. In situ hybridization studies demonstrate that the regional distribution pattern of mRNA in the kidney is identical for b(0,+)AT and 4F2hc. The pattern of rBAT mRNA expression is different from that of b(0,+)AT mRNA and 4F2hc mRNA, but there are regions in the kidney where b(0,+)AT mRNA expression overlaps with rBAT mRNA expression as well as with 4F2hc mRNA expression.     

The cationic amino acid transporter 2 is induced in inflammatory lung models and regulates lung fibrosis

Background

Cigarette smoke (CS) is known to initiate a cascade of mediator release and accumulation of immune and inflammatory cells in the lower airways. We investigated and compared the effects of CS on upper and lower airways, in a mouse model of subacute and chronic CS exposure.

Methods

C57BL/6 mice were whole-body exposed to mainstream CS or air, for 2, 4 and 24 weeks. Bronchoalveolar lavage fluid (BAL) was obtained and tissue cryosections from nasal turbinates were stained for neutrophils and T cells. Furthermore, we evaluated GCP-2, KC, MCP-1, MIP-3α, RORc, IL-17, FoxP3, and TGF-β1 in nasal turbinates and lungs by RT-PCR.

Results

In both upper and lower airways, subacute CS-exposure induced the expression of GCP-2, MCP-1, MIP-3α and resulted in a neutrophilic influx. However, after chronic CS-exposure, there was a significant downregulation of inflammation in the upper airways, while on the contrary, lower airway inflammation remained present. Whereas nasal FoxP3 mRNA levels already increased after 2 weeks, lung FoxP3 mRNA increased only after 4 weeks, suggesting that mechanisms to suppress inflammation occur earlier and are more efficient in nose than in lungs.

Conclusions

Altogether, these data demonstrate that CS induced inflammation may be differently regulated in the upper versus lower airways in mice. Furthermore, these data may help to identify new therapeutic targets in this disease model.     

SLC6A14 (ATB0,+) protein, a highly concentrative and broad specific amino acid transporter, is a novel and effective drug target for treatment of estrogen receptor-positive breast cancer

SLC6A14, also known as ATB(0,+), is an amino acid transporter with unique characteristics. It transports 18 of the 20 proteinogenic amino acids. However, this transporter is expressed only at low levels in normal tissues. Here, we show that the transporter is up-regulated specifically in estrogen receptor (ER)-positive breast cancer, demonstrable with primary human breast cancer tissues and human breast cancer cell lines. SLC6A14 is an estrogen/ER target. The transport features of SLC6A14 include concentrative transport of leucine (an activator of mTOR), glutamine (an essential amino acid for nucleotide biosynthesis and substrate for glutaminolysis), and arginine (an essential amino acid for tumor cells), suggesting that ER-positive breast cancer cells up-regulate SLC6A14 to meet their increased demand for these amino acids. Consequently, treatment of ER-positive breast cancer cells in vitro with α-methyl-DL-tryptophan (α-MT), a selective blocker of SLC6A14, induces amino acid deprivation, inhibits mTOR, and activates autophagy. Prolongation of the treatment with α-MT causes apoptosis. Addition of an autophagy inhibitor (3-methyladenine) during α-MT treatment also induces apoptosis. These effects of α-MT are specific to ER-positive breast cancer cells, which express the transporter. The ability of α-MT to cause amino acid deprivation is significantly attenuated in MCF-7 cells, an ER-positive breast cancer cell line, when SLC6A14 is silenced with shRNA. In mouse xenograft studies, α-MT by itself is able to reduce the growth of the ER-positive ZR-75-1 breast cancer cells. These studies identify SLC6A14 as a novel and effective drug target for the treatment of ER-positive breast cancer.     

Molecular cloning of mouse amino acid transport system B0, a neutral amino acid transporter related to Hartnup disorder

Resorption of amino acids in kidney and intestine is mediated by transporters, which prefer groups of amino acids with similar physico-chemical properties. It is generally assumed that most neutral amino acids are transported across the apical membrane of epithelial cells by system B(0). Here we have characterized a novel member of the Na(+)-dependent neurotransmitter transporter family (B(0)AT1) isolated from mouse kidney, which shows all properties of system B(0). Flux experiments showed that the transporter is Na(+)-dependent, electrogenic, and actively transports most neutral amino acids but not anionic or cationic amino acids. Superfusion of mB(0)AT1-expressing oocytes with neutral amino acids generated inward currents, which were proportional to the fluxes observed with labeled amino acids. In situ hybridization showed strong expression in intestinal microvilli and in the proximal tubule of the kidney. Expression of mouse B(0)AT1 was restricted to kidney, intestine, and skin. It is generally assumed that mutations of the system B(0) transporter underlie autosomal recessive Hartnup disorder. In support of this notion mB(0)AT1 is located on mouse chromosome 13 in a region syntenic to human chromosome 5p15, the locus of Hartnup disorder. Thus, the human homologue of this transporter is an excellent functional and positional candidate for Hartnup disorder.     

Expression cloning of a Na+-independent aromatic amino acid transporter with structural similarity to H+/monocarboxylate transporters

A cDNA was isolated from rat small intestine by expression cloning which encodes a novel Na+-independent transporter for aromatic amino acids. When expressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter 1) exhibited Na+-independent and low-affinity transport of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (Km values: approximately 5 mm), consistent with the properties of classical amino acid transport system T. TAT1 accepted some variations of aromatic side chains because it interacted with amino acid-related compounds such as l-DOPA and 3-O-methyl-DOPA. Because TAT1 accepted N-methyl- and N-acetyl-derivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions. Consistent with this, TAT1 exhibited sequence similarity (approximately 30% identity at the amino acid level) to H+/monocarboxylate transporters. Distinct from H+/monocarboxylate transporters, however, TAT1 was not coupled with the H+ transport but it mediated an electroneutral facilitated diffusion. TAT1 mRNA was strongly expressed in intestine, placenta, and liver. In rat small intestine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of aromatic amino acids. The identification of the amino acid transporter with distinct structural and functional characteristics will not only facilitate the expansion of amino acid transporter families but also provide new insights into the mechanisms of substrate recognition of organic solute transporters.     

Characterization of a novel Na+-independent amino acid transporter in horse erythrocytes.

Horse erythrocytes are polymorphic with respect to L-alanine permeability. The present investigation compared the specificity, kinetics and cation-dependence of erythrocyte amino acid transport in two groups of thoroughbred horses, those with erythrocyte L-alanine permeabilities in the range 5-15 mumol/h per litre of cells (0.2 mM extracellular L-alanine, 37 degrees C) (transport-negative type) and those with L-alanine permeabilities in the range 450-700 mumol/h per litre of cells (transport-positive type). Transport-positive cells are shown to possess a novel high-affinity, stereospecific, Na+-independent transporter selective for neutral amino acids of intermediate size. This carrier system (provisional designation asc) operates preferentially in an exchange mode and is functionally absent from erythrocytes of transport-negative-type horses.     

Expression of amino acid transporter genes in developmental stages and adult tissues of Antarctic echinoderms

Epithelial cells in the body wall of adult and developmental stages of marine invertebrates absorb dissolved organic material directly from seawater. Despite over a century of study, little is known about the molecular biological mechanisms responsible for this transport process. Previous studies on embryonic and larval Antarctic echinoderms show that amino acid uptake could provide an important supplement of metabolic substrates. In the present study, partial cDNA sequences of 11 putative amino acid transporter genes were isolated from six species of Antarctic echinoderms including the Antarctic sea stars Acodontaster hodgsoni, Diplasterias brucei, Odontaster meridionalis, Odontaster validus, and Perknaster fuscus, and the Antarctic sea urchin Sterechinus neumayeri. Conserved domains of cDNA-deduced amino acid sequences characterized these genes as being members of a family of amino acid transporters (solute carrier family 6). Expression of these genes was detected throughout embryonic and larval development of two species that have contrasting developmental modes (A. hodgsoni: lecithotrophic; O. meridionalis: planktotrophic). In all six species studied, the expression of amino acid transporter genes was detected in tube feet and digestive organs of adult animals, demonstrating that members of a single amino acid transporter gene family are expressed during the entire life history of a marine invertebrate. The identification of these genes is an important step toward developing a mechanistic understanding of amino acid transport capacities in Antarctic marine invertebrates.     

Intrinsic asymmetries of amino acid enantiomers and their peptides: a possible role in the origin of biochirality

L-amino acids and D-carbohydrates were incorporated into the first forms of life over 3.5 billion years ago, presumably from racemic mixtures of organic solutes produced by abiotic synthetic pathways. The process by which this choice occurred has not been established, but a consensus view is that it was a chance event, such that life could equally well have used D-amino acids and L sugars. In this review we will explore a second, less plausible alternative that minute differences in the physical properties of certain enantiomers made it more likely that L-amino acids and D-carbohydrates would be incorporated into early life. By all classical criteria, chiral isomers are perfect mirror image structures and, therefore, are expected to be identical in their macroscopic properties. However, scattered reports in the literature suggest that there may be slight differences in the physical properties of L- and D-amino acids and their polymers, which could lead to a preferred incorporation of L-amino acids into primitive forms of life. Here we present a literature survey of this issue and discuss its possible role in the origin of biochirality.