4F2 (CD98) heavy chain is associated covalently with an amino acid transporter and controls intracellular trafficking and membrane topology of 4F2 heterodimer

Differential expression of system L amino acid transporters during wound healing process in the skin of young and old rats

In order to elucidate the role of the system L-type amino acid transporters (LATs) in the wound healing process of aged and young subjects, we investigated the expression of LAT1, LAT2 and their subunit 4F2hc in the skin healing process after artificial wounds of dorsal skin in the young and old rats.

METHODS

The 1 cm full-thickness incisional wounds were made through the skin and panniculus carnosus muscle. The wounds were harvested at days 1, 3, 5 and 7 post-wounding, the experimental controls were harvested the skin of rat without wounds and the various analyses were performed.

RESULTS

In young rats, gradually and noticeable wound healing was detected, however, in old rats, wound healing was found to be greatly delayed. In young rats, the expression of LAT1 was increased rapidly on the day 1 after wound induction, on the other hand, in old rats, the expression of LAT1 after wound induction was not different from the control group. In young rats, the expression of LAT2 after the induction of wound was not different from the control group, however in old rats, the expression of LAT2 on the day 1 of wound induction was rapidly elevated.

CONCLUSION

These results suggest that the LAT1 and LAT2 increase in the wound healing process after cell injury in young and old rats, respectively.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Aminoacidurias: Clinical and molecular aspects

Inherited aminoacidurias are caused by defective amino-acid transport through renal (reabsorption) and in many cases also small intestinal epithelia (absorption). Recently, many of the genes causing this abnormal transport have been molecularly identified. In this review, we summarize the latest findings in the clinical and molecular aspects concerning the principal aminoacidurias, cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. Signs, symptoms, diagnosis, treatment, causative or candidate genes, functional characterization of the encoded transporters, and animal models are discussed.

Pharmacology of Glutamate Transport in the CNS: Substrates and Inhibitors of Excitatory Amino Acid Transporters (EAATs) and the Glutamate/Cystine Exchanger System x c −

As the primary excitatory neurotransmitter in the mammalian CNS, l-glutamateparticipates not only in standard fast synaptic communication, but also contributes to higher order signalprocessing, as well as neuropathology. Given this variety of functional roles, interest has been growingas to how the extracellular concentrations of l-glutamate surroundingneurons are regulated by cellular transporter proteins. This review focuses on two prominent systems, eachof which appears capable of influencing both the signaling and pathological actions of l-glutamatewithin the CNS: the sodium-dependent excitatory amino acid transporters (EAATs) and the glutamate/cystineexchanger, system x_c⁻(Sx_c⁻). Whilethe family of EAAT subtypes limit access to glutamate receptors by rapidly and efficiently sequesteringl-glutamate in neurons and glia, Sx_c⁻provides a route for the export of glutamate from cells into the extracellular environment. The primaryintent of this work is to provide an overview of the inhibitors and substrates that have been developedto delineate the pharmacological specificity of these transport systems, as well as be exploited as probeswith which to selectively investigate function. Particular attention is paid to the development of smallmolecule templates that mimic the structural properties of the endogenous substrates, l-glutamate,l-aspartate and l-cystine andhow strategic control of functional group position and/or the introduction of lipophilic R-groups can impactmultiple aspects of the transport process, including: subtype selectivity, inhibitory potency, and substrateactivity.

Cyclophilin B induces integrin-mediated cell adhesion by a mechanism involving CD98-dependent activation of protein kinase C-delta and p44/42 mitogen-activated protein kinases

Initially identified as a cyclosporin-A binding protein, cyclophilin B (CyPB) is an inflammatory mediator that induces adhesion of T lymphocytes to fibronectin, by a mechanism dependent on CD147 and alpha 4 beta 1 integrins. Recent findings have suggested that another cell membrane protein, CD98, may cooperate with CD147 to regulate beta1 integrin functions. Based on these functional relationships, we examined the contribution of CD98 in the pro-adhesive activity of CyPB, by utilizing the responsive promonocyte cell line THP-1. We demonstrated that cross-linking CD98 with CD98-AHN-18 antibody mimicked the responses induced by CyPB, i.e. homotypic aggregation, integrin-mediated adhesion to fibronectin and activation of p44/42 MAPK. Consistent with previous data, immunoprecipitation confirmed the existence of a heterocomplex wherein CD147, CD98 and beta1 integrins were associated. We then demonstrated that CyPB-induced cell adhesion and p44/42 MAPK activation were dependent on the participation of phosphoinositide 3-kinase and subsequent activation of protein kinase C-delta. Finally, silencing the expression of CD98 by RNA interference potently reduced CyPB-induced cell responses, thus confirming the role of CD98 in the pro-adhesive activity of CyPB. Altogether, our results support a model whereby CyPB induces integrin-mediated adhesion via interaction with a multimolecular unit formed by the association between CD147, CD98 and beta1 integrins.

xCT expression reduces the early cell cycle requirement for calcium signaling

Calcium has long been recognized as an important regulator of cell cycle transitions although the mechanisms are largely unknown. A functional genomic screen has identified genes involved in the regulation of early cell cycle progression by calcium. These genes when overexpressed confer the ability to bypass the G1/S arrest induced by Ca(2+)-channel antagonists in mouse fibroblasts. Overexpression of the cystine-glutamate exchanger, xCT, had the greatest ability to evade calcium antagonist-induced cell cycle arrest. xCT carries out the rate limiting step of glutathione synthesis in many cell types and is responsible for the uptake of cystine in most human cancer cell lines. Functional analysis indicates that the cystine uptake activity of xCT overcomes the G1/S arrest induced by Ca(2+)-channel antagonists by bypassing the requirement for calcium signaling. Since cells overexpressing xCT were found to have increased levels and activity of the AP-1 transcription factor in G1, redox stimulation of AP-1 activity accounts for the observed growth of these cells in the presence of calcium channel antagonists. These results suggest that reduced calcium signaling impairs AP-1 activation and that xCT expression may directly affect cell proliferation.

A plant triterpenoid, avicin D, induces autophagy by activation of AMP-activated protein kinase

Avicins, a family of plant triterpene electrophiles, can trigger apoptosis-associated tumor cell death, and suppress chemical-induced carcinogenesis by its anti-inflammatory, anti-mutagenic, and antioxidant properties. Here, we show that tumor cells treated with benzyloxycarbonylvalyl-alanyl-aspartic acid (O-methyl)-fluoro-methylketone, an apoptosis inhibitor, and Bax(-/-)Bak(-/-) apoptosis-resistant cells can still undergo cell death in response to avicin D treatment. We demonstrate that this non-apoptotic cell death is mediated by autophagy, which can be suppressed by chloroquine, an autophagy inhibitor, and by specific knockdown of autophagy-related gene-5 (Atg5) and Atg7. Avicin D decreases cellular ATP levels, stimulates the activation of AMP-activated protein kinase (AMPK), and inhibits mammalian target of rapamycin (mTOR) and S6 kinase activity. Suppression of AMPK by compound C and dominant-negative AMPK decreases avicin D-induced autophagic cell death. Furthermore, avicin D-induced autophagic cell death can be abrogated by knockdown of tuberous sclerosis complex 2 (TSC2), a key mediator linking AMPK to mTOR inhibition, suggesting that AMPK activation is a crucial event targeted by avicin D. These findings indicate the therapeutic potential of avicins by triggering autophagic cell death.

The structure of human 4F2hc ectodomain provides a model for homodimerization and electrostatic interaction with plasma membrane

4F2hc (CD98hc) is a multifunctional type II membrane glycoprotein involved in amino acid transport and cell fusion, adhesion, and transformation. The structure of the ectodomain of human 4F2hc has been solved using monoclinic (Protein Data Bank code 2DH2) and orthorhombic (Protein Data Bank code 2DH3) crystal forms at 2.1 and 2.8 A, respectively. It is composed of a (betaalpha)(8) barrel and an antiparallel beta(8) sandwich related to bacterial alpha-glycosidases, although lacking key catalytic residues and consequently catalytic activity. 2DH3 is a dimer with Zn(2+) coordination at the interface. Human 4F2hc expressed in several cell types resulted in cell surface and Cys(109) disulfide bridge-linked homodimers with major architectural features of the crystal dimer, as demonstrated by cross-linking experiments. 4F2hc has no significant hydrophobic patches at the surface. Monomer and homodimer have a polarized charged surface. The N terminus of the solved structure, including the position of Cys(109) residue located four residues apart from the transmembrane domain, is adjacent to the positive face of the ectodomain. This location of the N terminus and the Cys(109)-intervening disulfide bridge imposes space restrictions sufficient to support a model for electrostatic interaction of the 4F2hc ectodomain with membrane phospholipids. These results provide the first crystal structure of heteromeric amino acid transporters and suggest a dynamic interaction of the 4F2hc ectodomain with the plasma membrane.

Genetic polymorphisms in the amino acid transporters LAT1 and LAT2 in relation to the pharmacokinetics and side effects of melphalan

OBJECTIVES

Melphalan is widely used in the treatment of multiple myeloma. Pharmacokinetics of this alkylating drug shows high inter-individual variability. As melphalan is a phenylalanine derivative, the pharmacokinetic variability may be determined by genetic polymorphisms in the L-type amino acid transporters LAT1 (SLC7A5) and LAT2 (SLC7A8).

METHODS

Pharmacokinetics were analysed in 64 patients after first administration of intravenous melphalan. Severity of side effects was documented according to WHO criteria. Genomic DNA was analysed for polymorphisms in LAT1 and LAT2 by sequencing of the entire coding region, intron-exon boundaries and 2 kb upstream promoter region. Selected polymorphisms in the common heavy chain of both transporters, the protein 4F2hc (SLC3A2), were analysed by single nucleotide primer extension.

RESULTS

Melphalan pharmacokinetics was highly variable with up to 6.2-fold differences in total clearance. A total of 44 polymorphisms were identified in LAT1 and 21 polymorphisms in LAT2. From all variants, only five were in the coding region and only one heterozygous non-synonymous polymorphism (Ala94Thr) was found in LAT2. Numerous polymorphisms were found in the LAT1 and LAT2 5'-flanking regions but did not correlate with expression of the respective genes. No significant correlations could be observed between the polymorphisms in 4F2hc, LAT1, and LAT2 with melphalan pharmacokinetics or with melphalan side effects.

CONCLUSIONS

The study confirmed that these transporter genes are highly conserved, particularly in the coding sequences. Genetic variation in 4F2hc, LAT1, and LAT2 does not appear to be a major cause of inter-individual variability in pharmacokinetics and of adverse reactions to melphalan.

Polyamines upregulate the mRNA expression of cationic amino acid transporter-1 in human retinal pigment epithelial cells

We previously showed that ornithine was mainly transported via cationic amino acid transporter (CAT)-1 in human retinal pigment epithelial (RPE) cell line, human telomerase RT (hTERT)-RPE, and that CAT-1 was involved in ornithine cytotoxicity in ornithine-δ-aminotransferase (OAT)-deficient cell produced by a OAT specific inhibitor, 5-fluoromethylornithine (5-FMO). We showed here that CAT-1 mRNA expression was increased by ornithne in OAT-deficient RPE cells, which was reversed by an inhibitor of ornithine decarboxylase (ODC), α-difluoromethylornithine (DFMO). Polyamines, especially spermine, one of the metabolites of ODC, also enhanced the expression of CAT-1 mRNA. ODC mRNA expression was also increased by ornithine and polyamines, and gene silencing of ODC by siRNA decreased ornithine transport activity and its cytotoxicity. In addition, the mRNA of nuclear protein c-myc was also increased in 5-FMO- and ornithine-treated hTERT-RPE cells, and gene silencing of c-myc prevented the induction of CAT-1 and ODC. Increases in expression of CAT-1, ODC, and c-myc, and the inhibition of these stimulated expression by DFMO were also observed in primary porcine RPE cells. These results suggest that spermine plays an important role in stimulation of mRNA expression of CAT-1, which is a crucial role in ornithine cytotoxicity in OAT-deficient hTERT-RPE cells.

Tissue uptake of thyroid hormone by amino acid transporters

Thyroid hormones (THs) -- thyroxine (T4) and tri-iodothyronine (T3) -- are iodinated derivatives of the amino acid tyrosine, which regulates growth, development and critical metabolic functions. THs are taken up by target cells and act at the genomic level via nuclear thyroid receptors. Saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane. System L1 permease is a transporter of THs and amino acids in mammalian adipose tissue, placenta and brain. T(3) is also a substrate of a putative System T transporter, which is selective for aromatic amino acids. The activity and functional mechanisms of these transporters can be crucial to cells in determining both their hormone sensitivity and their responses to change in circulating hormone concentrations or availability of competing substrates (e.g. amino acids). TH transporters are potentially important pharmacological targets in the design of novel or improved therapies for thyroid-related disorders.

High-salt intake and the renal expression of amino acid transporters in spontaneously hypertensive rats

This study evaluated in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) the response to salt loading of the renal dopaminergic system and transcript abundance of Na+-independent (LAT1 and LAT2) and Na+-dependent (ASCT2 and B0AT1) amino acid transporters potentially involved in renal tubular uptake of l-DOPA. Rats were fed normal (NS)- or high (HS; 1% saline as drinking water)-salt intake for 24 h. Transcript abundance of amino acid transporters was age dependent, differently regulated in WKY and SHR and responded differently to salt intake. HS intake similarly increased urinary dopamine in 4-wk-old SHR and WKY. At 12 wk of age, HS intake increased urinary dopamine in SHR, but not in WKY. Changes in urinary dopamine paralleled changes in the uptake of l-DOPA in isolated renal tubules from 4- and 12-wk-old WKY and SHR on NS and HS intake. At 12 wk of age, HS intake was accompanied by decreases in LAT1 and LAT2 transcript abundance in WKY and SHR. ASCT2 and B0AT1 expression was significantly decreased in both 4- and 12-wk-old WKY and in 4-wk-old SHR on HS intake. By contrast, HS intake increased ASCT2 and B0AT1 expression in 12-wk-old SHR. It is concluded that salt-sensitive mechanisms influence LAT1, LAT2, ASCT2, and B0AT1 gene transcription. Differences in urinary dopamine and tubular uptake of l-DOPA between WKY and SHR during HS intake, namely in 12-wk-old animals, may result from increases in the ASCT2 and B0AT1 mRNA levels and less pronounced decreases in LAT2 expression.

Membrane trafficking of CD98 and its ligand galectin 3 in BeWo cells − implication for placental cell fusion

CD98 heavy chain (CD98hc), expressed at high levels in developing human trophoblasts, is an integral membrane protein with multiple N-linked glycosylation sites and known to be important for cell fusion, adhesion, and amino acid transport. Western blotting and flow cytometry were used to study the effect of brefeldin A, an inhibitor of protein translocation through the Golgi, on CD98hc in the human placental trophoblast cell line BeWo. Although brefeldin A treatment caused increased cell surface expression of CD98hc, a novel partially glycosylated form of the protein was found and, concomitantly, cell fusion was reduced. Western blotting showed that CD98 and galectin 3, a proposed ligand for the glycosylated extracellular domain of CD98hc, co-immunoprecipitated, and double-label immuno-electron microscopy confirmed that CD98hc associated with galectin 3. Furthermore, cell fusion was reduced (specifically) by the disaccharide lactose, a known ligand for the carbohydrate recognition domain of galectin 3, suggesting that the association was functional. Taken together, the data suggest that N-glycosylation of CD98 and subsequent interaction with galectin 3 is critical for aspects of placental cell biology, and provides a rationale for the observation that, in the mouse, truncation of the CD98hc extracellular domain leads to early embryonic lethality [Tsumura H, Suzuki N, Saito H, Kawano M, Otake S, Kozuka Y, Komada H, Tsurudome M & Ito Y (2003) Biochem Biophys Res Commun 308, 847-851].

Extracellular Interaction between hCD98 and the PDZ Class II Domain of hCASK in Intestinal Epithelia

The extracellular domain of the glycoprotein-associated integrin hCD98 protrudes into the basolateral extracellular space of the intestine and contains a PDZ class II-binding domain (GLLLRFPYAA, amino acids 520-529). Protein-protein interaction studies in vitro as well as in human colonic sections and Caco2-BBE cells have revealed that hCD98 coimmunoprecipitated with the basolateral membrane-associated guanylate kinase hCASK and that this interaction occurred in a PDZ domain-dependent manner. These novel results, which provide the first evidence for a PDZ domain-dependent interaction between a membrane protein and an extracellular protein, open a new field of investigation related to extracellular signaling in cell biology.

Differential effect of cross-linking the CD98 heavy chain on fusion and amino acid transport in the human placental trophoblast (BeWo) cell line

CD98 (otherwise known as 4F2) is an integral membrane protein with multiple functions including amino acid transport, integrin activation, cell fusion and cell activation. The molecular mechanisms coordinating these multiple functions remain unclear. We have studied CD98 heavy chain (hc) function in a human placental trophoblast cell line (BeWo). We show that cross-linking of CD98hc by incubation of cells in the presence of functional monoclonal antibodies causes cellular re-distribution of the protein from the cytoplasm to the plasma membrane as measured by flow cytometry, western blotting and quantitative immuno-electron microscopy. The latter technique also indicated that CD98hc is trafficked between cell surface and cytoplasmic pools in vesicles. Increased cell surface CD98 correlates with increased cellular fusion in BeWo cells. In addition, we show reduced LAT 1 surface expression and neutral amino acid transport in the presence of the CD98 mabs. The results thus suggest that the function of CD98 in cell fusion is distinct from its role in cellular nutrient delivery.

L-Type Amino Acid Transporter-1 Expressed in Human Astrocytomas, U343MGa

LAT1 (L-type amino acid transporter 1), one of the L-type amino acid transporters, transports the branched and aromatic amino acids. LAT1 requires the heavy chain of 4F2 antigen (4F2hc) for the functional expression as an amino acid transporter. The expression of this transporter is up-regulated in tumor cells and rapidly-growing cells to support their proliferation. Here, we studied the expression of LAT1 and 4F2hc in human cultured cells by real-time PCR and Western blot, and found that human brain astrocytomas, U343MGa, highly expressed LAT1 and 4F2hc mRNAs and proteins. The uptake of [14C]leucine by U343MGa cells is Na+-independent and inhibited by BCH (2-amino-2-norbornane carboxylic acid), and branched and aromatic amino acids, indicating that the LAT1 is expressed at the cell surface. Pulse chase labeling and surface labeling experiments of this cell line indicate that the protein synthesis of LAT1 and 4F2hc is slow, however, the heterodimeric complex assembled in the cells is very stable, and that the disulfide bond between the LAT1 and 4F2hc is not directly involved in the stability of the heterodimer.

L-type amino acid transporter 1 as a potential molecular target in human astrocytic tumors

L-type amino acid transporter 1 (LAT1) is a Na+-independent neutral amino acid transport agency and essential for the transport of large neutral amino acids. LAT1 has been identified as a light chain of the CD98 heterodimer from C6 glioma cells. LAT1 also corresponds to TA1, an oncofetal antigen that is expressed primarily in fetal tissues and cancer cells. We have investigated for the first time, the expression of the transporter in the human primary astrocytic tumor tissue from 60 patients. LAT1 is unique because it requires an additional single membrane spanning protein, the heavy chain of 4F2 cell surface antigen (4F2hc), for its functional expression. 4F2hc expression was also determined by immunohistochemistry. Kaplan-Meier analyses demonstrated that high LAT1 expression correlated with poor survival for the study group as a whole (p<0.0001) and for those with glioblastoma multiforme in particular (p=0.0001). Cox regression analyses demonstrated that LAT1 expression was one of significant predictors of outcome, independent of all other variables. On the basis of these findings, we also investigated the effect of the specific inhibitor to LAT1, 2-aminobicyclo-2 (2,2,1)-heptane-2-carboxylic acid (BCH), on the survival of C6 glioma cells in vitro and in vivo using a rat C6 glioma model. BCH inhibited the growth of C6 glioma cells in vitro and in vivo in a dose-dependent manner. Kaplan-Meier survival data of rats treated with BCH were significant. These findings suggest that LAT1 could be one of the molecular targets in glioma therapy.

Pharmacokinetics of Gabapentin during Delivery, in the Neonatal Period, and Lactation: Does a Fetal Accumulation Occur during Pregnancy?

PURPOSE

To study the pharmacokinetics of gabapentin (GBP) during delivery, lactation, and in the neonatal period.

METHODS

GBP concentrations in plasma and breast milk were determined with high-performance liquid chromatography in samples from six women treated with GBP and in their offspring. Blood samples were obtained at delivery from mothers, from the umbilical cord, and from the newborns on three occasions during 2 days after delivery. GBP concentration also was determined in breast milk and in blood collected from five of the mothers and suckling infants 2 weeks to 3 months after birth.

RESULTS

The umbilical cord/maternal plasma concentration ratios ranged from 1.3 to 2.1 (mean, 1.7). GBP plasma concentrations in the neonates declined with an estimated half-life of 14 h. Mean GBP plasma concentrations in the infants were 27% of the cord plasma levels (range, 12-36%) 24 h postpartum. The mean milk/maternal plasma concentration ratio was 1.0 (range, 0.7-1.3) from 2 weeks to 3 months. The infant dose of GBP was estimated to 0.2-1.3 mg/kg/day, equivalent to 1.3-3.8% of the weight-normalized dose received by the mother. The plasma concentrations in the breast-fed infants were approximately 12% of the mother's plasma levels, but no adverse effects were observed.

CONCLUSIONS

Our limited observations suggest an active transplacental transport of GBP, with accumulation in the fetus as a consequence. We suggest that this could be by the specific L-type amino acid transporter 1 (LAT-1), which is expressed in the placenta. Newborns seem to have a slightly lower capacity to eliminate GBP than do adults. Transfer of GBP to breast milk is extensive, but plasma concentrations appear to be low in suckling infants. No adverse effects were observed in the newborn. Although more data are needed, our observations suggest that breastfeeding in conjunction with GBP treatment is safe.

Thyroid hormone transporters in health and disease

Cellular entry is required for conversion of thyroid hormone by the intracellular deiodinases and for binding of 3,3',5-triiodothyronine (T(3)) to its nuclear receptors. Recently, several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have demonstrated that organic anion transporters (e.g., OATPs), and L-type amino acid transporters (LATs) facilitate thyroid hormone uptake. Among these, OATP1C1 has a high affinity and specificity for thyroxine (T(4)). OATP1C1 is expressed in capillaries throughout the brain, suggesting it is critical for transport of T(4) over the blood-brain barrier. We have also characterized a member of the monocarboxylate transporter family, MCT8, as a very active and specific thyroid hormone transporter. Human MCT8 shows preference for T(3) as the ligand. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. The MCT8 gene is located on the X chromosome. Recently, mutations in MCT8 have been found to be associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels.

Reabsorption of neutral amino acids mediated by amino acid transporter LAT2 and TAT1 in the basolateral membrane of proximal tubule

In order to understand the renal reabsorption mechanism of neutral amino acids via amino acid transporters, we have isolated human L-type amino acid transporter 2 (hLAT2) and human T-type amino acid transporter 1 (hTAT1) in human, then, we have examined and compared the gene structures, the functional characterizations and the localization in human kidney. Northern blot analysis showed that hLAT2 mRNA was expressed at high levels in the heart, brain, placenta, kidney, spleen, prostate, testis, ovary, lymph node and the fetal liver. The hTAT1 mRNA was detected at high levels in the heart, placenta, liver, skeletal muscle, kidney, pancreas, spleen, thymus and prostate. Immunohistochemical analysis on the human kidney revealed that the hLAT2 and hTAT1 proteins coexist in the basolateral membrane of the renal proximal tubules. The hLAT2 transports all neutral amino acids and hTAT1 transports aromatic amino acids. The basolateral location of the hLAT2 and hTAT1 proteins in the renal proximal tubule as well as the amino acid transport activity of hLAT2 and hTAT1 suggests that these transporters contribute to the renal reabsorption of neutral and aromatic amino acids in the basolateral domain of epithelial proximal tubule cells, respectively. Therefore, LAT2 and TAT1 play essential roles in the reabsorption of neutral amino acids from the epithelial cells to the blood stream in the kidney. Because LAT2 and TAT1 are essential to the efficient absorption of neutral amino acids from the kidney, their defects might be involved in the pathogenesis of disorders caused by a disruption in amino acid absorption such as blue diaper syndrome.

Activation of epithelial CD98 glycoprotein perpetuates colonic inflammation

Anomalies in the regulation and function of integrins have been implicated in the etiology of various pathologic conditions, including inflammatory disorders such as irritable bowel disease. Several classes of cell surface glycoproteins such as CD98 have been shown to play roles in integrins-mediated events. Here, we investigated the role of CD98 in intestinal inflammation using both in vivo and in vitro approaches. We found that in Caco2-BBE monolayers and colonic tissues, expression of CD98 was upregulated by the proinflammatory cytokine, interferon gamma (INF gamma). Furthermore, CD98 was highly upregulated in colonic tissues from mice with active colitis induced by dextran sodium sulfate (DSS), but not in DSS-treated INF gamma -/- mice. Administration of an anti-CD98 antibody worsened DSS-induced colitis in mice but had no effect on untreated control mice. Finally, we used Caco2-BBE cell monolayers to model intestinal epithelial wound healing, and found that activation of epithelial CD98 in DSS-treated monolayers inhibited monolayer reconstitution, but had no affect on untreated control monolayers. Our data collectively indicate that (i) CD98 upregulation is mediated by INF gamma during intestinal inflammation and (ii) activation of epithelial CD98 protein aggravates intestinal inflammation by reducing intestinal epithelial reconstitution. Overall, our data suggest that epithelial CD98 plays an important role in the perpetuation of intestinal inflammation.

System L-amino acid transporters are differently expressed in rat astrocyte and C6 glioma cells

The system L-amino acid transporter is a major nutrient transport system that is responsible for Na+-independent transport of neutral amino acids including several essential amino acids. We have compared and examined the expressions and functions of the system L-amino acid transporters in both rat astrocyte cultures and C6 glioma cells. The rat astrocyte cultures expressed the l-type amino acid transporter 2 (LAT2) with its subunit 4F2hc, whereas the l-type amino acid transporter 1 (LAT1) was not expressed in these cells. The C6 glioma cells expressed LAT1 but not LAT2 with 4F2hc. The [14C]l-leucine uptakes by the rat astrocyte cultures and C6 glioma cells were Na+-independent and were completely inhibited by the system l selective inhibitor, BCH. These results suggest that the transport of neutral amino acids including several essential amino acids into rat astrocyte cultures and C6 glioma cells are for the most part mediated by LAT2 and LAT1, respectively. Therefore, the rat astrocyte cultures and C6 glioma cells are excellent tools for examining the properties of LAT2 and LAT1, respectively. Moreover, the specific inhibition of LAT1 in cancer cells might be a new rationale for anti-cancer therapy.

Identification of stereoselective transporters for S-nitroso-L-cysteine: role of LAT1 and LAT2 in biological activity of S-nitrosothiols

Many of the biological effects of nitric oxide are mediated by S-nitrosothiols. However, the mechanisms by which S-nitrosothiols transduce their activity across cell membranes are unclear. We show that the pathway responsible for the cellular effects of S-nitrosothiols is specific for S-nitrosocysteine (CSNO), is stereoselective, and requires direct uptake of intact L-CSNO. Transport is independent of extracellular sodium, competitively inhibited by leucine, and blocked by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, a specific inhibitor of the system L amino acid transporter family. Other nitrosothiols such as S-nitrosoglutathione are not substrates for transport and require reaction with L-cysteine for activity. To show that system L family members mediate uptake, we expressed two members, LAT1 and LAT2, in Xenopus oocytes. Both LAT1 and LAT2, when co-expressed with 4F2 heavy chain, were found to efficiently transport L-CSNO. Mammalian cells were shown to express LAT1 and LAT2. A431 cells express both proteins, whereas T24 cells express only LAT1. Overexpression of LAT1 in T24 cells using recombinant adenoviruses led to increased uptake of L-CSNO, whereas knockdown using a specific small interfering RNA led to decreased uptake. These data definitively identify LAT1 and LAT2 as members of system L that mediate transmembrane movement of l-CSNO and suggest that system L family members are involved in the cellular activity of small molecular weight nitrosothiols.

CD98 modulates integrin beta1 function in polarized epithelial cells

The type II transmembrane protein CD98, best known as the heavy chain of the heterodimeric amino acid transporters (HAT), is required for the surface expression and basolateral localization of this transporter complex in polarized epithelial cells. CD98 also interacts with beta1 integrins resulting in an increase in their affinity for ligand. In this study we explored the role of the transmembrane and cytoplasmic domains of CD98 on integrin-dependent cell adhesion and migration in polarized renal epithelial cells. We demonstrate that the transmembrane domain of CD98 was sufficient, whereas the five N-terminal amino acids of this domain were required for CD98 interactions with beta1 integrins. Overexpression of either full-length CD98 or CD98 lacking its cytoplasmic tail increased cell adhesion and migration, whereas deletion of the five N-terminal amino acids of the transmembrane domain of CD98 abrogated this effect. CD98 and mutants that interacted with beta1 integrins increased both focal adhesion formation and FAK and AKT phosphorylation. CD98-induced cell adhesion and migration was inhibited by addition of phosphoinositol 3-OH kinase (PI3-K) inhibitors suggesting these cell functions are PI3-K-dependent. Finally, CD98 and mutants that interacted with beta1, induced marked changes in polarized renal epithelial cell branching morphogenesis in collagen gels. Thus, in polarized renal epithelial cells, CD98 might be viewed as a scaffolding protein that interacts with basolaterally expressed amino acid transporters and beta1 integrins and can alter diverse cellular functions such as amino acid transport as well as cell adhesion, migration and branching morphogenesis.

Thyroid hormone transporters

Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.

Amino acid transport system L is differently expressed in human normal oral keratinocytes and human oral cancer cells

Previously, we reported the expression and function of system L amino acid transporter in KB human oral epidermoid carcinoma cells. In the present study, therefore, we investigated the expression and function of system L amino acid transporter in human normal oral keratinocytes (HNOK) and compared the expressions and functions of system L amino acid transporters in HNOK and KB cells. The HNOK expressed L-type amino acid transporter 1 (LAT1) and L-type amino acid transporter 2 (LAT2) with their subunit 4F2hc in the plasma membrane but the expression of LAT1 was very weak, which is in contrast to the KB cells expressing LAT1 but not LAT2 with the 4F2hc in the plasma membrane. The [14C] L-leucine uptake by HNOK, as well as KB cells, was inhibited by the system L selective inhibitor BCH. The majority of [14C] L-leucine uptake was, therefore, mainly mediated by LAT2 in the HNOK and by LAT1 in the KB cells. These results suggest that the transport of neutral amino acids including several essential amino acids into the HNOK and KB cells are mainly mediated by LAT2 and LAT1, respectively. The specific inhibition of LAT1 in oral cancer cells could be a new rationale for anti-cancer therapy.

L-type amino acid transporter-1 overexpression and melphalan sensitivity in Barrett's adenocarcinoma

The L-type amino acid transporter-1 (LAT-1) has been associated with tumor growth. Using cDNA microarrays, overexpression of LAT-1 was found in 87.5% (7/8) of esophageal adenocarcinomas relative to 12 Barrett's samples (33% metaplasia and 66% dysplasia) and was confirmed in 100% (28/28) of Barrett's adenocarcinomas by quantitative reverse transcription polymerase chain reaction. Immunohistochemistry revealed LAT-1 staining in 37.5% (24/64) of esophageal adenocarcinomas on tissue microarray. LAT-1 also transports the amino acid-related chemotherapeutic agent, melphalan. Two esophageal adenocarcinoma and one esophageal squamous cell line, expressing LAT-1 on Western blot analysis, were sensitive to therapeutic doses of melphalan (P <.001). Simultaneous treatment with the competitive inhibitor, BCH [2-aminobicyclo-(2,1,1)-heptane-2-carboxylic acid], decreased sensitivity to melphalan (P <.05). In addition, confluent esophageal squamous cultures were less sensitive to melphalan (P <.001) and had a decrease in LAT-1 protein expression. Tumors from two esophageal adenocarcinoma cell lines grown in nude mice retained LAT-1 mRNA expression. These results demonstrate that LAT-1 is highly expressed in a subset of esophageal adenocarcinomas and that Barrett's adenocarcinoma cell lines expressing LAT-1 are sensitive to melphalan. LAT-1 expression is also retained in cell lines grown in nude mice providing a model to evaluate melphalan as a chemotherapeutic agent against esophageal adenocarcinomas expressing LAT-1.

CD98hc (SLC3A2) interaction with beta 1 integrins is required for transformation

CD98hc (SLC3A2) constitutively and specifically associates with beta(1) integrins and is highly expressed on the surface of human tumor cells irrespective of the tissue of origin. We have found here that expression of CD98hc promotes both anchorage- and serum-independent growth. This oncogenic activity is dependent on beta(1) integrin-mediated phosphoinositol 3-hydroxykinase stimulation and the level of surface expression of CD98hc. Using chimeras of CD98hc and the type II membrane protein CD69, we show that the transmembrane domain of CD98hc is necessary and sufficient for integrin association in cells. Furthermore, CD98hc/beta(1) integrin association is required for focal adhesion kinase-dependent phosphoinositol 3-hydroxykinase activation and cellular transformation. Amino acids 82-87 in the putative cytoplasmic/transmembrane region appear to be critical for the oncogenic potential of CD98hc and provide a novel mechanism for tumor promotion by integrins. These results explain how high expression of CD98hc in human cancers contributes to transformation; furthermore, the transmembrane association of CD98hc and beta(1) integrins may provide a new target for cancer therapy.

The amino acid transporter asc-1 is not involved in cystinuria

BACKGROUND

The human amino acid transporter asc-1 (SLC7A10) exhibits substrate selectivity for small neutral amino acids, including cysteine, is expressed in kidney, is located close to the cystinuria B gene and presents sequence variants (e.g., E112D) in some cystinuria patients. We have cloned human asc-1, assessed its transport characteristics, localized its expression in kidney, searched for mutations in cystinuria patients, and tested the transport function of variant E112D.

METHODS

We used an EST-based homology cloning strategy. Transport characteristics of asc-1 were assessed by coexpression with 4F2hc in Xenopus oocytes and HeLa cells. Localization of asc-1 mRNA in kidney was assessed by in situ hybridization. Exons and intron-exon boundaries were polymerase chain reaction (PCR)-amplified from blood cell DNA and mutational screening was performed by single-stranded conformational polymorphism (SSCP).

RESULTS

Asc-1 reaches the plasma membrane in HeLa cells, unlike in oocytes, most probably by interaction with endogenous 4F2hc and presents similar transport characteristics to those in oocytes coexpressing asc-1/4F2hc. Asc-1 mediates a substantial efflux of alanine in a facilitated diffusion mode of transport. Expression of asc-1 mRNA localized to Henle's loop, distal tubules, and collecting ducts. Finally, SLC7A10 polymorphisms were identified in cystinuria probands and the SLC7A10 sequence variant E112D showed full transport activity.

CONCLUSION

The lack of expression of asc-1 in the proximal tubule indicates that it plays no role in the bulk of renal reabsorption of amino acids. No mutations causing cystinuria have been found in SLC7A10. The facilitated diffusion mode of transport and the expression in distal nephron suggest a role for asc-1 in osmotic adaptation.

Lectin Receptors for Endometrial H-type 1 Antigen on Goat Conceptuses

PROBLEM

The goal of this study was to determine if caprine conceptuses express lectin-like receptors for endometrial H-type 1 (HT1) antigen.

METHOD OF STUDY

Conceptus tissues were collected during the apposition, adhesion and attachment phases of placentation and evaluated using immunofluorescence microscopy.

RESULTS

Conceptus staining for the trisaccharide lacto-N-fucopentaose-1 was strong and uniform during apposition of fetal and maternal tissues but changed by day 25 of pregnancy when large aggregates of intense staining were observed. Monoclonal antibodies to galectin-3 did not stain conceptus tissue during the apposition phase but intense punctate staining was observed after day 25. Strong uniform staining for Lewis Y antigen was detected only on day 17 of pregnancy.

CONCLUSION

Conceptus tissue expresses potential receptors for endometrial HT1 antigen. Carbohydrate-lectin interactions may facilitate attachment of the apical surfaces of uterine epithelial cells and trophectoderm during the early stages of placentation.

Lysophosphatidylcholine enhances cytokine production of endothelial cells via induction of L-type amino acid transporter 1 and cell surface antigen 4F2

OBJECTIVE

A diverse range of lipid oxidation products detected in oxidized low-density lipoprotein (oxLDL) and atherosclerotic lesions are capable of eliciting biological responses in vascular cells. We performed DNA microarray experiments to explore novel responses of human umbilical vein endothelial cells (HUVECs) to oxLDL and its components.

METHODS AND RESULTS

cDNA microarray analysis showed that oxLDL, lysophosphatidylcholine (LysoPC), 4-hydroxy-2-nonenal, and oxysterols altered gene expression specifically, but some genes were commonly induced in HUVECs. Solute carrier family 3 member 2 and family 7 member 5, encoding the heavy chain of the cell surface antigen 4F2 (4F2hc) and the L-type amino acid transporter 1 (LAT1), respectively, were induced by oxLDL and many oxidation products. LAT1 requires 4F2hc to form a heterodimeric functional complex to transport neutral amino acids into the cell. LysoPC increased membrane protein levels of LAT1 confirmed by Western blot analysis and also uptake of L-[(14)C]leucine, which was inhibited by a competitive inhibitor for LAT1. The release of interleukin 6 (IL-6) and IL-8 was increased in LysoPC-treated cells and was attenuated by the LAT1 inhibitor.

CONCLUSIONS

These findings suggest that an increase in uptake of neutral amino acids induced by LysoPC results in enhancement of inflammatory responses of endothelial cells.

Expression and functional characterization of the system L amino acid transporter in KB human oral epidermoid carcinoma cells

We have examined the expression and function of system l amino acid transporter in KB human oral epidermoid carcinoma cells. The KB cells express l-type amino acid transporter 1 (LAT1) in plasma membrane, but not l-type amino acid transporter 2 (LAT2). The [14C]l-leucine uptake by KB cells is inhibited by system l selective inhibitor BCH. The majority of [14C]l-leucine uptake is, therefore, mediated by LAT1. These results suggest that the transport of neutral amino acids including several essential amino acids into the KB cells mediated by LAT1 and the specific inhibition of LAT1 in oral cancer cells will be a new rationale for anti-cancer therapy.

Developmental regulation of the rabbit blood-brain barrier LAT1 large neutral amino acid transporter mRNA and protein

The expression of the blood-brain barrier (BBB) LAT1 large neutral amino acid transporter mRNA and protein was investigated in development in rabbits. The BBB LAT1 mRNA was down-regulated with postnatal development. However, the BBB immunoreactive LAT1 protein was unchanged in postnatal development, despite an up-regulation of the BBB GLUT1 glucose transporter protein during this period. The dissociation between LAT1 protein and mRNA levels in development is consistent with posttranscriptional regulation of BBB LAT1 gene expression.

Thiol modification of cysteine 327 in the eighth transmembrane domain of the light subunit xCT of the heteromeric cystine/glutamate antiporter suggests close proximity to the substrate binding site/permeation pathway

We measured sensitivity to thiol modification of the heteromeric glutamate/cystine transporter 4F2hc-xCT expressed in Xenopus oocytes. p-Chloromercuribenzoate (pCMB) and p-chloromercuribenzenesulfonate (pCMBS) rapidly blocked transport activity. Cys(327), located in the middle of the eighth transmembrane domain of the light subunit (xCT), was found to be the main target of inactivation. Cysteine, an impermeant reducing reagent, reversed pCMB and pCMBS effects only when applied from the extracellular medium. l-Glutamate and l-cystine, but not l-arginine, protected from the inactivation with an IC(50) similar to the K(m). Protection was not temperature-dependent, suggesting that it did not depend on large substrate-induced conformational changes. Mutation of Cys(327) to Ala and Ser slightly modified the K(m) and a C327L mutant abolished transport function without compromising transporter expression at the plasma membrane. The results indicate that Cys(327) is a functionally important residue accessible to the aqueous extracellular environment and is structurally linked to the permeation pathway and/or the substrate binding site.

Platelet-derived growth factor stimulates LAT1 gene expression in vascular smooth muscle: role in cell growth

Platelet-derived growth factor (PDGF) contributes to vascular disease by stimulating the growth of vascular smooth muscle cells (SMCs). Since amino acids are required for cell growth, the present study examined the effect of PDGF on system L amino acid transport, which is the predominant cellular pathway for the uptake of essential amino acids. System L amino acid transport was monitored by measuring the uptake of L-leucine. Treatment of SMCs with PDGF stimulated L-leucine transport in a concentration- and time-dependent manner, and this was associated with a selective increase in LAT1 mRNA and protein. PDGF failed to induce the expression of the other system L transport proteins, LAT2 and the heavy chain of the 4F2 cell surface antigen. The induction of LAT1 by PDGF was dependent on de novo RNA and protein synthesis and on mTOR activity. Serum, thrombin, and angiotensin II likewise stimulated L-leucine transport by inducing LAT1 expression. Inhibition of system L amino acid transport by the model substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid blocked growth factor-mediated SMC proliferation and induced SMC apoptosis, whereas it had no effect on quiescent cells. These results demonstrate that growth factors stimulate system L amino acid transport by inducing LAT1 gene expression and that system L amino acid transport is essential for SMC proliferation and survival. The capacity of vascular mitogens to induce LAT1 expression may represent a basic mechanism by which tho acid transport * apoptosis

Identification of a novel system L amino acid transporter structurally distinct from heterodimeric amino acid transporters

A cDNA that encodes a novel Na+-independent neutral amino acid transporter was isolated from FLC4 human hepatocarcinoma cells by expression cloning. When expressed in Xenopus oocytes, the encoded protein designated LAT3 (L-type amino acid transporter 3) transported neutral amino acids such as l-leucine, l-isoleucine, l-valine, and l-phenylalanine. The LAT3-mediated transport was Na+-independent and inhibited by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, consistent with the properties of system L. Distinct from already known system L transporters LAT1 and LAT2, which form heterodimeric complex with 4F2 heavy chain, LAT3 was functional by itself in Xenopus oocytes. The deduced amino acid sequence of LAT3 was identical to the gene product of POV1 reported as a prostate cancer-up-regulated gene whose function was not determined, whereas it did not exhibit significant similarity to already identified transporters. The Eadie-Hofstee plots of LAT3-mediated transport were curvilinear, whereas the low affinity component is predominant at physiological plasma amino acid concentration. In addition to amino acid substrates, LAT3 recognized amino acid alcohols. The transport of l-leucine was electroneutral and mediated by a facilitated diffusion. In contrast, l-leucinol, l-valinol, and l-phenylalaninol, which have a net positive charge induced inward currents under voltage clamp, suggesting these compounds are transported by LAT3. LAT3-mediated transport was inhibited by the pretreatment with N-ethylmaleimide, consistent with the property of system L2 originally characterized in hepatocyte primary culture. Based on the substrate selectivity, affinity, and N-ethylmaleimide sensitivity, LAT3 is proposed to be a transporter subserving system L2. LAT3 should denote a new family of organic solute transporters.

Activation of CEA-CAM-1-mediated cell adhesion via CD98: involvement of PKCδ

CD98 is a multifunctional protein involved in amino acid transport and regulation of integrin-mediated cell adhesion. Herein, we demonstrated that CD98 stimulation by anti-CD98 antibodies induced CEA-CAM-1-mediated cell adhesion in BaF3 cells expressing CEA-CAM-1, and suggest that this might be responsible for compact clumping of F9 embryonic carcinoma cells by CD98 stimulation. CEA-CAM-1 was co-immunoprecipitated by anti-CD98 antibody. CD98 stimulation induced the translocation of cytoplasmic protein kinase Cdelta (PKCdelta) to the cell adhesion sites, and rottlerin that inhibited the PKCdelta translocation abolished the cell aggregation without affecting integrin activation. The results suggested that CD98 stimulation could activate CEA-CAM-1-mediated cell adhesion independently of integrins.

Identification, functional characterization and expression of a LAT type amino acid transporter from the mosquito Aedes aegypti

We isolated two cDNAs from the mosquito Aedes aegypti, an L-amino acid transporter (AeaLAT) and a CD98 heavy chain (AeaCD98hc). Expression of AeaCD98hc or AeaLAT alone in Xenopus oocyte did not induce amino acid transport activity. However, co-expression of AeaCD98hc and AeaLAT, which are postulated to form a heterodimer protein linked through a disulfide bond, showed significant increase in amino acid transport activity. This heterodimeric protein showed uptake specificity for large neutral and basic amino acids. Small acidic neutral amino acids were poor substrates for this transporter. Neutral amino acid (leucine) uptake activity was partially Na+ dependent, because leucine uptake was approximately 44% lower in the absence of Na+ than in its presence. However, basic amino acid (lysine) uptake activity was completely Na+ independent at pH of 7.4. Extracellular amino acid concentration could be the main factor that determined amino acid transport. These results suggest the heteromeric protein is likely a uniporter mediating diffusion of amino acids in the absence of ions. The AeaLAT showed high level expression in the gastric caeca, Malpighian tubules and hindgut of larvae. In caeca and hindgut expression was in the apical cell membrane. However, in Malpighian tubules and in midgut, the latter showing low level expression, the transporter was detected in the basolateral membrane. This expression profile supports the conclusion that this AeaLAT is a nutrient amino acid transporter.

Molecular features, regulation, and function of monocarboxylate transporters: implications for drug delivery

The diffusion of monocarboxylates such as lactate and pyruvate across the plasma membrane of mammalian cells is facilitated by a family of integral membrane transport proteins, the monocarboxylate transporters (MCTs). Currently, at least eight unique members of the MCT family have been discovered and orthologs to each have been identified in a variety of species. Four MCTs (MCT1-MCT4) have been functionally characterized. Each isoform possesses unique biochemical properties such as kinetic constants and sensitivity to known MCT inhibitors. Several fold changes in the expression of MCTs may be evoked by altered physiological conditions, yet the molecular mechanisms underlying the regulation of MCTs are poorly understood. Post-translational regulation of MCT1 and MCT4 occurs, in part, by interaction with CD147, an accessory protein that is necessary for trafficking, localization, and functional expression of these transporters. Because of the physiological importance of monocarboxylates to the overall maintenance of metabolic homeostasis, the function of MCTs is significant to several pathologies that occur with disease, such as ischemic stroke and cancer. Finally, the expression of MCT1 in the epithelium of the small intestine and colon and in the blood-brain barrier may provide routes for the intestinal and blood to brain transfer of carboxylated pharmaceutical agents and other exogenous monocarboxylates.

Regulation of L-leucine transport in rat kidney by dexamethasone and triiodothyronine

We have investigated the transport mechanisms involved in the stimulation of renal tubular reabsorption of large amino acids by glucocorticoids in vivo through the examination of activity and expression of specific transport systems L and y(+)L for L-leucine in membrane preparations of rat kidneys. Kidneys were removed from adult female Wistar rats treated with dexamethasone or triiodothyronine, and the fractions of brush-border and basolateral membranes were isolated by density gradient centrifugation. Functional analysis of L-leucine uptake using rapid filtration technique revealed induction of a sodium-dependent, arginine-inhibitable system y(+)L transport component in the basolateral membrane in the dexamethasone-treated group. A minor sodium-independent, BCH-inhibitable, system L transport component was unaffected by glucocorticoids. L-leucine uptake remained unaffected in the triiodothyronine-treated group. Expression of both subunits of the system y(+)L transporter was increased in dexamethasone-treated rat kidneys: Western blot analysis showed a significant (46%) increase of 4F2hc protein abundance in the basolateral membrane fraction and competitive RT-PCR revealed an almost 4-times induced expression of y(+)LAT1 mRNA. Our results indicate that system y(+)L in rat kidney is regulated by glucocorticoids. We suggest that enhancement of both 4F2 heavy chain and y(+)LAT1 light chain is necessary for induction of this transport system in the kidney.

Hypoxia induces de-stabilization of the LAT1 large neutral amino acid transporter mRNA in brain capillary endothelial cells

Blood-brain barrier (BBB) transport of large neutral amino acids is mediated by the large neutral amino acid transporter type 1 (LAT1 transporter). Although the gene encoding the Glut1 glucose transporter is up-regulated in hypoxia, the response of the LAT1 gene to hypoxia is not known. The present study investigates the changes in the LAT1 mRNA in cultured bovine brain capillary endothelial cells exposed to 1% O2 for 24-48 h. The LAT1 mRNA was initially down-regulated in hypoxia with reciprocal changes in the Glut1 mRNA. No changes in the 4F2hc mRNA in hypoxia were observed. Hypoxia caused an initial de-stabilization of the LAT1 mRNA, and the t1/2 of the LAT1 mRNA in control and hypoxic cells was 6.4 +/- 0.5 and 2.4 +/- 0.1 h, respectively. To further explore post-transcriptional regulation of LAT1 gene expression, the polysome and cytosol fractions of the control and hypoxic endothelial cells were isolated, and LAT1 mRNA binding proteins were detected by ultraviolet light cross-linking. Whereas the cytosol contained no LAT1 mRNA binding proteins, the cell polysome fraction expressed several LAT1 mRNA binding proteins, including principal 40-, 70- and 80-kDa proteins. These studies are consistent with post-transcriptional de-stabilization of the LAT1 large neutral amino acid transporter in hypoxia.

Basolateral LAT-2 has a major role in the transepithelial flux of L-cystine in the renal proximal tubule cell line OK

During renal reabsorption, the amino acid transporters b(o,+) and y(+)L have a major role in the apical uptake of cystine and dibasic amino acids and in the basolateral efflux of dibasic amino acids, respectively. In contrast, the transporters responsible for the basolateral efflux of the apically transported cystine are unknown. This study shows the expression of system L and y(+)L transport activities in the basolateral domain of the proximal tubule-derived cell line OK and the cloning of the corresponding LAT-2 and y(+)LAT-1 cDNAs. Stable transfection with a LAT-2 antisense sequence demonstrated the specific role of LAT-2 in the basolateral system L amino acid exchange activity in OK cells. This partial reduction of LAT-2 expression decreased apical-to-basolateral trans-epithelial flux of cystine and resulted in a twofold to threefold increase in the intracellular content of cysteine. In contrast, the content of serine, threonine, and alanine showed a tendency to decrease, whereas other LAT-2 substrates were not affected. This demonstrates that LAT-2 plays a major specific role in the net basolateral efflux of cysteine and points to LAT-2 as a candidate gene to modulate cystine reabsorption.

Promoter analysis of the human SLC7A7 gene encoding y+L amino acid transporter-1 (y+LAT-1)

The human SLC7A7 gene on chromosome 14q11.2 encodes the y+L amino acid transporter-1 (y+LAT-1) protein that transports, together with the 4F2hc cell surface antigen, cationic amino acids through the basolateral membrane of epithelial cells in the small intestine and kidney. The SLC7A7 gene comprises 11 exons, but the first two are not translated. Mutations in the coding region of the SLC7A7 gene cause a rare autosomal disorder, lysinuric protein intolerance (LPI). We have now investigated the expression levels and putative 5' promoter elements of the SLC7A7. The 5' region of the first untranslated exon contains no TATA-box, Inr elements nor other classical promoter elements, but has instead other putative transcription factor binding sequences. The E-box and AP-2 elements were able to bind proteins in HEK293 cells and adult kidney tissue extracts, but not in fibroblasts. Using transient transfection and luciferase reporter gene studies, we showed that the first two introns located in the untranslated region contained transcriptional enhancer elements. Northern blot analysis showed low and equal SLC7A7 mRNA levels in the control and LPI patient fibroblastoid and lymphoblast cells.

Physical association and functional interaction between beta1 integrin and CD98 on human T lymphocytes

CD98 is a cell surface protein previously characterized as a cell activation marker, an amino acid transporter, and has recently been implicated in integrin-related functions. Integrins are cell surface proteins, important for homotypic cell aggregation, cell adhesion, and coactivation of T lymphocytes. We have previously shown that the anti-CD98 mAb 80A10, when coimmobilized with anti-CD3 mAb OKT3, is able to mediate human T cell coactivation that is inhibited by anti-beta1 integrin specific mAb 18D3. These results indicated a functional association of CD98 and beta1 integrin signaling but left open the question of a physical association. We now show the induction of homotypic aggregation through CD98 among human T cells and this aggregation was inhibited by anti-beta1 integrin mAb. Therefore, CD98-dependent lymphocyte proliferation and adhesion may involve integrins. Competitive binding assays and fluorescence colocalization analysis suggested that CD98 and beta1 integrin were physically associated. Differential extraction techniques and immunoprecipitations provided the first evidence that the alpha4beta1 integrin and CD98 are specifically associated on human T lymphocytes.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Involvement of the system L amino acid transporter on uptake of S-nitroso-L-cysteine, an endogenous S-nitrosothiol, in PC12 cells

Previously, we proposed that S-nitroso-L-cysteine, an endogenous S-nitrosothiol, was incorporated via the system L-like amino acid transporter(s) in rat brain slices. In this study, we investigated the effect of S-nitroso-L-cysteine on L-[3H]leucine uptake in PC12 cells (a neuronal cell line). L-[3H]Leucine uptake in PC12 cells was Na(+) independent and significantly inhibited by an inhibitor of system L and by L-phenylalanine, L-cysteine, L-methionine and L-leucine at 1 mM. The effects of L-alanine, L-serine and L-threonine were limited. S-Nitroso-L-cysteine, but not other nitric oxide compounds, inhibited L-[3H]leucine uptake, and this inhibitory effect was eliminated by washing with buffer. System L is composed of the 4F2 light chains (LAT1 or LAT2) and the heavy chain, and the transcripts of these components were detected in RNA from PC12 cells. These findings suggest that S-nitroso-L-cysteine is incorporated via the system L amino acid transporter and thus regulates cell responses in PC12 cells.

Characterization of the system L amino acid transporter in T24 human bladder carcinoma cells

System L is a major nutrient transport system responsible for the Na(+)-independent transport of large neutral amino acids including several essential amino acids. In malignant tumors, a system L transporter L-type amino acid transporter 1 (LAT1) is up-regulated to support tumor cell growth. LAT1 is also essential for the permeation of amino acids and amino acid-related drugs through the blood-brain barrier. To search for in vitro assay systems to examine the interaction of chemical compounds with LAT1, we have investigated the expression of system L transporters and the properties of [14C]L-leucine transport in T24 human bladder carcinoma cells. Northern blot, real-time quantitative PCR and immunofluorescence analyses have reveled that T24 cells express LAT1 in the plasma membrane together with its associating protein 4F2hc, whereas T24 cells do not express the other system L isoform LAT2. The uptake of [14C]L-leucine by T24 cells is Na(+)-independent and almost completely inhibited by system L selective inhibitor BCH. The profiles of the inhibition of [14C]L-leucine uptake by amino acids and amino acid-related compounds in T24 cells are comparable with those for the LAT1 expressed in Xenopus oocytes. The majority of [14C]L-leucine uptake is, therefore, mediated by LAT1 in T24 cells. Consistent with LAT1 in Xenopus oocytes, the efflux of preloaded [14C]L-leucine is induced by extracellularly applied substrates of LAT1 in T24 cells. This efflux measurement has been proven to be more sensitive than that in Xenopus oocytes, because triiodothyronine, thyroxine and melphalan were able to induce the efflux of preloaded [14C]L-leucine in T24 cells, which was not detected for Xenopus oocyte expression system. T24 cell is, therefore, proposed to be an excellent tool to examine the interaction of chemical compounds with LAT1.

rBAT-b(0,+)AT heterodimer is the main apical reabsorption system for cystine in the kidney

Mutations in the rBAT and b(0,+)AT genes cause type I and non-type I cystinuria, respectively. The disulfide-linked rBAT-b(0,+)AT heterodimer mediates high-affinity transport of cystine and dibasic amino acids (b(0,+)-like activity) in heterologous cell systems. However, the significance of this heterodimer for cystine reabsorption is unknown, as direct evidence for such a complex in vivo is lacking and the expression patterns of rBAT and b(0,+)AT along the proximal tubule are opposite. We addressed this issue by biochemical means. Western blot analysis of mouse and human kidney brush-border membranes showed that rBAT and b(0,+)AT were solely expressed as heterodimers of identical size and that both proteins coprecipitated. Moreover, quantitative immunopurification of b(0,+)AT followed by SDS-PAGE and mass spectrometry analysis established that b(0,+)AT heterodimerizes exclusively with rBAT. Together with cystine reabsorption data, our results demonstrate that a decreasing expression gradient of heterodimeric rBAT-b(0,+)AT along the proximal tubule is responsible for virtually all apical cystine reabsorption. As a corollary of the above, there should be an excess of rBAT expression over that of b(0,+)AT protein in the kidney. Indeed, complete immunodepletion of b(0,+)AT did not coprecipitate >20-30% of rBAT. Therefore, another rBAT-associated subunit may be present in latter parts of the proximal tubule.