The 4F2 antigen heavy chain induces uptake of neutral and dibasic amino acids in Xenopus oocytes

mTORC2-AKT-LAT1 signalling participates in methionine-induced β-CASEIN expression in mammary epithelial cells of dairy cows

This study investigated the role of the mammalian target of rapamycin complex 2 (mTORC2)-protein kinase B (AKT) signalling in methionine (Met)-induced L-type amino acid transporter 1 (LAT1) expression and milk protein production. Primary mammary epithelial cells (MECs) from mammary parenchymal tissues of three lactating cows and MAC-T bovine MECs were cultured with or without 0.6 mM Met. Rapamycin-insensitive companion of mTOR (RICTOR) siRNA, the mTORC1 inhibitor rapamycin and the AKT activator SC79 were used to evaluate the effects of mTORC2-AKT signalling on Met-induced LAT1 expression and function. Each experiment was performed three times. Data were analysed with a two-sided unpaired t test or ANOVA with the Bonferroni multiple-comparison test. Western blotting showed that Met stimulation increased RICTOR expression (~244.67%; p < 0.05; control, 0.15 ± 0.026; Met, 0.517 ± 0.109) and AKT-S473 levels (~281.42%; p < 0.01; control, 0.253 ± 0.067; Met, 0.965 ± 0.019) in both primary MECs and MAC-T cells. Rapamycin-induced mTORC1 signalling inhibition decreased only Met-induced β-CASEIN expression by ~21.24% (p < 0.01; Met, 0.777 ± 0.01; Met and rapamycin, 0.612 ± 0.04) and did not affect Met-stimulated AKT-S473 levels, suggesting that mTORC2-AKT activation upon Met stimulation also contributes to milk protein synthesis. LAT1 participates in Met-induced β-CASEIN expression. In dairy cow MECs, mTORC2 inhibition by RICTOR siRNA decreased LAT1 levels on the plasma membrane by ~45.13% (p < 0.01; control, 0.359 ± 0.006; siRICTOR, 0.197 ± 0.004). However, SC79-induced AKT activation had the opposite effect (p < 0.01). In primary MECs and MAC-T cells, Met stimulation increased cytosolic and plasma membrane LAT1 expression respectively (MECs, 113.98% and 58.43%; MAC-T, 165.85% and 396.39%; p < 0.05). However, RICTOR siRNA significantly reduced Met-induced plasma membrane LAT1 expression (~76.48%; Met, 0.539 ± 0.05; Met and siRICTOR, 0.127 ± 0.012; p < 0.05). Thus, Met increased LAT1 expression and function via mTORC2-AKT signalling, upregulating milk protein synthesis in dairy cow MECs.

N-glycosylation is crucial for trafficking and stability of SLC3A2 (CD98)

The type II glycoprotein CD98 (SLC3A2) is a membrane protein with pleiotropic roles in cells, ranging from modulation of inflammatory processes, host–pathogen interactions to association with membrane transporters of the SLC7 family. The recent resolution of CD98 structure in complex with LAT1 showed that four Asn residues, N365, N381, N424, N506, harbour N-glycosylation moieties. Then, the role of N-glycosylation on CD98 trafficking and stability was investigated by combining bioinformatics, site-directed mutagenesis and cell biology approach. Single, double, triple and quadruple mutants of the four Asn exhibited altered electrophoretic mobility, with apparent molecular masses from 95 to 70 kDa. The quadruple mutant displayed a single band of 70 kDa corresponding to the unglycosylated protein. The presence in the membrane and the trafficking of CD98 were evaluated by a biotinylation assay and a brefeldin assay, respectively. Taken together, the results highlighted that the quadruple mutation severely impaired both the stability and the trafficking of CD98 to the plasma membrane. The decreased presence of CD98 at the plasma membrane, correlated with a lower presence of LAT1 (SLC7A5) and its transport activity. This finding opens new perspectives for human therapy. Indeed, the inhibition of CD98 trafficking would act synergistically with LAT1 inhibitors that are under clinical trial for anticancer therapy.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Tissue-resident NK cells differ in their expression profile of the nutrient transporters Glut1, CD98 and CD71

Metabolism is a critical basis for immune cell functionality. It was recently shown that NK cell subsets from peripheral blood modulate their expression of nutrient receptors following cytokine stimulation, demonstrating that NK cells can adjust to changes in metabolic requirements. As nutrient availability in blood and tissues can significantly differ, we examined NK cells isolated from paired blood-liver and blood-spleen samples and compared expression of the nutrient transporters Glut1, CD98 and CD71. CD56^bright tissue-resident (CXCR6⁺) NK cells derived from livers and spleens expressed lower levels of Glut1 but higher levels of the amino acid transporter CD98 following stimulation than CD56^bright NK cells from peripheral blood. In line with that, CD56^dim NK cells, which constitute the main NK cell population in the peripheral blood, expressed higher levels of Glut1 and lower levels of CD98 and CD71 compared to liver CD56^bright NK cells. Our results show that NK cells from peripheral blood differ from liver- and spleen-resident NK cells in the expression profile of nutrient transporters, consistent with a cell-adaptation to the different nutritional environment in these compartments.

Intracellular boron accumulation in CHO-K1 cells using amino acid transport control

BPA used in BNCT has a similar structure to some essential amino acids and is transported into tumor cells by amino acid transport systems. Previous study groups have tried various techniques of loading BPA to increase intracellular boron concentration. CHO-K1 cells demonstrate system L (LAT1) activity and are suitable for specifying the transport system of a neutral amino acid. In this study, we examined the intracellular accumulation of boron in CHO-K1 cells by amino acid transport control, which involves co-loading with L-type amino acid esters. Intracellular boron accumulation in CHO-K1 cells showed the greatest increased upon co-loading 1.0mM BPA, with 1.0mM l-Tyr-O-Et and incubating for 60min. This increase is caused by activation of a system L amino acid exchanger between BPA and l-Tyr. The amino acid esters are metabolized to amino acids by intracellular hydrolytic enzymes that increase the concentrations of intracellular amino acids and stimulate exchange transportation. We expect that this amino acid transport control will be useful for enhancing intracellular boron accumulation.

Molecular characterization of cystinuria in south-eastern European countries

Cystinuria is an autosomal recessive disorder caused by defective transport of cystine and dibasic amino acids in the proximal renal tubules and small intestine. So far, more than 128 mutations in SLC3A1 gene, and 93 in SLC7A9 gene have been described as a cause of cystinuria. We present a molecular characterization of the cystinuria in 47 unrelated south-east European families. The molecular methodology included direct sequencing, single strand conformational polymorphism, and restriction fragment length polymorphism. A total of 93 (94.9 %) out of 98 unrelated cystinuric chromosomes have been characterized. Mutations in SLC3A1 gene account for 64.3 % and in SLC7A9 gene for 30.6 % of the cystinuric chromosomes. Ten different mutations in SLC3A1 gene were found, and two of them were novel (C242R and L573X), while in SLC7A9 gene seven mutations were found, of which three were novel (G73R, V375I and c.1048_1051delACTC). The most common mutations in this study were T216M (24.5 %), M467T (16.3 %) and R365L (11.2 %) in SLC3A1 and G105R (21.4 %) in SLC7A9 gene. A population specificity of cystinuria mutations was observed; T216M mutation was the only mutation present among Gypsies, G105R was the most common mutation among Albanians and Macedonians, and R365L among Serbs. The results of this study allowed introduction of rapid, simple and cost-effective genetic diagnosis of cystinuria that enables an early preventive care of affected patients and a prenatal diagnosis in affected families.

Functional characterization and substrate specificity of a novel gene encoding zinc finger-like protein, ZfLp, in Xenopus laevis oocytes

In the present study, we isolated and determined the pharmacological characteristics of a novel gene encoding the zinc finger-like protein (ZfLp). The isolated cDNA consisted of 1,581 base pairs that encoded a 526-amino acid protein. The amino acid sequence of ZfLp is 96% identical to that of zinc finger protein 415 isoform 5 (ZNF415-5). Reverse-transcription (RT)-polymerase chain reaction (PCR) analysis revealed that the ZfLp mRNA is expressed in the breast, lung, stomach, small intestine colon and ovary, but not in the liver. When expressed in Xenopus laevis oocytes, ZfLp mediated the high affinity transport of [(3)H]paclitaxel (taxol) in a sodium-independent manner (K(m) = 336.7 ± 190.0 nM). The uptake of [(3)H]paclitaxel (taxol) by ZfLp was trans-stimulated by glutarate and glutathione (GSH). A cis-inhibition experiment revealed that ZfLp-mediated transport of [(3)H]paclitaxel (taxol) is inhibited by several organic solutes specifically clotrimazole. Using several clotrimazole derivatives, we found that N-benzylimidazole would be a minimum unit for producing the inhibition of ZfLp-mediated drug uptake. Our results may provide insights into the novel role of soluble protein, such as ZNF, in the human body. Our results, therefore, would be expected to facilitate research on the novel role of ZNFs and on the discovery of novel drugs for targeting ZNF-related proteins such as ZfLp.

CD98 increases renal epithelial cell proliferation by activating MAPKs

CD98 heavy chain (CD98hc) is a multifunctional transmembrane spanning scaffolding protein whose extracellular domain binds with light chain amino acid transporters (Lats) to form the heterodimeric amino acid transporters (HATs). It also interacts with β1 and β3 integrins by its transmembrane and cytoplasmic domains. This interaction is proposed to be the mechanism whereby CD98 mediates cell survival and growth via currently undefined signaling pathways. In this study, we determined whether the critical function of CD98-dependent amino acid transport also plays a role in cell proliferation and defined the signaling pathways that mediate CD98-dependent proliferation of murine renal inner medullary collecting duct (IMCD) cells. We demonstrate that downregulating CD98hc expression resulted in IMCD cell death. Utilizing overexpression studies of CD98hc mutants that either lacked a cytoplasmic tail or were unable to bind to Lats we showed that CD98 increases serum-dependent cell proliferation by a mechanism that requires the CD98hc cytoplasmic tail. We further demonstrated that CD98-dependent amino acid transport increased renal tubular epithelial cell proliferation by a mechanism that does not require the CD98hc cytoplasmic tail. Both these mechanisms of increased renal tubular epithelial cell proliferation are mediated by Erk and p38 MAPK signaling. Although increased amino transport markedly activated mTor signaling, this pathway did not alter cell proliferation. Thus, these studies demonstrate that in IMCD cells, the cytoplasmic and extracellular domains of CD98hc regulate cell proliferation by distinct mechanisms that are mediated by common MAPK signaling pathways.

NIH3T3 cells overexpressing CD98 heavy chain resist early G1 arrest and apoptosis induced by serum starvation

CD98 is a heterodimeric glycoprotein of 125-kDa, which consists of a 90-kDa heavy chain (hc) subunit and 35-kDa to 55-kDa light chain (lc) subunits. It is strongly expressed on the surface of proliferating normal cells and almost all tumor cells. To investigate the participation of CD98 in cellular proliferation and malignant transformation, we analyzed cell-cycle progression of NIH3T3 clones transfected with cDNA of human CD98hc. Although NIH3T3 and control transfectant cells grown to the subconfluent state were arrested in the G0/G1 phase by serum starvation, considerable portions of CD98hc-transfected cells resided at S and G2/M phases. Under serum-starved and confluent conditions, significant fractions (20-25%) of NIH3T3 and control transfectant cells contained less than 2n content DNA, indicating occurrence of apoptosis, whereas no apoptotic cells were detected in CD98hc-transfectant cells. Under serum-starved conditions, a marked increase in the levels of cyclin D1 and cyclin E and a decrease in p16 were observed in CD98hc-transfectant cells. The reverse was true for NIH3T3 and control transfectant cells. Our results suggest that resistance to G1 arrest and apoptosis by CD98 overexpression are associated with high G1-cyclins and low p16 levels.

Ribosomal protein L3 mediated the transport of digoxin in Xenopus laevis oocyte

Ribosomal protein L3 (RPL3) is known to be an indispensable and essential component for the peptidyltransferase center. In the present study, we found a novel function of RPL3 using a Xenopus laevis oocyte expression system. When expressed in X. oocytes, RPL3 mediated the high affinity transport of [(3)H]digoxin (K(m) = 213.3 ± 46.8 nM) in a time-, concentration-, and sodium-dependent manners. The maximum velocity of the transport of [(3)H]digoxin via RPL3 produced at physiological pH. However, we did not observe RPL3-mediated transport of several organic solutes such as [(14)C]androstenedione, [(3)H]dexamethasone, [(3)H]dehydroepiandrosterone sulfate, [(3)H]L-tryptophan, [(14)C]L-ascorbic acid, [(14)C]α-ketoglutarate, [(14)C]glutarate, [(3)H]methotrexate, [(3)H]bumetanide, [(3)H]probenecid, [(14)C]salicylic acid, [(14)C]theophylline and [(3)H]valproate. Our results suggest that RPL3 functions as a drug carrier protein and may be involved in the digoxin toxicity in the human body.

Erythropoietin-induced neuroprotection requires cystine glutamate exchanger activity

Erythropoietin (Epo) has been used for many years in neonates for the treatment of anemia of prematurity. Epo has also been proposed for treatment of neonatal brain injury, as mounting evidence suggests neuroprotective properties for Epo. However, Epo's neuroprotective mechanism of action is poorly understood. In this study we hypothesized that Epo may confer neuroprotection by enhancing cellular redox defense brought about by cellular glutathione (GSH). This was examined in cultures of differentiated cortical neural stem cells and using the B104 cell line as model systems. Our data shows that Epo causes a time- and dose-dependent increase in expression and activity of system Xc(-), the transporter responsible for uptake of cystine for the production of glutathione. Cystine uptake increases 3-5 fold in differentiated neural stem cells and B104 cells treated with Epo. Exposure of cells to 100 microM kainate suppressed cellular GSH and caused excitotoxicity, but GSH levels and cell viability were completely restored by Epo in the continued presence of kainate. This rescue effect of Epo vanished if system Xc(-) was inhibited pharmacologically using S4-CPG in the presence of Epo leading to marked cell death of B104 cells and cultured mouse cortical neural stem cells. This could also be achieved using xCT siRNA to decrease xCT expression. This data suggests that system Xc(-) activity and protein expression are positively regulated by Epo directly explaining its neuroprotective effect.

Thyroid hormone transporters in the brain

Thyroid hormone plays an essential role in proper mammalian development of the central nervous system and peripheral tissues. Lack of sufficient thyroid hormone results in abnormal development of virtually all organ systems, a syndrome termed cretinism. In particular, hypothyroidism in the neonatal period causes serious damage to neural cells and leads to mental retardation. Although thyroxine is the major product secreted by the thyroid follicular cells, the action of thyroid hormone is mediated mainly through the deiodination of T(4) to the biologically active form 3,3', 5-triiodo-L-thyronine, followed by the binding of T(3) to a specific nuclear receptor. Before reaching the intracellular targets, thyroid hormone must cross the plasma membrane. Because of the lipophilic nature of thyroid hormone, it was thought that they traversed the plasma membrane by simple diffusion. However, in the past decade, a membrane transport system for thyroid hormone has been postulated to exist in various tissues. Several classes of transporters, organic anion transporter polypeptide (oatp) family, Na(+)/Taurocholate cotransporting polypeptide (ntcp) and amino acid transporters have been reported to transport thyroid hormones. Monocarboxylate transporter8 (MCT8) has recently been identified as an active and specific thyroid hormone transporter. Mutations in MCT8 are associated with severe X-linked psycomotor retardation and strongly elevated serum T3 levels in young male patients. Several other molecules should be contributed to exert the role of thyroid hormone in the central nervous 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.

Y+ and y+ L arginine transporters in neuronal cells expressing tyrosine hydroxylase

Arginine is a semi-essential amino acid that serves as sole substrate for enzymes involved in diverse cell processes including redox balance via nitric oxide synthase (NOS) and cell proliferation via arginase. Neurons that express nNOS require intracellular arginine to generate nitric oxide (NO). Using a TH+ neuronal cell line (CAD cells), we show that neuronal NO production is largely dependent on extracellular arginine. Although a small intracellular pool exists in CAD cells, the lack of mRNA for argininosuccinate synthase (AS), a rate limiting enzyme for arginine recycling, suggests that intracellular pools are not re-supplied by this mechanism in this sub-class of neurons. Rather, arginine is taken up from the extracellular media by two primary transport systems, the y+ and the y+ L systems. The expression of CAT1, CAT3, y+ LAT1 and y+ LAT2 mRNAs supports the presence of each system. CAD cell arginine transport is depressed by increased extracellular K+ levels and demonstrates that variations in membrane potential control neuronal arginine uptake. Short term exposure to the oxidizing agents, rotenone and Angeli's salt, but not FeSO4, increases arginine transport. The regulation of arginine uptake by physiological factors suggests that arginine supply adapts in a moment-to-moment fashion to the changing needs of the neuron.

Lysinuric protein intolerance: mechanisms of pathophysiology

Heteromeric amino acid transporters (HATs) are composed of two subunits, a polytopic membrane protein (the light subunit) and a disulfide-linked type II membrane glycoprotein (the heavy subunit). HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, xc-, and b(0,+)). The light subunits confer the amino acid transport specificity to the HAT. Two transporters of this family are relevant for inherited aminoacidurias. Mutations in any of the two genes coding for the subunits of system b(0,+) (rBAT and b(0,+)AT) lead to cystinuria (MIM 220100). Transport defects in a system y(+)L isoform, composed of 4F2hc and y(+)LAT-1, result in lysinuric protein intolerance (LPI) (MIM 222700). In this case, only mutations in the light subunit y(+)LAT-1, but not in the heavy chain 4F2hc, cause the disease. LPI, in addition to affecting intestinal and renal reabsorption of amino acids, is a multisystemic disease affecting the urea cycle and presents also with symptoms related to the immune system. The pathogenesis of these alterations is less well, or not understood at all.

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.

Rat kidney MAP17 induces cotransport of Na-mannose and Na-glucose in Xenopus laevis oocytes

Renal reabsorption is the main mechanism that controls mannose homeostasis. This takes place through a specific Na-coupled uphill transport system, the molecular identity of which is unknown. We prepared and screened a size-selected rat kidney cortex cDNA library through the expression of mannose transport in Xenopus laevis oocytes. We have identified a membrane protein that induces high-affinity and specific Na-dependent transport of d-mannose and d-glucose in X. laevis oocytes, most likely through stimulation of the capacity of an endogenous transport system of the oocyte. Sequencing has revealed that the cDNA encodes the counterpart of the human membrane-associated protein MAP17, previously known by its overexpression in renal, colon, lung, and breast carcinomas. We show that MAP17 is a 12.2-kDa nonglycosylated membrane protein that locates to the brush-border plasma membrane and the Golgi apparatus of transfected cells and that it is expressed in the proximal tubules of the kidney cortex and in the spermatids of the seminiferous tubules. It spans twice the cell membrane, with both termini inside the cell, and seems to form homodimers through intracellular Cys55, a residue also involved in transport expression. MAP17 is responsible for mannose transport expression in oocytes by rat kidney cortex mRNA. The induced transport has the functional characteristics of a Na-glucose cotransporter (SGLT), because d-glucose and alpha-methyl-d-glucopyranoside are also accepted substrates that are inhibited by phloridzin. The corresponding transporter from the proximal tubule remains to be identified, but it is different from the known mammalian SGLT-1, -2, and -3.

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.

Functional properties of multispecific amino acid transporters and their implications to transporter-mediated toxicity

The absorption, distribution and excretion of most of xenobiotics, drugs, environmental toxins and their metabolites are mediated by membrane transporters. Recent advances in the transporter molecular biology have made it possible to investigate the mechanisms of transport of those exogenous compounds and their transporter-mediated toxicity at the molecular level. Exogenous compounds including drugs and toxic substances occurring in the environment pass through the transporters with broad substrate selectivity, namely "multispecific" transporters, taking advantage of the multispecific nature to exert their toxic effects. The remarkable examples of such transporter-mediated toxicity are 1-methyl-4-phenyl-2,3-dihydropyridinium (MPP+)-neurotoxicity mediated by dopamine transporters, cephaloridine-nephrotoxicity mediated by organic anion transporters and methylmercury-toxicity mediated by system L amino acid transporters. The molecular identification of system L transporter LAT1 (L-type amino acid transporter 1) has lead to the understanding of the mechanisms of their multispecific substrate recognition and revealed their localization at the blood-brain barrier and placental barrier. LAT1 relies on the hydrophobic interaction between substrate amino acid side chains and the substrate binding site, so that many variations are possible for the substrate amino acid side chains, which is the basis of the broad substrate selectivity. System L transporters, thus, function as a path for the membrane permeation of drugs and toxic compounds occurring in the environment with amino acid-related structures. Beside methylmercury-cysteine conjugate, amino acid-related neurotoxins such as beta-N-methylamino-L-alanine, S-(1,2-dichlorovinyl)-L-cysteine and 3-hydroxykynurenine are proposed to pass through system L transporters to exert their toxicity. Because the presence of such transporters is crucial for the manifestation of the organ toxicity, the inhibition of the transporters would be expected to be beneficial to prevent the disorders caused by the transporter-mediated toxicity.

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.

Up-regulated expression of a novel gene in activated human peripheral blood mononuclear cells that is a truncated paralog of the human system L-amino acid transporter 1

INTRODUCTION AND AIMS

The human system L-amino acid transporter1 (hLAT1) is one of the CD98 light chains and its gene has been mapped to chromosome 16q24. Our preliminary findings have indicated that in HeLa S3 cells there are transcripts whose nucleotide sequences are very similar but not identical to that of the amino acid transporter. This study intends to examine whether these novel transcripts have biological significance through elucidating their genetic aspects and expression profiles in human cells.

METHODS

The expression levels of the transcripts were quantified by real-time PCR analysis. Chromosomal mapping of the gene was performed by fluorescence in situ hybridization (FISH).

RESULTS

Three types of transcripts were identified and their nucleotide sequences were aligned with the chromosome 16p12 clone with high identity. They encoded 180- or 190-amino acid proteins, showing 92-94% of amino acid identity to the amino-terminal region of the hLAT1 (507 amino acids). However, their 3' non-coding sequences did not show homology to the nucleotide sequence of the amino acid transporter. Their genes were mapped to chromosome 16p11.2-p13.1 as low-copy repeats (LCRs). The transcription of one of these genes in peripheral blood mononuclear cells was significantly up-regulated when the cells were stimulated with concanavalin A.

CONCLUSION

We have characterized the three truncated paralogs of the hLAT1 gene. It is suggested that the expression of one of these paralogs may play an important role in the activation of peripheral blood mononuclear cells.

Thyroid hormone transporters: recent advances

Thyroid hormones, being hydrophobic, were thought to enter target cell membranes by passive diffusion. However, recent studies have documented the existence of numerous organic anion transport systems, about half of which also transport thyroid hormones into (and possibly out of) a variety of target cells. Several of the genes encoding thyroid hormone transporters have been characterized by means of molecular approaches. Here, we discuss the classification of thyroid hormone transporters, with emphasis on how they are influenced by their ionic milieu and what their symported organic anions are.

The role of CD98 in astrocytic neoplasms

The high expression of CD98 was reported in some normal tissues, including blood brain barrier, activated lymphocytes, the basal layer of skin, proximal tubles of kidney, placenta, testis and a wide variety of tumors. The CD98 complex consists of an 80-85kD heavy chain (4F2hc/FRP-1) and a 40-45kD light chain. CD98hc, 4F2hc, and FRP-1 are the same glycosylated protein each other and define antigenicity of CD98. LAT1, the sodium-independent L-type amino acid transporter 1, 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 such as glioma cells. Increased LAT1 expression was found in various malignancies including human gliomas. Several studies implicated the important role of LAT1 and 4F2hc in malignant transformation and carcinogenesis. The LAT1-CD98 pathway may represent a unique therapeutic target for cancer intervention.

Heteromeric amino acid transporters: biochemistry, genetics, and physiology

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.

Function and structure of heterodimeric amino acid transporters

Heterodimeric amino acid transporters are comprised of two subunits, a polytopic membrane protein (light chain) and an associated type II membrane protein (heavy chain). The heavy chain rbAT (related to b(0,+) amino acid transporter) associates with the light chain b(0,+)AT (b(0,+) amino acid transporter) to form the amino acid transport system b(0,+), whereas the homologous heavy chain 4F2hc interacts with several light chains to form system L (with LAT1 and LAT2), system y(+)L (with y(+)LAT1 and y(+)LAT2), system x (with xAT), or system asc (with asc1). The association of light chains with the two heavy chains is not unambiguous. rbAT may interact with LAT2 and y(+)LAT1 and vice versa; 4F2hc may interact with b(0,+)AT when overexpressed. 4F2hc is necessary for trafficking of the light chain to the plasma membrane, whereas the light chains are thought to determine the transport characteristics of the respective heterodimer. In contrast to 4F2hc, mutations in rbAT suggest that rbAT itself takes part in the transport besides serving for the trafficking of the light chain to the cell surface. Heavy and light subunits are linked together by a disulfide bridge. The disulfide bridge, however, is not necessary for the trafficking of rbAT or 4F2 heterodimers to the membrane or for the functioning of the transporter. However, there is experimental evidence that the disulfide bridge in the 4F2hc/LAT1 heterodimer plays a role in the regulation of a cation channel. These results highlight complex interactions between the different subunits of heterodimeric amino acid transporters and suggest that despite high grades of homology, the interactions between rbAT and 4F2hc and their respective partners may be different.

Expression cloning of two genes that together mediate organic solute and steroid transport in the liver of a marine vertebrate

Uptake of organic solutes and xenobiotics by mammalian cells is mediated by ATP-independent transporters, and four families of transporters have now been identified. To search for novel organic solute transporters, a liver cDNA library from an evolutionarily primitive marine vertebrate, the little skate Raja erinacea, was screened for taurocholate transport activity by using Xenopus laevis oocytes. In contrast to the organic anion transporters identified to date, a transport activity was identified in this library that required the coexpression of two distinct gene products, termed organic solute transporter alpha and beta (Ostalpha, Ostbeta). Ostalpha cDNA encodes for a protein of 352 aa and seven putative transmembrane (TM) domains. Ostbeta contains 182 aa and has at least one and perhaps two TM domains. There is no significant sequence identity between Ostalpha and Ostbeta, and only low identity with sequences in the databases; however, Ostalpha bears a resemblance to some G protein-coupled receptors, and Ostbeta exhibits 22% amino acid identity with the C-terminal TM and intracellular domains of protocadherin-gamma, a cell surface glycoprotein. Xenopus oocytes injected with the cRNA for both Ostalpha and Ostbeta, but not each separately, were able to take up taurocholate, estrone sulfate, digoxin, and prostaglandin E(2), but not p-aminohippurate or S-dinitrophenyl glutathione. Transport was sodium-independent, saturable, and inhibited by organic anions and steroids, including the major skate bile salt, scymnol sulfate. These results identify an organic anion transporter composed of a putative seven-helix TM protein and an ancillary membrane polypeptide.

xCt cystine transporter expression in HEK293 cells: pharmacology and localization

xCT, the core subunit of the system x(c)(-) high affinity cystine transporter, belongs to a superfamily of glycoprotein-associated amino acid transporters. Although xCT was shown to promote cystine transport in Xenopus oocytes, little work has been done with mammalian cells (Sato, H., Tamba, M., Ishii, T., and Bannai, S. J. Biol. Chem. 274, 11455-11458, 1999). Therefore, we have constructed mammalian expression vectors for murine xCT and its accessory subunit 4F2hc and transfected them into HEK293 cells. We report that this transporter binds cystine with high affinity (81 microM) and displays a pharmacological profile expected for system x(c)(-). Surprisingly, xCT transport activity in HEK293 cells is not dependent on the co-expression of the exogenous 4F2hc. Expression of GFP-tagged xCT indicated a highly clustered plasma membrane and intracellular distribution suggesting the presence of subcellular domains associated with combating oxidative stress. Our results indicate that HEK293 cells transfected with the xCT subunit would be a useful vehicle for future structure-function and pharmacology experiments involving system x(c)(-).

Amino acid transporters in the human placenta

Amino acid transport across the human placenta is active, mediated by specific transporters in syncytiotrophoblast plasma membranes. Using functional criteria such as substrate specificity and sodium dependence, approximately 15 transport systems have been identified in the human placenta. Recently, the area of molecular biology of amino acid transporters has evolved rapidly and at least 25 cDNA clones coding for mammalian amino acid transporters or transporter subunits have been identified. The primary objective of this review is to integrate the available functional data on placental amino acid transport systems with recent molecular information on mammalian amino acid transporters. Furthermore, models for the mechanisms for net materno-fetal transfer of amino acids are discussed. Finally, the evidence to suggest that alterations in placental amino acid transport systems may play a crucial role in the regulation of fetal growth are presented briefly.

Enhanced tumorigenicity caused by truncation of the extracellular domain of GP125/CD98 heavy chain

GP125/CD98 is a heterodimeric 125-kDa glycoprotein, which consists of an 85-kDa heavy chain (hc) and a 40-kDa light chain (lc), and is strongly expressed on the cell surface of various tumor cells, irrespective of their tissue of origin. We have recently demonstrated that overexpression of the CD98hc cDNA causes malignant transformation of NIH3T3 cells. To investigate the function of the extracellular domain of CD98hc in cell proliferation and malignant transformation, we established two NIH3T3-derived clones transfected with human truncated CD98hc cDNAs, and compared their characteristics with parental NIH3T3 and clones transfected with full-length CD98hc cDNA. Truncated as well as full-length CD98hc-transfected clones grew to a higher saturation density than control cells. Efficiency of colony formation in soft agar was augmented in all CD98hc-transfected clones, and the degrees of augmented colony formation of the transfectants expressing full-length CD98hc of 529 a.a. or truncated CD98hc of 418 a.a. were reduced by anti-human CD98hc antibodies, while that of the transfectant expressing truncated CD98hc of 237 a.a. lacking the epitopes recognized by anti-human CD98hc antibodies was not affected by the addition of antibodies. CD98hc-transfected clones developed tumors in athymic mice, and tumor growth of truncated CD98hc-transfected clones was faster than that of full-length CD98hc-transfected clones.

Heteromeric amino acid transporters explain inherited aminoacidurias

In the past 5 years, the first genes responsible for aminoacidurias caused by defects in renal reabsorption transport mechanisms have been identified. These diseases are type I and non-type I cystinuria and lysinuric protein intolerance. This knowledge came from the molecular characterization of the first heteromeric amino acid transporters in mammals. In 1992, rBAT and 4F2hc (genes SLC3A1 and SLC3A2, respectively, in the nomenclature of the Human Genome Organization) were identified as putative heavy subunits of mammalian amino acid transporters. In 1994, it was demonstrated that mutations in SLC3A1 cause type I cystinuria. Very recently, several light subunits of the heteromeric amino acid transporters have been identified. In 1999, a putative light subunit of rBAT (the SLC7A9 gene; complementary DNA and protein termed amino acid transporter) and a light subunit of 4F2hc (the SLC7A7 gene; cDNA and protein termed y+LAT-1) were shown to be the non-type I cystinuria and lysinuric protein intolerance genes, respectively. In this review, the characteristics of these heteromeric amino acid transporters and their role in these inherited aminoacidurias is described.

Involvement of rBAT in Na(+)-dependent and -independent transport of the neurotransmitter candidate L-DOPA in Xenopus laevis oocytes injected with rabbit small intestinal epithelium poly A(+) RNA

Although L-3,4-dihydroxyphenylalanine (L-DOPA) is claimed to be a neurotransmitter in the central nervous system (CNS), receptor or transporter molecules for L-DOPA have not been determined. In an attempt to identify a transporter for L-DOPA, we examined whether or not an active and high affinity L-DOPA transport system is expressed in Xenopus laevis oocytes injected with poly A(+) RNA prepared from several tissues. Among the poly A(+) RNAs tested, rabbit intestinal epithelium poly A(+) RNA gave the highest transport activity for L-[(14)C]DOPA in the oocytes. The uptake was approximately five times higher than that of water-injected oocytes, and was partially Na(+)-dependent. L-Tyrosine, L-phenylalanine, L-leucine and L-lysine inhibited this transport activity, whereas D-DOPA, dopamine, glutamate and L-DOPA cyclohexylester, an L-DOPA antagonist did not affect this transport. Coinjection of an antisense cRNA, as well as oligonucleotide complementary to rabbit rBAT (NBAT) cDNA almost completely inhibited the uptake of L-[(14)C]DOPA in the oocytes. On the other hand, an antisense cRNA of rabbit 4F2hc barely affected this L-[(14)C]DOPA uptake activity. rBAT was thus responsible for the L-[(14)C]DOPA uptake activity expressed in X. laevis oocytes injected with poly A(+) RNA from rabbit intestinal epithelium. As rBAT is localized at the target regions of L-DOPA in the CNS, rBAT might be one of the components involved in L-DOPAergic neurotransmission.

Development and polarization of cationic amino acid transporters and regulators in the human placenta

We have investigated L-arginine transport systems in the human placental syncytiotrophoblast across gestation using purified microvillous (MVM) and basal (BM) plasma membrane vesicles. In MVM from first-trimester and term placentas, L-arginine transport was by systems y(+) and y(+)L. In BM (term placentas), however, there was evidence for system y(+)L only. The Michaelis constant of system y(+)L was significantly lower (P < 0.05) in first-trimester compared with term MVM and lower in term MVM compared with BM (P < 0.05). There was no functional evidence for system b(0+) in term MVM or BM. Cationic amino acid transporter (CAT) 1, CAT 4, and 4F2hc were detected using RT-PCR in placentas throughout gestation. rBAT was not detected in term placentas. An approximately 85-kDa and an approximately 135-kDa protein was detected by Western blotting in MVM under reducing and nonreducing conditions, respectively, consistent with the 4F2hc monomer and the 4F2hc-light chain dimer, and their expression was significantly higher (P < 0.05) in term compared with first-trimester MVM. These proteins were not detected in BM despite functional evidence for system y(+)L. These data suggest different roles for 4F2hc in the development and polarization of cationic amino acid transporters in the syncytiotrophoblast.

LAT2, a new basolateral 4F2hc/CD98-associated amino acid transporter of kidney and intestine

Glycoprotein-associated amino acid transporters (gpaAT) are permease-related proteins that require heterodimerization to express their function. So far, four vertebrate gpaATs have been shown to associate with 4F2hc/CD98 for functional expression, whereas one gpaAT specifically associates with rBAT. In this study, we characterized a novel gpaAT, LAT2, for which mouse and human cDNAs were identified by expressed sequence tag data base searches. The encoded ortholog proteins are 531 and 535 amino acids long and 92% identical. They share 52 and 48% residues with the gpaATs LAT1 and y(+)LAT1, respectively. When mouse LAT2 and human 4F2hc cRNAs were co-injected into Xenopus oocytes, disulfide-linked heterodimers were formed, and an L-type amino acid uptake was induced, which differed slightly from that produced by LAT1-4F2hc: the apparent affinity for L-phenylalanine was higher, and L-alanine was transported at physiological concentrations. In the presence of an external amino acid substrate, LAT2-4F2hc also mediated amino acid efflux. LAT2 mRNA is expressed mainly in kidney and intestine, whereas LAT1 mRNA is expressed widely. Immunofluorescence experiments showed colocalization of 4F2hc and LAT2 at the basolateral membrane of kidney proximal tubules and small intestine epithelia. In conclusion, LAT2 forms with LAT1 a subfamily of L-type gpaATs. We propose that LAT1 is involved in cellular amino acid uptake, whereas LAT2 plays a role in epithelial amino acid (re)absorption.

SLC7A8, a gene mapping within the lysinuric protein intolerance critical region, encodes a new member of the glycoprotein-associated amino acid transporter family

Using a bioinformatic approach, we have identified a new transcript, SLC7A8, mapping to 14q11.2, within the lysinuric protein intolerance (LPI) critical region. This gene is highly expressed in skeletal muscle, intestine, kidney, and placenta and encodes a predicted protein of 535 amino acids, homologous to the amino acid permease CD98 light chain and cationic amino acid transporters. RNA in situ hybridization data on mouse embryos confirm the expression in kidney and intestine and, interestingly, reveal that SLC7A8 is also highly expressed in eye, in retinal pigmented epithelium, and in tooth buds at day 16.5 of gestation. Mutational analysis excluded any direct involvement of the SLC7A8 gene product in LPI disease. The homology data and the expression pattern are in agreement with the hypothesis that SLC7A8 represents a novel light chain interacting with the 4F2 heavy chain in the multimeric complex mediating neutral and/or cationic amino acid transport and cystine/glutamate exchange.

Placental anionic and cationic amino acid transporter expression in growth hormone overexpressing and null IGF-II or null IGF-I receptor mice

The role of growth hormone (GH), insulin-like growth factor (IGF)-II and the IGF-I receptor (IGF-Ir) in the regulation of the in vivo expression of Na(+)-coupled anionic [System X-AG; GLAST1 (EAAT1), GLT1 (EAAT2), EAAC1 (EAAT3), EAAT4; where the human homologues of amino acid transport proteins first cloned in the rat are given in parentheses] and Na(+)-independent cationic (System y(+);CAT1) amino acid transport proteins was evaluated by comparing transporter expression in day 17 placentae of mice that overexpressed bovine GH (GH+) or that carried null gene mutations for IGF-II or IGF-Ir. Northern analysis revealed no apparent difference in the mRNA content of GLAST1 (EAAT1), EAAC1 (EAAT3), or EAAT4, in homogenates of GH+ placentae, but levels of GLT1 (EAAT2) and CAT1 mRNA were increased. Immunoblot analysis revealed that whole-placental steady-state GLAST1 (EAAT1), EAAC1 (EAAT3), and EAAT4 protein levels were not affected by GH+, whereas GLT1 (EAAT2) levels were increased. Immunohistochemical analysis showed that the cell-specific expression of the anionic and CAT1 transporters was not affected by overexpression of GH. Similar analyses of null IGF-II placentae demonstrated increases in GLAST1 (EAAT1), EAAT4 and CAT1 mRNAs. Parallel immunoblot analysis demonstrated decreased expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3) protein, but an increased expression of EAAT4. In null IGF-II and IGF-Ir placentae, however, GLT1 (EAAT2) and EAAC1 (EAAT3) protein content was decreased in junctional zone cells, whereas CAT1 content was increased in junctional and labyrinth zone cells. These data indicate that an excess level of GH stimulates GLT1 (EAAT2) expression and that a normal level of IGF-II is required for typical expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3), but that IGF-II downregulates the expression of EAAT4 and CAT1.

Selective expression of the large neutral amino acid transporter at the blood-brain barrier

Amino acid supply in brain is regulated by the activity of the large neutral amino acid transporter (LAT) at the brain capillary endothelial cell, which forms the blood-brain barrier (BBB) in vivo. Bovine BBB poly(A)(+) RNA was isolated from 2.0 kg of fresh bovine brain and size fractionated on a sucrose density gradient, and a size-fractionated bovine BBB cDNA library in the pSPORT vector was prepared. The full-length cDNA encoding the bovine BBB LAT was isolated from this library, and the predicted amino acid sequence was 89-92% identical to the LAT1 isoform. The bovine BBB LAT1 mRNA produced a 10-fold enhancement in tryptophan transport into frog oocytes coinjected with bovine BBB LAT1 mRNA and the mRNA for 4F2hc, which encodes the heavy chain of the heterodimer. Tryptophan transport into the mRNA-injected oocytes was sodium independent and was specifically inhibited by other large neutral amino acids, and the K(m) of tryptophan transport was 31.5 +/- 5.5 microM. Northern blotting with the bovine BBB LAT1 cDNA showed that the LAT1 mRNA is 100-fold higher in isolated bovine brain capillaries compared with C6 rat glioma cells or rat brain, and the LAT1 mRNA was not detected in rat liver, heart, lung, or kidney. These studies show that the LAT1 transcript is selectively expressed at the BBB compared with other tissues, and the abundance of the LAT1 mRNA at the BBB is manyfold higher than that of transcripts such as the 4F2hc antigen, actin, or the Glut1 glucose transporter.

Cloning and expression of a b(0,+)-like amino acid transporter functioning as a heterodimer with 4F2hc instead of rBAT. A new candidate gene for cystinuria

We have cloned a transporter protein from rabbit small intestine, which, when coexpressed with the 4F2 heavy chain (4F2hc) in mammalian cells, induces a b(0,+)-like amino acid transport activity. This protein (4F2-lc6 for the sixth member of the 4F2 light chain family) consists of 487 amino acids and has 12 putative transmembrane domains. At the level of amino acid sequence, 4F2-lc6 shows significant homology (44% identity) to the other five known members of the 4F2 light chain family, namely LAT1 (4F2-lc1), y(+)LAT1 (4F2-lc2), y(+)LAT2 (4F2-lc3), xCT (4F2-lc4), and LAT2 (4F2-lc5). The 4F2hc/4F2-lc6 complex-mediated transport process is Na(+)-independent and exhibits high affinity for neutral and cationic amino acids and cystine. These characteristics are similar to those of the b(0,+)-like amino acid transport activity previously shown to be associated with rBAT (protein related to b(0,+) amino acid transport system). However, the newly cloned 4F2-lc6 does not interact with rBAT. This is the first report of the existence of a b(0,+)-like amino acid transport process that is independent of rBAT. 4F2-lc6 is expressed predominantly in the small intestine and kidney. Based on the characteristics of the transport process mediated by the 4F2hc/4F2-lc6 complex and the expression pattern of 4F2-lc6 in mammalian tissues, we suggest that 4F2-lc6 is a new candidate gene for cystinuria.

Identification of an amino acid transporter associated with the cystinuria-related type II membrane glycoprotein

We identified an amino acid transporter that is associated with the cystinuria-related type II membrane glycoprotein, rBAT (related to b(0,+) amino acid transporter). The transporter designated BAT1 (b(0, +)-type amino acid transporter 1) from rat kidney was found to be structurally related to recently identified amino acid transporters for system L, system y(+)L, and system x(-)C, which are linked, via a disulfide bond, to the other type II membrane glycoprotein, 4F2hc (4F2 heavy chain). In the nonreducing condition, a 125-kDa band, which seems to correspond to the heterodimeric complex of BAT1 and rBAT, was detected in rat kidney with anti-BAT1 antibody. The band was shifted to 41 kDa in the reducing condition, confirming that BAT1 and rBAT are linked via a disulfide bond. The BAT1 and rBAT proteins were shown to be colocalized in the apical membrane of the renal proximal tubules where massive cystine transport had been proposed. When expressed in COS-7 cells with rBAT, but not with 4F2hc, BAT1 exhibited a Na(+)-independent transport of cystine as well as basic and neutral amino acids with the properties of system b(0,+). The results from the present investigation were used to establish a family of amino acid transporters associated with type II membrane glycoproteins.

Malignant transformation of NIH3T3 cells by overexpression of early lymphocyte activation antigen CD98

CD98, which forms a heterodimer of relative molecular mass (M(r)) 125, 000, was originally identified as an early T cell activation antigen. It consists of a heavy chain of M(r) 85,000 that bears the CD98 epitope and a light chain of M(r) 40, 000. CD98 is strongly expressed on the surface of activated lymphocytes and various tumor cells irrespective of tissue origins. To investigate the participation of CD98 in cellular proliferation and malignant transformation, we established and characterized human CD98-transfected NIH3T3 clones. Although the doubling times of the control cells and CD98-transfected clones were almost the same, CD98-transfected clones grew to a higher saturation density than control cells. Effciency of colony formation in soft agar was augmented in CD98-transfected clones, and this augmentation was significantly reduced by anti-human CD98 mAb. Furthermore, CD98-transfected clones developed tumors in athymic mice. These results indicated that overexpression of CD98 participates in the process of malignant transformation, suggesting that CD98 has oncogenic potential.

Cloning and functional expression of a human Na(+) and Cl(-)-dependent neutral and cationic amino acid transporter B(0+)

A Na(+)-dependent neutral and cationic amino acid transport system (B(0+)) plays an important role in many cells and tissues; however, the molecular basis for this transport system is still unknown. To identify new transporters, the expressed sequence tag database was queried, and cDNA fragments with sequence similarity to the Na(+)/Cl(-)-dependent neurotransmitter transporter family were identified. Based on these sequences, rapid amplification of cDNA ends of human mammary gland cDNA was used to obtain a cDNA of 4.5 kilobases (kb). The open reading frame encodes a 642-amino acid protein named amino acid transporter B(0+). Human ATB(0+) (hATB(0+)) is a novel member of the Na(+)/Cl(-)-dependent neurotransmitter transporter family with the highest sequence similarity to the glycine and proline transporters. Northern blot analysis identified transcripts of approximately 4.5 kb and approximately 2 kb in the lung. Another tissue survey suggests expression in the trachea, salivary gland, mammary gland, stomach, and pituitary gland. Electrophysiology and radiolabeled amino acid uptake measurements were used to functionally characterize the transporter expressed in Xenopus oocytes. hATB(0+) was found to transport both neutral and cationic amino acids, with the highest affinity for hydrophobic amino acids and the lowest affinity for proline. Amino acid transport was Na(+) and Cl(-)-dependent and was attenuated in the presence of 2-aminobicyclo-[2.2.1]-heptane-2-carboxylic acid, a system B(0+) inhibitor. These characteristics are consistent with system B(0+) amino acid transport. Thus, hATB(0+) is the first cloned B(0+) amino acid transporter.

Identification and immunological characterization of a novel 40-kDa protein linked to CD98 antigen

Monoclonal antibodies (mAbs) were obtained from hybridoma clones established by cell fusion between mouse myeloma cells and spleen cells from a mouse immunized against an affinity-purified 40-kDa component of rat 125-kDa glycoprotein (GP125). Two mAbs designated as 3F2 and 6B4 detected a 40-kDa and a 125-kDa band under reducing and nonreducing conditions, respectively, in extracts prepared from rat, mouse and human tumor cells. Association of the 40-kDa protein with CD98 was revealed by sandwich-type enzyme-linked immunosorbent assay. The two mAbs were strongly reactive with various tumor cells and activated lymphocytes, but were only weakly reactive with resting lymphocytes. Confocal microscopy indicated colocalization of CD98 and the 40-kDa protein defined with 3F2 and 6B4 at the cell surface and perinuclear regions. On immunohistochemical analysis of frozen sections of rat tongue, the anti-rat CD98 mAb B3 selectively stained the basal layer and 3F2 stained the upper epithelial part in addition to the basal layer, indicating the existence of CD98-unlinked 40-kDa protein.

Identification of a membrane protein, LAT-2, that Co-expresses with 4F2 heavy chain, an L-type amino acid transport activity with broad specificity for small and large zwitterionic amino acids

We have identified a new human cDNA, L-amino acid transporter-2 (LAT-2), that induces a system L transport activity with 4F2hc (the heavy chain of the surface antigen 4F2, also named CD98) in oocytes. Human LAT-2 is the fourth member of the family of amino acid transporters that are subunits of 4F2hc. The amino acid transport activity induced by the co-expression of 4F2hc and LAT-2 was sodium-independent and showed broad specificity for small and large zwitterionic amino acids, as well as bulky analogs (e.g. BCH (2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid)). This transport activity was highly trans-stimulated, suggesting an exchanger mechanism of transport. Expression of tagged N-myc-LAT-2 alone in oocytes did not induce amino acid transport, and the protein had an intracellular location. Co-expression of N-myc-LAT-2 and 4F2hc gave amino acid transport induction and expression of N-myc-LAT-2 at the plasma membrane of the oocytes. These data suggest that LAT-2 is an additional member of the family of 4F2 light chain subunits, which associates with 4F2hc to express a system L transport activity with broad specificity for zwitterionic amino acids. Human LAT-2 mRNA is expressed in kidney >>> placenta >> brain, liver > spleen, skeletal muscle, heart, small intestine, and lung. Human LAT-2 gene localizes at chromosome 14q11.2-13 (13 cR or approximately 286 kb from marker D14S1349). The high expression of LAT-2 mRNA in epithelial cells of proximal tubules, the basolateral location of 4F2hc in these cells, and the amino acid transport activity of LAT-2 suggest that this transporter contributes to the renal reabsorption of neutral amino acids in the basolateral domain of epithelial proximal tubule cells.

Suppression of FRP-1/CD98-mediated multinucleated giant cell and osteoclast formation by an anti-FRP-1/CD98 mAb, HBJ 127, that inhibits c-src expression

When anti-CD98 mAb 6-1-13, 4-5-1, or 38-2-2 was added to the culture fluids of monocytes, extensive cell aggregation and polykaryocyte formation were induced. These multinucleated giant cells were tartrate-resistant acid phosphatase (TRAP) positive. On the other hand, when monocytes were incubated with another anti-CD98 mAb, HBJ 127, polykaryocyte formation was not detected, although extensive cell aggregation was induced. When HBJ 127 and 6-1-13 were simultaneously added to the culture fluids, anti-CD98 mAb-induced cell fusion was inhibited almost completely. HBJ 127 suppressed formation of 6-1-13-induced cell fusion in a dose-dependent manner. If, however, HBJ 127 was added after incubation of monocytes with mAb 6-1-13 for 6 h, an appreciable degree of TRAP-positive polykaryocyte formation was found. The bindings of 6-1-13 and HBJ 127 were not mutually competed. When monocytes were incubated with 6-1-13 or HBJ 127, 6-1-13 induced c-src mRNA, while HBJ 127 did not. Furthermore, when monocytes were incubated with both 6-1-13 and HBJ 127, c-src mRNA could not be detected, showing that HBJ 127 suppresses c-src expression. Therefore, CD98-mediated osteoclast formation can be regulated by modification of CD98 system.

Cloning and expression of a plasma membrane cystine/glutamate exchange transporter composed of two distinct proteins

Transport system xc- found in plasma membrane of cultured mammalian cells is an exchange agency for anionic amino acids with high specificity for anionic form of cystine and glutamate. We have isolated cDNA encoding the transporter for system xc- from mouse activated macrophages by expression in Xenopus oocytes. The expression of system xc- activity in oocytes required two cDNA transcripts, and the sequence analysis revealed that one is identical with the heavy chain of 4F2 cell surface antigen (4F2hc) and the other is a novel protein of 502 amino acids with 12 putative transmembrane domains. The latter protein, named xCT, showed a significant homology with those recently reported to mediate cationic or zwitterionic amino acid transport when co-expressed with 4F2hc. Thus xCT is a new member of a family of amino acid transporters that form heteromultimeric complex with 4F2hc, with a striking difference in substrate specificity. The expression of system xc- was highly regulated, and Northern blot analysis demonstrated that the expression of both 4F2hc and xCT was enhanced in macrophages stimulated by lipopolysaccharide or an electrophilic agent. However, the expression of xCT was more directly correlated with the system xc- activity.