Expression of Na+-independent isoleucine transport activity from rat brain in Xenopus laevis oocytes

Interactions in the Metabolism of Glutamate and the Branched-Chain Amino Acids and Ketoacids in the CNS

Glutamatergic neurotransmission entails a tonic loss of glutamate from nerve endings into the synapse. Replacement of neuronal glutamate is essential in order to avoid depletion of the internal pool. In brain this occurs primarily via the glutamate-glutamine cycle, which invokes astrocytic synthesis of glutamine and hydrolysis of this amino acid via neuronal phosphate-dependent glutaminase. This cycle maintains constancy of internal pools, but it does not provide a mechanism for inevitable losses of glutamate N from brain. Import of glutamine or glutamate from blood does not occur to any appreciable extent. However, the branched-chain amino acids (BCAA) cross the blood-brain barrier swiftly. The brain possesses abundant branched-chain amino acid transaminase activity which replenishes brain glutamate and also generates branched-chain ketoacids. It seems probable that the branched-chain amino acids and ketoacids participate in a "glutamate-BCAA cycle" which involves shuttling of branched-chain amino acids and ketoacids between astrocytes and neurons. This mechanism not only supports the synthesis of glutamate, it also may constitute a mechanism by which high (and potentially toxic) concentrations of glutamate can be avoided by the re-amination of branched-chain ketoacids.

Upregulation of KCNQ1/KCNE1 K+ channels by Klotho

Klotho is a transmembrane protein expressed primarily in kidney, parathyroid gland, and choroid plexus. The extracellular domain could be cleaved off and released into the systemic circulation. Klotho is in part effective as β-glucuronidase regulating protein stability in the cell membrane. Klotho is a major determinant of aging and life span.Overexpression of Klotho increases and Klotho deficiency decreases life span. Klotho deficiency may further result in hearing loss and cardiac arrhythmia. The present study explored whether Klotho modifies activity and protein abundance of KCNQ1/KCNE1, a K(+) channel required for proper hearing and cardiac repolarization. To this end, cRNA encoding KCNQ1/KCNE1 was injected in Xenopus oocytes with or without additional injection of cRNA encoding Klotho. KCNQ1/KCNE1 expressing oocytes were treated with human recombinant Klotho protein (30 ng/mL) for 24 h. Moreover, oocytes which express both KCNQ1/KCNE1 and Klotho were treated with 10 μM DSA L (D-saccharic acid-1,4-lactone), a β-glucuronidase inhibitor. The KCNQ1/KCNE1 depolarization-induced current (I(Ks)) was determined utilizing dual electrode voltage clamp, while KCNQ1/KCNE1 protein abundance in the cell membrane was visualized utilizing specific antibody binding and quantified by chemiluminescence. KCNQ1/KCNE1 channel activity and KCNQ1/KCNE1 protein abundance were upregulated by coexpression of Klotho. The effect was mimicked by treatment with human recombinant Klotho protein (30 ng/mL) and inhibited by DSA L (10 μM). In conclusion, Klotho upregulates KCNQ1/KCNE1 channel activity by “mainly” enhancing channel protein abundance in the plasma cell membrane, an effect at least partially mediated through the β-glucuronidase activity of Klotho protein.

Downregulation of chloride channel ClC-2 by Janus kinase 3

Janus kinase-3 (JAK3) fosters proliferation and counteracts apoptosis of lymphocytes and tumor cells. The gain of function mutation (A572V)JAK3 has been discovered in acute megakaryoplastic leukemia. JAK3 is inactivated by replacement of lysine by alanine in the catalytic subunit ((K855A)JAK3). Regulation of cell proliferation and apoptosis involves altered activity of Cl(-) channels. The present study, thus, explored whether JAK3 modifies the function of the small conductance Cl(-) channel ClC-2. To this end, ClC-2 was expressed in Xenopus oocytes with or without wild-type JAK3, (A568V)JAK3 or (K851A)JAK3, and the Cl(-) channel activity determined by dual-electrode voltage clamp. Channel protein abundance in the cell membrane was determined utilizing chemiluminescence. As a result, expression of ClC-2 was followed by a marked increase of cell membrane conductance. The conductance was significantly decreased following coexpression of JAK3 or (A568V)JAK3, but not by coexpression of (K851A)JAK3. Exposure of the oocytes expressing ClC-2 together with (A568V)JAK3 to the JAK3 inhibitor WHI-P154 (4-[(3'-bromo-4'-hydroxyphenyl)amino]-6,7-dimethoxyquinazoline, 22 μM) increased the conductance. Coexpression of (A568V)JAK3 decreased the ClC-2 protein abundance in the cell membrane of ClC-2 expressing oocytes. The decline of conductance in ClC-2 and (A568V)JAK3 coexpressing oocytes following inhibition of channel protein insertion by brefeldin A (5 μM) was similar in oocytes expressing ClC-2 with (A568V)JAK3 and oocytes expressing ClC-2 alone, indicating that (A568V)JAK3 might slow channel protein insertion into rather than accelerating channel protein retrieval from the cell membrane. In conclusion, JAK3 downregulates ClC-2 activity and thus counteracts Cl(-) exit-an effect possibly influencing cell proliferation and apoptosis.

Up-regulation of hERG K⁺ channels by B-RAF

Human ether-a-go-go related-gene K⁺ channels (hERG) participate in the regulation of tumor cell proliferation and apoptosis. HERG channel activity is up-regulated by growth factors. Kinases sensitive to growth factor signaling include the serine/threonine protein kinase B-RAF. The present study thus explored whether B-RAF influences hERG channel expression and activity. To this end, hERG channels were expressed in Xenopus oocytes with or without wild-type B-RAF, hERG channel activity was determined utilizing dual-electrode voltage clamp and hERG protein abundance in the cell membrane was analyzed utilizing confocal microscopy as well as chemiluminescence. Moreover, in rhabdomyosarcoma RD cells the effect of B-RAF inhibitor PLX-4720 on hERG-mediated current was quantified by whole-cell patch clamp and hERG cell surface protein abundance by utilizing biotinylation of cell surface proteins as well as flow cytometry. As a result, co-expression of wild-type B-RAF in hERG-expressing Xenopus oocytes significantly increased hERG channel activity and hERG channel protein abundance in the cell membrane. Treatment for 24 hours of B-RAF and hERG-expressing Xenopus oocytes with B-RAF inhibitor PLX-4720 (10 µM) significantly decreased hERG-mediated current and hERG cell surface expression. Similarly, in rhabdomyosarcoma RD cells, treatment for 24 hours with B-RAF inhibitor PLX-4720 significantly decreased hERG cell membrane protein abundance and hERG-mediated current. In conclusion, B-RAF is a powerful regulator of hERG channel activity and cell surface hERG protein abundance.

Upregulation of the Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb by B-RAF

B-RAF, a serine/threonine protein kinase, contributes to signaling of insulin-like growth factor IGF1. Effects of IGF1 include stimulation of proximal renal tubular phosphate transport, accomplished in large part by Na⁺-coupled phosphate cotransporter NaPi-IIa. The related Na⁺-coupled phosphate cotransporter NaPi-IIb accomplishes phosphate transport in intestine and tumor cells. The present study explored whether B-RAF influences protein abundance and/or activity of type II Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb. cRNA encoding wild-type NaPi-IIa and wild-type NaPi-IIb was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild-type B-RAF, and electrogenic phosphate transport determined by dual-electrode voltage clamp. NaPi-IIa protein abundance in Xenopus oocyte cell membrane was visualized by confocal microscopy and quantified by chemiluminescence. Moreover, in HEK293 cells, the effect of B-RAF inhibitor PLX-4720 on NaPi-IIa cell surface protein abundance was quantified utilizing biotinylation of cell surface proteins and western blotting. In NaPi-IIa-expressing Xenopus oocytes, but not in oocytes injected with water, addition of phosphate to extracellular bath generated a current (I P), which was significantly increased following coexpression of B-RAF. According to kinetic analysis, coexpression of B-RAF enhanced the maximal IP. Coexpression of B-RAF further enhanced NaPi-IIa protein abundance in the Xenopus oocyte cell membrane. Treatment of HEK293 cells for 24 h with PLX-4720 significantly decreased NaPi-IIa cell membrane protein abundance. Coexpression of B-RAF, further significantly increased IP in NaPi-IIb-expressing Xenopus oocytes. Again, B-RAF coexpression enhanced the maximal IP. In conclusion, B-RAF is a powerful stimulator of the renal and intestinal type II Na⁺-coupled phosphate cotransporters NaPi-IIa and NaPi-IIb, respectively.

Upregulation of HERG channels by the serum and glucocorticoid inducible kinase isoform SGK3

Human ether-a-go-go (HERG) channels participate in the repolarization of the cardiac action potential. Loss of function mutations of HERG lead to delayed cardiac repolarization reflected by prolonged QT interval. HERG channels are regulated through a signaling cascade involving phosphatidylinositol 3 (PI3) kinase. Downstream targets of PI3 kinase include the serum and glucocorticoid inducible kinase (SGK) and protein kinase B (PKB) isoforms. The present study has been performed to explore whether SGK1 and SGK3 participate in the regulation of HERG channel activity. HERG was expressed in Xenopus oocytes with or without additional expression of SGK1 or SGK3. Chemiluminescence was employed to determine HERG plasma membrane protein abundance. Coexpression of SGK3 but not of SGK1 in Xenopus oocytes resulted in an increase of steady state current (I(HERG)) and enhanced cell membrane protein abundance without affecting gating kinetics of the channel. Replacement of serine by alanine at the two SGK consensus sites decreased I(HERG) but neither mutation abolished the stimulating effect of SGK3. In conclusion, SGK3 participates in the regulation of HERG by increasing HERG protein abundance in the plasma membrane and may thus modify the duration of the cardiac action potential.

Cystinuria-specific rBAT(R365W) mutation reveals two translocation pathways in the amino acid transporter rBAT-b0,+AT

Apical reabsorption of dibasic amino acids and cystine in kidney is mediated by the heteromeric amino acid antiporter rBAT/b(0,+)AT (system b(0,+)). Mutations in rBAT cause cystinuria type A, whereas mutations in b(0,+)AT cause cystinuria type B. b(0,+)AT is the catalytic subunit, whereas it is believed that rBAT helps the routing of the rBAT/b(0,+)AT heterodimeric complex to the plasma membrane. In the present study, we have functionally characterized the cystinuria-specific R365W (Arg(365)-->Trp) mutation of human rBAT, which in addition to a trafficking defect, alters functional properties of the b(0,+) transporter. In oocytes, where human rBAT interacts with the endogenous b(0,+)AT subunit to form an active transporter, the rBAT(R365W) mutation caused a defect of arginine efflux without altering arginine influx or apparent affinities for intracellular or extracellular arginine. Transport of lysine or leucine remained unaffected. In HeLa cells, functional expression of rBAT(R365W)/b(0,+)AT was observed only at the permissive temperature of 33 degrees C. Under these conditions, the mutated transporter showed 50% reduction of arginine influx and a similar decreased accumulation of dibasic amino acids. Efflux of arginine through the rBAT(R365W)/b(0,+)AT holotransporter was completely abolished. This supports a two-translocation-pathway model for antiporter b(0,+), in which the efflux pathway in the rBAT(R365W)/b(0,+)AT holotransporter is defective for arginine translocation or dissociation. This is the first direct evidence that mutations in rBAT may modify transport properties of system b(0,+).

Characterization of 3-[(123)I]iodo-L-alpha-methyl tyrosine transport in astrocytes of neonatal rats

3-[(123)I]Iodo-L-alpha-methyl tyrosine ((123)I-IMT) is used for diagnosis and monitoring of brain tumours by means of single-photon emission tomography. As recently shown, (123)I-IMT is predominantly mediated into rat C6 glioma cells by sodium-independent system L for large neutral amino acids. Until now, (123)I-IMT transport in non-neoplastic glial cells has not been examined. Therefore, the aim of this study was to examine the cellular pathways and precise transport kinetics of (123)I-IMT uptake into astrocytes of neonatal rats. In particular sodium-independent (123)I-IMT transport into neonatal astrocytes was compared with sodium-independent (123)I-IMT uptake into neoplastic rat C6 glioma cells. Competitive inhibition experiments showed that (123)I-IMT is exclusively transported via sodium-independent system L into the neonatal astrocytes (92%). Kinetic analysis of sodium-independent (123)I-IMT uptake into neonatal astrocytes and into C6 glioma cells revealed apparent Michaelis constants K(M) = 13.9 +/- 0.5 microM and K(M) = 33.9 +/- 4.1 microM, respectively, which are in the same range of K(M) values as those recently determined for amino acid transport into neoplastic and non-neoplastic glial cells. Indeed, the K(M) values in the micromolar range correspond to the expression of the LAT-1 subunit of system L both in the neonatal astrocytes and in C6 glioma cells. However, sodium-independent maximum transport velocities (V(max)) differed significantly between neonatal astrocytes and C6 glioma cells (11.1 +/- 0.3 and 39.9 +/- 3.3 nmol/mg protein/10 min, respectively).

The heterodimeric amino acid transporter 4F2hc/y+LAT2 mediates arginine efflux in exchange with glutamine

The cationic amino acid arginine, due to its positive charge, is usually accumulated in the cytosol. Nevertheless, arginine has to be released by a number of cell types, e.g. kidney cells, which supply other organs with this amino acid, or the endothelial cells of the blood-brain barrier which release arginine into the brain. Arginine release in mammalian cells can be mediated by two different transporters, y(+)LAT1 and y(+)LAT2. For insertion into the plasma membrane, these transporters have to be associated with the type-II membrane glycoprotein 4F2hc [Torrents, Estevez, Pineda, Fernandez, Lloberas, Shi, Zorzano and Palacin (1998) J. Biol. Chem. 273, 32437-32445]. The present study elucidates the function and distribution of y(+)LAT2. In contrast to y(+)LAT1, which is expressed mainly in kidney epithelial cells, lung and leucocytes, y(+)LAT2 has a wider tissue distribution, including brain, heart, testis, kidney, small intestine and parotis. When co-expressed with 4F2hc in Xenopus laevis oocytes, y(+)LAT2 mediated uptake of arginine, leucine and glutamine. Arginine uptake was inhibited strongly by lysine, glutamate, leucine, glutamine, methionine and histidine. Mutual inhibition was observed when leucine or glutamine was used as substrate. Inhibition of arginine uptake by neutral amino acids depended on the presence of Na(+), which is a hallmark of y(+)LAT-type transporters. Although arginine transport was inhibited strongly by glutamate, this anionic amino acid was only weakly transported by 4F2hc/y(+)LAT2. Amino acid transport via 4F2hc/y(+)LAT2 followed an antiport mechanism similar to the other members of this new family. Only preloaded arginine could be released in exchange for extracellular amino acids, whereas marginal release of glutamine or leucine was observed under identical conditions. These results indicated that arginine has the highest affinity for the intracellular binding site and that arginine release may be the main physiological function of this transporter.

Swelling-induced taurine release without chloride channel activity in Xenopus laevis oocytes expressing anion channels and transporters

Taurine is an important osmolyte involved in cell volume regulation. During regulatory volume decrease it is released via a volume-sensitive organic osmolyte/anion channel. Several molecules have been suggested as candidates for osmolyte release. In this study, we chose three of these, namely ClC-2, ClC-3 and ICln, because of their expression in rat astrocytes, a cell type which is known to release taurine under hypotonic stress, and their activation by hypotonic shock. As all three candidates were also suggested to be chloride channels, we investigated their permeability for both chloride and taurine under isotonic and hypotonic conditions using the Xenopus laevis oocyte expression system. We found a volume-sensitive increase of chloride permeability in ClC-2-expressing oocytes only. Yet, the taurine permeability was significantly increased under hypotonic conditions in oocytes expressing any of the tested candidates. Further experiments confirmed that the detected taurine efflux does not represent unspecific leakage. These results suggest that ClC-2, ClC-3 and ICln either participate in taurine transport themselves or upregulate an endogenous oocyte osmolyte channel. In either case, the taurine efflux of oocytes not being accompanied by an increased chloride flux suggests that taurine and chloride can be released via two separate pathways.

Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Transforming growth factor beta (TGF-beta) has been shown to participate in the pathophysiology of diabetic complications. As shown most recently, TGF-beta stimulates the expression of a distinct serine/threonine kinase (hSGK) which had previously been cloned as an early gene transcriptionally regulated by cell volume alterations. The present study was performed to elucidate transcription and function of hSGK in diabetic nephropathy. As shown by Northern blotting, an increase of extracellular glucose concentration increased hSGK mRNA levels in cultured cells, an effect qualitatively mimicked by osmotic cell shrinkage or treatment with TGF-beta (2 microgram/liter), phorbol 12,13-didecanoate (1 microM), or the Ca(2+) ionophore ionomycin (1 microM) and blunted by high concentrations of nifedipine (10 and 100 microM). In situ hybridization revealed that hSGK transcription was markedly enhanced in diabetic nephropathy, with particularly high expression in mesangial cells, interstitial cells, and cells in thick ascending limbs of Henle's loop and distal tubules. According to voltage clamp and tracer flux studies in Xenopus oocytes expressing the renal epithelial Na(+) channel ENaC or the mouse thick ascending limb Na(+),K(+),2Cl(-) cotransporter BSC-1, coexpression with hSGK stimulated ENaC and BSC-1 11-fold and 6-fold, respectively, effects reversed by kinase inhibitors staurosporine (1 microM) and chelerythrine (1 microM) and not elicited by inactive hSGK. In conclusion, excessive extracellular glucose concentrations enhance hSGK transcription, which in turn stimulates renal tubular Na(+) transport. These observations disclose an additional element in the pathophysiology of diabetic nephropathy.

The heterodimeric amino acid transporter 4F2hc/LAT1 is associated in Xenopus oocytes with a non-selective cation channel that is regulated by the serine/threonine kinase sgk-1

System L is the major Na(+)-independent amino acid transporter of mammalian cells. It is constituted of the type II membrane protein 4F2hc (CD98) which is covalently linked to the polytopic membrane protein LAT1 via a disulfide bridge. The transporter is known to be regulated by the mineral corticoid aldosterone in Xenopus A6 cells. To understand the regulation of the transporter, the 4F2hc/LAT1 heterodimer was functionally expressed in Xenopus laevis oocytes and its transport properties were analysed using flux measurements and the two-electrode voltage-clamp technique. Expression of 4F2hc/LAT1 resulted in a rapid increase in a Na(+)-independent neutral amino acid antiport activity and simultaneously gave rise to a cation conductance. The cation channel was non-rectifying and non-selective, conducting Li(+) > Cs(+) = Na(+) > K(+). After replacement of Na(+) by NMDG, however, the currents were suppressed almost completely. The cation channel was not inhibited by amiloride, Ba2(+), TEA, Hoe293B, flufenamic acid or substrates of the system L amino acid transporter. Significant inhibition, however, was observed in the presence of La3(+), Gd3(+) and quinidine. Channel activity was upregulated by coexpression of 4F2hc/LAT1 with the aldosterone-regulated protein kinase sgk-1. The cation conductance was sensitive to changes in the redox potential, being inhibited following incubation of the oocytes with DTE for 30 min. Mutation of either of the disulfide bridge-constituting cysteines to serine resulted in a loss of ion channel activity whereas amino acid transport was unaffected. It is concluded that the 4F2hc/LAT1 heterodimer regulates a closely associated cation channel or even constitutes a cation channel itself.

Characterization of the high-affinity monocarboxylate transporter MCT2 in Xenopus laevis oocytes

Observations on lactate transport in brain cells and cardiac myocytes indicate the presence of a high-affinity monocarboxylate transporter. The rat monocarboxylate transporter isoform MCT2 was analysed by expression in Xenopus laevis oocytes and the results were compared with the known characteristics of lactate transport in heart and brain. Monocarboxylate transport via MCT2 was driven by the H(+) gradient over the plasma membrane. Uptake of lactate strongly increased with decreasing pH, showing half-maximal stimulation at pH 7.2. A wide variety of monocarboxylates and ketone bodies, including lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, 2-oxoisovalerate and 2-oxoisohexanoate, were substrates of MCT2. All substrates had a high affinity for MCT2. For lactate a K(m) value of 0.74+/-0.07 mM was determined at pH 7.0. For the other substrates, K(i) values between 100 microM and 1 mM were measured for inhibition of lactate transport, which is about one-tenth of the corresponding values for the ubiquitously expressed monocarboxylate transporter isoform MCT1. Monocarboxylate transport via MCT2 could be inhibited by alpha-cyano-4-hydroxycinnamate, anion-channel inhibitors and flavonoids. It is suggested that cells which express MCT2 preferentially use lactate and ketone bodies as energy sources.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

The amino acid transport system y+L/4F2hc is a heteromultimeric complex

4F2hc is an almost ubiquitous transmembrane protein in mammalian cells; upon expression in Xenopus laevis oocytes, it induces amino acid transport with characteristics of system y+L. Indirect evidence fostered speculation that function requires the association of 4F2hc with another protein endogenous to oocytes and native tissues. We show that expression of system y+L-like amino acid transport activity by 4F2hc in oocytes is limited by an endogenous factor and that direct covalent modification of external cysteine residue(s) of an oocyte membrane protein blocks system y+L/4F2hc transport activity, based on the following. 1) Induction of system y+L-like activity saturates at very low doses of human 4F2hc cRNA (0.1 ng/oocyte). This saturation occurs with very low expression of 4F2hc at the oocyte surface, and further increased expression of the protein at the cell surface does not result in higher induction of system y+L-like activity. 2) Human 4F2hc contains only two cysteine residues (C109 and C330). We mutated these residues, singly and in combination, to serine (C109S; CS1, C330S; CS2 and C109S-C330S, Cys-less). Mutation CS2 had no effect on the expressed system y+L-like transport activity, whereas C109S-containing mutants (CS1 and Cys-less) retained only partial y+L-like transport activity (30 to 50% of wild type). 3) Hg2+, the organic mercury compounds pCMB, and the membrane-impermeant pCMBS almost completely inactivated system y+L-like induced by human 4F2hc wild type and all the mutants studied. This was reversed by ss-mercaptoethanol, indicating that external cysteine residue(s) are the target of this inactivation. 4) Sensitivity to Hg2+ inactivation is increased by pretreatment of oocytes with ss-mercaptoethanol or in the C109S-containing mutants (CS1 and Cys-less). The increased Hg2+ reactivity of C109S-containing mutants supports the possibility that C109 may be linked by a disulfide bond to the Hg2+-targeted cysteine residue of the associated protein. These results indicate that 4F2hc is intimately associated with a membrane oocyte protein for the expression of system y+L amino acid transport activity. To our knowledge, this is the first direct evidence for a heteromultimeric protein structure of an organic solute carrier in mammals.

Discrimination of two amino acid transport activities in 4F2 heavy chain- expressing Xenopus laevis oocytes

Expression of the type II membrane proteins of the rbAT/4F2hc family in Xenopus laevis oocytes results in the induction of amino acid transport activity. To elucidate the mechanism of action, amino acid transport was investigated in oocytes expressing the surface antigen 4F2hc. Leucine transport was mediated by a Na+-independent and a Na+-dependent transport mechanism. Both systems could be further discriminated by their stereochemical constraints. Isoleucine, with a branch at the beta-position, shared only the Na+-independent transport system with leucine. Both transport systems were sensitive to inhibition by arginine, but only the Na+-independent system was sensitive to inhibition by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid. When compared with known transport systems the two transport activities could be described as similar to, but not identical with, mammalian systems b0,+ and y+L. The Na+-independent b0,+-like transport system was found both in rbAT and 4F2hc expressing oocytes, indicating that both proteins act in a similar way.

Characterization of the monocarboxylate transporter 1 expressed in Xenopus laevis oocytes by changes in cytosolic pH

Several laboratories have investigated monocarboxylate transport in a variety of cell types. The characterization of the cloned transporter isoforms in a suitable expression system is nevertheless still lacking. H+/monocarboxylate co-transport was therefore investigated in monocarboxylate transporter 1 (MCT1)-expressing Xenopus laevis oocytes by using pH-sensitive microelectrodes and [14C]lactate. Superfusion with lactate resulted in intracellular acidification of MCT1-expressing oocytes, but not in non-injected control oocytes. The basic kinetic properties of lactate transport in MCT1-expressing oocytes were determined by analysing the rates of intracellular pH changes under different conditions. The results were in agreement with the known properties of the transporter, with respect to both the dependence on the lactate concentration and the external pH value. Besides lactate, MCT1 mediated the reversible transport of a wide variety of monocarboxylic acids including pyruvate, D,L-3-hydroxybutyrate, acetoacetate, alpha-oxoisohexanoate and alpha-oxoisovalerate, but not of dicarboxylic and tricarboxylic acids. The inhibitor alpha-cyano-4-hydroxycinnamate bound strongly to the transporter without being translocated, but could be displaced by the addition of lactate. In addition to changes in the intracellular pH, lactate transport also induced deviations from the resting membrane potential.

Cloning and functional expression of a cDNA from rat jejunal epithelium encoding a protein (4F2hc) with system y+L amino acid transport activity

Two different protein families, designated CAT (cationic amino acid transporter) and BAT (broad-specificity amino acid transporter) mediate the plasma membrane transport of cationic amino acids in animal cells. CAT transporters have 12-14 transmembrane domains and are selective for cationic amino acids. BAT proteins, in contrast, have one to four transmembrane domains and induce the transport of both cationic and zwitterionic amino acids when expressed in Xenopus oocytes. Mutations in the human BAT gene cause type I cystinuria, a disease affecting the ability of intestinal and renal brush border membranes to transport cationic amino acids and cystine. We have used functional expression cloning in oocytes to isolate a BAT-related cDNA from rat jejunal epithelium. The cDNA encodes the rat 4F2 heavy chain (4F2hc) cell-surface antigen, a 527-residue (60 kDa) protein that is 26% identical in amino acid sequence with rat renal BAT (also known as NBAT/D2). Expression of rat jejunal 4F2hc in oocytes induced the lysine-inhibitable Na+-dependent influx of leucine and the leucine-inhibitable Na+-independent influx of lysine. Lysine efflux was stimulated by extracellular (Na+ plus leucine). These characteristics identify the expressed amino acid transport activity as system y+L, a transporter that has been implicated in basal membrane transport of cationic amino acids in intestine, kidney and placenta.

Comparison of lactate transport in astroglial cells and monocarboxylate transporter 1 (MCT 1) expressing Xenopus laevis oocytes. Expression of two different monocarboxylate transporters in astroglial cells and neurons

The transport of lactate is an essential part of the concept of metabolic coupling between neurons and glia. Lactate transport in primary cultures of astroglial cells was shown to be mediated by a single saturable transport system with a Km value for lactate of 7.7 mM and a Vmax value of 250 nmol/(min x mg of protein). Transport was inhibited by a variety of monocarboxylates and by compounds known to inhibit monocarboxylate transport in other cell types, such as alpha-cyano-4-hydroxycinnamate and p-chloromercurbenzenesulfonate. Using reverse transcriptase-polymerase chain reaction and Northern blotting, the presence of mRNA coding for the monocarboxylate transporter 1 (MCT1) was demonstrated in primary cultures of astroglial cells. In contrast, neuron-rich primary cultures were found to contain the mRNA coding for the monocarboxylate transporter 2 (MCT2). MCT1 was cloned and expressed in Xenopus laevis oocytes. Comparison of lactate transport in MCT1 expressing oocytes with lactate transport in glial cells revealed that MCT1 can account for all characteristics of lactate transport in glial cells. These data provide further molecular support for the existence of a lactate shuttle between astrocytes and neurons.

Expression of the surface antigen 4F2hc affects system-L-like neutral-amino-acid-transport activity in mammalian cells

Mammalian cells possess a variety of amino acid-transport systems with overlapping substrate specificity. System L is one of the major amino acid-transport systems of non-epithelial cells. By expression cloning we have recently demonstrated that the surface antigen 4F2hc (CD98) is a necessary component for expression of system-L-like amino acid-transport activity in C6-BU-1 rat glioma cells [Bröer, Bröer and Hamprecht (1995) Biochem. J. 312, 863-870]. 4F2hc mRNA was detected in CHO cells, COS cells, activated lymphocytes isolated from mouse spleen and primary cultures of astrocytes. In all these cell types, Na+-independent isoleucine transport was mediated by system L. No contribution of system y+L to isoleucine or arginine transport was detected in C6-BU-1 cells. In lymphocytes, both system-L-like amino acid-transport activity and 4F2hc mRNA levels increased after treatment with phorbol ester plus ionomycin. Antisense oligonucleotides caused modest inhibition of Na+-independent isoleucine transport in C6-BU-1 cells and primary cultures of astroglial cells, whereas arginine transport was unaffected. Overexpression of 4F2hc cDNA in CHO cells resulted in an increase in Na+-independent isoleucine transport.

Derivatives of melphalan designed to enhance drug accumulation in cancer cells

The objective of this study was to develop chemical strategies to improve the uptake and accumulation of melphalan (L-Mel and D-Mel), a cytotoxic agent, into cancer cells. Dipeptides synthesized from L- (or D-) Mel and L-glutamic acid (L-Glu) or L-valine (L-Val) and their methyl or ethyl esters (all compounds were trifluoroacetic acid salts) were evaluated for cytotoxicity and cellular uptake using Caco-2 cells, a human colon carcinoma cell line, and RT-2 cells, a rat brain glioma cell line. Treatment of Caco-2 cells with L-Mel or D-Mel (0.5 mg/ml equivalent of melphalan) for 48 h resulted in approximately 50% cell survival. Treatment of the Caco-2 cells with dipeptide derivatives of L-Mel (or D-Mel) (11c-d, 12c-d and 13) caused similar cytotoxicity effects (approximately 50-70% of cell survival). When the cytotoxicities of the esters of L-Mel, D-Mel and their dipeptide derivatives (11a-b, 12a-b and 14) in Caco-2 cells were determined, less than 10% cell survival was observed. Similar results were observed in RT-2 cells. When the cellular uptake properties of these compounds were determined in Caco-2 cell monolayers, L-Glu-L-Mel (12c), L-Glu-D-Mel (12d), and L-Mel-L-Glu (11c) generated slightly lower intracellular levels of L-Mel or D-Mel than when the cell monolayer was treated with the amino acids (L-Mel or D-Mel). In Caco-2 cells treated with 11c, 12c or 12d, low levels of the dipeptides were also detected. Caco-2 cell monolayers treated with D-Mel-L-Glu (11d) or D-Mel-L-Val (13) showed very low levels of the amino acids (L-Mel or D-Mel), but generally higher levels of the dipeptides. In contrast to the amino acids (L-Mel, D-Mel) or the dipeptide derivatives (11c-d, 12c-d and 13), the ester derivatives of the amino acids [L-Mel(OEt), D-Mel(OEt)] or the dipeptides (11a-b, 12a-b and 14) produced 5-20 times higher intracellular concentrations of potentially cytotoxic metabolites (e.g., L-Mel, D-Mel, Mel-containing dipeptides or Mel-containing dipeptide monoesters). L-Mel(OEt), D-Mel(OEt), L-Glu(OEt)-L-Mel(OEt) (12a), L-Glu(OEt)-D-Mel(OEt) (12b), and L-Mel-L-Glu(OEt)2 (11a) accumulated mainly as either L-Mel or D-Mel, and the percentages of L-Mel or D-Mel were 99%, 99%, 90%, 75% and 98% of the total intracellular concentration of potentially cytotoxic agents, respectively. D-Mel-L-Glu(OEt)2 (11b) accumulated as its monoester (> 95%) and D-Mel-L-Val(OMe) (14) accumulated as its dipeptide metabolite (> 98%). Inclusion of Gly-Pro, carnosine, L-Phe or L-Glu did not inhibit uptake of the dipeptide derivatives of L-Mel (or D-Mel) or their esters. These results suggest that the cellular uptake of the dipeptide derivatives of melphalan and their esters is probably via passive diffusion rather than being facilitated by an amino acid transporter or a di/tripeptide transporter. The higher intracellular levels of cytotoxic agents generated from the ester derivatives of the amino acids and the dipeptides are probably due to their higher lipophilicity and the overall neutral charge of the esters and subsequent intracellular formation of the more polar amino acids (L- or D-Mel) and/or Mel-containing dipeptides. Finally, these studies suggest that dipeptides of D-Mel [11b, 11d, 13] have inherent cytotoxicity properties.

Expression of a renal type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic anions

Two distinct molecular types (I and II) of renal proximal tubular brush border Na+/Pi cotransporters have been identified by expression cloning on the basis of their capacity to induce Na+-dependent Pi influx in tracer experiments. Whereas the type II transporters (e.g., NaPi-2 and NaPi-3) resemble well known characteristics of brush border Na+/Pi cotransport, little is known about the properties of the type I transporter (NaPi-1). In contrast to type II, type I transporters produced electrogenic transport only at high extracellular Pi concentrations (> or =3 mM). On the other hand, expression of NaPi-1 induced a Cl- conductance in Xenopus laevis oocytes, which was inhibited by Cl- channel blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid >> 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. Further, the Cl- conductance was inhibited by the organic anions phenol red, benzylpenicillin (penicillin G), and probenecid. These organic anions induced outwardly directed currents in the absence of Cl-. In tracer studies, we observed uptake of benzylpenicillin with a Km of 0.22 mM; benzylpenicillin uptake was inhibited by NPPB and niflumic acid. These findings suggest that the type I Na+/Pi cotransporter functions also as a novel type of anion channel permeable not only for Cl- but also for organic anions. Such an apical anion channel could serve an important role in the transport of Cl- and the excretion of anionic xenobiotics.

The 4F2hc surface antigen is necessary for expression of system L-like neutral amino acid-transport activity in C6-BU-1 rat glioma cells: evidence from expression studies in Xenopus laevis oocytes

Mammalian cells possess a variety of amino acid-transport systems with overlapping substrate specificity. System L is one of the major amino acid-transport systems in all non-epithelial cells. Its molecular structure is not known. To clone the neutral amino acid-transporter system L, we followed an expression cloning strategy using Xenopus laevis oocytes. A cDNA library derived from C6-BU-1 rat glioma cells was used as a source, because high expression of system L activity could be demonstrated with polyadenylated RNA isolated from these cells, when injected into Xenopus laevis oocytes [Bröer, Bröer and Hamprecht (1994) Biochim. Biophys. Acta 1192, 95-100]. A single clone (ILAT) was identified, the sense cRNA of which, on injection into Xenopus laevis oocytes, stimulated sodium-independent isoleucine transport by about 100-fold. Further characterization revealed that transport of cationic amino acids was also stimulated. Sequencing of the cDNA showed that the identified clone is the heavy chain of the rat 4F2 surface antigen, a marker of tumour cells and activated lymphocytes. Uptake of neutral and cationic amino acids was not stimulated by the presence of Na+ ions. Antisense cRNA transcribed from this clone or antisense oligonucleotides, when co-injected with polyadenylated RNA from C6-BU-1 rat glioma cells, completely suppressed system L-like isoleucine-transport activity. We conclude that ILAT is necessary for expression of system L-like amino acid-transport activity by polyadenylated RNA from C6-BU-1 rat glioma cells.