Modulation of the activity of amino acid transport system L by phorbol esters and calmodulin antagonists in a human placental choriocarcinoma cell line

Protein kinase C activation upregulates human L-type amino acid transporter 2 function

L-type amino acid transporter 2 (LAT2) is a Na+-independent neutral amino acid transporter, whose function regulation system remains unclarified. Since protein kinase C (PKC) is known to regulate the functions of various transporters, we investigated whether human LAT2 (hLAT2) function is regulated by PKC. In mouse proximal tubule S2 cells, hLAT2 transport activity was upregulated by PKC activation. However, we found that the mRNA and protein expression of hLAT2 was not affected by PKC activation and that the upregulation was independent of the three potential PKC consensus sites in the hLAT2 amino acid sequence. Moreover, we found that PKC activation upregulated the Vmax value for hLAT2-mediated alanine transport, which was not accompanied by the induction of hLAT2 membrane insertion. In conclusion, we showed that hLAT2 function is upregulated by PKC activation, which is not related to either the de novo synthesis, the phosphorylation or the membrane insertion of hLAT2.

Regulation of placental amino acid transporter activity by mammalian target of rapamycin

The activity of placental amino acid transporters is decreased in intrauterine growth restriction (IUGR), but the underlying regulatory mechanisms have not been established. Inhibition of the mammalian target of rapamycin (mTOR) signaling pathway has been shown to decrease the activity of the system L amino acid transporter in human placental villous fragments, and placental mTOR activity is decreased in IUGR. In the present study, we used cultured primary trophoblast cells to study mTOR regulation of placental amino acid transporters in more detail and to test the hypothesis that mTOR alters amino acid transport activity by changes in transporter expression. Inhibition of mTOR by rapamycin significantly reduced the activity of system A (−17%), system L (−28%), and taurine (−40%) amino acid transporters. mRNA expression of isoforms of the three amino acid transporter systems in response to mTOR inhibition was measured using quantitative real-time PCR. mRNA expression of l-type amino acid transporter 1 (LAT1; a system L isoform) and taurine transporter was reduced by 13% and 50%, respectively; however, mTOR inhibition did not alter the mRNA expression of system A isoforms (sodium-coupled neutral amino acid transporter-1, -2, and -4), LAT2, or 4F2hc. Rapamycin treatment did not significantly affect the protein expression of any of the transporter isoforms. We conclude that mTOR signaling regulates the activity of key placental amino acid transporters and that this effect is not due to a decrease in total protein expression. These data suggest that mTOR regulates placental amino acid transporters by posttranslational modifications or by affecting transporter translocation to the plasma membrane.

Mammalian target of rapamycin in the human placenta regulates leucine transport and is down-regulated in restricted fetal growth

Pathological fetal growth is associated with perinatal morbidity and the development of diabetes and cardiovascular disease later in life. Placental nutrient transport is a primary determinant of fetal growth. In human intrauterine growth restriction (IUGR) the activity of key placental amino acid transporters, such as systems A and L, is decreased. However the mechanisms regulating placental nutrient transporters are poorly understood. We tested the hypothesis that the mammalian target of rapamycin (mTOR) signalling pathway regulates amino acid transport in the human placenta and that the activity of the placental mTOR pathway is reduced in IUGR. Using immunohistochemistry and culture of trophoblast cells, we show for the first time that the mTOR protein is expressed in the transporting epithelium of the human placenta. We further demonstrate that placental mTOR regulates activity of the l-amino acid transporter, but not system A or taurine transporters, by determining the mediated uptake of isotope-labelled leucine, methylaminoisobutyric acid and taurine in primary villous fragments after inhibition of mTOR using rapamycin. The protein expression of placental phospho-S6K1 (Thr-389), a measure of the activity of the mTOR signalling pathway, was markedly reduced in placentas obtained from pregnancies complicated by IUGR. These data identify mTOR as an important regulator of placental amino acid transport, and provide a mechanism for the changes in placental leucine transport in IUGR previously demonstrated in humans. We propose that mTOR functions as a placental nutrient sensor, matching fetal growth with maternal nutrient availability by regulating placental nutrient transport.

Regulated Internalization and Phosphorylation of the Native Norepinephrine Transporter in Response to Phorbol Esters

The effects of norepinephrine in the brain and periphery are terminated primarily by active reuptake of the catecholamine via cocaine- and amphetamine-sensitive norepinephrine transporters (NETs). Activation of protein kinase C (PKC) down-regulates NET by sequestering it from the plasma membrane, although the underlying mechanism is not yet known. Previously, we showed robust expression of endogenous NETs in rat placental trophoblasts (Jayanthi, L. D., Vargas, G., and DeFelice, L. J. (2002) Br. J. Pharmacol. 135, 1927-1934). Here we report a significant reduction in native NET function and surface expression in these cells following phorbol ester (beta-PMA) treatment. The beta-PMA-mediated down-regulation of NET occurs by a rapid sequestration of NETs from the plasma membrane and is calcium-independent. Reversible biotinylation experiments revealed a significant enhancement of NET endocytosis following beta-PMA treatment. Chemical treatments and expression of dominant negative mutants of dynamin 1 and 2 failed to prevent the beta-PMA effect, suggesting a clathrin-independent pathway. In contrast, treatment with the cholesterol-disrupting agent filipin, which blocks caveolae/lipid raft-mediated internalization, completely blocked the beta-PMA-mediated NET sequestration. Discontinuous sucrose density gradient centrifugation revealed NET in the lipid raft fractions. Following beta-PMA treatment, there was reduced NET levels in the lipid raft fractions suggesting that cholesterol-rich lipid rafts mediate PKC-triggered NET internalization. Metabolic labeling and immunoprecipitation studies revealed that NET phosphorylation is stimulated severalfold by PKC activation and protein phosphatase 1/2A inhibition. Together, these findings demonstrate for the first time that in trophoblasts (i) PKC activation regulates NET function and surface expression by an enhanced internalization process that is lipid raft-mediated and (ii) PKC and protein phosphatase(s) modulation regulates NET phosphorylation.

Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model

The regulation of neutral amino acid transport by cyclic AMP (cAMP) and calcium across the isolated duodenum of the lizard Gallotia galloti has been studied under short-circuit conditions. Active L-alanine transport was stimulated by forskolin, theophylline and dibutyryl cyclic AMP (db-cAMP). All these agents increased transmural potential difference (PD) and short-circuit current (I(sc)) in a manner consistent with the activation of a chloride secretory pathway. Both forskolin and theophylline increased intracellular cAMP levels in the lizard duodenal mucosa. Addition of calcium ionophore A23187 rapidly reduced mucosa-to-serosa L-alanine fluxes and diminished net L-alanine transport. Despite the reduction of alanine fluxes by A23187, transepithelial PD and I(sc) values were increased by the ionophore. Analyses of the responses of isolated transport pathways indicated that the Na(+)-independent L-alanine transport system was unaffected by db-cAMP or calcium ionophore. By contrast, Na(+)-dependent transport activities were profoundly modified by these agents. Thus, while system A [alpha-methylamino-isobutiric acid (MeAIB)-transporting pathway] was stimulated by increased calcium, system ASC activity was nearly abolished. Calcium ionophore also potentiated the electrogenic response of system A. Forskolin strongly stimulated system ASC activity but left system A activity unchanged. Activation of system ASC by forskolin was clearly electroneutral, as pre-incubation of the tissues with the chloride channel blocker diphenylamine-2-carboxilic acid (DPC) completely prevented forskolin-induced transepithelial electrical responses. It is concluded that intracellular messengers cAMP and calcium oppositely modulate active Na(+)-dependent (L)-alanine transport in the lizard intestine. The different sensitivity exhibited by individual transport pathways may well account for the changes observed in overall alanine transport.

Glial induction of blood-brain barrier-like L-system amino acid transport in the ECV304 cell line

The blood-brain barrier (BBB) is formed by the presence of tight junction complexes between brain endothelial cells that restrict paracellular permeability. As a consequence, a number of transport proteins are expressed on cerebral endothelial cells to facilitate the transport of nutrients into the brain. Although the modulation of barrier tight junction properties by glial-conditioned medium and by second messengers is well established, little is known about the effects of these factors on carrier-mediated BBB transport processes. The ECV304 cell line shows an endothelial phenotype and can be induced to upregulate certain BBB features in the presence of glial factors. In the present study, we have examined the effect of conditioned medium derived from rat C6-glioma cells (C6CM) on the function of the L-system amino acid transporter in ECV304 cells, using L-leucine as the model substrate, and have determined whether the changes observed can be mimicked by modulating intracellular cAMP levels. ECV304 cells exposed to C6CM exhibited a significant increase in both the affinity of leucine transport and the diffusional constant (Michaelis-Menten), while the maximal transport capacity remained unchanged. Conversely, acute exposure to modulators of the PKA and PKC second messenger pathways was found to reduce significantly the maximal transport capacity and diffusion constants, while transport affinity remained unchanged. In both cases, the maximal flux of leucine was increased, indicating transport of greater efficiency. This study indicates that exposure of ECV304 cells to C6CM provides an influence inducing L-system transport properties characteristic of brain endothelial cells. Furthermore, it appears that L-system-mediated transport of amino acids can be modulated by several distinct pathways.

Alterations in the activity of placental amino acid transporters in pregnancies complicated by diabetes

Alterations in placental transport may contribute to accelerated fetal growth in pregnancies complicated by diabetes. We studied the activity of the syncytiotrophoblast amino acid transporter system A and the transport of the essential amino acids leucine, lysine, and taurine. Syncytiotrophoblast microvillous plasma membranes (MVMs) and basal plasma membranes (BMs) were isolated from placentas obtained from normal pregnancies and pregnancies complicated by gestational diabetes mellitus (GDM) and type 1 diabetes, with and without large-for-gestational-age (LGA) fetuses. Amino acid transport was assessed using radio-labeled substrates and rapid filtration techniques. System A activity in MVM was increased (65-80%, P < 0.05) in all groups with diabetes independent of fetal overgrowth. However, MVM system A activity was unaffected in placentas of normal pregnancies with LGA fetuses. MVM leucine transport was increased in the GDM/LGA group. In BMs, amino acid transport was unaffected by diabetes. In conclusion, diabetes in pregnancy is associated with an increased system A activity in MVM, and MVM leucine transport is increased in the GDM/LGA group. We suggest that these changes result in an increased uptake of neutral amino acids across MVM, which may be used in placental metabolism or be delivered to the fetus. The increased MVM leucine uptake in the GDM/LGA group may contribute to accelerated fetal growth in these patients.

Expression and regulation of 4F2hc and hLAT1 in human trophoblasts

The neutral amino acid transport system L is a sodium-independent transport system in human placenta and choriocarcinoma cells. Recently, it was found that the heterodimer composed of hLAT1 (a light-chain protein) and 4F2 heavy chain (4F2hc), a type II transmembrane glycoprotein, is responsible for system L amino acid transport. We found that the mRNAs of 4F2hc and hLAT1 were expressed in the human placenta and a human choriocarcinoma cell line. The levels of the 4F2hc and hLAT1 proteins in the human placenta increased at full term compared with those at midtrimester. Immunohistochemical data showed that these proteins were localized mainly in the placental apical membrane. Data from leucine uptake experiments, Northern blot analysis, and immunoblot analysis showed that this transport system was partially regulated by protein kinase C and calcium ionophore in the human choriocarcinoma cell line. Our results suggest that the heterodimer of 4F2hc and hLAT1 may play an important role in placental amino acid transport system L.

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.

Proline transport in MDCK cells expressing a mutant regulatory subunit of cAMP-dependent protein kinase

cAMP-dependent protein kinase (cAK) regulates the activity of several membrane-bound ion channels and carriers. The role of cAK in regulating the transport of osmoprotective amino acids in the distal tubule is unknown. We examined the regulation of Na(+)- and Cl(-)-dependent proline transport in MDCK cells expressing a mutant murine regulatory subunit (RIalpha(AB)) of cAK. For this purpose, MDCK cells were transfected with an expression vector encoding RIalpha(AB) driven by the metallothionein 1 promoter together with neomycin-resistance (NEO) gene. Stable G418-resistant colonies were isolated that expressed RIalpha(AB) as demonstrated by Northern hybridization analysis using a cDNA probe for RIalpha and cAK assay that showed decreased enzyme activity. A clone constitutively expressing high levels of RIalpha(AB) (M(AB)) in a Zn-independent manner and a control clone transfected with the NEO gene alone (M(neo)) were selected for transport studies. We examined the effect of the cAMP-stimulating agents forskolin (F) and IBMX on NaCl-dependent uptake of [(3)H]proline by confluent monolayers of transfected MDCK cells. While F/IBMX-induced mean inhibition of proline transport in M(neo) cells was 48 and 45% at 5 and 15 min, respectively, inhibition of proline uptake in M(AB) cells was 9% (5 min) and 0% (15 min). These data demonstrate that the inhibition of NaCl-linked proline transport in response to elevated cAMP is reversed in MDCK clones that express mutant cAK and provide evidence that cAK mediates the modulatory action of cAMP on proline transport. cAK may play an important role in controlling transport of proline and other osmoprotective amino acids in the renal tubule.

Characteristics of L-glutamine transport during Caco-2 cell differentiation

Glutamine is the main fuel of intestinal epithelial cells, as well as a precursor for the intense nucleotide biosynthesis which arises with the rapid turnover of enterocytes. In order to determine whether glutamine uptake may vary as a function of metabolic demand, glutamine transport across the brush-border membrane of differentiating Caco-2 cells has been investigated. The uptake of L-[(3)H]glutamine was measured between day 7 and day 21 post-seeding. Kinetic analysis with glutamine concentrations ranging from 6.25 microM to 12.8 mM revealed the involvement of high affinity Na(+)-dependent (K(t)=110 microM) and low affinity Na(+)-independent (K(t)=900 microM) transport components at day 7. Both components were partially inhibited by L-lysine in a competitive fashion, suggesting that four different systems were responsible for glutamine uptake: B(0), B(0,+), b(0,+) and L. All four systems were present during the differentiation process, with systems L and B(0) being responsible for up to 80% of glutamine uptake. Caco-2 cell differentiation was associated with a marked decrease in L-glutamine uptake, which affected both the Na(+)-dependent and the Na(+)-independent components. In contrast to glucose uptake, the development of L-glutamine uptake across the brush-border membrane of Caco-2 cells may reflect an adjustment to cell metabolic demand rather than the progressive appearance of a vectorial transport process.

Cloning and functional characterization of a Na(+)-independent, broad-specific neutral amino acid transporter from mammalian intestine

We have isolated a cDNA from a rabbit intestinal cDNA library which, when co-expressed with the heavy chain of the human 4F2 antigen (4F2hc) in mammalian cells, induces system L-like amino acid transport activity. This protein, called LAT2, consists of 535 amino acids and is distinct from LAT1 which also interacts with 4F2hc to induce system L-like amino acid transport activity. LAT2 does not interact with rBAT, a protein with a significant structural similarity to 4F2hc. The 4F2hc/LAT2-mediated transport process differs from the 4F2hc/LAT1-mediated transport in substrate specificity, substrate affinity, tissue distribution, interaction with D-amino acids, and pH-dependence. The 4F2hc/LAT2-associated transport process has a broad specificity towards neutral amino acids with K(t) values in the range of 100-1000 microM, does not interact with D-amino acids to any significant extent, and is stimulated by acidic pH. In contrast, the 4F2hc/LAT1-associated transport process has a narrower specificity towards neutral amino acids, but with comparatively higher affinity (K(t) values in the range of 10-20 microM), interacts with some D-amino acids with high affinity, and is not influenced by pH. LAT2 is expressed primarily in the small intestine and kidney, whereas LAT1 exhibits a much broader tissue distribution.

Identification and functional characterization of a Na+-independent neutral amino acid transporter with broad substrate selectivity

We have isolated a cDNA from rat small intestine that encodes a novel Na+-independent neutral amino acid transporter with distinctive characteristics in substrate selectivity and transport property. The encoded protein, designated L-type amino acid transporter-2 (LAT-2), shows amino acid sequence similarity to the system L Na+-independent neutral amino acid transporter LAT-1 (Kanai, Y., Segawa, H., Miyamoto, K., Uchino, H., Takeda, E., and Endou, H. (1998) J. Biol. Chem. 273, 23629-23632) (50% identity) and the system y+L transporters y+LAT-1 (47%) and KIAA0245/y+LAT-2 (45%) (Torrents, D., Estevez, R., Pineda, M., Fernandez, E., Lloberas, J., Shi, Y.-B., Zorzano, A., and Palacin, M. (1998) J. Biol. Chem. 273, 32437-32445). LAT-2 is a nonglycosylated membrane protein. It requires 4F2 heavy chain, a type II membrane glycoprotein, for its functional expression in Xenopus oocytes. LAT-2-mediated transport is not dependent on Na+ or Cl- and is inhibited by a system L-specific inhibitor, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), indicating that LAT-2 is a second isoform of the system L transporter. Compared with LAT-1, which prefers large neutral amino acids with branched or aromatic side chains, LAT-2 exhibits remarkably broad substrate selectivity. It transports all of the L-isomers of neutral alpha-amino acids. LAT-2 exhibits higher affinity (Km = 30-50 microM) to Tyr, Phe, Trp, Thr, Asn, Ile, Cys, Ser, Leu, Val, and Gln and relatively lower affinity (Km = 180-300 microM) to His, Ala, Met, and Gly. In addition, LAT-2 mediates facilitated diffusion of substrate amino acids, as distinct from LAT-1, which mediates amino acid exchange. LAT-2-mediated transport is increased by lowering the pH level, with peak activity at pH 6.25, because of the decrease in the Km value without changing the Vmax value. Because of these functional properties and a high level of expression of LAT-2 in the small intestine, kidney, placenta, and brain, it is suggested that the heterodimeric complex of LAT-2 and 4F2 heavy chain is involved in the trans-cellular transport of neutral amino acids in epithelia and blood-tissue barriers.

Human LAT1, a subunit of system L amino acid transporter: molecular cloning and transport function

We report here on the cloning and functional characterization of human LAT1, a subunit of the amino acid transport system L. The hLAT1 cDNA, obtained from a human placental cDNA library, codes for a protein of 507 amino acids. When functionally expressed in mammalian cells together with the heavy chain of the rat 4F2 antigen (r4F2hc), hLAT1 induces the transport of neutral amino acids. When expressed independently, neither hLAT1 nor r4F2hc was capable of amino acid transport to any significant extent. Thus, the hLAT1-r4F2hc heterodimeric complex is responsible for the observed amino acid transport. The transport process induced by the heterodimer is Na+ independent and is not influenced by pH. It recognizes exclusively neutral amino acids with high affinity. LAT1-specific mRNA is expressed in most human tissues with the notable exception of the intestine.

Characterization of a sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin and lipoate in human placental choriocarcinoma cells

The characteristics of the uptake of the vitamin pantothenate into JAr human placental choriocarcinoma cells were investigated and these cells were found to accumulate the vitamin against a concentration gradient by a Na(+)-dependent process. Substitution of Na+ with over other monovalent cations abolished the uptake completely. The transport process showed no preference for any particular anion. Kinetic analysis indicated the presence of a single saturable transport system with a Michaelis-Menten constant of 2.1 +/- 0.2 microM and a maximal velocity of 341 +/- 12 pmol/mg of protein per 10 min. The dependence of the uptake rate of pantothenate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the transporter. The Hill coefficient for this process was calculated to be 1.6. The Na+/pantothenate coupling ratio being greater than unity suggests that the transport process is electrogenic, resulting in net transfer of positive charge across the membrane. This was confirmed in plasma membrane vesicles prepared from JAr cells where the uptake of pantothenate was found to be significantly stimulated by valinomycin-induced inside-negative K(+)-diffusion potential. Substrate specificity studies showed that, in addition to pantothenate, the transporter interacts with two other vitamins, namely biotin and lipoate. The characteristics of pantothenate uptake in the placental cell line BeWo was also investigated. These cells were also found to express a pantothenate transport system similar to that expressed in the JAr cells.

Sodium-dependent carnitine transport in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells were found to transport carnitine into the intracellular space by a Na(+)-dependent process. The transport showed no requirement for anions. The Na+-dependent process was saturable and the apparent Michaelis-Menten constant for carnitine was 12.3 +/- 0.5 microM. Na+ activated the transport by increasing the affinity of the transport system for carnitine. The transport system specifically interacted with L-carnitine, D-carnitine, acetyl-DL-carnitine and betaine. 6-N-Trimethyllysine and choline had little or no effect on carnitine transport. Of the total transport measured, transport into the intracellular space represented 90%. Plasma membrane vesicles prepared from JAR cells were found to bind carnitine in a Na(+)-dependent manner. The binding was saturable with an apparent dissociation constant of 0.66 +/- 0.08 microM. The binding process was specific for L-carnitine, D-carnitine, acetyl-DL-carnitine, and betaine. 6-N-Trimethyllysine and choline showed little or no affinity. It is concluded that the JAR cells express a Na(+)-dependent high-affinity system for carnitine transport and that the Na(+)-dependent high-affinity carnitine binding detected in purified JAR cell plasma membrane vesicles is possibly related to the transmembrane transport process.

Differential expression of protein kinase C isoforms in the human placenta

The extensive role played by protein kinase C (PKC) in signal transduction prompted this study of the expression and localization of PKC isoforms in human placental syncytiotrophoblast. Membranes prepared from these cells and samples of villous tissue were analysed by immunoblotting and immunocytochemistry using isoform-specific antibodies. PKC beta 2, gamma, epsilon and zeta were found to be present in both microvillous and basal membranes from term placenta. The alpha isoform was observed only on the basal membrane while the beta 1 isoform was confined to the microvillous membrane. The basal microvillous ratios for beta 2, gamma, epsilon and zeta ranged between 0.3 and 0.5, demonstrating a substantial asymmetry in plasma membrane localization. Immunocytochemistry supported the isoform identification and localization observed in the immunoblotting experiments. Moreover the cellular distribution showed that the majority of syncytical PKC was bound to the plasma membranes, in contrast to the other villous cell types. Immunoblotting experiments demonstrated significant increases in PKC beta 2 and epsilon on the microvillous membrane and PKC gamma and epsilon on the basal membrane between 16 and 40 of weeks gestation. This is the first detailed mapping of PKC isoform distribution in an epithelial cell type and demonstrates the potential for selectivity in signal transduction through phosphorylation of isoform specific and spatially-separated substrates.

JAR human placental choriocarcinoma cells actively synthesize, take up and release polyamines

Uptake of polyamines has been investigated extensively in many cells, but not in placenta, where the polyamine-polyamine oxidase system is supposed to have an immunoregulatory function in pregnancy. Due to the importance of the transfer in this tissue, we have started this study. JAR human placental choriocarcinoma cells in monolayer at confluency were used as a model for measuring the key enzymes of polyamine synthesis and interconversion, rate of uptake and efflux, and the polyamine content. Polyamines were taken up by JAR cells and released by an independent mechanism. Ornithine decarboxylase and spermidine acetyltransferase activities and the rate of transport in and out of the cell were much higher than in other cells, such as L1210 cells. However the systems used for uptake and release appear in many respects to be similar to those observed in L1210 cells, but different from others. The uptake appears to be regulated by an inhibitory protein. Moreover, protein kinase C appears to be involved in the process. The efflux also is regulated as in L1210 cells, through control of H+ and Ca2+ concentration. In conclusion, this study shows that, in JAR cells, ornithine decarboxylase and spermidine acetyltransferase activities were much higher than in other cells, and so was the rate of transport in and out of the cells. As a result, a much higher polyamine content was observed.

Regulation of taurine transport in rat astrocytes by protein kinase C: role of calcium and calmodulin

Phorbol 12-myristate 13-acetate, a potential stimulator of protein kinase C (PKC), inhibited taurine uptake in rat astrocytes. This effect was mimicked by 1-oleoyl-2-acetyl-sn-glycerol, an endogenous stimulator of PKC, and by r-59949, an inhibitor of diacylglycerol kinase. Maximal inhibition was obtained at microM phorbol 12-myristate 13-acetate (PMA) after 1 h of treatment. This effect was prevented by pretreatment of the cells with chelerythrine, a potent and selective inhibitor of PKC. The transport of beta-alanine, an amino acid that shares the same transporter as taurine, was inhibited to a comparable extent. The effect of PMA was potentiated by cotreatment of the cells with thapsigargin or the Ca2+ ionophore A-23187. However, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N1,N1-tetraacetic acid and verapamil did not prevent the PMA effect. Pretreatment of the cells with calmodulin antagonists W-13 or calmidazolium, prevented the PMA-induced inhibition of taurine uptake. This inhibition was not affected by cycloheximide, actinomycin D, colchicine, or cytochalasin D. The Na(+)-to-Cl(-)-to-taurine coupling ratio was unaffected. Dimethyl amiloride, a selective inhibitor of Na+/H+ antiport, was unable to prevent the effects of PMA. These effects were associated with a decrease in the maximal velocity and an increase in the Michaelis-Menten constant.

Placental amino acid transport

Normal fetal growth and development depend on a continuous supply of amino acids from the mother to the fetus. The placenta is responsible for the transfer of amino acids between the two circulations. The human placenta is hemomonochorial, meaning that the maternal and fetal circulations are separated by a single layer of polarized epithelium called the syncytiotrophoblast, which is in direct contact with maternal blood. Transport proteins located in the microvillous and basal membranes of the syncytiotrophoblast are the principal mechanism for transfer from maternal blood to fetal blood. Knowledge of the function and regulation of syncytiotrophoblast amino acid transporters is of great importance in understanding the mechanism of placental transport and potentially improving fetal and newborn outcomes. The development of methods for the isolation of microvillous and basal membrane vesicles from human placenta over the past two decades has contributed greatly to this understanding. Now a primary cultured trophoblast model is available to study amino acid transport and regulation as the cells differentiate. The types of amino acid transporters and their distribution between the syncytiotrophoblast microvillous and basal membranes are somewhat unique compared with other polarized epithelia. These differences may reflect the unusual circumstance of this epithelium that is exposed to blood on both sides. The current state of knowledge as to the types of transport systems present in syncytiotrophoblast, their regulation, and the effects of maternal consumption of drugs on transport are discussed.

Characterization of the glycine transport system GLYT 1 in human placental choriocarcinoma cells (JAR)

Transport of glycine in confluent monolayer cultures of JAR human placental choriocarcinoma cells was investigated. Glycine uptake in these cells was made up of two components, one being Na(+)-dependent with no requirement for Cl- and the other being dependent on Na+ as well as Cl-. Substrate specificity studies indicated that distinct transport systems were responsible for these two components. Alanine inhibited the Na(+)-dependent glycine uptake preferentially and the Na(+)- and Cl(-)-dependent glycine uptake represented > 95% of total uptake in the presence of 5 mM alanine. Competition experiments revealed that the Na(+)- and Cl(-)-dependent transport system exhibited a very narrow substrate specificity with affinity toward only glycine and its derivatives such as sarcosine, glycine methyl ester and glycine ethyl ester. These characteristics identify the transport system as GLYT 1. This system showed high affinity for glycine, with a Michaelis-Menten constant of 15 microM. The Na+:Cl-: glycine stoichiometry appeared to be 2:1:1. Treatment of JAR cells with calmodulin antagonists resulted in the inhibition of the transport function of GLYT 1 and this inhibition was solely due to a decrease in the maximal velocity of the system with no change in the substrate affinity. It is concluded that the placental choriocarcinoma cell line JAR expresses robust activity of the glycine transporter GLYT 1 and that the activity of this transporter is under the regulation of calmodulin-dependent cellular processes.

Selective expression of the high-affinity isoform of the folate receptor (FR-alpha) in the human placental syncytiotrophoblast and choriocarcinoma cells

The folate receptor (FR), an essential component in the process of folate uptake in various cells, is known to exist in three isoforms, FR-alpha, FR-beta and FR-gamma, with differential tissue expression. Transfer of folate across the human placenta from mother to fetus involves participation of a folate receptor expressed in the syncytiotrophoblast, but the isoform identity of this receptor has not been established. Based on the tissue/cell type from which these isoforms have been cloned, it is currently believed that FR-alpha is the isoform expressed in adult tissues whereas FR-beta is the isoform expressed in fetal tissues including placenta. The present study, undertaken primarily to establish the isoform identity of the FR expressed in the placental syncytiotrophoblast, does not support this currently prevailing nomenclature. Reverse transcription coupled with polymerase chain reaction (RT-PCR) of total/poly(A)+ RNA from placenta, cultured trophoblast cells and JAR choriocarcinoma cells with primer pairs specific for either FR-alpha or FR-beta reveals that while both isoforms are detectable in the whole placental tissue, only FR-alpha is present in the normal trophoblast cells and in the choriocarcinoma cells. Northern analysis with probes designed to distinguish between the mRNA transcripts coding for these two isoforms corroborate the RT-PCR findings. Furthermore, the nucleotide sequences of the PCR products obtained from the trophoblast cells and JAR cells are identical to the nucleotide sequence of the FR-alpha cDNA. These studies establish that it is the FR-alpha isoform, and not the FR-beta isoform, which is selectively expressed in the placental trophoblast cells. FR-beta, which is known to be present in the placenta, most likely arises from the maternal decidua normally associated with this tissue.

Molecular identity and calmodulin-mediated regulation of the taurine transporter in a human retinal pigment epithelial cell line

The molecular identity and calmodulin-mediated regulation of the taurine transporter were investigated in a human retinal pigment epithelial cell line (HRPE). Reverse transcription-polymerase chain reaction amplification of HRPE cell mRNA using primer specific for a taurine transporter cloned from human placenta yielded a product of expected size (approximately 0.9 kb) which hybridized to the placental cDNA probe under high stringency conditions. The nucleotide sequence of the product was identical to the sequence of the portion of the placental taurine transporter cDNA flanked by the specific primers. The taurine transporter expressed in the HRPE cell line thus appears to be identical to the transporter cloned from the placenta. Treatment of the HRPE cells with a selective calmodulin antagonist CGS 9343 B (CGS) led to a marked decrease in taurine transport activity. This effect could be reproduced with W-7, another calmodulin antagonist. The inhibition caused by CGS occurred rapidly (t1/2 approximately 10 min). Treatment of the cells with CGS did not affect the transport of leucine, and amino acid not recognized by the taurine transporter as a substrate. The CGS-induced inhibition of taurine transport was accompanied by a decrease in the maximal velocity of the transporter with no detectable change in the substrate affinity. The steady state levels of the transporter mRNA however remained unaffected by CGS treatment. It is concluded that the HRPE cell line expressed a taurine transporter identical to the transporter describe in the human placenta and that the function of this transporter is regulated by calmodulin-dependent processes.

Calmodulin-dependent modulation of pH sensitivity of the amino acid transport system L in human placental choriocarcinoma cells

The JAR human placental choriocarcinoma cells express the amino acid transport system L. The activity of this system is Na(+)-independent and is stimulated by acidic extracellular pH. Treatment of cells with the calmodulin antagonist CGS 9343B results in a marked stimulation of the system L activity. At a CGS 9343B concentration of 50 microM, the stimulation of activity measured at pH 7.5 is about 75-100%. This effect is not blocked by cycloheximide, actinomycin D, colchicine or cytochalasin D suggesting that the stimulation is not due to de novo synthesis of the carrier protein or recruitment of the carrier protein from an intracellular pool. The stimulatory effect of CGS 9343B is reproducible with other calmodulin antagonists. Treatment with CGS 9343B significantly modifies pH sensitivity of the system. The stimulatory effect of H+ is markedly reduced in treated cells compared to control cells. The stimulation of activity at pH 5.5 vs. pH 7.5 is 55% in control cells but only 8% in treated cells. Similarly, the stimulatory effect of CGS 9343B is reduced by H+. The stimulation of activity seen with 50 microM CGS 9343B is 80% at pH 8.0, but only 26% at pH 5.5. In addition, H+ and CGS 9343B affect the kinetic parameters of system L in a similar manner, the stimulation in both cases being primarily due to an increase in the maximal velocity. The apparent competitive nature between the effects of H+ and CGS 9343B is also observed with other calmodulin antagonists. These results show that the transport function and pH sensitivity of the amino acid transport system L in placental choriocarcinoma cells are modulated by calmodulin by processes which do not involve de novo synthesis nor recruitment of the carrier protein.

The role of protein phosphorylation in renal amino acid transport

Changes in tubular reabsorption of amino acids and other solutes are characteristic of the immature renal tubule and of various hereditary nephropathies. The cellular mechanisms governing these aberrations in renal amino acid transport have not been established. Calcium (Ca2+)-dependent protein kinases are known to phosphorylate membrane-bound carrier proteins, thereby modulating transport of various solutes by the proximal tubule. The role of these enzymes in regulating renal tubular amino acid transport, particularly during kidney development, is unknown. We investigated: (1) the effect of Ca(2+)- and phospholipid-dependent protein kinase [protein kinase C (PKC)] and Ca2+/calmodulin-dependent protein kinase II (CaMKII) on sodium chloride (NaCl)-linked proline transport by renal brush border membrane vesicles (BBMV) from adult rats using the "hypoosmotic shock" technique (lysis of vesicles); (2) the activity, expression and subcellular distribution (cytosol, particulate, BBM) of Ca(2+)-dependent protein kinases in kidneys from 7-day-old and adult rats using MBP 4-14 and autocamtide II phosphorylation assays for PKC and CaMKII, respectively, endogenous protein phosphorylation (using gel electrophoresis and autoradiography) and Western immunoblot analysis to detect PKC and CaMKII. The studies showed: (1) endogenous (membrane-bound) CaMKII and PKC as well as exogenous, highly purified PKC inhibit proline uptake by phosphorylated, lyzed/resealed BBMV when compared with control vesicles; the voltage-clamped, nonelectrogenic component of proline transport was inhibited by PKC- but not CaMKII-mediated phosphorylation; (2) a Ca(2+)-dependent activity of both kinases was evident in all subcellular fractions tested in immature and adult kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)