Immunolocalisation of nucleoside transporters in human placental trophoblast and endothelial cells: evidence for multiple transporter isoforms

Addressing the Clinical Importance of Equilibrative Nucleoside Transporters in Drug Discovery and Development

The US Food and Drug Administration (FDA), European Medicines Agency (EMA), and Pharmaceuticals and Medical Devices Agency (PMDA) guidances on small-molecule drug-drug interactions (DDIs), with input from the International Transporter Consortium (ITC), recommend the evaluation of nine drug transporters. Although other clinically relevant drug uptake and efflux transporters have been discussed in ITC white papers, they have been excluded from further recommendation by the ITC and are not included in current regulatory guidances. These include the ubiquitously expressed equilibrative nucleoside transporters (ENT) 1 and ENT2, which have been recognized by the ITC for their potential role in clinically relevant nucleoside analog drug interactions for patients with cancer. Although there is comparatively limited clinical evidence supporting their role in DDI risk or other adverse drug reactions (ADRs) compared with the nine highlighted transporters, several in vitro and in vivo studies have identified ENT interactions with non-nucleoside/non-nucleotide drugs, in addition to nucleoside/nucleotide analogs. Some noteworthy examples of compounds that interact with ENTs include cannabidiol and selected protein kinase inhibitors, as well as the nucleoside analogs remdesivir, EIDD-1931, gemcitabine, and fialuridine. Consequently, DDIs involving the ENTs may be responsible for therapeutic inefficacy or off-target toxicity. Evidence suggests that ENT1 and ENT2 should be considered as transporters potentially involved in clinically relevant DDIs and ADRs, thereby warranting further investigation and regulatory consideration.

Drug Transporters at the Human Blood-Testis Barrier

Transporters are involved in the movement of many physiologically important molecules across cell membranes and have a substantial impact on the pharmacological and toxicological effect of xenobiotics. Many transporters have been studied in the context of disposition to, or toxicity in, organs such as the kidney and liver; however, transporters in the testes are increasingly gaining recognition for their role in drug transport across the blood-testis barrier (BTB). The BTB is an epithelial membrane barrier formed by adjacent Sertoli cells (SCs) in the seminiferous tubules that form intercellular junctional complexes to protect developing germ cells from the external environment. Consequently, many charged or large polar molecules cannot cross this barrier without assistance from a transporter. SCs express a variety of drug uptake and efflux transporters to control the flux of endogenous and exogenous molecules across the BTB. Recent studies have identified several transport pathways in SCs that allow certain drugs to circumvent the human BTB. These pathways may exist in other species, such as rodents and nonhuman primates; however, there is (1) a lack of information on their expression and/or localization in these species, and (2) conflicting reports on localization of some transporters that have been evaluated in rodents compared with humans. This review outlines the current knowledge on the expression and localization of pharmacologically relevant drug transporters in human testes and calls attention to the insufficient and contradictory understanding of testicular transporters in other species that are commonly used in drug disposition and toxicity studies. SIGNIFICANCE STATEMENT: While the expression, localization, and function of many xenobiotic transporters have been studied in organs such as the kidney and liver, the characterization of transporters in the testes is scarce. This review summarizes the expression and localization of common pharmacologically-relevant transporters in human testes that have significant implications for the development of drugs that can cross the blood-testis barrier. Potential expression differences between humans and rodents highlighted here suggest rodents may be inappropriate for some testicular disposition and toxicity studies.

HIV in pregnancy: Mother-to-child transmission, pharmacotherapy, and toxicity

An estimated 1.3 million pregnant women were living with HIV in 2018. HIV infection is associated with adverse pregnancy outcomes and all HIV-positive pregnant women, regardless of their clinical stage, should receive a combination of antiretroviral drugs to suppress maternal viral load and prevent vertical fetal infection. Although antiretroviral treatment in pregnant women has undoubtedly minimized mother-to-child transmission of HIV, several uncertainties remain. For example, while pregnancy is accompanied by changes in pharmacokinetic parameters, relevant data from clinical studies are lacking. Similarly, long-term adverse effects of exposure to antiretrovirals on fetuses have not been studied in detail. Here, we review current knowledge on HIV effects on the placenta and developing fetus, recommended antiretroviral regimens, and pharmacokinetic considerations with particular focus on placental transport. We also discuss recent advances in antiretroviral research and potential effects of antiretroviral treatment on placental/fetal development and programming.

Contributions of Drug Transporters to Blood-Placental Barrier

The placenta is the only organ linking two different individuals, mother and fetus, termed as blood-placental barrier. The functions of the blood-placental barrier are to regulate material transfer between the maternal and fetal circulation. The main functional units are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. A series of drug transporters such as P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRP1, MRP2, MRP3, MRP4, and MRP5), organic anion-transporting polypeptides (OATP4A1, OATP1A2, OATP1B3, and OATP3A1), organic anion transporter 4 (OAT4), organic cation transporter 3 (OCT3), organic cation/carnitine transporters (OCTN1 and OCTN2), multidrug and toxin extrusion 1 (MATE1), and equilibrative nucleoside transporters (ENT1 and ENT2) have been demonstrated on the apical membrane of syncytiotrophoblast, some of which also expressed on the basolateral membrane of syncytiotrophoblast or fetal capillary endothelium. These transporters are involved in transport of most drugs in the placenta, in turn, affecting drug distribution in fetus. Moreover, expressions of these transporters in the placenta often vary along with the gestational ages and are also affected by pathophysiological factor. This chapter will mainly illustrate function and expression of these transporters in placentas, their contribution to drug distribution in fetus, and their clinical significance.

Chemotherapy in pregnancy: exploratory study of the effects of paclitaxel on the expression of placental drug transporters

Introduction The use of paclitaxel in pregnant cancer patients is feasible in terms of fetal safety, but little is known about the effects of paclitaxel on the placenta. Using three experimental models, we aimed to assess the effects of paclitaxel on the expression of placental drug transporters. Methods In the in vitro model (human primary trophoblast culture), trophoblasts were isolated from normal term placentas and subsequently exposed to paclitaxel. The transcriptional regulation of 84 genes encoding for drug transporters, and the protein expression of ABCB1/P-gp and ABCG2/BCRP were assessed. In the in vivo model, placental tissues isolated from pregnant cancer patients treated with paclitaxel were analyzed to assess the protein expression of ABCB1/P-gp and ABCG2/BCRP. The same parameters were assessed in extracts from human placental cotyledons perfused ex vivo with paclitaxel. Results In the in vitro model, the expression of twelve drug-transporters genes was found to be significantly down-regulated after exposure to paclitaxel, including ABCC10, SLC28A3, SLC29A2, and ATP7B (involved in the transport of taxanes, antimetabolites, and cisplatin, respectively). The protein expression of ABCB1/P-gp increased by 1.3-fold after paclitaxel administration. Finally, the protein expression of ABCB1/P-gp and ABCG2/BCRP was higher in cotyledons from mothers treated with multiple doses of paclitaxel during pregnancy than in cotyledons perfused with a single dose of paclitaxel. Discussion Paclitaxel modulates the expression of placental drug transporters involved in the disposition of various anticancer agents. Further studies will be needed to assess the impact of repeated or prolonged exposure to paclitaxel on the expression and function of placental drug transporters.

Equilibrative Nucleoside Transporter 1 (ENT1, SLC29A1) Facilitates Transfer of the Antiretroviral Drug Abacavir across the Placenta

Abacavir is a preferred antiretroviral drug for preventing mother-to-child human immunodeficiency virus transmission; however, mechanisms of its placental transfer have not been satisfactorily described to date. Because abacavir is a nucleoside-derived drug, we hypothesized that the nucleoside transporters, equilibrative nucleoside transporters (ENTs, SLC29A) and/or Na⁺-dependent concentrative nucleoside transporters (CNTs, SLC28A), may play a role in its passage across the placenta. To test this hypothesis, we performed uptake experiments using the choriocarcinoma-derived BeWo cell line, human fresh villous fragments, and microvillous plasma membrane (MVM) vesicles. Using endogenous substrates of nucleoside transporters, [³H]-adenosine (ENTs, CNT2, and CNT3) and [³H]-thymidine (ENTs, CNT1, and CNT3), we showed significant activity of ENT1 and CNT2 in BeWo cells, whereas experiments in the villous fragments and MVM vesicles, representing a model of the apical membrane of a syncytiotrophoblast, revealed only ENT1 activity. When testing [³H]-abacavir uptakes, we showed that of the nucleoside transporters, ENT1 plays the dominant role in abacavir uptake into placental tissues, whereas contribution of Na⁺-dependent transport, most likely mediated by CNTs, was observed only in BeWo cells. Subsequent experiments with dually perfused rat term placentas showed that Ent1 contributes significantly to overall [³H]-abacavir placental transport. Finally, we quantified the expression of SLC29A in first- and third-trimester placentas, revealing that SLC29A1 is the dominant isoform. Neither SLC29A1 nor SLC29A2 expression changed over the course of placental development, but there was considerable interindividual variability in their expression. Therefore, drug-drug interactions and the effect of interindividual variability in placental ENT1 expression on abacavir disposition into fetal circulation should be further investigated to guarantee safe and effective abacavir-based combination therapies in pregnancy.

Regulation of human placental drug transporters in HCV infection and their influence on direct acting antiviral medications

INTRODUCTION

The objectives of this study were to determine how HCV infection affects placental drug transporters, and to determine the role of drug transporters on the cellular accumulation of direct-acting antiviral drugs in human trophoblasts.

METHODS

Eighty-four ABC and SLC transporter genes were first screened in normal and HCV infected pregnant women using PCR profiler array. The changes in expression were confirmed by qPCR and Western blot. The impact of selected drug transporters on the cellular accumulation of radiolabeled antiviral drugs sofosbuvir, entecavir, and tenofovir was measured in primary human trophoblasts (PHT) and BeWo b30 cells in the presence or absence of transporter-specific inhibitors. PHT were then treated with CL097, ssRNA40, and imquimod to determine the impact of Toll-like receptor (TLR) 7/8 activation on drug transporter expression.

RESULTS

The expression of the ABC efflux transporters ABCB1/P-gp and ABCG2/BCRP was increased in placenta of women with HCV, while the nucleoside transporters SLC29A1/ENT1 and SLC29A2/ENT2 remained unchanged. The accumulation of sofosbuvir and tenofovir was unaffected by inhibition of these transporters in trophoblast cells. Entecavir accumulation was decreased by the inhibition of ENT2. P-gp and BCRP inhibition enhanced entecavir accumulation in BeWo b30, but not PHT. Overall, there was little effect of TLR7/8 activation on these drug transporters, and the accumulation of entecavir in PHT.

DISCUSSION

The data suggest that expression of placental drug transporters and selection of antiviral drug may impact fetal drug exposure in pregnancies complicated by HCV infections.

Expression of Concentrative Nucleoside Transporters ( SLC28A) in the Human Placenta: Effects of Gestation Age and Prototype Differentiation-Affecting Agents

Equilibrative ( SLC29A) and concentrative ( SLC28A) nucleoside transporters contribute to proper placental development and mediate uptake of nucleosides/nucleoside-derived drugs. We analyzed placental expression of SLC28A mRNA during gestation. Moreover, we studied in choriocarcinoma-derived BeWo cells whether SLC29A and SLC28A mRNA levels can be modulated by activity of adenylyl cyclase, retinoic acid receptor activation, CpG islands methylation, or histone acetylation, using forskolin, all- trans-retinoic acid, 5-azacytidine, and sodium butyrate/sodium valproate, respectively. We found that expression of SLC28A1, SLC28A2, and SLC28A3 increases during gestation and reveals considerable interindividual variability. SLC28A2 was shown to be a dominant subtype in the first-trimester and term human placenta, while SLC28A1 exhibited negligible expression in the term placenta only. In BeWo cells, we detected mRNA of SLC28A2 and SLC28A3. Levels of the latter were affected by 5-azacytidine and all- trans-retinoic acid, while the former was modulated by sodium valproate (but not sodium butyrate), all- trans-retinoic acid, 5-azacytidine, and forskolin that caused 25-fold increase in SLC28A2 mRNA; we documented by analysis of syncytin-1 that the observed changes in SLC28A expression do not correlate with the morphological differentiation state of BeWo cells. Upregulated SLC28A2 mRNA was reflected in elevated uptake of [³H]-adenosine, high-affinity substrate of concentrative nucleoside transporter 2. Using KT-5720 and inhibitors of phosphodiesterases, we subsequently confirmed importance of cAMP/protein kinase A pathway in SLC28A2 regulation. On the other hand, SLC29A genes exhibited constitutive expression and none of the tested compounds increased SLC28A1 expression to detectable levels. In conclusion, we provide the first evidence that methylation status and activation of retinoic acid receptor affect placental SLC28A2 and SLC28A3 transcription and substrates of concentrative nucleoside transporter 2 might be taken up in higher extent in placentas with overactivated cAMP/protein kinase A pathway and likely in the term placenta.

The role of the equilibrative nucleoside transporter 1 on tissue and fetal distribution of ribavirin in the mouse

Ribavirin is used for the treatment of hepatitis C virus (HCV) infection. The equilibrative nucleoside transporter 1 (ENT1) expressed in hepatocytes transports ribavirin into the liver, the site of efficacy of the drug. However, it is still unclear whether ENT1 plays a dominant role in the hepatic distribution of the drug in vivo. In addition, due to fetal toxicity, administration of ribavirin to pregnant women with HCV infection is contraindicated. ENT1 might play a role in the fetal distribution and therefore the fetal toxicity of ribavirin. The aim of the present study was to investigate the in vivo contribution of ENT1 to the tissue distribution of ribavirin. When compared with that in Ent1(+/+) mice, the ribavirin tissue to plasma concentration ratio (including phosphorylated metabolites) in Ent1(-/-) mice at 15 min and 6 h after intravenous [(3) H]-ribavirin (3 mg/kg) administration was consistently and significantly decreased in the liver and the pancreas. Likewise, when compared with the Ent1(+/+) mice, the fetal distribution of ribavirin at 15 min after administration was significantly reduced in Ent1(-/-) fetuses and placenta. In contrast, there was no significant difference between Ent1(+/+), Ent1(+/-) and Ent1(-/-) mice in the fetal or placental to maternal plasma ribavirin concentration ratio at 2 h after ribavirin administration. The findings in the present study suggest that ENT1 plays a pivotal role in the distribution of ribavirin into tissues including the liver and pancreas, but affects only the rate, but not the extent, of ribavirin distribution into the fetus. Copyright © 2016 John Wiley & Sons, Ltd.

Expression and distribution of nucleoside transporter proteins in the human syncytiotrophoblast

The plasma membrane distribution and related biological activity of nucleoside transporter proteins (NTs) were investigated in human syncytiotrophoblast from term placenta using a variety of approaches, including nucleoside uptake measurements into vesicles from selected plasma membrane domains, NT immunohistochemistry, and subcellular localization (basal, heavy, and light apical membranes as well as raft-enriched membranes from the apical domain). In contrast with other epithelia, in this epithelium, we have identified the high-affinity pyrimidine-preferring human concentrative nucleoside transporter (hCNT) 1 as the only hCNT-type protein expressed at both the basal and apical membranes. hCNT1 localization in lipid rafts is also dependent on its subcellular localization in the apical plasma membrane, suggesting a complex cellular and regional expression. Overall, this result favors the view that the placenta is a pyrimidine-preferring nucleoside sink from both maternal and fetal sides, and hCNT1 plays a major role in promoting pyrimidine salvage and placental growth. This finding may be of pharmacological relevance, because hCNT1 is known to interact with anticancer nucleoside-derived drugs and other molecules, such as nicotine and caffeine, for which a great variety of harmful effects on placental and fetal development, including intrauterine growth retardation, have been reported.

Active transport across the human placenta: impact on drug efficacy and toxicity

The human placenta expresses a large number of transport proteins. The ATP-binding cassette (ABC) family of active efflux pumps, predominantly localised to the maternal-facing syncytial membrane of placental microvilli, comprise the major placental drug efflux transporters. A variety of other transporters are also expressed in the placenta that can facilitate xenobiotic transfer in both the maternal and fetal directions. Many drugs administered in pregnancy are ABC transporter substrates, and many are either teratogenic or fetotoxic. The in vitro, in vivo and clinical evidence reviewed in this article argues that active efflux of drugs by placental transporters helps to maintain its barrier function, reducing the incidence of adverse fetal effects. ABC transporter polymorphisms may explain the wide variability observed in fetal drug concentrations, incidence of teratogenesis or drug failure in pregnancies exposed to therapeutic agents. Although our understanding of the molecular mechanics and dynamics of placental drug transfer is advancing, much work is needed to fully appreciate the significance of placental drug transporters in the face of increasing drug administration in pregnancy.

Excretion of biliary compounds during intrauterine life

In adults, the hepatobiliary system, together with the kidney, constitute the main routes for the elimination of several endogenous and xenobiotic compounds into bile and urine, respectively. However, during intrauterine life the biliary route of excretion for cholephilic compounds, such as bile acids and biliary pigments, is very poor. Although very early in pregnancy the fetal liver produces bile acids, bilirubin and biliverdin, these compounds cannot be efficiently eliminated by the fetal hepatobiliary system, owing to the immaturity of the excretory machinery in the fetal liver. Therefore, the potentially harmful accumulation of cholephilic compounds in the fetus is prevented by their elimination across the placenta. Owing to the presence of detoxifying enzymes and specific transport systems at different locations of the placental barrier, such as the endothelial cells of chorionic vessels and trophoblast cells, this organ plays an important role in the hepatobiliary-like function during intrauterine life. The relevance of this excretory function in normal fetal physiology is evident in situations where high concentrations of biliary compounds are accumulated in the mother. This may result in oxidative stress and apoptosis, mainly in the placenta and fetal liver, which might affect normal fetal development and challenge the fate of the pregnancy. The present article reviews current knowledge of the mechanisms underlying the hepatobiliary function of the fetal-placental unit and the repercussions of several pathological conditions on this tandem.

Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.

Potential of various drugs to inhibit nucleoside uptake in rat syncytiotrophoblast cell line, TR-TBT 18d-1

The placenta requires nucleosides as nutrients for fetal growth, so it is important to examine potential interactions between placental transports of nucleosides and drugs to ensure the safety of pharmacotherapy during pregnancy. The purposes of this study are to clarify the uptake mechanisms of nucleosides from the maternal side of the syncytiotrophoblast and to investigate the inhibitory effect of various drugs on nucleoside uptake, using the rat syncytiotrophoblast cell line TR-TBT 18d-1, which shows syncytial-like morphology and functional expression of several transporters. Initial uptake of [(3)H]uridine or [(3)H]adenosine from the apical side of TR-TBT 18d-1 was markedly reduced by an excess of the respective unlabelled compound, and was slightly reduced by replacement of Na(+) with N-methyl-d-glucamine, indicating that both uptakes were Na(+)-independent. [(3)H]Uridine and [(3)H]adenosine uptakes in the absence of Na(+) were significantly and concentration-dependently inhibited by both 0.1 microM and 100 microM nitrobenzylthioinosine, suggesting the involvement of equilibrative nucleoside transporters (ENTs, SLC29). Kinetic analysis of adenosine uptake yielded a K(m) value of approximately 17 microM. These results are consistent with the reported uptake characteristics of uridine and adenosine by ENT1 and ENT2. The uptakes were significantly reduced by high concentrations of several nucleoside drugs, including cytarabine, vidarabine, zidovudine, mizoribine, caffeine and amitriptyline, but the effects were small within the therapeutic concentration ranges. In summary, our results suggest that ENTs are involved in apical uptake of uridine and adenosine in the syncytiotrophoblast. However, therapeutic concentrations of the drugs tested in this study might have little influence on maternal-to-fetal nucleoside transfer.

In situ hybridization and immunolocalization of concentrative and equilibrative nucleoside transporters in the human intestine, liver, kidneys, and placenta

To better understand the role of human equilibrative (hENTs) and concentrative (hCNTs) nucleoside transporters in physiology and pharmacology, we investigated the regional, cellular, and spatial distribution of two hCNTs (hCNT1 and hCNT2) and two hENTs (hENT1 and hENT2) in four human tissues. Using in situ hybridization and immunohistochemical techniques, we found that the duodenum expressed hCNT1 and hCNT2 mRNAs in enterocytes and hENT1 and hENT2 mRNAs in crypt cells. In these cells, the hCNT and hENT proteins were predominantly localized in the apical and lateral membrane, respectively. Hepatocytes expressed higher levels of mRNAs of hENT1, hCNT1, and hENT2 than of hCNT2 and expressed all these proteins at hepatocyte cell borders and in the cytoplasm. While the kidney expressed hCNT1 and hCNT2 mRNAs in the proximal tubules, hENT1 and hENT2 mRNAs were present in the distal tubules, glomeruli, endothelial cells, and vascular smooth muscle cells. Proximal tubules adjacent to corticomedullary junctions expressed hENT1, hCNT1, and hCNT2 mRNA. Immunolocalization studies revealed predominant localization of hCNTs in the brush-border membrane of the proximal tubular epithelial cells and hENTs in the basolateral membrane of the distal tubular epithelial cells. Chorionic villi sections of human term placenta expressed mRNAs and proteins for hENT1 and hENT2 but only mRNA for hCNT2. Immunolocalization studies showed presence of hENT1 in the brush-border membrane of the syncytiotrophoblasts. These data are critical for a better understanding of the role of nucleoside transporters in the physiological and pharmacological effects of nucleosides and nucleoside drugs, respectively.

Glutamate mediates acute glucose transport inhibition in hippocampal neurons

Although it is known that brain activity is fueled by glucose, the identity of the cell type that preferentially metabolizes the sugar remains elusive. To address this question, glucose uptake was studied simultaneously in cultured hippocampal neurons and neighboring astrocytes using a real-time assay based on confocal epifluorescence microscopy and fluorescent glucose analogs. Glutamate, although stimulating glucose transport in astrocytes, strongly inhibited glucose transport in neurons, producing in few seconds a 12-fold increase in the ratio of astrocytic-to-neuronal uptake rate. Neuronal transport inhibition was reversible on removal of the neurotransmitter and displayed an IC50 of 5 microm, suggesting its occurrence at physiological glutamate concentrations. The phenomenon was abolished by CNQX and mimicked by AMPA, demonstrating a role for the cognate subset of ionotropic glutamate receptors. Transport inhibition required extracellular sodium and calcium and was mimicked by veratridine but not by membrane depolarization with high K+ or by calcium overloading with ionomycin. Therefore, glutamate inhibits glucose transport via AMPA receptor-mediated sodium entry, whereas calcium entry plays a permissive role. This phenomenon suggests that glutamate redistributes glucose toward astrocytes and away from neurons and represents a novel molecular mechanism that may be important for functional imaging of the brain using positron emission tomography.

Conditionally immortalized syncytiotrophoblast cell lines as new tools for study of the blood-placenta barrier

Syncytiotrophoblasts play an essential role in restriction of drug delivery through the blood-placenta barrier (BPB). Conditionally immortalized syncytiotrophoblast cell lines, TR-TBTs, were established at gestational day 18 from pregnant transgenic rats (Tg-rats) harboring the temperature-sensitive SV 40 (ts SV40) large T-antigen. TR-TBTs exhibit temperature-sensitive cell growth due to the expression of SV 40 large T-antigen, and thus the cell growth can be regulated by changing the culture temperature. TR-TBTs exhibit typical properties of syncytiotrophoblast cells, such as syncytium-like morphology, the expression of cytokeratins and hormones, and polarized expression of glucose transporter 1 (GLUT1) and GLUT3. TR-TBTs express in vivo influx and efflux transporters, such as taurine transporter (TauT), betaine/GABA transporter (BGT-1), amino acid transporter 2 (ATA2), organic anion transporting polypeptide 2 (oatp2), organic cation/carnitine transporter (OCTN2), P-glycoprotein (P-gp), and breast cancer resistance protein (BCRP/ABCG2). Moreover, TR-TBTs exhibit taurine, GABA, and DHEA-S uptake activity via TauT, BGT-1, and oatp2, respectively. Therefore, TR-TBTs can be used for the analysis of these functions and would be a good in vitro models for investigating carrier-mediated transport functions at the BPB.

Blood-brain barrier transport of synthetic adenosine A1 receptor agonists in vitro: structure transport relationships

Transport of 11 structurally related adenosine A(1) receptor agonists was determined in an in vitro BBB model of brain-capillary-endothelial-cells and astrocytes. Inhibitor S-(4-nitrobenzyl)-6-thioinosine (NBTI) was used to quantify the contribution of the es nucleoside transporter to the overall transport. The N(6)-substituted adenosine analogues N(6)-cyclobutyladenosine (CBA), N(6)-cyclopentyladenosine (CPA) and N(6)-cyclohexyladenosine (CHA) showed concentration-dependent clearance and their transport could be inhibited by NBTI. The V(max) was 1.5+/-0.2 pmol min(-1) and the Km values were 2.2+/-0.2, 1.8+/-0.3 and 15+/-4 microM for CBA, CPA and CHA, respectively. Further chemical modification such as substitution in the C8-position or modification at the ribose-moiety resulted in loss of affinity for the es nucleoside transporter. Transport by passive diffusion was slow with clearances ranging from 0.21+/-0.01 microl min(-1) for 8-(methylamino)-CPA (MCPA) to 1.8+/-0.18 microl min(-1) for 5'-deoxy-CPA (5'dCPA). Regression analysis showed no relationship between transport clearance by passive diffusion and the GTP-shift, a non-linear relationship between the transport clearance by passive diffusion and the dynamic polar surface area (Cl=0.469e(-0.071DPSA); R2=0.88) and a linear relationship between transport clearance and prediction of BBB transport on basis of the Abraham equation (logCl=1.53logBB-1.56; R2=0.83). It is concluded that the transport of synthetic A(1) adenosine derivatives across the blood-brain barrier is generally quite slow. In addition, transport by the es nucleoside transporter may contribute to the transport of certain structurally distinct analogues.

Equilibrative nucleoside transporters of Arabidopsis thaliana. cDNA cloning, expression pattern, and analysis of transport activities

Equilibrative nucleoside transporters (ENTs) occur in diverse organisms. In the model plant Arabidopsis thaliana, eight potential ENTs (AtENTs) have been predicted by genome sequencing. We here report the cloning of the cDNAs for AtENTs 2, 3, 4, 6, 7, and 8. Conceptual translation of the cDNAs of AtENTs 2, 3, 4, 6, 7, and 8 yielded polypeptides possessing strong similarities to ENTs characterized previously. Eleven putative transmembrane domains were identified in each of the six AtENTs. In suspension cells, the transcription of AtENTs 1, 3, 4, 6, and 8 was increased by two treatments (nitrogen deprivation, application of 5-fluorouracil and methotrexate) that inhibited the de novo pathway of nucleotide synthesis, indicating that multiple members of the Arabidopsis ENT family may function in the salvage pathway of nucleotide synthesis. Except for AtENT1, the transcription of the remaining six AtENTs showed varying degrees of organ specificity. However, all seven AtENTs were expressed in the leaf and flower. In plant, insect, and yeast cells, ectopically expressed AtENT3 was targeted to the plasma membrane. AtENT3 expressed in yeast cells transported adenosine and uridine with high affinity. Furthermore, the activities of AtENT3 appear not to require a transmembrane proton gradient because protonophores did not abolish adenosine or uridine transport. In competition experiments, the transport of [3H]adenosine by AtENT3 was most significantly inhibited by a number of different purine and pyrimidine nucleosides and 2'-deoxynucleosides, although certain nucleobases and nucleotides were also found to have some inhibitory effect. This indicates that AtENT3 may possess broad substrate specificity. Adenosine and uridine transport by AtENT3, although partly sensitive to the vasodilator drugs dilazep and dipyridamole, was resistant to the nucleoside analogue nitrobenzylmercaptopurine ribonucleoside. We conclude that AtENT3 represents the first ei type ENT characterized from higher plants. The potential functions of ENTs in the biology of A. thaliana are discussed.

Glutamate triggers rapid glucose transport stimulation in astrocytes as evidenced by real-time confocal microscopy

Glutamate stimulates glycolysis in astrocytes, a phenomenon that couples astrocytic metabolism with neuronal activity. However, it is not known whether glutamate also affects glucose transporter-1 (GLUT1), the transporter responsible for glucose entry into astrocytes. To address this question, two different real-time single-cell hexose uptake assays were applied to cultured hippocampal astrocytes using confocal epifluorescence microscopy. Glutamate caused a twofold to threefold increase in the zero-trans uptake rates of the fluorescent hexoses 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2-NBDG) and 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-6-deoxyglucose (6-NBDG). Galactose uptake, determined by the calcein volumetric assay, was stimulated to a similar extent, confirming the fluorescent hexose data, and also demonstrating that glutamate stimulation is a Vmax effect. Remarkably, glucose transport stimulation developed fully inside 10 sec, which is 100 times faster than acute stimulations of glucose transport in other cell types. Glutamate did not significantly affect the rate of 6-NBDG uptake by GLUT1-expressing epithelial Clone 9 cells, suggesting that an astrocyte-specific factor is required for transport stimulation. We conclude that glucose transport stimulation occurs early during astrocytic activation by glutamate, which provides a novel regulatory node to current models of brain energy metabolism. This mechanism should also be considered for the interpretation of functional imaging data based on hexoses.

The presence of xenobiotic transporters in rat placenta

Understanding the role of transporters in placental handling of xenobiotics across the maternal-fetal interface is essential to evaluate the pharmacological and toxicological potential of therapeutic agents, drugs of abuse, and other xenobiotics to which the mother is exposed during pregnancy. Therefore, the purpose of this study was to assess mRNA levels of various transporters in placenta and to compare these to levels in maternal liver and kidney, predominant organs of excretion, to determine which transporters are likely to have a role in xenobiotic transfer within the placenta. During late stage pregnancy, relative amounts of mRNA levels of 40 genes representing 11 families/group of transporters were assessed in placenta with respect to relative maternal liver and kidney mRNA levels. Members of the following transporter families were assessed: three multidrug resistance (Mdr), six multidrug resistance-associated protein (Mrp), eight organic anion-transporting polypeptide (Oatp), three organic anion transporters (Oat), five organic cation transporters (Oct), two bile acid transporters (Na(+)/taurocholate-cotransporting polypeptide and bile salt export protein), four metal (ZnT1, divalent metal transporter 1, Menkes and Wilsons), a prostaglandin, two peptide, two sterolin, and four nucleoside transporters. Of the 40 genes evaluated, 16 [Mdr1a and 1b, Mrp1 and 5, Oct3 and Octn1, Oatp3 and 12, four metal, a prostaglandin, AbcG8, equilibrative nucleoside transporter 1 (ENT1), and ENT2] were expressed in placenta at concentrations similar to or higher than in maternal liver and kidney. The abundance of these mRNA transcripts in placenta suggests a role for these transporters in placental transport of xenobiotics and supports their role in the transport of endogenous substances.

Polarized glucose transporters and mRNA expression properties in newly developed rat syncytiotrophoblast cell lines, TR-TBTs

In this study, we have established new syncytiotrophoblast cell lines (TR-TBTs) from the recently developed transgenic rat harboring temperature-sensitive simian virus 40 large T-antigen gene (Tg-rat). Four conditionally immortalized syncytiotrophoblast cell lines (TR-TBT 18d-1 approximately 4) were obtained from pregnant Tg-rats at gestational day 18. These cell lines had a syncytium-like morphology, could be prepared as monolayers, expressed cytokeratins and rat syncytiotrophoblast markers, and exhibited apical or basal GLUT1 localizations and apical GLUT3 localizations. TR-TBTs express large T-antigen and grow well at 33 degrees C with a doubling time of about 30 h. TR-TBTs have processes for the uptake of dehydroepiandrosteron-3-sulfate (DHEAS) and these are predominantly located on the basal side, and this is the first report of an in vitro model of blood placental barrier (BPB) able to incorporate DHEAS. Therefore, TR-TBTs are an appropriate in vitro model for investigating carrier-mediated transport functions at the BPB. Moreover, TR-TBTs express betaine/GABA transporter (GAT-2/BGT-1), concentrative nucleoside transporter 2 (CNT2), equilibrative nucleoside transporter 1 (ENT1), and ENT2 and the expression of these transporters has been reported in blood-brain barrier (BBB). Thus, the expression patterns of nucleoside and neurotransmitter transporters examined are quite similar in both the BPB and BBB.

Nutrient transport across the placenta

The placenta forms a selective barrier that functions to transport nutrients that are of critical use to the fetus. Nutrient transport across the placenta is regulated by many different active transporters found on the surface of both maternal and fetal facing membranes of the placenta. The presence of these transporters in the placenta has been implicated in the facilitation of nutrient diffusion and proper fetal growth. In this review, recent developments concerning nutrient transporters that regulate glucose, amino acid, fatty acid, and nucleoside transplacental movement are discussed.

Molecular cloning and functional characterization of nitrobenzylthioinosine (NBMPR)-sensitive (es) and NBMPR-insensitive (ei) equilibrative nucleoside transporter proteins (rENT1 and rENT2) from rat tissues

Equilibrative nucleoside transport processes in mammalian cells are either nitrobenzylthioinosine (NBMPR)-sensitive (es) or NBMPR-insensitive (ei). Previously, we isolated a cDNA from human placenta encoding the 456-residue glycoprotein hENT1. When expressed in Xenopus oocytes, hENT1 mediated es-type transport activity and was inhibited by coronary vasoactive drugs (dipyridamole and dilazep) that may compete with nucleosides and NBMPR for binding to the substrate binding site. We now report the molecular cloning and functional expression of es and ei homologs of hENT1 from rat tissues; rENT1 (457 residues) was 78% identical to hENT1 in amino acid sequence, and rENT2 (456 residues) was 49-50% identical to rENT1/hENT1 and corresponded to a full-length form of the delayed-early proliferative response gene product HNP36, a protein of unknown function previously cloned in truncated form. rENT1 was inhibited by NBMPR (IC50 = 4.6 nM at 10 microM uridine), whereas rENT2 was NBMPR-insensitive (IC50 > 1 microM). Both proteins mediated saturable uridine influx (Km = 0.15 and 0.30 mM, respectively), were broadly selective for purine and pyrimidine nucleosides, including adenosine, and were relatively insensitive to inhibition by dipyridamole and dilazep (IC50 > 1 microM). These observations demonstrate that es and ei nucleoside transport activities are mediated by separate, but homologous, proteins and establish a function for the HNP36 gene product.

Functional and molecular characteristics of Na(+)-dependent nucleoside transporters

Nucleoside transporters play a critical role in the absorption, disposition, and targeting of therapeutically used nucleosides and nucleoside analogs. This review is focused on the Na(+)-dependent, concentrative nucleoside transporters which are found in a variety of cells including renal, intestinal and hepatic epithelia. Five major Na(+)-dependent nucleoside transporter subtypes have been characterized in isolated tissue preparations: N1 is purine selective; N2 is pyrimidine selective and N3-N5 exhibit variable selectively for both purine and pyrimidine nucleosides. The recent cloning of N1 and N2 nucleoside transporters has provided the first information on the molecular function and structure of concentrative nucleoside transporters. In this manuscript we review the characteristics of the various subtypes of nucleoside transporters and the molecular structure, functional properties, and tissue distribution of the cloned Na(+)-dependent nucleoside transporters. In addition, the interactions of nucleosides and nucleoside analogs with the cloned transporters in mammalian and amphibian expression systems are presented. Mammalian expression systems may be particularly useful during drug development in screening potential compounds for improved bioavailability and tissue specific targeting. Finally, we present our view of future ares of study in the field of nucleoside transporters.

Cloning of a human nucleoside transporter implicated in the cellular uptake of adenosine and chemotherapeutic drugs

In most mammalian cells nucleoside uptake occurs primarily via broad-specificity, es (e, equilibrative; 5, sensitive to NBMPR inhibition) transporters that are potently inhibited by nitrobenzylthioinosine (NBMPR). These transporters are essential for nucleotide synthesis by salvage pathways in hemopoietic and other cells that lack de novo pathways and are the route of cellular uptake for many cytotoxic nucleosides used in cancer and viral chemotherapy. They play an important role in adenosine-mediated regulation of many physiological processes, including neurotransmission and platelet aggregation, and are a target for coronary vasodilator drugs. We have previously reported the purification of the prototypic es transporter from human erythrocytes and have shown that this glycoprotein of apparent M, 55,000 is immunologically related to nucleoside transporters from several other species and tissues, including human placenta. Here we report the isolation of a human placental cDNA encoding a 456-residue glycoprotein with functional characteristics typical of an es-type transporter. It is predicted to possess 11 membrane-spanning regions and is homologous to several proteins of unknown function in yeast, nematodes, plants and mammals. Because of its central role in the uptake both of adenosine and of chemotherapeutic nucleosides, study of this protein should not only provide insights into the physiological roles of nucleoside transport but also open the way to improved therapies.

Bovine adrenal endothelial cells express nucleoside transporters nonregulated by protein kinases A and C

The present investigation characterizes the nucleoside transporters in bovine adrenomedullary endothelial cells and their possible regulation by the action of protein kinases A and C to establish comparisons with the nucleoside transport system in chromaffin cells. The nucleoside transport proved to be a nitrobenzylthioinosine (NBTI)-sensitive facilitated-diffusion system with high affinity for adenosine. These endothelial cells had a high density of nucleoside transporters (660,000 +/- 130,000 transporters/ cell), measured by NBTI binding, and the efficiency was close to 2 adenosine molecules internalized transporter-1.s-1. The stimulation of the cells with bradykinin and P1,P4-di(adenosine-5')tetraphosphate, which raise the intracellular Ca2+ concentration, did not modulate the adenosine transport. When the cells were stimulated with signals coupled to adenosine 3',5'-cyclic monophosphate intracellular production, such as norepinephrine and isoproterenol, the adenosine transport was not modified. Furthermore, the treatment of the cells with direct activators of both protein kinases A and C had no effect on adenosine transport, in contrast to that reported in chromaffin cells.

Adenosine transporters

1. In mammals, nucleoside transport is an important determinant of the pharmacokinetics, plasma and tissue concentration, disposition and in vivo biological activity of adenosine as well as nucleoside analogues used in antiviral and anticancer therapies. 2. Two broad types of adenosine transporter exist, facilitated-diffusion carriers and active processes driven by the transmembrane sodium gradient. 3. Facilitated-diffusion adenosine carriers may be sensitive (es) or insensitive (ei) to nanomolar concentrations of the transport inhibitor nitrobenzylthioinosine (NBMPR). Dipyridamole, dilazep and lidoflazine analogues are also more potent inhibitors of the es carrier than the ei transporter in cells other than those derived from rat tissues. 4. The es transporter has a broad substrate specificity (apparent Km for adenosine approximately 25 microM in many cells at 25 degrees C), is a glycoprotein with an average apparent Mr of 57,000 in human erythrocytes that has been purified to near homogeneity and may exist in situ as a dimer. However, there is increasing evidence to suggest the presence of isoforms of the es transporter in different cells and species, based on kinetic and molecular properties. 5. The ei transporter also has a broad substrate specificity with a lower affinity for some nucleoside permeants than the es carrier, is genetically distinct from es but little information exists as to the molecular properties of the protein. 6. Sodium-dependent adenosine transport is present in many cell types and catalysed by four distinct systems, N1-N4, distinguished by substrate specificity, sodium coupling and tissue distribution. 7. Two genes have been identified which encode sodium-dependent adenosine transport proteins, SNST1 from the sodium/glucose cotransporter (SGLT1) gene family and the rat intestinal N2 transporter (cNT1) from a novel gene family including a bacterial nucleoside carrier (NupC). Transcripts of cNT1, which encodes a 648-residue protein, are found in intestine and kidney only. 8. Success in cloning the remaining adenosine transporter genes will improve our understanding of the diversity of nucleoside transport processes, with a view to better targeting of therapeutic nucleoside analogues and protective use of transport inhibitors.

Quantitation and immunolocalization of glucose transporters in the human placenta

The subcellular distributions of the mammalian passive glucose transporter isoforms GLUT1, GLUT3 and GLUT4, in the human placenta, were investigated using isoform-specific anti-peptide antibodies. On western blots of both basal and brush-border plasma membranes isolated from the syncytiotrophoblast, antibodies specific for GLUT1 labelled a broad band (apparent Mr 55,000) that co-migrated with the human erythrocyte GLUT1 glucose transporter. In contrast, no labelling was detectable when blots were probed with antibodies specific for the GLUT3 or GLUT4 isoforms. Densitometric analysis of blots showed that GLUT1 accounts for approximately 90 and 65 per cent of the D-glucose-sensitive cytochalasin B binding sites present in brush-border and basal membranes, respectively. Confocal immunofluorescence microscopy of fixed placental tissue showed that GLUT1 is abundant at both maternal- and fetal-facing surfaces of the syncytiotrophoblast whereas it was undetectable at the fetal capillary endothelium. In parallel experiments, no staining by antibodies against either the GLUT3 or the GLUT4 isoforms was detected in placental tissue. These results indicate that GLUT1 is the major isoform responsible for glucose transfer from mother to fetus. The absence of GLUT4 is consistent with the lack of insulin-sensitive glucose transport across the placenta.