Expression of members of the multidrug resistance protein family in human term placenta

Mercury exposure and public health

Mercury is a metal that is a liquid at room temperature. Mercury has a long and interesting history deriving from its use in medicine and industry, with the resultant toxicity produced. In high enough doses, all forms of mercury can produce toxicity. The most devastating tragedies related to mercury toxicity in recent history include Minamata Bay and Niagata, Japan in the 1950s, and Iraq in the 1970s. More recent mercury toxicity issues include the extreme toxicity of the dimethylmercury compound noted in 1998, the possible toxicity related to dental amalgams, and the disproved relationship between vaccines and autism related to the presence of the mercury-containing preservative, thimerosal.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Expression of the breast cancer resistance protein (Bcrp1/Abcg2) in tissues from pregnant mice: effects of pregnancy and correlations with nuclear receptors

The breast cancer resistance protein (BCRP) plays an important role in drug disposition, including limiting drug penetration across the placental barrier. Our goal was to investigate the effects of pregnancy on Bcrp1 expression in pregnant mice. We examined Bcrp1 expression in placenta, kidney, liver, and small intestine at various gestational ages. Bcrp1 protein levels peaked at gestation day (gd) 15 in placenta, at gd 10 and 15 in kidney, and at gd 15 in liver; however, Bcrp1 protein levels in small intestine did not change significantly with gestational ages. Immunohistochemistry analysis revealed that the cellular localization of Bcrp1 in placenta, kidney, liver, and small intestine was not influenced by pregnancy. Bcrp1 mRNA levels were analyzed by quantitative real-time RT-PCR. In general, the effects of pregnancy on Bcrp1 protein somewhat lagged behind the effects on Bcrp1 mRNA. To further investigate the possible roles of nuclear receptors in the regulation of the Bcrp1 gene during pregnancy, we examined mRNA levels of aryl hydrocarbon receptor (AhR), hypoxia-inducible factor 1alpha (HIF1alpha), estrogen receptor alpha (ERalpha), estrogen receptor beta (ERbeta), or progesterone receptor and compared them with those of Bcrp1. Bcrp1 mRNA was significantly correlated with mRNA of AhR, HIF1alpha, and ERbeta in placenta, with mRNA of HIF1alpha in kidney, and with mRNA of AhR and ERalpha in liver. These data suggest that Bcrp1 expression in mouse tissues can be altered by pregnancy in a gestational age-dependent manner. Such effects are likely mediated by certain nuclear receptors through a transcriptional mechanism.

P-glycoprotein in the placenta: Expression, localization, regulation and function

Detailed understanding of the mechanisms employed in transfer of drugs across the placenta is essential for optimization of pharmacotherapy during pregnancy. Disclosure of drug efflux transporters as an "active component" of the placental barrier has brought new important insights into the field of transplacental pharmacokinetics. P-glycoprotein (P-gp, MDR1) is the first discovered and so far the best characterized of drug efflux transporters, whose role in the regulation of drug disposition to the fetus has been extensively studied. Expression of P-gp in the placental trophoblast layer was confirmed at the mRNA and protein levels in all phases of pregnancy, and several in vitro and in vivo studies demonstrated functional activity of the transporter in materno-fetal drug transport. P-gp is able to actively pump drugs and other xenobiotics from trophoblast cells back to the maternal circulation, providing thus protection to the fetus. This review summarizes the current knowledge on the expression, localization and function of P-gp in the placenta. In addition, we include the latest data concerning transcriptional regulation of placental P-gp expression and polymorphisms of the MDR1 gene. Clinical significance of placental P-gp and its future perspectives for pharmacotherapy during pregnancy are also discussed.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

The apical conjugate efflux pump ABCC2 (MRP2)

ABCC2 is a member of the multidrug resistance protein subfamily localized exclusively to the apical membrane domain of polarized cells, such as hepatocytes, renal proximal tubule epithelia, and intestinal epithelia. This localization supports the function of ABCC2 in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly in the unidirectional efflux of substances conjugated with glutathione, glucuronate, or sulfate, as exemplified by leukotriene C(4), bilirubin glucuronosides, and some steroid sulfates. The hepatic ABCC2 pump contributes to the driving forces of bile flow. Acquired or hereditary deficiency of ABCC2, the latter known as Dubin-Johnson syndrome in humans, causes an increased concentration of bilirubin glucuronosides in blood because of their efflux from hepatocytes via the basolateral ABCC3, which compensates for the deficiency in ABCC2-mediated apical efflux. In this article we provide an overview on the molecular characteristics of ABCC2 and its expression in various tissues and species. We discuss the transcriptional and posttranscriptional regulation of ABCC2 and review approaches to the functional analysis providing information on its substrate specificity. A comprehensive list of sequence variants in the human ABCC2 gene summarizes predicted and proven functional consequences, including variants leading to Dubin-Johnson syndrome.

ABC drug transporter expression and functional activity in trophoblast-like cell lines and differentiating primary trophoblast

ATP-binding cassette (ABC) efflux transporters are expressed in the human placenta where they are thought to help protect the fetus from xenobiotics. To evaluate models for analysis of ABC transporter function and regulation in the placenta, we have characterized the expression and activity of multidrug resistance (MDR) 1/P glycoprotein (Pgp), MDR3/Pgp, breast cancer resistance protein (BCRP), and multidrug resistance proteins 1 and 2 (MRPs 1, 2) in differentiating primary trophoblast cells and BeWo and Jar cell lines. Real-time PCR and immunoblotting were used for analysis of mRNA and protein expression, respectively. Functional activity was measured using selective inhibitors of efflux of fluorescent substrates, calcein-AM (Pgp and MRPs) and Hoechst 33342 (BCRP). The levels of MDR1 mRNA and protein expression were much higher in trophoblast than in Jar and especially BeWo cells. Expression of MDR3 protein was also lower in BeWo cells. Levels of MDR3 expression were markedly higher than MDR1 levels in all tested cell types. Levels of both MDR1 and MDR3 expression decreased during trophoblast differentiation/syncytialization. BCRP was highly expressed in all cell types and increased with trophoblast differentiation. MRP1 expression was much lower in trophoblasts compared with both cell lines. In contrast to its abundant mRNA expression, MRP2 protein was practically undetectable in BeWo and Jar cells and was present only at very low levels in trophoblast. Functional studies confirmed the presence of active Pgp and BCRP in all studied cell types, whereas MRP functional activity was detected only in BeWo and Jar cells. Both cell lines may be useful models for studying various aspects of placental ABC transporter expression and function, but also have significant limitations. With respect to their ABC protein expression profile, Jar cells are more similar to nondifferentiated cytotrophoblast, whereas BeWo appear to more closely reflect differentiated syncytiotrophoblast.

Targeting multidrug resistance in cancer

Effective treatment of metastatic cancers usually requires the use of toxic chemotherapy. In most cases, multiple drugs are used, as resistance to single agents occurs almost universally. For this reason, elucidation of mechanisms that confer simultaneous resistance to different drugs with different targets and chemical structures - multidrug resistance - has been a major goal of cancer biologists during the past 35 years. Here, we review the most common of these mechanisms, one that relies on drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters. We describe various approaches to combating multidrug-resistant cancer, including the development of drugs that engage, evade or exploit efflux by ABC transporters.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Role of efflux pumps and metabolising enzymes in drug delivery

The impact of efflux pumps and metabolic enzymes on the therapeutic activity of various drugs has been well established. The presence of efflux pumps on various tissues and tumours has been shown to regulate the intracellular concentration needed to achieve therapeutic activity. The notable members of efflux proteins include P-glycoprotein, multi-drug resistance protein and breast cancer resistance protein. These efflux pumps play a pivotal role not only in extruding xenobiotics but also in maintaining the body's homeostasis by their ubiquitous presence and ability to coordinate among themselves. In this review, the role of efflux pumps in drug delivery and the importance of their tissue distribution is discussed in detail. To improve pharmacokinetic parameters of substrates, various strategies that modulate the activity of efflux proteins are also described. Drug metabolising enzymes mainly include the cytochrome P450 family of enzymes. Extensive drug metabolism due to the this family of enzymes is the leading cause of therapeutic inactivity. Therefore, the role of metabolising enzymes in drug delivery and disposition is extensively discussed in this review. The synergistic relationship between metabolising enzymes and efflux proteins is also described in detail. In summary, this review emphasises the urgent need to make changes in drug discovery and drug delivery as efflux pumps and metabolising enzymes play an important role in drug delivery and disposition.

Excretion of fetal biliverdin by the rat placenta-maternal liver tandem

Fetal liver immaturity is accompanied by active heme catabolism. Thus fetal biliary pigments must be excreted toward the mother by the placenta. To investigate biliverdin handling by the placenta-maternal liver tandem, biliverdin-IXalpha was administered to 21-day pregnant rats through the jugular vein or the umbilical artery of an in situ perfused placenta. Jugular administration resulted in the secretion into maternal bile of both bilirubin and biliverdin (3:1). However, when biliverdin was administered to the placenta, most of it was transformed into bilirubin before being transferred to the maternal blood. Injecting Xenopus laevis oocytes with mRNA from rat liver or placenta enhanced their ability to take up biliverdin, which was inhibited by estradiol 17beta-d-glucuronide. The expression of three OATP isoforms in this system revealed that they have a varying degrees of ability to transport biliverdin (Oatp1/1a1 > Oatp2/1a4 > Oatp4/1b2). The abundance of their mRNA in rat trophoblast was Oatp1/1a1 >> Oatp4/1b2 > Oatp2/1a4. The expression of biliverdin-IXalpha reductase in rat placenta was detected by RT-PCR/sequencing and Western blot analysis. The relative abundance of biliverdin-IXalpha reductase mRNA (determined by real-time quantitative RT-PCR) was fetal liver > placenta > maternal liver. Common bile duct ligation in the last week of pregnancy induced an upregulation of biliverdin-IXalpha reductase in maternal liver but had no effect on fetal liver and placenta. In conclusion, several members of the OATP family may contribute to the uptake of fetal biliverdin by the rat placenta. Before being transferred to the mother, biliverdin is extensively converted into bilirubin by biliverdin-IXalpha reductase, whose expression is maintained even though bilirubin excretion into maternal bile is impaired.

Apical/Basolateral Surface Expression of Drug Transporters and its Role in Vectorial Drug Transport

It is well known that transporter proteins play a key role in governing drug absorption, distribution, and elimination in the body, and, accordingly, they are now considered as causes of drug-drug interactions and interindividual differences in pharmacokinetic profiles. Polarized tissues directly involved in drug disposition (intestine, kidney, and liver) and restricted distribution to naive sanctuaries (blood-tissue barriers) asymmetrically express a variety of drug transporters on the apical and basolateral sides, resulting in vectorial drug transport. For example, the organic anion transporting polypeptide (OATP) family on the sinusoidal (basolateral) membrane and multidrug resistance-associated protein 2 (MRP2/ABCC2) on the apical bile canalicular membrane of hepatocytes take up and excrete organic anionic compounds from blood to bile. Such vectorial transcellular transport is fundamentally attributable to the asymmetrical distribution of transporter molecules in polarized cells. Besides the apical/basolateral sorting direction, distribution of the transporter protein between the membrane surface (active site) and the intracellular fraction (inactive site) is of practical importance for the quantitative evaluation of drug transport processes. The most characterized drug transporter associated with this issue is MRP2 on the hepatocyte canalicular (apical) membrane, and it is linked to a genetic disease. Dubin-Johnson syndrome is sometimes caused by impaired canalicular surface expression of MRP2 by a single amino acid substitution. Moreover, single nucleotide polymorphisms in OATP-C/SLC21A6 (SLCO1B1) also affect membrane surface expression, and actually lead to the altered pharmacokinetic profile of pravastatin in healthy subjects. In this review article, the asymmetrical transporter distribution and altered surface expression in polarized tissues are discussed.

Increased expression of MDR1 mRNAs and P-glycoprotein in placentas from HIV-1 infected women

P-glycoprotein transports several compounds including protease inhibitors, actually used in the clinical management of HIV-1 infection. Since P-glycoprotein is expressed in placental trophoblasts, its efflux activity could interfere with placental transfer of antiretrovirals. The purpose of this study was to investigate the expression of P-gp-encoding MDR1 gene and P-gp itself in full-term placentas from uninfected (n=35) and HIV-1 infected women (n=24). MDR1 transcripts were quantified by real-time PCR using relative (MDR1 normalized upon 28S levels) and absolute (copy number) determinations. P-glycoprotein localization and expression were evaluated by immunohistochemistry and western blot analysis, respectively. Relative or absolute PCR quantification showed a significant 3.3-fold (p<0.0009) or 3.7-fold (p<0.0002) mean increase in MDR1 placental transcription in HIV-infected compared to non-infected women, respectively. Ratios of individual HIV-positive values to HIV-negative mean ranged from 0.1 to 21.8. Moreover a significant 2.5-fold increased expression of immunoreactive P-glycoprotein was evidenced in placentas from HIV-infected women (p<0.0001). This MDR1 overexpression was observed in a similar extent in placentas from pregnant women treated with Zidovudine alone or in combination with Nelfinavir and/or Lamivudine. Our findings suggest that P-glycoprotein in placentas from HIV-infected women would contribute to modulate the materno-fetal transport of antiretrovirals across the placental barrier and consequently diminish fetal exposure to these compounds.

MDR1 P-gp expression and activity in intact human placental tissue; upregulation by retroviral transduction

This study investigates P-gp activity in placental villous fragments and the possibility of upregulating its expression and function by retroviral transduction. In fresh fragments, cyclosporin A caused a significant increase in 3H-vinblastine accumulation (187+/-48% at 180 min n=4), consistent with multi-drug resistance activity. After 7 days in culture, villous fragments showed a similar increase in 3H-vinblastine accumulation (143+/-10% at 180 min n=4), which was not significantly different from that in fresh tissue. Following transduction, immunohistochemistry revealed increased P-gp expression. However, the distribution of the protein differed from that in controls, with P-gp being located throughout the tissue as opposed to the normal specific location on the maternal facing plasma membrane. Transduced explants showed a significantly larger increase in 3H-vinblastine accumulation in the presence of cyclosporin A than control explants (245+/-15.5% at 180 min, n=4), suggesting reduced capacity to efflux vinblastine. This study demonstrates P-gp activity in intact placental tissue which is maintained in explant culture. Retroviral transduction of P-gp to such tissue leads to increased but undirected expression of the protein. The consequent increased activity at sites such as the basal, fetal facing, plasma membrane probably explains the increased substrate accumulation within the tissue.

Human placenta: a human organ for developmental toxicology research and biomonitoring

Pregnant mothers are exposed to a wide variety of foreign chemicals. This exposure is most commonly due to maternal medication, lifestyle factors, such as smoking, drug abuse, and alcohol consumption, or occupational and environmental sources. Foreign compounds may interfere with placental functions at many levels e.g. signaling, production and release of hormones and enzymes, transport of nutrients and waste products, implantation, cellular growth and maturation, and finally, at the terminal phase of placental life, i.e. delivery. Placental responses may also be due to pharmaco-/toxicodynamic responses to foreign chemicals, e.g. hypoxia. On the other hand, placental xenobiotic-metabolizing enzymes can detoxify or activate foreign chemicals, and transporters either enhance or prevent cellular accumulation and transfer across the placenta. The understanding of what xenobiotics do to the placenta and what the placenta does to the xenobiotics should provide the basis for the use of placenta as a tool to investigate and predict some aspects of developmental toxicity. This review aims to give an update of the fate and behavior of xenobiotics in the placenta from the viewpoint of xenobiotic-metabolizing enzymes and transporters. Their response levels will be described according to gestational status and methods used. The effects of foreign chemicals on placental metabolizing enzymes will be discussed. Also, interactions in the transporter protein level will be covered. The role of the placenta in contributing to developmental effects and fetotoxicity will be examined. The toxicological effects of maternal medications, smoking, and environmental exposures (dioxins, pesticides) as well as some possibilities for biomonitoring will be highlighted.

Role of P-glycoprotein in transplacental transfer of methadone

Methadone is the therapeutic agent of choice for treatment of the pregnant opiate addict. However, little is known on the factors affecting its concentration in the fetal circulation during pregnancy and how it might relate to neonatal outcome. Therefore, a better understanding of the function of placental metabolic enzymes and transporters should add to the knowledge of the role of the tissue in the disposition of methadone and its relation to neonatal outcome. We hypothesized that the expression and activity of the placental efflux transporter P-glycoprotein (P-gp) would affect the transfer of methadone to the fetal circulation. Data obtained utilizing dual perfusion of placental lobule and monolayers of Be-Wo cell line indicated that methadone is extruded by P-gp. Transfer of methadone to the fetal circuit was increased by 30% in the presence of the P-gp inhibitor GF120918 while the transfer of paclitaxel, a typical substrate of the glycoprotein, was increased by 50%. In the Be-Wo cell line, methadone and paclitaxel uptake was also increased in the presence of the P-gp inhibitor cyclosporin A. Moreover, the expression of P-gp in placental brush-border membranes varied between term placentas. Taken together, these data strongly suggest that the concentration of methadone in the fetal circulation is affected by the expression and activity of P-gp. It is reasonable to speculate that placental disposition of methadone affects its concentration in the fetal circulation. If true, this may also be directly related to the incidence and intensity of neonatal abstinence syndrome (NAS).

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Physiological, pharmacological and clinical features of the multidrug resistance protein 2

Multidrug resistance protein 2 (MRP2, ABCC2) is a drug efflux pump belonging to the ATP-binding cassette (ABC) transporter superfamily. MRP2 is present predominantly at the biliary pole of hepatocytes and is also expressed in the kidney and intestine. It plays a major role in hepato-biliary elimination of many structurally diverse xenobiotics, including organic anions and drug conjugates, and therefore most likely contributes to pharmacokinetic parameters of these compounds. MRP2 also handles endogenous molecules such as bilirubin, and its overexpression has been shown to confer a multidrug resistance phenotype to tumoral cells. MRP2 expression can be regulated by endogenous substances such as inflammatory cytokines and biliary acids. The MRP2 levels and activity can also be affected by a large panel of xenobiotics, including chemopreventive agents and ligands of the pregnane X receptor, which may be a potential source of drug-drug interactions and drug adverse effects. MRP2 appears therefore as one of the major drug efflux pumps of the organism, whose functional and regulatory features are important to consider, notably for drug disposition.

Expression, localization, and function of MRP5 (ABCC5), a transporter for cyclic nucleotides, in human placenta and cultured human trophoblasts: effects of gestational age and cellular differentiation

The placenta functions both as site for nutrition and protection of the fetus. Transport proteins, including members of the multidrug resistance protein (MRP)/ABCC subfamily, have been recognized to contribute to the latter function. MRP5 (ABCC5) was identified as transmembrane transport protein for cyclic nucleotides, especially 3',5'-cyclic GMP (cGMP), indicating an additional role in signal transduction and a potential role in placenta development. We therefore studied expression, localization, and function of MRP5 in placenta of different gestational ages. Quantitative real-time polymerase chain reaction revealed expression of MRP5 in all 60 samples from pre-term and term placenta, with a decreasing mean expression with gestational age (MRP5/18S-ratio x 1000; < 32 weeks: 2.91 +/- 0.73, n = 15; 32 to 37 weeks: 2.10 +/- 0.87, n = 15; > 37 weeks: 0.46 +/- 0.08, n = 30; P < 0.01). Immunofluorescence microscopy with an anti-MRP5 antibody indicated localization of MRP5 preferentially in the basal membrane of syncytiotrophoblasts and in and around fetal vessels. ATP-dependent [(3)H]cGMP transport as evidence for MRP5 function could be demonstrated in isolated basal membrane vesicles. Moreover, the influence of cellular differentiation on MRP5 expression was studied in isolated trophoblasts, revealing an increase of the MRP5 expression in parallel with the hCG production (MRP5/18S-ratio x 1000 was 2.4 +/- 0.5 at day 5 of culture and 1.45 +/- 0.5 at day 0 of culture, n = 3 preparations, significant difference with P < 0.05). In conclusion, MRP5 expression depends on gestational age and varies throughout the differentiation process. In view of the important role of cGMP for cellular differentiation, MRP5 may play a role in placental development in context with a specific need for cellular cGMP export.

Immuno-histochemical detection of MRPs in human lung cells in culture

The transport of molecules across membranes is an essential function of all living organisms and a large number of specific transporters have evolved to carry out this function. The largest transporter gene family is the ATP-binding cassette (ABC) transporter superfamily. The multidrug resistance-associated protein (MRP) family is comprised of nine related ABC transporters. The intra-cellular distribution of the different MRP isoforms in relation to their physiological and non physiological function is still a point of discussion. For this purpose we used normal human lung cells (bronchial epithelial cells, NHBEC, and peripheral lung cells, PLC) as well as tumor cell cultures as test tools to investigate the intracelluar localization of these proteins under classical culture conditions and under air-liquid interface by means of indirect fluorescence microscopy. Characterization of the cultured cells as lung epithelial cells was performed by means of immuno-histochemical analysis. MRP1 and MRP3 were localised to the cellular membrane in all tested lung cell types. In contrast to that MRP2, MRP4 and MRP5 could be described as intracellular proteins in NHBEC and PLC. All MRP1-MRP5 isoforms could be characterized in A549 tumor cell line as membrane proteins. In order to imitate the physiological in vivo circumstances in the lung, we have established a dry/wet method (air-liquid interface) for cell cultivation so that cultured cells have the option to polarize between air and basal membrane and this might influence the distribution pattern of MRP1 and MRP2 in NHBEC. Using confocal laser scanning techniques we could show that in cells kept under dry/wet conditions MRP1 was found to be localised to baso-lateral cell regions while MRP2 was localised to all cell regions. Under classical culture conditions MRP1 was not localized to particular membrane regions and MRP2 was found to be an intracellular protein.

P-glycoprotein expression of the human placenta during pregnancy

OBJECTIVE

To investigate whether the placental expression of P-glycoprotein shows a quantitative difference during pregnancy.

STUDY DESIGN

Villous tissue was collected from chorionic villus samples (13-14 weeks of gestation; n = 3 and 20-25 weeks of gestation; n = 4) and from full-term placentas (38-41 weeks of gestation; n = 28). P-glycoprotein was detected by western blot analysis and quantified by densitometry.

RESULTS

We showed for the first time a significant and progressive two-fold decrease in the mean expression of P-glycoprotein between early and late samples, with a major overlap of values.

CONCLUSION

As P-glycoprotein appears to be involved in drug extrusion, these data suggest that the placenta's ability to protect the fetus from xenobiotics is greater in early pregnancy than at term.

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.

Variable expression of P-glycoprotein in the human placenta and its association with mutations of the multidrug resistance 1 gene (MDR1, ABCB1)

The MDR1 gene product P-glycoprotein in the human placenta is important for protecting the fetus from unintended, harmful drug exposure, but also for limiting the access of therapeutic drugs to the fetus after maternal drug intake. A polymorphism in exon 26 of the MDR1 gene (C3435T) has previously been shown to be associated with reduced P-glycoprotein expression in the small intestine, kidney and lymphocytes. In the present study, we examined systematically whether MDR1 polymorphisms also have an impact on P-glycoprotein expression in the human placenta. MDR1 mRNA and P-glycoprotein were analysed in 73 full-term human placentas of Caucasians, as well as respective MDR1 genotypes/haplotypes, for the C3435T and G2677T/A polymorphisms of mothers and infants. MDR1 mRNA levels were not different between these genotype groups. However, P-glycoprotein expression was significantly lower when both mother and infant were homozygous for the 3435T allele (TT/tt) compared to maternal and fetal homozygotes for the C-allele (0.40 +/- 0.18 a.u. for TT/tt versus 0.66 +/- 0.30 a.u. for CC/cc, P = 0.01). Moreover, placentas from mothers carrying both polymorphisms (3435T and 2677T; TT/TT) also had a significantly lower P-glycoprotein expression (0.31 +/- 0.12 a.u.) compared to placentas of wild-type individuals (CC/GG, 0.71 +/- 0.31 a.u., P = 0.02). Taken together, the MDR1 polymorphisms C3435T and G2677T are associated with altered P-glycoprotein expression in the human placenta, and may have clinical consequences due to genetically determined, variable drug exposure of the fetus.

The ABC Transporters MDR1 and MRP2: Multiple Functions in Disposition of Xenobiotics and Drug Resistance

ATP-binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P-glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug-drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.

EXPRESSION, LOCALIZATION, AND FUNCTION OF THE CARNITINE TRANSPORTER OCTN2 (SLC22A5) IN HUMAN PLACENTA

L-carnitine is assumed to play an important role in fetal development, and there is evidence that carnitine is transported across the placenta. The protein involved in this transfer, however, has not been identified on a molecular level. We therefore characterized localization and function of the carnitine transporter OCTN2 in human placenta. Significant expression of OCTN2 mRNA was detected in human placenta applying real-time polymerase chain reaction technology. Confocal immunofluorescence microscopy using an antibody directed against the carboxy terminus of OCTN2 protein revealed that it is predominantly expressed in the apical membrane of syncytiotrophoblasts. This was confirmed by the costaining of organic anion-transporting polypeptide B and MRP2, which are known to be expressed mainly in the basal and apical syncytiotrophoblasts membrane, respectively. To further support this finding, we performed transport studies using basal and apical placenta membrane vesicles. We could demonstrate that the carnitine uptake into the apical vesicles was about eight times higher compared with the basal ones. Moreover, this uptake was sodium- and pH-dependent with an apparent K(m) value of 21 muM and inhibited by verapamil, which is in line with published data for recombinant OCTN2. Finally, experiments using trophoblasts in cell culture revealed that expression of OCTN2 paralleled human choriogonadotropin production and thus is modulated by cellular differentiation. In summary, we show expression and function of OCTN2 in human placenta. Moreover, several lines of evidence indicate that OCTN2 is localized in the apical membrane of syncytiotrophoblasts, thereby suggesting a major role in the uptake of carnitine during fetal development.

Temporal expression profiles of organic anion transport proteins in placenta and fetal liver of the rat

Physiological cholestasis linked to immature hepatobiliary transport systems for organic anions occurs in rat and human neonates. In utero, the placenta facilitates vectorial transfer of certain fetal-derived solutes to the maternal circulation for elimination. We compared the ontogenesis of organic anion transporters in the placenta and the fetal liver of the rat to assess their relative abundance throughout gestation and to determine whether the placenta compensates for the late maturation of transporters in the developing liver. The mRNA of members of the organic anion transporting polypeptide (Oatp) superfamily, the multidrug resistance protein (Mrp) family, one organic anion transporter (OAT), and the bile acid carriers Na(+)-taurocholate cotransporting polypeptide (Ntcp) and bile salt export pump (Bsep) was quantified by real-time PCR. The most abundant placental transporters were Oatp4a1, whose mRNA increased 10-fold during gestation, and Mrp1. Mrp1 immunolocalized predominantly to epithelial cells of the endoplacental yolk sac, suggesting an excretory role that sequesters fetal-derived solutes in the yolk sac cavity, and faintly to the basal syncytiotrophoblast surface. The mRNA levels of Oatp2b1, Mrp3, and Bsep in the placenta exceeded those in the fetal liver until day 20 of gestation, suggesting that the fetus relies on placental clearance of substrates when expression in the developing liver is low. Mrp3 immunolocalized to the epithelium of the endoplacental yolk sac and less abundantly in the labyrinth zone and endothelium of the maternal arteries. The placental expression of Oatp1a1, Oatp1a4, Oatp1a5, Oatp1b2, Oat, Ntcp, Mrp2, and Mrp6 was low.

P-glycoprotein expression and distribution in the rat placenta during pregnancy

P-glycoprotein (P-gp) is a drug efflux transporter that limits the entry of various potentially toxic drugs and xenobiotics into the fetus and is thus considered a placental protective mechanism. In this study, P-gp expression was investigated in the rat chorioallantoic placenta over the course of pregnancy. Three methods have been employed: real-time RT-PCR, western blotting and immunohistochemistry. The expression of mdr1a and mdr1b genes was demonstrated as early as on the 11th gestation day (gd) and increased with advancing gestation. Western blotting analysis revealed the presence of P-gp in the rat placenta starting from gd 13 onwards. P-gp was localized in the developing labyrinth zone of the placenta on gd 13; from gd 15 up to the term P-gp was seen as a dot like continuous line in the syncytiotrophoblast layers. Our data confirm the presence of P-gp in the rat chorioallantoic placenta starting soon after its development, which may signify the involvement of P-gp in transplacental pharmacokinetics during the whole period of placental maturing.

Clinical aspects of the MDR1 (ABCB1) gene polymorphism

Transporter proteins, in particular P-glycoprotein (Pgp), are important determinants in absorption, tissue targeting, and elimination of drugs. In addition to physiological and environmental factors, its expression and function are modified by genetic polymorphisms of the MDR1 gene. So far, several MDR1 SNPs have been identified, and mutations at positions 2677 and 3435 were associated with alteration of Pgp expression and/or function. In contrast to drug-metabolizing enzymes (eg, CYP2D6), for which loss of function mutations or gene amplification manifests as distinct phenotypes in the population, the impact of MDR1 polymorphisms on pharmacokinetics and pharmacodynamics of Pgp substrates is moderate. Clinical studies on the effects of the C3435T polymorphism and drug treatment with cardiac glycosides, the immunosuppressants cyclosporine and tacrolimus, HIV protease inhibitors, and tricyclic antidepressants are discussed.

Drug Transfer and Metabolism by the Human Placenta

The major function of the placenta is to transfer nutrients and oxygen from the mother to the foetus and to assist in the removal of waste products from the foetus to the mother. In addition, it plays an important role in the synthesis of hormones, peptides and steroids that are vital for a successful pregnancy. The placenta provides a link between the circulations of two distinct individuals but also acts as a barrier to protect the foetus from xenobiotics in the maternal blood. However, the impression that the placenta forms an impenetrable obstacle against most drugs is now widely regarded as false. It has been shown that that nearly all drugs that are administered during pregnancy will enter, to some degree, the circulation of the foetus via passive diffusion. In addition, some drugs are pumped across the placenta by various active transporters located on both the fetal and maternal side of the trophoblast layer. It is only in recent years that the impact of active transporters such as P-glycoprotein on the disposition of drugs has been demonstrated. Facilitated diffusion appears to be a minor transfer mechanism for some drugs, and pinocytosis and phagocytosis are considered too slow to have any significant effect on fetal drug concentrations. The extent to which drugs cross the placenta is also modulated by the actions of placental phase I and II drug-metabolising enzymes, which are present at levels that fluctuate throughout gestation. Cytochrome P450 (CYP) enzymes in particular have been well characterised in the placenta at the level of mRNA, protein, and enzyme activity. CYP1A1, 2E1, 3A4, 3A5, 3A7 and 4B1 have been detected in the term placenta. While much less is known about phase II enzymes in the placenta, some enzymes, in particular uridine diphosphate glucuronosyltransferases, have been detected and shown to have specific activity towards marker substrates, suggesting a significant role of this enzyme in placental drug detoxification. The increasing experimental data on placental drug transfer has enabled clinicians to make better informed decisions about which drugs significantly cross the placenta and develop dosage regimens that minimise fetal exposure to potentially toxic concentrations. Indeed, the foetus has now become the object of intended drug treatment. Extensive research on the placental transfer of drugs such as digoxin and zidovudine has assisted with the safe treatment of the foetus with these drugs in utero. Improved knowledge regarding transplacental drug transfer and metabolism will result in further expansion of pharmacological treatment of fetal conditions.

The MRP family of drug efflux pumps

The MRP family is comprised of nine related ABC transporters that are able to transport structurally diverse lipophilic anions and function as drug efflux pumps. Investigations of this family have provided insights not only into cellular resistance mechanisms associated with natural product chemotherapeutic agents, antifolates and nucleotide analogs, but also into factors that influence drug distribution in the body, membrane systems that are involved in the extrusion of reduced folates, cysteinyl leukotrienes and bile acids, and the molecular basis of two hereditary conditions in humans. The review will describe the biochemical properties, drug resistance activities and potential in vivo functions of these unusual pumps.

Role of MDR1 and MRP1 in trophoblast cells, elucidated using retroviral gene transfer

Natural differences in expression and retroviral transduction techniques were used to test the hypothesis that MDR1 P-glycoprotein (P-gp) and MRP1 (multidrug resistance-related protein) contribute to xenobiotic handling by placental trophoblast. RT-PCR and Western blotting in placenta, primary cytotrophoblast cell cultures, and BeWo, JAr, and JEG choriocarcinoma cell lines showed that MRP1 was ubiquitously expressed, whereas MDR1 was absent or minimally expressed in BeWo and JEG cell lines. In syncytiotrophoblast, P-gp was localized predominantly to the microvillous, maternal facing plasma membrane, and MRP1 to the basal, fetal facing plasma membrane. Functional studies showed that cyclosporin A-sensitive accumulation of [3H]vinblastine by cells containing both transport proteins was significantly different from those expressing predominantly MRP1. Retroviral gene transfer of MDR1 to BeWo cells confirmed that this difference was due to the relative expression of MDR1. Therefore, both P-gp and MRP1 contribute to xenobiotic handling by the trophoblast. Localization of P-gp to the microvillous membrane suggests an essential role in preventing xenobiotic accumulation by the syncytiotrophoblast and, therefore, in protecting the fetus.

Developmental and pediatric pharmacogenomics

Children, as well as adults, should benefit from the discoveries of the genomic era. Many diseases with complex etiologies originate during childhood (e.g., asthma, autism, attention deficit/hyperactivity disorder, epilepsy and juvenile rheumatoid arthritis) and persist into adulthood. Attempts to better understand the genetic basis of age-specific disease processes requires an appreciation that the period of human development encompasses the prenatal period through adolescence, and is a rapidly changing, dynamic process. As a result, pharmacologic modulation of developing gene networks may have unintended and unanticipated consequences that do not become apparent or relevant until later in life. Thus, there is considerable potential for large-scale pharmacogenomic technologies to impact the development and utilization of new therapeutic strategies in children.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Effects of maturation on RNA transcription and protein expression of four MRP genes in human placenta and in BeWo cells

The placenta is a multifunctional organ that protects the fetus from toxic compounds and the MRPs contribute to this function. The expression of MRP1, MRP2, MRP3, and MRP5 was compared in human placental tissue and in BeWo cells by real-time RT-PCR analysis; protein expression was assessed by Western blot. MRP1 and MRP3 were the most abundantly expressed genes in placenta but only MRP1 was highly expressed in the BeWo cells. Expression of MRP1 increased 4-fold in the third as compared with first trimester placental samples, and increased 20-fold with polarization of BeWo cells. MRP2, MRP3, and MRP5 were weakly expressed both in placenta and BeWo cells. Protein expression followed mRNA quantification for MRP1 and MRP5 but not for MRP2 and MRP3. These data indicated that MRP1 and MRP5 increase with trophoblast maturation, suggesting a particular role for these proteins in the organ functional development.

Bile acid stress in the mother and baby unit

Intrahepatic cholestasis of pregnancy (ICP) affects about 0.7% of deliveries in Britain. It is regarded as a benign condition for the mother but is associated with increased fetal mortality in late pregnancy and early delivery is advised. Ursodeoxycholic acid (UDCA) treatment is beneficial to the mother and does not appear to harm the fetus. ICP is often regarded as a disease of the maternal liver already made 'cholestatic' by high levels of circulating progesterone. We propose that ICP should be considered as a feto-maternal disease involving complex interactions between maternal and fetal bile acid metabolism across the placenta. During the late stages of gestation, when there is a rise in fetal and maternal bile acid levels, the placenta may fail to render potentially hepatotoxic bile acids water soluble and hence excretable. This might cause a vicious cycle leading to further cholestasis in the maternal liver already challenged by progesterone.

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.

Efflux transporters of the human placenta

The use of pharmaceuticals during pregnancy is often a necessity for the health of the mother. Until recently, the placenta was viewed as a passive organ through which molecules are passed indiscriminately between mother and fetus. In reality, the placenta contains a plethora of transporters, some of which appear to be specifically dedicated to removal of xenobiotics and toxic endogenous compounds. Drug efflux transporters such as P-glycoprotein (P-gp), several multidrug resistant associated proteins (MRPs) and breast cancer resistant protein (BCRP) may provide mechanisms that protect the developing fetus. Bile acid transporters may also play a role in exporting compounds back into the maternal compartment. Steroid hormones directly influence the level of expression and function in some of these transporters. Investigating the link between the hormones of pregnancy and these drug efflux transporters is one possible key in developing strategies to deliver drugs to the mother with minimal fetal risk.

Kinetic analysis of P-glycoprotein-mediated transport by using normal human placental brush-border membrane vesicles

PURPOSE

P-Glycoprotein (Pgp) plays an important role in drug disposition and excretion in various tissues such as the brain, intestine, and kidney. Moreover, we have demonstrated that Pgp is expressed on the brush-border membranes of trophoblast cells in the placenta and restricts drug transfer from the maternal circulation to the fetus. However, the transport kinetics of physiologically expressed Pgp has scarcely been investigated.

METHODS

In this study, we assessed the functional kinetics of transport mediated by Pgp that is physiologically expressed in normal tissue by using human placental brush-border membrane vesicles (BBMVs). Digoxin and vinblastine were used as typical substrates of Pgp.

RESULTS

The uptakes of [3H]digoxin and [3H]vinblastine into BBMVs were significantly increased in the presence of an ATP-regenerating system. The ATP-dependent uptakes of [3H]digoxin and [3H]vinblastine into BBMVs exhibited saturable kinetics. The Michaelis constants (Kt values) were 2.65 +/- 1.80 microM and 21.9 +/- 3.37 microM, respectively. In the presence of a Pgp inhibitor such as verapamil, cyclosporine A, or progesterone, the ATP-dependent uptakes of [3H]digoxin and [3H]vinblastine into BBMVs were significantly reduced. Anti-Pgp monoclonal antibody C219 completely inhibited the uptake of [3H]digoxin.

CONCLUSIONS

The transport kinetics of [3H]digoxin and [3H]vinblastine by physiologically expressed Pgp were successfully evaluated by using BBMVs prepared from normal human placenta. The present method enabled us to evaluate the function of physiologically expressed Pgp and is superior to the use of cultured transfectants in terms of the yield of vesicles. The present method may also be applicable to investigating the influence of various factors such as the genotype of the MDR1 gene or various pathophysiologic states of neonates on the function of Pgp.

Multidrug resistance protein 2 (MRP2) transports HIV protease inhibitors, and transport can be enhanced by other drugs

BACKGROUND

Various drug transporters of the ATP-binding cassette (ABC) family restrict the oral bioavailability and cellular, brain, testis, cerebrospinal fluid and fetal penetration of substrate drugs. MDRI P-glycoprotein (P-gp) has been demonstrated to transport most HIV protease inhibitors (HPI) and to reduce their oral bioavailability and lymphocyte, brain, testis and fetal penetration, possibly resulting in major limiting effects on the therapeutic efficacy of these drugs.

OBJECTIVES

To investigate whether the ABC transporters MRP1, MRP2, MRP3, MRP5 and breast cancer resistance protein 1 (Bcrp1) are efficient transporters of the HPI saquinavir, ritonavir and indinavir.

METHODS

Polarized epithelial non-human (canine) cell lines transduced with human or murine complementary DNA (cDNA) for each of the transporters were used to study transepithelial transport of the HPI.

RESULTS

MRP2 efficiently transported saquinavir, ritonavir and indinavir and this transport could be enhanced by probenecid. Sulfinpyrazone was also able to enhance MRP2-mediated saquinavir transport. In contrast, MRP1, MRP3, MRP5, or Bcrp1 did not efficiently transport the HPI tested.

CONCLUSIONS

Human MRP2 actively transports several HPI and could, based on its known and assumed tissue distribution, therefore reduce HPI oral bioavailability. It may also limit brain and fetal penetration of these drugs and increase their hepatobiliary, intestinal and renal clearance. MRP2 function and enhancement of its activity could adversely affect the therapeutic efficacy, including the pharmacological sanctuary penetration, of HPI. In vivo inhibition of MRP2 function might, therefore, improve HIV/AIDS therapy.

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.

Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

The characteristics of L-lactic acid transport across the trophoblast basal membrane were investigated and compared with those across the brush-border membrane by using membrane vesicles isolated from human placenta. The uptake of L-[(14)C]lactic acid into basal membrane vesicles was Na(+) independent, and an uphill transport was observed in the presence of a pH gradient ([H(+)](out) > [H(+)](in)). L-[(14)C]lactic acid uptake exhibited saturation kinetics with a K(m) value of 5.89 +/- 0.68 mM in the presence of a pH gradient. p-Chloromercuribenzenesulfonate and alpha-cyano-4-hydroxycinnamate inhibited the initial uptake, whereas phloretin or 4,4'-diisothiocyanostilbene-2,2'-disulfonate did not. Mono- and dicarboxylic acids suppressed the initial uptake. In conclusion, L-lactic acid transport in the basal membrane is H(+) dependent and Na(+) independent, as is also the case for the brush-border membrane transport, and its characteristics resemble those of monocarboxylic acid transporters. However, there were several differences in the effects of inhibitors between basal and brush-border membrane vesicles, suggesting that the transporter(s) involved in L-lactic acid transport in the basal membrane of placental trophoblast may differ from those in the brush-border membrane.

Regulation of expression of the multidrug resistance-associated protein 2 (MRP2) and its role in drug disposition

The multidrug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter accepting a diverse range of substrates, including glutathione, glucuronide, and sulfate conjugates of many endo- and xenobiotics. MRP2 generally performs excretory or protective roles, and it is expressed on the apical domain of hepatocytes, enterocytes of the proximal small intestine, and proximal renal tubular cells, as well as in the brain and the placenta. MRP2 is regulated at several levels, including membrane retrieval and reinsertion, translation, and transcription. In addition to transport of conjugates, MRP2 transports cancer chemotherapeutics, uricosurics, antibiotics, leukotrienes, glutathione, toxins, and heavy metals. Several mutagenesis studies have described critical residues for substrate binding and various naturally occurring mutations that eliminate MRP2 expression or function. MRP2 is important clinically as it modulates the pharmacokinetics of many drugs, and its expression and activity are also altered by certain drugs and disease states.

Characterization of an organic anion-transporting polypeptide (OATP-B) in human placenta

Organic anion-transporting polypeptides (OATPs) are a family of multispecific carriers that mediate the sodium-independent transport of steroid hormone and conjugates, drugs, and numerous anionic endogenous substrates. We investigated whether members of the OATP gene family could mediate fetal-maternal transfer of anionic steroid conjugates in the human placenta. OATP-B (gene symbol SLC21A9) was isolated from a placenta cDNA library. An antiserum to OATP-B detected an 85-kDa protein in basal but not apical syncytiotrophoblast membranes. Immunohistochemistry of first-, second-, and third-trimester placenta showed staining in the cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast. Trophoblasts that reacted with an antibody to Ki-67, a proliferation-associated antigen, expressed lower levels of OATP-B. OATP-B mRNA levels were measured in isolated trophoblasts under culture conditions that promoted syncytia formation. Real-time quantitative PCR estimated an 8-fold increase in OATP-B expression on differentiation to syncytia. The uptake of [(3)H]estrone-3-sulfate, a substrate for OATP-B, was measured in basal syncytiotrophoblast membrane vesicles. Transport was saturable and partially inhibited by pregnenolone sulfate, a progesterone precursor. Pregnenolone sulfate also partially inhibited OATP-B-mediated transport of estrone-3-sulfate in an oocyte expression system. These findings suggest a physiological role for OATP-B in the placental uptake of fetal-derived sulfated steroids.