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

Increased vitamin E intake is associated with higher alpha-tocopherol concentration in the maternal circulation but higher alpha-carboxyethyl hydroxychroman concentration in the fetal circulation

BACKGROUND

The transfer of vitamin E across the placenta is limited, but no data exist on the concentrations of vitamin E metabolites carboxyethyl hydroxychromans (α- and γ-CEHCs) in the fetal circulation.

OBJECTIVE

We measured α- and γ-CEHC concentrations in maternal and umbilical cord blood pairs and examined their relations to circulating vitamin E (α- and γ-tocopherol) and maternal dietary vitamin E intake.

DESIGN

Healthy, pregnant women were enrolled from Oregon Health and Science University's obstetric clinic (<22 wk gestation), and at least one fasting blood sample and a previous day's 24-h diet recall were collected during their pregnancy (n = 19). Umbilical cord blood samples were obtained at the time of delivery and were analyzed for α- and γ-tocopherol, α- and γ-CEHC, and total lipid concentrations.

RESULTS

Mean (±SD) concentrations of umbilical cord blood α-CEHC (30.2 ± 28.9 nmol/L) and γ-CEHC (104.5 ± 61.3 nmol/L) were not significantly different from maternal concentrations (P = 0.07 and 0.08, respectively), but metabolite:tocopherol ratios were significantly higher in cord blood (P < 0.01 and 0.001, respectively). Maternal α-tocopherol:total lipids ratios were correlated with cord blood α-CEHCs (r = 0.67, P = 0.004), and higher vitamin E intakes were associated with higher cord blood α-CEHC concentrations (r = 0.75, P < 0.003).

CONCLUSION

Higher maternal intake of vitamin E during pregnancy may result in increased metabolite concentrations in the fetal circulation, suggesting increased maternal or fetal liver metabolism of vitamin E. This trial was registered at clinicaltrials.gov as NCT00632476.

Quantitative immunofluorescent blotting of the multidrug resistance-associated protein 2 (MRP2)

INTRODUCTION

Quantitation of the expression levels of proteins involved in drug transport and disposition is needed to overcome limitations of film-based detection of chemiluminescent immunoblots.

PURPOSE

The purpose was to describe and validate a quantitative immunofluorescent blotting method for detection of ATP-binding cassette transporter isoform C2/multidrug resistance-associated protein 2 (ABCC2/MRP2).

METHODS

Western blotting was performed by electrophoresis of membrane vesicle protein isolated from Sf9 cells overexpressing MRP2 subsequently blotted with infrared labeled secondary antibody. The bound complex was detected using the Odyssey Infrared Imaging System (Li-Cor; Lincoln, NE). The images were analyzed using the Odyssey Application Software to obtain the integrated intensities, followed by linear regression of the intensity data.

RESULTS

The limits of quantitation for the time-insensitive technique described here were from 0.001 μg to 0.5 μg of total membrane protein, the coefficient of variation of the slope was 8.9%; r2 values were 0.986 ± 0.012. The utility and sensitivity of this technique were demonstrated in quantitating expression of MRP2 in human placental tissue samples, in which MRP2 was present in low abundance.

DISCUSSION

The immunofluorescent blotting technique described provides sensitive, reproducible, and quantitative determinations of large, integral membrane proteins such as MRP2, all with potential long-term cost savings.

Gestational and pregnane X receptor-mediated regulation of placental ATP-binding cassette drug transporters in mice

The ATP-binding cassette (ABC) drug transporters in the placenta are involved in controlling the exchange of endogenous and exogenous moieties. Pregnane X receptor (PXR) is a nuclear receptor that regulates the hepatic expression of several key ABC transporters, but it is unclear whether PXR is involved in the regulation of these transporters in the placenta. This study explores the role of PXR in the regulation of placental drug transporters. The placental mRNA expression of Mdr1a, Bcrp, and Mrp1, 2, and 3 was examined in PXR knockout (-/-), heterozygote (+/-), and wild-type (+/+) mice by quantitative PCR. The impact of PXR activation was examined in pregnant pregnane-16α-carbonitrile (PCN)-treated mice. Compared with that in controls, the basal expression of Mdr1a, Bcrp, Mrp1, and Mrp2 was significantly higher in (+/-) and (-/-) mice. Alterations in the expression of mdr1a, bcrp, and mrp1, 2, and 3 between gestational day (GD) 10 and GD 17 was dissimilar between (+/+) and (-/-) mice. Although PCN treatment induced maternal and fetal hepatic expression of Cyp3a11; placental expression of transporters were not significantly changed. Overall, our results suggest a repressive role of PXR in the basal expression of several placental transporters and a tissue-specific induction of these target genes after PXR activation.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Role of human placental apical membrane transporters in the efflux of glyburide, rosiglitazone, and metformin

OBJECTIVE

Substrates of placental efflux transporters could compete for a single transporter, which could result in an increase in the transfer of each substrate to the fetal circulation. Our aim was to determine the role of placental transporters in the biodisposition of oral hypoglycemic drugs that could be used as monotherapy or in combination therapy for gestational diabetes.

STUDY DESIGN

Inside-out brush border membrane vesicles from term placentas were used to determine the efflux of glyburide, rosiglitazone, and metformin by P-glycoprotein, breast cancer resistance protein, and multidrug resistance protein.

RESULTS

Glyburide was transported by multidrug resistance protein (43 +/- 4%); breast cancer resistance protein (25 +/- 5%); and P-glycoprotein (9 +/- 5%). Rosiglitazone was transported predominantly by P-glycoprotein (71 +/- 26%). Metformin was transported by P-glycoprotein (58 +/- 20%) and breast cancer resistance protein (25 +/- 14%).

CONCLUSION

Multiple placental transporters contribute to efflux of glyburide, rosiglitazone, and metformin. Administration of drug combinations could lead to their competition for efflux transporters.

cAMP efflux from human trophoblast cell lines: a role for multidrug resistance protein (MRP)1 transporter

Cyclic adenosine 3'-5'-monophosphate (cAMP) is a second messenger, which exerts an important role in the control of human first-trimester trophoblast functions. In the present study we demonstrate the existence of a mechanism that is able to extrude cAMP from trophoblast-derived cell lines, and show evidence indicating the involvement of multidrug resistance protein (MRP) 1, a transporter belonging to the ATP-binding cassette family, in cAMP egress. MRP1 is expressed in trophoblast cell lines and cAMP efflux is highly reduced by the MRP1 inhibitor, MK-571. In addition, interleukin-1beta and estrone are able to enhance MRP1 gene expression and influence extracellular cAMP concentration. The occurrence of a MRP1-dependent cAMP efflux is also shown in human first-trimester placenta explants. Extracellular cAMP could represent a source for adenosine formation, which in turn could regulate cAMP-dependent responses in placental tissue. Evidence is provided that adenosine receptor subtypes are present and functional in human trophoblast-derived cells. A role for cAMP egress mechanism in the fine modulation of the nucleotide homeostasis is therefore suggested.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Formation and efflux of ATP-binding cassette transporter substrate 2,4-dinitrophenyl-S-glutathione from cultured human term placental villous tissue fragments

Upon exposure to 1-chloro-2,4-dinitrobenzene (CDNB), the human placental tissue forms its glutathione conjugate 2,4-dinitrophenyl-S-glutathione (DNP-SG). The purpose of this study was to investigate the involvement of human placental ATP-binding cassette (ABC) transporters in the efflux of DNP-SG. Placental tissue samples were obtained from pregnant patients undergoing C-section deliveries following normal pregnancies; villous tissue was cultured in suspension, and DNP-SG formation and efflux upon exposure to 100 microM CDNB were measured by HPLC. DNP-SG efflux decreased by 69.1 (+/-11.3)%, 51.1 (+/-5.4)%, 56.7 (+/-8.3)% and 53.6 (+/-10.8)% (p < 0.05) in the presence of 5 mM sodium orthovanadate (ATPase inhibitor), 100 microM MK571 (MRP-inhibitor), 1 mM dipyridamole (BCRP/P-gp/MRP1-inhibitor) and 100 microM verapamil (P-gp/MRP1 inhibitor) respectively, without any change in DNP-SG formation, total tissue glutathione, GSH/GSSG ratio, tissue integrity or tissue viability. These data clearly established the role of ABC transporters in the human placental efflux of DNP-SG. To investigate the contribution of various ABC transporters toward DNP-SG transport, ATP-dependent transport of 3H-DNP-SG was determined in Sf9 membrane vesicles overexpressing P-gp, BCRP and the MRP proteins. MRP1-mediated DNP-SG transport was inhibited in the presence of sodium orthovanadate, MK571, dipyridamole and verapamil in the presence of glutathione. Furthermore, MRP1-mediated transport [K(t) = 11.3 +/- 1.3 microM and v(max) = 86.7 +/- 1.9 pmol/mg/min] was a high-affinity process compared to MRP2-mediated transport [K(t) = 168 +/- 7 microM and v(max) = 1367 +/- 18 pmol/mg/min]. The inhibition pattern and the kinetics of DNP-SG efflux in the placental villous tissue were consistent with MRP1-mediated DNP-SG efflux, suggesting a functional role and an apical localization for an MRP1-like transporter in the human placental syncytiotrophoblast.

Role of multidrug transporters in neurotherapeutics

Acquired resistance to antibiotics and other chemotherapeutic agents is a major problem in the practice of neurology and other branches of medicine. There are several mechanisms by which drug resistance is acquired. Multidrug transporters are important glycoproteins located in the cell membrane that actively transport small lipophilic molecules from one side of the cell membrane to the other, most often from the inside to the outside of a cell. They have important protective role yet may prove inconvenient in chemotherapy. In epilepsy and other disorders this mechanism augments the elimination of drugs from their target cells and leads to drug resistance. In this review, we have discussed the biochemical characteristics of multidrug transporters and the mechanisms by which these membrane bound proteins transport their target molecules from one side to the other side of the cell membrane. We have also briefly discussed the application of this knowledge in the understanding of drug resistance in various clinical situations with particular reference to neurological disorders. These proteins located in the placenta have important role in preventing the transplacental movement of drugs in to the fetus which may result in congenital malformations or other defects. The molecular genetic mechanisms that govern the expression of these important proteins are discussed briefly. The potential scope to develop targeted chemotherapeutic agents is also discussed.

Transport of inorganic mercury and methylmercury in target tissues and organs

Owing to the prevalence of mercury in the environment, the risk of human exposure to this toxic metal continues to increase. Following exposure to mercury, this metal accumulates in numerous organs, including brain, intestine, kidneys, liver, and placenta. Although a number of mechanisms for the transport of mercuric ions into target organs were proposed in recent years, these mechanisms have not been characterized completely. This review summarizes the current literature related to the transport of inorganic and organic forms of mercury in various tissues and organs. This review identifies known mechanisms of mercury transport and provides information on additional mechanisms that may potentially play a role in the transport of mercuric ions into target cells.

Molecular mechanism of ATP-dependent solute transport by multidrug resistance-associated protein 1

Millions of new cancer patients are diagnosed each year and over half of these patients die from this devastating disease. Thus, cancer causes a major public health problem worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Over-expression of ATP-binding cassette transporters, such as P-glycoprotein, breast cancer resistance protein and/or multidrug resistance-associated protein 1 (MRP1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs across the cell membrane barrier. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

Nitrofurantoin transport by placental choriocarcinoma JAr cells: involvement of BCRP, OATP2B1 and other MDR transporters

OBJECTIVE

To determine the role of BCRP in nitrofurantoin (NF) transport in JAr cells and the possible contribution of OATP2B1, P-gp and MRPs to this transport.

METHODS

Cells were incubated with various BCRP, P-gp, MRPs, organic anion transporting polypeptide (OAT) and OATP2B1 inhibitors for 15 min, followed by incubation for 30 min with NF, with or without the inhibitors mentioned earlier. NF cytotoxicity was examined using neutral red (NR) assay. Intracellular NF levels were analyzed by HPLC.

RESULTS

NR assay showed that incubation conditions with NF (as carried out in our experiments) were not cytotoxic. Incubation with specific inhibitors of BCRP (FTC, Chrysin and Novobiocin), showed a significant increase in NF accumulation in the cells. Inhibitors of OATP2B1 (EGCG and BSP) had no influence on NF accumulation. Specific inhibitors of P-gp and MRPs (Verapamil and Indomethacin, respectively) also had no influence on NF accumulation in JAr cells.

CONCLUSIONS

NF is probably a specific substrate of BCRP, and BCRP has a major active role in NF transport in JAr cells. For the first time, we showed, that P-gp, MRPs, and the OATP2B1, probably have a negligible contribution to NF transport in JAr cells.

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.

MRP isoforms and BCRP mediate sulfate conjugate efflux out of BeWo cells

The breast cancer resistance protein (BCRP) and the multidrug resistance-associated proteins (MRPs) have the ability to eliminate sulfate conjugates but it is not known if this constitutes one of their roles in the placenta. To determine this, the BeWo cell line was used as a model of placental trophoblast cells and the mechanisms of elimination of sulfate metabolites of two common sulfotransferase substrates, 4-nitrophenol and acetaminophen were examined. At 0.5-200 microM, neither 4-nitrophenyl sulfate nor acetaminophen sulfate affected the accumulation of the BCRP substrates BODIPY FL prazosin or mitoxantrone in BeWo monolayers, indicating a lack of interaction of BCRP with the sulfates. Examination of the effect of BCRP/MRP inhibitors on the efflux of intracellularly generated 4-nitrophenyl sulfate and acetaminophen sulfate, indicated that one or more of the MRP isoforms play a major role in the elimination of 4-nitrophenyl sulfate and acetaminophen sulfate across the basolateral (fetal-facing) and apical (maternal-facing) membranes respectively. BCRP played a minor role in the elimination of these two sulfate conjugates across the apical membrane. This study demonstrates that a yet undetermined role of trophoblast efflux transporters is the elimination of sulfate conjugates.

Drug transporters in the human blood-placental barrier

Studies on the increasing number of transporters found in the placental barrier are gaining momentum, because of their tissue-specific expression, significance in physiology and disease, and the possible utilization of the emerging knowledge in pharmacology. In the placenta, both syncytiotrophoblast and fetal capillary endothelium express transporters. Fetal exposure is determined by the net effect of combination of transporters, their nature and localization in relation to placental cells and their substrate specificity. Although the significance of placental transporters on human fetal drug exposure is almost an unstudied field so far, their potential use to design drugs that do not cross the placenta is already being pursued. It is thus of interest to review the existing knowledge of human placental transporters. Transporters in all groups which take part in drug transport are found in human placenta. Especially, ATP-binding cassette transporters ABCG2/breast cancer resistance protein, ABCB1/P-glycoprotein and ABCC2/MRP2 are all expressed at the apical surface of syncytiotrophoblast facing maternal blood and are putatively important protective proteins both for placental tissue and the fetus, because they are efflux transporters and their substrates include many drugs and also environmental chemicals. Such protective effect has been shown in animals, but these results cannot be directly extrapolated to humans due to interspecies differences in placental structure and function. Experimental models utilizing human placental tissue, especially human placental perfusion, offer valuable possibilities, which have been insufficiently studied so far.

Role of placental ATP-binding cassette (ABC) transporters in antiretroviral therapy during pregnancy

Highly active anti-retroviral therapy (HAART) is used to treat HIV-infected patients and involves administration of multiple antiretroviral drugs acting at different steps of the HIV life cycle. In treating HIV-infected pregnant patients, the aim of therapy is not only to treat the mother but also to prevent the transmission of the virus to the fetus. Among the antiretroviral drugs used, there are differences in the extent of transfer of these drugs across the placenta; HIV protease inhibitors are particularly poorly transferred. Activities of ABC transporters expressed in the human placenta as well as differences in plasma protein binding may account for the poor transplacental transfer of certain drugs. This review discusses factors affecting the extent of placental transfer of antiretroviral drugs during pregnancy. These issues may also apply to drugs in other therapeutic categories.

Effect of DMPS and DMSA on the placental and fetal disposition of methylmercury

Methylmercury (CH3Hg+) is a serious environmental toxicant. Exposure to this metal during pregnancy can cause serious neurological and developmental defects in a developing fetus. Surprisingly, little is known about the mechanisms by which mercuric ions are transported across the placenta. Although it has been shown that 2,3-dimercaptopropane-1-sulfonate (DMPS) and 2,3-dimercaptosuccinic acid (DMSA) are capable of extracting mercuric ions from various organs and cells, there is no evidence that they are able to extract mercury from placental or fetal tissues following maternal exposure to CH3Hg+. Therefore, the purpose of the current study was to evaluate the ability of DMPS and DMSA to extract mercuric ions from placental and fetal tissues following maternal exposure to CH3Hg+. Pregnant Wistar rats were exposed to CH3HgCl, containing [203Hg], on day 11 or day 17 of pregnancy and treated 24 h later with saline, DMPS or DMSA. Maternal organs, fetuses, and placentas were harvested 48 h after exposure to CH3HgCl. The disposition of mercuric ions in maternal organs and tissues was similar to that reported previously by our laboratory. The disposition of mercuric ions in placentas and fetuses appeared to be dependent upon the gestational age of the fetus. The fetal and placental burden of mercury increased as fetal age increased and was reduced by DMPS and DMSA, with DMPS being more effective. The disposition of mercury was examined in liver, total renal mass, and brain of fetuses harvested on gestational day 19. On a per gram tissue basis, the greatest amount of mercury was detected in the total renal mass of the fetus, followed by brain and liver. DMPS and DMSA reduced the burden of mercury in liver and brain while only DMPS was effective in the total renal mass. The results of the current study are the first to show that DMPS and DMSA are capable of extracting mercuric ions, not only from maternal tissues, but also from placental and fetal tissues following maternal exposure to CH3Hg+.

The effect of probenecid and MK-571 on the feto-maternal transfer of saquinavir in dually perfused human term placenta

Human placenta, particularly the blood-placenta barrier, with various transporters has crucial role to protect the fetus and, on the other hand, to facilitate movement of compounds towards the fetal circulation. This study aimed to characterize the role of basal transporters of the syncytiotrophoblast, which appear to be yet less studied, in the fetal-to-maternal transfer of saquinavir by use of dually perfused human placentas. A dual perfusion of human placenta was performed to study effect of MK-571 and probenecid, inhibitors of the MRP1 and OATP transporters, expressed in the basal trophoblast membrane, on the transfer of saquinavir. The fetal-to-maternal placental transfer of saquinavir in the control group as measured by TPT(AUC)% (absolute fraction of the dose crossing placenta) was 14.0%, which is 73% less than the transfer of the freely diffusible antipyrine. The two inhibitors, MK-571 and probenecid caused a non-significant (P = 0.34 for ANOVA) reduction of 43% and 24%, respectively, in the mean amount of saquinavir transferred from the fetal to the maternal side. MK-571 also somewhat (by 31%) reduced the TPT(AUC)% of antipyrine, but this finding did not reach statistical significance (P = 0.25). Neither of the employed inhibitors had an effect on the placental transfer index of saquinavir transfer (P = 0.77). The present results indicated lack of significant effect by MK-571 and probenecid on the fetal-to-maternal transfer of saquinavir and suggest that MRP1 and, possibly, OATP2B1 do not play a significant role in the fetal-to-maternal transfer of saquinavir.

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.

Posttranslational regulation of Abcc2 expression by SUMOylation system

The ATP-binding cassette transporter family C 2 (Abcc2) is a member of efflux transporters involved in the biliary excretion of organic anions from hepatocytes. Posttranslational regulation of Abcc2 has been implicated, although the molecular mechanism is not fully understood. In the present study, we performed yeast two-hybrid screening to identify novel protein(s) that particularly interacts with the linker region of Abcc2 located between the NH(2)-terminal nucleotide binding domain and the last membrane-spanning domain. The screening resulted in the identification of a series of small ubiquitin-like modifier (SUMO)-related enzymes and their substrates. In yeast experiments, all of these interactions were abolished by substituting the putative SUMO consensus site in the linker region (IKKE) in Abcc2 to IRKE. In vitro SUMOylation experiments confirmed that the Abcc2 linker was a substrate of Ubc9-mediated SUMOylation. It was also found that the IKKE sequence is the target of SUMOylation, since a mutant with IKKE is substituted by IRKE was not SUMOylated. Furthermore, we demonstrated for the first time that Abcc2, endogenously expressed in rat hepatoma-derived McARH7777 cells, is SUMOylated. Suppression of endogenous Ubc9 by small interfering RNA resulted in a selective 30% reduction in Abcc2 protein expression in the postnuclear supernatant, whereas subcellular localization of Abcc2 confirmed by semiquantitative immunofluorescence analysis was minimally affected. This is the first demonstration showing the regulation of ABC transporter expression by SUMOylation.

Pharmacologic and nonpharmacologic options for the management of HIV infection during pregnancy

Over the past decade, significant advances have been made in the treatment of HIV-1 infection using both pharmacologic and nonpharmacologic strategies to prevent mother-to-child transmission (MTCT). Optimal prevention of the MTCT of HIV requires antiretroviral drugs (ARV) during pregnancy, during labor, and to the infant. ARVs reduce viral replication, lowering maternal plasma viral load and thus the likelihood of MTCT. Postexposure prophylaxis of ARV agents in newborns protect against infection following potential exposure to maternal HIV during birth. In general, the choice of an ARV for treatment of HIV-infected women during pregnancy is complicated by the need to consider the effectiveness of the therapy for the maternal disease as well as the teratogenic or teratotoxic potential of these drugs. Clinicians managing HIV in pregnancy need to discuss the potential risks and benefits of available therapy options so that mothers can make informed decisions in choosing the best treatment regimen for themselves and for their children.

Influence of adenosine triphosphate and ABCB1 (MDR1) genotype on the P-glycoprotein-dependent transfer of saquinavir in the dually perfused human placenta

BACKGROUND

The ATP-dependent drug-efflux pump, P-glycoprotein (P-gp) encoded by ABCB1 (MDR1), plays a crucial role in several tissues forming blood-tissue barriers. Absence of a normally functioning P-gp can lead to a highly increased tissue penetration of a number of clinically important drugs.

METHODS

We have studied the dose-response effect of exogenous ATP on the placental transfer of the well-established P-gp substrate saquinavir in 17 dually perfused human term placentas. We have also studied the influence of the ABCB1 polymorphisms 2677G>T/A and 3435C>T on placental P-gp expression (n = 44) and the transfer (n = 16) of saquinavir.

RESULTS

The present results indicate that the addition of exogenous ATP to the perfusion medium does not affect the function of P-gp as measured by saquinavir transfer across the human placenta. The variant allele 3435T was associated with significantly higher placental P-gp expression than the wild-type alleles. However, neither polymorphism affected placental transfer of saquinavir nor there was any correlation between P-gp expression and saquinavir transfer.

CONCLUSIONS

Our results indicate that addition of exogenous ATP is not required for ATP-dependent transporter function in a dually perfused human placenta. Although the ABCB1 polymorphism 3435C>T altered the expression levels of P-gp in the human placenta, this did not have any consequences on P-gp-mediated placental transfer of saquinavir.

Placental transfer and fetal elimination of morphine-3-beta-glucuronide in the pregnant baboon

The glucuronide metabolites of several widely used drugs are detected in fetal plasma after maternal drug administration. However, the disposition of these metabolites is poorly understood and clinical concerns have been raised about accumulation of active metabolites in the fetus. For this reason, morphine-3-beta-glucuronide (M3G), an active metabolite of morphine, was studied to provide quantitative data on disposition. Maternal, fetal, and bidirectional placental clearances of M3G were measured in three pregnant baboons. During maternal infusion of M3G to steady-state, the glucuronide metabolite readily appeared in fetal plasma achieving a mean +/- S.D. fetal-to-maternal concentration ratio of 0.79 +/- 0.04. In paired maternal and fetal infusions, steady-state clearances were 53 +/- 3 (maternal), 1.5 +/- 0.5 (maternal-to-fetal), 2.6 +/- 0.1 (fetal-to-maternal), and -0.70 +/- 0.6 ml x min(-1) (fetal). These clearance values support bidirectional transfer of M3G across the placenta and indicate negligible direct clearance from the fetus. The clearance of M3G across the placenta is more than 20-fold less than that of morphine. Despite this low index of permeability, placental transfer contributes significantly to the glucuronide pool in the fetus. Placental transfer emerges as the major clearance pathway for the glucuronide from the fetus and suggests a component of active efflux. What is more, the results do not support the concept of sequestration in the fetal intestine as a significant route of clearance. Together these results clarify the distribution and clearance of glucuronides in the pregnant primate and facilitate prediction of fetal exposure to active metabolites.

Prominent expression of xenobiotic efflux transporters in mouse extraembryonic fetal membranes compared with placenta

Fetal exposure to xenobiotics can be restricted by transporters at the interface between maternal and fetal circulation. Previous work identified transporters in the placenta; however, less is known about the presence of these transporters in the fetal membranes (i.e., yolk sac and amniotic membranes). The purpose of this study was to quantify mRNA and protein expression of xenobiotic transporters in mouse placenta and fetal membranes during mid to late gestation. Concepti (placenta and fetal membranes, gestation day 11) or placenta and fetal membranes (gestation days 14 and 17) were collected from pregnant mice and analyzed for expression of multidrug resistance-associated proteins (Mrps), multidrug resistance proteins (Mdrs), multidrug and toxin extrusion proteins (Mates), breast cancer resistance protein (Bcrp), and organic anion-transporting polypeptides (Oatps). Maternal liver and kidneys were also collected at day 14 for mRNA and immunohistochemical analysis. mRNA expression of Mrp, Mdr, Bcrp, Mate-1, and Oatp isoforms was detected at day 11. The uptake carriers Oatp2a1, 3a1, 4a1, and 5a1 showed placenta-predominant expression. At days 14 and 17, fetal membranes expressed higher mRNA levels of the efflux transporters Mrp2 (7-fold), Mrp4 (5-fold), Mrp5 (3-fold), Mrp6 (12-fold), Bcrp (2-fold), and Mate-1 (7-fold) than placenta. Western blot analysis of Mrp2, Mrp4, Mrp6, and Bcrp confirmed higher expression in fetal membranes. Immunostaining revealed apical (Mrp2 and Bcrp) and basolateral (Mrp4, 5, and 6) cellular localization in epithelial cells of the yolk sac. In conclusion, xenobiotic transporters in the fetal membranes may provide an additional route to protect the fetus against endogenous chemicals and xenobiotics.

Pharmacogenomics of MRP transporters (ABCC1-5) and BCRP (ABCG2)

Elucidation of the key mechanisms that confer interindividual differences in drug response remains an important focus of drug disposition and clinical pharmacology research. We now know both environmental and host genetic factors contribute to the apparent variability in drug efficacy or in some cases, toxicity. In addition to the widely studied and recognized genes involved in the metabolism of drugs in clinical use today, we now recognize that membrane-bound proteins, broadly referred to as transporters, may be equally as important to the disposition of a substrate drug, and that genetic variation in drug transporter genes may be a major contributor of the apparent intersubject variation in drug response, both in terms of attained plasma and tissue drug level at target sites of action. Of particular relevance to drug disposition are members of the ATP Binding Cassette (ABC) superfamily of efflux transporters. In this review a comprehensive assessment and annotation of recent findings in relation to genetic variation in the Multidrug Resistance Proteins 1-5 (ABCC1-5) and Breast Cancer Resistance Protein (ABCG2) are described, with particular emphasis on the impact of such transporter genetic variation to drug disposition or efficacy.

Drug transport across the placenta, role of the ABC drug efflux transporters

The placenta serves an important role both as a protective barrier as well as in normal fetal development. The ATP-binding cassette (ABC) proteins perform crucial functions in the distribution of nutrients and exchange of waste metabolites across the placenta. They also protect the developing fetus from xenobiotics to which the pregnant mother is exposed. Recent studies in P-glycoprotein (P-gp) deficient mdr1a and mdr1b (-/-) CF-1 mice have shown pronounced increases in fetal exposure to P-gp substrates due to increased transplacental penetration demonstrating the important protective role of P-gp to the developing fetus. The role of placental ABC transporter proteins in protecting the fetus against maternal exposure to drugs, toxins and other xenobiotics is discussed. Overall, the paucity of information available on the transplacental transfer of drugs emphasizes the need to further employ preclinical in vivo models for drug development in order to best predict fetal outcomes of drug administration to pregnant mothers.

Simultaneous determination of 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenyl-S-glutathione and its metabolites for human placental disposition studies by high-performance liquid chromatography

We developed and validated an HPLC method for determination of 1-chloro-2,4-dinitrobenzene (CDNB) and its glutathione conjugate 2,4-dinitrophenyl-S-glutathione (DNP-SG) to study the kinetics and mechanisms involved in DNP-SG formation and efflux, as a probe for human placental metabolism and transport. This method combines use of 3 microm solid phase, rapid mobile phase gradient with dual wavelength ultraviolet detection to permit determination of a lipophilic parent compound and its hydrophilic metabolites in a single short run. The selectivity, linearity, accuracy, precision, relative recovery and stability of the assay are sufficient for determining CDNB, DNP-SG and its metabolites from buffer and tissue samples to support placental drug metabolism and transport studies.

The role of multiple drug resistance proteins in fetal and/or placental protection against teratogen-induced orofacial clefting

Previous studies have shown a role for multiple drug resistance proteins in protecting the fetus from a limited number of teratogens. We have expanded the number of proteins and teratogens examined by comparing the influence of the mdr1a and mdr2 proteins on teratogen-induced orofacial clefting using their respective knockouts in crosses with the A/J, high susceptibility strain. Western blots identified the presence of mdr1a and possibly mdr2 in the placenta and fetus. The mdr1a knockout, on its unique genetic background showed lower, similar, and higher incidences of clefting compared to A/J for Dilantin, hydrocortisone (HC), and 6-aminonicotinamide (6-AN), respectively. The mdr2 knockout did not affect 6-AN clefting when compared to A/J. In reciprocal crosses, when corrected for increased spontaneous clefting, maternally inherited A/J susceptibility genes predominated over the effects of the maternal absence of mdr1a (with 6-AN). Unlike mdr1a, which had a direct effect in the fetus as shown by genotyping of affected versus unaffected fetuses, an effect of mdr2 in the fetus was not found. The mdr1a knockout was backcrossed to the A/J inbred strain for 11 generations (congenics) to eliminate genetic background effects. Reciprocal crosses showed no maternal effect from the lack of mdr1a, confirming that mdr1a expression in the fetus, rather than the placenta, protects the fetus from teratogens. Mdr2 seems not to be involved in the protection of the fetus from teratogens.

Clinical trial: modulation of human placental multidrug resistance proteins in cholestasis of pregnancy by ursodeoxycholic acid

BACKGROUND

The effects of ursodeoxycholic acid on human placental bile acids and bilirubin transporters in intrahepatic cholestasis of pregnancy are still undefined.

AIM

To evaluate whether ursodeoxycholic acid affects MRP2, MRP3 and MRP4 expression in the placenta.

MATERIALS AND METHODS

Forty-three pregnant women were enrolled; fourteen subjects had physiological pregnancies. Intrahepatic cholestasis of pregnancy patients were divided into two groups: (i) 13 received ursodeoxycholic acid (20 mg/kg/day) and (ii) 16 untreated. Total bile acid and bilirubin in serum and cord blood were determined in each subject. Multidrug resistance proteins expression (immunoblot, quantitative real-time PCR) was evaluated in placentas collected at delivery. anova test was used for statistical analysis of data.

RESULTS

Ursodeoxycholic acid administration significantly improved maternal serum bile acid and cord blood bilirubin and bile acid levels. MRP2 protein and RNA expression was significantly increased in placentas from treated patients compared to controls (P < 0.001 and P < 0.01, respectively). MRP3 protein expression was not significantly different between the groups while RNA expression was significantly decreased in treated patients (P < 0.01). MRP4 did not show significant differences between the groups.

CONCLUSIONS

Ursodeoxycholic acid administration induces placental MRP2 expression, and reduces bilirubin and bile acid levels in cord blood.

Breast Cancer Resistance Protein (Bcrp1/Abcg2) in Mouse Placenta and Yolk Sac: Ontogeny and its Regulation by Progesterone

Breast Cancer Resistance Protein (BCRP), a recently-discovered transporter belonging to ABC superfamily, is highly expressed within the labyrinth of the placenta, the primary site of exchange between the maternal and fetal circulation. It has been proposed to function as an efflux pump protecting the fetus from a wide range of xenobiotics. It has also been recently shown that the yolk sac, in addition to the placenta, may be involved in transport of certain substances to and from the fetus. We hypothesised that there are changes in placental Bcrp1 (the mouse orthologue of human BCRP) expression during pregnancy and that these correlate with changes in progesterone production that occur in late gestation. We also hypothesised that Bcrp1 is expressed in the yolk sac, and that levels change with advancing gestation. Either whole concepti, or placenta and yolk sac, were collected from pregnant mice and analysed at embryonic (E) day 9.5, 12.5, 15.5 and 18.5 (term approximately E19.5). Peak expression of Bcrp1 mRNA was detected using in situ hybridisation within the placenta at E9.5 and the yolk sac at E12.5. There was a significant decrease thereafter in both tissues (p<0.001). In contrast, expression of Bcrp1 protein as assessed by immunohistochemistry and Western immunoblots did not change significantly during gestation either in the placenta nor the yolk sac, and no sex difference in Bcrp1 protein expression in either tissue was observed at E12.5. Daily progesterone treatment starting at E14.5 and continuing until E18.5 significantly increased maternal progesterone levels, but did not elicit any changes in the Bcrp1 mRNA or Bcrp1 protein expression either in the placenta or the yolk sac. Significant expression of Bcrp1 protein in fetal tissue was evident at the end of gestation, while expression in the fetal brain endothelium was evident as early as E12.5. We suggest that the placenta and the yolk sac, both of which express Bcrp1, may limit fetal exposure to the potentially adverse effects of xenobiotics including therapeutic drugs which the mother may be exposed to during pregnancy. The significant decrease in Bcrp1 mRNA expression in both the yolk sac and the placenta from mid to late gestation may be counter-balanced by an increase in Bcrp1 expression in fetal organs involved in absorption, excretion and protection.

Placental transfer of quetiapine in relation to P-glycoprotein activity

Atypical antipsychotic drugs are well tolerated and thus often preferred in women of fertile age; yet the information on their placental transfer and use during the prenatal period is limited. The aim of this study was to study the placental transfer of quetiapine, a widely used atypical antipsychotic, with special reference to the role of the placental transporter protein, P-glycoprotein (P-gp). This was performed in 18 dually perfused placentas, using the well established P-gp inhibitors PSC833 (valspodar) and GG918 to inhibit the function of P-gp. We also aimed to clarify the significance of two potentially functional ABCB1 single nuclear polymorphisms (SNPs), 2677G>T/A and 3435C>T, on the transplacental transfer (TPT) of quetiapine. The placental transfer of quetiapine in the control group as measured by TPT(AUC) % (absolute fraction of the dose crossing placenta) was 3.7%, which is 29% less than the transfer of the freely diffusible antipyrine, which was 5.2%. The P-gp inhibitors had no significant effect on the transfer of quetiapine as measured by TPT(AUC) % (P = 0.77). No correlation was found between the transplacental transfer of quetiapine (TPT(AUC) %) and placental P-gp expression (P = 0.61). The 3435T allele in exon 26 was associated with significantly higher placental transfer of quetiapine (P = 0.04). We conclude that quetiapine passes the human placenta but that the blood-placental barrier partially limits the transplacental transfer of quetiapine. Administration of P-gp inhibiting drugs with quetiapine is not likely to increase fetal exposure to quetiapine, although the ABCB1 C3435T polymorphism may contribute to inter-individual variation in fetal exposure to quetiapine.

Structure and function of the MRP2 (ABCC2) protein and its role in drug disposition

The multi-drug resistance protein 2 (MRP2; ABCC2) is an ATP-binding cassette transporter playing an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, glucuronate and sulfate, which are collectively known as phase II products of biotransformation. In addition, MRP2 can also transport uncharged compounds in cotransport with glutathione, and thus can modulate the pharmacokinetics of many drugs. The other way around, its expression and activity are also altered by certain drugs and disease states. Unlike other members of the MRP/ABCC family, MRP2 is specifically expressed on the apical membrane domain of polarised cells as hepatocytes, renal proximal tubular cells, enterocytes and syncytiotrophoblasts of the placenta. Several naturally occurring mutations leading to the absence of functional MRP2 protein from the apical membrane have been described causing the human Dubin-Johnson syndrome associated with conjugated hyperbilirubinaemia. Experimental mutation studies have revealed critical amino acids for substrate binding in the MRP2 molecule. This review is, therefore, focused on the structure and function of MRP2, the substrates transported and the clinical relevance of MRP2.

Antiepileptic medication during pregnancy: does fetal genotype affect outcome?

Congenital abnormalities and impaired development in childhood are attributable to fetal exposure to antiepileptic drugs (AEDs). Pregnancy registries set up to obtain information about the potential risks of fetal exposure to AEDs, in particular major congenital malformations (MCMs), suggest that valproate exposure increases the frequency of congenital malformations more than other AEDs. Furthermore, follow-up studies have drawn attention to cognitive impairments in later childhood after prenatal exposure to valproate. Fetal exposure to AEDs may be influenced by drug transporting proteins in the placenta, including P-glycoprotein (P-gp), multidrug resistance protein (MRP) 1, and breast cancer resistance protein (BCRP). Their location in the syncytiotrophoblast plasma membrane, at the interface of the maternal and fetal circulations, allows these transport proteins to efflux xenobiotics back to the mother and offers the fetus protection from medications taken during pregnancy. Genetic variations in the expression and activity of these transport proteins may influence fetal exposure to AEDs and thus the risk of teratogenicity. Identification of a hierarchy of haplotypes ranging from susceptible to protective of congenital abnormalities could assist genetic counseling, in assessing fetal risks from exposure to AEDs.

A molecular understanding of ATP-dependent solute transport by multidrug resistance-associated protein MRP1

Over a million new cases of cancers are diagnosed each year in the United States and over half of these patients die from these devastating diseases. Thus, cancers cause a major public health problem in the United States and worldwide. Chemotherapy remains the principal mode to treat many metastatic cancers. However, occurrence of cellular multidrug resistance (MDR) prevents efficient killing of cancer cells, leading to chemotherapeutic treatment failure. Numerous mechanisms of MDR exist in cancer cells, such as intrinsic or acquired MDR. Overexpression of ATP-binding cassette (ABC) drug transporters, such as P-glycoprotein (P-gp or ABCB1), breast cancer resistance protein (BCRP or ABCG2) and/or multidrug resistance-associated protein (MRP1 or ABCC1), confers an acquired MDR due to their capabilities of transporting a broad range of chemically diverse anticancer drugs. In addition to their roles in MDR, there is substantial evidence suggesting that these drug transporters have functions in tissue defense. Basically, these drug transporters are expressed in tissues important for absorption, such as in lung and gut, and for metabolism and elimination, such as in liver and kidney. In addition, these drug transporters play an important role in maintaining the barrier function of many tissues including blood-brain barrier, blood-cerebral spinal fluid barrier, blood-testis barrier and the maternal-fetal barrier. Thus, these ATP-dependent drug transporters play an important role in the absorption, disposition and elimination of the structurally diverse array of the endobiotics and xenobiotics. In this review, the molecular mechanism of ATP-dependent solute transport by MRP1 will be addressed.

Expression, localisation and activity of ATP binding cassette (ABC) family of drug transporters in human amnion membranes

Placental ATP-binding cassette (ABC) transporters limit fetal exposure to xenobiotics by regulating transplacental passage into the fetal circulation; their expression and function in fetal membranes, however, has not been studied. In the present study the expression, localisation and function of ABC transporters in human amnion was examined to explore their potential role in modulating amniotic fluid drug disposition in pregnancy. Single-assay oligo-microarrays were used to profile amnion gene expression, and drug transporters expressed at significant levels were identified and selected for further studies. The expression of ABCG2/breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRP) 1 (ABCC1), 2 (ABCC2) and 5 (ABCC5) was detected on the arrays, and verified by RT-PCR and immunoblotting. On confocal microscopy of fetal membrane cryosections, MRP1 and MRP5 were immunolocalised to both apical and basolateral surfaces of the amniotic epithelium, while MRP2 was expressed at low levels only in the apical membrane. BCRP in contrast showed cytoplasmic staining throughout the amniotic epithelium. In addition to the amnion, MRP1 and BCRP immunostaining was observed in the chorion and the decidua. Cell accumulation studies using selective MRP and BCRP inhibitors showed the transporters to be functionally active in amnion epithelial monolayer cultures. In contrast, transwell transport studies using intact amnion membranes did not show significant vectorial transport. These findings identify the amnion as a novel site of ABC drug transporter expression. Functional studies indicate that they may act primarily to prevent cellular xenobiotic accumulation, rather than to confer fetal protection through reduced accumulation in amniotic fluid.

Multidrug resistance proteins affect drug transmission across the placenta

OBJECTIVE

We studied the role of multidrug resistance proteins in regulating transplacental transmission of corticosteroids and protease inhibitors.

STUDY DESIGN

We performed quantitative polymerase chain reaction and FACS analyses to study MDR1 (encodes P-glycoprotein) and MRP-1 expression in extravillous (HTR-8/SVneo) and villous (BeWo) trophoblast cells treated with saquinavir, a multidrug resistance protein substrate. We measured H3-dexamethasone and H3-ritonavir transfer across confluent, syncytialized BeWo cells before and after treatment with agents that inhibit multidrug resistance proteins.

RESULTS

Compared with baseline expression, messenger RNA and protein levels were increased significantly in trophoblast cells after treatment with saquinavir. H3-dexamethasone and H3-ritonavir levels increased in BeWo cells after treatment with anti-P-glycoprotein antibodies or cyclosporine A. Transfer of H3-labeled drugs from the apical (eg, maternal) to basolateral (eg, fetal) side of the syncytialized BeWo cell monolayer was increased significantly when cells were pretreated with anti-P-glycoprotein antibodies.

CONCLUSION

Multidrug resistance proteins regulate drug levels in trophoblast cells and may mediate transmission of therapeutic agents across the placenta.