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

Hypoxia-Inducible Factor-Dependent and Independent Mechanisms Underlying Chemoresistance of Hypoxic Cancer Cells

In hypoxic regions of malignant solid tumors, cancer cells acquire resistance to conventional therapies, such as chemotherapy and radiotherapy, causing poor prognosis in patients with cancer. It is widely recognized that some of the key genes behind this are hypoxia-inducible transcription factors, e.g., hypoxia-inducible factor 1 (HIF-1). Since HIF-1 activity is suppressed by two representative 2-oxoglutarate-dependent dioxygenases (2-OGDDs), PHDs (prolyl-4-hydroxylases), and FIH-1 (factor inhibiting hypoxia-inducible factor 1), the inactivation of 2-OGDD has been associated with cancer therapy resistance by the activation of HIF-1. Recent studies have also revealed the importance of hypoxia-responsive mechanisms independent of HIF-1 and its isoforms (collectively, HIFs). In this article, we collate the accumulated knowledge of HIF-1-dependent and independent mechanisms responsible for resistance of hypoxic cancer cells to anticancer drugs and briefly discuss the interplay between hypoxia responses, like EMT and UPR, and chemoresistance. In addition, we introduce a novel HIF-independent mechanism, which is epigenetically mediated by an acetylated histone reader protein, ATAD2, which we recently clarified.

Physiologically-based pharmacokinetic modelling of long-acting injectable cabotegravir and rilpivirine in pregnancy

AIMS

Long-acting cabotegravir and rilpivirine have been approved to manage HIV in adults, but data regarding safe use in pregnancy are limited. Physiologically-based pharmacokinetic (PBPK) modelling was used to simulate the approved dosing regimens in pregnancy and explore if Ctrough was maintained above cabotegravir and rilpivirine target concentrations (664 and 50 ng/mL, respectively).

METHODS

An adult PBPK model was validated using clinical data of cabotegravir and rilpivirine in nonpregnant adults. This was modified by incorporating pregnancy-induced metabolic and physiological changes. The pregnancy PBPK model was validated with data on oral rilpivirine and raltegravir (UGT1A1 probe substrate) in pregnancy. Twelve weeks' disposition of monthly and bimonthly dosing of long-acting cabotegravir and rilpivirine was simulated at different trimesters and foetal exposure was also estimated.

RESULTS

Predicted Ctrough at week 12 for monthly long-acting cabotegravir was above 664 ng/mL throughout pregnancy, but below the target in 0.5% of the pregnant population in the third trimester with bimonthly long-acting cabotegravir. Predicted Ctrough at week 12 for monthly and bimonthly long-acting rilpivirine was below 50 ng/mL in at least 40% and over 90% of the pregnant population, respectively, throughout pregnancy. Predicted medians (range) of cord-to-maternal blood ratios were 1.71 (range, 1.55-1.79) for cabotegravir and 0.88 (0.78-0.93) for rilpivirine between weeks 38 and 40.

CONCLUSIONS

Model predictions suggest that monthly long-acting cabotegravir could maintain antiviral efficacy throughout pregnancy, but that bimonthly administration may require careful clinical evaluation. Both monthly and bimonthly long-acting rilpivirine may not adequately maintain antiviral efficacy in pregnancy.

Fetoplacental Disposition and Toxicity of Cadmium in Mice Lacking the Bcrp Transporter

The environmental toxicant cadmium (Cd) impairs the growth of rodents and humans in utero which in turn heightens susceptibility to diseases later in life. We previously demonstrated that the maternal-facing efflux transporter, breast cancer resistance protein (human BCRP/ABCG2, mouse Bcrp/Abcg2) confers resistance against Cd toxicity in human trophoblasts. In the current study, we sought to determine whether the absence of Bcrp alters the fetoplacental disposition and toxicity of Cd in mice. Pregnant female wild-type (WT) and Bcrp-null mice (n = 9-10/group) were administered a single injection of saline (5 ml/kg) or CdCl2 (5 mg/kg) on gestational day (GD) 9. Following Cd treatment, Bcrp-null offspring were shorter and accumulated more Cd in their placentas on GD 17 compared to WT mice. Because Cd can adversely impact placentation and transplacental nutrient delivery in mice, multiple pathways were assessed using morphometrics and immunohistochemistry including placenta zonation, vasculature development, and nutrient transporter expression. Most notably, the placentas of Bcrp-null mice had reduced immunostaining of the cell adhesion marker, β-catenin, and the trophoblast marker, cytokeratin, as well as decreased expression of divalent metal nutrient transporters (Dmt1, Zip14, and ZnT1) following Cd treatment. In summary, the absence of Bcrp expression increased placental concentrations of Cd which was associated with shorter fetal size that may be related to differential changes in molecular patterns of placental development and nutrition.

Changes in Expression and Function of Placental and Intestinal P-gp and BCRP Transporters during Pregnancy

ABC transporters are ubiquitous in the human body and are responsible for the efflux of drugs. They are present in the placenta, intestine, liver and kidney, which are the major organs that can affect the pharmacokinetic and pharmacologic properties of drugs. P-gp and BCRP transporters are the best-characterized transporters in the ABC superfamily, and they have a pivotal role in the barrier tissues due to their efflux mechanism. Moreover, during pregnancy, drug efflux is even more important because of the developing fetus. Recent studies have shown that placental and intestinal ABC transporters have great importance in drug absorption and distribution. Placental and intestinal P-gp and BCRP show gestational-age-dependent expression changes, which determine the drug concentration both in the mother and the fetus during pregnancy. They may have an impact on the efficacy of antibiotic, antiviral, antihistamine, antiemetic and oral antidiabetic therapies. In this review, we would like to provide an overview of the pharmacokinetically relevant expression alterations of placental and intestinal ABC transporters during pregnancy.

An update on the physiologic changes during pregnancy and their impact on drug pharmacokinetics and pharmacogenomics

For many years, the medical community has relied in clinical practice on historic data about the physiological changes that occur during pregnancy. However, some newer studies have disputed a number of assumptions in these data for not being evidence-based or derived from large prospective cohort-studies. Accurate knowledge of these physiological changes is important for three reasons: Firstly, it facilitates correct diagnosis of diseases during pregnancy; secondly, it enables us to answer questions about the effects of medication during pregnancy and the ways in which pregnancy alters pharmacokinetic and drug-effects; and thirdly, it allows for proper modeling of physiologically-based pharmacokinetic models, which are increasingly used to predict gestation-specific changes and drug-drug interactions, as well as develop new knowledge on the mode-of-action of drugs, the mechanisms underlying their interactions, and any adverse effects following drug exposure. This paper reviews new evidence regarding the physiologic changes during pregnancy in relation to existing knowledge.

Impact of in-utero antiretroviral drug exposure on expression of membrane-associated transporters in mouse placenta and fetal brain

OBJECTIVE

Although antiretroviral therapy (ART) during pregnancy is effective in limiting vertical HIV transmission, adverse outcomes persist amongst uninfected children exposed to antiretroviral drugs in utero. Membrane-associated drug transporters, metabolic enzymes, and tight junction proteins play important roles in adult antiretroviral drug disposition and toxicity; however, the fetal expression of these proteins in the context of ART, and their impact on in-utero antiretroviral drug distribution remain poorly understood. This study aimed to characterize the role of these proteins in modulating in-utero antiretroviral drug exposure.

METHODS

Pregnant mice were exposed to an ART regimen consisting of lamivudine, abacavir, atazanavir, and ritonavir, at clinically relevant doses. Fetal brain, liver, placenta amniotic fluid, and maternal plasma were collected on gestational day 18.5 and concentration of antiretroviral drugs in fetal tissues was measured by LC/MS/MS, whereas transporter expression was assessed by qPCR.

RESULTS

Abacavir and lamivudine were detected in fetal brain and amniotic fluid, whereas atazanavir and ritonavir were detected in amniotic fluid only. Robust mRNA expression of key transporters was observed in adult and fetal tissues, and sex differences were identified in the expression of Abcc1 and Slc29a1 in the placenta. Antiretroviral drug exposure was associated with a reduction in relative placental Abcg2, Abcc1, and Slc29a1 expression.

CONCLUSION

These findings identify a novel effect of fetal sex and antiretroviral drug treatment on the expression of placental transporters in a mouse model, and characterize the penetration of lamivudine and abacavir into fetal brain, uncovering a potential role of transporters in modulating fetal exposure to antiretroviral drugs.

Pharmacokinetics of tenofovir alafenamide with and without cobicistat in pregnant and postpartum women living with HIV

OBJECTIVE

To evaluate the pharmacokinetics of tenofovir alafenamide (TAF) 10 mg with cobicistat and 25 mg without boosting in pregnant and postpartum women with HIV and to characterize TAF placental transfer and infant washout pharmacokinetics.

DESIGN

Open-label, multicenter phase IV prospective study of TAF pharmacokinetics during pregnancy, postpartum, delivery, and infant washout.

METHODS

Pregnant women receiving TAF 10 mg with cobicistat or TAF 25 mg without boosting as part of clinical care had intensive pharmacokinetic assessments performed during the second and third trimesters, and 6-12 weeks postpartum. Maternal and cord blood samples were collected at delivery, and washout pharmacokinetic samples were collected in infants. TAF concentrations were quantified using liquid chromatography/mass spectrometry. Comparisons between pregnancy and postpartum were made using geometric mean ratios (90% confidence intervals) and Wilcoxon signed-rank tests.

RESULTS

Thirty-one pregnant women receiving TAF 10 mg with cobicistat-boosting and 27 women receiving TAF 25 mg without boosting were enrolled. TAF exposures did not significantly differ between pregnancy and postpartum when administered as 10 mg with cobicistat. Antepartum TAF exposures with the 25 mg dose were 33-43% lower in comparison with postpartum, but comparable with those measured in nonpregnant adults. TAF was below the lower limit of quantitation in 43 of 44 cord blood, 41 of 45 maternal blood at delivery, and all infant washout samples.

CONCLUSION

TAF exposures were comparable or higher than those measured in nonpregnant adults during pregnancy and postpartum. These findings provide reassurance on adequate TAF exposures during pregnancy, and support efforts to expand the use of TAF in pregnant women with HIV.

Low oxygen tension differentially regulates the expression of placental solute carriers and ABC transporters

Low oxygen concentration, or hypoxia, is an important physiological regulator of placental function including chemical disposition. Here, we compared the ability of low oxygen tension to alter the expression of solute carriers (SLC) and ABC transporters in two human placental models, namely BeWo cells and term placental explants. We found that exposure to low oxygen concentration differentially regulates transporter expression in BeWo cells, including downregulation of ENT1, OATP4A1, OCTN2, BCRP, and MRP2/3/5, and upregulation of CNT1, OAT4, OATP2B1, SERT, SOAT, and MRP1. Similar upregulation of MRP1 and downregulation of MRP5 and BCRP were observed in explants, whereas uptake transporters were decreased or unchanged. Furthermore, a screening of transcriptional regulators of transporters revealed that hypoxia leads to a decrease in the mRNA levels of aryl hydrocarbon receptor, nuclear factor erythroid 2-related factor 2, and retinoid x receptor alpha in both human placental models. These data suggest that transporter expression is differentially regulated by oxygen concentration across experimental human placental models.

Aryl hydrocarbon receptor (AhR) agonist β-naphthoflavone regulated gene networks in human primary trophoblasts

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is highly expressed in placenta. AhR belongs to a class of transcriptional regulators that control many developmental and physiological events (e.g. xenobiotic metabolism). Our study describes AhR regulated transcriptional responses in human primary trophoblast by using the AhR agonist, β-naphthoflavone (BNF). Human primary trophoblast cells were isolated from full term placenta after delivery. The trophoblasts were exposed to 25 μM of AhR agonist, BNF, for 72 hours. Gene expression profiling was conducted with Illumina HT-12 expression beadchips. Expression of selected genes was confirmed with RT-qPCR. Ingenuity pathway analysis (IPA) was used to predict functional pathways and upstream regulators of differentially expressed genes in order to identify regulatory networks associated with AhR. In response to BNF exposure, 64 genes were upregulated, and 257 genes were downregulated compared to control trophoblasts (±1.5-fold, p < 0.05). BNF regulated genes included placental hormones and genes implicated in immune- and inflammatory responses in addition to their well-known effects on xenobiotic metabolism, oxidative stress, antioxidant defense. In conclusion, these results show that BNF has wide-ranging effects on placental gene expression beyond xenobiotic metabolism e.g. disruption of inflammatory processes and hormones in the placenta.

An atlas of cell types in the mouse epididymis and vas deferens

Following testicular spermatogenesis, mammalian sperm continue to mature in a long epithelial tube known as the epididymis, which plays key roles in remodeling sperm protein, lipid, and RNA composition. To understand the roles for the epididymis in reproductive biology, we generated a single-cell atlas of the murine epididymis and vas deferens. We recovered key epithelial cell types including principal cells, clear cells, and basal cells, along with associated support cells that include fibroblasts, smooth muscle, macrophages and other immune cells. Moreover, our data illuminate extensive regional specialization of principal cell populations across the length of the epididymis. In addition to region-specific specialization of principal cells, we find evidence for functionally specialized subpopulations of stromal cells, and, most notably, two distinct populations of clear cells. Our dataset extends on existing knowledge of epididymal biology, and provides a wealth of information on potential regulatory and signaling factors that bear future investigation.

Transcription factor-mediated regulation of the BCRP/ABCG2 efflux transporter: a review across tissues and species

Introduction: The breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette superfamily of transporters. Using the energy garnered from the hydrolysis of ATP, BCRP actively removes drugs and endogenous molecules from the cell. With broad expression across the liver, kidney, brain, placenta, testes, and small intestines, BCRP can impact the pharmacokinetics and pharmacodynamics of xenobiotics.Areas covered: The purpose of this review is to summarize the transcriptional signaling pathways that regulate BCRP expression across various tissues and mammalian species. We will cover the endobiotic- and xenobiotic-activated transcription factors that regulate the expression and activity of BCRP. These include the estrogen receptor, progesterone receptor, peroxisome proliferator-activated receptor, constitutive androstane receptor, pregnane X receptor, nuclear factor e2-related factor 2, and aryl hydrocarbon receptor.Expert opinion: Key transcription factors regulate BCRP expression and function in response to hormones and xenobiotics. Understanding this regulation provides an opportunity to improve pharmacotherapeutic outcomes by enhancing the efficacy and reducing the toxicity of drugs that are substrates of this efflux transporter.

Contributions of Drug Transporters to Blood-Placental Barrier

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

An update on expression and function of P-gp/ABCB1 and BCRP/ABCG2 in the placenta and fetus

INTRODUCTION

P-glycoprotein (P-gp)/ABCB1 and breast cancer resistance protein (BCRP)/ABCG2 are highly expressed in the placenta and fetus throughout gestation and can modulate exposure and toxicity of drugs and xenobiotics to the vulnerable fetus during the sensitive times of growth and development. We aim to provide an update on current knowledge on placental and fetal expressions of the two transporters in different species, and to provide insight on interpreting transporter expression and fetal exposure relative to the concept of fraction of drug transported. Areas covered: Comprehensive literature review through PubMed (primarily from July 2010 to February 2018) on P-gp and BCRP expression and function in the placenta and fetus of primarily human, mouse, rat, and guinea pig. Expert opinion: While there are many commonalities in the expression and function of P-gp and BCRP in the placenta and fetal tissues across species, there are distinct differences in expression levels and temporal changes. Further studies are needed to quantify protein abundance of these transporters and functionally assess their activities at various gestational stages. Combining the knowledge of interspecies differences and the concept of fraction of drug transported, we may better predict the magnitude of impact these transporters have on fetal drug exposure.

Interindividual Regulation of the Breast Cancer Resistance Protein/ABCG2 Transporter in Term Human Placentas

The breast cancer resistance protein (BCRP/ABCG2) is a maternally-facing efflux transporter that regulates the placental disposition of chemicals. Transcription factors and gene variants are important regulatory factors that influence transporter expression. In this study, we sought to identify the genetic and transcriptional mechanisms underlying the interindividual expression of BCRP mRNA and protein across 137 term placentas from uncomplicated pregnancies. Placental expression of BCRP and regulatory transcription factor mRNAs was measured using multiplex-branched DNA analysis. BCRP expression and ABCG2 genotypes were determined using Western blot and Fluidigm Biomark genetic analysis, respectively. Placentas were obtained from a racially and ethnically diverse population, including Caucasian (33%), African American (14%), Asian (14%), Hispanic (15%), and mixed (16%) backgrounds, as well as unknown origins (7%). Between placentas, BCRP mRNA and protein varied up to 47-fold and 14-fold, respectively. In particular, BCRP mRNA correlated significantly with known transcription factor mRNAs, including nuclear factor erythroid 2-related factor 2 and aryl hydrocarbon receptor. Somewhat surprisingly, single-nucleotide polymorphisms (SNPs) in the ABCG2 noncoding regions were not associated with variation in placental BCRP mRNA or protein. Instead, the coding region polymorphism (C421A/Q141K) corresponded with 40%-50% lower BCRP protein in 421C/A and 421A/A placentas compared with wild types (421C/C). Although BCRP protein and mRNA expression weakly correlated (r = 0.25, P = 0.040), this relationship was absent in individuals expressing the C421A variant allele. Study results contribute to our understanding of the interindividual regulation of BCRP expression in term placentas and may help to identify infants at risk for increased fetal exposure to chemicals due to low expression of this efflux protein.

Pregnancy Increases Norbuprenorphine Clearance in Mice by Induction of Hepatic Glucuronidation

Norbuprenorphine (NBUP) is the major active metabolite of buprenorphine (BUP) that is commonly used to treat opiate addiction during pregnancy; it possesses 25% of BUP's analgesic activity and 10 times BUP's respiratory depression effect. To optimize BUP's dosing regimen during pregnancy with better efficacy and safety, it is important to understand how pregnancy affects NBUP disposition. In this study, we examined the pharmacokinetics of NBUP in pregnant and nonpregnant mice by administering the same amount of NBUP through retro-orbital injection. We demonstrated that the systemic clearance (CL) of NBUP in pregnant mice increased ∼2.5-fold compared with nonpregnant mice. Intrinsic CL of NBUP by glucuronidation in mouse liver microsomes from pregnant mice was ∼2 times greater than that from nonpregnant mice. Targeted liquid chromatography tandem-mass spectrometry proteomics quantification revealed that hepatic Ugt1a1 and Ugt2b1 protein levels in the same amount of total liver membrane proteins were significantly increased by ∼50% in pregnant mice versus nonpregnant mice. After scaling to the whole liver with consideration of the increase in liver protein content and liver weight, we found that the amounts of Ugt1a1, Ugt1a10, Ugt2b1, and Ugt2b35 protein in the whole liver of pregnant mice were significantly increased ∼2-fold compared with nonpregnant mice. These data suggest that the increased systemic CL of NBUP in pregnant mice is likely caused by an induction of hepatic Ugt expression and activity. The data provide a basis for further mechanistic analysis of pregnancy-induced changes in the disposition of NBUP and drugs that are predominately and extensively metabolized by Ugts.

Induction of ABCG2/BCRP restricts the distribution of zidovudine to the fetal brain in rats

Safety concerns for fetus development of zidovudine (AZT) administration as prophylaxis of vertical transmission of HIV persist. We evaluated the participation of the ATP-binding cassette efflux transporter ABCG2 in the penetration of AZT into the fetal brain and the relevance for drug safety. Oral daily doses of AZT (60mg/kg body weight) or its vehicle were administered between post gestational days 11 (E11) and 20 (E20) to Sprague-Dawley pregnant rats. At E21, animals received an intravenous bolus of 60mg AZT/kg body weight in the presence or absence of the ABCG2 inhibitor gefitinib (20mg/kg body weight, ip) and AZT in maternal plasma and fetal brain were measured by HPLC-UV. ABCG2 protein expression in placenta and fetal brain, as well as mitochondrial function and ultrastructure in fetal brain were also analyzed. In utero chronic exposure to AZT markedly induced ABCG2 expression in placenta and fetal brain whereas did not significantly alter mitochondrial functionality in the fetal brain. The area-under-the-concentration-time-curve of AZT significantly decreased in fetal brains isolated from AZT-exposed fetuses compared to control group, but this effect was abolished by ABCG2 inhibition. Our results suggest that the absence of mitochondrial toxicity in the fetal brain after chronic in utero administration of AZT could be attributed to its low accumulation in the tissue caused, at least in part, by ABCG2 overexpression. We propose that any interference with ABCG2 activity due to genetic, pathological or iatrogenic factors would increase the amount of AZT reaching the fetal brain, which could increase the risk of toxicity of this drug on the tissue.

P-gp/ABCB1 exerts differential impacts on brain and fetal exposure to norbuprenorphine

Norbuprenorphine is the major active metabolite of buprenorphine which is commonly used to treat opiate addiction during pregnancy. Norbuprenorphine produces marked respiratory depression and was 10 times more potent than buprenorphine. Therefore, it is important to understand the mechanism that controls fetal exposure to norbuprenorphine, as exposure to this compound may pose a significant risk to the developing fetus. P-gp/ABCB1 and BCRP/ABCG2 are two major efflux transporters regulating tissue distribution of drugs. Previous studies have shown that norbuprenorphine, but not buprenorphine, is a P-gp substrate. In this study, we systematically examined and compared the roles of P-gp and BCRP in determining maternal brain and fetal distribution of norbuprenorphine using transporter knockout mouse models. We administered 1mg/kg norbuprenorphine by retro-orbital injection to pregnant FVB wild-type, Abcb1a-/-/1b-/-, and Abcb1a-/-/1b-/-/Abcg2-/- mice on gestation day 15. The fetal AUC of norbuprenorphine was ∼64% of the maternal plasma AUC in wild-type mice, suggesting substantial fetal exposure to norbuprenorphine. The maternal plasma AUCs of norbuprenorphine in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were ∼2 times greater than that in wild-type mice. Fetal AUCs in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were also increased compared to wild-type mice; however, the fetal-to-maternal plasma AUC ratio remained relatively unchanged by the knockout of Abcb1a/1b or Abcb1a/1b/Abcg2. In contrast, the maternal brain-to-maternal plasma AUC ratio in Abcb1a-/-/1b-/- or Abcb1a-/-/1b-/-/Abcg2-/- mice was increased ∼30-fold compared to wild-type mice. Protein quantification by LC-MS/MS proteomics revealed significantly higher amounts of P-gp protein in the wild-type mice brain than that in the placenta. These results indicate that fetal exposure to norbuprenorphine is substantial and that P-gp has a minor impact on fetal exposure to norbuprenorphine, but plays a significant role in restricting its brain distribution. The differential impacts of P-gp on norbuprenorphine distribution into the brain and fetus are likely, at least in part, due to the differences in amounts of P-gp protein expressed in the blood-brain and blood-placental barriers. BCRP is not as important as P-gp in determining both the systemic and tissue exposure to norbuprenorphine. Finally, fetal AUCs of the metabolite norbuprenorphine-β-d-glucuronide were 3-7 times greater than maternal plasma AUCs, while the maternal brain AUCs were <50% of maternal plasma AUCs, suggesting that a reversible pool of conjugated metabolite in the fetus may contribute to the high fetal exposure to norbuprenorphine.

Down-regulation of the placental BCRP/ABCG2 transporter in response to hypoxia signaling

INTRODUCTION

The BCRP/ABCG2 efflux transporter protects the developing fetus by limiting the transplacental transfer of drugs and chemicals and prevents the apoptosis of trophoblasts. The purpose of this study was to determine whether hypoxia-related signaling alters placental BCRP expression and function in vitro and in human pregnancies.

METHODS

Human BeWo choriocarcinoma cells were treated with the hypoxia mimetic, cobalt chloride (CoCl₂), or 3% oxygen for 24-48 h. Activation of HIF-1α signaling and regulation of BCRP was assessed using qPCR, ELISA, western blotting and a fluorescent substrate transport assay. In addition, healthy term placentas from high altitude pregnancies with chronic hypoxia were assessed for BCRP expression.

RESULTS

CoCl₂ and 3% oxygen increased HIF-1α protein signaling and decreased the mRNA and protein expression of BCRP by 30-75% in BeWo cells. Reduced BCRP expression corresponded with impaired efflux activity during hypoxia as evidenced by accumulation of the substrate Hoechst 33342. A number of transcription factors known to regulate BCRP, including AHR, NRF2 and PPARγ, were also coordinately down-regulated by 3% oxygen in BeWo cells. Moreover, women who gave birth at a high altitude (3100 m) exhibited signs of chronic placental hypoxia, including enhanced protein expression of the HIF-1α target GLUT1, and had reduced BCRP levels in microvillous membranes compared to women at a moderate altitude (1600 m).

DISCUSSION

This study provides novel insight into the regulation of the placental BCRP transporter by hypoxia, which may be important for exposure of the fetus to chemicals during early development and in hypoxia-related pregnancy disorders.

Differential regulation of intestinal efflux transporters by pregnancy in mice

1. In the intestines, the nuclear receptors farnesoid X receptor (Fxr) and pregnane X receptor (Pxr) regulate the transcription of metabolizing enzymes and transporters that dictate the absorption of nutrients and xenobiotics. 2. Here, we sought to determine whether Fxr and Pxr signaling pathways are disrupted in response to high-circulating concentrations of steroid hormones late in pregnancy leading to altered transporter expression. To test this, ileum were collected from virgin and pregnant C57BL/6 mice on gestation days 14, 17 and 19. 3. Ileum from pregnant mice exhibited suppression of Fgf15 and Cyp3a11 mRNAs, which are the prototypical target genes for Fxr and Pxr, respectively. An overall reduction in the expression of apical efflux transporters, including Mdr1, Mrp2 and Bcrp, was observed in pregnant mice. To assess the ability of steroid hormones to alter intestinal nuclear receptor signaling, transporter mRNA expression was quantified in human intestinal LS174T adenocarcinoma cells. In vitro data demonstrated that progestins reduced CYP3A4, MDR1 and MRP2 mRNA expression by 30-40%. 4. These data suggest that progesterone may act as a mediator to negatively regulate efflux transporter expression in the mouse ileum during pregnancy possibly by reducing PXR/Pxr signaling. This may affect drug absorption and disposition during pregnancy.

Bioluminescent imaging of ABCG2 efflux activity at the blood-placenta barrier

Physiologic barriers such as the blood placenta barrier (BPB) and the blood brain barrier protect the underlying parenchyma from pathogens and toxins. ATP-binding cassette (ABC) transporters are transmembrane proteins found at these barriers, and function to efflux xenobiotics and maintain chemical homeostasis. Despite the plethora of ex vivo and in vitro data showing the function and expression of ABC transporters, no imaging modality exists to study ABC transporter activity in vivo at the BPB. In the present study, we show that in vitro models of the placenta possess ABCG2 activity and can specifically transport D-luciferin, the endogenous substrate of firefly luciferase. To test ABCG2 transport activity at the BPB, we devised a breeding strategy to generate a bioluminescent pregnant mouse model to demonstrate transporter function in vivo. We found that coadministering the ABCG2 inhibitors Ko143 and gefitinib with D-luciferin increased bioluminescent signal from fetuses and placentae, whereas the control P-gp inhibitor DCPQ had no effect. We believe that our bioluminescent pregnant mouse model will facilitate greater understanding of the BPB and ABCG2 activity in health and disease.

Polymorphisms of SLC22A9 (hOAT7) in Korean Females with Osteoporosis

Among solute carrier proteins, the organic anion transporters (OATs) play an important role for the elimination or reabsorption of endogenous and exogenous negatively charged anionic compounds. Among OATs, SLC22A9 (hOAT7) transports estrone sulfate with high affinity. The net decrease of estrogen, especially in post-menopausal women induces rapid bone loss. The present study was performed to search the SNP within exon regions of SLC22A9 in Korean females with osteoporosis. Fifty healthy controls and 50 osteoporosis patients were screened for the genetic polymorphism in the coding region of SLC22A9 using GC-clamped PCR and denaturing gradient gel electrophoresis (DGGE). Six SNPs were found on the SLC22A9 gene from Korean women with/without osteoporosis. The SNPs were located as follows: two SNPs in the osteoporosis group (A645G and T1277C), three SNPs in the control group (G1449T, C1467T and C1487T) and one SNP in both the osteoporosis and control groups (G767A). The G767A, T1277C and C1487T SNPs result in an amino acid substitution, from synonymous vs nonsynonymous substitution arginine to glutamine (R256Q), phenylalanine to serine (F426S) and proline to leucine (P496L), respectively. The Km values and Vmax of the wild type, R256Q, P496L and F426S were 8.84, 8.87, 9.83 and 12.74 µM, and 1.97, 1.96, 2.06 and 1.55 pmol/oocyte/h, respectively. The present study demonstrates that the SLC22A9 variant F426S is causing inter-individual variation that is leading to the differences in transport of the steroid sulfate conjugate (estrone sulfate) and, therefore this could be used as a marker for certain disease including osteoporosis.

Regulation of the expression of renal drug transporters in KEAP1-knockdown human tubular cells

The kidney secretes various xenobiotics through a well-established transport system. The transcription factor NF-E2-related factor 2 (NRF2) up-regulates a subset of genes encoding antioxidant and detoxification proteins. Kelch-like ECH-associated protein 1 (KEAP1) down-regulates NRF2 by facilitating continuous degradation of NRF2 protein. Here, we investigated the role of NRF2 in the expression of renal drug transporters by using a stable KEAP1 knockdown renal tubular HK-2 cell line (shKEAP1). KEAP1 knockdown resulted in a significant increase in the expression of four renal transporters, namely, multidrug resistance protein 1 (MDR1; ABCB1), breast cancer resistance protein (BCRP; ABCG2), multidrug resistance-associated protein 2 (MRP2; ABCC2), and MRP3 (ABCC3). In western blot and immunocytochemical analyses, protein levels of these transporters were also significantly higher in the knockdown group. Consequently, shKEAP1 cells released more Hoechst 33342 fluorescent dye and doxorubicin, and they were more resistant to doxorubicin than the control cells. In addition, cisplatin resistance of shKEAP1 decreased upon co-incubation with a transporter inhibitor. Whereas, a short term incubation (24h) with sulforaphane did not show noticeable changes in the expression of transporter. Collectively, these results indicate that NRF2 regulates the expression of MDR1, BCRP, MRP2, and MRP3 in human tubular epithelial cells. Altered expression of these transporters affects drug secretion in these cells, which may result in the renal cellular damage upon exposure to nephrotoxic xenobiotics.

Maternal-fetal disposition of glyburide in pregnant mice is dependent on gestational age

Gestational diabetes mellitus is a major complication of human pregnancy. The oral clearance (CL) of glyburide, an oral antidiabetic drug, increases 2-fold in pregnant women during late gestation versus nonpregnant controls. In this study, we examined gestational age-dependent changes in maternal-fetal pharmacokinetics (PK) of glyburide and metabolites in a pregnant mouse model. Nonpregnant and pregnant FVB mice were given glyburide by retro-orbital injection. Maternal plasma was collected over 240 minutes on gestation days (gd) 0, 7.5, 10, 15, and 19; fetuses were collected on gd 15 and 19. Glyburide and metabolites were quantified using high-performance liquid chromatography-mass spectrometry, and PK analyses were performed using a pooled data bootstrap approach. Maternal CL of glyburide increased approximately 2-fold on gd 10, 15, and 19 compared with nonpregnant controls. Intrinsic CL of glyburide in maternal liver microsomes also increased as gestation progressed. Maternal metabolite/glyburide area under the curve ratios were generally unchanged or slightly decreased throughout gestation. Total fetal exposure to glyburide was <5% of maternal plasma exposure, and was doubled on gd 19 versus gd 15. Fetal metabolite concentrations were below the limit of assay detection. This is the first evidence of gestational age-dependent changes in glyburide PK. Increased maternal glyburide clearance during gestation is attributable to increased hepatic metabolism. Metabolite elimination may also increase during pregnancy. In the mouse model, fetal exposure to glyburide is gestational age-dependent and low compared with maternal plasma exposure. These results indicate that maternal glyburide therapeutic strategies may require adjustments in a gestational age-dependent manner if these same changes occur in humans.

Effect of gestational age on mRNA and protein expression of polyspecific organic cation transporters during pregnancy

Polyspecific organic cation (OC) transporters play important roles in the disposition of clinically used drugs, including drugs used during pregnancy. Pregnancy is known to alter the expression of drug-metabolizing enzymes and transporters, but its specific effect on OC transporters has not been well defined. Using quantitative polymerase chain reaction and liquid chromatography coupled with tandem mass spectrometry targeted proteomics, we determined the effect of pregnancy and gestational age on mRNA and protein expression of major OC transporters in the kidney, liver, and placenta in mice with timed pregnancies. Human organic cation transporter 3 (hOCT3) expression was further investigated in human placentas from the first and second trimesters and at term. Our results showed that pregnancy had a marginal effect on renal mouse organic cation transporter 1/2 (mOct1/2) expression but significantly reduced mouse multidrug and toxin extrusion transporter 1 (mMate1) expression by 20%-40%. Hepatic expression of mOct1 and mMate1 was minimally affected by pregnancy. Human and mouse placentas predominantly expressed OCT3 with little expression of OCT1/2, MATE1/2, and plasma membrane monoamine transporter (PMAT). The hOCT3 protein in first and second trimester and term placentas was quantified to be 0.23 ± 0.033, 0.38 ± 0.072, and 0.36 ± 0.099 fmol/μg membrane protein, respectively. In contrast with the moderate increase in hOCT3 protein during human pregnancy, mOct3 expression in the mouse placenta was highly dependent on gestational age. Compared with gestational day (gd) 10, placental mOct3 mRNA increased by 37-fold and 46-fold at gd 15 and 19, leading to a 56-fold and 128-fold increase in mOct3 protein, respectively. Our study provides new insights into the effect of pregnancy on the expression of polyspecific OC transporters and supports an important role of OCT3 in OC transport at the placental barrier.

Bioavailability and pharmacokinetics of genistein: mechanistic studies on its ADME

Genistein, one of the most active natural flavonoids, exerts various biological effects including chemoprevention, antioxidation, antiproliferation and anticancer. More than 30 clinical trials of genistein with various disease indications have been conducted to evaluate its clinical efficacy. Based on many animals and human pharmacokinetic studies, it is well known that the most challenge issue for developing genistein as a chemoprevention agent is the low oral bioavailability, which may be the major reason relating to its ambiguous therapeutic effects and large interindividual variations in clinical trials. In order to better correlate pharmacokinetic to pharmacodynamics results in animals and clinical studies, an in-depth understanding of pharmacokinetic behavior of genistein and its ADME properties are needed. Numerous in vitro/in vivo ADME studies had been conducted to reveal the main factors contributing to the low oral bioavailability of genistein. Therefore, this review focuses on summarizing the most recent progress on mechanistic studies of genistein ADME and provides a systemic view of these processes to explain genistein pharmacokinetic behaviors in vivo. The better understanding of genistein ADME property may lead to development of proper strategy to improve genistein oral bioavailability via mechanism-based approaches.

Placental ABC transporters, cellular toxicity and stress in pregnancy

The human placenta, in addition to its roles as a nutrient transfer and endocrine organ, functions as a selective barrier to protect the fetus against the harmful effects of exogenous and endogenous toxins. Members of the ATP-binding cassette (ABC) family of transport proteins limit the entry of xenobiotics into the fetal circulation via vectorial efflux from the placenta to the maternal circulation. Several members of the ABC family, including proteins from the ABCA, ABCB, ABCC and ABCG subfamilies, have been shown to be functional in the placenta with clinically significant roles in xenobiotic efflux. However, recent findings suggest that these transporters also protect placental tissue by preventing the cellular accumulation of cytotoxic compounds such as lipids, sterols and their derivatives. Such protective functions are likely to be particularly important in pregnancies complicated by inflammatory or oxidative stress, where the generation of toxic metabolites is enhanced. For example, ABC transporters have been shown to protect against the harmful effects of hypoxia and oxidative stress through increased expression and efflux of oxysterols and glutathione conjugated xenobiotics. However, this protective capacity may be diminished in response to the same stressors. Several studies in primary human trophoblast cells and animal models have demonstrated decreased expression and activity of placental ABC transporters with inflammatory, oxidative or metabolic stress. Several clinical studies in pregnancies complicated by inflammatory conditions such as preeclampsia and gestational diabetes support these findings, although further studies are required to determine the clinical relevance of the relationships between placental ABC transporter expression and activity, and placental function in stressed pregnancies. Such studies are necessary to fully understand the consequences of pregnancy disorders on placental function and viability in order to optimise pregnancy care and maximise fetal growth and health.

Gestational age-dependent changes in gene expression of metabolic enzymes and transporters in pregnant mice

Pregnancy-induced changes in drug pharmacokinetics can be explained by changes in expression of drug-metabolizing enzymes and transporters and/or normal physiology. In this study, we determined gestational age-dependent expression profiles for all metabolic enzyme and transporter genes in the maternal liver, kidney, small intestine, and placenta of pregnant mice by microarray analysis. We specifically examined the expression of genes important for xenobiotic, bile acid, and steroid hormone metabolism and disposition, namely, cytochrome P450s (Cyp), UDP-glucuronosyltranserases (Ugt), sulfotransferases (Sult), and ATP-binding cassette (Abc), solute carrier (Slc), and solute carrier organic anion (Slco) transporters. Few Ugt and Sult genes were affected by pregnancy. Cyp17a1 expression in the maternal liver increased 3- to 10-fold during pregnancy, which was the largest observed change in the maternal tissues. Cyp1a2, most Cyp2 isoforms, Cyp3a11, and Cyp3a13 expression in the liver decreased on gestation days (gd) 15 and 19 compared with nonpregnant controls (gd 0). In contrast, Cyp2d40, Cyp3a16, Cyp3a41a, Cyp3a41b, and Cyp3a44 in the liver were induced throughout pregnancy. In the placenta, Cyp expression on gd 10 and 15 was upregulated compared with gd 19. Notable changes were also observed in Abc and Slc transporters. Abcc3 expression in the liver and Abcb1a, Abcc4, and Slco4c1 expression in the kidney were downregulated on gd 15 and 19. In the placenta, Slc22a3 (Oct3) expression on gd 10 was 90% lower than that on gd 15 and 19. This study demonstrates important gestational age-dependent expression of metabolic enzyme and transporter genes, which may have mechanistic relevance to drug disposition in human pregnancy.

Pharmacotherapy in pregnancy; effect of ABC and SLC transporters on drug transport across the placenta and fetal drug exposure

Pharmacotherapy during pregnancy is often inevitable for medical treatment of the mother, the fetus or both. The knowledge of drug transport across placenta is, therefore, an important topic to bear in mind when deciding treatment in pregnant women. Several drug transporters of the ABC and SLC families have been discovered in the placenta, such as P-glycoprotein, breast cancer resistance protein, or organic anion/cation transporters. It is thus evident that the passage of drugs across the placenta can no longer be predicted simply on the basis of their physical-chemical properties. Functional expression of placental drug transporters in the trophoblast and the possibility of drug-drug interactions must be considered to optimize pharmacotherapy during pregnancy. In this review we summarize current knowledge on the expression and function of ABC and SLC transporters in the trophoblast. Furthermore, we put this data into context with medical conditions that require maternal and/or fetal treatment during pregnancy, such as gestational diabetes, HIV infection, fetal arrhythmias and epilepsy. Proper understanding of the role of placental transporters should be of great interest not only to clinicians but also to pharmaceutical industry for future drug design and development to control the degree of fetal exposure.

Regulatory pathways for ATP-binding cassette transport proteins in kidney proximal tubules

The ATP-binding cassette transport proteins (ABC transporters) represent important determinants of drug excretion. Protective or excretory tissues where these transporters mediate substrate efflux include the kidney proximal tubule. Regulation of the transport proteins in this tissue requires elaborate signaling pathways, including genetic, epigenetic, nuclear receptor mediated, posttranscriptional gene regulation involving microRNAs, and non-genomic (kinases) pathways triggered by hormones and/or growth factors. This review discusses current knowledge on regulatory pathways for ABC transporters in kidney proximal tubules, with a main focus on P-glycoprotein, multidrug resistance proteins 2 and 4, and breast cancer resistance protein. Insight in these processes is of importance because variations in transporter activity due to certain (disease) conditions could lead to significant changes in drug efficacy or toxicity.

Endocrine and metabolic regulation of renal drug transporters

Renal xenobiotic transporters are important determinants of urinary secretion and reabsorption of chemicals. In addition to glomerular filtration, these processes are key to the overall renal clearance of a diverse array of drugs and toxins. Alterations in kidney transporter levels and function can influence the efficacy and toxicity of chemicals. Studies in experimental animals have revealed distinct patterns of renal transporter expression in response to sex hormones, pregnancy, and growth hormone. Likewise, a number of disease states including diabetes, obesity, and cholestasis alter the expression of kidney transporters. The goal of this review is to provide an overview of the major xenobiotic transporters expressed in the kidneys and an understanding of metabolic conditions and hormonal factors that regulate their expression and function.

Repression of hepatobiliary transporters and differential regulation of classic and alternative bile acid pathways in mice during pregnancy

During pregnancy, proper hepatobiliary transport and bile acid synthesis protect the liver from cholestatic injury and regulate the maternal and fetal exposure to bile acids, drugs, and environmental chemicals. The objective of this study was to determine the temporal messenger RNA (mRNA) and protein profiles of uptake and efflux transporters as well as bile acid synthetic and conjugating enzymes in livers from virgin and pregnant mice on gestational days (GD) 7, 11, 14, and 17 and postnatal days (PND) 1, 15, and 30. Compared with virgins, the mRNAs of most transporters were reduced approximately 50% in pregnant dams between GD11 and 17. Western blot and immunofluorescence staining confirmed the downregulation of Mrp3, 6, Bsep, and Ntcp proteins. One day after parturition, the mRNAs of many uptake and efflux hepatobiliary transporters remained low in pregnant mice. By PND30, the mRNAs of all transporters returned to virgin levels. mRNAs of the bile acid synthetic enzymes in the classic pathway, Cyp7a1 and 8b1, increased in pregnant mice, whereas mRNA and protein expression of enzymes in the alternative pathway of bile acid synthesis (Cyp27a1 and 39a1) and conjugating enzymes (Bal and Baat) decreased. Profiles of transporter and bile acid metabolism genes likely result from coordinated downregulation of transcription factor mRNA (CAR, LXR, PXR, PPARα, FXR) in pregnant mice on GD14 and 17. In conclusion, pregnancy caused a global downregulation of most hepatic transporters, which began as early as GD7 for some genes and was maximal by GD14 and 17, and was inversely related to increasing concentrations of circulating 17β-estradiol and progesterone as pregnancy progressed.

Down-regulation of brush border efflux transporter expression in the kidneys of pregnant mice

Pregnancy increases the urinary excretion of chemicals in women and rodents. It is unknown whether the enhanced clearance of drugs during pregnancy involves changes in the expression of transporters that mediate chemical secretion and reabsorption. The purpose of this study was to quantify the mRNA and protein expression of efflux transporters in kidneys from virgin and pregnant mice on gestational days 7, 11, 14, and 17 and postnatal days 1, 15, and 30 with use of quantitative polymerase chain reaction, Western blot, and immunofluorescence. Multidrug resistance protein (Mdr) 1b mRNA, multidrug resistance-associated protein (Mrp) 4 mRNA, and protein levels decreased significantly by 25-75% throughout pregnancy and lactation. Similarly, Mrp2 and multidrug and toxin extrusion transporter (Mate) 1 mRNA expression were down-regulated 20-40% during mid to late gestation but returned to control levels by postnatal day 15. In contrast, Mrp3 mRNA and protein increased 225% and 31%, respectively, at gestational day 14. Coordinated down-regulation of brush border transporters Mate1, Mrp2, and Mrp4 and up-regulation of the basolateral Mrp3 transporter would reduce chemical secretion into urine.

Effect of gender, pregnancy and exposure conditions on metabolism and distribution of zearalenone in rats

The mycotoxin zearalenone (ZEA) is produced by a variety of Fusarium fungi and contaminates numerous cereals, fruits and vegetables. Interacting with the oestrogen receptors, ZEA and reduced metabolites zearalenols (ZOLs) cause hormonal effects in animals, such as abnormalities in the development of the reproductive tract and mammary gland in female offspring. Limited information is available on the pharmacokinetics of ZEA and its metabolites, particularly in pregnant females, foetuses and newborns. Our study was conducted to characterise the tissue distribution and metabolism of ZEA in male and female rats in various physiological states (virgin female, pregnant female) and exposure conditions (subcutaneous versus oral exposure, single versus repeated exposure to 1 mg/kg ZEA). Respective placental and mammary transfer to foetuses and newborns was evaluated. In all states and exposure conditions, α-ZOL and the glucuronides of ZEA and α-ZOL were the predominant metabolites, mostly concentrated in the intestine, the liver and the urine. Toxins were very low or undetectable in most of the tissues 24 h after ZEA exposure, except in foetal livers. Absorption and intestinal glucuronidation of ZEA were higher in males than females. α-ZOL concentration was significantly higher in the intestine and liver of males and pregnant females, compared to virgin females. ZEA and all its metabolites easily crossed the placental barrier and transferred into the milk. ZEA was metabolised in the foetal and neonatal stages, glucuronides being the main form detected in all organs. Metabolite elimination was slower in foetal tissues than in maternal tissues. All toxin concentrations in the foetal and neonatal tissues strongly increased in cases of repeated maternal exposure. A better knowledge of the metabolism and transfer of ZEA in foetuses and newborns will help to evaluate the health risk that such endocrine disruptors represent in these stages.

ATP-binding cassette efflux transporters in human placenta

Pregnant women are often complicated with diseases including viral or bacterial infections, epilepsy, hypertension, or pregnancy-induced conditions such as depression and gestational diabetes that require treatment with medication. In addition, substance abuse during pregnancy remains a major public health problem. Many drugs used by pregnant women are off label without the necessary dose, efficacy, and safety data required for rational dosing regimens of these drugs. Thus, a major concern arising from the widespread use of drugs by pregnant women is the transfer of drugs across the placental barrier, leading to potential toxicity to the developing fetus. Knowledge regarding the ATP-binding cassette (ABC) efflux transporters, which play an important role in drug transfer across the placental barrier, is absolutely critical for optimizing the therapeutic strategy to treat the mother while protecting the fetus during pregnancy. Such transporters include P-glycoprotein (P-gp, gene symbol ABCB1), the breast cancer resistance protein (BCRP, gene symbol ABCG2), and the multidrug resistance proteins (MRPs, gene symbol ABCCs). In this review, we summarize the current knowledge with respect to developmental expression and regulation, membrane localization, functional significance, and genetic polymorphisms of these ABC transporters in the placenta and their relevance to fetal drug exposure and toxicity.

Aryl hydrocarbon receptor and aryl hydrocarbon nuclear translocator expression in human and rat placentas and transcription activity in human trophoblast cultures

Aryl hydrocarbon receptor (AHR) and its heterodimer aryl hydrocarbon nuclear translocator (ARNT) form a ligand-activated transcription complex that regulates expression of the AHR battery of target genes that includes the most important placental biotransformation enzyme cytochrome CYP1A1. Expression, placental localization, and ontogeny of AHR/Ahr and ARNT/Arnt have not been systematically studied in either human or rat placentas. Moreover, induction of such AHR target genes as CYP1A1, CYP1A2, CYP1B1, UGT1A1, and breast cancer resistance protein (BCRP), as well as of AHR, ARNT, and aryl hydrocarbon receptor repressor (AHRR) genes, after exposure to AHR ligands have not been studied in human placental trophoblast cultures. In this article, we show that only CYP1A1 messenger RNA (mRNA), but not CYP1A2, CYP1B1, UGT1A1, BCRP, AHR, ARNT, and AHRR mRNAs, is significantly induced in human term placental trophoblast cultures after exposure to prototype AHR ligands/activators 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3-methylcholanthrene, omeprazole, and β-naphthoflavone. We localized AHR/Ahr and ARNT/Arnt in rat placental trophoblasts throughout gestation and in first trimester and term human placental trophoblast, which comprise the crucial component of the maternal-fetal barrier. We demonstrate that rat Ahr and Cyp1a1 reached highest expression during gestation days 15 and 18, which might indicate different response to Ahr ligands in placental Cyp1a1 induction during rat gestation. We also propose the JEG3 choriocarcinoma cell line as a cellular model for human trophoblast induction studies through AHR. In conclusion, we describe expression and ontogeny of AHR/Ahr and ARNT/Arnt and systematically characterize induction of major AHR target genes in human placental trophoblast forming the placental maternal-fetal morphological and metabolic barrier.

Glucocorticoid regulation of placental breast cancer resistance protein (Bcrp1) in the mouse

Placental breast cancer resistance protein (Bcrp1; encoded by the Abcg2 gene) limits maternal-fetal transplacental transfer of numerous endogenous and exogenous substrates; however, the regulation of placental Abcg2 and Bcrp1 and is not well understood. Placental Abcg2 messenger RNA (mRNA) levels decrease with advancing gestation in the mouse, and this corresponds to increasing levels of maternal and fetal plasma glucocorticoid. Glucocorticoids, including dexamethasone (DEX), downregulate Bcrp1 expression and function in both breast cancer cell lines and the blood-brain barrier in vitro; whether this occurs in the placenta is not known. The potential regulatory role of synthetic glucocorticoids on placental Bcrp1 is of interest, given that approximately 10% of pregnant women are treated with synthetic glucocorticoid for threatened preterm labor. We hypothesized that (1) exposure of pregnant mice to DEX will downregulate placental Abcg2 mRNA and Bcrp1 protein, and (2) results in increased fetal accumulation of [(3)H]mitoxantrone. Pregnant mice were treated with DEX (low-dose: 0.1 mg/kg or high-dose: 1 mg/kg) or vehicle (saline) from embryonic day (E) E9.5 to E15.5 or E12.5 to E18.5. In placentae derived from female fetuses, high-dose DEX significantly downregulated Abcg2 mRNA expression on E15.5 (P < .05) and significantly inhibited Bcrp1 function (P < .05). Similarly, high-dose DEX significantly inhibited Bcrp1 function in the placentae derived from male fetuses (P < .05). In conclusion, there is a dose-dependent regulatory effect of synthetic glucocorticoid on placental Abcg2 mRNA and Bcrp1 function in vivo. Further, it appears that, at the level of Abcg2 gene expression, the female-derived placentae are more susceptible to the effects of DEX than male placentae.

ABCB1 and ABCG2 expression in the placenta and fetus: an interspecies comparison

IMPORTANCE OF THE FIELD

ABCB1 and ABCG2 are efflux transporters which have a major impact on the pharmacological behavior of numerous drugs. They are expressed, for example, in the intestine, liver, kidney, BBB and placenta. It has become evident that ABCB1 and ABCG2 modify the pharmaco/toxicokinetics in the placenta and fetus and may consequently affect the outcome of pregnancy.

AREAS COVERED IN THIS REVIEW

Comprehensive literature searches were done using PubMed (until June 2010) to identify publications on ABCB1 and ABCG2 expression in placenta and fetal tissues in human, mouse, rat, guinea-pig and rabbit.

WHAT THE READER WILL GAIN

In this review, we aim to provide an overview of the current knowledge on the ABCB1 and ABCG2 transporter expression profiles in the placenta and fetal tissues in humans relative to other species.

TAKE HOME MESSAGE

The available information on ABCB1 and ABCG2 temporal expression profiles in placenta and fetus indicates rather good correlation among human, mouse and rat although some specific differences have been reported. However, at this point no detailed comparisons or comparative functional data are available. Detailed knowledge on the expression patterns and functional activity of ABCB1 and ABCG2 transporters placenta and developing embryo/fetus in different species could possibly help the interspecies extrapolation.

Increased glyburide clearance in the pregnant mouse model

Glyburide (GLB) is an oral sulfonylurea, commonly used for the treatment of gestational diabetes mellitus. It has been reported that the clearance of GLB in pregnant women is significantly higher than that in nonpregnant women. The molecular mechanism by which pregnancy increases the clearance of GLB is not known, but it may be caused by increased CYP3A activity. Because liver tissue from pregnant women is not readily available, in the present study, we investigated the mechanism of such pregnancy-related changes in GLB disposition in a mouse model. We demonstrated that the systemic clearance of GLB in pregnant mice was increased approximately 2-fold (p < 0.01) compared with nonpregnant mice, a magnitude of change similar to that observed in the clinical study. Plasma protein binding of GLB in mice was not altered by pregnancy. The half-life of GLB depletion in hepatic S-9 fractions of pregnant mice was significantly shorter than that of nonpregnant mice. Moreover, GLB depletion was markedly inhibited by ketoconazole, a potent inhibitor of mouse Cyp3a, suggesting that GLB metabolism in mice is primarily mediated by hepatic Cyp3a. These data suggest that the increased systemic clearance of GLB in pregnant mice is likely caused by an increase in hepatic Cyp3a activity during pregnancy, and they provide a basis for further mechanistic understanding and analysis of pregnancy-induced alterations in the disposition of GLB and drugs that are predominantly and extensively metabolized by CYP3A/Cyp3a.

Estrogen receptor beta signaling through phosphatase and tensin homolog/phosphoinositide 3-kinase/Akt/glycogen synthase kinase 3 down-regulates blood-brain barrier breast cancer resistance protein

Breast cancer resistance protein (BCRP) is an ATP-driven efflux pump at the blood-brain barrier that limits central nervous system pharmacotherapy. Our previous studies showed rapid loss of BCRP transport activity in rat brain capillaries exposed to low concentrations of 17-beta-estradiol (E2); this occurred without acute change in BCRP protein expression. Here, we describe a pathway through which sustained, extended exposure to E2 signals down-regulation of BCRP at the blood-brain barrier. Six-hour exposure of isolated rat and mouse brain capillaries to E2 reduced BCRP transport activity and BCRP monomer and dimer expression. Experiments with brain capillaries from estrogen receptor (ER)alpha and ERbeta knockout mice and with ER agonists and antagonists showed that E2 signaled through ERbeta to down-regulate BCRP expression. In rat brain capillaries, E2 increased unphosphorylated, active phosphatase and tensin homolog (PTEN); decreased phosphorylated, active Akt; and increased phosphorylated, active glycogen synthase kinase (GSK)3. Consistent with this, inhibition of phosphoinositide 3-kinase (PI3K) or Akt decreased BCRP activity and protein expression, and inhibition of PTEN or GSK3 reversed the E2 effect on BCRP. Lactacystin, a proteasome inhibitor, abolished E2-mediated BCRP down-regulation, suggesting internalization followed by transporter degradation. Dosing mice with E2 reduced BCRP activity in brain capillaries within 1 h; this reduction persisted for 24 h. BCRP protein expression in brain capillaries was unchanged 1 h after E2 dosing but was substantially reduced 6 and 24 h after dosing. Thus, E2 signals through ERbeta, PTEN/PI3K/Akt/GSK3 to stimulate proteasomal degradation of BCRP. These in vitro and in vivo findings imply that E2-mediated down-regulation of blood-brain barrier BCRP has the potential to increase brain uptake of chemotherapeutics that are BCRP substrates.

Effect of pregnancy on nitrofurantoin disposition in mice

We investigated the effect of pregnancy on nitrofurantoin (NFT) disposition in wild-type and Bcrp1(-/-) mice. Pregnant and non-pregnant mice were administered NFT intravenously (5 mg/kg) or orally (10 mg/kg). Blood samples were collected at various times (5-60 min) after drug administration, plasma NFT concentrations determined by HPLC/UV, and pharmacokinetic parameters estimated. Dose-normalized area under the plasma concentration-time curve (AUC), terminal plasma half-life (T(1/2)), total plasma clearance (CL), and steady-state volume of distribution (V(ss)) of intravenous NFT in wild-type or Bcrp1(-/-) mice were not altered by pregnancy. After oral administration, pregnancy did not affect dose-normalized AUC of NFT in wild-type mice; however, dose-normalized AUC in Bcrp1(-/-) mice was decreased by approximately 70% by pregnancy. In conclusion, since Bcrp1 plays a minor role in the systemic clearance of NFT in female mice, pregnancy did not affect disposition of intravenous NFT despite the fact that Bcrp1 expression in the liver and kidney of mice is significantly induced by pregnancy. On the other hand, pregnancy may affect expression and activity of certain intestinal efflux transporters and/or metabolic enzymes in Bcrp1(-/-) mice, resulting in a drastic decrease in the systemic exposure of oral NFT in pregnant Bcrp1(-/-) mice.

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.