Regulation of human organic anion transporter 4 by progesterone and protein kinase C in human placental BeWo cells

A Literature Review of Changes in Phase II Drug-Metabolizing Enzyme and Drug Transporter Expression during Pregnancy

The purpose of this literature review is to comprehensively summarize changes in the expression of phase II drug-metabolizing enzymes and drug transporters in both the pregnant woman and the placenta. Using PubMed®, a systematic search was conducted to identify literature relevant to drug metabolism and transport in pregnancy. PubMed was searched with pre-specified terms during the period of 26 May 2023 to 10 July 2023. The final dataset of 142 manuscripts was evaluated for evidence regarding the effect of gestational age and hormonal regulation on the expression of phase II enzymes (n = 16) and drug transporters (n = 38) in the pregnant woman and in the placenta. This comprehensive review exposes gaps in current knowledge of phase II enzyme and drug transporter localization, expression, and regulation during pregnancy, which emphasizes the need for further research. Moreover, the information collected in this review regarding phase II drug-metabolizing enzyme and drug transporter changes will aid in optimizing pregnancy physiologically based pharmacokinetic (PBPK) models to inform dose selection in the pregnant population.

The regulation of human organic anion transporter 4 by insulin-like growth factor 1 and protein kinase B signaling

Human organic anion transporter 4 (hOAT4), mainly expressed in the kidney and placenta, is essential for the disposition of numerous drugs, toxins, and endogenous substances. Insulin-like growth factor 1 (IGF-1) is a hormone generated in the liver and plays important roles in systemic growth, development, and metabolism. In the current study, we explored the regulatory effects of IGF-1 and downstream signaling on the transport activity, protein expression, and SUMOylation of hOAT4. We showed that IGF-1 significantly increased the transport activity, expression, and maximal transport velocity Vmax of hOAT4 in kidney-derived cells. This stimulatory effect of IGF-1 on hOAT4 activity was also confirmed in cells derived from the human placenta. The increased activity and expression were correlated well with the reduced degradation rate of hOAT4 at the cell surface. Furthermore, IGF-1 significantly increased hOAT4 SUMOylation, and protein kinase B (PKB)-specific inhibitors blocked the IGF-1-induced regulations on hOAT4. In conclusion, our study demonstrates that the hepatic hormone IGF-1 regulates hOAT4 expressed in the kidney and placenta through the PKB signaling pathway. Our results support the remote sensing and signaling theory, where OATs play a central role in the remote communications among distal tissues.

Transporter Regulation in Critical Protective Barriers: Focus on Brain and Placenta

Drug transporters play an important role in the maintenance of chemical balance and homeostasis in different tissues. In addition to their physiological functions, they are crucial for the absorption, distribution, and elimination of many clinically important drugs, thereby impacting therapeutic efficacy and toxicity. Increasing evidence has demonstrated that infectious, metabolic, inflammatory, and neurodegenerative diseases alter the expression and function of drug transporters. However, the current knowledge on transporter regulation in critical protective barriers, such as the brain and placenta, is still limited and requires more research. For instance, while many studies have examined P-glycoprotein, it is evident that research on the regulation of highly expressed transporters in the blood–brain barrier and blood–placental barrier are lacking. The aim of this review is to summarize the currently available literature in order to better understand transporter regulation in these critical barriers.

Is It Time to Use Modeling of Cellular Transporter Homeostasis to Inform Drug-Drug Interaction Studies: Theoretical Considerations

Mathematical modeling has been an important tool in pharmaceutical research for 50 + years and there is increased emphasis over the last decade on using modeling to improve the efficiency and effectiveness of drug development. In an earlier commentary, we applied a multiscale model linking 6 scales (whole body, tumor, vasculature, cell, spatial location, time), together with literature data on nanoparticle and tumor properties, to demonstrate the effects of nanoparticle particles on systemic disposition. The current commentary used a 4-scale model (cell membrane, intracellular organelles, spatial location, time) together with literature data on the intracellular processing of membrane receptors and transporters to demonstrate disruption of transporter homeostasis can lead to drug-drug interaction (DDI) between victim drug (VD) and perpetrator drug (PD), including changes in the area-under-concentration-time-curve of VD in cells that are considered significant by the US Food and Drug Administration (FDA). The model comprised 3 computational components: (a) intracellular transporter homeostasis, (b) pharmacokinetics of extracellular and intracellular VD/PD concentrations, and (c) pharmacodynamics of PD-induced stimulation or inhibition of an intracellular kinetic process. Model-based simulations showed that (a) among the five major endocytic processes, perturbation of transporter internalization or recycling led to the highest incidence and most extensive DDI, with minor DDI for perturbing transporter synthesis and early-to-late endosome and no DDI for perturbing transporter degradation and (b) three experimental conditions (spatial transporter distribution in cells, VD/PD co-incubation time, extracellular PD concentrations) were determinants of DDI detection. We propose modeling is a useful tool for hypothesis generation and for designing experiments to identify potential DDI; its application further aligns with the model-informed drug development paradigm advocated by FDA.

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

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

Post-translational regulation of the major drug transporters in the families of organic anion transporters and organic anion-transporting polypeptides

The organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs) belong to the solute carrier (SLC) transporter superfamily and play important roles in handling various endogenous and exogenous compounds of anionic charge. The OATs and OATPs are often implicated in drug therapy by impacting the pharmacokinetics of clinically important drugs and, thereby, drug exposure in the target organs or cells. Various mechanisms (e.g. genetic, environmental, and disease-related factors, drug-drug interactions, and food-drug interactions) can lead to variations in the expression and activity of the anion drug-transporting proteins of OATs and OATPs, possibly impacting the therapeutic outcomes. Previous investigations mainly focused on the regulation at the transcriptional level and drug-drug interactions as competing substrates or inhibitors. Recently, evidence has accumulated that cellular trafficking, post-translational modification, and degradation mechanisms serve as another important layer for the mechanisms underlying the variations in the OATs and OATPs. This review will provide a brief overview of the major OATs and OATPs implicated in drug therapy and summarize recent progress in our understanding of the post-translational modifications, in particular ubiquitination and degradation pathways of the individual OATs and OATPs implicated in drug therapy.

Impaired Transport Activity of Human Organic Anion Transporters (OATs) and Organic Anion Transporting Polypeptides (OATPs) by Wnt Inhibitors

The Wnt/β-catenin signaling pathway is dysregulated in diseases and Wnt inhibitors like PRI-724 are in clinical development. This study evaluated the regulatory actions of PRI-724 and other Wnt inhibitors on the transport activity of human renal Organic anion transporters (OATs) and Organic anion transporting polypeptides (OATPs). The substrate uptake by OAT4 and OATP2B1 was markedly decreased by PRI-724 (V_max/K_m: ∼26% and ∼17% of corresponding control), with less pronounced decreases in OAT1, OAT3 and OAT1A2. PRI-724 decreased the plasma membrane expression of inhibited OATs/OATPs but didn't affect their total cellular expression. Two model Wnt inhibitors - FH535 and 21H7 - were also tested in comparative studies. Like PRI-724, they also strongly decreased the activities and membrane expression of multiple OATs/OATPs. In contrast, FH535 didn't affect the substrate uptake by organic cation transporters. In control studies, the EGFR inhibitor lapatinib did not inhibit the function of some OATs/OATPs. Together these findings suggest that Wnt inhibitors selectively modulate the function of multiple organic anions transporters, so their clinical use may have unanticipated effects on drug entry into cells. These findings are pertinent to current clinical trials that have been designed to understand the safety and efficacy of new Wnt inhibitor drugs.

Pregnancy-induced changes in serum concentrations of perfluoroalkyl substances and the influence of kidney function

BACKGROUND

Epidemiological associations between maternal concentrations of perfluoroalkyl substances (PFAS) and birth weight are inconsistent. There is concern that studies based on samples collected in late pregnancy may be confounded by kidney function but studies of the relation between pregnancy-induced changes in PFAS and kidney function are lacking. Our aims were to investigate changes in serum concentrations of perfluorononanoic acid (PFNA), perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS) and perfluorohexane sulfonate (PFHxS) from early to late pregnancy and to explore relations to changes in glomerular filtration rate (GFR) and glomerular pore size.

METHODS

We conducted the study in a cohort of 73 pregnancies of normal-weight Swedish women without gestational diabetes and preeclampsia, enrolled 2009-2014. Blood was collected in median weeks 11 and 36, respectively, and analysed PFAS using liquid chromatography-tandem-mass-spectrometry. We estimated GFR based on creatinine and cystatin C and used the ratio eGFR_{cystatin C}/eGFR_creatinine to indicate glomerular pore size. We used Wilcoxon signed-rank test to compare early and late measures and partial Spearman rank correlations to explore relations between changes in PFAS and kidney function.

RESULTS

Median concentrations of PFNA, PFOA and PFOS decreased by 15-21% but changes were uncorrelated to changes in kidney function (partial R = - 0.06-0.11). The observed increase in median PFHxS concentration of 69% was likely an artefact of systematic measurement error caused by coeluting endogenous inferences.

CONCLUSIONS

Serum concentrations of PFNA, PFOA and PFOS decrease during pregnancy but the magnitudes of change are unrelated to parallel changes in eGFR and glomerular pore size, suggesting that changes in these indicators of kidney function are not important confounders in studies of PFAS and birth weight in pregnancies without gestational diabetes and preeclampsia.

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.

Trafficking and other regulatory mechanisms for organic anion transporting polypeptides and organic anion transporters that modulate cellular drug and xenobiotic influx and that are dysregulated in disease

Organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs), encoded by a number of solute carrier (SLC)22A and SLC organic anion (SLCO) genes, mediate the absorption and distribution of drugs and other xenobiotics. The regulation of OATs and OATPs is complex, comprising both transcriptional and post-translational mechanisms. Plasma membrane expression is required for cellular substrate influx by OATs/OATPs. Thus, interest in post-translational regulatory processes, including membrane targeting, endocytosis, recycling and degradation of transporter proteins, is increasing because these are critical for plasma membrane expression. After being synthesized, transporters undergo N-glycosylation in the endoplasmic reticulum and Golgi apparatus and are delivered to the plasma membrane by vesicular transport. Their expression at the cell surface is maintained by de novo synthesis and recycling, which occurs after clathrin- and/or caveolin-dependent endocytosis of existing protein. Several studies have shown that phosphorylation by signalling kinases is important for the internalization and recycling processes, although the transporter protein does not appear to be directly phosphorylated. After internalization, transporters that are targeted for degradation undergo ubiquitination, most likely on intracellular loop residues. Epigenetic mechanisms, including methylation of gene regulatory regions and transcription from alternate promoters, are also significant in the regulation of certain SLC22A/SLCO genes. The membrane expression of OATs/OATPs is dysregulated in disease, which affects drug efficacy and detoxification. Several transporters are expressed in the cytoplasmic subcompartment in disease states, which suggests that membrane targeting/internalization/recycling may be impaired. This article focuses on recent developments in OAT and OATP regulation, their dysregulation in disease and the significance for drug therapy.

Serum- and glucocorticoid-inducible kinase SGK2 regulates human organic anion transporters 4 via ubiquitin ligase Nedd4-2

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the kidney and placenta. In the current study, we examined the regulation of hOAT4 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT4 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity Vmax without significant change in substrate-binding affinity Km. We further showed that regulation of hOAT4 activity by sgk2 was mediated by ubiquitin ligase Nedd4-2. Overexpression of Nedd4-2 enhanced hOAT4 ubiquitination, and inhibited hOAT4 transport activity, whereas overexpression of ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on hOAT4. Our co-immunoprecipitation experiment revealed that sgk2 weakened the association between hOAT4 and Nedd4-2. In conclusion, our study demonstrated for the first time that sgk2 stimulated hOAT4 transport activity by abrogating the inhibitory effect of Nedd4-2 on the transporter.

Human oligopeptide transporter 2 (PEPT2) mediates cellular uptake of polymyxins

OBJECTIVES

Polymyxins are a last-line therapy to treat MDR Gram-negative bacterial infections. Nephrotoxicity is the dose-limiting factor for polymyxins and recent studies demonstrated significant accumulation of polymyxins in renal tubular cells. However, little is known about the mechanism of polymyxin uptake into these cells. Oligopeptide transporter 2 (PEPT2) is a solute carrier transporter (SLC) expressed at the apical membrane of renal proximal tubular cells and facilitates drug reabsorption in the kidney. In this study, we examined the role of PEPT2 in polymyxin uptake into renal tubular cells.

METHODS

We investigated the inhibitory effects of colistin and polymyxin B on the substrate uptake mediated through 15 essential SLCs in overexpressing HEK293 cells. The inhibitory potency of both polymyxins on PEPT2-mediated substrate uptake was measured. Fluorescence imaging was employed to investigate PEPT2-mediated uptake of the polymyxin fluorescent probe MIPS-9541 and a transport assay was conducted with MIPS-9541 and [(3)H]polymyxin B1.

RESULTS

Colistin and polymyxin B potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 11.4 ± 3.1 and 18.3 ± 4.2 μM, respectively). In contrast, they had no or only mild inhibitory effects on the transport activity of the other 14 SLCs evaluated. MIPS-9541 potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 15.9 μM) and is also a substrate of PEPT2 (Km 74.9 μM). [(3)H]polymyxin B1 was also significantly taken up by PEPT2-expressing cells (Km 87.3 μM).

CONCLUSIONS

Our study provides the first evidence of PEPT2-mediated uptake of polymyxins and contributes to a better understanding of the accumulation of polymyxins in renal tubular cells.

What do drug transporters really do?

Potential drug-drug interactions mediated by the ATP-binding cassette (ABC) transporter and solute carrier (SLC) transporter families are of clinical and regulatory concern. However, the endogenous functions of these drug transporters are not well understood. Discussed here is evidence for the roles of ABC and SLC transporters in the handling of diverse substrates, including metabolites, antioxidants, signalling molecules, hormones, nutrients and neurotransmitters. It is suggested that these transporters may be part of a larger system of remote communication ('remote sensing and signalling') between cells, organs, body fluid compartments and perhaps even separate organisms. This broader view may help to clarify disease mechanisms, drug-metabolite interactions and drug effects relevant to diabetes, chronic kidney disease, metabolic syndrome, hypertension, gout, liver disease, neuropsychiatric disorders, inflammatory syndromes and organ injury, as well as prenatal and postnatal development.

Novel mechanism of impaired function of organic anion-transporting polypeptide 1B3 in human hepatocytes: post-translational regulation of OATP1B3 by protein kinase C activation

The organic anion-transporting polypeptide (OATP) 1B3 is a membrane transport protein that mediates hepatic uptake of many drugs and endogenous compounds. Currently, determination of OATP-mediated drug-drug interactions in vitro is focused primarily on direct substrate inhibition. Indirect inhibition of OATP1B3 activity is under-appreciated. OATP1B3 has putative protein kinase C (PKC) phosphorylation sites. Studies were designed to determine the effect of PKC activation on OATP1B3-mediated transport in human hepatocytes using cholecystokinin-8 (CCK-8), a specific OATP1B3 substrate, as the probe. A PKC activator, phorbol-12-myristate-13-acetate (PMA), did not directly inhibit [(3)H]CCK-8 accumulation in human sandwich-cultured hepatocytes (SCH). However, pretreatment with PMA for as little as 10 minutes rapidly decreased [(3)H]CCK-8 accumulation. Treatment with a PKC inhibitor bisindolylmaleimide (BIM) I prior to PMA treatment blocked the inhibitory effect of PMA, indicating PKC activation is essential for downregulating OATP1B3 activity. PMA pretreatment did not affect OATP1B3 mRNA or total protein levels. To determine the mechanism(s) underlying the indirect inhibition of OATP1B3 activity upon PKC activation, adenoviral vectors expressing FLAG-Myc-tagged OATP1B3 (Ad-OATP1B3) were transduced into human hepatocytes; surface expression and phosphorylation of OATP1B3 were determined by biotinylation and by an anti-phosphor-Ser/Thr/Tyr antibody, respectively. PMA pretreatment markedly increased OATP1B3 phosphorylation without affecting surface or total OATP1B3 protein levels. In conclusion, PKC activation rapidly decreases OATP1B3 transport activity by post-translational regulation of OATP1B3. These studies elucidate a novel indirect inhibitory mechanism affecting hepatic uptake mediated by OATP1B3, and provide new insights into predicting OATP-mediated drug interactions between OATP substrates and kinase modulator drugs/endogenous compounds.

SLC22, SLC44, and SLC47 transporters--organic anion and cation transporters: molecular and cellular properties

Transporters within the SLC22, SLC44, and SLC47 families of solute carriers mediate transport of a structurally diverse array of organic electrolytes, that is, molecules that are generally charged (cationic, anionic, or zwitterionic) at physiological pH. Transporters in the SLC22 family--all of which are members of the major facilitator superfamily (MFS) of transporters--represent a mechanistically diverse set of processes, including the organic anion transporters (OATs and URAT1) that physiologically operate as organic anion (OA) exchangers, the organic cation transporters (OCTs) that operate as electrogenic uniporters of organic cations (OCs), and the so-called "novel" organic cation transporters (OCTNs) that support Na-cotransport of selected zwitterions. Whereas the OCTNs display a high degree of substrate selectivity, the physiological hallmark of the OATs and OCTs is their multiselectivity--consistent with a principal role in renal and hepatic clearance of a wide array of both endogenous and xenobiotic compounds. SLC47 consists of members of the multidrug and toxin extruder (MATE) family, which are carriers that are obligatory exchangers and that physiologically support electroneutral H⁺ exchange. The MATEs also display a characteristic multiselectivity and are frequently paired with OCTs to mediate transepithelial OC secretion, with the OCTs typically supporting basolateral OC entry and the MATEs supporting apical OC efflux. The SLC44 family contains the choline transporter-like (CTL) transporters. Largely restricted to choline and a limited set of structural congeners, the CTLs appear to support the Na-independent, electrogenic uniport of choline, thereby providing choline for membrane biogenesis. The solution of X-ray crystal structures of representative prokaryotic MFS and MATE transporters has led to the development of homology models of mammalian OAT, OCT, and MATE transporters that, in turn, have supplemented studies of the molecular basis of the complex interactions of ligands with these multiselective proteins.

Protein kinase C regulates the internalization and function of the human organic anion transporting polypeptide 1A2

BACKGROUND AND PURPOSE

The human organic anion transporting polypeptide 1A2 (OATP1A2) is expressed in cells from several regions of the human body, including the kidney, cholangiocytes and the blood-brain barrier, and mediates the cellular flux of various anionic substances, including drugs in clinical use. Several related mammalian transporters have been shown to be subject to post-translational regulation, including kinase-induced internalization. In the present study the role of protein kinase C (PKC) in the regulation of OATP1A2 was investigated in an in vitro cell model.

EXPERIMENTAL APPROACH

COS-7 cells in which OATP1A2 was overexpressed were treated with the PKC-specific activator (phorbol 12-myristate 13-acetate; PMA) and the PKC-specific inhibitor (Go6976). The impact of these treatments on the function and regulation of OATP1A2 was determined.

KEY RESULTS

PKC activation decreased the transport function of OATP1A2 in a time- and concentration-dependent manner. PMA (0.1 µM) decreased the V(max) of oestrone-3-sulphate uptake and decreased the cell surface expression of OATP1A2 immunoreactive protein; these effects of PMA were prevented by the PKC specific inhibitor Go6976. In further studies, PMA treatment accelerated the internalization of OATP1A2 but did not affect its recycling. The disruption of clathrine-dependent endocytosis attenuated both the constitutive and PKC-modulated internalization of OATP1A2. In contrast, blocking the caveolin-dependent pathway was without effect.

CONCLUSIONS AND IMPLICATIONS

PKC regulates the transport function of OATP1A2 by modulating protein internalization; this effect of PKC is mediated in part by clathrine-dependent pathways.

In vitro and in vivo evidence of the importance of organic anion transporters (OATs) in drug therapy

Organic anion transporters 1-10 (OAT1-10) and the urate transporter 1 (URAT1) belong to the SLC22A gene family and accept a huge variety of chemically unrelated endogenous and exogenous organic anions including many frequently described drugs. OAT1 and OAT3 are located in the basolateral membrane of renal proximal tubule cells and are responsible for drug uptake from the blood into the cells. OAT4 in the apical membrane of human proximal tubule cells is related to drug exit into the lumen and to uptake of estrone sulfate and urate from the lumen into the cell. URAT1 is the major urate-absorbing transporter in the apical membrane and is a target for uricosuric drugs. OAT10, also located in the luminal membrane, transports nicotinate with high affinity and interacts with drugs. Major extrarenal locations of OATs include the blood-brain barrier for OAT3, the placenta for OAT4, the nasal epithelium for OAT6, and the liver for OAT2 and OAT7. For all transporters we provide information on cloning, tissue distribution, factors influencing OAT abundance, interaction with endogenous compounds and different drug classes, drug/drug interactions and, if known, single nucleotide polymorphisms.

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.

Regulation of human organic anion transporter 4 by protein kinase C and NHERF-1: altering the endocytosis of the transporter

PURPOSE

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs. We have previously shown that the activity of hOAT4 was down-regulated by activation of PKC and up-regulated by PDZ protein NHERF-1. Here, we investigated the mechanisms underlying such regulations.

METHODS

COS-7 cells expressing hOAT4 were treated with PKC activator phorbol 12-myristate 13-acetate (PMA) or transfected with dominant negative mutants of dynamin-2 or Eps15 or transfected with NHERF-1. The internalization and the function of hOAT4 were then determined.

RESULTS

We showed that hOAT4 constitutively internalized from and recycled back to plasma membrane. Transfection of dominant negative mutants of dynamin-2 or Eps15 into the cells, all of which block clathrin-dependent endocytotic pathway, significantly blocked hOAT4 internalization. Treatment of cells with PMA accelerated hOAT4 internalization, whereas transfection of cells with NHERF-1 attenuated hOAT4 internalization.

CONCLUSION

Our studies demonstrated that i) hOAT4 undergoes constitutive trafficking between cell surface and intracellular compartments, ii) hOAT4 internalization partly occurs through clathrin-dependent pathway, iii) the down-regulation of hOAT4 activity by activation of PKC and the up-regulation of hOAT4 activity by NHERF-1 are mediated through alteration of hOAT4 internalization.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Angiotensin II inhibits activity of human organic anion transporter 3 through activation of protein kinase Calpha: accelerating endocytosis of the transporter

Human organic anion transporter 3 (hOAT3) belongs to a family of organic anion transporters that play critical roles in the body disposition of numerous clinically important drugs. In the kidney, hOAT3 functions through a tertiary transport mechanism involving two other membrane proteins Na/K-ATPase and Na-dicarboxylate cotransporter. In the current study, we established COS-7 cells stably expressing hOAT3 and examined the regulation of hOAT3 by protein kinase C (PKC) and angiotensin II. Both PKC activation and angiotensin II inhibited hOAT3 transport activity. Angiotensin II induced inhibition of hOAT3 activity could be prevented by treating hOAT3-expressing cells with staurosporine, a general inhibitor for PKC, and with Gö6976 (5,6,7,13-Tetrahydro-13-methyl-5-oxo-12H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-12-propanenitrile), a PKCalpha-specific inhibitor. Examination of hOAT3 expression and transport kinetics revealed that angiotensin II induced inhibition of hOAT3 activity mainly resulted from a decreased cell surface expression kinetically reflected as a decreased V(max) without a significant change in K(m). Such angiotensin II induced decrease in cell surface expression of hOAT3 was caused by an increase in hOAT3 endocytosis. However, angiotensin II induced endocytosis of Na/K-ATPase did not occur under such condition. We concluded that angiotensin II inhibited hOAT3 activity through the activation of PKCalpha, which led to an acceleration of hOAT3 endocytosis.

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.

Short-term regulation of organic anion transporters

Organic anion transporters (OATs), which belong to the superfamily SLC22A, are key determinants in the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, and, therefore, are critical for the survival of mammalian species. Alteration in the function of these drug transporters plays important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. As a result, the activity of OATs must be under tight regulation so as to carry out their normal functions. This review article highlights the recent advances from our laboratory as well as from others in delineating the short-term regulation of OATs. These advances provide important insights into strategies to maximize therapeutic efficacy in drug development.

Toward a systems level understanding of organic anion and other multispecific drug transporters: a remote sensing and signaling hypothesis

Organic anion transporters (Oats) are located in the barrier epithelia of diverse organs, where they mediate the absorption and excretion of a wide range of metabolites, signaling molecules, and xenobiotics. Although their interactions with a broad group of substrates have been extensively studied and described, the primary physiological role of Oats remains elusive. The presence of overlapping substrate specificities among the different Oat isoforms, together with recent metabolomic data from the Oat1, Oat3, and renal-specific transporter (RST/URAT1) knockout mice, suggests a possible role in remote signaling wherein substrates excreted through one Oat isoform in one organ are taken up by another Oat isoform located in a different organ, thereby mediating communication between different organ systems, or even between different organisms. Here we further develop this "remote sensing and signaling hypothesis" and suggest how the regulation of SLC22 subfamily members (including those of the organic cation, organic carnitine, and unknown substrate transporter subfamilies) can be better understood by considering the organism's broader need to communicate between epithelial and other tissues by simultaneous regulation of transport of metabolites, signaling molecules, drugs, and toxins. This systems biology perspective of remote signaling (sensing) could help reconcile an enormous array of tissue-specific data for various SLC22 family genes and, possibly, other multispecific transporters, such as those of the organic anion transporting polypeptide (OATP, SLC21) and multidrug resistance-associated protein (MRP) families.

Novobiocin is a potent inhibitor for human organic anion transporters

Organic anion transporters (OATs) mediate the body disposition of a diverse array of environmental toxins and clinically important drugs. Previous studies have shown that novobiocin, an inhibitor for breast cancer resistance proteins (BCRP), inhibited organic anion transport. However, its interactions with specific OATs are unknown. In the current study, we characterized the inhibitory effects of novobiocin on the function of human OATs (hOAT)1, hOAT3, and hOAT4. Kinetic study revealed that novobiocin inhibited OAT-mediated uptake in a competitive manner, with K(i) of 14.87 +/- 0.40 microM for hOAT1, K(i) of 4.77 +/- 1.12 microM for hOAT3, and K(i) of 90.50 +/- 7.50 microM for hOAT4. Furthermore, the cis- and trans-inhibition feature of novobiocin demonstrated that novobiocin was a potent inhibitor but not a substrate for hOAT1 (IC(50) = 34.76 +/- 0.31 microM), hOAT3 (IC(50) = 4.987 +/- 0.35 microM), and hOAT4 (IC(50) = 92.68 +/- 0.34 microM). We further showed that the effects of novobiocin on OATs were not mediated through a change in transporter protein abundance on the plasma membrane. Taken together, we conclude that novobiocin seems to interact with the substrate-binding sites of OATs from both the intracellular and the extracellular sides, and this interaction interferes with the substrate-binding site(s) on respective carriers, leading to an apparent reduction in carriers available for the substrates. Because BCRP is often expressed in the same tissue where multiple OATs are identified such as liver, kidney and placenta, when dissecting the contribution of BCRP to drug disposition using novobiocin as an inhibitor, its inhibitory effect to OATs has to be taken into consideration.

Regulation of human organic anion transporter 1 by ANG II: involvement of protein kinase Calpha

Human organic anion transporter 1 (hOAT1) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT1 is abundantly expressed in the kidney. In the current study, we examined the regulation of hOAT1 by ANG II in kidney COS-7 cells. ANG II induced a concentration- and time-dependent inhibition of hOAT1 transport activity. Such inhibition mainly resulted from a decreased cell surface expression without a change in total cell expression of the transporter, kinetically revealed as a decreased maximal velocity without significant change in Michaelis constant. ANG II-induced inhibition of hOAT1 activity could be prevented by treating hOAT1-expressing cells with staurosporine, a general protein kinase C (PKC) inhibitor. To obtain further information on which PKC isoform mediates ANG II regulation of hOAT1 activity, cellular distribution of various PKC isoforms was examined in cells treated with or without ANG II. We showed that ANG II treatment resulted in a significant translocation of PKCalpha from cytosol to membrane, and such translocation was blocked by treating hOAT1-expressing cells with Gö-6976, a PKCalpha-specific inhibitor. We further showed that ANG II-induced inhibition of hOAT1 activity and retrieval of hOAT1 from the cell surface could also be prevented by treating hOAT1-expressing cells with Gö-6976. We concluded that ANG II inhibited hOAT1 activity through activation of PKCalpha, which led to the redistribution of the transporter from the cell surface to the intracellular compartments.

Co-localization and interaction of human organic anion transporter 4 with caveolin-1 in primary cultured human placental trophoblasts

The human organic anion transporter 4 (hOAT4) has been identified as the fourth isoform of OAT family. hOAT4 contributes to move several negatively charged organic compounds between cells and their extracellular milieu. The functional characteristics and regulatory mechanisms of hOAT4 remain to be elucidated. It is well known that caveolin plays a role in modulating proteins having some biological functions. To address this issue, we investigated the co-localization and interaction between hOAT4 and caveolin-1. hOAT4 and caveolin-1 (mRNA and protein expression) were observed in cultured human placental trophoblasts isolated from placenta. The confocal microscopy of immuno-cytochemistry using primary cultured human trophoblasts showed hOAT4 and caveolin-1 were co-localized at the plasma membrane of the cell. This finding was confirmed by Western blot analysis using isolated caveolae-enriched membrane fractions and immune-precipitates from the trophoblasts. When synthesized cRNA of hOAT4 along with scrambled- or antisense-oligodeoxynucleotide (ODN) of Xenopus caveolin-1 were co-injected to Xenopus oocytes, the [3H]estrone sulfate uptake was significantly decreased by the co-injection of antisense ODN but not by scrambled ODN. These findings suggest that hOAT4 and caveolin-1 share a cellular expression in the plasma membrane and caveolin-1 up-regulates the organic anionic compound uptake by hOAT4 under the normal physiological condition.

Update on the molecular physiology of organic anion transporters

PURPOSE OF REVIEW

Organic anion transporters (OATs) mediate the renal absorption and excretion of a wide range of metabolites and xenobiotics. We discuss the recent advances that have been made in elucidating the binding and transport characteristics of OATs, new insights into their physiological role and regulation by various factors, and pharmacogenetics.

RECENT FINDINGS

Overlapping substrate specificity among the OATs is well established. However, recent findings have suggested distinct differences in the structural binding determinants among the OATs, which have important implications for understanding drug interactions and drug design. A potential role for OATs in blood pressure regulation and remote sensing has been reported. Meanwhile, factors regulating the expression of OATs continue to be identified and characterized. The effect of renal ischemia on OAT expression and function is currently being explored. Finally, recent studies identifying various OAT polymorphisms may facilitate prediction of individual drug response and toxicity.

SUMMARY

As progress is made in unveiling the many functional aspects of the OATs, it is becoming clear that their significance is not only limited to a role in drug elimination from the body, but also extends to other vital physiological roles. Further delineation of the function and regulation of the OATs will uncover enormous potential clinical and pharmacological applications.

Organic anion transporter OAT1 undergoes constitutive and protein kinase C-regulated trafficking through a dynamin- and clathrin-dependent pathway

Organic anion transporter 1 (OAT1) mediates the body disposition of a diverse array of environmental toxins and clinically important drugs. Therefore, understanding the regulation of this transporter has profound clinical significance. We previously demonstrate that OAT1 activity was down-regulated by activation of protein kinase C (PKC), kinetically revealed as a decrease in the maximum transport velocity V(max) without significant change in the substrate affinity K(m) of the transporter. In the current study, we showed that OAT1 constitutively internalized from and recycled back to the plasma membrane, and PKC activation accelerated OAT1 internalization without affecting OAT1 recycling. We further showed that treatment of OAT1-expressing cells with concanavalin A, depletion of K(+) from the cells, or transfection of dominant negative mutants of dynamin-2 or Eps15 into the cells, all of which block the clathrin-dependent endocytotic pathway, significantly blocked constitutive and PKC-regulated OAT1 internalization. We finally showed that OAT1 colocalized with transferrin, a marker for clathrin-dependent endocytosis, at the cell surface and in the EEA1-positive early endosomes. Together, our findings demonstrated for the first time that (i) OAT1 constitutively traffics between plasma membrane and recycling endosomes, (ii) PKC activation down-regulates OAT1 activity by altering already existent OAT1 trafficking, and (iii) OAT1 internalization occurs partly through a dynamin- and clathrin-dependent pathway.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

Molecular cloning and characterization of prostaglandin (PG) transporter in ovine endometrium: role for multiple cell signaling pathways in transport of PGF2alpha

In ruminants, endometrial prostaglandin F(2alpha) (PGF(2alpha)) is the luteolytic hormone. Cellular transport of PGF(2alpha) in the uterine endometrium is critical for regulation of the estrous cycle. Molecular mechanisms responsible for control of PGF(2alpha) transport in endometrium during luteolysis are largely unknown. In the present study, we characterized the prostaglandin transporter (PGT) in ovine endometrium. Ovine PGT cDNA consists of 1935 nucleotides that encode 644 amino acids. In ovine endometria, PGT is highly expressed during the period of luteolysis, between d 14 and 16 of the estrous cycle, in luminal and glandular epithelia. Pharmacological and genomic inhibition of PGT indicates that it is responsible for influx and efflux of PGF(2alpha) in ovine endometrial epithelial cells. Inhibition of PGT during the period of luteolysis prevents the release of oxytocin-induced PGF(2alpha) pulses, and maintains functional corpus luteum and its secretion of progesterone. In ovine endometrial epithelial cells, protein kinase A and protein kinase C pathways are involved in regulating the influx of PGF(2alpha), whereas epidermal growth factor receptor pathways are implicated in regulation of influx and efflux of PGF(2alpha.) The ERK1/2 pathway is associated with efflux of PGF(2alpha), whereas Jun-amino-terminal kinase/stress-activated protein kinase pathways are involved in both efflux and influx of PGF(2alpha.) Phosphatidylinositol 3-kinase pathways are not involved in either influx or efflux of PGF(2alpha) in ovine endometrial epithelial cells. These are the first results to demonstrate a functional role for PGT in regulation of PGF(2alpha) efflux and influx in ovine endometrial cells that influence luteolytic mechanisms in ruminants.

Comparison of the Interaction of Human Organic Anion Transporter hOAT4 with PDZ Proteins between Kidney Cells and Placental Cells

PURPOSE

To compare the interaction of human organic anion transporter hOAT4 with PDZ proteins between kidney cells and placental cells.

MATERIALS AND METHODS

PDZ proteins PDZK1 and NHERF1 were transfected into kidney LLC-PK1 cells and placental BeWo cells expressing hOAT4 or hOAT4-Delta, which lacks the PDZ consensus binding site. The interaction of PDZK1 and NHERF1 with hOAT4 and hOAT4-Delta was investigated by measurement of [3H] estrone sulfate uptake, cell surface and total cell expression of hOAT4.

RESULTS

PDZK1 and NHERF1 enhanced hOAT4 activity in LLC-PK1 cells by increasing the cell surface expression of the transporter. In contrasts, these two PDZ proteins had no effect on hOAT4 activity in BeWo cells.

CONCLUSION

The interaction of PDZ proteins with hOAT4 may be cell-specific. In placenta, a different set of interacting proteins from PDZK1 and NHERF1 may be required to modulate hOAT4 activity.