Multidrug resistance protein mrp2 mediates ATP-dependent transport of classic renal organic anion p-aminohippurate

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Flavonoids are inhibitors of human organic anion transporter 1 (OAT1)-mediated transport

Organic anion transporter 1 (OAT1) has been reported to be involved in the nephrotoxicity of many anionic xenobiotics. As current clinically used OAT1 inhibitors are often associated with safety issues, identifying potent OAT1 inhibitors with little toxicity is of great value in reducing OAT1-mediated drug nephrotoxicity. Flavonoids are a class of polyphenolic compounds with exceptional safety records. Our objective was to evaluate the effects of 18 naturally occurring flavonoids, and some of their glycosides, on the uptake of para-aminohippuric acid (PAH) in both OAT1-expressing and OAT1-negative LLC-PK1 cells. Most flavonoid aglycones produced substantial decreases in PAH uptake in OAT1-expressing cells. Among the flavonoids screened, fisetin, luteolin, morin, and quercetin exhibited the strongest effect and produced complete inhibition of OAT1-mediated PAH uptake at a concentration of 50 μM. Further concentration-dependent studies revealed that both morin and luteolin are potent OAT1 inhibitors, with IC50 values of <0.3 and 0.47 μM, respectively. In contrast to the tested flavonoid aglycones, all flavonoid glycosides had negligible or small effects on OAT1. In addition, the role of OAT1 in the uptake of fisetin, luteolin, morin, and quercetin was investigated and fisetin was found to be a substrate of OAT1. Taken together, our results indicate that flavonoids are a novel class of OAT1 modulators. Considering the high consumption of flavonoids in the diet and in herbal products, OAT1-mediated flavonoid-drug interactions may be clinically relevant. Further investigation is warranted to evaluate the nephroprotective role of flavonoids in relation to drug-induced nephrotoxicity mediated by the OAT1 pathway.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Expression and function of renal and hepatic organic anion transporters in extrahepatic cholestasis

Obstructive jaundice occurs in patients suffering from cholelithiasis and from neoplasms affecting the pancreas and the common bile duct. The absorption, distribution and elimination of drugs are impaired during this pathology. Prolonged cholestasis may alter both liver and kidney function. Lactam antibiotics, diuretics, non-steroidal anti-inflammatory drugs, several antiviral drugs as well as endogenous compounds are classified as organic anions. The hepatic and renal organic anion transport pathways play a key role in the pharmacokinetics of these compounds. It has been demonstrated that acute extrahepatic cholestasis is associated with increased renal elimination of organic anions. The present work describes the molecular mechanisms involved in the regulation of the expression and function of the renal and hepatic organic anion transporters in extrahepatic cholestasis, such as multidrug resistance-associated protein 2, organic anion transporting polypeptide 1, organic anion transporter 3, bilitranslocase, bromosulfophthalein/bilirubin binding protein, organic anion transporter 1 and sodium dependent bile salt transporter. The modulation in the expression of renal organic anion transporters constitutes a compensatory mechanism to overcome the hepatic dysfunction in the elimination of organic anions.

The interaction of gut microbes with host ABC transporters

ATP binding cassette (ABC) transporters are increasingly recognized for their ability to modulate the absorption, distribution, metabolism, secretion and toxicity of xenobiotics. In addition to their essential function in drug resistance, there is also emerging evidence documenting the important role ABC transporters play in tissue defense. In this respect, the gastrointestinal tract represents a critical vanguard of defense against oral exposure of drugs while at the same time functions as a physical barrier between the lumenal contents (including bacteria) and the intestinal epithelium. Given emerging evidence suggesting that multidrug resistance protein (MDR) plays an important role in host-bacterial interactions in the gastrointestinal tract, this review will discuss the interplay between MDR of the intestinal epithelial cell barrier and gut microbes in health and disease. In particular, we will explore host-microbe interactions involving three apically restricted ABC transporters of the intestinal epithelium; P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP2) and cystic fibrosis transmembrane regulator (CFTR).

Function and Regulation of Multidrug Resistance Proteins (MRPs) in the Renal Elimination of Organic Anions

The reabsorptive and excretory capacity of the kidney has an important influence on the systemic concentration of drugs. Multidrug resistance proteins (MRP/ABCC) expressed in the kidney play a critical role in the tubular efflux of a wide variety of drugs and toxicants, and, in particular, of their negatively charged phase II metabolites. Nine structurally and functionally related MRP family members have been identified (MRP1-9), which differ from each other by their localization, expression levels, and substrate specificity. During altered physiological circumstances, adaptations in these transporters are required to avoid systemic toxicity as well as renal tubular damage. Key players in these events are hormones, protein kinases, nuclear receptors, and disease conditions, which all may affect transporter protein expression levels. This review discusses current knowledge on the renal characteristics of MRP1-9, with specific focus on their regulation.

Increase in Mrp1 expression and 4-hydroxy-2-nonenal adduction in heart tissue of Adriamycin-treated C57BL/6 mice

Multidrug resistance-associated protein 1 (MRP1) mediates the ATP-dependent efflux of endobiotics and xenobiotics, including estradiol 17-(beta-d-glucuronide), leukotriene C(4), and the reduced glutathione conjugate of 4-hydroxy-2-nonenal (HNE), a highly reactive product of lipid peroxidation. Adriamycin is an effective cancer chemotherapeutic drug whose use is limited by cardiotoxicity. Adriamycin induces oxidative stress and production of HNE in cardiac tissue, which may contribute to cardiomyopathy. We investigated the role of Mrp1 in Adriamycin-induced oxidative stress in cardiac tissue. Mice were treated with Adriamycin (20 mg/kg, i.p.), and heart homogenate and sarcolemma membranes were assayed for Mrp1 expression and ATP-dependent transport activity. Expression of Mrp1 was increased at 6 and 24 hours after Adriamycin treatment compared with saline treatment. HNE-adducted proteins were significantly increased (P < 0.001) in the homogenates at 6 hours after Adriamycin treatment and accumulated further with time; HNE adduction of a 190-kDa protein was evident 3 days after Adriamycin treatment. Mrp1 was localized predominately in sarcolemma as shown by confocal and Western blot analysis. Sarcolemma membrane vesicles transported leukotriene C(4) with a K(m) and V(max) of 51.8 nmol/L and 94.1 pmol/min/mg, respectively, and MK571 (10 micromol/L) inhibited the transport activity by 65%. Exposure of HEK(Mrp1) membranes to HNE (10 micromol/L) significantly decreased the V(max) for estradiol 17-(beta-d-glucuronide) transport by 50%. These results show that expression of Mrp1 in the mouse heart is localized predominantly in sarcolemma. Adriamycin treatment increased Mrp1 expression and HNE adduction of Mrp1. Cardiac Mrp1 may play a role in protecting the heart from Adriamycin-induced cardiomyopathy by effluxing HNE conjugates.

Impact of transporter-mediated drug absorption, distribution, elimination and drug interactions in antimicrobial chemotherapy

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis. Membrane transporters play important roles in determining the pharmacokinetic aspects of intestinal absorption, tissue distribution, and the urinary and biliary excretions of a wide variety of therapeutic drugs. The identification and characterization of transporters responsible for the transfer of nutrients and xenobiotics, including drugs, is expected to provide a scientific basis for understanding drug disposition, as well as the molecular mechanisms of drug-drug/drug-food/drug-protein interactions and inter-individual/inter-species differences. This review focuses on the influence of transporters on the pharmacokinetics of beta-lactam antibiotics, new quinolones, and other antimicrobial agents, as well as focusing on the drug-drug interactions associated with transporter-mediated uptake from the small intestine and transporter-mediated elimination from the kidney and liver.

Characterization of prostaglandin E1 transport in rat renal brush-border membrane

Transport of prostaglandin E(1) (PGE(1)) was investigated in rat renal brush-border membrane vesicles. The uptake of [(3)H]PGE(1) was sensitive to osmosis and temperature. This uptake was saturable and mediated by high-affinity (K(m)=2.1 microM)/low-capacity (V(max)=17.4 pmol/mg protein/30 sec) and low-affinity (K(m)=526.5 microM)/high-capacity (V(max)=1,032.5 pmol/mg protein/30 sec) transport systems. [(3)H]PGE(1) uptake was Na(+)-independent and inhibited by various eicosanoids including PGE(2) and PGF(2alpha). Bromcresol green and sulfobromophthalein, potent inhibitors of prostaglandin transporter (PGT), significantly decreased [(3)H]PGE(1) uptake. Uptake was also inhibited by indomethacin and probenecid, which reportedly have little effect on PGT. Benzylpenicillin and taurocholate decreased the uptake of [(3)H]PGE(1). Like p-[(14)C]aminohippurate (PAH) uptake by vesicles, the uptake of [(3)H]PGE(1) was stimulated by an inside-positive membrane potential, created by applying an inward K(+) gradient and valinomycin. However, the uptake of [(3)H]PGE(1) was not inhibited by PAH, suggesting that PAH and PGE(1) are transported by separate transport systems. [(3)H]PGE(1) uptake was not stimulated by outwardly directed gradients of Cl(-) nor unlabeled PGE(1), indicating that an anion exchanger may not be involved in PGE(1) transport. These findings suggest that the transport of PGE(1) in rat renal brush-border membrane is mediated by specific transport system(s), at least in part, by a potential-sensitive transport system.

The apical conjugate efflux pump ABCC2 (MRP2)

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

Up-regulation of Mrp4 expression in kidney of Mrp2-deficient TR- rats

Multidrug resistance-associated proteins (Mrps) are a group of ATP-dependent efflux transporters for organic anions. Mrp2 and Mrp4 are co-localized to the apical (brush-border) membrane domain of renal proximal tubules, where they may function together in the urinary excretion of organic anions. Previous reports showed that urinary excretion of some organic anions is not impaired in transport-deficient (TR-) rats, which lack Mrp2, suggesting that up-regulation of other transporter(s) may compensate for the loss of Mrp2 function. The purpose of this study was to determine whether Mrp4 expression in kidney is altered in TR- rats. Mrp4 mRNA expression was quantified using the high-throughput branched DNA signal amplification assay. Mrp4 protein expression was determined by Western blot and immunohistochemical analysis. Mrp4 mRNA in kidney of TR- rats was 100% higher than normal Wistar rats. Western blot analysis showed a 200% increase in Mrp4 protein expression in kidney of the mutant rats compared to normal rats. Immunohistochemical analysis of Mrp4 protein demonstrated apical localization of Mrp4 on renal proximal tubules, and that the immunoreactivity was more intense in kidney sections from TR- rats than those from normal rats. In summary, the results of the present study demonstrate that renal Mrp4 expression is up-regulated in TR- rats, which may explain why urinary excretion of some organic anions remains normal in the mutant rats.

Modulation of Renal Apical Organic Anion Transporter 4 Function by Two PDZ Domain–Containing Proteins

Human organic anion transporter 4 (OAT4) is an apical organic anion/dicarboxylate exchanger in the renal proximal tubules and mediates high-affinity transport of steroid sulfates such as estrone-3-sulfate (E1S) and dehydroepiandrosterone sulfate. Here, two multivalent PDZ (PSD-95/Discs Large/ZO-1) proteins PDZK1 and NHERF1 were examined as interactors of OAT4 by a yeast two-hybrid assay. These interactions require the extreme C-terminal region of OAT4 and the first and fourth PDZ domains of PDZK1 and the first PDZ domain of NHERF1. These interactions were confirmed by surface plasmon resonance assays (K(D): 36 nM, 1.2 microM, and 41.7 microM, respectively). In vitro binding assays and co-immunoprecipitation studies revealed that the OAT4 wild-type but not a mutant lacking the PDZ motif interacted directly with both PDZK1 and NHERF1. OAT4, PDZK1, and NHERF1 proteins were shown to be localized at the apical membrane of renal proximal tubules. The association with PDZK1 or NHERF1 enhanced OAT4-mediated E1S transport activities in HEK293 cells (1.2- to 1.4-fold), and the deletion of the OAT4 C-terminal PDZ motif abolished this effect. The augmentation of the transport activity was accompanied by alteration in V(max) of E(1)S transport via OAT4 and was associated with the increased surface expression level of OAT4 protein. This study indicates that the functional activity of OAT4 is modulated through the PDZ interaction with the network of PDZK1 and NHERF1 and suggests that OAT4 is involved in the regulated apical organic anion handling in the renal proximal tubules, provided by the PDZ scaffold.

Human organic anion transporter 4 is a renal apical organic anion/dicarboxylate exchanger in the proximal tubules

Human organic anion transporter OAT4 is expressed in the kidney and placenta and mediates high-affinity transport of estrone-3-sulfate (E1S). Because a previous study demonstrated no trans-stimulatory effects by E1S, the mode of organic anion transport via OAT4 remains still unclear. In the present study, we examined the driving force of OAT4 using mouse proximal tubular cells stably expressing OAT4 (S2 OAT4). OAT4-mediated E1S uptake was inhibited by glutarate (GA) (IC50:1.25 mM) and [14C]GA uptake via S2 OAT4 was significantly trans-stimulated by unlabeled GA (5 mM) (P<0.001). [3H]E1S uptake via S2 OAT4 was significantly trans-stimulated by preloaded GA (P<0.001) and its [14C]GA efflux was significantly trans-stimulated by unlabeled E1S in the medium (P<0.05). In addition, both the uptake and efflux of [14C]p-aminohippuric acid (PAH) and [14C]GA via S2 OAT4 were significantly trans-stimulated by unlabeled GA or PAH. The immunoreactivities of OAT4 were observed in the apical membrane of proximal tubules along with those of basolateral organic anion/dicarboxylate exchangers such as hOAT1 and hOAT3 in the same tubular population. These results indicate that OAT4 is an apical organic anion/dicarboxylate exchanger and mainly functions as an apical pathway for the reabsorption of some organic anions in renal proximal tubules driven by an outwardly directed dicarboxylate gradient.

Human organic anion transporter MRP4 (ABCC4) is an efflux pump for the purine end metabolite urate with multiple allosteric substrate binding sites

The end product of human purine metabolism is urate, which is produced primarily in the liver and excreted by the kidney through a well-defined basolateral blood-to-cell uptake step. However, the apical cell-to-urine efflux mechanism is as yet unidentified. Here, we show that the renal apical organic anion efflux transporter human multidrug resistance protein 4 (MRP4), but not apical MRP2, mediates ATP-dependent urate transport via a positive cooperative mechanism (K(m) of 1.5 +/- 0.3 mM, V(max) of 47 +/- 7 pmol x mg(-1) x min(-1), and Hill coefficient of 1.7 +/- 0.2). In HEK293 cells overexpressing MRP4, intracellular urate levels were lower than in control cells. Urate inhibited methotrexate transport (IC50 of 235 +/- 8 microM) by MRP4, did not affect cAMP transport, whereas cGMP transport was stimulated. Urate shifted cGMP transport by MRP4 from positive cooperativity (K(m) and V(max) value of 180 +/- 20 microM and 58 +/- 4 pmol x mg(-1) x min(-1), respectively, Hill coefficient of 1.4 +/- 0.1) to single binding site kinetics (K(m) and V(max) value of 2.2 +/- 0.9 mM and 280 +/- 50 pmol x mg(-1) x min(-1), respectively). Finally, MRP4 could transport urate simultaneously with cAMP or cGMP. We conclude that human MRP4 is a unidirectional efflux pump for urate with multiple allosteric substrate binding sites. We propose MRP4 as a candidate transporter for urinary urate excretion and suggest that MRP4 may also mediate hepatic export of urate into the circulation, because of its basolateral expression in the liver.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Cl- uptake promoting depolarizing GABA actions in immature rat neocortical neurones is mediated by NKCC1

GABA is the principal inhibitory neurotransmitter in the mature brain, but during early postnatal development the elevated [Cl(-)](i) in immature neocortical neurones causes GABA(A) receptor activation to be depolarizing. The molecular mechanisms underlying this intracellular Cl(-) accumulation remain controversial. Therefore, the GABA reversal potential (E(GABA)) or [Cl(-)](i) in early postnatal rat neocortical neurones was measured by the gramicidin-perforated patch-clamp method, and the relative expression levels of the cation-Cl(-) cotransporter mRNAs (in the same cells) were examined by semiquantitative single-cell multiplex RT-PCR to look for statistical correlations with [Cl(-)](i). The mRNA expression levels were positively (the Cl(-) accumulating Na(+),K(+)-2Cl(-) cotransporter NKCC1) or negatively (the Cl(-) extruding K(+)-Cl(-) cotransporter KCC2) correlated with [Cl(-)](i). NKCC1 mRNA expression was high in early postnatal days, but decreased during postnatal development, whereas KCC2 mRNA expression displayed the opposite pattern. [Cl(-)](i) and NKCC1 mRNA expression were each higher in cortical plate (CP) neurones than in the presumably older layer V/VI pyramidal neurones in a given slice. The pharmacological effects of bumetanide on E(GABA) were consistent with the different expression levels of NKCC1 mRNA. These data suggest that NKCC1 may play a pivotal role in the generation of GABA-mediated depolarization in immature CP cells, while KCC2 promotes the later maturation of GABAergic inhibition in the rat neocortex.

The ABCs of drug transport in intestine and liver: efflux proteins limiting drug absorption and bioavailability

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (Pgp; MDR1) and MRP2, may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into blood. Drugs may also be modified by intracellular phase I and phase II metabolising enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for Pgp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subject to further metabolism and biliary excretion, often by a similar system of ATP-binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolising enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. The effectiveness of this system is optimised through dynamic regulation of transporter and enzyme expression; tissues have a remarkable capacity to regulate the amounts of protein both at transcriptional and post-transcriptional levels in order to maintain homeostasis. This review addresses the progress to date on what is known about the role and regulation of drug efflux mechanisms in the intestine and liver.

Contribution of organic anion transporter 3 (Slc22a8) to the elimination of p-aminohippuric acid and benzylpenicillin across the blood-brain barrier

The role of rat organic anion transporter 3 (rOat3; Slc22a8) in the efflux transport at the blood-brain barrier (BBB) was characterized. The expression of rOat1, rOat2, and rOat3 in the brain capillary endothelial cells (BCEC) was examined using reverse transcription-polymerase chain reaction analysis, which showed that there was no expression of rOat1 or rOat2, but moderate expression of rOat3. The expression of rOat3 in the BCEC was further confirmed by Western blotting. Immunohistochemical staining showed that rOat3 is located on the abluminal and, possibly, luminal membrane of the BCEC. The contribution of rOat3 to the efflux of para-aminohippuric acid (PAH) and benzylpenicillin (PCG), substrates of rOat3, from the cerebrum into the blood circulation across the BBB was evaluated using the Brain Efflux Index method. PAH and PCG were eliminated from the cerebrum with rate constants of 0.039 and 0.043 min-1, respectively, and the elimination was saturated at high substrate concentrations. Taking account of the dilution in the brain, the Km values for the elimination of PAH and PCG were estimated to be 168 and 29 micro M, respectively. The efflux of PAH and PCG across the BBB was inhibited in a dose-dependent manner by unlabeled PCG and PAH, respectively. The Ki value of PAH for the efflux of PCG was 106 micro M and that of PCG for the efflux of PAH was 58 micro M. These values were comparable with their Km values, suggesting that they share the same efflux mechanism at the BBB. Furthermore, cimetidine and pravastatin, which are also substrates and inhibitors of rOat3, significantly inhibited the efflux of PAH and PCG from the cerebrum. These results suggest that rOat3 is responsible for the elimination of PAH and PCG from the brain across the BBB.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Role of BSP/bilirubin binding protein on p-aminohippurate transport in rat kidney

BSP/Bilirubin binding protein (BBBP) is a protein located on the sinusoidal membrane of the liver that transport several organic anions. The aim of this investigation was to determine whether BBBP is present in the kidney and its role in p-aminohippurate transport (PAH). Anti-BBBP antibodies inhibited PAH uptake in brush border membrane vesicles (BBMV) and Na(+)-independent PAH uptake in basolateral membrane vesicles (BLMV). Western blot studies revealed positivity to antiBBBP antibodies in both BBMV and BLMV. So BBBP is also expressed in the kidneys and accounts, at least in part, for the renal tubular transport of PAH.

Role of organic anion transporters in the tubular transport of indoxyl sulfate and the induction of its nephrotoxicity

In uremic patients, various uremic toxins are accumulated and exert various biologic effects on uremia. Indoxyl sulfate (IS) is one of uremic toxins that is derived from dietary protein, and serum levels of IS are markedly increased in both uremic rats and patients. It has been previously reported that the accumulation of IS promotes the progression of chronic renal failure (CRF). This study demonstrates the role of rat organic anion transporters (rOATs) in the transport of IS and the induction of its nephrotoxicity. The administration of IS to 5/6-nephrectomized rats caused a faster progression of CRF, and immunohistochemistry revealed that IS was detected in the proximal and distal tubules where rOAT1 (proximal tubules) and/or rOAT3 (proximal and distal tubules) were also shown to be localized. In in vitro study, the proximal tubular cells derived from mouse that stably express rOAT1 (S2 rOAT1) and rOAT3 (S2 rOAT3) were established. IS inhibited organic anion uptake by S2 rOAT1 and S2 rOAT3, and the Ki values were 34.2 and 74.4 microM, respectively. Compared with mock, S2 rOAT1 and S2 rOAT3 exhibited higher levels of IS uptake, which was inhibited by probenecid and cilastatin, organic anion transport inhibitors. The addition of IS induced a decrease in the viability of S2 rOAT1 and S2 rOAT3 as compared with the mock, which was rescued by probenecid. These results suggest that rOAT1 and rOAT3 play an important role in the transcellular transport of IS and the induction of its nephrotoxicity.

Molecular aspects of renal anionic drug transport

Multiple organic anion transporters in the proximal tubule of the kidney are involved in the secretion of drugs, toxic compounds, and their metabolites. Many of these compounds are potentially hazardous on accumulation, and it is therefore not surprising that the proximal tubule is also an important target for toxicity. In the past few years, considerable progress has been made in the cloning of these transporters and their functional characterization following heterologous expression. Members of the organic anion transporter (OAT), organic anion transporting polypeptide (OATP), multidrug resistance protein (MRP), sodium-phosphate transporter (NPT), and peptide transporter (PEPT) families have been identified in the kidney. In this review, we summarize our current knowledge on their localization, molecular and functional characteristics, and substrate and inhibitor specificity. A major challenge for the future will be to understand how these transporters work in concert to accomplish the renal secretion of specific anionic substrates.

Shiga-like toxin II derived from Escherichia coli O157:H7 modifies renal handling of levofloxacin in rats

The effect of Shiga-like toxin II (SLT-II) (2 microg/animal), which was derived from Escherichia coli O157:H7, on renal handling of levofloxacin (LVX), a model drug for quinolone antimicrobial agents, was investigated in rats 24 h after intravenous injection. In histopathological examination, acute tubular injury was observed in SLT-II-treated rats, but the glomeruli were not injured. SLT-II significantly increased the steady-state concentration of LVX in plasma to 1.5-fold that of control rats. SLT-II induced significant decreases in the glomerular filtration rate (GFR) and renal clearance (CL(R)) of LVX. SLT-II slightly, but significantly, increased the unbound fraction and decreased renal plasma flow with no change in the extraction ratio of p-aminohippurate. SLT-II significantly increased concentrations of tumor necrosis factor alpha (TNF-alpha) and nitrite and nitrate (NOx) in plasma. The TNF-alpha inhibitor pentoxifylline partly, but significantly, inhibited SLT-II-induced decreases in the GFR and CL(R) of LVX; in contrast, S-methylisothiourea, a selective inhibitor of inducible nitric oxide synthase, did not. Western blotting analysis revealed that SLT-II did not alter the levels of multidrug resistance-associated protein 2 (Mrp2) and P-glycoprotein in kidneys 24 h after injection, assuming the lack of involvement of Mrp2 and P-glycoprotein in SLT-II-induced acute renal tubular injury and renal handling of LVX observed 24 h after SLT-II injection. The present study suggests that SLT-II impairs the renal handling of LVX by decreasing GFR and causing decreased renal plasma flow.

Sex differences in p-aminohippuric acid transport in rat kidney: role of membrane fluidity and expression of OAT1

Sex differences in the kinetic parameters of p-aminohippuric acid (PAH) transport in brush border (BBMV) and basolateral (BLMV) membrane vesicles from kidney cortex have been observed. Membrane fluidity of BBMV was higher in females as compared with male rats as indicated by anisotropy values (0.1897 +/- 0.0010 vs. 0.2003 +/- 0.0014, p < 0.05, for females and males respectively). Membrane fluidity of BLMV were similar in both sexes. Western blot studies revealed that OAT1 protein in female BLMV was present at only 40% of level found in BLMV from male rats. The lower expression of OAT1 in BLMV in association with the higher BBMV fluidity (which may affect the affinity of PAH transporter in this membrane domain) observed in females may be responsible, at least in part, for the gender difference described in renal PAH secretion.