Identification and functional assessment of the novel murine organic anion transporter Oat5 (Slc22a19) expressed in kidney

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.

Stevioside and related compounds: therapeutic benefits beyond sweetness

Stevioside, an abundant component of Stevia rebaudiana leaf, has become well-known for its intense sweetness (250-300 times sweeter than sucrose) and is used as a non-caloric sweetener in several countries. A number of studies have suggested that, beside sweetness, stevioside along with related compounds, which include rebaudioside A (second most abundant component of S. rebaudiana leaf), steviol and isosteviol (metabolic components of stevioside) may also offer therapeutic benefits, as they have anti-hyperglycemic, anti-hypertensive, anti-inflammatory, anti-tumor, anti-diarrheal, diuretic, and immunomodulatory actions. It is of interest to note that their effects on plasma glucose level and blood pressure are only observed when these parameters are higher than normal. As steviol can interact with drug transporters, its role as a drug modulator is proposed. This review summarizes the current knowledge of the pharmacological actions, therapeutic applications, pharmacokinetics and safety of stevioside and related compounds. Although much progress has been made concerning their biological and pharmacological effects, questions regarding chemical purity and safety remain unsolved. These issues are discussed to help guide future research directions.

Oat5 and NaDC1 protein abundance in kidney and urine after renal ischemic reperfusion injury

The aim of this study was to evaluate the abundance of the organic anion transporter 5 (Oat5) and the sodium-dicarboxylate cotransporter 1 (NaDC1) in kidney and urine after renal ischemic reperfusion injury. Renal injury was induced in male Wistar rats by occlusion of both renal pedicles for 0 (Group Sham), 5 (Group I5R60), or 60 (Group I60R60) min. The studies were performed after 60 min of reperfusion. The expression of Oat5 and NaDC1 was evaluated by IHC and Western blotting. Oat5 and NaDC1 abundance and alkaline phosphatase activity (AP) were assayed in urine. A decreased expression in renal homogenates and apical membranes and an increase in urinary excretion of Oat5 and NaDC1 were observed in I60R60 rats, as well as alterations of other widely used parameters for renal dysfunction and injury (plasma creatinine, urinary AP activity, kidney weight, histological lesions). In contrast, in the I5R60 group, only an increase in urinary excretion of Oat5 and mild histopathological damage was detected. This is the first study on Oat5 and NaDC1 detection in urine. These results suggest that urinary excretion of Oat5 might be an early indicator of renal dysfunction, which is useful for detection of even minor alterations in renal structural and functional integrity.

Organic anion transporter 3 (oat3/slc22a8) interacts with carboxyfluoroquinolones, and deletion increases systemic exposure to ciprofloxacin

Carboxyfluoroquinolones, such as ciprofloxacin, are used for the treatment of numerous infectious diseases. Renal secretion is a major determinant of their systemic and urinary concentration, but the specific transporters involved are virtually unknown. In vivo studies implicate the organic anion transporter (OAT) family as a pivotal component of carboxyfluoroquinolone renal secretion. Therefore, this study identified the specific renal basolateral OAT(s) involved, thereby highlighting potential sources of carboxyfluoroquinolone-drug interactions and variable efficacy. Two heterologous expression systems, Xenopus laevis oocytes and cell monolayers, were used to determine the roles of murine and human renal basolateral mOat1/hOAT1 and mOat3/hOAT3. Ciprofloxacin was transported by mOat3 in both systems (K(m) value, 70 +/- 6 microM) and demonstrated no interaction with mOat1 or hOAT1. Furthermore, ciprofloxacin, norfloxacin, ofloxacin, and gatifloxacin exhibited concentration-dependent inhibition of transport on mOat3 in cells with inhibition constants of 198 +/- 39, 558 +/- 75, 745 +/- 165, and 941 +/- 232 microM, respectively. Ciprofloxacin and gatifloxacin also inhibited hOAT3. Thereafter, in vivo elimination of ciprofloxacin was assessed in wild-type and Oat3 null mice [Oat3-/-]. Oat3-/- mice exhibited significantly elevated plasma levels of ciprofloxacin at clinically relevant concentrations (P < 0.05, male mice; P < 0.01, female mice). Oat3-/- mice also demonstrated a reduced volume of distribution (27%, P < 0.01, male mice; 14%, P < 0.01, female mice) and increased area under the concentration-time curve (25%, P < 0.05, male mice; 33%, P < 0.01, female mice). Female Oat3-/- mice had a 35% (P < 0.01) reduction in total clearance of ciprofloxacin relative to wild type. In addition, putative ciprofloxacin metabolites were significantly elevated in Oat3-/- mice. The present findings indicate that polymorphisms of and drug interactions on hOAT3 may influence carboxyfluoroquinolone efficacy, especially in urinary tract infections.

Identification of six putative human transporters with structural similarity to the drug transporter SLC22 family

The solute carrier family 22 (SLC22) is a large family of organic cation and anion transporters. These are transmembrane proteins expressed predominantly in kidneys and liver and mediate the uptake and excretion of environmental toxins, endogenous substances, and drugs from the body. Through a comprehensive database search we identified six human proteins not yet cloned or annotated in the reference sequence databases. Five of these belong to the SLC22 family, SLC22A20, SLC22A23, SLC22A24, SLC22A25, and SPNS3, and the sixth gene, SVOPL, is a paralog to the synaptic vesicle protein SVOP. We identified the orthologs for these genes in mouse and rat and additional homologous proteins and performed the first phylogenetic analysis on the entire SLC22 family in human, mouse, and rat. In addition, we performed a phylogenetic analysis which showed that SVOP and SV2A-C are, in a comparison with all vertebrate proteins, most similar to the SLC22 family. Finally, we performed a tissue localization study on 15 genes on a panel of 30 rat tissues using quantitative real-time polymerase chain reaction.

Xenobiotic transporters: ascribing function from gene knockout and mutation studies

Transporter-mediated absorption, secretion, and reabsorption of chemicals are increasingly recognized as important determinants in the biological activities of many xenobiotics. In recent years, the rapid progress in generating and characterizing mice with targeted deletion of transporters has greatly increased our knowledge of the functions of transporters in the pharmacokinetics/toxicokinetics of xenobiotics. In this introduction, we focus on functions of transporters learned from experiments on knockout mice as well as humans and rodents with natural mutations of these transporters. We limit our discussion to transporters that either directly transport xenobiotics or are important in biliary excretion or cellular defenses, namely multidrug resistance, multidrug resistance-associated proteins, breast cancer resistance protein, organic anion transporting polypeptides, organic anion transporters, organic cation transporters, nucleoside transporters, peptide transporters, bile acid transporters, cholesterol transporters, and phospholipid transporters, as well as metal transporters. Efflux transporters in intestine, liver, kidney, brain, testes, and placenta can efflux xenobiotics out of cells and serve as barriers against the entrance of xenobiotics into cells, whereas many xenobiotics enter the biological system via uptake transporters. The functional importance of a given transporter in each tissue depends on its substrate specificity, expression level, and the presence/absence of other transporters with overlapping substrate preferences. Nevertheless, a transporter may affect a tissue independent of its local expression by altering systemic metabolism. Further studies on the gene regulation and function of transporters, as well as the interrelationship between transporters and phase I/II xenobiotic-metabolizing enzymes, will provide a complete framework for developing novel strategies to protect us from xenobiotic insults.

Gender differences in kidney function

Sex hormones influence the development of female (F) and male (M) specific traits and primarily affect the structure and function of gender-specific organs. Recent studies also indicated their important roles in regulating structure and/or function of nearly every tissue and organ in the mammalian body, including the kidneys, causing gender differences in a variety of characteristics. Clinical observations in humans and studies in experimental animals in vivo and in models in vitro have shown that renal structure and functions under various physiological, pharmacological, and toxicological conditions are different in M and F, and that these differences may be related to the sex-hormone-regulated expression and action of transporters in the apical and basolateral membrane of nephron epithelial cells. In this review we have collected published data on gender differences in renal functions, transporters and other related parameters, and present our own microarray data on messenger RNA expression for various transporters in the kidney cortex of M and F rats. With these data we would like to emphasize the importance of sex hormones in regulation of a variety of renal transport functions and to initiate further studies of gender-related differences in kidney structure and functions, which would enable us to better understand occurrence and development of various renal diseases, pharmacotherapy, and drug-induced nephrotoxicity in humans and animals.

Ochratoxin A: An overview on toxicity and carcinogenicity in animals and humans

Ochratoxin A (OTA) is a ubiquitous mycotoxin produced by fungi of improperly stored food products. OTA is nephrotoxic and is suspected of being the main etiological agent responsible for human Balkan endemic nephropathy (BEN) and associated urinary tract tumours. Striking similarities between OTA-induced porcine nephropathy in pigs and BEN in humans are observed. International Agency for Research on Cancer (IARC) has classified OTA as a possible human carcinogen (group 2B). Currently, the mode of carcinogenic action by OTA is unknown. OTA is genotoxic following oxidative metabolism. This activity is thought to play a central role in OTA-mediated carcinogenesis and may be divided into direct (covalent DNA adduction) and indirect (oxidative DNA damage) mechanisms of action. Evidence for a direct mode of genotoxicity has been derived from the sensitive 32P-postlabelling assay. OTA facilitates guanine-specific DNA adducts in vitro and in rat and pig kidney orally dosed, one adduct comigrates with a synthetic carbon (C)-bonded C8-dG OTA adduct standard. In this paper, our current understanding of OTA toxicity and carcinogenicity are reviewed. The available evidence suggests that OTA is a genotoxic carcinogen by induction of oxidative DNA lesions coupled with direct DNA adducts via quinone formation. This mechanism of action should be used to establish acceptable intake levels of OTA from human food sources.

Organic anion transporters of the SLC22 family: biopharmaceutical, physiological, and pathological roles

The human organic anion transporters OAT1, OAT2, OAT3, OAT4 and URAT1 belong to a family of poly-specific transporters mainly located in kidneys. Selected OATs occur also in liver, placenta, and brain. OATs interact with endogenous metabolic end products such as urate and acidic neutrotransmitter metabolites, as well as with a multitude of widely used drugs, including antibiotics, antihypertensives, antivirals, anti-inflammatory drugs, diuretics and uricosurics. Thereby, OATs play an important role in renal drug elimination and have an impact on pharmacokinetics. In this review we focus on the interaction of human OATs with drugs. We report the affinities of human OATs for drug classes and compare the putative importance of individual OATs for renal drug excretion. The role of OATs as sites of drug-drug interaction and mediators cell toxicity, their gender-dependent regulation in health and diseased states, and the possible impact of single nucleotide polymorphisms are also dealt with.

Alteration in Renal Organic Anion Transporter 1 After Ischemia/Reperfusion in Cadaveric Renal Allografts

We have previously shown that postischemic injury to renal allografts results in profound impairment of p-aminohippuric acid (PAH) extraction. To elucidate the cellular integrity of the human organic anion transporter 1 (hOAT1) in postischemic acute renal failure (ARF), immunohistochemical analysis of hOAT1 was performed in cadaveric renal allografts using confocal microscopy for three-dimensional reconstruction of serial optical images. Biopsy samples were obtained from 10 cadaveric renal allografts 1 hr after reperfusion during transplant operation. Control tissues were obtained from four living donors of healthy kidneys immediately before an arterial clamp was applied to the renal artery. Control tissues demonstrated hOAT1 distributed to basolateral membrane of proximal tubule cells. In contrast, maldistribution of hOAT1 to cytoplasm and/or diminution of the protein was noted in cadaveric allografts. Characteristics of maldistribution were variable: disappearance of lateral distribution, diffuse cytoplasmic aggregates, apical cytoplasmic aggregates, and disappearance of the staining. In addition, iothalamate and PAH clearances were performed on posttransplant days 3–7 in 18 recipients of a cadaveric renal allograft. PAH clearance was depressed <250 ml/min in all but three subjects. We conclude that reperfused, transplanted kidneys exhibit maldistribution of hOAT1 in proximal tubule cells, resulting in impairment of PAH clearance. This manuscript contains online supplemental material at http://www.jhc.org . Please visit this article online to view these materials.

Human renal organic anion transporter 4 operates as an asymmetric urate transporter

Human organic anion transporter 4 (hOAT4) is located at the apical membrane of proximal tubule cells and involved in renal secretion and reabsorption of endogenous substances as well as many drugs and xenobiotics. This study reevaluated the physiologic role, transport mode, and driving forces of hOAT4. 6-Carboxyfluorescein (6-CF) uptake into HEK293 cells that stably expressed hOAT4 was saturable, resulting in a K(m) of 108 muM. 6-CF as well as [(3)H]estrone sulfate ([(3)H]ES) accumulation by HEK293-hOAT4 cells were abolished by ES, dehydroepiandrosterone sulfate, sulfinpyrazone, benzbromarone, and probenecid, whereas several OA, including p-aminohippurate (PAH), lactate, pyrazinoate, nicotinate, glutarate, and the diuretic hydrochlorothiazide (HCTZ) exhibited a slight or a NS inhibitory effect. PAH and glutarate are not taken up by HEK293-hOAT4 cells, but they trans-stimulated 6-CF and [(3)H]ES uptake, indicating an asymmetric interaction of hOAT4 with these substrates. In chloride-free medium, HEK293-hOAT4-mediated [(3)H]PAH efflux was almost abolished, whereas addition of ES restored it comparable to Ringer solution, consistent with a physiologic ES/PAH or PAH/Cl(-) exchange mode of hOAT4. Moreover, an acidification of the uptake medium increased 6-CF as well as [(3)H]ES uptake, which was reduced by nigericin, suggesting that hOAT4 also can operate as an OA/OH(-) exchanger. hOAT4 facilitates substantial uptake of [(14)C]urate, which was elevated 2.6-fold by intracellular HCTZ. Thus, hOAT4 is the long-postulated, low-affinity apical urate anion exchanger that facilitates HCTZ-associated hyperuricemia.

Determination of the external loops and the cellular orientation of the N- and the C-termini of the human organic anion transporter hOAT1

The OAT (organic anion transporter) family mediates the absorption, distribution and excretion of a diverse array of environmental toxins and clinically important drugs. OAT dysfunction significantly contributes to renal, hepatic, neurological and fetal toxicity and disease. As a first step to establish the topological model of hOAT1 (human OAT1), we investigated the external loops and the cellular orientation of the N- and the C-termini of this transporter. Combined approaches of immunofluorescence studies and site-directed chemical labelling were used for such purpose. Immunofluorescence microscopy of Myc-tagged hOAT1 expressed in cultured cells identified that both the N- and the C-termini of the transporter were located in the cytoplasm. Replacement of Lys59 in the predicted extracellular loop I with arginine resulted in a mutant (K59R), which was largely inaccessible for labelling by membrane-impermeable NHS (N-hydroxysuccinimido)-SS (dithio)-biotin present in the extracellular medium. This result suggests that loop I faces outside of the cell membrane. A single lysine residue introduced into putative extracellular loops III, V and VI of mutant K59R, which is devoid of extracellular lysine, reacted readily with membrane-impermeable NHS-SS-biotin, suggesting that these putative extracellular loops are in the extracellular domains of the protein. These studies provided the first experimental evidence on the extracellular loops and the cellular orientation of the N- and the C-termini of hOAT1.

Molecular insights into the structure-function relationship of organic anion transporters OATs

The organic anion transporter (OAT) family encoded by SLC22A mediates the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories, and therefore is critical for the survival of mammalian species. Several OATs have been identified: OAT1 (SLC22A6), OAT2 (SLC22A7), OAT3 (SLC22A8), OAT4 (SLC22A11), OAT5 (SLC22A19) OAT6 (SLC22A20) and URAT1 (SLC22A12). The expressions of these OATs have been detected in key organs such as kidney, liver, brain and placenta. OAT dysfunction in these organs may contribute to the renal, hepatic, neurological and fetal toxicity and diseases. In this review, we summarize, according to the work done by our laboratory as well as by others, the most updated molecular studies on these OAT members, especially on the aspect of their structure-function relationships. The functional roles of N-glycosylation, transmembrane domains and individual amino acids, cell surface assembly, as well as associating proteins will be discussed. In addition, we will show the recent analyses of coding region polymorphisms of OATs, which give us information on the genetic variants of OATs and their potential effects on OAT functions.

Organic anion transporter family: current knowledge

Organic anion transporters (OATs) play an essential role in the elimination of numerous endogenous and exogenous organic anions from the body. The renal OATs contribute to the excretion of many drugs and their metabolites that are important in clinical medicine. Several families of multispecific organic anion and cation transporters, including OAT family transporters, have recently been identified by molecular cloning. The OAT family consists of six isoforms (OAT1 - 4, URAT1, and rodent Oat5) and they are all expressed in the kidney, while some are also expressed in the liver, brain, and placenta. The OAT family represents mainly the renal secretory and reabsorptive pathway for organic anions and is also involved in the distribution of organic anions in the body, drug-drug interactions, and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. In this review, current knowledge of and recent progress in the understanding of several aspects of OAT family members are discussed.

Functional characterization of a human organic anion transporter hOAT4 in placental BeWo cells

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters which play critical roles in the body disposition of clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. hOAT4 is expressed in the placenta and kidney. In the current study, we stably transfected hOAT4 into human placental BeWo cells and the functional properties of hOAT4 and its regulation were investigated in these cells. hOAT4-mediated uptake of estrone sulfate, a protypical organic anion for hOAT4, was dose- and time-dependent, and saturable (Km=4.2 microM). The substrate specificity of hOAT4 includes various steroid sulfates, such as beta-estradiol-3,17-disulfate, 17-beta-estradiol-3-sulfate, beta-estradiol-3-sulfate, and dehydroepiandrosterone-3-sulfate (DHEAS), but does not include p-aminohippuric acid (PAH) and tetraethylammonium (TEA). Pre-incubation of hOAT4-expressing BeWo cells with phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), both of which are protein kinase C (PKC) activators, acutely inhibited the transport activity. The inhibition by PDBu resulted in a decreased Vmax without significant affecting the Km. Establishment of hOAT4-expressing BeWo cells provided useful tool for further pharmacological and molecular biological studies of placental transport of organic anions mediated by this carrier.

Transport of estrone sulfate by the novel organic anion transporter Oat6 (Slc22a20)

Recently, a novel Slc22 gene family member expressed in murine olfactory mucosa was identified and based on sequence homology proposed to be an organic anion transporter [Oat6 (Slc22a20); J. C. Monte, M. A. Nagle, S. A. Eraly, and S. K. Nigam. Biochem Biophys Res Commun 323: 429-436, 2004]. However, no functional data for Oat6 was reported. In the present study, we demonstrate that murine Oat6 mediates the inhibitable transport of estrone sulfate using both Xenopus oocyte expression assay and Chinese hamster ovary (CHO) cells stably transfected with mOat6 (CHO-mOat6). Uptake was virtually eliminated by probenecid and the anionic herbicide 2,4-dichlorophenoxyacetate. The organic anions ochratoxin A, salicylate, penicillin G, p-aminohippurate, and urate inhibited mOat6-mediated accumulation to varying degrees. Transport of estrone sulfate by mOat6 was demonstrated to be saturable, and K(m) estimates of 109.8 +/- 22.6 microM in oocytes and 44.8 +/- 7.3 microM in CHO-mOat6 cells were obtained. Inhibitory constants for 2,4-dichlorophenoxyacetate (15.7 +/- 2.0 microM), salicylate (49.0 +/- 4.4 microM), probenecid (8.3 +/- 2.5 microM), and penicillin G (1,450 +/- 480 microM) were also determined. Accumulation of estrone sulfate mediated by mOat6 was significantly trans-stimulated by glutarate, indicating that mOat6 functions as an organic anion/dicarboxylate exchanger. These data demonstrate for the first time that the novel murine gene Oat6 (Slc22a20) encodes a functional organic anion transporter and mOat6 is indeed the newest member of the OAT gene family.

Toxicokinetics and toxicodynamics of ochratoxin A, an update

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic and immunotoxic to several species of animals and to cause kidney and liver tumours in mice and rats. Because of differences in the physiology of animal species, wide variations are seen in the toxicokinetic patterns of absorption, distribution and elimination of the toxin. Biotransformation of OTA has not been entirely elucidated. At present, data regarding OTA metabolism are controversial. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. Several major mechanisms have been shown as involved in the toxicity of OTA: inhibition of protein synthesis, promotion of membrane peroxidation, disruption of calcium homeostasis, inhibition of mitochondrial respiration and DNA damage. The contribution of metabolites in OTA genotoxicity and carcinogenicity is still unclear. The genotoxic status of OTA is still controversial because contradictory results were obtained in various microbial and mammalian tests, notably regarding the formation of DNA adducts. More recent studies are focused on the OTA ability to disturb cellular signalling and regulation, to modulate physiological signals and thereby to influence cells viability and proliferation. The present paper offers an update on these different issues. In addition since humans and animals are likely to be simultaneously exposed to several mycotoxins, especially through their diet, the little information available on the combined effects of OTA and other mycotoxins has also been reviewed.

Prostaglandin E2 inhibits its own renal transport by downregulation of organic anion transporters rOAT1 and rOAT3

Prostaglandin E2 (PGE2) is the principal mediator of fever and inflammation. Recently, evidence emerged that during febrile response, PGE2 that is generated in the periphery enters the hypothalamus and contributes to the maintenance of fever. In a rat model of fever generation, peripheral PGE2 is increased, whereas clearance by metabolism of peripheral PGE2 is downregulated. The major route of PGE2 excretion is via the renal proximal tubular organic anion secretory system, where basolateral uptake that is mediated by renal organic anion transporter 1 (rOAT1) and rOAT3 is rate limiting. Therefore, it was hypothesized that PGE2 itself will abolish its excretion by rOAT1 or rOAT3. Fluorescein was used as a prototypic organic anion, and NRK-52E cells from rat served as a proximal tubular model system. PGE2 time-dependently downregulates basolateral organic anion uptake, without affecting cell volume or cell protein, recirculation of counter ions, or proximal tubular transport systems in general. In addition, PGE2 diminishes expression of both rOAT1 and rOAT3. Both organic anion uptake and expression of rOAT1 and rOAT3 are dose-dependently downregulated by PGE2. These findings suggest that during fever or inflammation, renal secretory transport of PGE2 is reduced, contributing to elevated PGE2 levels in blood. These data fit into the hypothetical concept of peripheral PGE2's playing a significant role in fever. The described regulatory mechanism may also be of relevance in chronic inflammatory events. Moreover, the data presented could explain why increased plasma urate levels occur in diseases that go along with increased levels of PGE2.

Characterization of mouse organic anion transporter 5 as a renal steroid sulfate transporter

A family of organic anion transporters (OAT) recently identified has important roles for the excretion or reabsorption of endogenous and exogenous compounds, and several new isoforms have been reported in this decade. Although the transepithelial transport properties of organic anions are gradually being understood, many portions of their functional characteristics in functions remain to be elucidated. A recently reported new cDNA encoding a mouse OAT5 (mOAT5) was constructed, using 3'-RACE PCR, with the total RNA isolated from a mouse kidney. When mOAT5 was expressed in Xenopus oocytes, mOAT5 transported estrone sulfate, dehydroepiandrosterone sulfate and ochratoxin A. Estrone sulfate uptake by mOAT5 displayed a time-dependent and sodium-independent manner. The Km values of estrone sulfate and dehydroepiandrosterone sulfate were 2.2 and 3.8 microM, respectively. mOAT5 interacted with chemically heterogeneous steroid or organic sulfates, such as nitrophenyl sulfate, methylumbelliferyl sulfate and estradiol sulfates. In contrast to the sulfate conjugates, mOAT5-mediated estrone sulfate uptake was not inhibited by the steroid or organic glucuronides. The mOAT5 protein having about 85 kDa molecular weight was shown to be mainly localized in the apical membrane of the proximal tubules of the outer medulla. These results suggest an important role of mOAT5 for the excretion or reabsorption of steroid sulfates in the kidney.

Mouse organic anion transporter 2 and 3 (mOAT2/3[Slc22a7/8]) mediates the renal transport of bumetanide

Multispecific organic anion transporters play an important role in the excretion and the elimination of a wide variety of endogenous and exogenous substrates. To date, five murine OAT homologs such as mouse organic anion transporters 1-3, 5, and 6 (mOAT1-3, 5 and 6) have been isolated and well characterized. With the exception of mOAT6, other mOAT isoforms are predominantly expressed in the kidney. The aim of this study was to examine whether mOAT2/3, as well as hOAT2/3, transports the diuretic bumetanide using a Xenopus laevis oocyte expression system. When expressed in Xenopus oocytes, mOAT2/3 mediated the high affinity transport of bumetanide. The apparent K(m) values for the uptake of bumetanide via mOAT2 and mOAT3 were 9.12 +/- 2.42 microM and 1.01 +/- 0.27 microM, respectively. Immunohistochemical analysis revealed that mOAT2 is expressed on the luminal membrane site of the proximal tubule. Our results indicate that mOAT2 and 3, as well as human homologs, are molecules for the transport of bumetanide on the luminal membranes of kidney proximal tubules.

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Integrated physiology of proximal tubular organic anion transport

PURPOSE OF REVIEW

Renal organic anion transport proteins play important roles in the reabsorption and the secretion of endogenous and exogenous compounds. This review focuses on the interpretation of the physiological integration of identified transport molecules in the renal proximal tubules.

RECENT FINDINGS

To date, molecular identification of organic anion transport proteins is still continuing: rodent organic anion transporter 5, organic anion-transporting polypeptide 4C1, voltage-driven organic anion transporter 1, multidrug resistance-associated protein 4, and sodium-coupled monocarboxylate transporter have yielded additional information in this field. In addition, particularly at the apical membrane of the proximal tubules, the importance of the PDZ (PSD-95, DglA, and ZO-1) binding domain proteins has emerged in the formation of the multimolecular complex as a functional unit of membrane transport. Finally, discovery of dicarboxylate receptors in the renal tubular cells raises the possibility that dicarboxylate anions function as intrarenal signaling molecules. This novel aspect of renal organic anion transport, the potential modulation of signaling via dicarboxylate receptors, may be of significant relevance to renovascular hypertension and other renal diseases.

SUMMARY

Comprehensive understanding of the multimolecular complex, which is composed of transporters and their related signaling elements and is supported by the scaffold proteins underneath the plasma membrane, may be useful in clarifying complex transport phenomena such as renal apical organic anion handling. In addition to the recent proteomics approaches and conventional molecular physiology, it is necessary to develop novel methods to analyze the overall function of the multimolecular complex for the post-genomic era.

Evidence for rat organic anion transporter 3 association with caveolin-1 in rat kidney

The rat organic anion transporter 3 (rOAT3) has recently been identified as the third isoform of the OAT family. The mechanisms that regulate rOAT3's functions remain to be elucidated. rOAT3 contributes for moving a number of negatively charged organic compounds between cells and their extracellular milieu. Caveolin (Cav) also plays a role as a membrane transporter. To address the relationship of these two proteins, we investigated the protein-protein interaction between rOAT3 and Cav-1. The rOAT3 mRNA and protein expression were observed in the rat kidney, and the expressions of Cav-1 mRNA and protein were also detected in the kidney. Confocal microscopy of the immuno-cytochemistry experiments using primary cultured renal proximal tubular cells showed that rOAT3 and Cav-1 were co-localized at the plasma membrane. This finding was confirmed by Western blot analysis using isolated caveolae-enriched membrane fractions from the rat kidney and immuno-precipitation experimentation. When rOAT3's synthesized cRNA of rOAT3 along with the antisense oligo deoxynucleotide ofXenopusCav-1 were co-injected intoXenopusoocytes, the [(3)H] estrone sulfate uptake was significantly decreased. These findings suggest that rOAT3 and caveolin-1 share a cellular expression in the plasma membrane and Cav-1 up-regulates the organic anionic compound uptake via rOAT3 under normal physiological conditions.

The multispecific organic anion transporter family: properties and pharmacological significance

Physiological and pharmacological studies indicate that the renal and hepatic organic anion transport systems are responsible for the elimination of numerous compounds, such as drugs, environmental substances and metabolites of both endogenous and exogenous origins. Recently, the molecular identity of the organic anion transport system, the OAT family, was revealed. To date, six OAT members have been identified and shown to have important roles not only in detoxification in the kidneys, liver and brain, but also in the reabsorption of essential compounds such as urate. The OAT family members are closely associated with the pharmacokinetics, drug-drug interactions and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. The molecular characterization of the OAT family encoded by SLC22A will be discussed.

Identification of a novel murine organic anion transporter family member, OAT6, expressed in olfactory mucosa

The organic anion and cation transporters (OATs and OCTs) are a large family (SLC22) of transmembrane proteins that are able to transport a variety of compounds including drugs, environmental toxins, and endogenous metabolites. OATs are expressed in various tissues, primarily kidney and liver, but also in placenta, small intestine, and choroid plexus, which are all epithelial tissues that transport xenobiotics. The upper airway, particularly the nose, is also a site of frequent exposure to environmental toxins. Many drugs are administered intranasally. This raises the possibility that the olfactory epithelium contains OATs and OCTs. Here, we report the identification of a novel putative transporter, mouse OAT6, expressed predominantly in olfactory mucosa but not in kidney or brain. Sequence comparisons and intron phasing analysis indicated that OAT6 is closely related to OAT1 and OAT3. Unlike many other slc22 genes, OAT6 is unpaired in the genome, although it is in proximity to the OAT1/OAT3 gene pair. Expression of OAT6 was also observed in testis. Embryonic expression was observed at day 7, but not later in embryogenesis. This might be due to the need for a key metabolite transported by OAT6. The data raise the possibility that the olfactory mucosa may have a significant transport apparatus which could be important in the design of new therapeutic approaches for direct nose-to-brain transfer of drugs and olfaction. Supporting this possibility, we have demonstrated that OAT1, OCT1-2, and OCTN1-3 are also expressed in olfactory mucosa. Furthermore, e-blot data suggest very different expression of individual OATs, OCTs and OCTNs in kidney, brain, liver, and eye.