The multispecific organic anion transporter family: properties and pharmacological significance

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.

Torsemide renal clearance and genetic variation in luminal and basolateral organic anion transporters

AIMS

To investigate the association between torsemide renal clearance and genetic variation in the basolaterally expressed renal organic anion transporters OAT1 and OAT3 and in the luminally situated OAT4.

METHODS

We analysed 22 polymorphisms in the OAT coding genes SLC22A6, SLC22A8 and SLC22A11 and their haplotypes and measured torsemide renal clearance in 95 healthy men. In addition, the effect of torsemide on the OAT-mediated transport was studied in vitro.

RESULTS

In stably transfected HEK293 cells torsemide (100 microm) inhibited the uptake by human OAT1, OAT3 and OAT4 by 63.1, 58.1 and 68.0%, respectively. Torsemide renal clearance ranged from 6.5 to 43.1 ml min(-1) with a log-normal distribution and a geometric mean of 15.6 ml min(-1) (15.0-16.1 +/- SEM). No clear outlier group was observed. AA carriers of the polymorphism rs11231809 in SLC22A11 had a torsemide renal clearance of 13.3 ml min(-1) (12.7-13.9) compared with 15.1 ml min(-1) (14.5-15.8) in AT and 18.0 ml min(-1) (16.7-19.5) in TT carriers (P = 0.002). The two most frequent haplotypes at SLC22A11 showed an association with torsemide renal clearance. Homozygous carriage of these two haplotypes resulted in renal clearances of 21.2 ml min(-1) (19.0-23.7) and 11.8 ml min(-1) (10.5-13.5), respectively. No association between reanl clearance and genetic variation in SLC22A6 or SLC22A8 was observed.

CONCLUSIONS

Genetic variation in the gene encoding the luminally expressed OAT4 rather than in the basolaterally expressed OATs may affect the renal clearance of torsemide.

Isolation and characterization of polyspecific mouse organic solute carrier protein 1 (mOscp1)

We succeeded in isolating the cDNA-encoding mouse organic solute carrier protein 1 (mOscp1) from a mouse testis cDNA library. mOscp1 consisted of 1137 base pairs that encoded a 379-amino acid protein, and the amino acid sequence was 85% identical to that of human OSCP1 (hOSCP1). Northern blot analysis revealed that the gene coding for mOscp1 is highly expressed in the testis, but not in other tissues. When expressed in Xenopus laevis oocytes, mOscp1 mediated the high-affinity transport of p-aminohippurate (PAH) (K(m) = 18.8 +/- 4.1 microM) with Na(+) independence. mOscp1 transported various kinds of structurally dissimilar drugs and chemicals such as probenecid, dehydroepiandrosterone sulfate, and glutarate with some differences in substrate specificity compared with hOSCP1. Cyclophosphamide inhibited the mOscp1-mediated PAH uptake. Immunohistochemical analysis revealed that the mOscp1 protein is localized in the plasma membrane side of Sertoli cells in the testis. Our results indicate that isolated mOscp1 is a polyspecific organic solute carrier protein and may be a key molecule for the testicular handling of organic solutes.

Adverse effects of the classic antioxidant uric acid in adipocytes: NADPH oxidase-mediated oxidative/nitrosative stress

Uric acid is considered a major antioxidant in human blood that may protect against aging and oxidative stress. Despite its proposed protective properties, elevated levels of uric acid are commonly associated with increased risk for cardiovascular disease and mortality. Furthermore, recent experimental studies suggest that uric acid may have a causal role in hypertension and metabolic syndrome. All these conditions are thought to be mediated by oxidative stress. In this study we demonstrate that differentiation of cultured mouse adipocytes is associated with increased production of reactive oxygen species (ROS) and uptake of uric acid. Soluble uric acid stimulated an increase in NADPH oxidase activity and ROS production in mature adipocytes but not in preadipocytes. The stimulation of NADPH oxidase-dependent ROS by uric acid resulted in activation of MAP kinases p38 and ERK1/2, a decrease in nitric oxide bioavailability, and an increase in protein nitrosylation and lipid oxidation. Collectively, our results suggest that hyperuricemia induces redox-dependent signaling and oxidative stress in adipocytes. Since oxidative stress in the adipose tissue has recently been recognized as a major cause of insulin resistance and cardiovascular disease, hyperuricemia-induced alterations in oxidative homeostasis in the adipose tissue might play an important role in these derangements.

Renal Transport of Adefovir, Cidofovir, and Tenofovir by SLC22A Family Members (hOAT1, hOAT3, and hOCT2)

PURPOSE

The nephrotoxicity of the nucleotide antivirals adefovir, cidofovir and tenofovir is considered to depend on the renal tubular transport of them. Although it is known that the antivirals are substrates of the human renal organic anion transporter hOAT1 (SLC22A6), there is no information available on other organic ion transporters. The aim of the present study was to investigate whether the other renal organic anion transporter hOAT3 (SLC22A8) and organic cation transporter hOCT2 (SLC22A2) transport the antivirals.

MATERIALS AND METHODS

Uptake experiments were performed using HEK293 cells transfected with cDNA of the organic ion transporters.

RESULTS

The uptake of adefovir, cidofovir and tenofovir in monolayers stably expressing hOAT3 increased time-dependently, compared with control. Probenecid, a typical inhibitor of organic anion transporters, completely inhibited their transport. The amounts of the antivirals taken up by hOAT3 were much lower than those by hOAT1. The transient expression of hOCT2 did not increase uptake of the antivirals.

CONCLUSION

These results indicate that adefovir, cidofovir and tenofovir are substrates of hOAT3 as well as hOAT1, but that quantitatively hOAT1 is the major renal transporter for these drugs.

Brain ECF antioxidant interactions in hamsters during arousal from hibernation

Warming from hibernation to cenothermia involves intense metabolic activity and large fluxes in regional blood flow and volume. During this transition, levels of the antioxidants, ascorbate (AA), urate and glutathione (GSH) in brain tissue, extracellular fluid (ECF) and plasma change substantially. Striatal ECF was sampled and manipulated using very slow perfusion microdialysis to examine the mechanisms that influence the changing profile of striatal ECF AA, urate and GSH levels during arousal from hibernation to cenothermia in Syrian hamsters (Mesocricetus auratus). Omission of glucose from the perfusate had no effect upon the respective decrease, increase and transient increase in striatal ECF levels of AA, GSH and urate observed during arousal from hibernation to cenothermia. In contrast, inhibition of xanthine dehydrogenase/oxidase (XOR) activity by reverse dialysis with oxypurinol, itself a free radical scavenger, decreased ECF urate and preserved ECF AA levels. This suggests that some ECF AA is oxidized by free radical products of XOR flux and/or by other free radical producing processes activated during the transition from hibernation to cenothermia. Local supplementation of ECF AA, GSH and cystiene had no effect upon the profile of transient increase of ECF urate observed during arousal from hibernation. The production of free radicals by XOR and the disappearance of AA from the ECF continues for at least 2h immediately after the hamster has attained cenothermia. The hamster, immediately after arousal from hibernation, can be utilized as a natural model to study free radical production and effective scavenging at cenothermia.

Genetic polymorphisms of drug transporters: pharmacokinetic and pharmacodynamic consequences in pharmacotherapy

There has been increasing appreciation of the role of drug transporters in pharmacokinetic and pharmacodynamic consequences in pharmacotherapy. The clinical relevance of drug transporters depends on the localisation in human tissues (i.e., vectorial movement), the therapeutic index of the substrates and inherent interindividual variability. With regard to variability, polymorphisms of drug transporter genes have recently been reported to be associated with alterations in the pharmacokinetics and pharmacodynamics of clinically useful drugs. A growing number of preclinical and clinical studies have demonstrated that the application of genetic information may be useful in individualised pharmacotherapy for numerous diseases. However, the reported effects of variants in certain drug transporter genes have been inconsistent and, in some cases, conflicting among studies. Furthermore, the incidence of almost all known variants in transporter genes tends to be racially dependent. These observations suggest the necessity of considering interethnic variability before extrapolating pharmacokinetic data obtained in one ethic group to another, especially in the early phase of drug development. This review focuses on the impact of genetic variations in the function of drug transporters (ABC, organic anion and cation transporters) and the implications of these variations for pharmacotherapy from pharmacokinetic and pharmacodynamic viewpoints.

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.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

Anti-influenza prodrug oseltamivir is activated by carboxylesterase human carboxylesterase 1, and the activation is inhibited by antiplatelet agent clopidogrel

Oseltamivir is the main medicine recommended by the World Health Organization in anticipation of next influenza pandemic. This anti-influenza viral agent is an ester prodrug, and the antiviral activity is achieved by its hydrolytic metabolite: oseltamivir carboxylate. In this study, we report that the hydrolytic activation is catalyzed by carboxylesterase human carboxylesterase (HCE) 1. Liver microsomes rapidly hydrolyzed oseltamivir, but no hydrolysis was detected with intestinal microsomes or plasma. The overall rate of the hydrolysis varied among individual liver samples and was correlated well with the level of HCE1. Recombinant HCE1 but not HCE2 hydrolyzed this prodrug and produced similar kinetic parameters as the liver microsomes. Several HCE1 natural variants differed from the wild-type enzyme on the hydrolysis of oseltamivir. In the presence of antiplatelet agent clopidogrel, the hydrolysis of oseltamivir was inhibited by as much as 90% when the equal concentration was assayed. Given the fact that hydrolysis of oseltamivir is required for its therapeutic activity, concurrent use of both drugs would inhibit the activation of oseltamivir, thus making this antiviral agent therapeutically inactive. This is epidemiologically of significance because people who receive oseltamivir and clopidogrel simultaneously may maintain susceptibility to influenza infection or a source of spreading influenza virus if already infected.

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.

Duration and degree of cyclosporin induced P-glycoprotein inhibition in the rat blood-brain barrier can be studied with PET

Active efflux transporters in the blood-brain barrier lower the brain concentrations of many drug molecules and endogenous substances and thus affect their central action. The objective of this investigation was to study the dynamics of the entire inhibition process of the efflux transporter P-glycoprotein (P-gp), using positron emission tomography (PET). The P-gp marker [(11)C]verapamil was administered to anesthetized rats as an i.v. bolus dose followed by graded infusions via a computerized pump system to obtain a steady-state concentration of [(11)C]verapamil in brain. The P-gp modulator cyclosporin A (CsA) (3, 10 and 25 mg/kg) was administered as a short bolus injection 30 min after the start of the [(11)C]verapamil infusion. The CsA pharmacokinetics was studied in whole blood in a parallel group of rats. The CsA blood concentrations were used as input to model P-gp inhibition. The inhibition of P-gp was observed as a rapid increase in brain concentrations of [(11)C]verapamil, with a maximum after 5, 7.5 and 17.5 min for the respective doses. The respective increases in maximal [(11)C]verapamil concentrations were 1.5, 2.5 and 4 times the baseline concentration. A model in which CsA inhibited P-gp by decreasing the transport of [(11)C]verapamil out from the brain resulted in the best fit. Our data suggest that it is not the CsA concentration in blood, but rather the CsA concentration in an effect compartment, probably the endothelial cells of the blood-brain barrier that is responsible for the inhibition of P-gp.

Implications of genetic polymorphisms in drug transporters for pharmacotherapy

Drug transporters are increasingly recognized as a key determinant of drug disposition and response. It is now widely appreciated that expression of the ATP-dependent efflux transporter, MDR1 (ABCB1, P-glycoprotein), in organs such as the gastrointestinal tract, liver and kidney significantly alters the extent of drug absorption and excretion. Moreover, expression of MDR1 at the level of the blood-brain barrier limits the entry of many drugs into the central nervous system. Given such an important role of MDR1 in the drug disposition process, it is not surprising to see increasing focus on the role of single nucleotide polymorphisms (SNPs) in this transporter as a potential determinant of interindividual variability in drug disposition and pharmacological response. However, drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. A growing list of membrane-spanning proteins involved in the in- or outward transport of a large variety of drugs has been recognized and characterized over the past few years in almost all tissues, including organic anion and cation transporters (OAT, OCT, solute carrier family SLC22A), organic anion transport proteins (OATP, solute carrier family SLCO, formerly SLC21A), and MRPs (ABCCs), other members of the ATP-binding cassette family. We are just beginning to appreciate their role for drug delivery and disposition and the contribution of genetic polymorphisms in these transport proteins to interindividual variability in the efficacy and safety for pharmacotherapy. This review summarizes the consequences of inherited differences in drug transport for pharmacotherapy. With the main focus on ABCB1, an update of recent advances is given and clinically relevant examples are used to illustrate how heritable differential drug transport can help to explain individual variability in drug response. The pharmacogenetics of other transporters is briefly introduced.

Molecular physiology of renal organic anion transporters

Recent advances in molecular biology have identified three organic anion transporter families: the organic anion transporter (OAT) family encoded by SLC22A, the organic anion transporting peptide (OATP) family encoded by SLC21A (SLCO), and the multidrug resistance-associated protein (MRP) family encoded by ABCC. These families play critical roles in the transepithelial transport of organic anions in the kidneys as well as in other tissues such as the liver and brain. Among these families, the OAT family plays the central role in renal organic anion transport. Knowledge of these three families at the molecular level, such as substrate selectivity, tissue distribution, and gene localization, is rapidly increasing. In this review, we will give an overview of molecular information on renal organic anion transporters and describe recent topics such as the regulatory mechanisms and molecular physiology of urate transport. We will also discuss the physiological roles of each organic anion transporter in the light of the transepithelial transport of organic anions in the kidneys.

Carrier-mediated uptake of grepafloxacin, a fluoroquinolone antibiotic, by the isolated rat lung cells

Grepafloxacin (GPFX) is a new quinolone antibiotic (NQ) which is highly distributed to the lung and other tissues. In the present study, to characterize the distribution mechanism of GPFX to the lung, the uptake of GPFX by isolated rat lung cells was examined in vitro. GPFX was rapidly taken up by the cells, and the uptake reached a steady-state within 5 min. The cell-to-medium concentration ratio at equilibrium was 56.8+/-1.9 microL/mg protein, which was much higher than the cellular volume. GPFX uptake consisted of a saturable component (Km: 264+/-181 microM, Vmax: 2.94+/-2.33 nmol/min/mg protein) and a nonsaturable component (Pdif: 7.04+/-2.17 microL/min/mg protein). The uptake of GPFX was reduced in the presence of ATP-depletors (FCCP and Rotenone) and by the replacement of sodium with choline in the medium, suggesting that GPFX uptake is at least partially mediated by an Na+- and energy-dependent process. GPFX uptake tended to be reduced in the presence of other NQs such as levofloxacin, lomefloxacin and sparfloxacin, but was only minimally affected by the substrates of several uptake mechanisms already identified in the liver and kidney such as taurocholate, p-aminohippurate, L-carnitine and tetraethylammonium. These results suggested that GPFX is taken up by the lung partially via carrier-mediated transport system(s), distinct from the identified transporters, and such active transport systems may at least partially account for the efficient distribution of GPFX to the lung.

Inhibition of oat3-mediated renal uptake as a mechanism for drug-drug interaction between fexofenadine and probenecid

Fexofenadine, a nonsedating antihistamine drug, is effective for the treatment of seasonal allergic rhinitis and chronic urticaria. Simultaneous administration of probenecid increases the plasma concentration of fexofenadine due to an inhibition of its renal elimination in healthy volunteers (Clin Pharmacol Ther 77:17-23, 2005). The purpose of the present study is to investigate the possibility that the drug-drug interaction between fexofenadine and probenecid involves the renal basolateral uptake process. The uptake of fexofenadine was determined in HEK293 cells expressing human organic anion transporter 1 (OAT1/SLC22A6), OAT2 (SLC22A7), OAT3 (SLC22A8), and organic cation transporter 2 (OCT2/SLC22A2). Only hOAT3-HEK showed a significantly greater accumulation of fexofenadine than that in vector-HEK, which was saturable with K(m) and V(max) values of 70.2 microM and 120 pmol/min/mg protein, respectively. Inhibition potency of probenecid for the uptake of fexofenadine was compared between hOAT3 and organic anion-transporting peptide 1B3 (hOATP1B3), a transporter responsible for the hepatic uptake of fexofenadine (Drug Metab Dispos 33:1477-1481, 2005). The K(i) values were determined to be 1.30 and 130 microM for hOAT3 and hOATP1B3, respectively, with Hill coefficients of 0.76 and 0.64, respectively. The K(i) value of probenecid for hOAT3, but not for hOATP1B3, was significantly lower than the maximum unbound plasma concentration of probenecid at clinical dosages. These results suggest that the renal drug-drug interaction between fexofenadine and probenecid is probably explained by an inhibition of the renal uptake of fexofenadine via hOAT3, at least in part.

Effects of renal failure on drug transport and metabolism

Renal failure not only alters the renal elimination, but also the non-renal disposition of drugs that are extensively metabolized by the liver. Although reduced metabolic enzyme activity in some cases can be responsible for the reduced drug clearance, alterations in the transporter systems may also be involved in the process. With the development of renal failure, the renal secretion of organic ions mediated by organic anion transporters (OATs) and organic cation transporters (OCTs) is decreased. 3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and other organic anionic uremic toxins may directly inhibit the renal excretion of various drugs and endogenous organic acids by competitively inhibiting OATs. In addition, the expression of OAT1 and OCT2 was reduced in chronic renal failure (CRF) rats. Renal failure also impairs the liver uptake of drugs and organic anions, such as bromosulphophthalein (BSP), indocyanine green (ICG), and thyroxine, where organic anion transport polypeptides (OATPs) are the major transporters. Most previous studies have been done in animals or cell culture, very often in rat models, but these are presumed to reflect the presentation of advanced renal disease in humans as well. Recent studies demonstrate that the uremic toxins CMPF and indoxyl sulfate (IS) can directly inhibit rOatp2 and hOATP-C in hepatocytes. The protein content of the liver uptake transporters Oatp1, 2, and 4 were significantly decreased in CRF rats. Decreased activity of the intestinal efflux transporter, P-glycoprotein (P-gp), was also observed in CRF rats, with no significant change of protein content, suggesting that uremic toxins may suppress P-gp function. However, increased protein levels of multidrug resistance-associated protein (MRP) 2 in the kidney and MRP3 in the liver were found in CRF rats, suggesting an adaptive response that may serve as a protective mechanism. Increases in drug areas under the curve (AUCs) in subjects with advanced renal disease for drugs that are not renally excreted are consistent with uremic toxin effects on either intestinal or hepatic cell transporters, metabolizing enzymes, or both. In conclusion, alterations of drug transporters, as well as metabolic enzymes, in patients with renal failure can be responsible for reduced drug clearance.

Is the Monkey an Appropriate Animal Model to Examine Drug-Drug Interactions Involving Renal Clearance? Effect of Probenecid on the Renal Elimination of H2Receptor Antagonists

The renal drug-drug interaction between famotidine (an H(2) receptor antagonist) and probenecid has not been reproduced in rats. We have proposed that this is caused by a species difference in the transport activity by human/rat organic anion transporter (OAT) 3 and the expression of organic cation transporter (OCT) 1 in the rodent kidney. Since monkey OATs (mkOATs) exhibit similar transport activities to human orthologs, it is hypothesized that in vivo studies in monkeys will allow a more precise prediction of renal drug-drug interactions in humans. Famotidine and cimetidine were efficiently taken up by mkOAT3-expressing human embryonic kidney cells (Km, 154 and 71 microM, respectively), and their uptake was strongly inhibited by probenecid (Ki, 3.0-5.7 microM). Quantification of mkOCT1 and mkOCT2 mRNAs in the monkey kidney using real-time reverse transcription-polymerase chain reaction revealed their predominant expression in the liver and kidney, respectively. Crossover studies were conducted in cynomolgus monkeys. Famotidine was given by i.v. administration, with or without probenecid. Probenecid treatment caused a 65% reduction in the renal clearance (0.426 +/- 0.079 versus 0.165 +/- 0.027 l/h/kg) and a 90% reduction in the tubular secretion clearance (0.275 +/- 0.075 versus 0.0230 +/- 0.0217 l/h/kg), whereas it had no effect on the renal clearance of cimetidine. In contrast to the species-dependent effect of probenecid, allometric scaling using animal data (rat, dog, and monkey) successfully predicted the renal and tubular secretion clearance of famotidine in humans. These results suggest that monkeys are more appropriate animal species for predicting the renal drug-drug interactions in humans.

The human organic anion transporter 3 (OAT3; SLC22A8): genetic variation and functional genomics

The human organic anion transporter, OAT3 (SLC22A8), plays a critical role in renal drug elimination, by mediating the entry of a wide variety of organic anions, including a number of commonly used pharmaceuticals, into the renal proximal tubular cells. To understand the nature and extent of genetic variation in OAT3, and to determine whether such variation affects its function, we identified OAT3 variants in a large, ethnically diverse sample population and studied their transport activities in cellular assays. We identified a total of 10 distinct coding-region variants, which altered the encoded amino acid sequence, in DNA samples from 270 individuals (80 African-Americans, 80 European-Americans, 60 Asian-Americans, and 50 Mexican-Americans). The overall prevalence of these OAT3 variants was relatively low among the screened population, with only three variants having allele frequencies of >1% in a particular ethnic group. Clones of each variant were created by site-directed mutagenesis, expressed in HEK-293 cells, and tested for function using the model substrates, estrone sulfate (ES) and cimetidine (CIM). The results revealed a high degree of functional heterogeneity among OAT3 variants, with three variants (p. Arg149Ser, p. Gln239Stop, and p. Ile260Arg) that resulted in complete loss of function, and several others with significantly reduced function. One of the more common variants (p. Ile305Phe), found in 3.5% of Asian-Americans, appeared to have altered substrate specificity. This variant exhibited a reduced ability to transport ES, but a preserved ability to transport CIM. These data suggest that genetic variation in OAT3 may contribute to variation in the disposition of drugs.

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.

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.

Isoflavone conjugates are underestimated in tissues using enzymatic hydrolysis

Many health effects of soy foods are attributed to isoflavones. Isoflavones upon absorption present as free form, glucuronide, and sulfate conjugates in blood, urine, and bile. Little is known about the molecular forms and the relative concentrations of soy isoflavones in target organs. Acid hydrolysis or enzymatic hydrolysis (glucuronidases and sulfatases) was used to study isoflavone contents in the heart, brain, epididymis, fat, lung, testis, liver, pituitary gland, prostate gland, mammary glands, uterus, and kidney from rats fed diets made with soy protein isolate. The heart had the lowest isoflavone contents (undetectable), and the kidney had the highest (1.8 +/- 0.6 nmol/g total genistein; 3.0 +/- 1.1 nmol/g total daidzein). Acid hydrolysis released 20-60% more aglycon in tissues than enzymatic digestion (p < 0.05), and both hydrolysis methods gave the same level of isoflavones in serum. Approximately 28-44% of the total isoflavone content within the liver was unconjugated aglycon, and the remainder was conjugated mainly as glucuronide. The subcellular distribution of total isoflavones was 55-60% cytosolic and 13-16% in each of the nuclear, mitochondrial, and microsomal fractions. These results demonstrated that (1) soy isoflavones distribute in a wide variety of tissues as aglycon and conjugates and (2) the concentrations of isoflavone aglycons, which are thought to be the bioactive molecules, are in the 0.2-0.25 nmol/g range, far below the concentrations required for most in vitro effects of genistein or daidzein.

A species difference in the transport activities of H2 receptor antagonists by rat and human renal organic anion and cation transporters

A clinical drug-drug interaction between famotidine (a H2 receptor antagonist) and probenecid has not been reproduced in rats. The present study hypothesized that the species-dependent probenecid sensitivity is due to a species difference in the contribution of renal organic anion and cation transporters. The transport activities of the H2 receptor antagonists (cimetidine, famotidine, and ranitidine) by rat and human basolateral organic anion and cation transporters [human organic anion transporter (hOAT) 1, hOAT2, r/hOAT3, rat organic cation transporter (rOct) 1, and r/hOCT2] were compared using their cDNA transfectants. The transport activities (Vmax/Km) of famotidine (Km, 345 microM) by rOat3 were 8- and 15-fold lower than those of cimetidine (Km, 91 microM) and ranitidine (Km, 155 microM), respectively, whereas the activity by hOAT3 (Km, 124 microM) was 3-fold lower than that of cimetidine (Km, 149 microM) but similar to that of ranitidine (Km, 234 microM). Comparison of the relative transport activity with regard to that of cimetidine suggests that famotidine was more efficiently transported by hOAT3 than rOat3, and vice versa, for ranitidine. Only ranitidine was efficiently transported by hOAT2 (Km, 396 microM). rOct1 accepts all of the H2 receptor antagonists with a similar activity, whereas the transport activities of ranitidine and famotidine (Km, 61/56 microM) by r/hOCT2 were markedly lower than that of cimetidine (Km, 69/73 microM). Probenecid was a potent inhibitor of r/OAT3 (Ki, 2.6-5.8 microM), whereas it did not interact with OCTs. These results suggest that, in addition to the absence of OCT1 in human kidney, a species difference in the transport activity by hOAT3 and rOat3 accounts, at least in part, for the species difference in the drug-drug interaction between famotidine and probenecid.

Renal Clearance of Endogenous Hippurate Correlates with Expression Levels of Renal Organic Anion Transporters in Uremic Rats

Hippurate (HA) is a harmful uremic toxin that accumulates during chronic renal failure, and failure of the excretion system for uremic toxins is thought to be responsible. Recently, we reported that rat organic anion transporter 1 (rOat1) is the primary mediator of HA uptake in the kidney, and so now we have studied the pharmacokinetics and tissue distribution of HA after a single i.v. dose of HA to normal and 5/6 nephrectomized rats (5/6Nx rats). In control rats, the renal and biliary clearances of HA were 18.1 and 0.1 ml/min/kg, respectively. Plasma clearance decreased as dosage increased from 0.1 to 5 mg/kg, which suggests that renal tubular secretion is the primary route for elimination of HA. The plasma clearance of HA was significantly decreased in 5/6 Nx rats compared with normal rats. In 5/6 Nx rats, renal clearance of endogenous HA correlated more closely with clearance of p-aminohippurate than with that of creatinine. Protein expression of rOat1 and rOat3, assessed by Western blot analysis, was decreased in 5/6 Nx rats. Furthermore, in 5/6 Nx rats, the renal secretory clearance of endogenous HA correlated closely with protein expression of renal rOats. Thus, HA is primarily eliminated from the plasma via the kidney by active tubular secretion. The renal clearance of endogenous HA seems to be a useful indicator of changes in renal secretion that accompany the reduced levels of OAT protein in chronic renal failure.