Urate, a naturally occurring product of purine metabolism, is a scavenger of biological oxidants implicated in numerous disease processes, as demonstrated by its capacity of neuroprotection. It is present at higher levels in human blood (200 500 microM) than in other mammals, because humans have an effective renal urate reabsorption system, despite their evolutionary loss of hepatic uricase by mutational silencing. The molecular basis for urate handling in the human kidney remains unclear because of difficulties in understanding diverse urate transport systems and species differences. Here we identify the long-hypothesized urate transporter in the human kidney (URAT1, encoded by SLC22A12), a urate anion exchanger regulating blood urate levels and targeted by uricosuric and antiuricosuric agents (which affect excretion of uric acid). Moreover, we provide evidence that patients with idiopathic renal hypouricaemia (lack of blood uric acid) have defects in SLC22A12. Identification of URAT1 should provide insights into the nature of urate homeostasis, as well as lead to the development of better agents against hyperuricaemia, a disadvantage concomitant with human evolution.

Abstract. 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 M, 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.

Renal excretion is an important elimination pathway for antiviral agents, such as acyclovir (ACV), ganciclovir (GCV), and zidovudine (AZT). The purpose of this study was to elucidate the molecular mechanisms of renal ACV, GCV, and AZT transport using cells stably expressing human organic anion transporter 1 (hOAT1), hOAT2, hOAT3, and hOAT4, and human organic cation transporter 1 (hOCT1) and hOCT2. Time-and concentration-dependent uptake of ACV and GCV was observed in hOAT1-and hOCT1-expressing cells. In contrast, uptake of valacyclovir, L-valyl ester of ACV, was observed only in hOAT3-expressing cells. On the other hand, AZT uptake was observed in hOAT1-, hOAT2-, hOAT3-, and hOAT4-expressing cells. The K m values of ACV uptake by hOAT1 and hOCT1 were 342.3 and 151.2 M, respectively, whereas those of GCV uptake by hOAT1 and hOCT1 were 895.5 and 516.2 M, respectively. On the other hand, the K m values of AZT uptake by hOAT1, hOAT2, hOAT3, and hOAT4 were 45.9, 26.8, 145.1, and 151.8 M, respectively. In addition, probenecid weakly inhibited the hOAT1-mediated ACV uptake. In conclusion, these results suggest that hOAT1 and hOCT1 mediate renal ACV and GCV transport, whereas hOAT1, hOAT2, hOAT3, and hOAT4 mediate renal AZT transport. In addition, L-valyl ester appears to be important in differential substrate recognition between hOAT1 and hOAT3. hOAT1 may not be the molecule responsible for the drug interaction between ACV and probenecid.

Abstract. Human organic anion transporter OAT4 is expressed in the kidney and placenta and mediates high-affinity transport of estrone-3-sulfate (E 1 S). Because a previous study demonstrated no trans-stimulatory effects by E 1 S, 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 (S 2 OAT4). OAT4-mediated E 1 S uptake was inhibited by glutarate (GA) (IC 50 : 1.25 mM) and [14 C]GA uptake via S 2 OAT4 was significantly trans-stimulated by unlabeled GA (5 mM) (P<0.001).[3 H]E 1 S uptake via S 2 OAT4 was significantly trans-stimulated by preloaded GA (P<0.001) and its [ 14 C]GA efflux was significantly trans-stimulated by unlabeled E 1 S in the medium (P<0.05). In additon, both the uptake and efflux of [14 C]p-aminohippuric acid (PAH) and [14 C]GA via S 2 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.

Life-threatening drug interactions are known to occur between methotrexate and nonsteroidal anti-inflammatory drugs (NSAIDs), probenecid, and penicillin G. The purpose of this study was to characterize methotrexate transport, as well as to determine the site and the mechanism of drug interactions in the proximal tubule. Mouse proximal tubule cells stably expressing basolateral human organic anion transporters (hOAT1 and hOAT3) and apical hOAT (hOAT4) were established. The K m values for hOAT1-, hOAT3-, and hOAT4-mediated methotrexate uptake were 553.8 M, 21.1 M, and 17.8 M, respectively. NSAIDs (salicylate, ibuprofen, ketoprofen, phenylbutazone, piroxicam, and indomethacin), probenecid, and penicillin G dose dependently inhibited methotrexate uptake mediated by hOAT1, hOAT3, and hOAT4. Kinetic analysis of inhibitory effects of these drugs on hOAT3-mediated methotrexate uptake revealed that these inhibitions were competitive. The K i values for the effects of salicylate, phenylbutazone, indomethacin, and probenecid on hOAT3-mediated methotrexate uptake were comparable with therapeutically relevant plasma concentrations of unbound drugs. In addition, in the presence of human serum albumin, the K i values were comparable with therapeutically relevant total plasma concentrations of drugs. In conclusion, these results suggest that methotrexate is taken up via hOAT3 and hOAT1 at the basolateral side of the proximal tubule and effluxed or taken up at the apical side via hOAT4. In addition, hOAT1, hOAT3, and hOAT4 are the sites of drug interactions between methotrexate and NSAIDs, probenecid, and penicillin G. Furthermore, it was predicted that hOAT3 is the site of drug interactions between methotrexate and salicylate, phenylbutazone, indomethacin, and probenecid in vivo.Methotrexate is widely used at high dosages in the treatment of malignancies, whereas it is used at low dosages in rheumatoid arthritis. Methotrexate is eliminated almost entirely in an unchanged form in urine, which involves glomerular filtration and active tubular secretion (Shen and Azarnoff, 1978). Therefore, renal insufficiency or drug interactions, which reduce the clearance of methotrexate, are potentially toxic events.Interactions between methotrexate and drugs including nonsteroidal anti-inflammatory drugs (NSAIDs), probenecid, and penicillin G have been reported by several groups of investigators (Ellison and Servi, 1985;Thyss et al., 1986;Basin et al., 1991;Frenia and Long, 1992;Tracy et al., 1992;Kremer and Hamilton, 1995). Severe and even life-threatening interactions have been observed, including bone marrow suppression and acute renal failure (Ellison and Servi, 1985;Thyss et al., 1986;Basin et al., 1991;Frenia and Long, 1992). The interactions may have been caused by protein binding displacement, inhibitory effects on the renal secretion of methotrexate, and a decline in glomerular filtration as a result of inhibition of prostaglandin synthesis (Tracy et al., 1992;Kremer and Hamilton, 1995). Among these possible causes, the renal...

The purpose of this study was to elucidate the interactions of human organic anion transporters (hOATs) and human organic cation transporters (hOCTs) with nonsteroidal anti-inflammatory drugs (NSAIDs) using cells stably expressing hOATs and hOCTs. NSAIDs tested were acetaminophen, acetylsalicylate, salicylate, diclofenac, ibuprofen, indomethacin, ketoprofen, mefenamic acid, naproxen, piroxicam, phenacetin, and sulindac. These NSAIDs inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4. By comparing the IC 50 values of NSAIDs for hOATs, it was found that hOAT1 and hOAT3 exhibited higher affinity interactions with NSAIDs than did hOAT2 and hOAT4. HOAT1, hOAT2, hOAT3, and hOAT4 mediated the uptake of either ibuprofen, indomethacin, ketoprofen, or salicylate, but not acetylsalicylate. Although organic cation uptake mediated by hOCT1 and hOCT2 was also inhibited by some NSAIDs, hOCT1 and hOCT2 did not mediate the uptake of NSAIDs. In conclusion, hOATs and hOCTs interacted with various NSAIDs, whereas hOATs but not hOCTs mediated the transport of some of these NSAIDs. Considering the localization of hOATs, it was suggested that the interactions of hOATs with NSAIDs are associated with the pharmacokinetics and the induction of adverse reactions of NSAIDs.Nonsteroidal anti-inflammatory drugs (NSAIDs) have been widely used for their anti-inflammatory and analgesic properties. The indications of NSAIDs are broadening from rheumatic diseases and various pain states, such as cancer pain, and biliary and colic pain, to include possibly Alzheimer's disease and colon cancer prevention (Day et al., 2000). Table 1 shows the chemical structures of NSAIDs tested in the current study. Although all of these NSAIDs are weak organic acids, they are grouped in several classes based on their chemical structures. Although the chemical diversity yields a broad range of pharmacokinetic characteristics (Frust and Munster, 2000), they have some general properties in common. NSAIDs have been shown to induce various forms of adverse drug reactions including adverse gastrointestinal effects (Day et al., 2000), renal dysfunction and nephrotoxicity (Day et al., 2000), liver damage (Zimmerman, 1981;Wood et al., 1985;Purcell et al., 1991;Day et al., 2000), adverse neurological effects (Hoppman et al., 1991;Day et al., 2000), and rhabdomyolysis (Ross and Hoppel, 1987;Leventhal et al., 1989;Delrio et al., 1996).The secretion of numerous organic anions and cations, including endogenous metabolites, drugs, and xenobiotics, is an important physiological function of the renal proximal tubule. The process of secreting organic anions and cations through the proximal tubule cells is achieved via unidirectional transcellular transport involving the uptake of organic anions and cations into the cells from the blood across the basolateral membrane, followed by extrusion across the brush-border membrane into the proximal tubule fluid (Pritchard and Miller, 1993). Recently, cDNAs encoding the human organic anion transporter (hOAT) fa...

The tubular secretion of diuretics in the proximal tubule has been shown to be critical for the action of drugs. To elucidate the molecular mechanisms for the tubular excretion of diuretics, we have elucidated the interactions of human organic anion transporters (hOATs) with diuretics using cells stably expressing hOATs. Diuretics tested were thiazides, including chlorothiazide, cyclothiazide, hydrochlorothiazide, and trichlormethiazide; loop diuretics, including bumetanide, ethacrynic acid, and furosemide; and carbonic anhydrase inhibitors, including acetazolamide and methazolamide. These diuretics inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4 in a competitive manner. hOAT1 exhibited the highest affinity interactions for thiazides, whereas hOAT3 did those for loop diuretics. hOAT1, hOAT3, and hOAT4 but not hOAT2, mediated the uptake of bumetanide. hOAT3 and hOAT4, but not hOAT1 mediated the efflux of bumetanide. hOAT1 and hOAT3, but not hOAT2 and hOAT4 mediated the uptake of furosemide. In conclusion, it was suggested that hOAT1 may play an important role in the basolateral uptake of thiazides, and hOAT3 in the uptake of loop diuretics. In addition, it was also suggested that bumetanide taken up by hOAT3 and/or hOAT1 is excreted into the urine by hOAT4.

The organic anion transport system is involved in the tubular excretion and reabsorption of various drugs and substances. The purpose of this study was to characterize the effects of various organic anion transport inhibitors on renal organic anion transport using proximal tubule cells stably expressing human organic anion transporter 2 (hOAT2) and hOAT4. Immunohistochemical analysis revealed that hOAT2 is localized to the basolateral side of the proximal tubule in the kidney. hOAT2 mediated a time-and concentration-dependent increase in prostaglandin F 2␣ (PGF 2␣ ) uptake. The organic anion transport inhibitors used for this study were probenecid, 8-(noradamantan-3-yl)-1,3-dipropylxanthine (KW-3902), betamipron, and cilastatin. Probenecid, but not KW-3902, betamipron, and cilastatin, significantly inhibited hOAT2-mediated PGF 2␣ uptake. In contrast, probenecid, KW-3902, and betamipron, but not cilastatin, inhibited hOAT4-mediated estrone sulfate (ES) uptake. Kinetic analyses revealed that these inhibitions were competitive. The K i value of probenecid for hOAT2 was 766 M, whereas those of probenecid, KW-3902, and betamipron for hOAT4 were 54.9, 20.7, and 502 M, respectively. These results suggest that probenecid, KW-3902, and betamipron could inhibit hOAT4-mediated ES uptake in vitro, whereas probenecid alone could inhibit the hOAT2-mediated PGF 2␣ uptake. Comparing the K i values with the therapeutically relevant concentrations of unbound inhibitors in the plasma, probenecid alone was predicted to inhibit hOAT4-mediated organic anion transport in vivo.Various organic anion transport inhibitors are used experimentally and clinically. Probenecid is a conventional and standard organic anion transport inhibitor experimentally, but it is used as a uricosuric drug clinically. In addition, KW-3902, developed as an adenosine A1 receptor antagonist , was also shown to inhibit organic anion transport in the basolateral membrane of opossum kidney cells derived from the American opossum kidney (Nagai et al., 1999). On the other hand, betamipron and cilastatin are administered in combination with carbapenem antibiotics, panipenem, and imipenem, respectively (Birnbaum et al., 1985;Shiba et al., 1991). Betamipron inhibits the uptake of panipenem and imipenem into proximal tubule cells (Hirouchi et al., 1994). On the other hand, imipenem is degraded by human renal dehydropeptidase-I and therefore must be administered in combination with cilastatin, a dehydropeptidase-I inhibitor, to prevent loss of antimicrobial activity in urine and limit potential nephrotoxicity associated with renal metabolism (Craig, 1997).In our previous study, we elucidated the interaction of human organic anion transporter 1 (hOAT1) and hOAT3 with organic anion transport inhibitors including probenecid, KW-3902, betamipron, and cilastatin (Takeda et al., 2001). Thus, the purpose of this study was to characterize the interaction of hOAT2 and hOAT4 with these organic anion transport inhibitors using cells derived from the second portion of the pr...