A novel member of the organic anion transporter (OAT) family, Oat5 (Slc22a19), has been reported to transport a naturally occurring mycotoxin, ochratoxin A (OTA). However, neither its endogenous substrate and driving force nor physiological functions have been determined. Herein, we report the functional characterization of rat Oat5 (rOat5), as well as its intrarenal distribution and membrane localization. When expressed in Xenopus laevis oocytes, rOat5 mediated the transport of sulfate conjugates of steroids such as estrone-3-sulfate (E 1 S; K m ϭ 18.9 Ϯ 3.9 M) and dehydroepiandrosterone sulfate (K m ϭ 2.3 Ϯ 0.2 M) in a sodium-independent manner, in addition to OTA. The rOat5-mediated E 1 S transport was strongly inhibited by four-carbon (C4) dicarboxylate succinate and longer dicarboxylates (C7-C9). The uptake of [ 3 H]E 1 S via rOat5 was significantly trans-stimulated by succinate, and the efflux of [ 14 C]succinate was significantly trans-stimulated by E 1 S. A similar trans-stimulatory effect of preloaded succinate on E 1 S uptake was also detected in cells stably expressing rOat5 (S 2 rOat5). rOat5 interacted with chemically heterogenous anionic compounds. The rOat5-mediated E 1 S transport was inhibited by several sulfate conjugates, such as 4-methylumbelliferyl sulfate and ␤-estradiol sulfate, but not by glucuronide conjugates. An immunohistochemical study showed that rOat5 was localized at the apical membrane of renal proximal tubules in the corticomedullary region. rOat5 mRNA was expressed in the late segments (S 2 and S 3 ) of proximal tubules. These results indicate that rOat5 is renal organic anion/dicarboxylates exchanger and, under physiological conditions, may function as an apical reabsorptive pathway for organic anions in proximal tubules driven by an outward gradient of dicarboxylates.The kidney plays an important role in the elimination of harmful endogenous compounds and xenobiotics from the body. The proximal tubule is the primary site where numerous organic anions are taken up from the blood and excreted into the urine (Wright and Dantzler, 2004). The transcellular secretion of organic anions in the proximal tubule involves a two-step process: 1) the uptake of organic anions across the basolateral membrane of proximal tubular cells and 2) their excretion across the apical membrane into the tubular lumen. The former is energetically uphill and is accomplished by a tertiary active process via the organic anion/dicarboxylate exchanger that uses the outwardly directed gradient of

We identified a novel prostaglandin (PG)-specific organic anion transporter (OAT) in the OAT group of the SLC22 family. The transporter designated OAT-PG from mouse kidney exhibited Na ؉ -independent and saturable transport of PGE 2 when expressed in a proximal tubule cell line (S 2 ). Unusual for OAT members, OAT-PG showed narrow substrate selectivity and high affinity for a specific subset of PGs, including PGE 2 , PGF 2␣ , and PGD 2 . Similar to PGE 2 receptor and PGT, a structurally distinct PG transporter, OAT-PG requires for its substrates an ␣-carboxyl group, with a double bond between C13 and C14 as well as a (S)-hydroxyl group at C15. Unlike the PGE 2 receptor, however, the hydroxyl group at C11 in a cyclopentane ring is not essential for OAT-PG substrates. Addition of a hydroxyl group at C19 or C20 impairs the interaction with OAT-PG, whereas an ethyl group at C20 enhances the interaction, suggesting the importance of hydrophobicity around the -tail tip forming a "hydrophobic core" accompanied by a negative charge, which is essential for substrates of OAT members. OAT-PG-mediated transport is concentrative in nature, although OAT-PG mediates both facilitative and exchange transport. OAT-PG is kidney-specific and localized on the basolateral membrane of proximal tubules where a PG-inactivating enzyme, 15-hydroxyprostaglandin dehydrogenase, is expressed. Because of the fact that 15-keto-PGE 2 , the metabolite of PGE 2 produced by 15-hydroxyprostaglandin dehydrogenase, is not a substrate of OAT-PG, the transport-metabolism coupling would make unidirectional PGE 2 transport more efficient. By removing extracellular PGE 2 , OAT-PG is proposed to be involved in the local PGE 2 clearance and metabolism for the inactivation of PG signals in the kidney cortex.

Abstract. Renal organic anion transporters play an important role in the handling of a number of endogenous and exogenous anionic substances in the kidney. In this study, we investigated prostaglandin E 2 (PGE 2 ) transport properties and intrarenal localization of mouse organic anion transporter 3 (mOat3). When expressed in Xenopus oocytes, mOat3 mediated the time-and concentration-dependent transport of PGE 2 (K m : 1.48 µM). PGE 2 transport mediated by mOat3 was trans-stimulated by intracellular glutarate injected into the oocytes. PGE 2 efflux via mOat3 was also trans-stimulated by extracellular glutarate. Thus, mOat3 was shown to mediate the bidirectional transport of PGE 2 , partly coupled to the dicarboxylate exchange mechanism. Immunohistochemical study revealed that mOat3 protein was localized at the basolateral membrane of renal proximal and distal tubules. Furthermore, diffuse expression of mOat3, including expression in the basolateral membrane in macula densa (MD) cells, was observed. These results indicate that mOat3 plays an important role as a basolateral transport pathway of PGE 2 in the distal nephron including MD cells that may constitute one of the indispensable steps for renin release and regulation of the tubuloglomerular feedback mechanism.

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