Human Renal Organic Anion Transporter 4 Operates as an Asymmetric Urate Transporter Yohannes Hagos,* Daniel Stein,* Bernhard Ugele, † Gerhard Burckhardt,* and Andrew Bahn* *Zentrum Physiologie und Pathophysiologie, Abteilung Vegetative Physiologie und Pathophysiologie, Go ¨ttingen, and † Klinikum der Universita ¨t Mu ¨ nchen, I. Frauenklinik-Innenstadt, Mu ¨ nchen, Germany 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 M. 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 pro- benecid, 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. J Am Soc Nephrol 18: 430 – 439, 2007. doi: 10.1681/ASN.2006040415 A ctive tubular transport processes determine secretion or reabsorption of a large variety of substances, a substantial fraction of which is represented by or- ganic anions (OA), such as antibiotics, diuretics, antiviral drugs, cytostatics, and several endogenous metabolites. Pro- teins that are known to deal with these multiple compounds belong to the family of organic anion transporters (OAT) (1–3). They are expressed along the proximal tubule of the kidneys and in other bordering epithelia as such blood-brain barrier, choroid plexus, and placenta. OAT1 and OAT3 are located at the basolateral membrane of renal proximal tubule cells. Both transporters show overlapping substrate specificities and share the mode of transport and driving force, exchanging OA against -ketoglutarate (4), representing the first step of renal OA secretion. The luminal exit of OA is not well characterized. Besides OAT2 (luminal localization in rodents [5]), the voltage- dependent OAT v 1 (related to NPT1 in human [6,7]), and the multidrug resistance–associated proteins 2 and 4 (8), other OAT members, namely hOAT4, OAT5, and URAT1, have been doc- umented to be involved in apical OA release as well as in reabsorption processes (9 –12). hURAT1 was discovered recently as apical urate transporter, playing a pivotal role in urate homeostasis (11,13,14). Mutations in the hURAT1 gene diminish renal urate reabsorption, resulting in hypouricemia (11). hURAT1 operates as an exchanger, which takes up urate from the primary urine in exchange for intracellular nicotinate or lactate but not -ketoglutarate, indicating different driving forces for OAT at the luminal and basolateral side of proximal tubule cells. Accordingly, the apically expressed rOAT5 exchanges organic anions against succinate, a dicarboxylate that is not accepted by OAT1 and OAT3 (10). hOAT4 was identified as apical transporter in the proximal tubule cells (15), which is expressed only in higher primates, including human. It facilitates the reabsorption of conjugated steroids such as estrone sulfate (ES) or dehydroepiandrosterone sulfate with high affinity (12). Moreover, hOAT4 was detected in placenta, where it may be involved in the uptake of fetal- derived steroid sulfates (16). The driving force of hOAT4-me- diated transport is controversial, and its physiologic role is still unknown. hOAT4 is closely related to hURAT1, and their genes are located as a tandem on chromosome 11q13 (17). Recently, Eraly et al. (17) postulated that OAT, which are genomically clustered in pairs, should have similar functional characteris- tics. This assumption holds for OAT1 and OAT3, but published functional data of hOAT4 and URAT1 so far do not support this hypothesis (11,12). Received May 11, 2006. Accepted November 8, 2006. Published online ahead of print. Publication date available at www.jasn.org. Address correspondence to: Dr. Yohannes Hagos, Zentrum fu ¨ r Physiologie und Pathophysiologie, Abt. Vegetative Physiologie und Pathophysiologie, Universita ¨t Go ¨ ttingen, Humboldtallee 23, 37073 Go ¨ ttingen, Germany. Phone: +49-551- 395894; Fax: +49-551-395883; E-mail: hagos@physiol.med.uni-goettingen.de Copyright © 2007 by the American Society of Nephrology ISSN: 1046-6673/1802-0430