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.

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 (E 1 S) 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 M, and 41.7 M, 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 E 1 S transport activities in HEK293 cells T he human organic anion transporter OAT4 (encoded by SLC22A11) is expressed in the kidney and the placenta and mediates the high-affinity transport of steroid sulfates such as estrone-3-sulfate (E 1 S) and dehydroepiandrosterone sulfate (DHEAS) (1). Because of its apical membrane localization in the renal proximal tubules (2), OAT4 had been presumed to be the apical exit pathway of organic anions that are taken up by the basolateral entrance pathway such as OAT1 and OAT3 (3). Recently, Ugele et al. (4) found the OAT4 protein expression at the fetal side of the syncytiotrophoblasts in the placenta and proposed a role for OAT4 in the placental uptake of fetal-derived steroid sulfates. On the basis of our finding that OAT4 is an apical organic anion/dicarboxylate exchanger (5), we suggested that OAT4 mainly functions as an apical entrance pathway for some organic anions in renal proximal tubules driven by an outwardly directed dicarboxylate gradient created by Na ϩ /dicarboxylate co-transporters (6). One possible role of OAT4 is as an apical backflux pathway (7) for some organic anions such as steroid sulfates, functionally coupled to the apical efflux transporters for organic anions, such as MRP2, MRP4, and NPT1 (putative human homologue of OAT V 1) (8 -13).At the extreme C-terminal (CT) end, OAT4 has a specific protein-protein interaction peptide sequence named the PDZ (PSD-95/Discs Large/ZO-1) motif (S-T-S-L) (14). PDZ domains have been identified in various proteins and are known to be modular protein-protein recognition domains that play roles in protein targeting and protein complex assembly (15-17). These multidomain molecules not only target and provide scaffolds for protein-protein interactions but also modulate the function of receptors and ion channels, by which they associate. Recently, we reported that the urate/anion exchanger URAT1 (18), which has a similar PDZ motif (S-T-Q-F) at its C-terminus, interacts with multivalent PDZ protein PDZK1 (19,20). In the same study, w...