Ho Jung Shin

Novel liver‐specific organic anion transporter OAT7 that operates the exchange of sulfate conjugates for short chain fatty acid butyrate

The liver plays an important role in the elimination of endogenous and exogenous lipophilic organic compounds from the body, which is mediated by various carrier proteins that differ in substrate specificity and kinetic properties. Here, we have characterized a novel member of the organic anion transporter family (SLC22) isolated from human liver. The transporter named organic anion transporter 7 (OAT7/ SLC22A9) showed 35% to 46% identities to those of other organic anion transporters of SLC22 family. When expressed in Xenopus oocytes, OAT7 mediated Na+‐independent, high‐affinity transport of sulfate‐conjugated steroids, estrone sulfate (ES; Km = 8.7 μM), and dehydroepiandrosterone sulfate (Km = 2.2 μM). In addition, OAT7 interacted with negatively charged sulfobromophthalein, indocyanine green, and several sulfate‐conjugated xenobiotics. In contrast, glucuronide and glutathione conjugates exhibited no inhibitory effects on OAT7‐mediated [3H]ES transport. OAT7‐mediated [3H]ES transport was trans‐stimulated by three‐carbon to five‐carbon (C3 to C5) short‐chain fatty acids. The efflux of [14C]butyrate (C4) via OAT7 was significantly trans‐stimulated by extracellular ES. Furthermore, OAT7 mediated [14C]butyrate uptake and [3H]ES efflux in exchange for extracellular butyrate both in Xenopus oocytes and OAT7‐stably expressing cells. OAT7 protein was localized in the sinusoidal membrane of hepatocytes by immunohistochemical analysis. Conclusion: OAT7 is the first liver‐specific transporter among members of the organic anion transporters of SLC22 family. Our findings suggest a new class of substrates for organic anion transporters and provide evidence for the transport of anionic substances such as sulfate‐conjugates in exchange for butyrate in hepatocytes. (HEPATOLOGY 2007;45:1046–1055.)

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.

Interactions of urate transporter URAT1 in human kidney with uricosuric drugs.

Aim:  Hyperuricaemia is a significant factor in a variety of diseases, including gout and cardiovascular diseases. The kidney plays a dominant role in maintaining plasma urate levels through the excretion process. Human renal urate transporter URAT1 is thought to be an essential molecule that mediates the reabsorption of urate on the apical side of the proximal tubule. In this study the pharmacological characteristics and clinical implications of URAT1 were elucidated.

Identification of a Novel Organic Anion Transporter Mediating Carnitine Transport in Mouse Liver and Kidney

This study identifies a novel organic anion transporter Oat9 expressed in mouse liver and kidney. Two variants were detected by screening a mouse liver cDNA library; these varients consist of 1815 (designated Oat9S) and 2165 (Oat9L) base pairs which encode 443 and 551 amino acid proteins, respectively. Oat9S has a predicted structure containing eight transmembrane domains (TMD); whereas, Oat9L possesses twelve TMD. Oat9 mRNA expression was detected in kidney and liver. This transporter was located at the apical side of the late portion of proximal tubules and at the sinusoidal side of hepatocytes. When expressed in Xenopus oocytes, Oat9S mediated the transport of L-carnitine (Km = 2.9 µM), a representative zwitterion, as well as cimetidine (Km = 16.1 µM) and salicylic acid (Km = 175.5 µM), while Oat9L did not show any transport activity. Oat9S-mediated L-carnitine uptake was inhibited by D-carnitine, acetylcarnitine, octanoylcarnitine, betaine, and other organic compounds, suggesting that quaternary ammonium cation bulkiness and relative hydrophobicity are important factors for Oat9S-substrate interactions. Among OATs, Oat9S appears to be the first member to mediate the transport of carnitine and possesses eight TMD. Overall, these new results provide added insight into the structure-activity relationship comprising the organic ion-permeation pathway.