Human renal organic anion transporter 4 operates as an asymmetric urate transporter

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 mu M. 6-CF as well as [H-3]estrone sulfate ([H-3]ES) accumulation by HEK293-hOAT4 cells were abolished by ES, dehydroepiandrosterone sulfate, sulfinpyrazone, benzbromarone, and probenecid, 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 [H-3]ES uptake, indicating an asymmetric interaction of hOAT4 with these substrates. In chloride-free medium, HEK293-hOAT4-mediated [H-3]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 [H-3]ES uptake, which was reduced by nigericin, suggesting that hOAT4 also can operate as an OA/OH- exchanger. hOAT4 facilitates substantial uptake of [C-14]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.