Functional characterization of human organic anion transporter 4 (hOAT4) as a pH-dependent citrate transporter in renal proximal tubules

Citrate plays a crucial role in preventing calcium stone formation by chelating calcium and inhibiting crystal aggregation. Organic anion transporters (OATs), expressed in renal proximal tubules, mediate the transport of various organic anions, including drugs. However, whether OATs transport citrate and oxalate-key metabolites involved in urinary stone formation-remains unclear. This study aimed to determine whether OATs contribute to citrate and oxalate transport and to clarify their underlying mechanisms. Uptake experiments were conducted using S2 cells stably expressing human OAT1, OAT3, or OAT4. Substrate uptake was measured using radiolabeled compounds, and [¹⁴C]citrate uptake was evaluated for time and concentration dependence, and inhibition by various compounds. OAT4, localized to the apical membrane, exhibited significantly increased [¹⁴C]citrate uptake under acidic conditions (pH 6.0), whereas OAT1 and OAT3 showed minimal pH dependence. OAT4-mediated citrate uptake increased in a time-dependent manner and showed biphasic kinetics. Citrate transport was unaffected by endogenous dicarboxylates but inhibited by diuretics and nonsteroidal anti-inflammatory drugs. These findings identify OAT4 as a novel citrate transporter. Enhanced OAT4 activity under acidic conditions may promote citrate reabsorption, thereby increasing the risk of calcium stone formation. Targeting OAT4 with specific inhibitors could represent a new therapeutic strategy for preventing urinary stone recurrence.