Characterization of hOAT4 and mOat5 as functional orthologs for renal anion uptake and efflux transport

Organic anions (OA) are compounds including drugs or toxicants that are negatively charged at physiological pH and are typically transported by Organic Anion Transporters (OATs). Human OAT4 (SLC22A11) is expressed in the apical membrane of renal proximal tubules. Although there is no rodent ortholog of hOAT4, rodents express Oat5 (Slc22a19), an anion exchanger that is also localized to the apical membrane of renal proximal tubule cells. The purpose of this study was to determine the functional similarity between mouse Oat5 and human OAT4. Chinese hamster ovary (CHO) cells expressing SLC22A11 or Slc22a19 were used to assess the transport characteristics of radiolabeled ochratoxin (OTA). We determined the kinetics of OTA transport; the resulting K_t and J_max values were very similar for both hOAT4 and mOat5: K_t 3.9 and 7.2 µM, respectively, & J_max 4.4 and 3.9 pmol/cm², respectively. For the profile of OTA inhibition by OAs, IC₅₀ values were determined for several clinically important drugs and toxicants. The resulting IC₅₀ values ranged from 9 µM for indomethacin to ~600 µM for the diuretic hydrochlorothiazide. We measured the efflux of OTA from preloaded cells; both hOAT4 and mOat5 supported the efflux of OTA. These data support the hypothesis that OAT4 and Oat5 are functional orthologs and share selectivity for OTA both for reabsorption and secretion. Significance Statement This study compares the selectivity profile between human OAT4 and mouse Oat5. Our data revealed a similar selectivity profile for OTA reabsorption and secretion by these two transporters, thereby supporting the hypothesis that hOAT4 and mOat5, while not genetic orthologs, behave as functional orthologs for both uptake and efflux. These data will be instrumental in selecting an appropriate animal model when studying the renal disposition of anionic drugs and toxicants.