Functional coupling of organic anion transporter OAT10 (SLC22A13) and monocarboxylate transporter MCT1 (SLC16A1) influencing the transport function of OAT10.

OAT10 (SLC22A13) is a transporter highly expressed in renal tubules and transporting organic anions including nicotinate, β-hydroxybutyrate, p-aminohippurate, and orotate. In transport assays using Xenopus oocytes and HEK293 cells, we found that apparent substrate selectivity of OAT10 was different between the expression systems, particularly less pronounced uptake of β-hydroxybutyrate in HEK293 cells. Because functional coupling between transporters may interfere with functional properties of the transporter, we searched for endogenous transporters in HEK293 cells that could affect OAT10. By means of comprehensive approach with co-immunoprecipitation followed by LC-MS/MS analysis, we identified monocarboxylate transporter MCT1 (SLC16A1) as physically coupled with OAT10. The knockdown of MCT1 in OAT10-expressing HEK293 cells increased the uptake of β-hydroxybutyrate and nicotinate, common substrates of OAT10 and MCT1, whereas the uptake of orotate, a substrate of only OAT10, was not affected. MCT1 is supposed to act as an escape route and mediate the efflux of nicotinate and β-hydroxybutyrate taken up by OAT10 localized nearby MCT1, as suggested by co-immunoprecipitation. This functional coupling would explain altered apparent substrate selectivity in HEK293 cells compared with Xenopus oocytes. The findings in this study warn in transporter studies that the expression system can interfere with assessing correct transport properties due to unexpected interactions with endogenous transporters.