The Blood-Brain Barrier Thyroxine Transporter Organic Anion-Transporting Polypeptide 1c1 Displays Atypical Transport Kinetics

Organic anion-transporting polypeptide (Oatp) 1c1 is a high-affinity T-4 transporter expressed in brain barrier cells. Oatp1c1 transports a variety of additional ligands including the conjugated sterol estradiol 17 beta-glucuronide (E(2)17 beta G). Intriguingly, published data suggest that E(2)17 beta G inhibition of Oatp1c1-mediated T-4 transport exhibits characteristics suggestive of atypical transport kinetics. To determine whether Oatp1c1 exhibits atypical transport kinetics, we first performed detailed T-4 and E(2)17 beta G uptake assays using Oatp1c1 stably transfected HEK293 cells and a wide range of T-4 and E(2)17 beta G concentrations (100 pM to 300 nM and 27 nM to 200 mu M, respectively). Eadie-Hofstee plots derived from these detailed T-4 and E(2)17 beta G uptake experiments display a biphasic profile consistent with atypical transport kinetics. These data along with T-4 and E(2)17 beta G cis-inhibition dose-response measurements revealed shared high-and low-affinity Oatp1c1 binding sites for T-4 and E(2)17 beta G. T-4 and E(2)17 beta G recognized these Oatp1c1 binding sites with opposite preferences. In addition, sterols glucuronidated in the 17 or 21 position, exhibited preferential substrate-dependent inhibition of Oatp1c1 transport, inhibiting Oatp1c1-mediated E(2)17 beta G transport more strongly than T-4 transport. Together these data reveal that Oatp1c1-dependent substrate transport is a complex process involving substrate interaction with multiple binding sites and competition for binding with a variety of other substrates. A thorough understanding of atypical Oatp1c1 transport processes and substrate-dependent inhibition will allow better prediction of endo-and xenobiotic interactions with the Oatp transporter. (Endocrinology 150: 5153-5162, 2009)