Identification of thyroid hormone transporters.

Thyroid hormone action and metabolism are intracellular events that require transport of the hormone across the plasma membrane. We tested the possible involvement of the Na+/taurocholate cotransporting polypeptide (Ntcp) and organic anion transporting polypeptide (oatp1) in the hepatic uptake of the prohormone T4, the active hormone T3, and the metabolites rT3 and 3,3'-T2. Xenopus laevis oocytes were injected with 2.3 ng Ntcp or oatp1 cRNA and, after 2-3 days, incubated for 1 h at 25 degrees C with usually 0.1 microM 125I-labeled ligand. Uninjected oocytes showed marked uptake of iodothyronines and this was further increased by Ntcp and oatp1 cRNA, i.e., 1.9- and 2.8-fold for T4, 1.7- and 1.7-fold for T3, 1.8- and 6.0-fold for rT3, and 1.3- and 1.4-fold for 3,3'-T2, respectively. Mostly due to much lower uptake by uninjected oocytes, Ntcp and oatp1 cRNA induced larger, 12- to 76-fold increases in uptake of iodothyronine sulfates. The Ntcp cRNA-induced iodothyronine uptake was completely inhibited in Na+-deplete medium, whereas the oatp1 cRNA-induced uptake was not affected. These results suggest that hepatic uptake of thyroid hormones and their metabolites is mediated at least in part by Ntcp and oatp1. Copyright 1999 Academic Press.