Identification of monocarboxylate transporter 8 as a specific thyroid hormone transporter

Transport of thyroid hormone across the cell membrane is required for its action and metabolism. Recently, a T-type amino acid transporter was cloned which transports aromatic amino acids but not iodothyronines. This transporter belongs to the monocarboxylate transporter (MCT) family and is most homologous with MCT8 (SLC16A2). Therefore, we cloned rat MCT8 and tested it for thyroid hormone transport in Xenopus laevis oocytes. Oocytes were injected with rat MCT8 cRNA, and after 3 days immunofluorescence microscopy demonstrated expression of the protein at the plasma membrane. MCT8 cRNA induced an similar to10-fold increase in uptake of 10 nM I-125-labeled thyroxine (T-4), 3,3',5-triiodothyronine (T-3), 3,3',5similar to-triiodothyronine (rT(3)) and 3,3'-diiodothyronine. Because of the rapid uptake of the ligands, transport was only linear with time for <4 min. MCT8 did not transport Leu, Phe, Trp, or Tyr. [I-125]T-4 transport was strongly inhibited by L-T-4, D-T-4, L-T-3, D-T-3, 3,3',5-triiodothyroacetic acid, N-bromoacetyl-T-3, and bromosulfophthalein. T-3 transport was less affected by these inhibitors. Iodothyronine uptake in uninjected oocytes was reduced by albumin, but the stimulation induced by MCT8 was markedly increased. Saturation analysis provided apparent K-m values of 2-5 μM for T-4, T-3, and rT(3). Immunohistochemistry showed high expression in liver, kidney, brain, and heart. In conclusion, we have identified MCT8 as a very active and specific thyroid hormone transporter.