Thyroid Hormone Transport by the Rat Fatty Acid Translocase

We examined the hypothesis that rat fatty acid translocase (rFAT) mediates the cellular uptake of T3 and other iodothyronines. Uninjected Xenopus laevis oocytes and oocytes injected 4 d previously with rFAT cRNA were incubated for 60 min at 25 C in medium containing 0.01–10 μm [125I]T3 and 0.1% BSA, or 1–100 μm [3H]oleic acid and 0.5% BSA. Injection of rFAT cRNA resulted in a 1.9-fold increase in uptake of T3 (10 nm) and a 1.4-fold increase in uptake of oleic acid (100 μm). Total T3 uptake was lower in the presence than in the absence of BSA, but relative to the free T3 concentration, uptake was increased by BSA. The fold induction of T3 uptake by rFAT was not influenced by BSA. By analyzing uptake as a function of the ligand concentration, we estimated a Km value of 3.6 μm for (total) T3 and 56 μm for (total) oleic acid. In addition to T3, rFAT mediates the uptake of T4, rT3, 3,3′-diiodothyronine, and T3 sulfate. The injection of human type III deiodinase cRNA with or without rFAT cRNA resulted in the complete deiodination of T3 taken up by the oocytes, indicating that T3 is indeed transported to the cytoplasm. In conclusion, our results demonstrate transport of T3 and other iodothyronines by rFAT.