Thyroid Hormone Transport by the Heterodimeric Human System L Amino Acid Transporter

Abstract

Transport of thyroid hormone across the cell membrane is required for thyroid hormone action and metabolism. We have investigated the possible transport of iodothyronines by the human system L amino acid transporter, a protein consisting of the human 4F2 heavy chain and the human LAT1 light chain. Xenopus oocytes were injected with the cRNAs coding for human 4F2 heavy chain and/or human LAT1 light chain, and after 2 d were incubated at 25 C with 0.01-10 microM [(125)I]T(4), [(125)I]T(3), [(125)I]rT(3), or [(125)I]3,3'-diiodothyronine or with 10-100 microM [(3)H]arginine, [(3)H]leucine, [(3)H]phenylalanine, [(3)H]tyrosine, or [(3)H]tryptophan. Injection of human 4F2 heavy chain cRNA alone stimulated the uptake of leucine and arginine due to dimerization of human 4F2 heavy chain with an endogenous Xenopus light chain, but did not affect the uptake of other ligands. Injection of human LAT1 light chain cRNA alone did not stimulate the uptake of any ligand. Coinjection of cRNAs for human 4F2 heavy chain and human LAT1 light chain stimulated the uptake of phenylalanine > tyrosine > leucine > tryptophan (100 microM) and of 3,3'-diiodothyronine > rT(3) approximately T(3) > T(4) (10 nM), which in all cases was Na(+) independent. Saturation analysis provided apparent Michaelis constant (K(m)) values of 7.9 microM for T(4), 0.8 microM for T(3), 12.5 microM for rT(3), 7.9 microM for 3,3'-diiodothyronine, 46 microM for leucine, and 19 microM for tryptophan. Uptake of leucine, tyrosine, and tryptophan (10 microM) was inhibited by the different iodothyronines (10 microM), in particular T(3). Vice versa, uptake of 0.1 microM T(3) was almost completely blocked by coincubation with 100 microM leucine, tryptophan, tyrosine, or phenylalanine. Our results demonstrate stereospecific Na(+)-independent transport of iodothyronines by the human heterodimeric system L amino acid transporter.

Involvement of the L-Type Amino Acid Transporter Lat2 in the Transport of 3,3′-Diiodothyronine across the Plasma Membrane

“…By testing different iodothyronine concentrations (10-100 n M ) we could not enhance the uptake of T 3 , which remained low, while rT 3 and T 4 are not transported by Lat2. These results support and extend similar studies with human LAT1 and human LAT2 [13,18,19]. …”

“…Oocytes represent a well-described system that is widely used in transport studies. They have been used to characterize the thyroid hormone transporters MCT8 and LAT1 in vitro [13,16]. The endogenous expression of transport proteins in this system is low, leading to only minor levels of background transport activity [17].…”

“…In contrast to Lat2, LAT1 markedly induced the uptake of rT 3 when coexpressed with human CD98 in X. laevis oocytes, but only to a small extent the uptake of T 3 and T 4 [13]. As previously shown, thyroid hormones T 3 and T 4 show a better competition for leucine uptake in LAT1 than in LAT2 [18,19].…”

“…Lat2 is not only expressed in the brain, but also in the kidney, intestine and liver [7,8]. As already mentioned, studies indicate the involvement of Lat2 in the transport of thyroid hormones [7,13], but its role in the transport of these hormones, such as T 3 or the metabolite 3,3′-T 2 , including its thyroid hormone substrate specificity, is still unclear. So far, transport studies by LAT2 have mainly been performed in oocytes from Xenopus laevis [10,11,13].…”

“…As already mentioned, studies indicate the involvement of Lat2 in the transport of thyroid hormones [7,13], but its role in the transport of these hormones, such as T 3 or the metabolite 3,3′-T 2 , including its thyroid hormone substrate specificity, is still unclear. So far, transport studies by LAT2 have mainly been performed in oocytes from Xenopus laevis [10,11,13]. Using this cell system and the known LAT inhibitor 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) we observed a preferential uptake of 3,3′-T 2 by Lat2 and its strong competition by thyroid hormone metabolites, such as 3-T 1 and other T 2 isomers.…”