Comparison of mechanisms mediating uptake and efflux of thyroid hormones in the human choriocarcinoma cell line, JAR

Abstract

We compared the specificities of transport mechanisms for uptake and efflux of thyroid hormones in cells of the human choriocarcinoma cell line, JAR, to determine whether triiodothyronine (T 3 ), thyroxine (T 4 ) and reverse T 3 (rT 3 ) are carried by the same transport mechanism. Uptake of 125 I-T 3 , 125 I-T 4 and 125 I-rT 3 was saturable and stereospecific, but not specific for T 3 , T 4 and rT 3 , as unlabelled -stereoisomers of the thyroid hormones inhibited uptake of each of the radiolabelled hormones. Efflux of 125 I-T 3 was also saturable and stereospecific and was inhibited by T 4 and rT 3 . Efflux of 125 I-T 4 or 125 I-rT 3 was, in contrast, not significantly inhibited by any of the unlabelled thyroid hormones tested. A range of compounds known to interfere with receptor-mediated thyroid hormone uptake in cells inhibited uptake of 125 I-T 3 and 125 I-rT 3 , but not 125 I-T 4 . We conclude that in JAR cells uptake and efflux of 125 I-T 3 are mediated by saturable and stereospecific membrane transport processes. In contrast, the uptake, but not the efflux, of 125 I-T 4 and 125 I-rT 3 is saturable and stereospecific, indicating that uptake and efflux of T 4 and rT 3 in JAR cells occur by different mechanisms. These results suggest that in JAR cells thyroid hormones may be transported by at least two types of transporters: a low affinity iodothyronine transporter (Michaelis constant, K m , around 1 µM) which interacts with T 3 , T 4 and rT 3 , but not amino acids, and an amino acid transporter which takes up T 3 , but not T 4 or rT 3 . Efflux of T 4 and rT 3 appears to occur by passive diffusion in these cells.

Different transporters for tri-iodothyronine (T(3)) and thyroxine (T(4)) in the human choriocarcinoma cell line, JAR

“…The present study confirms our previous findings of at least two types of transporters for thyroid hormones in JAR cells (Mitchell et al 1999b). Uptake of T 3 and T 4 in JAR cells was inhibited by the L-system amino acids tryptophan, phenylalanine, leucine and the synthetic amino acid analogue BCH.…”

“…We had previously reported a preliminary account of the affects of nitrendipine on thyroid hormone uptake in JAR cells (Mitchell et al 1999b). A larger range of structurally similar but pharmacologically diverse compounds (verapamil, phenytoin, mefenamic acid, meclofenamic acid, nifedipine, iopanoic acid and diazepam) were tested for effects on thyroid hormone uptake in the present study.…”

Thyroid hormone export from cells: contribution of P-glycoprotein

“…In addition, the calcium channel blocker verapamil, which is not selective for MDR, was used as an inhibitor of MDR transport. We have previously shown in the human choriocarcinoma cell line, JAR, significant inhibition of both uptake and efflux of T 3 by calcium channel blockers nitrendipine and verapamil (Mitchell et al 1999). JAR cells express minimal levels of P-gp (Mylona et al 1996, Atkinson et al 2003.…”

Synthesis of Thyroid Hormone Binding Proteins Transthyretin and Albumin by Human Trophoblast

“…Secreted trophoblastic TTR could bind maternal T 4 and then be taken up by a megalin-mediated process into trophoblast, perhaps then providing T 4 to the fetal circulation. Secreted TTR could also facilitate thyroid hormone efflux (26), which in choriocarcinoma cells appears to occur by passive diffusion (27).…”