Molecular mechanisms of thyroid hormone transport : the role of amino acid transporters

Thyroid hormones (TH) exert their multitude of effects on body development, growth and metabolism largely via transcriptional regulatory pathways. TH-induced transcription is controlled by receptors present in the cell nucleus, therefore extracellular TH must first cross the plasma membrane to gain entry into the cell. The exact mechanisms of TH transport across the plasma membrane are only beginning to be clarified, but it is likely that transport may be an important control step for the effects of TH on transcription. Members of the recently cloned organic anion transporting polypeptide (OATP) family have been shown to transport TH. Inhibitor studies indicate that both the aromatic amino acid System T-type transporter, and the broad scope neutral amino acid transporter System L are mediators of TH uptake into various cell types. However cloned amino acid transporters have not been studied to demonstrate directly whether they can accept TH as substrates.