title: A role for thyroid hormone transporters in transcriptional regulation by thyroid hormone receptors.
abstract: Thyroid hormones (THs) must be taken up by target cells to act at the genomic level through binding to nuclear thyroid hormone receptors (TRs). Extensive study has been made of mechanisms by which TH-bound TRs regulate transcription, yet little is known about the critical upstream step, i.e. how THs enter the cell. Growing evidence suggests that saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane and have important roles in the regulation of TH bioavailability. For example, System L is a multifunctional transport system serving as a plasma membrane transporter of THs and amino acids in mammalian cells. We have used two complementary systems, the Xenopus oocyte (which has negligible basal System L activity) and the mammalian BeWo cell line (which has System L activity for TH transport), to investigate the role of this representative TH transporter in nuclear action of THs. We demonstrate that overexpression of System L in Xenopus oocytes increases both cytoplasmic and nuclear delivery of THs from external medium and also enhances transcriptional activation by TRs. Conversely, blocking endogenous System L activity in BeWo cells with specific inhibitors reduces both TH uptake and TR function. These results indicate that plasma membrane TH transporters such as System L may have important roles in gene regulation by TRs.
authors.affiliation: School of Life Sciences, University of Dundee, MSI/WTB Complex, Dundee DD1 5EH, Scotland, United Kingdom.
authors.forename: J W A
authors.initials: JW
authors.lastname: Ritchie
authors.forename: Y-B
authors.initials: YB
authors.lastname: Shi
authors.forename: Y
authors.initials: Y
authors.lastname: Hayashi
authors.forename: F E
authors.initials: FE
authors.lastname: Baird
authors.forename: R W
authors.initials: RW
authors.lastname: Muchekehu
authors.forename: G R
authors.initials: GR
authors.lastname: Christie
authors.forename: P M
authors.initials: PM
authors.lastname: Taylor
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D002822:Choriocarcinoma / Q000378:metabolism; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D017874:Immediate-Early Proteins / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009865:Oocytes; D018168:Receptors, Retinoic Acid / Q000235:genetics / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000235:genetics* / Q000378:metabolism; D047488:Retinoid X Receptors; D013963:Thyroid Hormones / Q000378:metabolism* / Q000493:pharmacokinetics; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014982:Xenopus laevis
chemical_list: D000596:Amino Acids; D002352:Carrier Proteins; C110598:IU12 protein, Xenopus; D017874:Immediate-Early Proteins; D008565:Membrane Proteins; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D013963:Thyroid Hormones; D014157:Transcription Factors