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Submitted on May 16, 2002
Accepted on January 13, 2003
1 Division of Molecular Physiology, School of Life Sciences, University of Dundee, Dundee, DD1 5EH, Scotland, UK; Laboratory of Gene Regulation and Development, NICHD/NIH, Bethesda, MD 20892-5431, USA; Research Institute of Environmental Medicine, Nagoya University, Nagoya, 464-8601 Japan
* To whom correspondence should be addressed. E-mail: p.m.taylor{at}dundee.ac.uk.
Thyroid hormones (TH) 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 TH 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 TH 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 TH. We demonstrate that overexpression of System L in Xenopus oocytes increases both cytoplasmic and nuclear delivery of TH from external medium and also enhances transcriptional activation by TR. 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 TR.
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