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0090-9556/04/3208-779-782$20.00
DMD 32:779-782, 2004

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SHORT COMMUNICATION

LEVOTHYROXINE UP-REGULATES P-GLYCOPROTEIN INDEPENDENT OF THE PREGNANE X RECEPTOR

Tim Mitin, Lisa L. von Moltke, Michael H. Court, and David J. Greenblatt

Department of Pharmacology & Experimental Therapeutics, Tufts University School of Medicine, Boston, Massachusetts

P-Glycoprotein (P-gp) and cytochrome P450 3A4 (CYP3A4) constitute a physiologic barrier in the intestine for many of the same substrates. Their expression can be influenced by nuclear receptor NR1I2 (pregnane X receptor; PXR), which acts as a receptor for various endobiotics and xenobiotics. However, P-gp and CYP3A4 are not identical in anatomic localization, suggesting unique as well as shared regulatory mechanisms of gene expression. We used established human colon carcinoma cell lines (LS180 and Caco-2) and measured mRNA and protein levels in cells after exposures to levothyroxine (L-T4), triiodo-L-thyronine (L-T3), and rifampin. Results indicate that L-T4, L-T3, and rifampin can upregulate the expression of P-gp mRNA and protein in LS180 cells, but only L-T4 and L-T3 can produce the same effect in Caco-2 cells, which are relatively lacking in PXR. In addition, L-T4 and L-T3 did not affect the expression of CYP3A4 in either cell line. We conclude that P-gp, but not CYP3A4, can be up-regulated by thyroid hormones in vitro by a PXR-independent mechanism. Considering the widespread prescription use of L-T4 preparations in the older adult population, these results may be important for the clinical consideration of drug-drug interactions mediated by P-gp.

Address correspondence to: David J. Greenblatt, Department of Pharmacology & Experimental Therapeutics, Tufts University School of Medicine, 136 Harrison Avenue, Boston, MA 02111. E-mail: dj.greenblatt{at}tufts.edu




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