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Julika Lietzow, Janine Golchert, Georg Homuth, Uwe Völker, Wenke Jonas, and Josef Köhrle

The endogenous thyroid hormone (TH) metabolite 3,5-diiodo-l-thyronine (3,5-T2) acts as a metabolically active substance affecting whole-body energy metabolism and hepatic lipid handling in a desirable manner. Considering possible adverse effects regarding thyromimetic action of 3,5-T2 treatment in rodents, the current literature remains largely controversial. To obtain further insights into molecular mechanisms and to identify novel target genes of 3,5-T2 in liver, we performed a microarray-based liver tissue transcriptome analysis of male lean and diet-induced obese euthyroid mice treated for 4 weeks with a dose of 2.5 µg/g bw 3,5-T2. Our results revealed that 3,5-T2 modulates the expression of genes encoding Phase I and Phase II enzymes as well as Phase III transporters, which play central roles in metabolism and detoxification of xenobiotics. Additionally, 3,5-T2 changes the expression of TH responsive genes, suggesting a thyromimetic action of 3,5-T2 in mouse liver. Interestingly, 3,5-T2 in obese but not in lean mice influences the expression of genes relevant for cholesterol and steroid biosynthesis, suggesting a novel role of 3,5-T2 in steroid metabolism of obese mice. We concluded that treatment with 3,5-T2 in lean and diet-induced obese male mice alters the expression of genes encoding hepatic xenobiotic-metabolizing enzymes that play a substantial role in catabolism and inactivation of xenobiotics and TH and are also involved in hepatic steroid and lipid metabolism. The administration of this high dose of 3,5-T2 might exert adverse hepatic effects. Accordingly, the conceivable use of 3,5-T2 as pharmacological hypolipidemic agent should be considered with caution.

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Vaishnavi Venugopalan, Maren Rehders, Jonas Weber, Lisa Rodermund, Alaa Al-Hashimi, Tonia Bargmann, Janine Golchert, Vivien Reinecke, Georg Homuth, Uwe Völker, Francois Verrey, Janine Kirstein, Heike Heuer, Ulrich Schweizer, Doreen Braun, Eva K Wirth, and Klaudia Brix

Proteolytic cleavage of thyroglobulin (Tg) for thyroid hormone (TH) liberation is followed by TH release from thyroid follicles into the circulation, enabled by TH transporters. The existence of a functional link between Tg-processing cathepsin proteases and TH transporters has been shown to be independent of the hypothalamus–pituitary–thyroid axis. Thus, lack of cathepsin K, combined with genetic defects in the TH transporters Mct8 and Mct10, that is the Ctsk−/−/Mct8−/y/Mct10−/− genotype, results in persistent Tg proteolysis due to autophagy induction. Because amino acid transport by L-type amino acid transporter 2 (Lat2) has been described to regulate autophagy, we asked whether Lat2 availability is affected in Ctsk−/−/Mct8−/y/Mct10−/− thyroid glands. Our data revealed that while mRNA amounts and subcellular localization of Lat2 remained unaltered in thyroid tissue of Ctsk−/−/Mct8−/y/Mct10−/− mice in comparison to WT controls, the Lat2 protein amounts were significantly reduced. These data suggest a direct link between Lat2 function and autophagy induction in Ctsk−/−/Mct8−/y/Mct10−/− mice. Indeed, thyroid tissue of Lat2−/− mice showed enhanced endo-lysosomal cathepsin activities, increased autophagosome formation, and enhanced autophagic flux. Collectively, these results suggest a mechanistic link between insufficient Lat2 protein function and autophagy induction in the thyroid gland of male mice.