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Thyroid hormone action starts and ends by deiodination.

Summary: Antonio Bianco is interested in characterizing fundamental aspects of the pathways controlling thyroid hormone action that are regulated by the iodothyronine deiodinases D1, D2 and D3. His laboratory is currently investigating the molecular and cellular biology of these enzymes to better understand their physiological and pathophysiological roles as well as to develop therapeutic approaches in order to regulate thyroid hormone action in a tissue specific fashion.

Thyroid hormone modulates gene expression in virtually every vertebrate cell through ligand-dependent transcription factors, the T3 receptors. Thyroid hormone is secreted as a pro-hormone (T4) that can be activated to T3 in a stage- and tissue-specific manner by two iodothyronine deiodinases, D1 and D2, while a third deiodinase, D3, prevents T4 activation and terminates T3 action. These three deiodinases are dimeric integral membrane proteins composed of a single N-terminal trans-membrane segment connected to a larger globular domain that contains the active center embedded in a thioredoxin fold. A striking feature of this pocket is the presence of the rare amino acid Selenocysteine (Sec) that is critical for catalysis.

D2 is the key thyroid hormone activating enzyme. It is localized in the endoplasmic reticulum (ER) and thus has the potential to increase the supply of T3 to the cell nucleus, directly affecting the expression of T3-responsive genes. D2 regulation is achieved by transcriptional control and also by ubiquitination, i.e. the covalent attachment of mono- or polyubiquitin chains to proteins, known in eukaryotic cells as a critical method by which the function and fate of proteins may be altered. Ubiquin conjugation to D2 is catalyzed by UBC6 or UBC7 in yeast and mammalian cells. This inactivates D2 and targets the protein for degradation in the proteasomes. However, inactive ubiquitinated D2 can be reactivated by the USP33 and USP20 deubiquitinating enzymes and rescued from terminal proteasomal degradation.

D2 ubiquitination is mediated by WSB-1, a SOCS-box-containing WD-40 protein that is induced by hedgehog signaling in embryonic structures during development. WSB-1 is the substrate recognition subunit of a much larger ubiquitinating catalytic core complex, modeled as Elongin BC-Cul5-Rbx1 (ECSWSB-1). D2 is also ubiquitinated via TEB4, a large ER-resident ligase that is involved in unfolding protein response and ER stress.

In developmental settings and in the brain, hedgehog-stimulated D2 ubiquitination locally decreases thyroid hormone signaling and thus the expression of T3-responsive genes. This effect that hedgehog molecules have on thyroid hormone signaling via D2 ubiquitination is potentiated by simultaneous transcriptional upregulation of D3, thus accelerating inactivation of thyroid hormone.

Deiodinase-mediated thyroid hormone activation or inactivation also play a role in energy homeostasis. Mammals can rapidly increase the utilization of energy substrates and metabolic rate in response to different signals, including thyroid hormone. This is possible because D2 is expressed in tissues that promote these changes such as brown adipose tissue and skeletal muscle. Targeted disruption of the Dio2 gene in mice is associated with impaired adaptive thermogenesis, cold intolerance and obesity. At the same time, hypoxic injury to the brain and myocardium leads to ectopic HIF1-induced D3 expression, reducing thyroid hormone signaling and energy expenditure.

Selected Publications

1. Freitas BCG, Gereben B, Castillo M, Kalló I, Zeöld A, Egri P, Liposits Z, Zavacki AM, Maciel RMB, Jo S, Singru P, Sanchez E, Lechan RM, Bianco AC. Glial activation of thyroxine defines a thyroid hormone footprint in neuronal gene expression. J Clin Invest 2010;120:2206-17.

2. Zavacki AM, Drigo R, Freitas B, Harney JW, Bianco AC. The E3 Ubiquitin ligase TEB4 mediates the degradation of the type 2 iodothyrosine deiodinase (D2). Mol Cell Biol 2009;29:5339-47.

3. Huang SA, Bianco AC. Reawakened interest in type III iodothyronine deiodinase in critical illness and injury. Nature Clin Pract Endocrinol Metab 2008; 4:148-55.

4. Gereben B, Zavacki AM, Ribich S, Kim BW, Huang S, SImonides WS, Zeold A, Bianco AC. Cellular and molecular basis of deiodinase-regulated thyroid homrone signaling. Endoc Rev 2008;29:898-938.

5. da-Silva WS, Harney JW, Kim BW, Li J, Bianco SD, Crescenzi A, Christoffolete MA, Huang SA, Bianco AC. The small polyphenolic molecule Kaempferol increases thyroid hormone activation and cellular energy expenditure. Diabetes 2007; 56:767-776

6. Sagar GDV, Gereben B, Callebaut I, Mornon J-P, Zeöld A, da Silva WS, Luongo C, Dentice M, Tente SM, Freitas BCG, Harney JW, Zavacki AM, Bianco AC. Ubiquitination-induced conformational change within the deiodinase dimer is a switch regulating enzyme activity. Mol Cell Biol 2007; 27:4774-83.

7. Bianco AC, Kim BW-S. Deiodinases: Implications of the local control of thyroid hormone action. J Clin Invest 2006; 116:2571-2579.

8. Watanabe M, Houten SM, Mataki C, Christoffolete MA, Kim BW, Bianco AC, Auwerx J. Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation. Nature 2006; 439:484-489.

9. Dentice M, Bandyopadhyay A, Gereben B, Callebaut I, Christoffolete MA, Kim BW, Nissim S, Mornon J-P, Zavacki AM, Zeöld A, Curcio-Morelli C, Ribeiro R, Harney JW, Tabin CJ and Bianco AC. The Hedgehog-inducible ubiquitin ligase subunit WSB-1 modulates thyroid hormone activation and PTHrP secretion in the developing growth plate. Nature Cell Biol 2005; 7:698-705.

10. Kim BW-S, Zavacki AM, Morelli C, Dentice M, Harney JW, Larsen PR, Bianco AC. Endoplasmic reticulum-associated degradation of the human type 2 iodothyronine deiodinase (D2) is mediated via an association between mammalian UBC7 and the carboxyl region of D2. Mol Endocrinol 2003; 17:2603-12.

11. Callebaut I, Curcio-Morelli C, Mornon JP, Gereben B, Buettner C, Huang S, Castro B, Fonseca TL, Harney JW, Larsen PR, Bianco AC. The iodothyronine selenodeiodinases are thioredoxin-fold family proteins containing a glycoside hydrolase-clan GH-A-like structure. J Biol Chem 2003; 278:36887-96.

12. Curcio-Morelli C, Zavacki AM, Christofollete M, Gereben B, Freitas B, Harney JW, Li Z, Wu G, Bianco AC. Deubiquitination of D2 by pVHL-interacting deubiquitinating enzymes regulates thyroid hormone activation. J Clin Invest 2003;112:189-96.

13. de Jesus LA, Carvalho SD, Ribeiro MO, Schneider M, Kim S-W, Harney JW, Larsen PR, Bianco AC. The type 2 deiodinase is essential for adaptive thermogenesis in brown adipose tissue. J Clin Invest 2001;108:1379-85.