3,5-Diiodothyronine in vivo maintains euthyroidal expression of type 2 iodothyronine deiodinase, growth hormone, and thyroid hormone receptor β1 in the killifish

Abstract

Until recently,3, has been considered an inactive by-product of triiodothyronine (T3) deiodination. However, studies from several laboratories have shown that 3,5-T2 has specific, nongenomic effects on mitochondrial oxidative capacity and respiration rate that are distinct from those due to T3. Nevertheless, little is known about the putative genomic effects of 3,5-T2. We have previously shown that hyperthyroidism induced by supraphysiological doses of 3,5-T2 inhibits hepatic iodothyronine deiodinase type 2 (D2) activity and lowers mRNA levels in the killifish in the same manner as T3 and T4, suggesting a pretranslational effect of 3,5-T2 (Garcia-G C, Jeziorski MC, Valverde-R C, Orozco A. Gen Comp Endocrinol 135: 201-209, 2004). The question remains as to whether 3,5-T2 would have effects under conditions similar to those that are physiological for T3. To this end, intact killifish were rendered hypothyroid by administering methimazole. Groups of hypothyroid animals simultaneously received 30 nM of either T3, reverse T3, or 3,5-T2. Under these conditions, we expected that, if it were bioactive, 3,5-T2 would mimic T3 and thus reverse the compensatory upregulation of D2 and tyroid receptor ␤1 and downregulation of growth hormone that characterize hypothyroidism. Our results demonstrate that 3,5-T2 is indeed bioactive, reversing both hepatic D2 and growth hormone responses during a hypothyroidal state. Furthermore, we observed that 3,5-T2 and T3 recruit two distinct populations of transcription factors to typical palindromic and DR4 thyroid hormone response elements. Taken together, these results add further evidence to support the notion that 3,5-T 2 is a bioactive iodothyronine. deiodinase type 2; thyroid hormone receptor ␤1; thyroid hormone response element; killifish IODOTHYRONINES OR THYROID hormones (TH) are essential in regulating energy expenditure and development. Triiodothyronine (T 3 ) is the bioactive TH, which modulates gene expression in virtually every vertebrate tissue through ligand-dependent transcription factors, the TH receptors (TR). Sequential deiodination of thyroxine (T 4 ) generates T 3 as well as other iodothyronines that have been considered inactive by-products, but, recently, interest has grown in identifying bioactive iodothyronines in addition to T 4 and T 3 . Studies from several laboratories have suggested that 3,5-diiodothyronine (3,5-T 2 ), a putative product of the deiodination pathway involved in T 3 metabolism, could be a peripheral mediator of some effects of TH on mitochondrial oxidative capacity and respiration rate. To date, results in mammals suggest that 3,5-T 2 has specific actions on oxygen consumption that are distinct from those of T 3 : they are not attenuated by inhibition of protein synthesis and are more rapid than those due to T 3 (for review, see Ref. 12). Genomic effects of 3,5-T 2 have been analyzed in only a few classic iodothyronine-dependent genes, such as thyroid stimulating hormone (TSH), thyroid receptor ␤2 (TR␤2), iodothyronine deiodinase type 1 (D1)...

3,5-di-iodothyronine stimulates tilapia growth through an alternate isoform of thyroid hormone receptor β1

“…A major finding in this study consists in the demonstration that, through interacting with a specific and distinct TRb1 isoform, T 2 effectively triggers an outburst of body growth in tilapia fingerlings, with an efficiency comparable to that of T 3 . These results confirm and extend previous reports from our laboratory and others suggesting that T 2 is also an important transcriptional regulator, at least in teleosts (García-G et al 2004, 2007, Little et al 2013, Mendoza et al 2013. The lack of available data on circulating or intratissular T 2 concentrations has masked the importance of this TH.…”

“…Accordingly, studies from several laboratories have suggested that 3,5-di-iodothyronine (3,5-T 2 or T 2 ), a putative product of the outer ring deiodination pathway involved in T 3 metabolism, also possesses bioactivity (Goglia 2005). In fact, studies from our laboratory have shown that both T 3 and T 2 act directly on TH-dependent genes in teleost liver (García-G et al 2007) and that these genomic actions are mediated by different isoforms of TRb1 (Mendoza et al 2013). Indeed, results from binding and transactivating assays revealed that the effects of T 2 on genomic regulation appear to be mediated by a different isoform of TRb1 that is found only in fish and contains a 9-amino acid insert in its ligand-binding domain, and which we have denominated long TRb1 (L-TRb1).…”

Translating Pharmacological Findings from Hypothyroid Rodents to Euthyroid Humans: Is There a Functional Role of Endogenous 3,5-T2?

“…However, several animal studies in rodents and other species, for example killifish, indicated that 3,5-T2 interferes with the hypothalamus-pituitary-thyroid axis at several levels such as pituitary, thyroid, and peripheral action, thereby displaying a broad spectrum of mechanisms involved such as classical interaction with TR and also rapid effects at the cell membrane and mitochondria (10,12,14,(34)(35)(36)(37)(38)(39)(40) (for a review, see (3)). Action of 3,5-T2 via these different targets and molecular mechanisms might occur at different 3,5-T2 concentrations and depend on acute or chronic exposure (35,37,40), thus possibly explaining the unexpected bell-shaped relationship with serum TSH.…”

3,5-T2 alters murine genes relevant for xenobiotic, steroid, and thyroid hormone metabolism

“…The results in diet-induced obese mice by Jonas et al (2014) strongly suggest that 3,5-T 2 dose-dependently influences the expression of TH responsive genes possibly through binding and activating the TH receptor (TR) as shown for teleosts (Garcia et al 2007, Mendoza et al 2013, Navarrete-Ramirez et al 2014 and rats (Ball et al 1997).…”