Role of the type 2 iodothyronine deiodinase (D2) in the control of thyroid hormone signaling

Background

Thyroid hormone signaling is critical for development, growth and metabolic control in vertebrates. Although serum concentration of thyroid hormone is remarkable stable, deiodinases modulate thyroid hormone signaling on a time- and cell-specific fashion by controlling the activation and inactivation of thyroid hormone.

Scope of the review

This review covers the recent advances in D2 biology, a member of the iodothyronine deiodinase family, thioredoxin fold‐containing selenoenzymes that modify thyroid hormone signaling in a time- and cell-specific manner.

Major conclusions

D2-catalyzed T3 production increases thyroid hormone signaling whereas blocking D2 activity or disruption of the Dio2 gene leads to a state of localized hypothyroidism. D2 expression is regulated by different developmental, metabolic or environmental cues such as the hedgehog pathway, the adrenergic- and the TGR5-activated cAMP pathway, by xenobiotic molecules such as flavonols and by stress in the endoplasmic reticulum, which specifically reduces de novo synthesis of D2 via an eIF2a-mediated mechanism. Thus, D2 plays a central role in important physiological processes such as determining T3 content in developing tissues and in the adult brain, and promoting adaptive thermogenesis in brown adipose tissue. Notably, D2 is critical in the T4-mediated negative feed-back at the pituitary and hypothalamic levels, whereby T4 inhibits TSH and TRH expression, respectively. Notably, ubiquitination is a major step in the control of D2 activity, whereby T4 binding to and/or T4 catalysis triggers D2 inactivation by ubiquitination that is mediated by the E3 ubiquitin ligases WSB-1 and/or TEB4. Ubiquitinated D2 can be either targeted to proteasomal degradation or reactivated by deubiquitination, a process that is mediated by the deubiquitinases USP20/33 and is important in adaptive thermogenesis.

General significance

Here we review the recent advances in the understanding of D2 biology focusing on the mechanisms that regulate its expression and their biological significance in metabolically relevant tissues. This article is part of a Special Issue entitled Thyroid hormone signalling.     

Differential expression of iodothyronine deiodinase type 2 in growth plates of chickens divergently selected for incidence of tibial dyschondroplasia

Tibial dyschondroplasia (TD) is a genetic leg defect in broilers with a lesion of avascular, non-calcified cartilage below the growth plate of the proximal tibiatarsus. This disease is considered to result from the inability of chondrocytes to undergo terminal differentiation. Thyroid hormones are required for chondrocyte differentiation. The thyroid gland produces and secrets mostly L-thyroxine or T4 and T4 plays most of its biological activities through conversion to triiodothyronine or T3 in local tissues by iodothyronine deiodinases type 1 or type 2, which are tissue specific. In this study, no differences were found in the plasma concentrations of total T3 and T4 between two chicken lines divergently selected for the incidence of TD. Plasma T4 was higher than T3, especially in older chickens. Younger birds had much higher T3 than older birds, but there were no significant age differences in T4. The expression level of deiodinase type 2 in the growth plates of broilers with TD was one-eighth of those birds without the disease. The expression levels of deiodinase type 2 (DIO2) in commercial broilers without the disease were much higher than those with TD and lower than those without the disease in the susceptible and resistant lines, respectively. These results indicate that the inadequate expression of DIO2 in the growth plates contributes to the pathogenesis of TD in broilers and that TD is a tissue-specific hypothyroidism.     

Functional characterization of the rat growth hormone promoter elements required for induction by thyroid hormone with and without a co-transfected beta type thyroid hormone receptor

We have extensively characterized the sequences of the rat growth hormone (rGH) promoter required for induction by T3 (thyroid hormone, 3,5,3'-L-triiodothyronine) in a transient transfection system. Oligonucleotides containing portions of the rGH promoter sequence with various deletions and point mutations were placed upstream of the first 137 base pairs of the rGH promoter or the heterologous herpes virus thymidine kinase promoter in chloramphenicol acetyltransferase expression vectors. The rGH137 and thymidine kinase promoters show no or minimal response to T3 in the basal state. The constructs were tested in GH4C1 rat pituitary cells and COS cells (functionally deficient in thyroid hormone receptor) with and without a co-transfected plasmid expressing a beta type c-erbA gene coding for a functional T3 receptor. Oligonucleotides containing the T3 receptor binding site confer hormone-dependent induction in a manner that is independent of either orientation or variation in position on the helix relative to the promoter. Point mutations in the sequence -189 to -173 result in loss of T3 induction, and bases between -173 and -167 were also required for a full T3 response. The minimal length to confer T3 induction to the rGH promoter was 23 base pairs (-190 to -167). Point mutations creating a perfect duplication of 7 base pairs within the receptor binding site conferred 12-fold T3 response to the rGH137 promoter, 3-fold greater than the wild type rGH237 construct. T3 inductibility was also transferred to the thymidine kinase promoter by an oligonucleotide containing the sequence -200 to -157, demonstrating that cell type specific elements located 3' to 157 of the rGH promoter are not required for thyroid hormone responsiveness.     

Early expression of thyroid hormone receptor beta and retinoid X receptor gamma in the Xenopus embryo

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor beta (XTR beta) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor gamma (RXR gamma), a potential dimerization partner for XTR beta, was also found to be expressed in the retina at early stages, and at later stages RXR gamma was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTR beta and RXR gamma, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTR beta and RXR gamma in the developing Xenopus retina.     

Effects of selenium and tellurium on the activity of selenoenzymes glutathione peroxidase and Type I iodothyronine deiodinase, trace element thyroid level, and thyroid hormone status in rats

Tellurium (Te) and selenium (Se) belong chemically to the VIa group of elements. Se represents an essential element closely related to thyroid function. Te has growing application in industrial processes. Little is known about the Te biological activity, particularly with respect to potential chemical interactions with Se-containing components in the organism. In this study, female Wistar rats (body weight: 115-120 g) received sodium selenite pentahydrate (10 mg/L) or sodium tellurite (9.4 mg/L) in drinking water for 6 wk. Additional groups of rats received their combination with zinc sulfate heptahydrate (515 mg/L). The stimulation of 5’-DI-I activity due to selenite (to 158%, p<0.01) or tellurite treatment (to 197%, p<0.01) was seen; however, no effect on glutathione peroxidase was demonstrated in this experiment. An elevation of T4, T3, and rT3 serum levels was measured in the Se+Te-treated group; T4 and rT3 levels were elevated in the Te+Zn-treated group. Te accumulates in the thyroid gland and influences the zinc thyroid level. Te treatment alone and in combination with Se or Zn decreased the iodine thyroid concentration to 65-70% of the control value. Further studies are needed to clarify the nature and effects of these events.     

An intron control region differentially regulates expression of thyroid hormone receptor beta2 in the cochlea, pituitary, and cone photoreceptors

The Thrb gene, encoding thyroid hormone receptor beta (TRbeta), serves key roles in endocrine regulation and the development of the senses of hearing and color vision. The versatile functions of this gene depend upon its expression of distinct receptor isoforms by differential promoter activation. The TRbeta2 isoform has a particularly specialized distribution including in the anterior pituitary and cochlea. TRbeta2 is also found in immature cone photoreceptors where it has a unique role in programming the expression pattern of opsin photopigments that mediate color vision. Given the importance of precise, tissue-specific expression for the function of TRbeta2, we investigated the genomic control elements that direct this expression in vivo using lacZ reporter transgenes in mice. The TRbeta2 promoter region is sufficient for cochlear expression, whereas a complex intron control region is necessary for pituitary and retinal expression. In the retina, the intron region directs peak expression in the embryo in postmitotic, immature cones. The retinal control region is further subdivided into domains that specify and amplify expression, respectively, indicating that timely, cone-specific expression reflects an integrated response to complex signals. The mammalian Thrb gene has therefore incorporated several mechanisms into a multifunctional intron control region that regulates developmental induction of the distant promoter. This specialized genomic organization underlies the unique expression pattern and functions of TRbeta2.     

Contrasting developmental and tissue-specific expression of alpha and beta thyroid hormone receptor genes.

Thyroid hormones and their receptors (TRs) have critical functions in development. Here we show that a chicken TR beta cDNA clone encodes a receptor with a novel, short N-terminal domain. In vitro-expressed TR beta protein bound thyroid hormone with similar affinity as the chicken TR alpha. Comparison of expression of TR alpha and TR beta mRNAs throughout chicken development until 3 weeks post-hatching revealed ubiquitous expression of TR alpha mRNAs (in 14 different tissues) with some variations in levels, from early embryonic stages. In contast, expression of TR beta mRNA was restricted, occurring notably in brain, eye, lung, yolk sac and kidney, and was subject to striking developmental control, especially in brain where levels increased 30-fold upon hatching. Levels also sharply increased in late embryonic lung, but were relatively high earlier in embryonic eye and yolk sac. RNase protection analyses detected no obvious mRNAs for alpha and beta TRs with variant C-termini as demonstrated previously for the rat TR alpha gene. The data suggest a general role for TR alpha and specific developmental functions for TR beta, and that thyroid-dependent development involves temporal and tissue-specific expression of the TR beta gene.     

Expression of type II iodothyronine deiodinase marks the time that a tissue responds to thyroid hormone-induced metamorphosis in Xenopus laevis

The thyroid gland synthesizes thyroxine (T4), which passes through the larval tadpole's circulatory system. The enzyme type II iodothyronine deiodinase (D2) converts thyroxine (T4) to the active hormone 3,5,3'-triiodothyronine (T3) in peripheral tissues. An early response to thyroid hormone (TH) in the Xenopus laevis tadpole is the stimulation of cell division in cells that line the brain ventricles, the lumen of the spinal cord, and the limb buds. These cells express constitutively high levels of D2 mRNA. Exogenous T4 induces early DNA synthesis in brain, spinal cord, and limb buds as efficiently as T3. The deiodinase inhibitor iopanoic acid blocks T4- but not T3-induced cell division. At metamorphic climax, both TH-induced cell division and D2 expression decrease in the brain. Then D2 expression appears in late-responding tissues including the anterior pituitary, the intestine, and the tail where cell division is reduced or absent. Therefore, constitutive expression of D2 occurs in the earliest target tissues of TH that will grow and differentiate, while TH-induced expression of D2 takes place in late-responding tissues that will remodel or die. This pattern of constitutive and induced D2 expression contributes to the timing of metamorphic changes in these tissues.     

Genomic imprinting of the type 3 thyroid hormone deiodinase gene: Regulation and developmental implications

Background

In recent years, findings in a number of animal and human models have ignited renewed interest in the type 3 deiodinase (D3), the main enzyme responsible for the inactivation of thyroid hormones. The induction of D3 in models of illness and injury has raised critical questions about the physiological significance of reduced thyroid hormone availability in those states. Phenotypes in transgenic mice lacking this enzyme also point to important developmental roles for D3. A critical determinant of D3 expression is genomic imprinting, an epigenetic phenomenon that regulates a small number of dosage-critical genes in the mammalian genome. The D3 gene (Dio3) is imprinted and preferentially expressed from one of the alleles in most tissues.

Scope of review

In the context of the physiological significance of D3 and the characteristics and purported origins of genomic imprinting, we review the current knowledge about the epigenetic mechanisms specifying gene dosage in the Dio3 locus.

Major conclusions

Altered Dio3 dosage is detrimental to development, suggesting that the level of thyroid hormone action needs to be exquisitely tailored in a timely fashion to the requirements of particular tissues. An appropriate Dio3 dosage is the result of the coordinated action of certain genomic elements and epigenetic marks in the Dlk1-Dio3 domain.

General significance

The imprinting of Dio3 prompts intriguing questions about why the level of thyroid hormone signaling should be regulated in this rare epigenetic manner, and to what extent altered Dio3 expression due to aberrant imprinting may be implicated in human conditions. This article is part of a Special Issue entitled Thyroid hormone signalling.     

Functional properties of human thyroid hormone receptor beta 1 overexpressed using baculovirus

We have overexpressed the human beta 1 thyroid hormone receptor in insect cells using a recombinant baculovirus to a level of 5-10% of total cellular protein. The recombinant protein migrates as a 50 kDa band by SDS-PAGE and Western blot analysis. The expressed receptor binds to L-T3 with a Kd of 1.3 +/- 0.4 x 10(-10) M and to thyroid hormone analogues with an affinity hierarchy of TRIAC greater than L-T3 greater than L-T4 greater than rT3. Gel retardation assays show highly specific receptor binding to a TRE which is modified by the presence of ligand and avidin-biotin complex DNA analysis shows a Kd of 6.2 +/- 2.0 x 10(-10) M for this interaction. These results indicate high level expression of hTR beta with authentic hormone and DNA binding properties.