New insights into thyroid hormone action

Thyroid hormone - triiodothyronine - has contrary effect on proliferation of human proximal tubules cell line (HK2) and renal cancer cell lines (Caki-2, Caki-1) - role of E2F4, E2F5 and p107, p130

Background

Triiodothyronine regulates proliferation acting as stimulator or inhibitor. E2F4 and E2F5 in complexes with pocket proteins p107 or p130 stop cells in G1, repressing transcription of genes important for cell cycle progression. p107 and p130 inhibits activity of cyclin/cdk2 complexes. Expression of all those proteins could be regulated by triiodothyronine. In clear cell renal cell carcinoma many disturbances in T3 signaling pathway was described, in that type of cancer also expression of some key G1 to S phase progression regulators was shown.

Methods

We investigated role of T3 and its receptors in regulation of proliferation of HK2, Caki-2, Caki-1 cell lines (cell counting, cytometric analysis of DNA content) and expression of thyroid hormone receptors, E2F4, E2F5, p107 and p130 (western blot and semi-quantitative real time PCR). Statistical analysis was performed using one-way ANOVA.

Results and Conclusion

We show that T3 inhibits proliferation of HK2, and stimulates it in Caki lines. Those differences are result of disturbed expression of TR causing improper regulation of E2F4, E2F5, p107 and p130 in cancer cells.

To bind or not to bind - how to down-regulate target genes by liganded thyroid hormone receptor?

The terrain is well explored regarding genes whose gene expression is up-regulated upon binding of thyroid hormone (TH) to its nuclear receptor. This regulation mechanism has been intensively studied and is well understood. In contrast, a lot of white spots remain on the map when it comes to target genes whose expression is down-regulated upon binding of TH to the thyroid hormone receptor (TR). Since no consistent mechanism has been proposed to explain ligand-dependent down-regulation of target gene transcription several working hypotheses favour different molecular mechanisms. Some working theories suggest a direct binding of TR to regulatory elements of target genes. Others favour models that are independent of a direct DNA binding event. However recent data suggested that a direct binding of TR to DNA is dispensable for TH-dependent negative gene transcription.

Thyroid hormone action in chronic kidney disease

PURPOSE OF REVIEW

Chronic kidney disease is characterized by multiple abnormalities in the thyroid hormone physiology. In the present review, we will briefly discuss the effects of uremia on thyroid hormone synthesis, metabolism, transport, and action.

RECENT FINDINGS

Uremic toxins have been shown to interfere at various levels of the thyroid hormone action, including thyroid hormone transport across plasma membrane and thyroid hormone receptor activity. These abnormalities could explain the resistance to thyroid hormone action in uremia, at least in some tissues.

SUMMARY

The pathogenesis of thyroid axis abnormalities in uremia is incompletely understood, and its clinical significance remains unclear. The increasing prevalence of chronic kidney disease underscores the need for further efforts to understand the metabolic consequences of uremia and address questions such as the impact of thyroid hormone therapy.

Regulation of mammary gland sensitivity to thyroid hormones during the transition from pregnancy to lactation

Thyroid hormones are galactopoietic and help to establish the mammary gland's metabolic priority during lactation. Expression patterns for genes that can alter tissue sensitivity to thyroid hormones and thyroid hormone activity were evaluated in the mammary gland and liver of cows at 53, 35, 20, and 7 days before expected parturition, and 14 and 90 days into the subsequent lactation. Transcript abundance for the three isoforms of iodothyronine deiodinase, type I (DIO1), type II (DIO2) and type III (DIO3), thyroid hormone receptors alpha1 (TRalpha1), alpha2 (TRalpha2) and beta1 (TRbeta1), and retinoic acid receptors alpha (RXRalpha) and gamma (RXRgamma), which act as coregulators of thyroid hormone receptor action, were evaluated by quantitative RT-PCR. The DIO3 is a 5-deiodinase that produces inactive iodothyronine metabolites, whereas DIO1 and DIO2 generate the active thyroid hormone, triiodothyronine, from the relatively inactive precursor, thyroxine. Low copy numbers of DIO3 transcripts were present in mammary gland and liver. DIO2 was the predominant isoform expressed in mammary gland and DIO1 was the predominant isoform expressed in liver. Quantity of DIO1 mRNA in liver tissues did not differ with physiological state, but tended to be lowest during lactation. Quantity of DIO2 mRNA in mammary gland increased during lactation (P < 0.05), with copy numbers at 90 days of lactation 6-fold greater than at 35 and 20 days prepartum. When ratios of DIO2/DIO3 mRNA were evaluated, the increase was more pronounced (>100-fold). Quantity of TRbeta1 mRNA in mammary gland increased with onset of lactation, whereas TRalpha1 and TRalpha2 transcripts did not vary with physiological state. Conversely, quantity of RXRalpha mRNA decreased during late gestation to low levels during early lactation. Data suggest that increased expression of mammary TRbeta1 and DIO2, and decreased RXRalpha, provide a mechanism to increase thyroid hormone activity within the mammary gland during lactation.

A common variation in deiodinase 1 gene DIO1 is associated with the relative levels of free thyroxine and triiodothyronine

INTRODUCTION

Genetic factors influence circulating thyroid hormone levels, but the common gene variants involved have not been conclusively identified. The genes encoding the iodothyronine deiodinases are good candidates because they alter the balance of thyroid hormones. We aimed to thoroughly examine the role of common variation across the three deiodinase genes in relation to thyroid hormones.

METHODS

We used HapMap data to select single-nucleotide polymorphisms (SNPs) that captured a large proportion of the common genetic variation across the three deiodinase genes. We analyzed these initially in a cohort of 552 people on T(4) replacement. Suggestive findings were taken forward into three additional studies in people not on T(4) (total n = 2513) and metaanalyzed for confirmation.

RESULTS

A SNP in the DIO1 gene, rs2235544, was associated with the free T(3) to free T(4) ratio with genome-wide levels of significance (P = 3.6 x 10(-13)). The C-allele of this SNP was associated with increased deiodinase 1 (D1) function with resulting increase in free T(3)/T(4) ratio and free T(3) and decrease in free T(4) and rT(3). There was no effect on serum TSH levels. None of the SNPs in the genes coding for D2 or D3 had any influence on hormone levels.

CONCLUSIONS

This study provides convincing evidence that common genetic variation in DIO1 alters deiodinase function, resulting in an alteration in the balance of circulating free T(3) to free T(4). This should prove a valuable tool to assess the relative effects of circulating free T(3) vs. free T(4) on a wide range of biological parameters.

The thyroid hormone-inactivating deiodinase functions as a homodimer

The type 3 deiodinase (D3) inactivates thyroid hormone action by catalyzing tissue-specific inner ring deiodination, predominantly during embryonic development. D3 has gained much attention as a player in the euthyroid sick syndrome, given its robust reactivation during injury and/or illness. Whereas much of the structure biology of the deiodinases is derived from studies with D2, a dimeric endoplasmic reticulum obligatory activating deiodinase, little is known about the holostructure of the plasma membrane resident D3, the deiodinase capable of thyroid hormone inactivation. Here we used fluorescence resonance energy transfer in live cells to demonstrate that D3 exists as homodimer. While D3 homodimerized in its native state, minor heterodimerization was also observed between D3:D1 and D3:D2 in intact cells, the significance of which remains elusive. Incubation with 0.5-1.2 m urea resulted in loss of D3 homodimerization as assessed by bioluminescence resonance energy transfer and a proportional loss of enzyme activity, to a maximum of approximately 50%. Protein modeling using a D2-based scaffold identified potential dimerization surfaces in the transmembrane and globular domains. Truncation of the transmembrane domain (DeltaD3) abrogated dimerization and deiodinase activity except when coexpressed with full-length catalytically inactive deiodinase, thus assembled as DeltaD3:D3 dimer; thus the D3 globular domain also exhibits dimerization surfaces. In conclusion, the inactivating deiodinase D3 exists as homo- or heterodimer in living intact cells, a feature that is critical for their catalytic activities.

Thyroid hormone receptor expression during metamorphosis of Atlantic halibut (Hippoglossus hippoglossus)

Flatfish such as the Atlantic halibut (Hippoglossus hippoglossus) undergo a dramatic metamorphosis that transforms the pelagic, symmetric larva into a benthic, cranially asymmetric juvenile. In common with amphibian metamorphosis, flatfish metamorphosis is under endocrine control with thyroid hormones being particularly important. In this report we confirm that tri-iodothyronine (T(3)) levels peak at metamorphic climax during halibut metamorphosis. Moreover, we have isolated cDNA clones of TRalpha and TRbeta genes and confirmed the presence in halibut of two TRalpha isoforms (representing the products of distinct genes) and two TRbeta isoforms (generated from a single gene by alternative splicing). Real-time PCR was used to assess expression of these genes during metamorphosis. TRbeta shows the most dramatic expression profile, with a peak occurring during metamorphic climax.

High-selenium yeast supplementation in free-living North American men: no effect on thyroid hormone metabolism or body composition

In a prior study, we observed decreased serum 3,3',5-triiodothyronine (T(3)), increased serum thyrotropin and increased body weight in five men fed 297 microg/d of selenium (Se) in foods naturally high in Se while confined in a metabolic research unit. In an attempt to replicate and confirm those observations, we conducted a randomized study of high-Se yeast supplements (300 microg/d) or placebo yeast administered to 42 healthy free-living men for 48 weeks. Serum thyroxine, T(3) and thyrotropin did not change in supplemented or control subjects. Body weight increased in both groups during the 48-week treatment period and remained elevated for the 48-week follow-up period. Body fat increased by 1.2 kg in both groups. Energy intake and voluntary activity levels were not different between the groups and remained unchanged during the treatment period. Dietary intakes of Se, macronutrients and micronutrients were not different between groups and remained unchanged during the treatment period. These results suggest that our previous observation of a hypothyroidal response to high-Se foods was confounded by some aspect of the particular foods used, or were merely chance observations. Because of the high dose and long administration period, the present study suggests that the effects of Se supplements on thyroid hormone metabolism and energy metabolism in healthy North American men with adequate Se status do not represent a significant risk for unhealthy weight gain.