Plasma thyroxine-binding proteins and thyroid hormone levels in primate species; is callithricidae thyroid hormone resistant?

Thyroxine(T4)-binding to serum proteins in primates; catarrhini, prosimiae, and platyrrhini were studied by polyacrylamide gel electrophoresis T4 binding analysis. From the electrophoretic analysis, it was shown that thyroxine-binding proteins similar to human thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) were present in catarrhini and prosimiae species, but not in platyrrhini (callithricidae and cebidae). T4-binding analysis also revealed that catarrhini and prosimiae have a high affinity T4-binding protein similar to human TBG. The association constant (Ka) for T4 of the plasma proteins in these species was approximately 2.0 X 10(10) M-1. On the other hand, it was unable to demonstrate a high affinity binding site for T4 in the plasma of platyrrhini species. Both the total and free thyroid hormone concentrations in catarrhini and prosimiae were similar to those in human. Total T4 in cebidae, one of the platyrrhini species, was extremely low. Among 8 animals examined, T4 in 6 was undetectable by radioimmunoassay and the mean T4 of the other two was 2.8 micrograms/dl. However, free thyroid hormone concentrations were similar to those in human. In callithricidae, another platyrrhini species, T4 in plasma was 6.90 +/- 2.11, which is comparable to the level in normal human subjects. However, in this species, high-affinity T4-binding protein was lacking and free thyroid hormone concentrations were extremely high (most were higher than the assay limit). Although the thyroid function of callithricidae remains to be studied, it will be interesting if callithricidae is resistant to thyroid hormone action.     

The critical period for thyroid hormone responsiveness through thyroid hormone receptor isoform α in the postnatal small intestine

During second and third weeks after birth in rats, serum thyroid hormone level is elevated. In this study, we investigated the jejunal expression of thyroid hormone receptor (TR) α in developing rats. The TRα-1 mRNA level and TRα-1/TRα-2 mRNA ratio increased two-fold from 5 to 13 days after birth. This high level of TRα-1 mRNA was maintained until 20 days and then decreased to the basal level by the end of weaning period at 27 days; however, the level of TRα-2 mRNA remained unchanged throughout the developmental period. The increase in the TRα-1/TRα-2 mRNA ratio from 5 to 13 days was accompanied by an initial rise in the levels of mRNA for hexose transporters in the jejunum. Administration of T₃ during the suckling period (8–13 days) caused a 50% increase in the TRα-1/TRα-2 mRNA ratio, while administration of T₃ on days 12–17 and days 16–21, but not on days 22–27, caused a two to four-fold increase in the levels of mRNA for hexose transporters. These results suggest that a transient variation in the TRα-1/TRα-2 expression ratio is closely related to the critical period of thyroid hormone responsiveness for hexose transporters expression in the developing rat jejunum.     

Identification of thyroid hormone response elements in?vivo using mice expressing a tagged thyroid hormone receptor α1

TRα1 (thyroid hormone receptor α1) is well recognized for its importance in brain development. However, due to the difficulties in predicting TREs (thyroid hormone response elements) in silico and the lack of suitable antibodies against TRα1 for ChIP (chromatin immunoprecipitation), only a few direct TRα1 target genes have been identified in the brain. Here we demonstrate that mice expressing a TRα1–GFP (green fluorescent protein) fusion protein from the endogenous TRα locus provide a valuable animal model to identify TRα1 target genes. To this end, we analysed DNA–TRα1 interactions in vivo using ChIP with an anti-GFP antibody. We validated our system using established TREs from neurogranin and hairless, and by verifying additional TREs from known TRα1 target genes in brain and heart. Moreover, our model system enabled the identification of novel TRα1 target genes such as RNF166 (ring finger protein 166). Our results demonstrate that transgenic mice expressing a tagged nuclear receptor constitute a feasible approach to study receptor–DNA interactions in vivo, circumventing the need for specific antibodies. Models like the TRα1–GFP mice may thus pave the way for genome-wide mapping of nuclear receptor-binding sites, and advance the identification of novel target genes in vivo.     

Down-regulation of thyroid hormone receptor β1 gene expression in gastric cancer involves promoter methylation

Hypermethylation has been shown in the promoter region of the thyroid hormone receptor β1 (TRβ1) gene in several human tumors. However, its role in gastric cancer formation is still unclear. In the study, we analyzed mRNA expression of TRβ1 gene using real-time quantitative PCR (qPCR). A quantitative methylation-specific PCR (Q-MSP) assay was used to determine the methylation status of the TRβ1 gene promoter region in 46 pair-matched gastric neoplastic and adjacent non-neoplastic tissues. The results showed that TRβ1 mRNA expression was significantly reduced in gastric cancer specimens. The methylation of promoter of TRβ1 gene in gastric cancer tissues was significantly higher than in adjacent normal tissues. Promoter hypermethylation of the TRβ1 gene correlated with tumor infiltration, lymph node metastasis, and distant metastasis, but it was not associated with other clinicopathological characteristics. We treated gastric cancer cell lines MKN-45, MKN-28, SGC-7901, NCI-N87, and SNU-1 with 5-Aza-2-deoxycytidine (5-Aza-dC). The results showed the expression of TRβ1 mRNA was increased in MKN-45, MKN-28, SGC-7901, but not increased in NCI-N87 and SNU-1. These results suggest that the TRβ1 gene plays important roles in the development of gastric cancer partially through epigenetic mechanisms.     

Selective thyroid hormone receptor β agonists with oxadiazolone acid isosteres

Use of the oxadiazolone acid isostere in triiodothyronine analogs yielded potent and selective agonists for the thyroid hormone receptor β. Selected examples showed good in-vivo efficacy in a rat hypercholesterolemic model. One compound was further profiled in a diet-induced mouse model of nonalcoholic steatohepatitis (NASH) and showed robust target engagement and significant histological improvements in both liver steatosis and fibrosis.     

Thyrotropin-releasing hormone receptor expression in thyroid follicular cells: a new paracrine role of C-cells?

Thyrotropin-releasing hormone (TRH) synthesized in the hypothalamus has the capability of inducing the release of thyroid-stimulating hormone (TSH) from the anterior pituitary, which in turn stimulates the production of thyroid hormones in the thyroid gland. Immunoreactivity for TRH and TRH-like peptides has been found in some tissues outside the nervous system, including thyroid. It has been demonstrated that thyroid C-cells express authentic TRH, affecting thyroid hormone secretion by follicular cells. Therefore, C-cells could have a paracrine role in thyroid homeostasis. If this hypothesis is true, follicular cells should express TRH receptors (TRH-Rs) for the paracrine modulation carried out by C-cells. In order to elucidate whether or not C-cell TRH production could act over follicular cells modulating thyroid function, we studied TRH-Rs expression in PC C13 follicular cells from rat thyroid, by means of immunofluorescence technique and RT-PCR analysis. We also investigated the possibility that C-cells present TRH-Rs for the autocrine control of its own TRH production. Our results showed consistent expression for both receptors, TRH-R1 and TRH-R2, in 6-23 C-cells, and only for TRH-R2 in PC C13 follicular cells. Our data provide new evidence for a novel intrathyroidal regulatory pathway of thyroid hormone secretion via paracrine/autocrine TRH signaling.     

Inhibition of thyroid hormone receptor α1 impairs post-ischemic cardiac performance after myocardial infarction in mice

Thyroid hormone receptor α1 (TRα1) is shown to be critical for the maturation of cardiomyocytes and for the cellular response to stress. TRα1 is altered during post ischemic cardiac remodeling but the physiological significance of this response is not fully understood. Thus, the present study explored the potential consequences of selective pharmacological inhibition of TRα1 on the mechanical performance of the post-infarcted heart. Acute myocardial infarction was induced in mice (AMI), while sham operated animals served as controls (SHAM). A group of mice was treated with debutyl-dronedarone (DBD), a selective TRα1 inhibitor (AMI–DBD). AMI resulted in low T3 levels in plasma and in down-regulation of TRα1 and TRβ1 expression. Left ventricular ejection fraction (LVEF%) was significantly reduced in AMI [33 (SEM 2.1) vs 79(2.5) in SHAM, p < 0.05] and was further declined in AMI–DBD [22(1.1) vs 33(2.1), respectively, p < 0.05]. Cardiac mass was increased in AMI but not in AMI–DBD hearts, resulting in significant increase in wall tension index. This increase in wall stress was accompanied by marked activation of p38 MAPK, a kinase that is sensitive to mechanical stretch and exerts negative inotropic effect. Furthermore, AMI resulted in β-myosin heavy chain overexpression and reduction in the ratio of SR(Ca)ATPase to phospholamban (PLB). The latter further declined in AMI–DBD mainly due to increased expression of PLB. AMI induces downregulation of thyroid hormone signaling and pharmacological inhibition of TRα1 further depresses post-ischemic cardiac function. p38 MAPK and PLB may, at least in part, be involved in this response.     

The thyroid hormone receptor β induces DNA damage and premature senescence

There is increasing evidence that the thyroid hormone (TH) receptors (THRs) can play a role in aging, cancer and degenerative diseases. In this paper, we demonstrate that binding of TH T3 (triiodothyronine) to THRB induces senescence and deoxyribonucleic acid (DNA) damage in cultured cells and in tissues of young hyperthyroid mice. T3 induces a rapid activation of ATM (ataxia telangiectasia mutated)/PRKAA (adenosine monophosphate–activated protein kinase) signal transduction and recruitment of the NRF1 (nuclear respiratory factor 1) and THRB to the promoters of genes with a key role on mitochondrial respiration. Increased respiration leads to production of mitochondrial reactive oxygen species, which in turn causes oxidative stress and DNA double-strand breaks and triggers a DNA damage response that ultimately leads to premature senescence of susceptible cells. Our findings provide a mechanism for integrating metabolic effects of THs with the tumor suppressor activity of THRB, the effect of thyroidal status on longevity, and the occurrence of tissue damage in hyperthyroidism.     

Developmental changes in regulation of the Na+, K+-ATPase α3 isoform by thyroid hormone in ferret heart

Ferret heart expresses the α1- as well as the α3-isoform of the Na⁺, K⁺-ATPase. We have shown previously that the α3 isoform is differentially upregulated during postnatal cardiac development and that in adult ferrets expression of α3 is not responsive to regulation by thyroid hormone (TH). Since developmental-stage dependent effects of TH have been reported previously, the present study examined whether effects of TH on expression of the Na⁺, K⁺-ATPase isoforms in ferret heart is modulated during development and possible mechanisms were examined. Ferrets of different age groups were treated with TH and the relative abundance of Na⁺, K⁺-ATPase isoforms in ferret myocardium was determined by immunoblotting. Thyroid hormone (T3; 50 μg/100 g body weight on 3 alternating days, s.c.) increased protein levels of the α3 isoform, but not that of α1 or β1, in myocardium of 5-day-old and 3-week-old ferrets. By contrast, in myocardium of 6- and 8-week-old ferrets T3 failed to increase protein levels of α1 and α3. To determine whether elevated plasma levels of TH during development plays a role in the transition, mature ferrets were first made hypothyroid before TH treatment. In these hypothyroid ferrets expression of the α3 isoform remained unresponsive to TH (T4, 0.5 mg/kg for 7 days, s.c.). The transition from TH-responsive to TH-unresponsive appears to be isoform-specific because in skeletal muscle of 8-week-old ferrets and in hypothyroid ferrets the α2 isoform is upregulated by TH. Finally, there appears to be functional thyroid hormone receptors throughout development because in each age group TH effectively induced expression of α-MHC in the myocardium. In conclusion, these findings demonstrate that expression of α3 isoform in the myocardium of newborn ferret is responsive to TH; however, the responsiveness terminates between 3- and 6-weeks of age. Neither elevated endogenous TH level nor a lack of functional thyroid hormone receptor appears to be responsible for the transition from TH-responsive to TH-unresponsive.     

Mechanisms of thyroid hormone control over sensitivity and maximal contractile responsiveness to β-adrenergic agonists in atria

This paper discusses the mechanisms of two basic effects of thyroid hormones on atrial responses to -adrenergic agonists, i.e. increased inotropic sensitivity and decreased maximal contractile responsiveness. The increased sensitivity of atria to -adrenergic agonists under thyroid hormones appears to be related to increases in -adrenoceptor density and G_s/G_i protein ratio, leading to activation of G_s-mediated pathway, but suppression of G_i-mediated pathway of adenylate cyclase regulation. Therefore, the i/c concentrations of cAMP and corresponding inotropic responses achieve their maximums at lower doses of -adrenergic agonist. Thyroid hormones also decrease the expression of phospholamban, but increase the expression of sarcoplasmic reticulum Ca⁺²-pump. As a result, the basal activity of sarcoplasmic reticulum Ca⁺²-pump increases, but its -adrenergic activation through phosphorylation of phospholamban decreases. It is suggested that these changes are causal for decreased maximal inotropic and lusitropic responses of atria to -adrenergic agonists.     

A sensitive immunoradiometric assay for serum thyroid stimulating hormone: a replacement for the thyrotrophin releasing hormone test?

The value as a thyroid function test of a new, rapid, and highly sensitive immunoradiometric assay for thyroid stimulating hormone (TSH) was assessed in 188 consecutive new patients with suspected hyperthyroidism. The diagnosis was made on clinical grounds and on the basis of serum total triiodothyronine and thyroxine concentrations and the response of TSH to thyrotrophin releasing hormone (TRH) as measured by radioimmunoassay. In all except one patient the basal TSH concentration by immunoradiometric assay predicted the response of TSH by radioimmunoassay to TRH, an undetectable value being recorded in patients with a subnormal response and a measurable value in those with a normal test result. This clear relation was not observed for basal TSH concentrations as measured by radioimmunoassay. In a series of 39 hospital inpatients with acute or chronic non-thyroidal illness, of whom 11 had low concentrations of total thyroxine or triiodothyronine, or both, basal TSH concentrations were detectable by both radioimmunoassay and immunoradiometric assay in all cases and were associated with normal responses to TRH. The immunoradiometric assay for TSH, which is commercially available, may therefore obviate the need for the more time consuming TRH test and simplify the approach to thyroid function testing in patients with suspected hyperthyroidism.     

Do thyroid hormones function in insects?

Earlier work demonstrated that phenoxy-phenyl compounds such as fenoxycarb and thyroxine mimicked the effects of JH III in causing a reduction in volume of the follicle cells of Locusta migratoria. While these compounds were only moderately effective, a derivative of thyroxine, 3,3',5-triiodothyronine (T3) was as effective as JH III, and T3 has been shown to bind to the same membrane receptor and activate the same pathway as JH III. The current paper shows that other thyroxine derivatives vary in activity. 3,3', 5'-Triiodothyronine (reverse T3) is inactive. 3,5-Diiodothyronine (T2) is more active than JH III, while its relatives (iodines at 3', 5' or at 3,3') are inactive. When follicles are exposed in vitro to rhodamine conjugated T3, the fluorescent compound can be seen to enter the cells and accumulate there: this process is inhibited by cycloheximide or by a temperature of 0 degrees C. The accumulation is antagonised by JH III but not JH I (which does not bind to the JH III membrane receptor) and by an antiserum raised against the putative membrane receptor protein. The action of T3, but not T2, is inhibited by 6-n-propyl-2-thiouracil or by aurothioglucose, both known to inhibit deiodinases. The activity of T3, but not of T2, increases with time of exposure to the follicle cells. These facts suggest that T3 enters the cells by receptor mediated endocytosis and is converted to a more active compound. Immunoreactivity to T3, but not thyroxine, can be detected in the haemolymph of locusts, and the titre varies slightly with the gonotrophic cycle. The food shows immunoreactivity for both thyroxine and T3. These findings suggest that thyroid hormones are ingested by locusts and have the potential to be used as hormonal signals in the control of egg production.