Thyroid hormone regulation of metabolism

Emerging regulation and function of betatrophin

Betatrophin, also known as TD26/RIFL/lipasin/ANGPTL8/C19orf80, is a novel protein predominantly expressed in human liver. To date, several betatrophin orthologs have been identified in mammals. Increasing evidence has revealed an association between betatrophin expression and serum lipid profiles, particularly in patients with obesity or diabetes. Stimulators of betatrophin, such as insulin, thyroid hormone, irisin and caloric intake, are usually relevant to energy expenditure or thermogenesis. In murine models, serum triglyceride levels as well as pancreatic cell proliferation are potently enhanced by betatrophin. Intriguingly, conflicting phenomena have also been reported that betatrophin suppresses hepatic triglyceride levels, suggesting that betatrophin function is mediated by complex regulatory processes. However, its precise physiological role remains unclear at present. In this review, we have summarized the current findings on betatrophin and their implications.

Proteomic approaches for the study of tissue specific effects of 3,5,3'-triiodo-L-thyronine and 3,5-diiodo-L-thyronine in conditions of altered energy metabolism

In vertebrates and, specifically, in mammals, energy homeostasis is achieved by the integration of metabolic and neuroendocrine signals linked to one another in an intricate network hierarchically responding to the tight modulating action of hormones among which thyroid hormones (THs) play a central role. At the cellular level, 3,5,3′-triiodo-L-thyronine (T3) acts mainly by binding to specific nuclear receptors (TRs) but actually it is becoming more and more evident that some T3- actions are independent of TRs and that other iodothyronines, such as 3,5-diiodo-L-thyronine (T2), affect energy metabolism and adiposity. In the postgenomic era, clinical and basic biological researches are increasingly benefiting from the recently developed new omics approaches including, among the others, proteomics. Considering the recognized value of proteins as excellent targets in physiology, the functional and simultaneous analysis of the expression level and the cellular localization of multiple proteins can actually be considered fundamental in the understanding of complex mechanisms such as those involved in thyroid control of metabolism. Here, we will discuss new leads (i.e., target proteins and metabolic pathways) emerging in applying proteomics to the actions of T3 and T2 in conditions of altered energy metabolism in animal tissues having a central role in the control of energy balance.

Altered thyroid hormone profile in offspring after exposure to high estradiol environment during the first trimester of pregnancy: a cross-sectional study

BACKGROUND

The increasing number of babies conceived by in vitro fertilization and embryo transfer (IVF-ET) shifts concern from pregnancy outcomes to long-time health of offspring. Maternal high estradiol (E2) is a major characteristic of IVF-ET and lasts throughout the first trimester of pregnancy. The fetal thyroid develops during this period and may thus be affected by exposure to the supra-physiological E2. The aim of this study is to investigate whether the high E2 maternal environment in the first trimester increases the risk of thyroid dysfunction in children born following IVF-ET.

METHODS

A cross-sectional survey design was used to carry out face-to-face interviews with consecutive children attending the hospital. A total of 949 singletons born after fresh embryo transfer (ET) (n=357), frozen ET (n=212), and natural conception (NC) (n=380), aged 3 to 10 years old, were included. All children were thoroughly examined. Meanwhile, another 183 newborns, including 55 fresh ET, 48 frozen ET, and 80 NC were studied. Levels of serum T3, FT3, T4, FT4, and TSH and levels of maternal E2 at different stages of the first trimester were examined.

RESULTS

The mean serum E2 levels of women undergoing fresh ET during the first trimester of pregnancy were significantly higher than those of the women undergoing frozen ET or following NC. The thyroid hormone profile, especially the levels of T4, FT4, and TSH, were significantly increased in 3- to 10-year-old children conceived by fresh ET compared to NC. The same tendency was confirmed in newborns. However, levels of T4 and TSH in the frozen ET group were nearer to that of the NC group. Furthermore, levels of T4 and FT4 in fresh ET were positively correlated with maternal serum levels of E2 during early pregnancy.

CONCLUSIONS

The maternal high E2 environment in the first trimester is correlated with increased risk of thyroid dysfunction. Frozen ET could reduce risks of thyroid damage in children conceived by IVF. Further studies are needed to confirm these findings and to better determine the underlying molecular mechanisms and clinical significance.

TRIAL REGISTRATION

ChicCTR-OCC-14004682 (22-05-2014).

Effects of dietary creatine supplementation on systemic microvascular density and reactivity in healthy young adults

Background

Dietary creatine supplementation (CrS) is a practice commonly adopted by physically active individuals. However, the effects of CrS on systemic microvascular reactivity and density have never been reported. Additionally, CrS is able to influence blood levels of homocysteine, resulting in presumed effects on vascular endothelial function. Thus, we investigated the effects of CrS on the systemic microcirculation and on homocysteine levels in healthy young individuals.

Methods

This open-label study was performed on a group of 40 healthy male, moderately physically active subjects aged 27.7 ± 13.4 years who received one week of CrS at a dose of 20 g/day of commercially available micronized creatine monohydrate. Laser speckle contrast imaging was used in the evaluation of cutaneous microvascular reactivity, and intra-vital video microscopy was used to evaluate skin capillary density and reactivity, before and after CrS.

Results

CrS did not alter plasma levels of homocysteine, although CrS increased creatinine (p = 0.0001) and decreased uric acid (p = 0.0004) plasma levels. Significant changes in total cholesterol (p = 0.0486) and LDL-cholesterol (p = 0.0027) were also observed along with a reduction in plasma levels of T3 (p = 0.0074) and an increase in T4 levels (p = 0.0003). Skin functional capillary density (p = 0.0496) and capillary recruitment during post-occlusive reactive hyperemia (p = 0.0043) increased after CrS. Increases in cutaneous microvascular vasodilation induced by post-occlusive reactive hyperemia (p = 0.0078) were also observed.

Conclusions

Oral supplementation with creatine in healthy, moderately physically active young adults improves systemic endothelial-dependent microvascular reactivity and increases skin capillary density and recruitment. These effects are not concurrent with changes in plasma homocysteine levels.

Adipose tissues and thyroid hormones

The maintenance of energy balance is regulated by complex homeostatic mechanisms, including those emanating from adipose tissue. The main function of the adipose tissue is to store the excess of metabolic energy in the form of fat. The energy stored as fat can be mobilized during periods of energy deprivation (hunger, fasting, diseases). The adipose tissue has also a homeostatic role regulating energy balance and functioning as endocrine organ that secretes substances that control body homeostasis. Two adipose tissues have been identified: white and brown adipose tissues (WAT and BAT) with different phenotype, function and regulation. WAT stores energy, while BAT dissipates energy as heat. Brown and white adipocytes have different ontogenetic origin and lineage and specific markers of WAT and BAT have been identified. “Brite” or beige adipose tissue has been identified in WAT with some properties of BAT. Thyroid hormones exert pleiotropic actions, regulating the differentiation process in many tissues including the adipose tissue. Adipogenesis gives raise to mature adipocytes and is regulated by several transcription factors (c/EBPs, PPARs) that coordinately activate specific genes, resulting in the adipocyte phenotype. T3 regulates several genes involved in lipid mobilization and storage and in thermogenesis. Both WAT and BAT are targets of thyroid hormones, which regulate genes crucial for their proper function: lipogenesis, lipolysis, thermogenesis, mitochondrial function, transcription factors, the availability of nutrients. T3 acts directly through specific TREs in the gene promoters, regulating transcription factors. The deiodinases D3, D2, and D1 regulate the availability of T3. D3 is activated during proliferation, while D2 is linked to the adipocyte differentiation program, providing T3 needed for lipogenesis and thermogenesis. We examine the differences between BAT, WAT and brite/beige adipocytes and the process that lead to activation of UCP1 in WAT and the presence of BAT in humans and its relevance.

New avenues for regulation of lipid metabolism by thyroid hormones and analogs

Weight loss due to negative energy balance is a goal in counteracting obesity and type 2 diabetes mellitus. The thyroid is known to be an important regulator of energy metabolism through the action of thyroid hormones (THs). The classic, active TH, 3,5,3'-triiodo-L-thyronine (T3) acts predominantly by binding to nuclear receptors termed TH receptors (TRs), that recognize TH response elements (TREs) on the DNA, and so regulate transcription. T3 also acts through "non-genomic" pathways that do not necessarily involve TRs. Lipid-lowering therapies have been suggested to have potential benefits, however, the establishment of comprehensive therapeutic strategies is still awaited. One drawback of using T3 in counteracting obesity has been the occurrence of heart rhythm disturbances. These are mediated through one TR, termed TRα. The end of the previous century saw the exploration of TH mimetics that specifically bind to TR beta in order to prevent cardiac disturbances, and TH derivatives such as 3,5-diiodo-L-thyronine (T2), that possess interesting biological activities. Several TH derivatives and functional analogs have low affinity for the TRs, and are suggested to act predominantly through non-genomic pathways. All this has opened new perspectives in thyroid physiology and TH derivative usage as anti-obesity therapies. This review addresses the pros and cons of these compounds, in light of their effects on energy balance regulation and on lipid/cholesterol metabolism.

Early postnatal exposure to a low dose of decabromodiphenyl ether affects expression of androgen and thyroid hormone receptor-alpha and its splicing variants in mouse Sertoli cells

Decabromodiphenyl ether (decaBDE) adversely affects reproduction and development. Our previous study showed that postnatal exposure to a low dose of decaBDE (0.025 mg/kg body weight/day) by subcutaneous injection on postnatal days (PNDs) 1 through 5 leads to reductions in testicular size and number of Sertoli cells and sperm, while higher dose of decaBDE (2.5 mg/kg body weight/day) had no significant differences about these. In the present study, we examined the molecular mechanism of these effects on mouse testes following postnatal exposure to a low decaBDE dose. We hypothesized that postnatal exposure to decaBDE may alter levels of serum thyroid hormones (THs) and testosterone, or the level of TH receptor alpha (Thra) transcripts and its splicing variants and androgen receptor (Ar) in Sertoli cells, adversely affecting spermatogenesis. To test this hypothesis, we examined serum TH and testosterone levels and the levels of transcripts of the Ar, Thra and its splicing variants, and Thra splicing factors (Hnrnpa1, Srsf1, and Hnrnph1) with qPCR in isolated mouse Sertoli cells exposed postnatally to decaBDE (0.025, 0.25, and 2.5 mg/kg). Levels of serum testosterone and transcripts encoding Ar, Thra, and its variant, Thra1, declined significantly in Sertoli cells of mice exposed to 0.025 mg decaBDE/kg. No significant differences in serum TH level or Thra2, Hnrnph1, or Srsf1 transcript levels were observed between control and decaBDE-exposed mice. However, the Thra1:Thra2 and Hnrnpa1:Srsf1 ratios were altered in Sertoli cells of mice exposed to 0.025 mg decaBDE/kg but not in cells exposed to 0.25 or 2.5 mg decaBDE/kg. These results indicate that postnatal exposure to a low dose of decaBDE on PNDs 1 through 5 lowers the testosterone level and the levels of Ar and Thra transcripts in Sertoli cells, accompanied by an imbalance in the ratios of Thra splicing variants, resulting in smaller testicular size and impaired spermatogenesis.

An acute injection of corticosterone increases thyrotrophin-releasing hormone expression in the paraventricular nucleus of the hypothalamus but interferes with the rapid hypothalamus pituitary thyroid axis response to cold in male rats

The activity of the hypothalamic-pituitary-thyroid (HPT) axis is rapidly adjusted by energy balance alterations. Glucocorticoids can interfere with this activity, although the timing of this interaction is unknown. In vitro studies indicate that, albeit incubation with either glucocorticoid receptor (GR) agonists or protein kinase A (PKA) activators enhances pro-thyrotrophin-releasing hormone (pro-TRH) transcription, co-incubation with both stimuli reduces this enhancement. In the present study, we used primary cultures of hypothalamic cells to test whether the order of these stimuli alters the cross-talk. We observed that a simultaneous or 1-h prior (but not later) activation of GR is necessary to inhibit the stimulatory effect of PKA activation on pro-TRH expression. We tested these in vitro results in the context of a physiological stimulus on the HPT axis in adult male rats. Cold exposure for 1 h enhanced pro-TRH mRNA expression in neurones of the hypophysiotrophic and rostral subdivisions of the paraventricular nucleus (PVN) of the hypothalamus, thyrotrophin (TSH) serum levels and deiodinase 2 (D2) activity in brown adipose tissue (BAT). An i.p. injection of corticosterone stimulated pro-TRH expression in the PVN of rats kept at ambient temperature, more pronouncedly in hypophysiotrophic neurones that no longer responded to cold exposure. In corticosterone-pretreated rats, the cold-induced increase in pro-TRH expression was detected only in the rostral PVN. Corticosterone blunted the increase in serum TSH levels and D2 activity in BAT produced by cold in vehicle-injected animals. Thus, increased serum corticosterone levels rapidly restrain cold stress-induced activation of TRH hypophysiotrophic neurones, which may contribute to changing energy expenditure. Interestingly, TRH neurones of the rostral PVN responded to both corticosterone and cold exposure with an amplified expression of pro-TRH mRNA, suggesting that these neurones integrate stress and temperature distinctly from the hypophysiotrophic neurones.

Thyroid circadian timing: roles in physiology and thyroid malignancies

The circadian clock represents an anticipatory mechanism, well preserved in evolution. It has a critical impact on most aspects of the physiology of light-sensitive organisms. These rhythmic processes are governed by environmental cues (fluctuations in light intensity and temperature), an internal circadian timing system, and interactions between this timekeeping system and environmental signals. Endocrine body rhythms, including hypothalamic-pituitary-thyroid (HPT) axis rhythms, are tightly regulated by the circadian system. Although the circadian profiles of thyroid-releasing hormone (TRH), thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) in blood have been well described, relatively few studies have analyzed molecular mechanisms governing the circadian regulation of HPT axis function. In this review, we will discuss the latest findings in the area of complex regulation of thyroid gland function by the circadian oscillator. We will also highlight the molecular makeup of the human thyroid oscillator as well as the potential link between thyroid malignant transformation and alterations in the clockwork.

Restoration of thyroid hormone balance: a game changer in the treatment of heart failure?

The link between low thyroid hormone (TH) function and heart failure is reviewed in the present report. The idea that TH dysfunction may contribute to diseases leading to HF has been discussed for over 60 yr. A growing body of evidence from animal and human studies, particularly in recent years, suggests that TH treatment may improve clinical outcomes. Indeed, if a similar amount of positive information were available for a newly developed heart drug, there is little doubt that large-scale clinical trials would be underway with considerable excitement. THs offer the promise of improving ventricular contraction and relaxation, improving coronary blood flow, and inhibiting atherosclerosis, and new results suggest they may even reduce the incidence of arrhythmias in heart diseases. Are the potential clinical benefits worth the risk of possible overdosing? After so many years, why has this question not been answered? Clearly, the concept has not been disproven. This review explores the body of clinical evidence related to TH dysfunction and heart failure, discuss insights into pathophysiological, cellular, and molecular mechanisms provided by animal research, and discuss what is needed to resolve this long-standing issue in cardiology and move forward.

Update on 3-iodothyronamine and its neurological and metabolic actions

3-iodothyronamine (T1AM) is an endogenous amine, that has been detected in many rodent tissues, and in human blood. It has been hypothesized to derive from thyroid hormone metabolism, but this hypothesis still requires validation. T1AM is not a ligand for nuclear thyroid hormone receptors, but stimulates with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor. With a lower affinity it interacts with alpha2A adrenergic receptors. Additional targets are represented by apolipoprotein B100, mitochondrial ATP synthase, and membrane monoamine transporters, but the functional relevance of these interactions is still uncertain. Among the effects reported after administration of exogenous T1AM to experimental animals, metabolic and neurological responses deserve special attention, because they were obtained at low dosages, which increased endogenous tissue concentration by about one order of magnitude. Systemic T1AM administration favored fatty acid over glucose catabolism, increased ketogenesis and increased blood glucose. Similar responses were elicited by intracerebral infusion, which inhibited insulin secretion and stimulated glucagon secretion. However, T1AM administration increased ketogenesis and gluconeogenesis also in hepatic cell lines and in perfused liver preparations, providing evidence for a peripheral action, as well. In the central nervous system, T1AM behaved as a neuromodulator, affecting adrenergic and/or histaminergic neurons. Intracerebral T1AM administration favored learning and memory, modulated sleep and feeding, and decreased the pain threshold. In conclusion T1AM should be considered as a component of thyroid hormone signaling and might play a significant physiological and/or pathophysiological role. T1AM analogs have already been synthetized and their therapeutical potential is currently under investigation. 3-iodothyronamine (T1AM) is a biogenic amine whose structure is closely related to that of thyroid hormone (3,5,3′-triiodothyronine, or T3). The differences with T3 are the absence of the carboxylate group and the substitution of iodine with hydrogen in 5 and 3′ positions (Figure 1). In this paper we will review the evidence supporting the hypothesis that T1AM is a chemical messenger, namely that it is an endogenous substance able to interact with specific receptors producing significant functional effects. Special emphasis will be placed on neurological and metabolic effects, which are likely to have physiological and pathophysiological importance.

Thyroid hormone regulation of hepatic lipid and carbohydrate metabolism

Thyroid hormone (TH) has important roles in regulating hepatic lipid, cholesterol, and glucose metabolism. Recent findings suggest that clinical conditions such as non-alcoholic fatty liver disease and type 2 diabetes mellitus, which are associated with dysregulated hepatic metabolism, may involve altered intracellular TH action. In addition, TH has key roles in lipophagy in lipid metabolism, mitochondrial quality control, and the regulation of metabolic genes. In this review, we discuss recent findings regarding the functions of TH in hepatic metabolism, the relationship between TH and metabolic disorders, and the potential therapeutic use of thyromimetics to treat metabolic dysfunction in the liver.

Assessing the association of single nucleotide polymorphisms in thyroglobulin gene with age of puberty in bulls

Puberty is a stage of sexual development determined by the interaction of many loci and environmental factors. Identification of genes contributing to genetic variation in this character can assist with selection for early pubertal bulls, improving genetic progress in livestock breeding. Thyroid hormones play an important role in sexual development and spermatogenic function. The objective of this study was to evaluate the association between single nucleotide polymorphisms (SNPs) located in thyroglobulin(TG) gene with age of puberty in Angus bulls. Four SNPs were genotyped in 273 animals using SEQUENOM technology and the association between markers and puberty age was analyzed. Results showed a significant association (P < 0.05) between these markers and puberty age estimated at a sperm concentration of 50 million and a progressive motility of 10%. This is the first report of an association of TG polymorphisms with age of puberty in bulls, and results suggest the importance of thyroidal regulation in bovine sexual development and arrival to puberty.