Thyroid hormone and norepinephrine signaling in brown adipose tissue. II: Differential effects of thyroid hormone on beta 3-adrenergic receptors in brown and white adipose tissue

Spexin may induce mitochondrial biogenesis in white and brown adipocytes via the hypothalamus-pituitary-thyroid (HPT) axis

AIM

The present study aimed to investigate the effect of the intracerebroventricular (icv) administration of spexin on the hypothalamus-pituitary-thyroid (HPT) axis (TRH, TSH, T4 and T3 hormones) and energy expenditure (PGC-1α and UCP1 genes) in white adipose (WAT) and brown adipose tissues (BAT) in rats. Furthermore, the study aimed to determine the effects of spexin on food-water consumption and body weight of rats.

MATERIAL AND METHOD

The study was conducted with 40 male rats that were divided into 4 groups: Control, Sham, Spexin 30 and Spexin 100 (n = 10). Spexin (1 μl/hour) was administered to rats other than those in the control group for 7 days with osmotic minipumps intracerebroventricularly, artificial cerebrospinal fluid (vehicle) was administered to the Sham group, and 30 nMol and 100 nMol spexin was infused to the Spexin 30 and Spexin 100 groups, respectively. Food-water consumption and body weight of the rats were monitored during the experiments. After the seven-day infusion, the rats were decapitated and serum TSH, fT4 and fT3 levels were determined with ELISA on rat blood samples. Also, TRH gene expression levels from the hypothalamus tissues and PGC-1α and UCP1 expression levels from WAT and BAT were determined by real-time PCR.

FINDINGS

It was determined that icv spexin infusion reduced daily food consumption and body weight without leading to a significant change in water consumption (p < 0.05). Icv spexin infusion significantly decreased serum TSH, and increased fT4 and fT3 levels when compared to control and sham groups (p < 0.05). Moreover, icv spexin infusion increased the TRH expressions in the hypothalamus tissues and PGC-1α UCP1 in the WAT and BAT (p < 0.05).

CONCLUSION

Icv Spexin infusion may have effects on food consumption and body weight as well as, thyroid hormones and energy metabolism.

The adipocyte supersystem of insulin and cAMP signaling

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

RNAseq Analysis of Brown Adipose Tissue and Thyroid of Newborn Lambs Subjected to Short-Term Cold Exposure Reveals Signs of Early Whitening of Adipose Tissue

During the early postnatal period, lambs have the ability to thermoregulate body temperature via non-shivering thermogenesis through brown adipose tissue (BAT), which soon after birth begins to transform into white adipose tissue. An RNA seq approach was used to characterize the transcriptome of BAT and thyroid tissue in newborn lambs exposed to cold conditions. Fifteen newborn Romney lambs were selected and divided into three groups: group 1 (n = 3) was a control, and groups 2 and 3 (n = 6 each) were kept indoors for two days at an ambient temperature (20–22 °C) or at a cold temperature (4 °C), respectively. Sequencing was performed using a paired-end strategy through the BGISEQ-500 platform, followed by the identification of differentially expressed genes using DESeq2 and an enrichment analysis by g:Profiler. This study provides an in-depth expression network of the main characters involved in the thermogenesis and fat-whitening mechanisms that take place in the newborn lamb. Data revealed no significant differential expression of key thermogenic factors such as uncoupling protein 1, suggesting that the heat production peak under cold exposure might occur so rapidly and in such an immediate way that it may seem undetectable in BAT by day three of life. Moreover, these changes in expression might indicate the start of the whitening process of the adipose tissue, concluding the non-shivering thermogenesis period.

Leptin counteracts hypothermia in hypothyroidism through its pyrexic effects and by stabilizing serum thyroid hormone levels

Objective

Thyroid hormones (TH) are essential for the homeostatic control of energy metabolism and the regulation of body temperature. The hypothalamic–pituitary–thyroid (HPT) axis is regulated by negative feedback mechanisms, ensuring that TH levels are maintained at a constant level. However, the feedback mechanisms underlying the resetting of the HPT axis regulation in the control of body temperature are still not fully understood. Here, we aimed to determine the thermoregulatory response in hypothyroid mice to different environmental temperatures and the underlying mechanisms.

Methods

Distinct thermogenic challenges were induced in hypothyroid female C57BL/6N and leptin-deficient ob/ob mice through housing at either room temperature or thermoneutrality. The thermogenic and metabolic effects were analyzed through metabolic chambers, 18F-FDG-PET/MRI, infrared thermography, metabolic profiling, histology, gene expression and Western blot analysis.

Results

In hypothyroid mice maintained at room temperature, high leptin serum levels induce a pyrexic effect leading to the stabilization of body temperature through brown adipose tissue thermogenesis and white adipose tissue browning. Housing at thermoneutrality leads to the normalization of leptin levels and a reduction of the central temperature set point, resulting in decreased thermogenesis in brown and white adipose tissue and skeletal muscle and a significant decline in body temperature. Furthermore, anapyrexia in hypothyroid leptin-deficient ob/ob mice indicates that besides its pyrexic actions, leptin exerts a stimulatory effect on the HPT axis to stabilize the remaining TH serum levels in hypothyroid mice.

Conclusion

This study led to the identification of a previously unknown endocrine loop in which leptin acts in concert with the HPT axis to stabilize body temperature in hypothyroid mice.

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Thyroid hormones are essential for the regulation of body temperature.

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Thyroid hormone-deficient (hypothyroid) mice show distinct leptin serum concentrations in response to changes in ambient housing temperature.

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High leptin serum levels confer a stimulatory effect on the hypothalamic-pituitary-thyroid axis.

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High leptin serum level prevents fall in body temperature in hypothyroid mice at room temperature through its pyrexic effects.

Control of Adipocyte Thermogenesis and Lipogenesis through β3-Adrenergic and Thyroid Hormone Signal Integration

Here, we show that β adrenergic signaling coordinately upregulates de novo lipogenesis (DNL) and thermogenesis in subcutaneous white adipose tissue (sWAT), and both effects are blocked in mice lacking the cAMP-generating G protein-coupled receptor Gs (Adipo-GsαKO) in adipocytes. However, UCP1 expression but not DNL activation requires rapamycin-sensitive mTORC1. Furthermore, β3-adrenergic agonist CL316243 readily upregulates thermogenic but not lipogenic genes in cultured adipocytes, indicating that additional regulators must operate on DNL in sWAT in vivo. We identify one such factor as thyroid hormone T3, which is elevated locally by adrenergic signaling. T3 administration to wild-type mice enhances both thermogenesis and DNL in sWAT. Mechanistically, T3 action on UCP1 expression in sWAT depends upon cAMP and is blocked in Adipo-GsαKO mice even as elevated DNL persists. Thus, T3 enhances sWAT thermogenesis by amplifying cAMP signaling, while its control of adipocyte DNL can be mediated independently of both cAMP and rapamycin-sensitive mTORC1.

The Unity of Redox and Structural Remodeling of Brown Adipose Tissue in Hypothyroidism

Brown adipose tissue (BAT) is important for maintaining whole-body metabolic and energy homeostasis. However, the effects of hypothyroidism, one of the most common diseases worldwide, which increases the risk of several metabolic disorders, on BAT redox and metabolic homeostasis remain mostly unknown. We aimed to investigate the dynamics of protein expression, enzyme activity, and localization of antioxidant defense (AD) enzymes in rat interscapular BAT upon induction of hypothyroidism by antithyroid drug methimazole for 7, 15, and 21 days. Our results showed an increased protein expression of CuZn- and Mn-superoxide dismutase, catalase, glutamyl-cysteine ligase, thioredoxin, total glutathione content, and activity of catalase and thioredoxin reductase in hypothyroid rats, compared to euthyroid control. Concomitant with the increase in AD, newly established nuclear, mitochondrial, and peroxisomal localization of AD enzymes was found. Hypothyroidism also potentiated associations between mitochondria, peroxisomes, and lipid bodies, creating specific structural-functional units. Moreover, hypothyroidism induced protein expression and nuclear translocation of a master regulator of redox-metabolic homeostasis, nuclear factor erythroid 2-related factor 2 (Nrf2), and an increased amount of 4-hydroxynonenal (4-HNE) protein adducts. The results indicate that spatiotemporal overlap in the remodeling of AD is orchestrated by Nrf2, implicating the role of 4-HNE in this process and suggesting the potential mechanism of redox-structural remodeling during BAT adaptation in hypothyroidism.

Effects of Hyperthyroidism on Adipose Tissue Activity and Distribution in Adults

Background: Positron emission tomography (PET) has provided evidence that adult humans retain metabolically active brown adipose tissue (BAT) depots. Thyroid hormones (TH) stimulate BAT thermogenesis by central and peripheral mechanisms. However, the effect of hyperthyroidism on BAT activity and BAT volume in humans is yet not fully understood. The aim of this study was to investigate the effect of TH on (i) the metabolic activity of brown and white adipose tissue (WAT) depots, (ii) on abdominal visceral and subcutaneous adipose tissue area, and (iii) on serum levels of metabolically active cytokines. Methods: Nineteen patients with overt hyperthyroidism were investigated through repeated 2-[¹⁸F]fluoro-2-deoxy-d-glucose positron emission tomography/computed tomography (2-[¹⁸F]FDG PET/CT) in the hyperthyroid and in the euthyroid state. The 2-[¹⁸F]FDG uptake was calculated as standard uptake ratio with blood pool as reference. Fat areas were quantified by means of CT segmentation. Serum levels of fetuin A and B, fibroblast growth factor 21, adipocyte fatty acid-binding protein (AFABP), retinol-binding protein 4, pro-enkephalin, pro-neurotensin, and neuregulin 4 were determined in the hyperthyroid and in the euthyroid state for each subject. Results: 2-[¹⁸F]FDG uptake was increased in the hyperthyroid state in BAT in comparison with the euthyroid phase (p = 0.001). There was no correlation between serum free triiodothyronine (fT3) and free thyroxine (fT4) levels and 2-[¹⁸F]FDG uptake in BAT or WAT. In the hyperthyroid state, fT3 levels were positively associated with skeletal muscle standardized uptake value ratios. Areas of visceral adipose tissue and skeletal muscle were significantly decreased in hyperthyroidism. AFABP levels correlated positively with fT3 (p = 0.031, β = 0.28) and fT4 (p = 0.037, β = 0.27) in the hyperthyroid state. Conclusions: Our results suggest that the contribution of increased TH levels to the glucose uptake of BAT and WAT is low compared with that of the skeletal muscle. Hyperthyroid subjects have reduced areas of visceral adipose tissue and increased AFABP levels.

Thyroid hormones in the regulation of brown adipose tissue thermogenesis

A normal thyroid status is crucial for body temperature homeostasis, as thyroid hormone regulates both heat loss and conservation as well as heat production in the thermogenic tissues. Brown adipose tissue (BAT) is the major site of non-shivering thermogenesis and an important target of thyroid hormone action. Thyroid hormone not only regulates the tissue's sensitivity to sympathetic stimulation by norepinephrine but also the expression of uncoupling protein 1, the key driver of BAT thermogenesis. Vice versa, sympathetic stimulation of BAT triggers the expression of deiodinase type II, an enzyme that enhances local thyroid hormone availability and signaling. This review summarizes the current knowledge on how thyroid hormone controls BAT thermogenesis, aiming to dissect the direct actions of the hormone in BAT and its indirect actions via the CNS, browning of white adipose tissue or heat loss over body surfaces. Of particular relevance is the apparent dose dependency of the observed effects, as we find that minor or moderate changes in thyroid hormone levels often have different effects as compared to high pharmacological doses. Moreover, we conclude that the more recent findings require a reevaluation of older studies, as key aspects such as heat loss or central BAT activation may not have received the necessary attention during the interpretation of these early findings. Finally, we provide a list of what we believe are the most relevant questions in the field that to date are still enigmatic and require further studies.

Takotsubo Syndrome: Clinical Manifestations, Etiology and Pathogenesis

The purpose of the review is the analysis of clinical and experimental data on the etiology and pathogenesis of takotsubo syndrome (TS). TS is characterized by contractile dysfunction, which usually affects the apical region of the heart without obstruction of coronary artery, moderate increase in myocardial necrosis markers, prolonged QTc interval (in 50% of patients), sometimes elevation of ST segment (in 19% of patients), increase N-Terminal Pro-B-Type Natriuretic Peptide level, microvascular dysfunction, sometimes spasm of the epicardial coronary arteries (in 10% of patients), myocardial edema, and life-threatening ventricular arrhythmias (in 11% of patients). Stress cardiomyopathy is a rare disease, it is observed in 0.6 - 2.5% of patients with acute coronary syndrome. The occurrence of takotsubo syndrome is 9 times higher in women, who are aged 60-70 years old, than in men. The hospital mortality among patients with TS corresponds to 3.5% - 12%. Physical or emotional stress do not precede disease in all patients with TS. Most of patients with TS have neurological or mental illnesses. The level of catecholamines is increased in patients with TS, therefore, the occurrence of TS is associated with excessive activation of the adrenergic system. The negative inotropic effect of catecholamines is associated with the activation of β2 adrenergic receptors. An important role of the adrenergic system in the pathogenesis of TS is confirmed by studies which were performed using 125I-metaiodobenzylguanidine (125I -MIBG). TS causes edema and inflammation of the myocardium. The inflammatory response in TS is systemic. TS causes impaired coronary microcirculation and reduces coronary reserve. There is a reason to believe that an increase in blood viscosity may play an important role in the pathogenesis of microcirculatory dysfunction in patients with TS. Epicardial coronary artery spasm is not obligatory for the occurrence of TS. Cortisol, endothelin-1 and microRNAs are challengers for the role of TS triggers. A decrease in estrogen levels is a factor contributing to the onset of TS. The central nervous system appears to play an important role in the pathogenesis of TS.

[Clinical Manifestation of Stressful Cardiomyopathy (Takotsubo Syndrome) and the Problem of Differential Diagnosis with Acute Myocardial Infarction]

The presented data show that tacotsubo syndrome (TS) is characterized by the absence of coronary artery obstruction, cardiac contractile dysfunction, apical ballooning, and heart failure, and in some patients, ST-segment elevation and prolongation of the QTc interval. Every tenth patient with TS develops ventricular arrhythmias. Most of TS patients have elevated markers of necrosis (troponin I, troponin Т, and creatine kinase МВ (CK-МВ), which are considerably lower than in patients with acute myocardial infarction (AMI) with ST-segment elevation. The level of N-terminal pro-B-type natriuretic peptide (NT-proBNP), in contrast, is considerably higher in patients with TS than with AMI. Differential diagnosis of TS and AMI should be based on a multifaceted approach using coronary angiography, echocardiography, analysis of ECG, magnetic resonance imaging, single-photon emission computed tomography, and measurement of troponins, CK-MB, and NT-proBNP.

Thyroid hormones and the mechanisms of adaptation to cold

The thyroid gland plays a crucial role in the regulation of metabolism, oxygen consumption, and the release of energy in the form of heat to maintain the body. Even at rest, these processes are sensitive to changes in thyroid function. This means that along with the adrenergic system, thyroid function determines the organism's ability to adapt to cold. Cold adaptation causes deiodination of thyroxine (T₄) and thus promotes an increase in blood triiodothyronine (T₃) levels in humans and animals. Triiodothyronine is an inductor of iodothyronine deiodinase expression in brown fat, liver, and kidney. Iodothyronine deiodinase plays an important role in adaptation of the organism to cold by contributing to high adrenergic reactivity of brown fat. T₃ also leads to an increase in expression of uncoupling proteins and uncoupling oxidative phosphorylation and an increase in heat production. The aim of this article is to review the available literature regarding the role of thyroid hormones in adaptation to cold and to present the current knowledge of the understanding of the molecular mechanism underlying their action during cold adaptation.

Nutritional status and follicular-derived thyroid cancer: An update

The incidence of differentiated thyroid cancer has been increasing in the last decades all over the world. Such a steady growth cannot be entirely attributable to more intensive thyroid nodule screening and more sensitive diagnostic procedures. Several environmental factors have changed with sufficient rapidity in the same time frame and may represent credible candidates for this increase. They include modified iodine intake, lifestyle-associated risk factors, exposure to various toxic compounds, pollutants and xenobiotics, nutritional deficiencies, eating habits and comorbidities. Foremost, nutritional patterns have gained high interest as possible promoters and modifiable risk factors for thyroid cancer in recent years. The aim of this narrative review is to focus on the relationship between thyroid cancer and nutritional factors, dietary habits and obesity. Low iodine intake has been associated to increased risk of thyroid cancer, favoring the development of more aggressive histotypes. Moreover, correction of iodine deficiency can shift thyroid cancer subtypes toward less aggressive forms, without affecting the overall risk for cancer. Actually, evidence regarding the association between selenium and vitamin D deficiency and thyroid cancer is very limited, despite their well-known anti-cancer potentials, and the clinical usefulness of their supplementation is still uncertain in this setting. Albeit the relationship between single foods and thyroid cancer is difficult to examine, fish and iodine-rich foods, vegetables, and fruits might exert protective effects on thyroid cancer risk. Conversely, no clear association has been found for other foods to date. Lastly, a clear association between obesity and the risk of thyroid cancer, with more aggressive behavior, seems to emerge from most studies, likely involving variations in thyroid function and chronic inflammation mediated by cytokines, insulin, leptin and adiponectins. Although no definite association between dietary factors and thyroid cancer has been firmly established so far, some nutritional patterns, together with excessive weight, seem to play a relevant role in thyroid cancer carcinogenesis as well as in its severity and aggressiveness. These effects may play an additive role to the well-established one exerted by environmental carcinogens, such as pollutants and radiation exposure.

Novel Aspects of White Adipose Tissue Browning by Thyroid Hormones

Thyroid hormones are essential for the full thermogenic capacity of brown adipose tissue. The thermogenic response of brown adipocytes to thyroid hormones is resulting from the synergistic interaction of thyroid hormones with the sympathetic nervous system. In recent years, evidence has been provided that thyroid hormones also induce the browning of white adipose tissues. This review will provide a brief overview about the recent findings regarding the effects of thyroid hormones on adipose tissue thermogenesis including central and peripheral regulation of white adipose tissue browning.

Voluntary Exercise-Induced Activation of Thyroid Axis and Reduction of White Fat Depots Is Attenuated by Chronic Stress in a Sex Dimorphic Pattern in Adult Rats

The activity of the hypothalamus-pituitary-thyroid (HPT) axis is inhibited by energy deficit, by acute or chronic stress, but activated by cold exposure or exercise. Because stress curtails acute cold induced activation of HPT, we evaluated the effect of chronic stress on HPT axis response to voluntary exercise, a persistent energy-demanding situation. Adult male and female Wistar rats were exposed to restraint stress, 30 min/day for 2 weeks, or to isolation (Iso) [post-natal day [PND] 30-63]. Exercise was performed (7 p.m.-7 a.m.) in a running wheel, sedentary controls stayed in individual cages (Sed); at 7 a.m. they were housed with their cage mate or individually (Iso); food intake by the exercised group was measured day and night to pair-fed Sed. At sacrifice, hormones, mRNA levels and tissue weights were quantified. Control or restrained adult rats had access to running wheel daily for 2 weeks. Compared to C, exercise decreased white adipose tissue (WAT) mass in females and males, increased hypothalamic paraventricular nucleus (PVN)-Trh expression in males proportionally to exercise performed, and increased TSH and T4 serum concentration in females. These changes were not detected in restrained groups. Starting at PND 63 control (2/cage) and isolated (1/cage) rats either exercised on 10 alternated nights or were sedentary. In control male animals, compared to Sed rats, exercise did not decrease WAT mass, nor changed HPT axis activity, but increased Pomc and deiodinase 2 (Dio2) expression in mediobasal hypothalamus (MBH), adrenergic receptor β3 and uncoupling protein-1 in brown adipose tissue. In control female animals, exercise decreased WAT mass, increased Pomc, Dio2, and Trhde expression in MBH, and TSH serum concentration. Iso females had lower TSH and T4 serum concentration, Dio2 and Trhde expression in MBH than controls. The stress response was higher in isolated males than females, but in males it did not alter the effects of exercise, in contrast to isolated females that had a blunted response to exercise compared to controls. In conclusion, chronic stress interferes with metabolic effects produced by exercise, such as loss of WAT mass, coincident with dampening of HPT activity.

Effects of Diets Differing in Composition of 18-C Fatty Acids on Adipose Tissue Thermogenic Gene Expression in Mice Fed High-Fat Diets

Dietary fatty acids play important roles in the regulation of fat accumulation or metabolic phenotype of adipocytes, either as brown or beige fat. However, a systematic comparison of effects of diets with different composition of 18-C fatty acids on browning/beiging phenotype has not been done. In this study, we compared the effects of different dietary fats, rich in specific 18-carbon fatty acids, on thermogenesis and lipid metabolism. Male C57BL/6 mice were fed a control diet containing 5.6% kcal fat from lard and 4.4% kcal fat from soybean oil (CON) or high-fat diets (HFD) containing 25% kcal from lard and 20% kcal fat from shea butter (stearic acid-rich fat; SHB), olive oil (oleic acid-rich oil; OO), safflower oil (linoleic acid-rich oil; SFO), or soybean oil (mixed oleic, linoleic, and α-linolenic acids; SBO) ad libitum for 12 weeks, with or without a terminal 4-h norepinephrine (NE) treatment. When compared to SHB, feeding OO, SFO, and SBO resulted in lower body weight gain. The OO fed group had the highest thermogenesis level, which resulted in lower body fat accumulation and improved glucose and lipid metabolism. Feeding SFO downregulated expression of lipid oxidation-related genes and upregulated expression of lipogenic genes, perhaps due to its high n-6:n-3 ratio. In general, HFD-feeding downregulated Ucp1 expression in both subcutaneous and epididymal white adipose tissue, and suppressed NE-induced Pgc1a expression in brown adipose tissue. These results suggest that the position of double bonds in dietary fatty acids, as well as the quantity of dietary fat, may have a significant effect on the regulation of oxidative and thermogenic conditions in vivo.

Effect of Iodothyronines on Thermogenesis: Focus on Brown Adipose Tissue

Thyroid hormones significantly influence energy expenditure by affecting the activity of metabolic active tissues, among which, mammalian brown adipose tissue (BAT) plays a significant role. For a long time, the modulation of BAT activity by 3,3',5-triiodo-l-thyronine (T3) has been ascribed to its direct actions on this tissue; however, recent evidence indicates that T3, by stimulating specific brain centers, activates the metabolism of BAT via the sympathetic nervous system. These distinct mechanisms of action are not mutually exclusive. New evidence indicates that 3,5-diiodo-l-thyronine (3,5-T2), a thyroid hormone derivative, exerts thermogenic effects, by influencing mitochondrial activity in metabolically active tissues, such as liver, skeletal muscle, and BAT. At the moment, due to the absence of experiments finalized to render a clear cut discrimination between peripheral and central effects induced by 3,5-T2, it is not possible to exclude that some of the metabolic effects exerted by 3,5-T2 may be mediated centrally. Despite this, some evidence suggests that 3,5-T2 plays a role in adrenergic stimulation of thermogenesis in BAT. This mini-review provides an overview of the effects induced by T3 and 3,5-T2 on BAT thermogenesis, with a focus on data suggesting the involvement of central adrenergic stimulation. These aspects may reveal new perspectives in thyroid physiology and in the control of energy metabolism.

3,5-diiodothyronine (3,5-T2) reduces blood glucose independently of insulin sensitization in obese mice

AIM

Thyroid hormones regulate metabolic response. While triiodothyronine (T3) is usually considered to be the active form of thyroid hormone, one form of diiodothyronine (3,5-T2) exerts T3-like effects on energy consumption and lipid metabolism. 3,5-T2 also improves glucose tolerance in rats and 3,5-T2 levels correlate with fasting glucose in humans. Presently, however, little is known about mechanisms of 3,5-T2 effects on glucose metabolism. Here, we set out to compare effects of T3, 3,5-T2 and another form of T2 (3,3-T2) in a mouse model of diet-induced obesity and determined effects of T3 and 3,5-T2 on markers of classical insulin sensitization to understand how diiodothyronines influence blood glucose.

METHODS

Cell- and protein-based assays of thyroid hormone action. Assays of metabolic parameters in mice. Analysis of transcript and protein levels in different tissues by qRT-PCR and Western blot.

RESULTS

T3 and 3,5-T2 both reduce body weight, adiposity and body temperature despite increased food intake. 3,3'-T2 lacks these effects. T3 and 3,5-T2 reduce blood glucose levels, whereas 3,3'-T2 worsens glucose tolerance. Neither T3 nor 3,5-T2 affects markers of insulin sensitization in skeletal muscle or white adipose tissue (WAT), but both reduce hepatic GLUT2 glucose transporter levels and glucose output. T3 and 3,5-T2 also induce expression of mitochondrial uncoupling proteins (UCPs) 3 and 1 in skeletal muscle and WAT respectively.

CONCLUSIONS

3,5-T2 influences glucose metabolism in a manner that is distinct from insulin sensitization and involves reductions in hepatic glucose output and changes in energy utilization.

LDB1 Regulates Energy Homeostasis During Diet-Induced Obesity

The broadly expressed transcriptional coregulator LDB1 is essential for β-cell development and glucose homeostasis. However, it is unclear whether LDB1 has metabolic roles beyond the β-cell, especially under metabolic stress. Global Ldb1 deletion results in early embryonic lethality; thus, we used global heterozygous Ldb1+/- and inducible β-cell-specific Ldb1-deficient (Ldb1Δβ-cell) mice. We assessed glucose and insulin tolerance, body composition, feeding, and energy expenditure during high-fat diet exposure. Brown adipose tissue (BAT) biology was evaluated by thermogenic gene expression and LDB1 chromatin immunoprecipitation analysis. We found that partial loss of Ldb1 does not impair the maintenance of glucose homeostasis; rather, we observed improved insulin sensitivity in these mice. Partial loss of Ldb1 also uncovered defects in energy expenditure in lean and diet-induced obese (DIO) mice. This decreased energy expenditure during DIO was associated with significantly altered BAT gene expression, specifically Cidea, Elovl3, Cox7a1, and Dio2. Remarkably, the observed changes in energy balance during DIO were absent in Ldb1Δβ-cell mice, despite a similar reduction in plasma insulin, suggesting a role for LDB1 in BAT. Indeed, LDB1 is expressed in brown adipocytes and occupies a regulatory domain of Elovl3, a gene crucial to normal BAT function. We conclude that LDB1 regulates energy homeostasis, in part through transcriptional modulation of critical regulators in BAT function.

A-FABP mediates adaptive thermogenesis by promoting intracellular activation of thyroid hormones in brown adipocytes

The adipokine adipocyte fatty acid-binding protein (A-FABP) has been implicated in obesity-related cardio-metabolic complications. Here we show that A-FABP increases thermogenesis by promoting the conversion of T4 to T3 in brown adipocytes. We find that A-FABP levels are increased in both white (WAT) and brown (BAT) adipose tissues and the bloodstream in response to thermogenic stimuli. A-FABP knockout mice have reduced thermogenesis and whole-body energy expenditure after cold stress or after feeding a high-fat diet, which can be reversed by infusion of recombinant A-FABP. Mechanistically, A-FABP induces the expression of type-II iodothyronine deiodinase in BAT via inhibition of the nuclear receptor liver X receptor α, thereby leading to the conversion of thyroid hormone from its inactive form T4 to active T3. The thermogenic responses to T4 are abrogated in A-FABP KO mice, but enhanced by A-FABP. Thus, A-FABP acts as a physiological stimulator of BAT-mediated adaptive thermogenesis.

Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming

The hypothalamus-pituitary-thyroid feedback control is a dynamic, adaptive system. In situations of illness and deprivation of energy representing type 1 allostasis, the stress response operates to alter both its set point and peripheral transfer parameters. In contrast, type 2 allostatic load, typically effective in psychosocial stress, pregnancy, metabolic syndrome, and adaptation to cold, produces a nearly opposite phenotype of predictive plasticity. The non-thyroidal illness syndrome (NTIS) or thyroid allostasis in critical illness, tumors, uremia, and starvation (TACITUS), commonly observed in hospitalized patients, displays a historically well-studied pattern of allostatic thyroid response. This is characterized by decreased total and free thyroid hormone concentrations and varying levels of thyroid-stimulating hormone (TSH) ranging from decreased (in severe cases) to normal or even elevated (mainly in the recovery phase) TSH concentrations. An acute versus chronic stage (wasting syndrome) of TACITUS can be discerned. The two types differ in molecular mechanisms and prognosis. The acute adaptation of thyroid hormone metabolism to critical illness may prove beneficial to the organism, whereas the far more complex molecular alterations associated with chronic illness frequently lead to allostatic overload. The latter is associated with poor outcome, independently of the underlying disease. Adaptive responses of thyroid homeostasis extend to alterations in thyroid hormone concentrations during fetal life, periods of weight gain or loss, thermoregulation, physical exercise, and psychiatric diseases. The various forms of thyroid allostasis pose serious problems in differential diagnosis of thyroid disease. This review article provides an overview of physiological mechanisms as well as major diagnostic and therapeutic implications of thyroid allostasis under a variety of developmental and straining conditions.

Phytochemicals as novel agents for the induction of browning in white adipose tissue

Obesity and its associated metabolic syndrome continue to be a health epidemic in westernized societies and is catching up in the developing world. Despite such increases, little headway has been made to reverse adverse weight gain in the global population. Few medical options exist for the treatment of obesity which points to the necessity for exploration of anti-obesity therapies including pharmaceutical and nutraceutical compounds. Defects in brown adipose tissue, a major energy dissipating organ, has been identified in the obese and is hypothesized to contribute to the overall metabolic deficit observed in obesity. Not surprisingly, considerable attention has been placed on the discovery of methods to activate brown adipose tissue. A variety of plant-derived, natural compounds have shown promise to regulate brown adipose tissue activity and enhance the lipolytic and catabolic potential of white adipose tissue. Through activation of the sympathetic nervous system, thyroid hormone signaling, and transcriptional regulation of metabolism, natural compounds such as capsaicin and resveratrol may provide a relatively safe and effective option to upregulate energy expenditure. Through utilizing the energy dissipating potential of such nutraceutical compounds, the possibility exists to provide a therapeutic solution to correct the energy imbalance that underlines obesity.

The sympathetic nervous system is controlled by transient receptor potential vanilloid 1 in the regulation of body temperature

Transient receptor potential vanilloid 1 (TRPV1) is involved in sensory nerve nociceptive signaling. Recently, it has been discovered that TRPV1 receptors also regulate basal body temperature in multiple species from mice to humans. In the present study, we investigated whether TRPV1 modulates basal sympathetic nervous system (SNS) activity. C57BL6/J wild-type (WT) mice and TRPV1 knockout (KO) mice were implanted with radiotelemetry probes for measurement of core body temperature. AMG9810 (50 mg/kg) or vehicle (2% DMSO/5% Tween 80/10 ml/kg saline) was injected intraperitoneally. Adrenoceptor antagonists or vehicle (5 ml/kg saline) was injected subcutaneously. In WT mice, the TRPV1 antagonist, AMG9810, caused significant hyperthermia, associated with increased noradrenaline concentrations in brown adipose tissue. The hyperthermia was significantly attenuated by the β-adrenoceptor antagonist propranolol, the mixed α-/β-adrenoceptor antagonist labetalol, and the α1-adrenoceptor antagonist prazosin. TRPV1 KO mice have a normal basal body temperature, indicative of developmental compensation. d-Amphetamine (potent sympathomimetic) caused hyperthermia in WT mice, which was reduced in TRPV1 KO mice, suggesting a decreased sympathetic drive in KOs. This study provides new evidence that TRPV1 controls thermoregulation upstream of the SNS, providing a potential therapeutic target for sympathetic hyperactivity thermoregulatory disorders.

Obesity is associated with a decrease in expression but not with the hypermethylation of thermogenesis-related genes in adipose tissues

Background

Impaired thermogenesis can promote obesity. Therefore, the aim of this study was to investigate whether the expression of thermogenesis-related genes is altered in adipose tissues of obese individuals and whether excessive methylation of their promoters is involved in this phenomenon.

Methods

The expression of genes encoding β adrenergic receptors (ADRBs), thyroid hormone receptors (THRs), 5’-iodothyronine deiodinases (DIOs), and uncoupling proteins (UCPs) was measured by real-time PCR in visceral and in subcutaneous adipose tissues of 58 obese (BMI >40 kg/m²) and 50 slim (BMI 20-24.9 kg/m²) individuals. The methylation status of these genes was studied by the methylation-sensitive digestion/real-time PCR method.

Results

The expression of ADRB2, ADRB3, THRA, THRB, DIO2, UCP2 was significantly lower in the adipose tissues of obese patients than in tissues of normal-weight individuals (P < 0.00001). In the obese, the expression of ADRB2, ADRB3, DIO2 was lower in visceral adipose tissue than in subcutaneous adipose tissue (P = 0.008, P = 0.002, P = 0.001, respectively). However, the mean methylation of CpG islands of these genes was similar in tissues with their high and low expression, and there was no correlation between the level of expression and the level of methylation.

Conclusions

Decreased expression of thermogenesis-related genes in adipose tissues of obese patients might result in the reduced reactivity to both hormonal and adrenergic stimuli and therefore in a lower potential to activate thermogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0395-2) contains supplementary material, which is available to authorized users.

White-to-brown metabolic conversion of human adipocytes by JAK inhibition

The rising incidence of obesity and related disorders such as diabetes and heart disease has focused considerable attention on the discovery of new therapeutics. One promising approach has been to increase the number or activity of brown-like adipocytes in white adipose depots, as this has been shown to prevent diet-induced obesity and reduce the incidence and severity of type 2 diabetes. Thus, the conversion of fat-storing cells into metabolically active thermogenic cells has become an appealing therapeutic strategy to combat obesity. Here, we report a screening platform for the identification of small molecules capable of promoting a white-to-brown metabolic conversion in human adipocytes. We identified two inhibitors of Janus kinase (JAK) activity with no precedent in adipose tissue biology that stably confer brown-like metabolic activity to white adipocytes. Importantly, these metabolically converted adipocytes exhibit elevated UCP1 expression and increased mitochondrial activity. We further found that repression of interferon signalling and activation of hedgehog signalling in JAK-inactivated adipocytes contributes to the metabolic conversion observed in these cells. Our findings highlight a previously unknown role for the JAK-STAT pathway in the control of adipocyte function and establish a platform to identify compounds for the treatment of obesity.

Thermogenic effect of glucose in hypothyroid subjects

The importance of thyroid hormone, catecholamines, and insulin in modification of the thermogenic effect of glucose (TEG) was examined in 34 healthy and 32 hypothyroid subjects. We calculated the energy expenditure at rest and during oral glucose tolerance test. Blood samples for determinations of glucose, plasma insulin, adrenaline (A), and noradrenaline (NA) were collected. It was found that TEG was lower in hypothyroid than in control group (19.68 ± 3.90 versus 55.40 ± 7.32 kJ, resp., P < 0.0004). Mean values of glucose and insulin areas under the curve were higher in women with hypothyroidism than in control group (286.79 ± 23.65 versus 188.41 ± 15.84 mmol/L·min, P < 0.003 and 7563.27 ± 863.65 versus 4987.72 ± 583.88 mU/L·min, P < 0.03 resp.). Maximal levels of catecholamines after glucose ingestion were higher in hypothyroid patients than in control subjects (Amax-0.69 ± 0.08 versus 0.30 ± 0.07 nmol/L, P < 0.0001, and NAmax-6.42 ± 0.86 versus 2.54 ± 0.30 nmol/L, P < 0.0002). It can be concluded that in hypothyroidism TEG and glucose tolerance are decreased while the adrenergic response to glucose administration is enhanced. Presumably, these changes are related to decreased insulin sensitivity and responsiveness to catecholamine action.

Expression of uncoupling protein 1 in mouse brown adipose tissue is thyroid hormone receptor-beta isoform specific and required for adaptive thermogenesis

Cold-induced adaptive (or nonshivering) thermogenesis in small mammals is produced primarily in brown adipose tissue (BAT). BAT has been identified in humans and becomes more active after cold exposure. Heat production from BAT requires sympathetic nervous system stimulation, T(3), and uncoupling protein 1 (UCP1) expression. Our previous studies with a thyroid hormone receptor-beta (TR beta) isoform-selective agonist demonstrated that after TR beta stimulation alone, adaptive thermogenesis was markedly impaired, although UCP-1 expression in BAT was normal. We used mice with a dominant-negative TR beta PV mutation (frameshift mutation in resistance to thyroid hormone patient PV) to determine the role of TR beta in adaptive thermogenesis and UCP1 expression. Wild-type and PV mutant mice were made hypothyroid and replaced with T(3) (7 ng/g x d) for 10 d to produce similar serum thyroid hormone concentration in the wild-type and mutant mice. The thermogenic response of interscapular BAT, as determined by heat production during iv infusions of norepinephrine, was reduced in PV beta heterozygous and homozygous mutant mice. The level of UCP1, the key thermogenic protein in BAT, was progressively reduced in PV beta(+/-) and PV beta(-/-) mutant mice. Brown adipocytes isolated from PV mutant mice had some reduction in cAMP and glycerol production in response to adrenergic stimulation. Defective adaptive thermogenesis in TR beta PV mutant mice is due to reduced UCP1 expression and reduced adrenergic responsiveness. TR beta mediates T(3) regulation of UCP1 in BAT and is required for adaptive thermogenesis.

Differential control of lipogenesis and lipolysis during development of ovine preadipocytesin vitro

The stromovascular fraction of adipose tissue from sheep, like that of other species, contains preadipocytes which can be induced to differentiate in culture, providing a potentially useful system for studying adipocyte development. Differentiation of ruminant preadipocytes has only been partly characterized previously so we have investigated the factors regulating the development of lipogenesis and lipolysis in sheep cells. Insulin, rosiglitazone (a peroxisome proliferation activated receptor-γ agonist) and either dexamethasone or a lipid suplement are required during differentiation for maximum rates of lipogenesis, whereas all four components are required to achieve maximum rates of catecholamine-stimulated lipolysis. Tri-iodothyronine had no effect on the development of lipogenesis but resulted in a reduced rate of catecholamine-stimulated lipolysis. Lipogenesis and lipolysis also differed in that the rate of lipogenesis increased to a maximum at about 10 days of differentiation and then fell, whereas the rate of lipolysis reached a plateau at about 10 days. By contrast to catecholamine-stimulated lipolysis, there is little or no evidence for development of the adenosine-based antilipolytic system; this may be because response to adenosine develops very late during preadipocyte differentiation or additional, unidentified factors are required to induce this antilipolytic system. Lipogenesis in differentiated preadipocytes responded to both insulin and growth hormone. These studies show that the development of lipogenesis and lipolysis are under distinct control systems. Furthermore, while preadipocytes differentiatedin vitroshow many of the characteristics of adipocytes differentiatedin vivo, there are still significant differences.

Thermogenic mechanisms and their hormonal regulation

Increased heat generation from biological processes is inherent to homeothermy. Homeothermic species produce more heat from sustaining a more active metabolism as well as from reducing fuel efficiency. This article reviews the mechanisms used by homeothermic species to generate more heat and their regulation largely by thyroid hormone (TH) and the sympathetic nervous system (SNS). Thermogenic mechanisms antecede homeothermy, but in homeothermic species they are activated and regulated. Some of these mechanisms increase ATP utilization (same amount of heat per ATP), whereas others increase the heat resulting from aerobic ATP synthesis (more heat per ATP). Among the former, ATP utilization in the maintenance of ionic gradient through membranes seems quantitatively more important, particularly in birds. Regulated reduction of the proton-motive force to produce heat, originally believed specific to brown adipose tissue, is indeed an ancient thermogenic mechanism. A regulated proton leak has been described in the mitochondria of several tissues, but its precise mechanism remains undefined. This leak is more active in homeothermic species and is regulated by TH, explaining a significant fraction of its thermogenic effect. Homeothermic species generate additional heat, in a facultative manner, when obligatory thermogenesis and heat-saving mechanisms become limiting. Facultative thermogenesis is activated by the SNS but is modulated by TH. The type II iodothyronine deiodinase plays a critical role in modulating the amount of the active TH, T(3), in BAT, thereby modulating the responses to SNS. Other hormones affect thermogenesis in an indirect or permissive manner, providing fuel and modulating thermogenesis depending on food availability, but they do not seem to have a primary role in temperature homeostasis. Thermogenesis has a very high energy cost. Cold adaptation and food availability may have been conflicting selection pressures accounting for the variability of thermogenesis in humans.

Pharmacological characterization of alpha1- and beta-adrenergic synergism of 5'DII activity in rat brown adipocytes

The role of adrenoceptor subtypes was studied in rat brown adipose tissue (BAT). The type II 5'-deiodinase (5'DII) was activated in response to simultaneous stimulation by beta3- and alpha1-adrenergic agonists, BRL 37344 or CGP 12177, and cirazoline, in brown adipocytes. Inhibition of the alpha1- and beta-adrenergic phenylephrine-stimulated 5'DII activity was obtained by the alpha1-adrenergic antagonists in the order of prazosin >/= wb 4101 > 5-methylurapidil. In comparison, the binding of [3H]prazosin to rat BAT plasma membranes was inhibited by alpha1-adrenergic antagonists in the order of prazosin > WB 4101 = benoxathian > 5-methylurapidil. Although the order of the alpha1-adrenergic competition seemed to be rather typical for the alpha1B-adrenergic receptors, a molecular analysis on adrenoceptor mRNAs should be made to confirm the exact alpha1-adrenergic subtypes at the level of brown adipocytes, since the possibility of a mixture of different receptor subtypes in brown fat cells and/or tissue may interact with the pharmacological characterization. Thus, specific alpha1- and beta-adrenoceptor subtypes participate in the regulation of 5'DII activity in the rat brown adipocytes, and therefore, an impaired alpha1- and beta-adrenergic co-work may be involved in a defective BAT function, e.g., in obese Zucker rats, too. An interesting possibility is that the decreased number of alpha1-adrenoceptors in the BAT of obese Zucker rats is due to the decrease in the alpha1B-adrenoceptor subtype which would further be involved especially in the regulation of BAT 5'DII activity.

Thyroid hormones as molecular determinants of thermogenesis

Thyroid hormones (TH) are major modulators of energy metabolism and thermogenesis. It is generally believed that 3,5,3'-triiodo-l-thyronine (T3) is the only active form of TH, and that most of its effects are mediated by nuclear T3 receptors, which chiefly affect the transcription of target genes. Some of these genes encode for the proteins involved in energy metabolism. However, a growing volume of evidence now indicates that other iodothyronines may be biologically active. Several mechanisms have been proposed to explain the calorigenic effect of TH, but none has received universal acceptance. Cold acclimation/exposure and altered nutritional status are physiological conditions in which a modulation of energy expenditure is particularly important. TH seem to be deeply involved in this modulation, and this article will review some aspects of their possible influence in these conditions.

Adaptive activation of thyroid hormone and energy expenditure

The mechanisms by which thyroid hormone accelerates energy expenditure are poorly understood. In the brown adipose tissue (BAT), activation of thyroid hormone by type 2 iodothyronine deiodinase (D2) has been known to play a role in adaptive energy expenditure during cold exposure in human newborns and other small mammals. Although BAT is not present in significant amounts in normal adult humans, recent studies have found substantial amounts of D2 in skeletal muscle, a metabolically relevant tissue in humans. This article reviews current biological knowledge about D2 and adaptive T3 production and their roles in energy expenditure.

Alpha1- and beta1-adrenoceptor signaling fully compensates for beta3-adrenoceptor deficiency in brown adipocyte norepinephrine-stimulated glucose uptake

To assess the relative roles and potential contribution of adrenergic receptor subtypes other than the beta3-adrenergic receptor in norepinephrine-mediated glucose uptake in brown adipocytes, we have here analyzed adrenergic activation of glucose uptake in primary cultures of brown adipocytes from wild-type and beta3-adrenergic receptor knockout (KO) mice. In control cells in addition to high levels of beta3-adrenergic receptor mRNA, there were relatively low alpha1A-, alpha1D-, and moderate beta1-adrenergic receptor mRNA levels with no apparent expression of other adrenergic receptors. The levels of alpha1A-, alpha1D-, and beta1-adrenergic receptor mRNA were not changed in the beta3-KO brown adipocytes, indicating that the beta3-adrenergic receptor ablation does not influence adrenergic gene expression in brown adipocytes in culture. As expected, the beta3-adrenergic receptor agonists BRL-37344 and CL-316 243 did not induce 2-deoxy-d-glucose uptake in beta3-KO brown adipocytes. Surprisingly, the endogenous adrenergic neurotransmitter norepinephrine induced the same concentration-dependent 2-deoxy-D-glucose uptake in wild-type and beta3-KO brown adipocytes. This study demonstrates that beta1-adrenergic receptors, and to a smaller degree alpha1-adrenergic receptors, functionally compensate for the lack of beta3-adrenergic receptors in glucose uptake. Beta1-adrenergic receptors activate glucose uptake through a cAMP/protein kinase A/phosphatidylinositol 3-kinase pathway, stimulating conventional and novel protein kinase Cs. The alpha1-adrenergic receptor component (that is not evident in wild-type cells) stimulates glucose uptake through a phosphatidylinositol 3-kinase and protein kinase C pathway in the beta3-KO cells.

Mice with targeted disruption of the Dio2 gene have cold-induced overexpression of the uncoupling protein 1 gene but fail to increase brown adipose tissue lipogenesis and adaptive thermogenesis

The Dio2 gene encodes the type 2 deiodinase (D2) that activates thyroxine (T4) to 3,3',5-triiodothyronine (T3), the disruption of which (Dio2(-/-)) results in brown adipose tissue (BAT)-specific hypothyroidism in an otherwise euthyroid animal. In the present studies, cold exposure increased Dio2(-/-) BAT sympathetic stimulation approximately 10-fold (normal approximately 4-fold); as a result, lipolysis, as well as the mRNA levels of uncoupling protein 1, guanosine monophosphate reductase, and peroxisome proliferator-activated receptor gamma coactivator 1, increased well above the levels detected in the cold-exposed wild-type animals. The sustained Dio2(-/-) BAT adrenergic hyperresponse suppressed the three- to fourfold stimulation of BAT lipogenesis normally seen after 24-48 h in the cold. Pharmacological suppression of lipogenesis with betabeta'-methyl-substituted alpha-omega-dicarboxylic acids of C14-C18 in wild-type animals also impaired adaptive thermogenesis in the BAT. These data constitute the first evidence that reduced adrenergic responsiveness does not limit cold-induced adaptive thermogenesis. Instead, the resulting compensatory hyperadrenergic stimulation prevents the otherwise normal stimulation in BAT lipogenesis during cold exposure, rapidly exhausting the availability of fatty acids. The latter is the preponderant determinant of the impaired adaptive thermogenesis and hypothermia in cold-exposed Dio2(-/-) mice.

Effects of hypo- and hyperthyroidism on noradrenergic activity and glycerol concentrations in human subcutaneous abdominal adipose tissue assessed with microdialysis

Thyroid hormones play a major role in lipid metabolism. However, whether they directly affect lipolysis locally in the adipose tissue remains unknown. Therefore, we measured abdominal sc adipose tissue norepinephrine (NE), basal, and isoprenaline-stimulated lipolysis in 12 hypothyroid patients (HYPO), six hyperthyroid patients (HYPER), and 12 healthy controls by in vivo microdialysis. Adipose tissue NE was decreased in HYPO and increased in HYPER compared with controls (90.4 +/- 2.9 and 458.0 +/- 69.1 vs. 294.9 +/- 19.5 pmol/liter, P < 0.01). Similarly, basal lipolysis, assessed by glycerol assay, was lower in HYPO and higher in HYPER than in controls (88.2 +/- 9.9 and 566.0 +/- 42.0 vs. 214.3 +/- 5.1 micromol/liter P < 0.01). The relative magnitude of isoprenaline-induced glycerol increase was smaller in HYPO (39 +/- 19.4%, P < 0.05 vs. basal) and higher in HYPER (277 +/- 30.4%, P < 0.01) than in controls (117 +/- 5.6%, P < 0.01). The corresponding changes in NE after isoprenaline stimulation were as follows: 120 +/- 9.2% (P < 0.05), 503 +/- 113% (P < 0.01), and 267 +/- 17.2 (P < 0.01). In summary, by affecting local NE levels and adrenergic postreceptor signaling, thyroid hormones may influence the lipolysis rate in the abdominal sc adipose tissue.

Triiodothyronine is required for the stimulation of type II 5'-deiodinase mRNA in rat brown adipocytes

Type II 5'-iodothyronine deiodinase (D2), produces triiodothyronine (T(3)) and is stimulated by cold exposure via norepinephrine (NE) release in brown adipose tissue. Cultured rat brown adipocytes require T(3) for the adrenergic stimulation of D2 activity. D2 mRNA expression in cultured brown adipocytes is undetectable with the use of basal conditions or NE without T(3). Full D2 expression is achieved using NE + T(3), especially after prolonged T(3) exposure. beta(3)-Adrenergic agonists mimic the NE action, whereas cAMP analogs do not. Prolonged exposure to T(3) alone increases D2 mRNA. High T(3) doses (500 nM) inhibit the adrenergic stimulation of D2 activity while increasing D2 mRNA. The effects obtained with NE + T(3) or T(3) alone are suppressed by actinomycin, but not by cycloheximide, which leads to accumulation of short D2 mRNA transcripts. Prolonged or short exposure to T(3) did not change D2 mRNA half-life, but T(3) seemed to elongate it. In conclusion, T(3) is an absolute requirement for the adrenergic stimulation of D2 mRNA in brown adipocytes. T(3) upregulates D2 mRNA, an effect that might involve stimulation of factors required for transcription or for stabilization of D2 mRNA.

Association between a novel variant of the human type 2 deiodinase gene Thr92Ala and insulin resistance: evidence of interaction with the Trp64Arg variant of the beta-3-adrenergic receptor

Thyroid hormone action is an important determinant of energy and glucose metabolism. T4 metabolism is regulated by the deiodinases of which type 2 is expressed in humans in skeletal muscle and brown adipose tissue, where its transcription is stimulated by the beta-3 adrenergic pathway. We performed molecular scanning of the human type 2 deiodinase (DIO2) gene and evaluated a novel variant for associations with obesity and insulin resistance, assessing both the main effect and interaction with the Trp64Arg beta-3--adrenergic receptor (ADRB3) variant. Molecular scanning of DIO2 in 50 obese Caucasians demonstrated a Thr92Ala variant. Association studies in 972 nondiabetic patients, 135 of whom underwent euglycemic-hyperinsulinemic clamps, showed that subjects with the Thr92Ala variant had lower glucose disposal rate (0.54 plus minus 0.02 mg center dot min(-1) center dot kg(-1) fat-free mass Ala92 homozygotes vs. 0.44 plus minus 0.02 Ala92 heterozygotes vs. 0.42 plus minus 0.04 Thr92 homozygotes, P = 0.0088). Association analysis of the entire group showed significant evidence for a synergistic effect between the Thr92Ala DIO2 and Trp64Arg ADRB3 variants on BMI (both variants 34.3 plus minus 0.9 kg/m(2) vs. neither variant 33.1 plus minus 0.4 kg/m(2), P = 0.04 for interaction). To our knowledge, Thr92Ala is the first description of a missense mutation of DIO2. This variant strongly associates with insulin resistance and, in subjects with the Trp64Arg ADRB3 variant, an increased BMI, suggesting an interaction between these two common gene variants.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

The type 2 iodothyronine deiodinase is essential for adaptive thermogenesis in brown adipose tissue

Type 2 iodothyronine deiodinase (D2) is a selenoenzyme, the product of the recently cloned cAMP-dependent Dio2 gene, which increases 10- to 50-fold during cold stress only in brown adipose tissue (BAT). Here we report that despite a normal plasma 3,5,3'-triiodothyronine (T3) concentration, cold-exposed mice with targeted disruption of the Dio2 gene (Dio2(-/-)) become hypothermic due to impaired BAT thermogenesis and survive by compensatory shivering with consequent acute weight loss. This occurs despite normal basal mitochondrial uncoupling protein 1 (UCP1) concentration. In Dio2(-/-) brown adipocytes, the acute norepinephrine-, CL316,243-, or forskolin-induced increases in lipolysis, UCP1 mRNA, and O(2) consumption are all reduced due to impaired cAMP generation. These hypothyroid-like abnormalities are completely reversed by a single injection of T3 14 hours earlier. Recent studies suggest that UCP1 is primarily dependent on thyroid hormone receptor beta (TR beta) while the normal sympathetic response of brown adipocytes requires TR alpha. Intracellularly generated T3 may be required to saturate the TR alpha, which has an approximately fourfold lower T3-binding affinity than does TR beta. Thus, D2 is an essential component in the thyroid-sympathetic synergism required for thermal homeostasis in small mammals.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Thyroid hormone--sympathetic interaction and adaptive thermogenesis are thyroid hormone receptor isoform--specific

In newborns and small mammals, cold-induced adaptive (or nonshivering) thermogenesis is produced primarily in brown adipose tissue (BAT). Heat production is stimulated by the sympathetic nervous system, but it has an absolute requirement for thyroid hormone. We used the thyroid hormone receptor-beta--selective (TR-beta--selective) ligand, GC-1, to determine by a pharmacological approach whether adaptive thermogenesis was TR isoform--specific. Hypothyroid mice were treated for 10 days with varying doses of T3 or GC-1. The level of uncoupling protein 1 (UCP1), the key thermogenic protein in BAT, was restored by either T3 or GC-1 treatment. However, whereas interscapular BAT in T3-treated mice showed a 3.0 degrees C elevation upon infusion of norepinephrine, indicating normal thermogenesis, the temperature did not increase (<0.5 degrees C) in GC-1--treated mice. When exposed to cold (4 degrees C), GC-1--treated mice also failed to maintain core body temperature and had reduced stimulation of BAT UCP1 mRNA, indicating impaired adrenergic responsiveness. Brown adipocytes isolated from hypothyroid mice replaced with T3, but not from those replaced with GC-1, had normal cAMP production in response to adrenergic stimulation in vitro. We conclude that two distinct thyroid-dependent pathways, stimulation of UCP1 and augmentation of adrenergic responsiveness, are mediated by different TR isoforms in the same tissue.

Calorigenic actions of leptin are additive to, but not dependent on, those of thyroid hormones

We examined a possible mechanistic interaction between leptin and thyroid hormones in rats with hypothyroidism induced by thyroidectomy (TX) and propylthiouracil administration. In study 1, the TX rats were treated by vehicle (V, n = 9) or by recombinant murine leptin (L, 0.3 mg. kg(-1). day(-1), n = 9) or were pair-fed (PF, n = 9) against L. In study 2, the TX rats were all given 3, 3'5'-triiodo-L-thyronine (T(3)) replacement (T, 5 microg. kg(-1). day(-1)) to correct hypothyroidism. They were then subdivided into three groups, namely, vehicle (T+V, n = 9), leptin (T+L, n = 10), and pair-feeding (T+PF, n = 9), similar to study 1 except for T(3) (T). Reduced food consumption and weight gain in the TX rats were reversed by T(3) replacement. Leptin suppressed food intake in the TX rats regardless of T(3) replacement. O(2) consumption (VO(2)) and CO(2) production (VCO(2)) were reduced in TX rats (P < 0.05 vs. normal) but were normalized by either T(3) or leptin treatment. T+L additively increased VO(2) and VCO(2) (P < 0.05 vs. TX, T(3), and L). The respiratory exchange ratio was unaltered in TX rats, with and without T(3), but was significantly reduced by L or T+L treatments. These results indicate that the metabolic actions of leptin are not dependent on a normal thyroid status and that the effects of leptin and T(3) on oxidative metabolism are additive.

Evidence of UCP1-independent regulation of norepinephrine-induced thermogenesis in brown fat

To study the thermal response of interscapular brown fat (IBF) to norepinephrine (NE), urethan-anesthetized rats (1.2 g/kg ip) maintained at 28-30 degrees C received a constant venous infusion of NE (0-2 x 10(4) pmol/min) over a period of 60 min. IBF temperatures (T(IBF)) were recorded with a small thermistor fixed under the IBF pad. Data were plotted against time and expressed as maximal variation (Deltat degrees C). Saline-injected rats showed a decrease in T(IBF) of approximately 0.6 degrees C. NE infusion increased T(IBF) by a maximum of approximately 3.0 degrees C at a dose of 10(4) pmol x min(-1) x 100 g body wt(-1). Surgically thyroidectomized (Tx) rats kept on 0.05% methimazole showed a flat response to NE. Treatment with thyroxine (T(4), 0.8 microg x 100 g(-1) x day(-1)) for 2-15 days normalized mitochondrial UCP1 (Western blotting) and IBF thermal response to NE, whereas iopanoic acid (5 mg x 100 g body wt(-1) x day(-1)) blocked the effects of T(4). Treatment with 3,5, 3'-triiodothyronine (T(3), 0.6 microg x 100 g body wt(-1) x day(-1)) for up to 15 days did not normalize UCP1 levels. However, these animals showed a normal IBF thermal response to NE. Cold exposure for 5 days or feeding a cafeteria diet for 20 days increased UCP1 levels by approximately 3.5-fold. Nevertheless, the IBF thermal response was only greater than that of controls when maximal doses of NE (2 x 10(4) pmol/min and higher) were used.

CONCLUSIONS

1) hypothyroidism is associated with a blunted IBF thermal response to NE; 2) two- to fourfold changes in mitochondrial UCP1 concentration are not necessarily translated into heat production during NE infusion.

Triiodothyronine amplifies the adrenergic stimulation of uncoupling protein expression in rat brown adipocytes

Uncoupling protein (UCP), the mitochondrial protein specific to brown adipose tissue, is activated transcriptionally in response to cold and adrenergic agents. We studied the role of triiodothyronine (T(3)) on the adrenergic stimulation of UCP mRNA expression by use of primary cultures of rat brown adipocytes. Basal UCP mRNA levels are undetectable. Norepinephrine (NE) increases UCP mRNA during differentiation, not during proliferation. In hypothyroid conditions, UCP mRNA response to NE is almost absent. The presence of T(3) (0.2-20 nM) greatly increases the adrenergic response (30-fold). The sensitivity of UCP mRNA responses to NE is potentiated approximately 100-fold by the presence of T(3). The effect is proportional to the dose and time of preexposure to T(3). The increases obtained with NE and T(3) are prevented by actinomycin and cycloheximide. T(3) greatly stabilizes UCP mRNA transcripts. The effects of thyroxine and retinoic acid are weaker than those of T(3). In conclusion, in cultured rat brown adipocytes, T(3) is required and both synergizes with NE to increase UCP mRNA and stabilizes its mRNA transcripts.

Regulation of beta 1- and beta 3-adrenergic agonist-stimulated lipolytic response in hyperthyroid and hypothyroid rat white adipocytes

1. This study examined the effects of thyroid status on the lipolytic responses of rat white adipocytes to beta-adrenoceptor (beta-AR) stimulation. The beta 1- and beta 3-AR mRNAs and proteins were measured by Northern and saturation analyses, respectively. Glycerol production and adenyl cyclase (AC) activity induced by various non-selective and selective beta 1/beta 3-AR agonists and drugs which act distal to the receptor in the signalling cascade were measured in cells from untreated, triiodothyronine (T3)-treated and thyroidectomized rats. 2. The beta 3-AR density was enhanced (72%) by T3-treatment and reduced (50%) by introduction of a hypothyroid state while beta 1-AR number remained unaffected. The beta 1- and beta 3-AR density was correlated with the specific mRNA level in all thyroid status. 3. The lipolytic responses to isoprenaline, noradrenaline (beta 1/beta 3/beta 3-AR agonists) and BRL 37344 (beta 3-AR agonist) were potentiated by 48, 58 and 48%, respectively in hyperthyroidism and reduced by about 80% in hypothyroidism. 4. T3-treatment increased the maximal lipolytic response to the partial beta 3-AR (CGP 12177) and beta 1-AR (xamoterol) agonists by 234 and 260%, respectively, increasing their efficacy (intrinsic activity: 0.95 versus 0.43 and 1.02 versus 0.42). The maximal AC response to these agonists was increased by 84 and 58%, respectively, without changing their efficacy. 5. In the hypothyroid state, the maximal lipolytic and AC responses were decreased with CGP (0.17 +/- 0.03 versus 0.41 +/- 0.08 mumol glycerol/10(6) adipocytes; 0.048 +/- 0.005 versus 0.114 +/- 0.006 pmol cyclic AMP min-1 mg-1) but not changed with xamoterol. 6. The changes in lipolytic responses to postreceptor-acting agents (forskolin, enprofylline and dibutenyl cyclic AMP, (Bu)2cAMP) suggest the modifications on receptor coupling and phosphodiesterase levels in both thyroid states. 7. Thyroid status affects lipolysis by modifying beta 3-AR density and postreceptor events without changes in the beta 1-AR functionality.