Adipose Tissue Remodeling in Obesity: An Overview of the Actions of Thyroid Hormones and Their Derivatives

Metabolic syndrome and obesity have become important health issues of epidemic proportions and are often the cause of related pathologies such as type 2 diabetes (T2DM), hypertension, and cardiovascular disease. Adipose tissues (ATs) are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. An ample body of evidence indicates that in some pathophysiological conditions, the aberrant remodeling of adipose tissue may provoke dysregulation in the production of various adipocytokines and metabolites, thus leading to disorders in metabolic organs. Thyroid hormones (THs) and some of their derivatives, such as 3,5-diiodo-l-thyronine (T2), exert numerous functions in a variety of tissues, including adipose tissues. It is known that they can improve serum lipid profiles and reduce fat accumulation. The thyroid hormone acts on the brown and/or white adipose tissues to induce uncoupled respiration through the induction of the uncoupling protein 1 (UCP1) to generate heat. Multitudinous investigations suggest that 3,3',5-triiodothyronine (T3) induces the recruitment of brown adipocytes in white adipose depots, causing the activation of a process known as "browning". Moreover, in vivo studies on adipose tissues show that T2, in addition to activating brown adipose tissue (BAT) thermogenesis, may further promote the browning of white adipose tissue (WAT), and affect adipocyte morphology, tissue vascularization, and the adipose inflammatory state in rats receiving a high-fat diet (HFD). In this review, we summarize the mechanism by which THs and thyroid hormone derivatives mediate adipose tissue activity and remodeling, thus providing noteworthy perspectives on their efficacy as therapeutic agents to counteract such morbidities as obesity, hypercholesterolemia, hypertriglyceridemia, and insulin resistance.

Thermogenic adipose tissue in energy regulation and metabolic health

The ability to generate thermogenic fat could be a targeted therapy to thwart obesity and improve metabolic health. Brown and beige adipocytes are two types of thermogenic fat cells that regulate energy balance. Both adipocytes share common morphological, biochemical, and thermogenic properties. Yet, recent evidence suggests unique features exist between brown and beige adipocytes, such as their cellular origin and thermogenic regulatory processes. Beige adipocytes also appear highly plastic, responding to environmental stimuli and interconverting between beige and white adipocyte states. Additionally, beige adipocytes appear to be metabolically heterogenic and have substrate specificity. Nevertheless, obese and aged individuals cannot develop beige adipocytes in response to thermogenic fat-inducers, creating a key clinical hurdle to their therapeutic promise. Thus, elucidating the underlying developmental, molecular, and functional mechanisms that govern thermogenic fat cells will improve our understanding of systemic energy regulation and strive for new targeted therapies to generate thermogenic fat. This review will examine the recent advances in thermogenic fat biogenesis, molecular regulation, and the potential mechanisms for their failure.

Temperature determines the shift of thermal neutral zone and influences thermogenic capacity in striped hamsters

The thermoneutral zone (TNZ) reflects the adaptation of mammals to their natural habitat. However, it remains unclear how TNZ shifts in response to variations in ambient temperature. To test the hypothesis that ambient temperature plays a key role in determining TNZ variations between seasons, we measured metabolic rate, body temperature, and cytochrome c oxidase (COX) activity of several visceral organs in striped hamsters (Cricetulus barabensis) either acclimated to semi-natural conditions over a year, or subjected to a gradual decrease in mean temperature from 30 ± 1°C to -15 ± 1°C. The TNZ range in striped hamsters differed seasonally, with a wider TNZ and a lower lower-critical temperature in winter compared to summer. The hamsters showed a considerable leftward shift of lower-critical temperature from 30°C to 20°C after the ambient temperature of acclimation from 30°C down to -15°C, whereas the upper-critical temperature of TNZ remained fixed at 32.5°C. The resting metabolic rate in thermoneutral zone (RMRt), nonshivering thermogenesis (NST), and COX activity of brown adipose tissue, liver, skeletal muscle, brain, and kidneys, increased significantly in hamsters acclimated at lower ambient temperatures. Following acute exposure to 5°C and -15°C, hamsters acclimated to 32.5°C had significantly lower maximal NST and lower serum thyroid tri-iodothyronine (T₃ ) levels compared to those kept at 23°C. These findings suggest that acclimation to the upper-critical temperature of TNZ impairs the hamsters' thermogenic capacity to cope with extreme cold temperature. Reduced ambient temperature was mainly responsible for the leftward shift of TNZ in striped hamsters, which reflects the adaptation to cold environments.

A 7-Week Summer Camp in Antarctica Induces Fluctuations on Human Oral Microbiome, Pro-Inflammatory Markers and Metabolic Hormones Profile

Antarctic camps pose psychophysiological challenges related to isolated, confined, and extreme (ICE) conditions, including meals composed of sealed food. ICE conditions can influence the microbiome and inflammatory responses. Seven expeditioners took part in a 7-week Antarctic summer camp (Nelson Island) and were evaluated at Pre-Camp (i.e., at the beginning of the ship travel), Camp-Initial (i.e., 4th and 5th day in camp), Camp-Middle (i.e., 19th-20th, and 33rd-34th days), Camp-Final (i.e., 45th-46th day), and at the Post-Camp (on the ship). At the Pre-Camp, Camp-Initial, and Camp-Final, we assessed microbiome and inflammatory markers. Catecholamines were accessed Pre- and Post-Camp. Heart rate variability (HRV), leptin, thyroid stimulating hormone (TSH), and thyroxine (T4) were accessed at all time points. Students' t-tests or repeated-measures analysis of variance (one or two-way ANOVA) followed by Student-Newman-Keuls (post hoc) were used for parametric analysis. Kruskal-Wallis test was applied for non-parametric analysis. Microbiome analysis showed a predominance of Pseudomonadota (34.01%), Bacillota (29.82%), and Bacteroidota (18.54%), followed by Actinomycetota (5.85%), and Fusobacteria (5.74%). Staying in a long-term Antarctic camp resulted in microbiome fluctuations with a reduction in Pseudomonadota-a "microbial signature" of disease. However, the pro-inflammatory marker leptin and IL-8 tended to increase, and the angiogenic factor VEGF was reduced during camp. These results suggest that distinct Antarctic natural environments and behavioral factors modulate oral microbiome and inflammation.

Triiodothyronine (T3) promotes brown fat hyperplasia via thyroid hormone receptor α mediated adipocyte progenitor cell proliferation

The thyroid hormone (TH)-controlled recruitment process of brown adipose tissue (BAT) is not fully understood. Here, we show that long-term treatment of T3, the active form of TH, increases the recruitment of thermogenic capacity in interscapular BAT of male mice through hyperplasia by promoting the TH receptor α-mediated adipocyte progenitor cell proliferation. Our single-cell analysis reveals the heterogeneous nature and hierarchical trajectory within adipocyte progenitor cells of interscapular BAT. Further analyses suggest that T3 facilitates cell state transition from a more stem-like state towards a more committed adipogenic state and promotes cell cycle progression towards a mitotic state in adipocyte progenitor cells, through mechanisms involving the action of Myc on glycolysis. Our findings elucidate the mechanisms underlying the TH action in adipocyte progenitors residing in BAT and provide a framework for better understanding of the TH effects on hyperplastic growth and adaptive thermogenesis in BAT depot at a single-cell level.

Thyroid hormone (TH) action regulates brown adipose tissue thermogenic capacity through incompletely understood mechanisms. Here the authors report that T3, the active form of TH, increases thermogenic capacity via thyroid hormone receptor α-mediated hyperplasia of brown adipose tissue adipocyte progenitor cells.

Heart rate variability, thyroid hormone concentration, and neuropsychological responses in Brazilian navy divers: a case report of diving in Antarctic freezing waters

Open-water diving in a polar environment is a psychophysiological challenge to the human organism. We evaluated the effect of short-term diving (i.e., 10 min) in Antarctic waters on autonomic cardiac control, thyroid hormone concentration, body temperatures, mood, and neuropsychological responses (working memory and sleepiness). Data collection was carried out at baseline, before, and after diving in four individuals divided into the supporting (n=2) and diving (n=2) groups. In the latter group, autonomic cardiac control (by measuring heart rate variability) was also assessed during diving. Diving decreased thyroid-stimulating hormone (effect size = 1.6) and thyroxine (effect size = 2.1) concentrations; these responses were not observed for the supporting group. Diving also reduced both the parasympathetic (effect size = 2.6) and sympathetic activities to the heart (ES > 3.0). Besides, diving reduced auricular (effect size > 3.0), skin [i.e., hand (effect size = 1.2) and face (effect size = 1.5)] temperatures compared to pre-dive and reduced sleepiness state (effect size = 1.3) compared to basal, without changing performance in the working memory test. In conclusion, short-term diving in icy waters affects the hypothalamic-pituitary-thyroid axis, modulates autonomic cardiac control, and reduces body temperature, which seems to decrease sleepiness.

Orally Induced Hyperthyroidism Regulates Hypothalamic AMP-Activated Protein Kinase

Besides their direct effects on peripheral metabolic tissues, thyroid hormones (TH) act on the hypothalamus to modulate energy homeostasis. However, since most of the hypothalamic actions of TH have been addressed in studies with direct central administration, the estimation of the relative contribution of the central vs. peripheral effects in physiologic conditions of peripheral release (or administration) of TH remains unclear. In this study we used two different models of peripherally induced hyperthyroidism (i.e., T4 and T3 oral administration) to assess and compare the serum and hypothalamic TH status and relate them to the metabolic effects of the treatment. Peripheral TH treatment affected feeding behavior, overall growth, core body temperature, body composition, brown adipose tissue (BAT) morphology and uncoupling protein 1 (UCP1) levels and metabolic activity, white adipose tissue (WAT) browning and liver metabolism. This resulted in an increased overall uncoupling capacity and a shift of the lipid metabolism from WAT accumulation to BAT fueling. Both peripheral treatment protocols induced significant changes in TH concentrations within the hypothalamus, with T3 eliciting a downregulation of hypothalamic AMP-activated protein kinase (AMPK), supporting the existence of a central action of peripheral TH. Altogether, these data suggest that peripherally administered TH modulate energy balance by various mechanisms; they also provide a unifying vision of the centrally mediated and the direct local metabolic effect of TH in the context of hyperthyroidism.

Thyroid wars: the rise of central actions

In the field of thyroid hormone (TH) action on energy balance, huge advances have been achieved in the past decade, from human, animal, and in vitro studies. A key achievement was the demonstration of the TH 'central' metabolic action, which was recently discovered in rodent models and challenged the previous 'peripheral' paradigm. In this opinion, we dissect and try to unify the two paradigms, from analyzing the respective bench models to extrapolating the possible translational bedside implications.

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.

Effect of Liothyronine Treatment on Dermal Temperature and Activation of Brown Adipose Tissue in Female Hypothyroid Patients: A Randomized Crossover Study

BACKGROUND

Thyroid hormones are essential for the full thermogenic response of brown adipose tissue (BAT) and have been implicated in dermal temperature regulation. Nevertheless, persistent cold-intolerance exists among a substantial proportion of hypothyroid patients on adequate levothyroxine (LT4) substitution.

MATERIALS AND METHODS

To assess if skin temperature and activation of BAT during treatment with liothyronine (LT3) differs from that of LT4 treatment, fifty-nine female hypothyroid patients with residual symptoms on LT4 or LT4/LT3 combination therapy were randomly assigned in a non-blinded crossover study to receive monotherapy with LT4 or LT3 for 12 weeks each. Change in supraclavicular (SCV) skin temperature overlying BAT, and sternal skin temperature not overlying BAT, during rest and cold stimulation were assessed by infrared thermography (IRT). In addition, abundance of exosomal miR-92a, a biomarker of BAT activation, was estimated as a secondary outcome.

RESULTS

Cold stimulated skin temperatures decreased less with LT3 vs. LT4 in both SCV (mean 0.009°C/min [95% CI: 0.004, 0.014]; P<0.001) and sternal areas (mean 0.014°C/min [95% CI: 0.008, 0.020]; P<0.001). No difference in serum exosomal miR-92a abundance was observed between the two treatment groups.

CONCLUSION

LT3 may reduce dermal heat loss. Thermography data suggested increased BAT activation in hypothyroid patients with cold-intolerance. However, this finding was not corroborated by assessment of the microRNA biomarker of BAT activation.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, identifier NCT03627611.

Association of residual feed intake with growth performance, carcass traits, meat quality, and blood variables in native chickens

Improving feed efficiency is a primary goal in poultry breeding strategies. Residual feed intake (RFI) in chickens typically calculated during the growing period is a measure of feed efficiency that is independent of the level of production. The objective of this study was to evaluate phenotypic correlations of growth performance, carcass traits, meat quality, and blood variables with RFI in growing native chickens. A total of 1,008 chickens were selected for the experiment to derive RFI. After the RFI measurement period of 42 d, 25 chickens with low RFI values, 25 chickens with medium RFI values, and 25 chickens with high RFI values were selected. The RFI was significantly positively correlated with feed conversion ratio and average daily feed intake, while it was not significantly correlated with initial body weight (BW), final BW, average daily body weight gain, and metabolic BW0.75. The abdominal fat weight and yield of high RFI group were significantly greater than those of medium and low RFI groups, and the abdominal fat yield was significantly positively correlated with RFI. Moreover, the plasma insulin-like growth factor 1 (IGF-1) content of low RFI group was significantly greater than those of high and medium RFI groups. The plasma concentrations of adrenocorticotropic hormone, triiodothyronine (T3), and cortisol of high RFI birds were significantly greater than that of low RFI birds. RFI was significantly positively correlated with plasma concentrations of T3 and cortisol, while it was significantly negatively correlated with plasma concentration of IGF-1. In addition, the serum levels of glucose and triglyceride of high RFI birds were significantly lower than that of low RFI birds. The serum low-density lipoprotein cholesterol (LDL-C) content of high RFI group was significantly greater than that of medium and low RFI groups, and it was significantly positively correlated with RFI. Our data suggested that selection of chickens with low RFI values may be beneficial to reduce fat deposition in native chickens without affecting the meat quality. Circulating IGF-1, T3, cortisol, and LDL-C concentrations can be used as indirect selection indicators of feed efficiency in native chickens. The effect of IGF-1, T3, cortisol, and LDL-C on feed efficiency of native chickens should be carefully examined and validated in future breeding programs.

Evolutionary adaptation revealed by comparative genome analysis of woolly mammoths and elephants

Comparative genomics studies typically limit their focus to single nucleotide variants (SNVs) and that was the case for previous comparisons of woolly mammoth genomes. We extended the analysis to systematically identify not only SNVs but also larger structural variants (SVs) and indels and found multiple mammoth-specific deletions and duplications affecting exons or even complete genes. The most prominent SV found was an amplification of RNase L (with different copy numbers in different mammoth genomes, up to 9-fold), involved in antiviral defense and inflammasome function. This amplification was accompanied by mutations affecting several domains of the protein including the active site and produced different sets of RNase L paralogs in four mammoth genomes likely contributing to adaptations to environmental threats. In addition to immunity and defense, we found many other unique genetic changes in woolly mammoths that suggest adaptations to life in harsh Arctic conditions, including variants involving lipid metabolism, circadian rhythms, and skeletal and body features. Together, these variants paint a complex picture of evolution of the mammoth species and may be relevant in the studies of their population history and extinction.

IL37 dampens the IL1β-induced catabolic status of human OA chondrocytes

Objective

A crucial feature of OA is cartilage degradation. This process is mediated by pro-inflammatory cytokines, among other factors, via induction of matrix-degrading enzymes. Interleukin 37 (IL37) is an anti-inflammatory cytokine and is efficient in blocking the production of pro-inflammatory cytokines during innate immune responses. We hypothesize that IL37 is therapeutic in treating the inflammatory cytokine cascade in human OA chondrocytes and can act as a counter-regulatory cytokine to reduce cartilage degradation in OA.

Methods

Human OA cartilage was obtained from patients undergoing total knee or hip arthroplasty. Immunohistochemistry was applied to study IL37 protein expression in cartilage biopsies from OA patients. Induction of IL37 expression by IL1β, OA synovium-conditioned medium and TNFα was investigated in human OA chondrocytes. Adenoviral overexpression of IL37 followed by IL1β stimulation was performed to investigate the anti-inflammatory potential of IL37.

Results

IL37 expression was detected in cartilage biopsies of OA patients and induced by IL1β. After IL1β stimulation, increased IL1β, IL6 and IL8 expression was observed in OA chondrocytes. Elevated IL37 levels diminished the IL1β-induced IL1β , IL6 and IL8 gene levels and IL1β and IL8 protein levels. In addition to the reduction in pro-inflammatory cytokine expression, IL37 reduced MMP1 , MMP3 , MMP13 and disintegrin and metalloproteinase with thrombospondin motifs 5 gene levels and MMP3 and MMP13 protein levels.

Conclusion

IL37 is induced by IL1β, and IL37 itself reduced IL1β, IL6 and IL8 production, indicating that IL37 is able to induce a counter-regulatory anti-inflammatory feedback loop in chondrocytes. In addition, IL37 dampens catabolic enzyme expression. This supports IL37 as a potential therapeutic target in OA.

Transiently Altered Distribution of F-18 FDG in a Patient with Subacute Thyroiditis

F-18 fluorodeoxyglucose (FDG) is a highly influential radiotracer that provides valuable information in many cancer types. However, the normal biodistribution of F-18 FDG is often variable and can be altered by intrinsic or iatrogenic factors. We report a case of diffuse symmetrically increased skeletal muscle uptake and relatively decreased hepatic uptake on F-18 FDG PET/CT in a 57-year-old female with pulmonary adenocarcinoma. Detailed clinical evaluation and retrospective radiologic evaluation revealed that she had been diagnosed with subacute thyroiditis 2 weeks ago. After 6 weeks, F-18 FDG distribution was normalized at the follow-up PET/CT study.

Differential responses of the somatotropic and thyroid axes to environmental temperature changes in the green iguana

Growth hormone (GH), together with thyroid hormones (TH), regulates growth and development, and has critical effects on vertebrate metabolism. In ectotherms, these physiological processes are strongly influenced by environmental temperature. In reptiles, however, little is known about the direct influences of this factor on the somatotropic and thyroid axes. Therefore, the aim of this study was to describe the effects of both acute (48h) and chronic (2weeks) exposure to sub-optimal temperatures (25 and 18°C) upon somatotropic and thyroid axis function of the green iguana, in comparison to the control temperature (30-35°C). We found a significant increase in GH release (2.0-fold at 25°C and 1.9-fold at 18°C) and GH mRNA expression (up to 3.7-fold), mainly under chronic exposure conditions. The serum concentration of insulin-like growth factor-I (IGF-I) was significantly greater after chronic exposure (18.5±2.3 at 25°C; 15.92±3.4 at 18°C; vs. 9.3±1.21ng/ml at 35°C), while hepatic IGF-I mRNA expression increased up to 6.8-fold. Somatotropic axis may be regulated, under acute conditions, by thyrotropin-releasing hormone (TRH) that significantly increased its hypothalamic concentration (1.45 times) and mRNA expression (0.9-fold above control), respectively; and somatostatin (mRNA expression increased 1.0-1.2 times above control); and under chronic treatment, by pituitary adenylate cyclase-activating peptide (PACAP mRNA expression was increased from 0.4 to 0.6 times). Also, it was shown that, under control conditions, injection of TRH stimulated a significant increase in circulating GH. On the other hand, while there was a significant rise in the hypothalamic content of TRH and its mRNA expression, this hormone did not appear to influence the thyroid axis activity, which showed a severe diminution in all conditions of cold exposure, as indicated by the decreases in thyrotropin (TSH) mRNA expression (up to one-eight of the control), serum T4 (from 11.6±1.09 to 5.3±0.58ng/ml, after 2weeks at 18°C) and T3 (from 0.87±0.09 to 0.05±0.01ng/ml, under chronic conditions at 25°C), and Type-2 deiodinase (D2) activity (from 992.5±224 to 213.6±26.4fmolI(125)T4/mgh). The reduction in thyroid activity correlates with the down-regulation of metabolism as suggested by the decrease in the serum glucose and free fatty acid levels. These changes apparently were independent of a possible stress response, at least under acute exposure to both temperatures and in chronic treatment to 25°C, since serum corticosterone had no significant changes in these conditions, while at chronic 18°C exposure, a slight increase (0.38 times above control) was found. Thus, these data suggest that the reptilian somatotropic and thyroid axes have differential responses to cold exposure, and that GH and TRH may play important roles associated to adaptation mechanisms that support temperature acclimation in the green iguana.

Hypothyroidism Induces Hypophagia Associated with Alterations in Protein Expression of Neuropeptide Y and Proopiomelanocortin in the Arcuate Nucleus, Independently of Hypothalamic Nuclei-Specific Changes in Leptin Signaling

BACKGROUND

Thyroid hormone and leptin are essential regulators of energy homeostasis. Both hormones stimulate energy expenditure but have opposite effects on appetite. The mechanisms behind food intake regulation in thyroid dysfunctions are poorly understood. It has been shown that hypothyroid rats exhibited impaired leptin anorexigenic effect and signaling in total hypothalamus, even though they were hypophagic. It was hypothesized that hypothyroidism modulates the expression of neuropeptides: orexigenic neuropeptide Y (NPY) and anorexigenic proopiomelanocortin (POMC), independently of inducing nuclei-specific changes in hypothalamic leptin signaling.

METHODS

Adult male rats were rendered hypothyroid by administration of 0.03% methimazole in the drinking water for 21 days. Protein content of NPY, POMC, and leptin signaling (the signal transducer and activator of transcription 3 [STAT3] pathway) were evaluated by Western blot, and mRNA levels by real time reverse transcription polymerase chain reaction in arcuate (ARC), ventromedial (VMN), and paraventricular (PVN) hypothalamic nuclei isolated from euthyroid (eu) and hypothyroid (hypo) rats. Leptin anorexigenic effect was tested by recording food intake for two hours after intracerebroventricular (i.c.v.) administration of leptin. Statistical differences were considered significant at p ≤ 0.05.

RESULTS

Hypothyroidism was confirmed by decreased serum triiodothyronine, thyroxine, and increased thyrotropin, in addition to increased levels of pro-TRH mRNA in PVN and Dio2 mRNA in the ARC of hypo rats. Hypothyroidism decreased body weight and food intake associated with decreased protein content of NPY and increased content of POMC in the ARC. Conversely, hypothyroidism induced central resistance to the acute anorexigenic effect of leptin, since while euthyroid rats displayed reduced food intake after leptin i.c.v. injection, hypothyroid rats showed no response. Hypothyroid rats exhibited decreased leptin receptor (ObRb) protein content in ARC and VMN but not in PVN nucleus. ObRb protein changes were concomitant with decreased phosphorylated STAT3 in the ARC, and decreased total STAT3 in VMN and PVN. However, hypothyroidism did not affect mRNA levels of Lepr or Stat3 in the hypothalamic nuclei.

CONCLUSIONS

Experimental hypothyroidism induced a negative energy balance accompanied by decreased NPY and increased POMC protein content in the ARC, resulting in predominance of anorexigenic pathways, despite central leptin resistance and impairment of the leptin signaling cascade in a nuclei-specific manner.

Hypothalamic effects of thyroid hormones on metabolism

Over the past few decades, obesity and its related metabolic disorders have increased at an epidemic rate in the developed and developing world. New signals and factors involved in the modulation of energy balance and metabolism are continuously being discovered, providing potential novel drug targets for the treatment of metabolic disease. A parallel strategy is to better understand how hormonal signals, with an already established role in energy metabolism, work, and how manipulation of the pathways involved may lead to amelioration of metabolic dysfunction. The thyroid hormones belong to the latter category, with dysregulation of the thyroid axis leading to marked alterations in energy balance. The potential of thyroid hormones in the treatment of obesity has been known for decades, but their therapeutic use has been hampered because of side-effects. Data gleaned over the past few years, however, have uncovered new features at the mechanisms of action involved in thyroid hormones. Sophisticated neurobiological approaches have allowed the identification of specific energy sensors, such as AMP-activated protein kinase and mechanistic target of rapamycin, acting in specific groups of hypothalamic neurons, mediating many of the effects of thyroid hormones on food intake, energy expenditure, glucose, lipid metabolism, and cardiovascular function. More extensive knowledge about these molecular mechanisms will be of great relevance for the treatment of obesity and metabolic syndrome.

Triiodothyronine regulates distribution of thyroid hormone receptors by activating AMP-activated protein kinase in 3T3-L1 adipocytes and induces uncoupling protein-1 expression

The purposes of this study were to examine whether thermogenesis in 3T3-L1 adipocytes is related to variations in thyroid hormone receptors (TRs) that are differently regulated by triiodothyronine (T3), and the possible role of AMP-activated protein (AMPK) in thermogenesis after cell differentiation. Differentiated 3T3-L1 adipocytes were maintained under four conditions: normal control group, T3 treatment group, AMPK agonist (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside) treatment group, and T3 and AMPK inhibitor (Compound C) treatment group. Real-time polymerase chain reaction was then performed to evaluate the changes in TRα and TRβ mRNA levels in the cells, as well as marker genes for brown adipose tissue including uncoupling protein (UCP)-1 and Cidea. Western blotting was carried out for the cells to detect the expressions of TRα, TRβ, and AMPK protein levels. After T3 treatment, the mRNA and protein levels of TRα decreased compared with the control group, while TRβ mRNA and protein levels increased markedly at the same time. We also found elevated mRNA levels of UCP-1 and Cidea after exposure to T3. However, the distribution of TRs was reversed by Compound C. AMPK protein levels were clearly activated by T3. Our results suggest that the distribution of TRs is related to thermogenesis, and AMPK may participate in the alterations.

Hyperthyroid state or in vitro thyroxine treatment modulates TH1/TH2 responses during exposure to HSV-1 antigens

Increasingly in recent years, thyroid hormones (THs) have been considered to be important regulators of the immune system. However, their roles in host defense against viral infections are not clearly established. Therefore, this study was undertaken to examine proliferative activity and cytokine production by lymphocytes isolated from hyperthyroid and euthyroid Balb/c mice in response to herpes simplex virus-1 (HSV-1). Lymphocytes of hyperthyroid animals showed a significantly higher rate of proliferation and interferon (IFN)-γ production when compared with that by lymphocytes from euthyroid mice. In vitro thyroxine (T4) treatment was similarly effective in the potentiation of proliferation, but not IFNγ production, by euthyroid lymphocytes. Furthermore, the hyperthyroid state significantly attenuated ConA-, but not HSV-1-, induced interleukin (IL)-10 release; in vitro T4 treatment synergized this effect. These findings suggest that supra-physiologic TH levels (i.e. as occur in hyper-thyroid states) or in vitro TH treatment modulate T-helper (TH)1/TH2 lymphocyte responses and thereby amplifies host defenses against viral infections. One may also conclude that THs may have a potential application in viral immunization and/or treatment of viral infections.

Cartilage to bone transitions in health and disease

Aberrant redeployment of the 'transient' events responsible for bone development and postnatal longitudinal growth has been reported in some diseases in what is otherwise inherently 'stable' cartilage. Lessons may be learnt from the molecular mechanisms underpinning transient chondrocyte differentiation and function, and their application may better identify disease aetiology. Here, we review the current evidence supporting this possibility. We firstly outline endochondral ossification and the cellular and physiological mechanisms by which it is controlled in the postnatal growth plate. We then compare the biology of these transient cartilaginous structures to the inherently stable articular cartilage. Finally, we highlight specific scenarios in which the redeployment of these embryonic processes may contribute to disease development, with the foresight that deciphering those mechanisms regulating pathological changes and loss of cartilage stability will aid future research into effective disease-modifying therapies.

The significance of alteration 2-[fluorine-18]fluoro-2-deoxy-(D)-glucose uptake in the liver and skeletal muscles of patients with hyperthyroidism

RATIONALE AND OBJECTIVES

Hyperthyroidism leads to an enhanced demand for glucose. The hypothesis of the study is that 2-[fluorine-18]fluoro-2-deoxy-d-glucose (FDG) positron emission tomography (PET) can demonstrate the alteration of systemic glucose metabolism in hyperthyroidism patients by measuring the FDG standard uptake value (SUV) in liver and skeletal muscle.

MATERIALS AND METHODS

Forty-eight active hyperthyroidism patients and 30 control participants were recruited for the study. The intensity of FDG uptake in the liver and thigh muscles was graded subjectively, comprising three groups: group I, higher FDG uptake in the liver; group II, equal FDG uptake in the liver and muscles; and group III, higher FDG uptake in the muscles. Ten subjects with FDG PET scans at hyperthyroid and euthyroid status were analyzed. Serum levels of thyroxine (T4) and triiodothyronine (T3) correlated to the SUVs of the liver and muscles.

RESULTS

Forty-one patients (41/48, 85.4%) showed symmetrically increased FDG uptake in the muscles (22 in group I, 9 in group II, and 17 in group III). Group I patients were significantly older than group II (P = .02) and group III (P = .001) patients. The correlation coefficient between the serum T3, T4, and SUV levels in the muscles was significant (r = 0.47-0.77, P < .01), particularly in liver and muscle FDG uptake between hyperthyroid and euthyroid states. In the 30 control subjects, there was normal physiological FDG uptake in the liver and muscles.

CONCLUSION

In PET scans showing a pattern of decreased liver and increased skeletal muscle FDG uptake in hyperthyroidism patients, this change of FDG distribution is correspondence to the severity of hyperthyroidism status.

Mast cell-restricted, tetramer-forming tryptases induce aggrecanolysis in articular cartilage by activating matrix metalloproteinase-3 and -13 zymogens

Mouse mast cell protease (mMCP)-6-null C57BL/6 mice lost less aggrecan proteoglycan from the extracellular matrix of their articular cartilage during inflammatory arthritis than wild-type (WT) C57BL/6 mice, suggesting that this mast cell (MC)-specific mouse tryptase plays prominent roles in articular cartilage catabolism. We used ex vivo mouse femoral head explants to determine how mMCP-6 and its human ortholog hTryptase-β mediate aggrecanolysis. Exposure of the explants to recombinant hTryptase-β, recombinant mMCP-6, or lysates harvested from WT mouse peritoneal MCs (PMCs) significantly increased the levels of enzymatically active matrix metalloproteinases (MMP) in cartilage and significantly induced aggrecan loss into the conditioned media, relative to replicate explants exposed to medium alone or lysates collected from mMCP-6-null PMCs. Treatment of cartilage explants with tetramer-forming tryptases generated aggrecan fragments that contained C-terminal DIPEN and N-terminal FFGVG neoepitopes, consistent with MMP-dependent aggrecanolysis. In support of these data, hTryptase-β was unable to induce aggrecan release from the femoral head explants obtained from Chloe mice that resist MMP cleavage at the DIPEN↓FFGVG site in the interglobular domain of aggrecan. In addition, the abilities of mMCP-6-containing lysates from WT PMCs to induce aggrecanolysis were prevented by inhibitors of MMP-3 and MMP-13. Finally, recombinant hTryptase-β was able to activate latent pro-MMP-3 and pro-MMP-13 in vitro. The accumulated data suggest that human and mouse tetramer-forming tryptases are MMP convertases that mediate cartilage damage and the proteolytic loss of aggrecan proteoglycans in arthritis, in part, by activating the zymogen forms of MMP-3 and MMP-13, which are constitutively present in articular cartilage.

UCP1: its involvement and utility in obesity

Energy balance to prevent the development of obesity is dependent on energy expenditure. Although physical activity is the dominant mechanism for dissipating excess energy, a system of thermogenesis that evolved to protect the body from hypothermia is based upon the uncoupling of oxidative phosphorylation in brown adipocytes by the mitochondrial uncoupling protein (UCP1). It has been shown that upregulation of UCP1 by genetic manipulations or pharmacological agents can reduce obesity and improve insulin sensitivity. Recent evidence has shown the existence of two sources for brown adipocytes, one appearing as discrete brown fat depots during fetal development and the other appears during post-natal development as diffuse populations in traditional white fat depots. The latter can be induced by adrenergic stimulation depending on the genetic background of the animals and the nutritional environment. Understanding the biological and environmental factors controlling the expression of these two brown adipocyte populations promises to provide new strategies by which enhanced thermogenesis can be used to reduce obesity.International Journal of Obesity (2008) 32, S32-S38; doi:10.1038/ijo.2008.236.

[hGH treatment impact on adrenal and thyroid functions]

Somatotrophic status is a major determinant of both thyrotrophic and corticotrophic axis. In growth hormone deficient patients, somatotrophic replacement increases the conversion rate of the inactive form of the thyroid hormone (T4) to its active form (T3), whereas the same replacement induces the conversion of cortisol, which is hormonally active, in cortisone, its inactive form. This review details the effects of GH on these two hormonal axis, possible mechanisms and clinical implications for the management of hypopituitary patients.

Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate

It has long been accepted that thyroid hormone is an important determinant of overall energy expenditure and the basal metabolic rate. Indeed, regulating thermogenesis is one of the major tasks of thyroid hormone in adult humans. A wealth of data have demonstrated the effects of thyroid hormone on cellular processes involved with energy expenditure, yet in spite of this body of work it remains unclear which 3,3'-triiodothyronine-responsive energetic processes are most relevant for the determination of the basal metabolic rate. Recently, a novel metabolic role for thyroid hormone has been recognized based on the observation that bile acids can activate local production of thyroid hormone via induction of the type 2 deiodinase. Nevertheless, more work must be done before it can be fully explained how thyroid hormone determines the metabolic rate.

The TRH neuron: a hypothalamic integrator of energy metabolism

Thyrotropin-releasing hormone (TRH) has an important role in the regulation of energy homeostasis not only through effects on thyroid function orchestrated through hypophysiotropic neurons in the hypothalamic paraventricular nucleus (PVN), but also through central effects on feeding behavior, thermogenesis, locomotor activation and autonomic regulation. Hypophysiotropic TRH neurons are located in the medial and periventricular parvocellular subdivisions of the PVN and receive direct monosynaptic projections from two, separate, populations of leptin-responsive neurons in the hypothalamic arcuate nucleus containing either alpha-melanocyte-stimulating hormone (alpha-MSH) and cocaine- and amphetamine-regulated transcript (CART), peptides that promote weight loss and increase energy expenditure, or neuropeptide Y (NPY) and agouti-related protein (AGRP), peptides that promote weight gain and reduce energy expenditure. During fasting, the reduction in TRH mRNA in hypophysiotropic neurons mediated by suppression of alpha-MSH/CART simultaneously with an increase in NPY/AGRP gene expression in arcuate nucleus neurons contributes to the fall in circulating thyroid hormone levels, presumably by increasing the sensitivity of the TRH gene to negative feedback inhibition by thyroid hormone. Endotoxin administration, however, has the paradoxical effect of increasing circulating levels of leptin and melanocortin signaling and CART gene expression in arcuate nucleus neurons, but inhibiting TRH gene expression in hypophysiotropic neurons. This may be explained by an overriding inhibitory effect of endotoxin to increase type 2 iodothyroine deiodinase (D2) in a population of specialized glial cells, tanycytes, located in the base and infralateral walls of the third ventricle. By increasing the conversion of T4 into T3, tanycytes may increase local tissue concenetrations of thyroid hormone, and thereby induce a state of local tissue hyperthyroidism in the region of hypophysisotrophic TRH neurons. Other regions of the brain may also serve as metabolic sensors for hypophysiostropic TRH neurons including the ventrolateral medulla and dorsomedial nucleus of the hypothalamus that have direct monosynaptic projections to the PVN. TRH also exerts a number of effects within the central nervous system that may contribute to the regulation of energy homeostasis. Included are an increase in core body temperature mediated through neurons in the anterior hypothalamic-preoptic area that coordinate a variety of autonomic responses; arousal and locomotor activation through cholinergic and dopaminergic mechanisms on the septum and nucleus accumbens, respectively; and regulation of the cephalic phase of digestion. While the latter responses are largely mediated through cholinergic mechanisms via TRH neurons in the brainstem medullary raphe and dorsal motor nucleus of the vagus, effects of TRH on autonomic loci in the hypothalamic PVN may also be important. Contrary to the actions of T3 to increase appetite, TRH has central effects to reduce food intake in normal, fasting and stressed animals. The precise locus where TRH mediates this response is unknown. However, evidence that an anatomically separate population of nonhypophysiotropic TRH neurons in the anterior parvocellular subdivision of the PVN is integrated into the leptin regulatory control system by the same arcuate nucleus neuronal populations that innervate hypophysiotropic TRH neurons, raises the possibility that anterior parvocellular TRH neurons may be involved, possibly through interactions with the limbic nervous system.

Metabolic adaptation in the crew of the Hesperides on their Antarctic journey

AIMS

We studied the metabolic changes that took place in the crew of the Hesperides vessel in their 2001-2002 Antarctic journey, comparing two periods differing in diet and physical activity.

METHODS AND RESULTS

Lipid profile, vitamin and hormone levels were analyzed in 17 subjects who completed the study in its two phases. In phase I the participants spent 47 days sailing with hard work and rough seas, and the diet was rich in fat and poor in fresh foods. In this phase, glucose decreased and HDL-cholesterol, apo-AI, and TSH increased. Plasma retinol and alpha-tocopherol levels remained stable, gamma-tocopherol, alpha-carotene and beta-carotene significantly decreased, and lycopene significantly increased. Phase II lasted 49 days including a 7-day long stop in port. This meant that a more varied diet was available and fresh foods were present in the hold. There was also less extreme physical activity. The metabolic pattern changed direction, glucose rose, HDL-cholesterol and apo-AI decreased and the levels of the vitamins that dropped in phase I started to increase. Lycopene significantly decreased.

CONCLUSION

Contrary to popular beliefs about navigation at extreme latitudes, the metabolic changes described may be explained by the intense physical activity in a cold environment and a high-fat diet poor in fresh products.

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.

Role of sarco/endoplasmic reticulum Ca(2+)-ATPase in thermogenesis

Enzymes are able to handle the energy derived from the hydrolysis of phosphate compounds in such a way as to determine the parcel that is used for work and the fraction that is converted into heat. The sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCA) is a family of membrane-bound ATPases that are able to transport Ca(2+) ion across the membrane using the chemical energy derived from ATP hydrolysis. The heat released during ATP hydrolysis by SERCA may vary from 10 up to 30 kcal/mol depending on the SERCA isoform used and on whether or not a Ca(2+) gradient is formed across the membrane. Drugs such as heparin, dimethyl sulfoxide and the platelet-activating factor (PAF) are able to modify the fraction of the chemical energy released during ATP hydrolysis that is used for Ca(2+) transport and the fraction that is dissipated in the surrounding medium as heat. The thyroid hormone 3,5,3'-triiodo L: -thyronine (T(3)) regulates the expression and function of the thermogenic SERCA isoforms. Modulation of heat production by SERCA might be one of the mechanisms involved in the increased thermogenesis found in hyperthyroidism.

Interactions between the melanocortin system and the hypothalamo-pituitary-thyroid axis

Recent studies of transgenic mice and humans have provided compelling evidence for the importance of the hypothalamic melanocortin system in the regulation of energy balance. Energy homeostasis is a balance between food intake (energy input) and energy expenditure. The melanocortin system regulates feeding via effects of the endogenous agonist, alpha-melanocyte stimulating hormone (alpha-MSH) and the endogenous antagonist agouti-related protein (AGRP) on melanocortin 3 and 4 receptors (MC3-Rs and MC4-Rs). It has been demonstrated that the melanocortin system interacts with the hypothalamo-pituitary-thyroid (HPT) axis. Thyroid hormones influence metabolism and hence energy expenditure. Therefore, an interaction between the HPT axis and the melanocortin system would allow control of both sides of the energy balance equation, by the regulation of both energy input and energy expenditure. Here we will discuss the evidence demonstrating interactions between the melanocortin system and the HPT axis.

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.

Transcriptional synergy and the regulation of Ucp1 during brown adipocyte induction in white fat depots

Induction of brown adipocytes in white fat depots by adrenergic stimulation is a complex genetic trait in mice that affects the ability of the animal to regulate body weight. An 80-fold difference in expression of the mitochondrial uncoupling gene (Ucp1) at the mRNA and protein levels between A/J and C57BL/6J (B6) mice is controlled by allelic interactions among nine quantitative trait loci (QTLs) on eight chromosomes. Overlapping patterns of these QTLs also regulate expression levels of Pgc-1alpha, Pparalpha, and type 2 deiodinase. Independent validation that PPARalpha is associated with Ucp1 induction was obtained by treating mice with the PPARalpha agonist clofibrate, but not from the analysis of PPARalpha knockout mice. The most upstream sites of regulation for Ucp1 that differed between A/J and B6 were the phosphorylation of p38 mitogen-activated protein kinase and CREB and then followed by downstream changes in levels of mRNA for PPARgamma, PPARalpha, PGC-1alpha, and type 2 deiodinase. However, compared to Ucp1 expression, the two- to fourfold differences in the expression of these regulatory components are very modest. It is proposed that small variations in the levels of several transcriptional components of the Ucp1 enhanceosome interact synergistically to achieve large differences in Ucp1 expression.

c-Cbl-deficient mice have reduced adiposity, higher energy expenditure, and improved peripheral insulin action

Casitas b-lineage lymphoma (c-Cbl) is an E3 ubiquitin ligase that has an important role in regulating the degradation of cell surface receptors. In the present study we have examined the role of c-Cbl in whole-body energy homeostasis. c-Cbl-/- mice exhibited a profound increase in whole-body energy expenditure as determined by increased core temperature and whole-body oxygen consumption. As a consequence, these mice displayed a decrease in adiposity, primarily due to a reduction in cell size despite an increase in food intake. These changes were accompanied by a significant increase in activity (2- to 3-fold). In addition, c-Cbl-/- mice displayed a marked improvement in whole-body insulin action, primarily due to changes in muscle metabolism. We observed increased protein levels of the insulin receptor (4-fold) and uncoupling protein-3 (2-fold) in skeletal muscle and a significant increase in the phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxylase. These findings suggest that c-Cbl plays an integral role in whole-body fuel homeostasis by regulating whole-body energy expenditure and insulin action.

The metabolic and cardiovascular effects of hyperthyroidism are largely independent of beta-adrenergic stimulation

Hyperthyroidism and states of adrenergic hyperactivity have many common clinical features, suggesting similar pathogenic mechanisms of action. The widespread use of beta-adrenergic receptor (betaAR) antagonists (beta-blockers) to treat hyperthyroidism has led to the belief that the physiological consequences of thyroid hormone (TH) excess are mediated in part via catecholamine signaling through betaARs. To test this hypothesis, we compared the response to TH excess in mice lacking the three known betaARs (beta-less) vs. wild-type (WT) mice. Although beta-less mice had a lower heart rate at baseline in comparison to WT mice, the metabolic and cardiovascular responses to hyperthyroidism were equivalent in both WT and beta-less mice. These data indicate that the metabolic and cardiovascular effects of TH excess are largely independent of betaARs. These findings suggest that the efficacy of clinical treatment of hyperthyroidism with beta-blockers is due to antagonism of sympathetic signaling, and that this process functions independently of TH action.

Nonexercise activity thermogenesis (NEAT): environment and biology

Nonexercise activity thermogenesis (NEAT) is the energy expended for everything that is not sleeping, eating, or sports-like exercise. It includes the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks, and fidgeting. NEAT can be measured by one of two approaches. The first is to measure or estimate total NEAT. Here, total daily energy expenditure is measured, and from it "basal metabolic rate-plus-thermic effect of food" is subtracted. The second is the factoral approach, whereby the components of NEAT are quantified, and total NEAT is calculated by summing these components. The amount of NEAT that humans perform represents the product of the amount and types of physical activities and the thermogenic cost of each activity. The factors that impact a human's NEAT are readily divisible into environmental factors, such as occupation or dwelling within a "concrete jungle," and biological factors such as weight, gender, and body composition. The combined impact of these factors explains the substantial variance in human NEAT. The variability in NEAT might be viewed as random, but human and animal data contradict this. It appears that changes in NEAT subtly accompany experimentally induced changes in energy balance and are important in the physiology of weight change. Inadequate modulation of NEAT plus a sedentary lifestyle may thus be important in obesity. It then becomes intriguing to dissect mechanistic studies that delineate how NEAT is regulated into neural, peripheral, and humoral factors. A scheme is described in this review in which NEAT corresponds to a carefully regulated "tank" of physical activity that is crucial for weight control.

Sequestration of thermogenic transcription factors in the cytoplasm during development of brown adipose tissue

Transcription factors that regulate gene expression during adipogenesis also control the expression of genes of thermogenesis in brown adipose tissue, in particular, the mitochondrial uncoupling protein gene (Ucp1). There is evidence that a plasticity exists among adipocytes in which activation of the Ucp1 gene together with mitochondrial biogenesis can increase the brown adipocyte character of white fat. To understand this process, we have characterized the changes in transcription that occur in interscapular brown adipocytes during development. We have found dramatic reductions in both DNA-binding activity to probes and immunoreactive protein for peroxisome proliferator-activated receptor, retinoid X receptor, CCAAT/enhancer binding protein, and cAMP-response element-binding protein regulatory motifs in nuclear extracts when mice reach adulthood. Exposure of adult mice to the cold, which reactivates Ucp1 expression, leads to a re-accumulation of factors in the nucleus. We propose that transcription factors are sequestered in the cytoplasm as mice age under conditions of reduced thermogenesis. Changes in isoform sub-types for peroxisome proliferator-activated receptor-gamma and cAMP-response element-binding proteins indicate an additional level of control on gene expression during thermogenesis. The increased movement of the RIIbeta protein kinase A regulatory subunit into the nucleus with age suggests a mechanism for regulating the phosphorylation of transcription factors in the nucleus in response to the thermogenic requirements of the animal. Nuclear factor-kappaB has been used as a model to demonstrate that the nuclear localization of transcription factors in brown fat are reduced during post-natal development. Furthermore, it was found by immunofluorescence that adrenergic stimulation of primary adipocyte cultures causes an increase of both the protein kinase A catalytic alpha-subunit and nuclear factor-kappaB into the nucleus.

Green Tea Polyphenol Epigallocatechin-3-gallate (EGCG) Differentially Inhibits Interleukin-1β-Induced Expression of Matrix Metalloproteinase-1 and -13 in Human Chondrocytes

Interleukin-1beta (IL-1beta)-induced inflammatory response in arthritic joints include the enhanced expression and activity of matrix metalloproteinases (MMPs), and their matrix degrading activity contribute to the irreversible loss of cartilage and may also be associated with sustained chronic inflammation. We have earlier shown that green tea (Camellia sinensis) polyphenol epigallocatechin-3-gallate (EGCG) was non-toxic to human chondrocytes [Singh R, Ahmed S, Islam N, Goldberg VM, and Haqqi TM (2002) Arthritis Rheum 46: 2079-2086] and inhibits the expression of inflammatory mediators in arthritic joints [Haqqi TM, Anthony DD, Gupta S, Ahmed N, Lee MS, Kumar GK, and Mukhtar H (1999) Proc Natl Acad Sci USA 96: 4524-4529]. Here we show that EGCG at micromolar concentrations was highly effective in inhibiting the IL-1beta-induced glycosaminoglycan (GAG) release from human cartilage explants in vitro. EGCG also inhibited the IL-1beta-induced mRNA and protein expression of MMP-1 and MMP-13 in human chondrocytes. Importantly, EGCG showed a differential, dose-dependent inhibitory effect on the expression and activity of MMP-13 and MMP-1. A similar differential dose-dependent inhibition of transcription factors NF-kappaB and AP-1 by EGCG was also noted. These results for the first time demonstrate a differential dose-dependent effect of EGCG on the expression and activity of MMPs and on the activities of transcription factors NF-kappaB and AP-1 and provide insights into the molecular basis of the reported anti-inflammatory effects of EGCG. These results also suggest that EGCG or compounds derived from it may be therapeutically effective inhibitors of IL-1beta-induced production of matrix-degrading enzymes in arthritis.

Mitochondrial respiration in muscle and liver from cold-acclimated hypothyroid rats

The effects of long-term cold exposure on muscle and liver mitochondrial oxygen consumption in hypothyroid and normal rats were examined. Thyroid ablation was performed after 8-wk acclimation to 4 degrees C. Hypothyroid and normal controls remained in the cold for an additional 8 wk. At the end of 16-wk cold exposure, all hypothyroid rats were alive and normothermic and had normal body weight. At ambient temperature (24 degrees C), thyroid ablation induced a 65% fall in muscle mitochondrial oxygen consumption, which was reversed by thyroxine but not by norepinephrine administration. After cold acclimation was reached, suppression of thyroid function reduced muscle mitochondrial respiration by 30%, but the hypothyroid values remained about threefold higher than those in hypothyroid muscle in the warm. Blockade of beta- and alpha1-adrenergic receptors in both hypothyroid and normal rats produced hypothermia in vivo and a fall in muscle, liver, and brown adipose tissue mitochondria respiration in vitro. In normal rats, cold acclimation enhanced muscle respiration by 35%, in liver 18%, and in brown adipose tissue 450% over values in the warm. The results demonstrate that thyroid hormones, in the presence of norepinephrine, are major determinants of thermogenic activity in muscle and liver of cold-acclimated rats. After thyroid ablation, cold-induced nonshivering thermogenesis replaced 3,5,3'-triiodothyronine-induced thermogenesis, and normal body temperature was maintained.

Effect of thyroid hormone on gene expression

PURPOSE OF REVIEW

Thyroid hormones are key regulators of development and metabolism that modulate transcription via nuclear receptors. Although the molecular actions of thyroid hormones have been thoroughly studied, their pleiotropic effects are mediated by complex changes in expression of numerous, but still largely unknown, target genes. This review summarizes the recent advances in the characterization of target genes in different organs.

RECENT FINDINGS

New patterns of gene expression regulation have been described in tissues with known physiological actions of thyroid hormone, that is brain, liver, skeletal and cardiac muscles, and brown and white adipose tissues. The studies have benefited from the numerous transgenic models with altered thyroid hormone receptor expression and the application of DNA microarray technology to mouse and human tissues.

SUMMARY

Data on thyroid hormone-mediated control of gene expression and on the roles of the different thyroid hormone receptor isoforms bring new clues to our understanding of the molecular mechanisms of thyroid hormone action in physiological situations and, most importantly, in diseases associated with alterations of the thyroid status.

Phenyl N-tert-butylnitrone down-regulates interleukin-1 beta-stimulated matrix metalloproteinase-13 gene expression in human chondrocytes: suppression of c-Jun NH2-terminal kinase, p38-mitogen-activated protein kinase and activating protein-1

Cytokine-mediated induction and overexpression of matrix metalloproteinases (MMPs) is recognized as an important factor in the pathogenesis of arthritis. Interleukin (IL)-1 beta is a proinflammatory cytokine that is known to superinduce the expression and production of MMP-13 in many cell types. Phenyl N-tert-butylnitrone (PBN), a spin trap agent, inhibited the IL-1 beta-induced expression of MMP-13 in human osteoarthritis (OA) chondrocytes. Down-regulation of MMP-13 expression correlated with the inhibition of mitogen-activated protein kinase (MAPK) subgroups c-Jun NH2-terminal kinase (JNK) and p38-MAPK activation, accumulation of phospho-c-jun, and the DNA binding activity of activating protein-1 (AP-1). Results of in vitro kinase assays showed that exogenously added PBN completely blocked the c-Jun phosphorylating activity of JNK. Interestingly, using in vitro kinase assay, we also found that chondrocyte p38-MAPK phosphorylate c-Jun and that PBN was not very effective in inhibiting c-Jun phosphorylating activity of p38-MAPK. In addition, PBN did not block the ATF-2 phosphorylating activity of p38-MAPK and Elk-1 phosphorylating activity of extracellular regulated kinase p44/p42 in vitro, indicating that PBN may act selectively to inhibit the phosphorylation of c-Jun in OA chondrocytes. Together, our results for the first time demonstrate that PBN suppresses the IL-1 beta-stimulated expression of MMP-13 in OA chondrocytes and that this was achieved by inhibiting the activation of JNK and AP-1. These results suggest that use of PBN or compounds derived from it may be of potential benefit in inhibiting signaling events associated with cartilage degradation in arthritis.

Effect of hyperthyroidism on spontaneous physical activity and energy expenditure in rats

Thyroid hormone excess is associated with increased energy expenditure. The contributions of increases in spontaneous physical activity and nonexercise activity thermogenesis (NEAT) to this effect have not been defined. To address the hypothesis that hyperthyroidism is associated with increased spontaneous physical activity and NEAT, we rendered rats hyperthyroid by using continuous infusion of high-dose triiodothyronine for 14 days and measured the effects on physical activity and NEAT. On day 14, in the hyperthyroid group the mean +/- SD triiodothyronine concentration was 755 +/- 137 (range 574-919) ng/dl and in the control group 59 +/- 0.5 (58-59) ng/dl. Over the 14-day treatment period, mean spontaneous physical activity increased in the hyperthyroid rats from 24 +/- 7 to 36 +/- 6 activity units (AU)/min; P < 0.001 but did not increase in the controls (23 +/- 7 vs. 22 +/- 4 AU/min). Also, over the 14-day period, daily NEAT increased in the hyperthyroid rats from 8.1 +/- 2.8 to 19.7 +/- 5.0 kcal/day (P < 0.001) but did not increase in the controls (8.7 +/- 3.5 cf 9.4 +/- 1.7 kcal/day; not significant). In conclusion, hyperthyroidism is associated with increased spontaneous physical activity and NEAT.

Central stimulatory effect of leptin on T3 production is mediated by brown adipose tissue type II deiodinase

To assess whether intracerebroventricular leptin administration affects monodeiodinase type II (D2) activity in the tissues where it is expressed [cerebral cortex, hypothalamus, pituitary, and brown adipose tissue (BAT)], hepatic monodeiodinase type I (D1) activity was inhibited with propylthiouracil (PTU), and small doses of thyroxine (T4; 0.6 nmol. 100 g body wt(-1). day(-1)) were supplemented to compensate for the PTU-induced hypothyroidism. Two groups of rats were infused with leptin for 6 days, one of them being additionally treated with reverse triiodothyronine (rT3), an inhibitor of D2. Control rats were infused with vehicle and pair-fed the amount of food consumed by leptin-infused animals. Central leptin administration produced marked increases in D2 mRNA expression and activity in BAT, changes that were likely responsible for increased plasma T3 and decreased plasma T4 levels. Indeed, plasma T3 and T4 concentrations were unaltered by central leptin administration in the presence of rT3. The additional observation of a leptin-induced increased mRNA expression of BAT uncoupling protein-1 suggested that the effect on BAT D2 may be mediated by the sympathetic nervous system.