Two thyroid hormone-mediated gene expression patterns in vivo identified by cDNA expression arrays in rat

Hypothyroidism/subclinical hypothyroidism and metabolic dysfunction-associated steatotic liver disease: advances in mechanism and treatment

Hypothyroidism is a risk factor for metabolic dysfunction-associated steatotic liver disease (MASLD) but it is not clear whether subclinical hypothyroidism (SCH) increases the risk of MASLD and whether SCH patients with MASLD require treatment. In this study, we reviewed articles published in PubMed from 2013 to 2024 with SCH/hypothyroidism and MASLD as keywords. According to the studies retrieved, SCH increases the likelihood of developing MASLD. Thyroid hormones influence energy metabolism and storage in adipose tissues, as well as fatty acid and cholesterol metabolism and transport in the liver. L-T4 replacement therapy reduces the prevalence of MASLD, especially in patients with severe SCH or mild SCH with dyslipidemia. Recent studies showed that thyroid hormone analogues and thyroid hormone β receptor agonists obtained positive results in the treatment of MASLD in animal models and clinical trials, and Resmetirom has been approved by the US. Food and Drug Administration (FDA) under the name Rezdiffra for use in conjunction with dietary and exercise regimens for managing non-cirrhotic NASH in adults with moderate to advanced fibrosis.

Irisin in thyroid diseases

Irisin, a hormone-like adipo-myokine, has garnered considerable attention in recent years for its potential impact in metabolic diseases. Its physiological effects are similar to those of thyroid hormones, prompting numerous investigations into potential correlations and interactions between irisin and thyroid function through various in vitro and animal experiments. However, existing studies suggest that the relationship between irisin and thyroid diseases is highly complex and multifaceted. In this paper, we have summarized the research results on serum irisin and thyroid function, providing an overview of advancements and constraints in current research on irisin and thyroid hormones. The aim is to offer insights and directions for future clinical trials in this field.

Breast cancer cells utilize T3 to trigger proliferation through cellular Ca2+ modulation

High levels of thyroid hormones are linked to increased risk and advanced stages of breast cancer. Our previous work demonstrated that the biologically active triiodothyronine (T3) facilitates mitochondrial ATP production by upregulating Ca2+ handling proteins, thereby boosting mitochondrial Ca2+ uptake and Krebs cycle activity. In this study, different cell types were utilized to investigate whether T3 activates a Ca2+-induced signaling pathway to boost cancer cell proliferation. Using live-cell imaging, biochemical assays, and molecular profiling, differences in intracellular signaling among MCF7 and MDA-MB-468 breast cancer cells, non-cancerous breast cells hTERT-HME1, and PC3 prostate carcinoma cells, previously found to be insensitive to thyroid hormones in terms of proliferation, were investigated. Our findings revealed that T3 upregulates 1,4,5-trisphosphate receptor 3 via thyroid hormone receptor α. This boosts mitochondrial Ca2+ uptake, reduction equivalent yield, and mitochondrial ATP production, supporting the viability and proliferation of breast cancer cells without affecting non-cancerous hTERT-HME1 or PC3 prostate carcinoma cells. Understanding the interplay between T3 signaling, organellar interaction, and breast cancer metabolism could lead to targeted therapies that exploit cancer cell vulnerabilities. Our findings highlight T3 as a crucial regulator of cancer metabolism, reinforcing its potential as a therapeutic target in breast cancer.

Maternal Exercise Before and During Pregnancy Facilitates Embryonic Myogenesis by Enhancing Thyroid Hormone Signaling

Background: Maternal exercise (ME) improves fetal and offspring muscle development, but mechanisms remain to be established. Since the thyroid hormone (TH) is critical for cell differentiation during embryonic development, we hypothesized that ME elevates TH receptor (THR) signaling in embryos, which promotes embryonic myogenesis. Methods: Female mice were exercised daily on a treadmill or received a daily TH, triiodothyronine (T3) injection. Embryos (embryonic day 12.5 [E12.5]) and P19 cells were used for studying effects of TH on embryonic myogenesis. TH levels in serum and embryos after ME or T3I were analyzed. Expression of TH signaling related genes and myogenic genes was assessed. THRα binding to the promoters of myogenic genes was investigated by chromatin immunoprecipitation-qantitative polymerase chain reaction (ChIP-qPCR). A CRISPR/CAS9 plasmid was utilized to knock out THRα in P19 cells. Results: ME elevated TH levels in both maternal circulation and embryos, which were correlated with enhanced TH signaling and myogenesis. At E12.5, both myogenic determinants (Pax3, Pax7) and myogenic regulatory factors (Myf5, Myod) were upregulated in ME embryos. ME increased THRα content and elevated messenger RNA (mRNA) expression of TH transporter Slc16a2 and deiodinase Dio2. In addition, the THRα binding to the promoters of Pax3/7 was increased. In P19 embryoid bodies, T3 promoted myogenic differentiation, which was abolished by ablating THRα. Furthermore, maternal daily injection of T3 at a level matching exercised mothers promoted embryonic myogenesis. Conclusions: ME promotes TH delivery to the embryos and enhances embryonic myogenesis, which is partially mediated by enhanced TH signaling in ME embryos.

Bioenergetic Aspects of Mitochondrial Actions of Thyroid Hormones

Much is known, but there is also much more to discover, about the actions that thyroid hormones (TH) exert on metabolism. Indeed, despite the fact that thyroid hormones are recognized as one of the most important regulators of metabolic rate, much remains to be clarified on which mechanisms control/regulate these actions. Given their actions on energy metabolism and that mitochondria are the main cellular site where metabolic transformations take place, these organelles have been the subject of extensive investigations. In relatively recent times, new knowledge concerning both thyroid hormones (such as the mechanisms of action, the existence of metabolically active TH derivatives) and the mechanisms of energy transduction such as (among others) dynamics, respiratory chain organization in supercomplexes and cristes organization, have opened new pathways of investigation in the field of the control of energy metabolism and of the mechanisms of action of TH at cellular level. In this review, we highlight the knowledge and approaches about the complex relationship between TH, including some of their derivatives, and the mitochondrial respiratory chain.

T3-induced enhancement of mitochondrial Ca2+ uptake as a boost for mitochondrial metabolism

Thyroid hormones act as master regulators of cellular metabolism. Thereby, the biologically active triiodothyronine (T3) induces the expression of genes to enhance mitochondrial metabolic function. Notably, Ca²⁺ ions are necessary for the activity of dehydrogenases of the tricarboxylic acid cycle and, thus, mitochondrial respiration. We investigated whether treating HeLa cells with T3 causes alterations in mitochondrial Ca²⁺ ([Ca²⁺]_mito) levels. Real-time measurements by fluorescence microscopy revealed that treatment with T3 for 3 h induces a significant increase in basal [Ca²⁺]_mito levels and [Ca²⁺]_mito uptake upon the depletion of the endoplasmic reticulum (ER) Ca²⁺ store, while cytosolic Ca²⁺ levels remained unchanged. T3 incubation was found to upregulate mRNA expression levels of uncoupling proteins 2 and 3 (UCP2, UCP3) and of protein arginine methyltransferase 1 (PRMT1). Live-cell imaging revealed that T3-induced enhancement of mitochondrial Ca²⁺ uptake depends on the mitochondrial Ca²⁺ uniporter (MCU), UCP2, and PRMT1 that are essential for increased mitochondrial ATP ([ATP]_mito) production after T3 treatment. Besides, increased [Ca²⁺]_mito and [ATP]_mito levels correlated with enhanced production of reactive oxygen species (ROS) in mitochondria. Notably, ROS scavenging causes mitochondrial Ca²⁺ elevation and outplays the impact of T3 on [Ca²⁺]_mito homeostasis. Based on these results, we assume that thyroid hormones adjust [Ca²⁺]_mito homeostasis by modulating the UCP2- and PRMT1-balanced [Ca²⁺]_mito uptake via MCU in case of physiological ROS levels to convey their impact on mitochondrial ATP and ROS production.

Comparative Analysis of the Effects of Long-Term 3,5-diiodothyronine Treatment on the Murine Hepatic Proteome and Transcriptome Under Conditions of Normal Diet and High-Fat Diet

Background: The thyroid hormone (TH) metabolite 3,5-diiodothyronine (3,5-T2) is considered as a potential drug for treatment of nonalcoholic fatty liver disease (NAFLD) based on its prominent antisteatotic effects in murine models of obesity without the detrimental thyromimetic side effects known for classical TH. To expand our understanding of its mode of action, we comprehensively characterized the effects of 3,5-T2 on hepatic gene expression in a diet-induced murine model of obesity by a combined liver proteome and transcriptome analysis. Materials and Methods: Male C57BL/6 mice fed high-fat diet (HFD) to induce NAFLD or standard diet (SD) as control were treated with 2.5 μg/g body weight 3,5-T2 or saline for 4 weeks. We performed mass spectrometry analyses and integrated those proteome data with earlier published microarray-based transcriptome data from the same animals. In addition, concentrations of several sex steroids in serum and different tissues were determined by gas chromatography-tandem mass spectrometry. Results: We observed limited concordance between transcripts and proteins exhibiting differential abundance under 3,5-T2 treatment, which was only partially explainable by methodological reasons and might, therefore, reflect noncanonical post-transcriptional events. The treatment affected the levels of more and partially different proteins under HFD as compared with SD, demonstrating response modulation by the hepatic lipid load. The hepatic physiological signatures of 3,5-T2 treatment inferable from the omics data comprised the reduction of oxidative stress and alteration of apolipoprotein profiles, both due to decreased liver fat content. In addition, induction of several classical TH target genes and genes involved in the biosynthesis of cholesterol, bile acids (BAs), and male sex steroids was observed. The latter finding was supported by hepatic sex steroid measurements. Conclusion: While confirming the beneficial hepatic liver fat reduction by 3,5-T2 treatment, our data suggest that besides the well-known induction of fatty acid oxidation the stimulation of cholesterol- and BA synthesis with subsequent excretion of the latter through bile might represent a further important mechanism in this context. The obvious intensified male sex steroid exposition of the liver in 3,5-T2-treated HFD animals can be predicted to cause enhanced hepatic "masculinization," with not yet clear but potentially detrimental physiological consequences.

Role of irisin in Chinese patients with hypothyroidism: an interventional study

Objective

Irisin is a myokine that greatly affects energy expenditure and systemic metabolism. While thyroid hormone is likely associated with irisin, a direct relationship remains to be fully elucidated. This study aimed to investigate plasma irisin levels in Chinese patients with hypothyroidism.

Methods

A total of 155 subjects were divided into the hypothyroidism group or the control group. Fifty-seven patients in the hypothyroidism group received levothyroxine treatment. Baseline irisin levels were measured in the two groups and post-treatment levels were measured in the hypothyroidism group.

Results

Irisin levels were significantly lower in the hypothyroidism group than in the control group. In the hypothyroidism group, irisin levels were positively associated with free triiodothyronine and free thyroxine levels, and negatively associated with thyrotropin levels. In the hypothyroidism group, irisin levels were significantly increased after levothyroxine treatment. Multiple linear regression models showed that total cholesterol and free thyroxine levels were the only significant predictors of serum irisin levels.

Conclusions

Irisin levels are decreased in patients with hypothyroidism. Our results suggest that decreased irisin levels are directly associated with reduced thyroid hormone levels. These values may be restored after levothyroxine treatment in Chinese patients with hypothyroidism.

Hepatic thyroid signaling of heat-stressed late pregnant and early lactating cows

During the transition between late gestation and early lactation, dairy cows experience severe metabolic stress due to the high energy and nutrient requirements of the fetus and the mammary gland. Additional thermal stress that occurs with rising temperatures during the ongoing climate change has further adverse implications on energy intake, metabolism and welfare. The thyroid hormone (TH)-mediated cellular signaling has a pivotal role in regulation of body temperature, energy intake and metabolic adaptation to heat. To distinguish between energy intake and heat stress-related effects, Holstein cows were first kept at thermoneutrality at 15°C followed by exposure to heat stress (HS) at 28°C or pair-feeding (PF) at 15°C for 6 days, in late pregnancy and again in early lactation. Herein, we focused on hepatic metabolic changes associated with alterations in the hypothalamic-pituitary-thyroid axis in HS and PF animals. T₃ and T₄ levels dropped with HS or PF; however, in HS animals, this decline was more pronounced. Thyroid-stimulating hormone (TSH) levels remain unaffected, while plasma cholesterol concentrations were lower in HS than PF animals. Hepatic marker genes for TH action (THRA, DIO1 and PPARGC1) decreased after HS and were lower compared to PF cows but only post-partum. Proteomics data revealed reduced hepatic amino acid catabolism ante-partum and a shift toward activated beta-oxidation and gluconeogenesis but declined oxidative stress defense post-partum. Thus, liver metabolism of HS and PF cows adapts differently to diminished energy intake both ante-partum and post-partum, and a different TH sensitivity is involved in the regulation of catabolic processes.

Regulating role of fetal thyroid hormones on placental mitochondrial DNA methylation: epidemiological evidence from the ENVIRONAGE birth cohort study

BACKGROUND

Fetal development largely depends on thyroid hormone availability and proper placental function with an important role played by placental mitochondria. The biological mechanisms by which thyroid hormones exert their effects on mitochondrial function are not well understood. We investigated the role of fetal thyroid hormones on placental mitochondrial DNA (mtDNA) content and mtDNA methylation. We collected placental tissue and cord blood from 305 mother-child pairs that were enrolled between February 2010 and June 2014 in the ENVIRONAGE (ENVIRonmental influence ON early AGEing) birth cohort (province of Limburg, Belgium). Placental mtDNA content was determined by qPCR and placental mtDNA methylation by bisulfite-pyrosequencing in two regions, i.e., the D-loop control region and 12S ribosomal RNA (MT-RNR1). The levels of free thyroid hormones (FT₃, FT₄) and thyroid-stimulating hormone (TSH) were measured in cord blood.

RESULTS

Cord blood FT₃ and FT₄ were inversely associated with placental mtDNA methylation at the MT-RNR1 (p ≤ 0.01) and D-loop (p ≤ 0.05) regions, whereas a positive association was observed for both hormones with placental mtDNA content (p ≤ 0.04). Assuming causality, we estimated that MT-RNR1 and D-loop methylation mediated, respectively, 77% [indirect effect +14.61% (95% CI 2.64 to 27.98%, p = 0.01)] and 47% [indirect effect +8.60% (95% CI 1.23 to 16.50%, p = 0.02] of the positive association between FT₃ and placental mtDNA content. Mediation models with FT₄ gave similar results but the estimated effect proportions were smaller compared with those of FT₃ (54% and 24%, respectively).

CONCLUSIONS

We showed that epigenetic modification at specific loci of the mitochondrial genome could intervene with the thyroid-dependent regulation of mitochondrial DNA copy numbers.

Exogenous thyroxine improves glucose intolerance in insulin-resistant rats

Both hypothyroidism and hyperthyroidism are associated with glucose intolerance, calling into question the contribution of thyroid hormones (TH) on glucose regulation. TH analogues and derivatives may be effective treatment options for glucose intolerance and insulin resistance (IR), but their potential glucoregulatory effects during conditions of impaired metabolism are not well described. To assess the effects of thyroxine (T4) on glucose intolerance in a model of insulin resistance, an oral glucose tolerance test (oGTT) was performed on three groups of rats (n = 8): (1) lean, Long Evans Tokushima Otsuka (LETO), (2) obese, Otsuka Long Evans Tokushima Fatty (OLETF) and (3) OLETF + T4 (8.0 µg/100 g BM/day × 5 weeks). T4 attenuated glucose intolerance by 15% and decreased IR index (IRI) by 34% in T4-treated OLETF compared to untreated OLETF despite a 31% decrease in muscle Glut4 mRNA expression. T4 increased the mRNA expressions of muscle monocarboxylate transporter 10 (Mct10), deiodinase type 2 (Di2), sirtuin 1 (Sirt1) and uncoupling protein 2 (Ucp2) by 1.8-, 2.2-, 2.7- and 1.4-fold, respectively, compared to OLETF. Activation of AMP-activated protein kinase (AMPK) and insulin receptor were not significantly altered suggesting that the improvements in glucose intolerance and IR were independent of enhanced insulin-mediated signaling. The results suggest that T4 treatment increased the influx of T4 in skeletal muscle and, with an increase of DI2, increased the availability of the biologically active T3 to upregulate key factors such SIRT1 and UCP2 involved in cellular metabolism and glucose homeostasis.

Plasma proteome and metabolome characterization of an experimental human thyrotoxicosis model

BACKGROUND

Determinations of thyrotropin (TSH) and free thyroxine (FT4) represent the gold standard in evaluation of thyroid function. To screen for novel peripheral biomarkers of thyroid function and to characterize FT4-associated physiological signatures in human plasma we used an untargeted OMICS approach in a thyrotoxicosis model.

METHODS

A sample of 16 healthy young men were treated with levothyroxine for 8 weeks and plasma was sampled before the intake was started as well as at two points during treatment and after its completion, respectively. Mass spectrometry-derived metabolite and protein levels were related to FT4 serum concentrations using mixed-effect linear regression models in a robust setting. To compile a molecular signature discriminating between thyrotoxicosis and euthyroidism, a random forest was trained and validated in a two-stage cross-validation procedure.

RESULTS

Despite the absence of obvious clinical symptoms, mass spectrometry analyses detected 65 metabolites and 63 proteins exhibiting significant associations with serum FT4. A subset of 15 molecules allowed a robust and good prediction of thyroid hormone function (AUC = 0.86) without prior information on TSH or FT4. Main FT4-associated signatures indicated increased resting energy expenditure, augmented defense against systemic oxidative stress, decreased lipoprotein particle levels, and increased levels of complement system proteins and coagulation factors. Further association findings question the reliability of kidney function assessment under hyperthyroid conditions and suggest a link between hyperthyroidism and cardiovascular diseases via increased dimethylarginine levels.

CONCLUSION

Our results emphasize the power of untargeted OMICs approaches to detect novel pathways of thyroid hormone action. Furthermore, beyond TSH and FT4, we demonstrated the potential of such analyses to identify new molecular signatures for diagnosis and treatment of thyroid disorders. This study was registered at the German Clinical Trials Register (DRKS) [DRKS00011275] on the 16th of November 2016.

Regulation of skeletal muscle mitochondrial activity by thyroid hormones: focus on the "old" triiodothyronine and the "emerging" 3,5-diiodothyronine

3,5,3′-Triiodo-L-thyronine (T3) plays a crucial role in regulating metabolic rate and fuel oxidation; however, the mechanisms by which it affects whole-body energy metabolism are still not completely understood. Skeletal muscle (SKM) plays a relevant role in energy metabolism and responds to thyroid state by remodeling the metabolic characteristics and cytoarchitecture of myocytes. These processes are coordinated with changes in mitochondrial content, bioenergetics, substrate oxidation rate, and oxidative phosphorylation efficiency. Recent data indicate that “emerging” iodothyronines have biological activity. Among these, 3,5-diiodo-L-thyronine (T2) affects energy metabolism, SKM substrate utilization, and mitochondrial functionality. The effects it exerts on SKM mitochondria involve more aspects of mitochondrial bioenergetics; among these, respiratory chain activity, mitochondrial thermogenesis, and lipid-handling are stimulated rapidly. This mini review focuses on signaling and biochemical pathways activated by T3 and T2 in SKM that influence the above processes. These novel aspects of thyroid physiology could reveal new perspectives for understanding the involvement of SKM mitochondria in hypo- and hyper-thyroidism.

Functional maturation of human pluripotent stem cell derived cardiomyocytes in vitro--correlation between contraction force and electrophysiology

Cardiomyocytes from human pluripotent stem cells (hPSC-CM) have many potential applications in disease modelling and drug target discovery but their phenotypic similarity to early fetal stages of cardiac development limits their applicability. In this study we compared contraction stresses of hPSC-CM to 2nd trimester human fetal derived cardiomyocytes (hFetal-CM) by imaging displacement of fluorescent beads by single contracting hPSC-CM, aligned by microcontact-printing on polyacrylamide gels. hPSC-CM showed distinctly lower contraction stress than cardiomyocytes isolated from hFetal-CM. To improve maturation of hPSC-CM in vitro we made use of commercial media optimized for cardiomyocyte maturation, which promoted significantly higher contraction stress in hPSC-compared with hFetal-CM. Accordingly, other features of cardiomyocyte maturation were observed, most strikingly increased upstroke velocities and action potential amplitudes, lower resting membrane potentials, improved sarcomeric organization and alterations in cardiac-specific gene expression. Performing contraction force and electrophysiology measurements on individual cardiomyocytes revealed strong correlations between an increase in contraction force and a rise of the upstroke velocity and action potential amplitude and with a decrease in the resting membrane potential. We showed that under standard differentiation conditions hPSC-CM display lower contractile force than primary hFetal-CM and identified conditions under which a commercially available culture medium could induce molecular, morphological and functional maturation of hPSC-CM in vitro. These results are an important contribution for full implementation of hPSC-CM in cardiac disease modelling and drug discovery.

Hepatoprotective effect of satureja khuzestanica essential oil and vitamin e in experimental hyperthyroid rats: evidence for role of antioxidant effect

BACKGROUND

Hyperthyroidism is associated with liver oxidative stress causing liver dysfunction in many hyperthyroid patients. The hepatoprotective effect of Satureja Khuzestanica Essential Oil (SKEO), as herbal origin antioxidant and anti-inflammatory agent on the hyperthyroidism induced hepatotoxicity and oxidative stress is investigated.

METHODS

Adult male sprague dawley rats were divided into categories of; control (group C), hyperthyroid (group H), hyperthyroid with olive oil (group H+O), hyperthyroid with vitamin E (group H+E), hyperthyroid with SKEO (group H+S), combination of hyperthyroid with vitamin E and SKEO (group H+S+E). Hepatoprotective and antioxidant properties of SKEO with or without vitamin E in hyperthyroid rats were then investigated.

RESULTS

Serum Aspartate Transaminase (AST) and Alanine Transaminase (ALT) activities reduced significantly in H+O, H+E, H+S and H+S+E groups in comparison with hyperthyroid rats. Enzymes activities returned to normal in H+S+E group. Hepatic Malondialdehyde (MDA) was reduced in H+E, H+S and H+S+E groups in comparison with hyperthyroid rats. The most significant MDA reduction was in the H+S+E group. Glutathione Peroxidase (GPx) and Glutathione Reductase (GR) activities increased in H+E, H+S and H+S+E groups in comparison with group H. The largest increment in GPx and GR activities were in the H+S+E group. Glutathione level did not change in any group in comparison with the control group.

CONCLUSION

Administration of SKEO has hepatoprotective effect in hyperthyroid rats and is more effective when used in combination with vitamin E.

SIRT1 is a direct coactivator of thyroid hormone receptor β1 with gene-specific actions

Sirtuin 1 (SIRT1) NAD⁺-dependent deacetylase regulates energy metabolism by modulating expression of genes involved in gluconeogenesis and other liver fasting responses. While many effects of SIRT1 on gene expression are mediated by deacetylation and activation of peroxisome proliferator activated receptor coactivator α (PGC-1α), SIRT1 also binds directly to DNA bound transcription factors, including nuclear receptors (NRs), to modulate their activity. Since thyroid hormone receptor β1 (TRβ1) regulates several SIRT1 target genes in liver and interacts with PGC-1α, we hypothesized that SIRT1 may influence TRβ1. Here, we confirm that SIRT1 cooperates with PGC-1α to enhance response to triiodothyronine, T₃. We also find, however, that SIRT1 stimulates TRβ1 activity in a manner that is independent of PGC-1α but requires SIRT1 deacetylase activity. SIRT1 interacts with TRβ1 in vitro, promotes TRβ1 deacetylation in the presence of T₃ and enhances ubiquitin-dependent TRβ1 turnover; a common response of NRs to activating ligands. More surprisingly, SIRT1 knockdown only strongly inhibits T₃ response of a subset of TRβ1 target genes, including glucose 6 phosphatase (G-6-Pc), and this is associated with blockade of TRβ1 binding to the G-6-Pc promoter. Drugs that target the SIRT1 pathway, resveratrol and nicotinamide, modulate T₃ response at dual TRβ1/SIRT1 target genes. We propose that SIRT1 is a gene-specific TRβ1 co-regulator and TRβ1/SIRT1 interactions could play important roles in regulation of liver metabolic response. Our results open possibilities for modulation of subsets of TR target genes with drugs that influence the SIRT1 pathway.

FoxO6 and PGC-1α form a regulatory loop in myogenic cells

Transcription factors of the FoxO (forkhead box O) family regulate a wide range of cellular physiological processes, including metabolic adaptation and myogenic differentiation. The transcriptional activity of most FoxO members is inhibitory to myogenic differentiation and overexpression of FoxO1 inhibits the development of oxidative type I fibres in vivo. In this study, we found that FoxO6, the last discovered FoxO family member, is expressed ubiquitously in various tissues but with higher expression levels in oxidative tissues, such as brain and oxidative muscles. Both the expression level and promoter activity of FoxO6 were found to be enhanced by PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α), thus explained its enriched expression in oxidative tissues. We further demonstrated that FoxO6 represses the expression of PGC-1α via direct binding to an upstream A/T-rich element (AAGATATCAAAACA,−2228–2215) in the PGC-1α promoter. Oxidative low-intensity exercise induced PGC-1α but reduced FoxO6 expression levels in hind leg muscles, and the binding of FoxO6 to PGC-1α promoter was also prevented by exercise. As FoxO6 promoter can be co-activated by PGC-1α and its promoter in turn can be repressed by FoxO6, it suggests that FoxO6 and PGC-1α form a regulatory loop for setting oxidative metabolism level in the skeletal muscle, which can be entrained by exercise.

Nuclear corepressors mediate the repression of phospholipase A2 group IIa gene transcription by thyroid hormone

Secretory phospholipase A2 group IIa (PLA2g2a) is associated with inflammation, hyperlipidemia, and atherogenesis. Transcription of the PLA2g2a gene is induced by multiple cytokines. Here, we report the surprising observation that thyroid hormone (T3) inhibited PLA2g2a gene expression in human and rat hepatocytes as well as in rat liver. Moreover, T3 reduced the cytokine-mediated induction of PLA2g2a, suggesting that the thyroid status may modulate aspects of the inflammatory response. In an effort to dissect the mechanism of repression by T3, we cloned the PLA2g2a gene and identified a negative T3 response element in the promoter. This T3 receptor (TRβ)-binding site differed considerably from consensus T3 stimulatory elements. Using in vitro and in vivo binding assays, we found that TRβ bound directly to the PLA2g2a promoter as a heterodimer with the retinoid X receptor. Knockdown of nuclear corepressor or silencing mediator for retinoid and thyroid receptors by siRNA blocked the T3 inhibition of PLA2g2a. Using chromatin immunoprecipitation assays, we showed that nuclear corepressor and silencing mediator for retinoid and thyroid receptors were associated with the PLA2g2a gene in the presence of T3. In contrast with the established role of T3 to promote coactivator association with TRβ, our experiments demonstrate a novel inverse recruitment mechanism in which liganded TRβ recruits corepressors to inhibit PLA2g2a expression.

Distinct ligand-dependent and independent modes of thyroid hormone receptor (TR)/PGC-1α interaction

Thyroid hormone receptor (TR)/peroxisome proliferator activated receptor coactivator (PGC-1α) interactions are required for T(3)-dependent transcriptional responses involved in adaptive thermogenesis and liver. Thus, it is important to define TR/PGC-1α contact modes and to understand their significance in gene expression. Previous studies have shown that TRβ1 recruits PGC-1α to target promoters via contacts between the hormone-dependent TRβ1 activation function 2 (AF-2) in the C-terminal ligand binding domain (LBD) and a major PGC-1α nuclear receptor (NR) interaction box (consensus LxxLL) at amino acids 142-146. While our studies verify the existence and importance of this interaction, we present evidence that TRβ1 also binds PGC-1α in a second ligand and LxxLL motif independent mode and show that this interaction requires the TRβ1 N-terminal domain (NTD) and the PGC-1α N-terminal activation domain (AD) at amino acids 1-130. Transfection assays suggest that optimal PGC-1α coactivation requires the TRβ1 NTD and that these contacts are needed for utilization of the PGC-1α C-terminal AD, which does not bind TR and is implicated in basal transcription machinery contacts. We propose that TR AF-1/PGC-1α contacts are needed for transition between activities of PGC-1α N-and C-terminal ADs in gene expression. Our findings provide insights into possible roles for TR and NR AF-1 in gene expression.

The Relationship between Type 2 Diabetes Mellitus and Related Thyroid Diseases

Type 2 diabetes mellitus (T2DM) has an intersecting underlying pathology with thyroid dysfunction. The literature is punctuated with evidence indicating a contribution of abnormalities of thyroid hormones to type 2 DM. The most probable mechanism leading to T2DM in thyroid dysfunction could be attributed to perturbed genetic expression of a constellation of genes along with physiological aberrations leading to impaired glucose utilization and disposal in muscles, overproduction of hepatic glucose output, and enhanced absorption of splanchnic glucose. These factors contribute to insulin resistance. Insulin resistance is also associated with thyroid dysfunction. Hyper- and hypothyroidism have been associated with insulin resistance which has been reported to be the major cause of impaired glucose metabolism in T2DM. The state-of-art evidence suggests a pivotal role of insulin resistance in underlining the relation between T2DM and thyroid dysfunction. A plethora of preclinical, molecular, and clinical studies have evidenced an undeniable role of thyroid malfunctioning as a comorbid disorder of T2DM. It has been investigated that specifically designed thyroid hormone analogues can be looked upon as the potential therapeutic strategies to alleviate diabetes, obesity, and atherosclerosis. These molecules are in final stages of preclinical and clinical evaluation and may pave the way to unveil a distinct class of drugs to treat metabolic disorders.

PPARγ activation attenuates cold-induced upregulation of thyroid status and brown adipose tissue PGC-1α and D2

Here, we investigated whether pharmacological PPARγ activation modulates key early events in brown adipose tissue (BAT) recruitment induced by acute cold exposure with the aim of unraveling the interrelationships between sympathetic and PPARγ signaling. Sprague-Dawley rats treated or not with the PPARγ ligand rosiglitazone (15 mg·kg(-1)·day(-1), 7 days) were kept at 23°C or exposed to cold (5°C) for 24 h and evaluated for BAT gene expression, sympathetic activity, thyroid status, and adrenergic signaling. Rosiglitazone did not affect the reduction in body weight gain and the increase in feed efficiency, Vo(2), and BAT sympathetic activity induced by 24-h cold exposure. Rosiglitazone strongly attenuated the increase in serum total and free T4 and T3 levels and BAT iodothyronine deiodinase type 2 (D2) and PGC-1α mRNA levels and potentiated the reduction in BAT thyroid hormone receptor (THR) β mRNA levels induced by cold. Administration of T3 to rosiglitazone-treated rats exacerbated the cold-induced increase in energy expenditure but did not restore a proper activation of D2 and PGC-1α, nor further increased uncoupling protein 1 expression. Regarding adrenergic signaling, rosiglitazone did not affect the changes in BAT cAMP content and PKA activity induced by cold. Rosiglitazone alone or in combination with cold increased CREB binding to DNA, but it markedly reduced the expression of one of its major coactivators, CREB binding protein. In conclusion, pharmacological PPARγ activation impairs short-term cold elicitation of BAT adrenergic and thyroid signaling, which may result in abnormal tissue recruitment and thermogenic activity.

Skeletal muscle mitochondrial energetics in obesity and type 2 diabetes mellitus: endocrine aspects

During the development of type 2 diabetes mellitus, skeletal muscle is a major site of insulin resistance. The latter has been linked to mitochondrial dysfunction and impaired fatty acid oxidation. Some hormones like insulin, thyroid hormones and adipokines (e.g., leptin, adiponectin) have positive effects on muscle mitochondrial bioenergetics through their direct or indirect effects on mitochondrial biogenesis, mitochondrial protein expression, mitochondrial enzyme activities and/or AMPK pathway activation--all of which can improve fatty acid oxidation. It is therefore not surprising that treatment with these hormones has been proposed to improve muscle and whole body insulin sensitivity. However, treatment of diabetic patients with leptin and adiponectin has no effect on muscle mitochondrial bioenergetics showing resistance to these hormones during type 2 diabetes. Furthermore, treatment with most thyroid hormones has unexpectedly revealed negative effects on muscle insulin sensitivity. Future research should focus on development of agents that improve metabolic dysfunction downstream of hormone receptors.

Transcriptional regulation of temperature-induced remodeling of muscle bioenergetics in goldfish

Central to mammalian mitochondrial biogenesis is the transcriptional master regulator peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), and a network of DNA-binding proteins it coactivates. We explored the role of this pathway in muscle mitochondrial biogenesis in response to thermal acclimation in goldfish (Carassius auratus). We investigated the transcriptional response of PGC-1α, PGC-1β, and their antagonist the nuclear receptor interacting protein 1 (RIP140), as well as the mRNA and protein patterns of DNA-binding proteins that bind PGC-1, including nuclear respiratory factors (NRF) 1 and 2, retinoid X receptor α (RXRα), estrogen-related receptor α (ERRα), thyroid receptor α-1 (TRα-1), PPARα, and PPARβ/δ, and the host cell factor 1 (HCF1), which links PGC-1 and NRF-2. Cold-acclimated (4°C) fish had higher COX activities (4.5-fold) and COX4-1 mRNA levels (3.5-fold per total RNA; 6.5-fold per gram tissue) than warm-acclimated (32°C) fish. The transcription factor patterns were profoundly influenced by changes in RNA per gram tissue (2-fold higher in cold fish) and nuclear protein content (2-fold higher in warm fish). In cold-acclimated fish, mRNA per gram tissue was elevated for PGC-1β, RIP140, NRF-1, HCF1, NRF-2α, NRF-2β-2, ERRα, PPAR β/δ, and RXRα, but other transcriptional regulators either did not change (PGC-1α, PPARα) or even decreased (TRα-1). Nuclear protein levels in cold-acclimated fish were higher only for NRF-1; other proteins were either unaffected (NRF-2α, ERRα) or decreased (NRF-2β1/2, TRα, RXRα). Collectively, these data support the role for NRF-1 in regulating cold-induced mitochondrial biogenesis in goldfish, with effects mediated by PGC-1β, rather than PGC-1α.

Why can insulin resistance be a natural consequence of thyroid dysfunction?

Evidence for a relationship between T4 and T3 and glucose metabolism appeared over 100 years ago when the influence of thyroid hormone excess in the deterioration of glucose metabolism was first noticed. Since then, it has been known that hyperthyroidism is associated with insulin resistance. More recently, hypothyroidism has also been linked to decreased insulin sensitivity. The explanation to this apparent paradox may lie in the differential effects of thyroid hormones at the liver and peripheral tissues level. The purpose of this paper is to explore the effects of thyroid hormones in glucose metabolism and analyze the mechanisms whereby alterations of thyroid hormones lead to insulin resistance.

Coordination of mitochondrial biogenesis by thyroid hormone

Thyroid hormone (TH) has profound influence on metabolism that is closely linked to its effect on mitochondrial biogenesis and function. After a single injection of TH into mammals, physiological alterations (e.g. changes in oxygen consumption rates) are detectable after a lag period of ∼48h. This characteristic lag period is somewhat surprising since non-genomic responses are already detectable within minutes, and first genomic responses within some hours after administration of TH. This review provides a model to explain the characteristic lag period: TH regulates a first series of TH target genes via classical activation of gene expression by binding to thyroid hormone response elements. Some directly regulated target genes serve as intermediate factors and subsequently regulate a second series of indirect TH target genes. Intermediate factors are transcription factors (such as NRF-1, NRF-2 and PPARγ) and transcriptional coactivators (such as PGC-1α and PGC-1β). In concert with several post-translational modifications, these intermediate factors orchestrate the physiological response to thyroid hormone in vivo.

Proteomic characterization of early changes induced by triiodothyronine in rat liver

High doses of T3 are mitogenic in liver, causing hyperplasia that has numerous differences from the compensatory regeneration induced by partial hepatectomy (PH). T3 binds to the thyroid hormone receptor (TR), which directly regulates transcription, while PH acts indirectly through signal transduction pathways. We therefore carried out a proteomic analysis to compare early effects of the two treatments. Transcriptome analysis by DNA microarray also confirmed the observed proteomic changes, demonstrating that they were caused by transcriptional regulation. Among the differentially expressed proteins, many are directly or indirectly involved in energy metabolism and response to oxidative stress. Several enzymes of lipid metabolism (e.g., Acaa2, Acads, Hadh, and Echs1) were differentially regulated by T3. In addition, altered expression levels of several mitochondrial proteins (e.g., Hspa9, Atp5b, Cps1, Glud1, Aldh2, Ak2, Acads) demonstrated the known increase of mitochondrial biogenesis mediated by T3. The present results provide insights in changes in metabolic balance occurring following T3-stimulation and define a basis for dissecting the molecular pathways of hepatocyte hyperplasia.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by CCAAT/enhancer-binding protein beta (C/EBPbeta)

The conversion of pyruvate to acetyl-CoA in mitochondria is catalyzed by the pyruvate dehydrogenase complex (PDC). Activity of PDC is inhibited by phosphorylation via the pyruvate dehydrogenase kinases (PDKs). Here, we examined the regulation of Pdk4 gene expression by the CCAAT/enhancer-binding protein β (C/EBPβ). C/EBPβ modulates the expression of multiple hepatic genes including those involved in metabolism, development, and inflammation. We found that C/EBPβ induced Pdk4 gene expression and decreased PDC activity. This transcriptional induction was mediated through two C/EBPβ binding sites in the Pdk4 promoter. C/EBPβ participates in the hormonal regulation of gluconeogenic genes. Previously, we reported that Pdk4 was induced by thyroid hormone (T(3)). Therefore, we investigated the role of C/EBPβ in the T(3) regulation of Pdk4. T(3) increased C/EBPβ abundance in primary rat hepatocytes. Knockdown of C/EBPβ with siRNA diminished the T(3) induction of the Pdk4 and carnitine palmitoyltransferase (Cpt1a) genes. CPT1a is an initiating step in the mitochondrial oxidation of long chain fatty acids. Our results indicate that C/EBPβ stimulates Pdk4 expression and participates in the T(3) induction of the Cpt1a and Pdk4 genes.

Studies of complex biological systems with applications to molecular medicine: the need to integrate transcriptomic and proteomic approaches

Omics approaches to the study of complex biological systems with potential applications to molecular medicine are attracting great interest in clinical as well as in basic biological research. Genomics, transcriptomics and proteomics are characterized by the lack of an a priori definition of scope, and this gives sufficient leeway for investigators (a) to discern all at once a globally altered pattern of gene/protein expression and (b) to examine the complex interactions that regulate entire biological processes. Two popular platforms in "omics" are DNA microarrays, which measure messenger RNA transcript levels, and proteomic analyses, which identify and quantify proteins. Because of their intrinsic strengths and weaknesses, no single approach can fully unravel the complexities of fundamental biological events. However, an appropriate combination of different tools could lead to integrative analyses that would furnish new insights not accessible through one-dimensional datasets. In this review, we will outline some of the challenges associated with integrative analyses relating to the changes in metabolic pathways that occur in complex pathophysiological conditions (viz. ageing and altered thyroid state) in relevant metabolically active tissues. In addition, we discuss several new applications of proteomic analysis to the investigation of mitochondrial activity.

Hormetic responses of thyroid hormone calorigenesis in the liver: Association with oxidative stress

Thyroid hormone (L-3,3',5-triiodothyronine, T(3)) exerts calorigenic effects by accelerating mitochondrial O(2) consumption through transcriptional activation of respiratory genes, with consequent increased reactive oxygen species (ROS) production. In the liver, ROS generation occurs at different sites of hepatocytes and in the respiratory burst of Kupffer cells, triggering the activation of the transcription factors nuclear factor-kappaB, signal transducer and activator of transcription 3, and activating protein 1. Under these conditions, the redox upregulation of Kupffer cell-dependent expression of cytokines [tumor necrosis factor-alpha, interleukin (IL)-1, and IL-6] is achieved, which upon interaction with specific receptors in hepatocytes trigger the expression of antioxidant enzymes (manganese superoxide dismutase, inducible nitric oxide synthase), antiapoptotic proteins (Bcl-2), and acute-phase proteins (haptoglobin, beta-fibrinogen). These responses and the promotion of hepatocyte and Kupffer cell proliferation observed represent hormetic effects re-establishing redox homeostasis, promoting cell survival, and protecting the liver against ischemia-reperfusion (IR) injury. It is proposed that hormesis underlying T(3) action may constitute a novel preconditioning strategy for IR injury during liver surgery in man or in liver transplantation using reduced-size grafts from living donors, considering that (i) with the exception of the controversial ischemic preconditioning, all other studied strategies have failed to reach the clinical setting and (ii) T(3) is a well-tolerated therapeutic agent that either lacks major adverse effects or has minimal and controlled side effects.

Dissecting the Relation between a nuclear receptor and GATA: binding affinity studies of thyroid hormone receptor and GATA2 on TSHβ promoter

Background

Much is known about how genes regulated by nuclear receptors (NRs) are switched on in the presence of a ligand. However, the molecular mechanism for gene down-regulation by liganded NRs remains a conundrum. The interaction between two zinc-finger transcription factors, Nuclear Receptor and GATA, was described almost a decade ago as a strategy adopted by the cell to up- or down-regulate gene expression. More recently, cell-based assays have shown that the Zn-finger region of GATA2 (GATA2-Zf) has an important role in down-regulation of the thyrotropin gene (TSHβ) by liganded thyroid hormone receptor (TR).

Methodology/Principal Findings

In an effort to better understand the mechanism that drives TSHβ down-regulation by a liganded TR and GATA2, we have carried out equilibrium binding assays using fluorescence anisotropy to study the interaction of recombinant TR and GATA2-Zf with regulatory elements present in the TSHβ promoter. Surprisingly, we observed that ligand (T3) weakens TR binding to a negative regulatory element (NRE) present in the TSHβ promoter. We also show that TR may interact with GATA2-Zf in the absence of ligand, but T3 is crucial for increasing the affinity of this complex for different GATA response elements (GATA-REs). Importantly, these results indicate that TR complex formation enhances DNA binding of the TR-GATA2 in a ligand-dependent manner.

Conclusions

Our findings extend previous results obtained in vivo, further improving our understanding of how liganded nuclear receptors down-regulate gene transcription, with the cooperative binding of transcription factors to DNA forming the core of this process.

Absence of thyroid hormone activation during development underlies a permanent defect in adaptive thermogenesis

Type 2 deiodinase (D2), which is highly expressed in brown adipose tissue (BAT), is an enzyme that amplifies thyroid hormone signaling in individual cells. Mice with inactivation of the D2 pathway (D2KO) exhibit dramatically impaired thermogenesis in BAT, leading to hypothermia during cold exposure and a greater susceptibility to diet-induced obesity. This was interpreted as a result of defective acute activation of BAT D2. Here we report that the adult D2KO BAT has a permanent thermogenic defect that stems from impaired embryonic BAT development. D2KO embryos have normal serum T3 but due to lack of D2-generated T3 in BAT, this tissue exhibits decreased expression of genes defining BAT identity [i.e. UCP1, PGC-1alpha and Dio2 (nonfunctional)], which results in impaired differentiation and oxidative capacity. Coinciding with a reduction of these T3-responsive genes, there is oxidative stress that in a cell model of brown adipogenesis can be linked to decreased insulin signaling and decreased adipogenesis. This discovery highlights the importance of deiodinase-controlled thyroid hormone signaling in BAT development, where it has important metabolic repercussions for energy homeostasis in adulthood.

Evidence of a bigenomic regulation of mitochondrial gene expression by thyroid hormone during rat brain development

Hypothyroidism during early mammalian brain development is associated with decreased expression of various mitochondrial encoded genes along with evidence for mitochondrial dysfunction. However, in-spite of the similarities between neurological disorders caused by perinatal hypothyroidism and those caused by various genetic mitochondrial defects we still do not know as to how thyroid hormone (TH) regulates mitochondrial transcription during development and whether this regulation by TH is nuclear mediated or through mitochondrial TH receptors? We here in rat cerebellum show that hypothyroidism causes reduction in expression of nuclear encoded genes controlling mitochondrial biogenesis like PGC-1alpha, NRF-1alpha and Tfam. Also, we for the first time demonstrate a mitochondrial localization of thyroid hormone receptor (mTR) isoform in developing brain capable of binding a TH response element (DR2) present in D-loop region of mitochondrial DNA. These results thus indicate an integrated nuclear-mitochondrial cross talk in regulation of mitochondrial transcription by TH during brain development.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by thyroid hormone: role of the peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha)

PDK4 (pyruvate dehydrogenase kinase 4) regulates pyruvate oxidation through the phosphorylation and inhibition of the pyruvate dehydrogenase complex (PDC). PDC catalyzes the conversion of pyruvate to acetyl-CoA and is an important control point in glucose and pyruvate metabolism. PDK4 gene expression is stimulated by thyroid hormone (T(3)), glucocorticoids, and long chain fatty acids. The effects of T(3) on gene expression in the liver are mediated via the thyroid hormone receptor. Here, we have identified two binding sites for thyroid hormone receptor beta in the promoter of the rat PDK4 (rPDK4) gene. In addition, we have investigated the role of transcriptional coactivators and found that the PGC-1 alpha (peroxisome proliferator-activated receptor gamma coactivator) enhances the T(3) induction of rPDK4. Following T(3) administration, there is an increase in the association of PGC-1 alpha with the rPDK4 promoter. Interestingly, this increased association is with the proximal rPDK4 promoter rather than the distal region of the gene that contains the T(3) response elements. Administration of T(3) to hypothyroid rats elevated the abundance of PGC-1 alpha mRNA and protein in the liver. In addition, we observed greater association of PGC-1 alpha not only with the rPDK4 gene but also with phosphoenolpyruvate carboxykinase and CPT-1a (carnitine palmitoyltransferase 1a) genes. Knockdown of PGC-1 alpha in rat hepatocytes reduced the T(3) induction of PDK4, PEPCK, and CPT-1a genes. Our results indicate that T(3) regulates PGC-1 alpha abundance and association with hepatic genes, and in turn PGC-1 alpha is an important participant in the T(3) induction of selected genes.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

Isoform-specific transcriptional activity of overlapping target genes that respond to thyroid hormone receptors alpha1 and beta1

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of physiology and development. TRs are expressed in vertebrates as a series of distinct isoforms that exert distinct biological roles. We wished to determine whether the two most widely expressed isoforms, TR alpha 1 and TR beta 1, exert their different biological effects by regulating different sets of target genes. Using stably transformed HepG2 cells and a microarray analysis, we were able to demonstrate that TR alpha 1 and TR beta 1 regulate a largely overlapping repertoire of target genes in response to T(3) hormone. However, these two isoforms display very different transcriptional properties on each individual target gene, ranging from a much greater T(3)-mediated regulation by TR alpha 1 than by TR beta 1, to near equal regulation by both isoforms. We also identified TR alpha 1 and TR beta 1 target genes that were regulated by these receptors in a hormone-independent fashion. We suggest that it is this gene-specific, isoform-specific amplitude of transcriptional regulation that is the likely basis for the appearance and maintenance of TR alpha 1 and TR beta 1 over evolutionary time. In essence, TR alpha 1 and TR beta 1 adjust the magnitude of the transcriptional response at different target genes to different levels; by altering the ratio of these isoforms in different tissues or at different developmental times, the intensity of T(3) response can be individually tailored to different physiological and developmental requirements.

Involvement of PGC-1, NRF-1, and NRF-2 in metabolic response by rat liver to hormonal and environmental signals

We studied liver oxidative capacity and O2 consumption in hypothyroid rats treated for 10 days with T4, or T3, or treated for 10 days with T3 and exposed to cold for the last 2 days. The metabolic response of homogenates and mitochondria indicated that all treatments increased the synthesis of respiratory chain components, whereas only the cold-induced mitochondrial proliferation. Determination of mRNA and protein expression of transcription factor activators, such as NRF-1 and NRF-2, and coactivators, such as PGC-1, showed that mRNA levels, except PGC-1 ones, were not related to aerobic capacities. Conversely, a strong correlation was found between cytochrome oxidase activity and PGC-1 or NRF-2 protein levels. Such a correlation was not found for NRF-1. Our results strongly support the view that in rat liver PGC-1 and NRFs are responsible for the iodothyronine-induced increases in respiratory chain components, whereas their role in cold-induced mitochondrial proliferation needs to be further on clarified.

Estrogenic control of mitochondrial function and biogenesis

Estrogens have cell-specific effects on a variety of physiological endpoints including regulation of mitochondrial biogenesis and activity. Estrogens regulate gene transcription by the classical genomic mechanism of binding to estrogen receptors alpha and beta (ERalpha and ERbeta) as well as the more recently described nongenomic pathways involving plasma membrane-associated ERs that activate intracellular protein kinase-mediated phosphorylation signaling cascades. Here I will review the rapid and longer-term effects of estrogen on mitochondrial function. The identification of ERalpha and ERbeta within mitochondria of various cells and tissues is discussed with a model of estrogen regulation of the transcription of nuclear respiratory factor-1 (NRF-1, NRF1). NRF-1 subsequently promotes transcription of mitochondrial transcription factor Tfam (mtDNA maintenance factor, also called mtTFA) and then Tfam targets mtDNA-encoded genes. The nuclear effects of estrogens on gene expression directly controlling mitochondrial biogenesis, oxygen consumption, mtDNA transcription, and apoptosis are reviewed. Overall, we are just beginning to evaluate the many direct and indirect effects of estrogens on mitochondrial activities.

Microarray analysis of mitogenic effects of T3 on the rat liver

BACKGROUND AND AIM

A single dose of the thyroid hormone tri-iodothyronine, T3, can enhance both size and function of normal rodent liver, which is potentially of value in the treatment of liver disease. However the mechanism of this has not been fully elucidated, and it cannot be modeled in vitro. We therefore investigated the transcriptome response to T3 in rat liver in vivo.

METHODS

After adult rats were administered 5 microg T3 subcutaneously, a whole rat genome microarray comparing global hepatic gene expression against vehicle-only treated liver after 3 h was performed.

RESULTS

Informative transcripts which had identifiable gene ontology biological processes were grouped according to function, broadly reflecting general metabolic effects and those linked to cell-proliferation control. We then compared the transcriptome response after 5-microg T3 initiating hepatocyte DNA synthesis (mitogenic) with that after 0.1 microg T3, a supraphysiological amount not initiating hepatocyte DNA synthesis.

CONCLUSIONS

We compared the results with published results of the response to other primary mitogens, and identified the Gadd45beta/MyD118 gene as a common early factor upregulated during proliferation.

Effect of thyroid hormone on mitochondrial properties and oxidative stress in cells from patients with mtDNA defects

Mitochondrial (mt)DNA mutations contribute to various disease states characterized by low ATP production. In contrast, thyroid hormone [3,3',5-triiodothyronine (T(3))] induces mitochondrial biogenesis and enhances ATP generation within cells. To evaluate the role of T(3)-mediated mitochondrial biogenesis in patients with mtDNA mutations, three fibroblast cell lines with mtDNA mutations were evaluated, including two patients with Leigh's syndrome and one with hypertrophic cardiomyopathy. Compared with control cells, patient fibroblasts displayed similar levels of mitochondrial mass, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), mitochondrial transcription factor A (Tfam), and uncoupling protein 2 (UCP2) protein expression. However, patient cells exhibited a 1.6-fold elevation in ROS production, a 1.7-fold elevation in cytoplasmic Ca2+ levels, a 1.2-fold elevation in mitochondrial membrane potential, and 30% less complex V activity compared with control cells. Patient cells also displayed 20-25% reductions in both cytochrome c oxidase (COX) activity and MnSOD protein levels compared with control cells. After T(3) treatment of patient cells, ROS production was decreased by 40%, cytoplasmic Ca2+ was reduced by 20%, COX activity was increased by 1.3-fold, and ATP levels were elevated by 1.6-fold, despite the absence of a change in mitochondrial mass. There were no significant alterations in the protein expression of PGC-1alpha, Tfam, or UCP2 in either T(3)-treated patient or control cells. However, T(3) restored the mitochondrial membrane potential, complex V activity, and levels of MnSOD to normal values in patient cells and elevated MnSOD levels by 21% in control cells. These results suggest that T(3) acts to reduce cellular oxidative stress, which may help attenuate ROS-mediated damage, along with improving mitochondrial function and energy status in cells with mtDNA defects.

Role of nuclear receptors in the modulation of insulin secretion in lipid-induced insulin resistance

In healthy individuals, a hyperbolic relationship exists between whole-body insulin-sensitivity and insulin secretion. Thus, for any difference in insulin-sensitivity, a reciprocal proportionate change occurs in insulin secretion. Such a feedback loop is evident in healthy individuals ingesting diets high in saturated fat and in late pregnancy where, despite lipid-induced insulin resistance, glucose tolerance is maintained through augmented GSIS (glucose-stimulated insulin secretion). NRs (nuclear receptors) are members of a superfamily of ligand-regulated and orphan transcription factors. On activation by a cognate ligand, many ligand-activated NRs recruit the RXR (retinoid X receptor) for heterodimer formation. Such NRs include the PPARs (peroxisome-proliferator-activated receptors), which are involved in lipid sensing and liporegulation. PPARs exert important lipid-lowering effects in vivo, thereby opposing the development of lipid-induced insulin resistance by relieving the inhibition of insulin-stimulated glucose disposal by muscle and lowering the necessity for augmented GSIS to counter lipid-induced insulin resistance. Long-chain fatty acids are proposed as natural PPAR ligands and some specific endogenous pathways of lipid metabolism are believed to generate PPAR agonists. Other NRs, e.g. the LXR (liver X receptor), which senses expansion of the metabolically active pool of cholesterol, and the FXR (farnesoid X receptor; NR1H4), which, like the LXR, is involved in sterol metabolism, also modulate systemic lipid levels and insulin-sensitivity. In this review, we discuss how these NRs impact insulin secretion via effects on the insulin-sensitivity-insulin secretion feedback loop and, in some cases, via direct effects on the islet itself. In addition, we discuss interactions between these nutrient/metabolite-responsive NRs and NRs that are central to the action of metabolically important hormones, including (i) the glucocorticoid receptor, critical for maintaining glucose homoeostasis in stress, inflammation and during fasting, and (ii) the thyroid hormone receptors, vital for maintenance of oxidative functions. We present data indicating that the RXR occupies a key role in directly modulating islet function and that its heterodimerization with at least two of its partners modulates GSIS.

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

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

T3-mediated expression of PGC-1alpha via a far upstream located thyroid hormone response element

Thyroid hormone (T3) has a profound influence on normal development, differentiation and metabolism. T3 induces complex gene expression patterns raises the question of how these expression patterns might be regulated. Since the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) induces very similar cellular energy metabolic pathways, we investigated the molecular mechanism of T3 regulation of PGC-1alpha. PGC-1alpha is rapidly regulated by T3, both in vivo and in cell culture. Transient transfection experiments demonstrated binding of the thyroid hormone receptor (TR) to a response element located at -4kb upstream of the transcriptional start site within the PGC-1alpha gene. Introducing of a single copy of the -4kb TRE in a heterologous promoter context is sufficient to maintain T3 responsiveness. Chromatin immunoprecipitation analysis revealed increased histone acetylation upon stimulation of T3. Finally, TR binds the -4kb TRE in electrophoretic mobility shift assays, identifying PGC-1alpha as a direct target of TR action. Since T3 directly regulates PGC-1alpha and PGC-1alpha coactivates liganded TR, we suggest an autoregulatory feed-forward loop of PGC-1alpha activation upon T3 treatment.

Transcriptional control of mitochondrial biogenesis: the central role of PGC-1alpha

Although the concept of energy starvation in the failing heart was proposed decades ago, still very little is known about the origin of energetic failure. Recent advances in molecular biology have started to elucidate the transcriptional events governing mitochondrial biogenesis. In particular, a great step was taken with the discovery that peroxisome proliferator-activated receptor gamma co-activator (PGC-1alpha) is the master regulator of mitochondrial biogenesis. The molecular mechanisms underlying the downregulation of PGC-1alpha and the consequent decrease in mitochondrial function in heart failure are, however, still poorly understood. Indeed, the main pathways involved in mitochondrial biogenesis are thought to be up- rather than down-regulated in pathological hypertrophy and heart failure. The current review summarizes recent advances in this field and is restricted to the heart when cardiac data are available.

Links between thyroid hormone action, oxidative metabolism, and diabetes risk?

A key phenotype associated with type 2 diabetes in humans is impaired mitochondrial oxidative metabolism in skeletal muscle, a pattern potentially contributing to increased lipid accumulation and impaired metabolic flexibility-in turn, central features of both insulin resistance and diabetes. Since thyroid hormone regulates mitochondrial gene expression and function in skeletal muscle, reductions in T3-mediated transcription may contribute to diabetes-related impairments in oxidative metabolism. We review the evidence for relationships between thyroid hormone action and diabetes risk, and discuss potential mechanisms linking intracellular thyroid hormone availability, thyroid receptor action, and the transcriptional coactivator PGC1 in regulating oxidative metabolism.

Thyroid hormone is required for the phenotype transitions induced by the pharmacological inhibition of calcineurin in adult soleus muscle of rats

The present experiment was designed to examine the effects of hypothyroidism and calcineurin inhibition induced by cyclosporin A (CsA) administration on both contractile and metabolic soleus muscle phenotypes, with a novel approach to the signaling pathway controlling mitochondrial biogenesis. Twenty-eight rats were randomly assigned to four groups, normothyroid, hypothyroid, and orally treated with either CsA (25 mg/kg, N-CsA and H-CsA) or vehicle (N-Vh and H-Vh), for 3 wk. Muscle phenotype was estimated by the MHC profile and activities of oxidative and glycolytic enzymes. We measured mRNA levels of the peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), the major regulator of mitochondrial content. We also studied the expression of the catalytic A-subunit of calcineurin (CnA) both at protein and transcript levels and mRNA levels of modulatory calcineurin inhibitor proteins (MCIP)-1 and -2, which are differentially regulated by calcineurin activity and thyroid hormone, respectively. CsA-administration induced a slow-to-fast MHC transition limited to the type IIA isoform, which is associated with increased oxidative capacities. Hypothyroidism strongly decreased both the expression of fast MHC isoforms and oxidative capacities. Effects of CsA administration on muscle phenotype were blocked in conditions of thyroid hormone deficiency. Changes in the oxidative profile were strongly related to PGC-1 alpha changes and associated with phosphorylation of p38 MAPK. Calcineurin and MCIPs mRNA levels were decreased by both hypothyroidism and CsA without additive effects. Taken together, these results suggest that adult muscle phenotype is primarily under the control of thyroid state. Physiological levels of thyroid hormone are required for the effects of calcineurin inhibition on slow oxidative muscle phenotype.