Rapid activation by 3,5,3'-L-triiodothyronine of adenosine 5'-monophosphate-activated protein kinase/acetyl-coenzyme a carboxylase and akt/protein kinase B signaling pathways: relation to changes in fuel metabolism and myosin heavy-chain protein content in rat gastrocnemius muscle in vivo

The key roles of thyroid hormone in mitochondrial regulation, at interface of human health and disease

Mitochondria are highly plastic and dynamic organelles long known as the powerhouse of cellular bioenergetics, but also endowed with a critical role in stress responses and homeostasis maintenance, supporting and integrating activities across multifaced cellular processes. As a such, mitochondria dysfunctions are leading causes of a wide range of diseases and pathologies. Thyroid hormones (THs) are endocrine regulators of cellular metabolism, regulating intracellular nutrients fueling of sugars, amino acids and fatty acids. For instance, THs regulate the balance between the anabolism and catabolism of all the macro-molecules, influencing energy homeostasis during different nutritional conditions. Noteworthy, not only most of the TH-dependent metabolic modulations act via the mitochondria, but also THs have been proved to regulate the mitochondrial biosynthesis, dynamics and function. The significance of such an interplay is different in the context of specific tissues and strongly impacts on cellular homeostasis. Thus, a comprehensive understanding of THs-dependent mitochondrial functions and dynamics is required to develop more precise strategies for targeting mitochondrial function. Herein, we describe the mechanisms of TH-dependent metabolic regulation with a focus on mitochondrial action, in different tissue contexts, thus providing new insights for targeted modulation of mitochondrial dynamics.

Mild Endurance Exercise during Fasting Increases Gastrocnemius Muscle and Prefrontal Cortex Thyroid Hormone Levels through Differential BHB and BCAA-Mediated BDNF-mTOR Signaling in Rats

Mild endurance exercise has been shown to compensate for declined muscle quality and may positively affect the brain under conditions of energy restriction. Whether this involves brain-derived neurotrophic factor (BDNF) and mammalian target of rapamycin (mTOR) activation in relation to central and peripheral tissue levels of associated factors such as beta hydroxy butyrate (BHB), branched-chain amino acids (BCAA) and thyroid hormone (T3) has not been studied. Thus, a subset of male Wistar rats housed at thermoneutrality that were fed or fasted was submitted to 30-min-mild treadmill exercise bouts (five in total, twice daily, 15 m/min, 0° inclination) over a period of 66 h. Prefrontal cortex and gastrocnemius muscle BHB, BCAA, and thyroid hormone were measured by LC-MS/MS analysis and were related to BDNF and mammalian target of rapamycin (mTOR) signaling. In gastrocnemius muscle, mild endurance exercise during fasting maintained the fasting-induced elevated BHB levels and BDNF-CREB activity and unlocked the downstream Akt-mTORC1 pathway associated with increased tissue BCAA. Consequently, deiodinase 3 mRNA levels decreased whereas increased phosphorylation of the mTORC2 target FOXO1 was associated with increased deiodinase 2 mRNA levels, accounting for the increased T3 tissue levels. These events were related to increased expression of CREB and T3 target genes beneficial for muscle quality previously observed in this condition. In rat L6 myoblasts, BHB directly induced BDNF transcription and maturation. Mild endurance exercise during fasting did not increase prefrontal cortex BHB levels nor was BDNF activated, whereas increased leucine levels were associated with Akt-independent increased phosphorylation of the mTORC1 target P70S6K. The associated increased T3 levels modulated the expression of known T3-target genes involved in brain tissue maintenance. Our observation that mild endurance exercise modulates BDNF, mTOR and T3 during fasting provides molecular clues to explain the observed beneficial effects of mild endurance exercise in settings of energy restriction.

Thyroid Hormone Receptor α Regulates Autophagy, Mitochondrial Biogenesis, and Fatty Acid Use in Skeletal Muscle

Skeletal muscle (SM) weakness occurs in hypothyroidism and resistance to thyroid hormone α (RTHα) syndrome. However, the cell signaling and molecular mechanism(s) underlying muscle weakness under these conditions is not well understood. We thus examined the role of thyroid hormone receptor α (TRα), the predominant TR isoform in SM, on autophagy, mitochondrial biogenesis, and metabolism to demonstrate the molecular mechanism(s) underlying muscle weakness in these two conditions. Two genetic mouse models were used in this study: TRα1PV/+ mice, which express the mutant Thra1PV gene ubiquitously, and SM-TRα1L400R/+ mice, which express TRα1L400R in a muscle-specific manner. Gastrocnemius muscle from TRα1PV/+, SM-TRα1L400R/+, and their control mice was harvested for analyses. We demonstrated that loss of TRα1 signaling in gastrocnemius muscle from both the genetic mouse models led to decreased autophagy as evidenced by accumulation of p62 and decreased expression of lysosomal markers (lysosomal-associated membrane protein [LAMP]-1 and LAMP-2) and lysosomal proteases (cathepsin B and cathepsin D). The expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), mitochondrial transcription factor A (TFAM), and estrogen-related receptor α (ERRα), key factors contributing to mitochondrial biogenesis as well as mitochondrial proteins, were decreased, suggesting that there was reduced mitochondrial biogenesis due to the expression of mutant TRα1. Transcriptomic and metabolomic analyses of SM suggested that lipid catabolism was impaired and was associated with decreased acylcarnitines and tricarboxylic acid cycle intermediates in the SM from the mouse line expressing SM-specific mutant TRα1. Our results provide new insight into TRα1-mediated cell signaling, molecular, and metabolic changes that occur in SM when TR action is impaired.

A breakthrough-like effect of metformin reduces peripheral resistance to triiodothyronine in euthyroid, non-insulin-resistant, type 2 diabetic patients

BACKGROUND

Type 2 diabetes is characterized, beyond the insulin resistance, by polyhormonal resistance. Thyroid hormonal resistance has not yet been described in this population of patients. Metformin is used to decrease insulin resistance, and at present, it is assumed to influence the effect of triiodothyronine, as well.

METHODS

In this open-label, pilot, hypothesis-generating, follow-up study, 21 patients were included; all of them were euthyroid with drug naïve, newly diagnosed type 2 diabetes. Before and after 4 weeks of metformin therapy, fructosamine, homeostasis model assessment for insulin resistance (HOMA-IR), thyroid hormones, T3/T4 ratio, and TSH, as well as blood pressure and heart rate using ambulatory blood pressure monitor were measured. We also conducted an in vitro study to investigate the possible mechanisms of T3 resistance, assessing T3-induced Akt phosphorylation among normal (5 mM) and high (25 mM) glucose levels with or without metformin treatment in a human embryonal kidney cell line.

RESULTS

Metformin decreased the level of T3 (P < 0.001), the ratio of T3/T4 (P = 0.038), fructosamine (P = 0.008) and HOMA-IR (P = 0.022). All these changes were accompanied by an unchanged TSH, T4, triglyceride, plasma glucose, bodyweight, blood pressure, and heart rate. In our in vitro study, T3-induced Akt phosphorylation decreased in cells grown in 25 mM glucose medium compared to those in 5 mM. Metformin could not reverse this effect.

CONCLUSION

Metformin seems to improve T3 sensitivity in the cardiovascular system in euthyroid, type 2 diabetic patients, the mechanism of which may be supracellular.

Exercise with food withdrawal at thermoneutrality impacts fuel use, the microbiome, AMPK phosphorylation, muscle fibers, and thyroid hormone levels in rats

Exercise under fasting conditions induces a switch to lipid metabolism, eliciting beneficial metabolic effects. Knowledge of signaling responses underlying metabolic adjustments in such conditions may help to identify therapeutic strategies. Therefore, we studied the effect of mild exercise on rats submitted to food withdrawal at thermoneutrality (28°C) for 3 days. Animals were housed at thermoneutrality rather than the standard housing temperature (22°C) to avoid beta-adrenergic signaling responses that themselves affect metabolism and well-being. Quantitative analysis of multi-organ mRNA levels, myofibers, and serum metabolites shows that this protocol (a) boosts fat oxidation in muscle and liver, (b) reduces lipogenesis and increases gluconeogenesis in liver, (c) increases serum acylcarnitines (especially C4 OH) and ketone bodies and the use of the latter as fuel in muscle, (d) increases Type I myofibers, and (e) is associated with an increased thyroid hormone uptake and metabolism in muscle. In addition, stool microbiome DNA analysis revealed that food withdrawal dramatically alters the presence of bacterial genera associated with ketone metabolism. Taken together, this protocol induces a drastic switch toward increased lipid and ketone metabolism compared to exercise or food withdrawal alone, which may prove beneficial and may involve local thyroid hormones, which may be regarded as exercise mimetics.

Thyroid Hormone Status Regulates Skeletal Muscle Response to Chronic Motor Nerve Stimulation

Although both exercise and thyroid hormone (TH) status can cause cellular and metabolic changes in skeletal muscle, the impact of TH status on exercise-associated changes is not well understood. Here, we examined the effects of TH status on muscle fiber type, cell signaling, and metabolism in a rabbit model of exercise training – chronic motor nerve stimulation (CMNS). Five rabbits were rendered hypothyroid for 7–8 weeks and three rabbits were made hyperthyroid for 2 weeks prior to CMNS of the left peroneal nerve for 10 days. We then measured markers of muscle fiber type, autophagy, and nutrient- or energy-sensing proteins, and metabolic intermediates. CMNS increased MHC-I expression in hypothyroid rabbits, whereas it was unchanged in hyperthyroid rabbits. CMNS also increased p-AMPK, p-ATGL, CPT-1α, p-Akt, GLUT4, and p-70S6K in hypothyroid rabbits. In contrast, p-AMPK and p-AKT were increased at baseline in hyperthyroid rabbits, but CMNS did not further increase them or any of the other markers. CMNS also increased TCA cycle and acylcarnitine metabolites in hypothyroid rabbits; whereas, acylcarnitines were already elevated in hyperthyroid rabbits, and were only slightly increased further by CMNS. In summary, CMNS effects on cell signaling and metabolism of skeletal muscle were more pronounced in the hypothyroid than the hyperthyroid state. Interestingly, in the hypothyroid state, CMNS caused concomitant activation of two signaling pathways that are usually reciprocally regulated – AMPK and mTOR signaling – which manifested as increased β-oxidation, MHC-I expression, and protein synthesis. Thus, our findings provide insight into the role of TH status on exercise response in muscle. Our observations suggest that TH status of patients may be an important determinant and predictor of their response to exercise training in skeletal muscle.

Both 3,3',5-triiodothyronine and 3,5-diodo-L-thyronine Are Able to Repair Mitochondrial DNA Damage but by Different Mechanisms

This study evaluated the effect of 3,5-diiodo-L-thyronine (T2) and 3,5,3'-triiodo-L-thyronine (T3) on rat liver mitochondrial DNA (mtDNA) oxidative damage and repair and to investigate their ability to induce protective effects against oxidative stress. Control rats, rats receiving a daily injection of T2 (N+T2) for 1 week and rats receiving a daily injection of T3 (N+T3) for 1 week, were used throughout the study. In the liver, mtDNA oxidative damage [by measuring mtDNA lesion frequency and expression of DNA polymerase γ (POLG)], mtDNA copy number, mitochondrial biogenesis [by measuring amplification of mtDNA/nDNA and expression of peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC-1α)], and oxidative stress [by measuring serum levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG)] were detected. T2 reduces mtDNA lesion frequency and increases the expression of POLG, and it does not change the mtDNA copy number, the expression of PGC-1α, or the serum levels of 8-OHdG. Therefore, T2, by stimulating the major mtDNA repair enzyme, maintains genomic integrity. Similar to T2, T3 decreases mtDNA lesion frequency but increases the serum levels of 8-OHdG, and it decreases the expression of POLG. Moreover, as expected, T3 increases the mtDNA copy number and the expression of PGC-1α. Thus, in T3-treated rats, the increase of 8-OHdG and the decrease of POLG indicate that there is increased oxidative damage and that the decreased mtDNA lesion frequency might be a consequence of increased mitochondrial biogenesis. These data demonstrate that both T2 and T3 are able to decrease in the liver mtDNA oxidative damage, but they act via different mechanisms.

Similarities and Differences in the Peripheral Actions of Thyroid Hormones and Their Metabolites

Thyroxine (T4) and 3,5,3'-triiodothyronine (T3) are secreted by the thyroid gland, while T3 is also generated from the peripheral metabolism of T4 by iodothyronine deiodinases types I and II. Several conditions like stress, diseases, and physical exercise can promote changes in local TH metabolism, leading to different target tissue effects that depend on the presence of tissue-specific enzymatic activities. The newly discovered physiological and pharmacological actions of T4 and T3 metabolites, such as 3,5-diiodothyronine (3,5-T2), and 3-iodothyronamine (T1AM) are of great interest. A classical thyroid hormone effect is the ability of T3 to increase oxygen consumption in almost all cell types studied. Approximately 30 years ago, a seminal report has shown that 3,5-T2 increased oxygen consumption more rapidly than T3 in hepatocytes. Other studies demonstrated that exogenous 3,5-T2 administration was able to increase whole body energy expenditure in rodents and humans. In fact, 3,5-T2 treatment prevents diabetic nephropathy, hepatic steatosis induced by high fat diet, insulin resistance, and weight gain during aging in Wistar male rats. The regulation of mitochondria is likely one of the most important actions of T3 and its metabolite 3,5-T2, which was able to restore the thermogenic program of brown adipose tissue (BAT) in hypothyroid rats, just as T3 does, while T1AM administration induced rapid hypothermia. T3 increases heart rate and cardiac contractility, which are hallmark effects of hyperthyroidism involved in cardiac arrhythmia. These deleterious cardiac effects were not observed with the use of 3,5-T2 pharmacological doses, and in contrast T1AM was shown to promote a negative inotropic and chronotropic action at micromolar concentrations in isolated hearts. Furthermore, T1AM has a cardioprotective effect in a model of ischemic/reperfusion injury in isolated hearts, such as occurs with T3 administration. Despite the encouraging possible therapeutic use of TH metabolites, further studies are needed to better understand their peripheral effects, when compared to T3 itself, in order to establish their risk and benefit. On this basis, the main peripheral effects of thyroid hormones and their metabolites in tissues, such as heart, liver, skeletal muscle, and BAT are discussed herein.

Both 3,5-Diiodo-L-Thyronine and 3,5,3'-Triiodo-L-Thyronine Prevent Short-term Hepatic Lipid Accumulation via Distinct Mechanisms in Rats Being Fed a High-Fat Diet

3,3′,5-triiodo-L-thyronine (T3) improves hepatic lipid accumulation by increasing lipid catabolism but it also increases lipogenesis, which at first glance appears contradictory. Recent studies have shown that 3,5-diiodothyronine (T2), a natural thyroid hormone derivative, also has the capacity to stimulate hepatic lipid catabolism, however, little is known about its possible effects on lipogenic gene expression. Because genes classically involved in hepatic lipogenesis such as SPOT14, acetyl-CoA-carboxylase (ACC), and fatty acid synthase (FAS) contain thyroid hormone response elements (TREs), we studied their transcriptional regulation, focusing on TRE-mediated effects of T3 compared to T2 in rats receiving high-fat diet (HFD) for 1 week. HFD rats showed a marked lipid accumulation in the liver, which was significantly reduced upon simultaneous administration of either T3 or T2 with the diet. When administered to HFD rats, T2, in contrast with T3, markedly downregulated the expression of the above-mentioned genes. T2 downregulated expression of the transcription factors carbohydrate-response element-binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c) involved in activation of transcription of these genes, which explains the suppressed expression of their target genes involved in lipogenesis. T3, however, did not repress expression of the TRE-containing ChREBP gene but repressed SREBP-1c expression. Despite suppression of SREBP-1c expression by T3 (which can be explained by the presence of nTRE in its promoter), the target genes were not suppressed, but normalized to HFD reference levels or even upregulated (ACC), partly due to the presence of TREs on the promoters of these genes and partly to the lack of suppression of ChREBP. Thus, T2 and T3 probably act by different molecular mechanisms to achieve inhibition of hepatic lipid accumulation.

Thyroid states regulate subcellular glucose phosphorylation activity in male mice

The thyroid hormones (THs), triiodothyronine (T₃) and thyroxine (T₄), are very important in organism metabolism and regulate glucose utilization. Hexokinase (HK) is responsible for the first step of glycolysis, catalyzing the conversion of glucose to glucose 6-phosphate. HK has been found in different cellular compartments, and new functions have been attributed to this enzyme. The effects of hyperthyroidism on subcellular glucose phosphorylation in mouse tissues were examined. Tissues were removed, subcellular fractions were isolated from eu- and hyperthyroid (T₃, 0.25 µg/g, i.p. during 21 days) mice and HK activity was assayed. Glucose phosphorylation was increased in the particulate fraction in soleus (312.4% ± 67.1, n = 10), gastrocnemius (369.2% ± 112.4, n = 10) and heart (142.2% ± 13.6, n = 10) muscle in the hyperthyroid group compared to the control group. Hexokinase activity was not affected in brain or liver. No relevant changes were observed in HK activity in the soluble fraction for all tissues investigated. Acute T₃ administration (single dose of T₃, 1.25 µg/g, i.p.) did not modulate HK activity. Interestingly, HK mRNA levels remained unchanged and HK bound to mitochondria was increased by T₃ treatment, suggesting a posttranscriptional mechanism. Analysis of the AKT pathway showed a 2.5-fold increase in AKT and GSK3B phosphorylation in the gastrocnemius muscle in the hyperthyroid group compared to the euthyroid group. Taken together, we show for the first time that THs modulate HK activity specifically in particulate fractions and that this action seems to be under the control of the AKT and GSK3B pathways.

The Kv1.3 channel blocker Vm24 enhances muscle glucose transporter 4 mobilization but does not reduce body-weight gain in diet-induced obese male rats

AIMS

Voltage-gated potassium channels 1.3 (Kv1.3) can be targeted to reduce diet-induced obesity and insulin resistance in mice. Since species-specific differences in Kv1.3 expression and pharmacology have been observed, we tested the effect of Vm24, a high-affinity specific blocker of Kv1.3 channels from Vaejovis mexicanus smithi, on body weight (BW), glucose tolerance and insulin resistance in diet-induced obese rats.

MATERIALS AND METHODS

Young adult male Wistar rats were switched to a high-fat/high-fructose (HFF) diet. Eighteen days later animals were divided in two groups: vehicle and Vm24 group. Subcutaneous injections were applied every other day until sacrifice 2months later. An additional cohort was maintained on standard chow.

KEY FINDINGS

The HFF diet promoted obesity. Treatment with Vm24 did not alter various metabolic parameters such as food intake, BW gain, visceral white adipose tissue mass, adipocyte diameter, serum glucose, leptin and thyroid hormone concentrations, brown adipose tissue mass or uncoupling protein-1 expression, and insulin tolerance. Vm24 did reduce basal and glucose-stimulated serum insulin concentrations, serum C-peptide concentration, increased QUICKI, and tended to lower HOMA-IR. Vm24 treatment did not change the activation of insulin receptor substrate-1, but enhanced protein-kinase B activation and membrane glucose-transporter 4 (GLUT4) protein levels in skeletal muscle.

SIGNIFICANCE

In conclusion, in male rats, long-term blockade of Kv1.3 channels with Vm24 does not reduce weight gain and visceral adiposity induced by HFF diet; instead, it reduces serum insulin concentration, and enhances GLUT4 mobilization in skeletal muscle.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

Stimulation of astrocyte fatty acid oxidation by thyroid hormone is protective against ischemic stroke-induced damage

We previously demonstrated that stimulation of astrocyte mitochondrial ATP production via P2Y₁ receptor agonists was neuroprotective after cerebral ischemic stroke. Another mechanism that increases ATP production is fatty acid oxidation (FAO). We show that in primary human astrocytes, FAO and ATP production are stimulated by 3,3,5 triiodo-l-thyronine (T3). We tested whether T3-stimulated FAO enhances neuroprotection, and show that T3 increased astrocyte survival after either hydrogen peroxide exposure or oxygen glucose deprivation. T3-mediated ATP production and protection were both eliminated with etomoxir, an inhibitor of FAO. T3-mediated protection in vitro was also dependent on astrocytes expressing HADHA (hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase), which we previously showed was critical for T3-mediated FAO in fibroblasts. Consistent with previous reports, T3-treatment decreased stroke volumes in mice. While T3 decreased stroke volume in etomoxir-treated mice, T3 had no protective effect on stroke volume in HADHA +/- mice or in mice unable to upregulate astrocyte-specific energy production. In vivo, 95% of HADHA co-localize with glial-fibrillary acidic protein, suggesting the effect of HADHA is astrocyte mediated. These results suggest that astrocyte-FAO modulates lesion size and is required for T3-mediated neuroprotection post-stroke. To our knowledge, this is the first report of a neuroprotective role for FAO in the brain.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 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.

Thyroid Hormone Mediated Modulation of Energy Expenditure

Thyroid hormone (TH) has diverse effects on mitochondria and energy expenditure (EE), generating great interest and research effort into understanding and harnessing these actions for the amelioration and treatment of metabolic disorders, such as obesity and diabetes. Direct effects on ATP utilization are a result of TH's actions on metabolic cycles and increased cell membrane ion permeability. However, the majority of TH induced EE is thought to be a result of indirect effects, which, in turn, increase capacity for EE. This review discusses the direct actions of TH on EE, and places special emphasis on the indirect actions of TH, which include mitochondrial biogenesis and reduced metabolic efficiency through mitochondrial uncoupling mechanisms. TH analogs and the metabolic actions of T2 are also discussed in the context of targeted modulation of EE. Finally, clinical correlates of TH actions on metabolism are briefly presented.

Thyroid hormone in the frontier of cell protection, survival and functional recovery

Thyroid hormone (TH) exerts important actions on cellular energy metabolism, accelerating O2consumption with consequent reactive oxygen species (ROS) generation and redox signalling affording cell protection, a response that is contributed by redox-independent mechanisms. These processes underlie genomic and non-genomic pathways, which are integrated and exhibit hierarchical organisation. ROS production led to the activation of the redox-sensitive transcription factors nuclear factor-κB, signal transducer and activator of transcription 3, activating protein 1 and nuclear factor erythroid 2-related factor 2, promoting cell protection and survival by TH. These features involve enhancement in the homeostatic potential including antioxidant, antiapoptotic, antiinflammatory and cell proliferation responses, besides higher detoxification capabilities and energy supply through AMP-activated protein kinase upregulation. The above aspects constitute the molecular basis for TH-induced preconditioning of the liver that exerts protection against ischemia-reperfusion injury, a strategy also observed in extrahepatic organs of experimental animals and with other types of injury, which awaits application in the clinical setting. Noteworthy, re-adjusting TH to normal levels results in several beneficial effects; for example, it lengthens the cold storage time of organs for transplantation from brain-dead donors; allows a superior neurological outcome in infants of <28 weeks of gestation; reduces the cognitive side-effects of lithium and improves electroconvulsive therapy in patients with bipolar disorders.

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

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

Oestrogen-dependent satellite cell activation and proliferation following a running exercise occurs via the PI3K signalling pathway and not IGF-1

AIM

The purpose of this study was to determine whether 17β-estradiol (E2) enhances the activation, proliferation and differentiation of muscle satellite cells (SC) following eccentric exercise either via insulin-like growth factor-1 (IGF-1) or through phosphatidylinositol 3-kinase (PI3K) signalling.

METHODS

This study used 64, 9-week-old, ovariectomized Sprague-Dawley rats that were divided into eight treatments groups based on oestrogen status (0.25 mg oestrogen pellet or sham), exercise status (90 min run @ 17 m min(-1), -13.5° or unexercised) and PI3K signalling inhibition (0.7 mg wortmannin kg(-1) body weight or DMSO control).

RESULTS

Significant increases in total SCs were found in both soleus and white gastrocnemius muscles (immunofluorescent co-localization of Pax7(+) nuclei) 72 h following eccentric exercise (P < 0.05). Oestrogen supplementation caused a further enhancement in total SCs in exercised rats (P < 0.05). In animals where the PI3K pathway was inhibited, regardless of oestrogen or exercise status, there was no significant enhancement of SC number in both the soleus or white gastrocnemius muscles. Interestingly, oestrogen supplementation lowered muscle levels of IGF-1 with this effect being most prominent in the soleus muscle. While IGF-1 was increased following exercise (P < 0.05), oestrogen supplementation abrogated this increase back to sedentary levels.

CONCLUSION

These data suggest that the increase in SC population following exercise in oestrogen-supplemented females may be mediated via PI3K pathway signalling and not IGF-1.

Thyroid: biological actions of 'nonclassical' thyroid hormones

Thyroid hormones (THs) are produced by the thyroid gland and converted in peripheral organs by deiodinases. THs regulate cell functions through two distinct mechanisms: genomic (nuclear) and nongenomic (non-nuclear). Many TH effects are mediated by the genomic pathway--a mechanism that requires TH activation of nuclear thyroid hormone receptors. The overall nongenomic processes, emerging as important accessory mechanisms in TH actions, have been observed at the plasma membrane, in the cytoplasm and cytoskeleton, and in organelles. Some products of peripheral TH metabolism (besides triiodo-L-thyronine), now termed 'nonclassical THs', were previously considered as inactive breakdown products. However, several reports have recently shown that they may have relevant biological effects. The recent accumulation of knowledge on how classical and nonclassical THs modulate the activity of membrane receptors, components of the mitochondrial respiratory chain, kinases and deacetylases, opened the door to the discovery of new pathways through which they act. We reviewed the current state-of-the-art on the actions of the nonclassical THs, discussing the role that these endogenous TH metabolites may have in the modulation of thyroid-related effects in organisms with differing complexity, ranging from nonmammals to humans.

Thyroid hormones and mitochondria: with a brief look at derivatives and analogues

Thyroid hormones (TH) have a multiplicity of effects. Early in life, they mainly affect development and differentiation, while later on they have particularly important influences over metabolic processes in almost all tissues. It is now quite widely accepted that thyroid hormones have two types of effects on mitochondria. The first is a rapid stimulation of respiration, which is evident within minutes/hours after hormone treatment, and it is probable that extranuclear/non-genomic mechanisms underlie this effect. The second response occurs one to several days after hormone treatment, and leads to mitochondrial biogenesis and to a change in mitochondrial mass. The hormone signal for the second response involves both T3-responsive nuclear genes and a direct action of T3 at mitochondrial binding sites. T3, by binding to a specific mitochondrial receptor and affecting the transcription apparatus, may thus act in a coordinated manner with the T3 nuclear pathway to regulate mitochondrial biogenesis and turnover. Transcription factors, coactivators, corepressors, signaling pathways and, perhaps, all play roles in these mechanisms. This review article focuses chiefly on TH, but also looks briefly at some analogues and derivatives (on which the data is still somewhat patchy). We summarize data obtained recently and in the past to try to obtain an updated picture of the current research position concerning the metabolic effects of TH, with particular emphasis on those exerted via mitochondria.

Thyroid hormone improves the mechanical performance of the post-infarcted diabetic myocardium: a response associated with up-regulation of Akt/mTOR and AMPK activation

OBJECTIVE

Thyroid hormone (TH) is shown to be protective against cardiac and pancreatic injury. Thus, this study explored the potential effects of TH treatment on the functional status of the postinfarcted diabetic myocardium. Diabetic patients have worse prognosis after acute myocardial infarction (AMI).

MATERIALS/METHODS

AMI was induced by left coronary ligation in rats previously treated with 35 mg/kg streptozotocin (STZ), (DM-AMI). TH treatment was initiated at 2 weeks after AMI and continued for 6 weeks (DM-AMI+TH), while sham-operated animals served as control (DM-SHAM).

RESULTS

TH treatment increased cardiac mass, improved wall stress and favorably changed cardiac geometry. TH significantly increased echocardiographic left ventricular ejection fraction (LVEF%): [54.2 (6.5) for DM-AMI+TH vs 37 (2.0) for DM-AMI, p<0.05]. TH treatment resulted in significantly increased insulin and decreased glucose levels in serum. The ratios of phosphorylated (p)-Akt/total Akt and p-mTOR/total mTOR were increased 2.0 fold and 2.7 fold in DM-AMI+TH vs DM-AMI respectively, p<0.05. Furthermore, the ratio of p-AMPK/total AMPK was found to be increased 1.6 fold in DM-AMI+TH vs DM-AMI, p<0.05.

CONCLUSION

TH treatment improved the mechanical performance of the post-infarcted myocardium in rats with STZ-induced diabetes, an effect which was associated with Akt/mTOR and AMPK activation.

(Healthy) ageing: focus on iodothyronines

The activity of the thyroid gland diminishes during ageing, but a certain tissue reserve of T3 and its metabolites is maintained. This reserve is thought to play a regulatory role in energy homeostasis during ageing. This review critically assesses this notion. T3 was thought to act predominantly through pathways that require transcriptional regulation by thyroid hormone receptors (TRs). However, in recent years, it has emerged that T3 and its metabolites can also act through non-genomic mechanisms, including cytosolic signaling. Interestingly, differences may exist in the non-genomic pathways utilized by thyroid hormone metabolites and T3. For instance, one particular thyroid hormone metabolite, namely 3,5-diiodo-l-thyronine (T2), increases the activity of the redox-sensitive protein deacetylase SIRT1, which has been associated with improvements in healthy ageing, whereas evidence exists that T3 may have the opposite effect. Findings suggesting that T3, T2, and their signaling pathways, such as those involving SIRT1 and AMP-activated protein kinase (AMPK), are associated with improvements in diet-induced obesity and insulin resistance emphasize the potential importance of the thyroid during ageing and in ageing-associated metabolic diseases.

Non-classic thyroid hormone signalling involved in hepatic lipid metabolism

Thyroid hormones are important modulators of lipid metabolism because the liver is a primary hormonal target. The hypolipidaemic effects of thyroid hormones result from the balance between direct and indirect actions resulting in stimulation of lipid synthesis and lipid oxidation, which favours degradation pathways. Originally, it was believed that thyroid hormone activity was only transduced by alteration of gene transcription mediated by the nuclear receptor thyroid hormone receptors, comprising the classic action of thyroid hormone. However, the discovery of other effects independent of this classic mechanism characterised a new model of thyroid hormone action, the non-classic mechanism that involves other signalling pathways. To date, this mechanism and its relevance have been intensively described. Considering the increasing evidence for non-classic signalling of thyroid hormones and the major influence of these hormones in the regulation of lipid metabolism, we reviewed the role of thyroid hormone in cytosolic signalling cascades, focusing on the regulation of second messengers, and the activity of effector proteins and the implication of these mechanisms on the control of hepatic lipid metabolism.

3,5-Diiodo-L-thyronine administration to hypothyroid rats rapidly enhances fatty acid oxidation rate and bioenergetic parameters in liver cells

Growing evidence shows that, among triiodothyronine derivatives, 3,5 diiodo-L-thyronine (T(2)) plays an important role in energy metabolism and fat storage. In the present study, short-term effects of T(2) administration to hypothyroid rats on fatty acid oxidation rate and bioenergetic parameters were investigated. Within 1 h following T(2) injection, state 3 and state 4 respiration rates, which were reduced in hypothyroid mitochondria, were noticeably increased particularly in succinate- with respect to glutamate/malate-energized mitochondria. Maximal respiratory activity, observed when glutamate/malate/succinate were simultaneously present in the respiratory medium, was significantly stimulated by T(2) treatment. A T(2)-induced increase in respiratory rates was also observed when palmitoyl-CoA or L-palmitoylcarnitine were used as substrates. No significant change in respiratory control index and ADP/O ratio was observed. The activities of the mitochondrial respiratory chain complexes, especially Complex II, were increased in T(2)-treated rats. In the latter, Complex V activities, assayed in both ATP synthesis and hydrolysis direction, were enhanced. The rate of fatty acid oxidation, followed by conversion of [(14)C]palmitate to CO(2) and ketone bodies, was higher in hepatocytes isolated from T(2)-treated rats. This increase occurs in parallel with the raise in the activity of carnitine palmitoyltransferase-I, the rate limiting enzyme of fatty acid β-oxidation, assayed in situ in digitonin-permeabilized hepatocytes. Overall, these results indicate that T(2) rapidly increases the ability of mitochondria to import and oxidize fatty acids. An emerging idea in the literature is the ability of T(2) to reduce adiposity and dyslipidemia and to prevent the development in liver steatosis. The results of the present study, showing a rapid T(2)-induced increase in the ability of mitochondria to import and oxidize fatty acids, may contribute to understand the biochemical mechanisms of T(2)-metabolic effects.

Responses of skeletal muscle lipid metabolism in rat gastrocnemius to hypothyroidism and iodothyronine administration: a putative role for FAT/CD36

Iodothyronines such as triiodothyronine (T(3)) and 3,5-diiodothyronine (T(2)) influence energy expenditure and lipid metabolism. Skeletal muscle contributes significantly to energy homeostasis, and the above iodothyronines are known to act on this tissue. However, little is known about the cellular/molecular events underlying the effects of T(3) and T(2) on skeletal muscle lipid handling. Since FAT/CD36 is involved in the utilization of free fatty acids by skeletal muscle, specifically in their import into that tissue and presumably their oxidation at the mitochondrial level, we hypothesized that related changes in lipid handling and in FAT/CD36 expression and subcellular redistribution would occur due to hypothyroidism and to T(3) or T(2) administration to hypothyroid rats. In gastrocnemius muscles isolated from hypothyroid rats, FAT/CD36 was upregulated (mRNA levels and total tissue, sarcolemmal, and mitochondrial protein levels). Administration of either T(3) or T(2) to hypothyroid rats resulted in 1) little or no change in FAT/CD36 mRNA level, 2) a decreased total FAT/CD36 protein level, and 3) further increases in FAT/CD36 protein level in sarcolemma and mitochondria. Thus, the main effect of each iodothyronine seemed to be exerted at the level of FAT/CD36 cellular distribution. The effect of further increases in FAT/CD36 protein level in sarcolemma and mitochondria was already evident at 1 h after iodothyronine administration. Each iodothyronine increased the mitochondrial fatty acid oxidation rate. However, the mechanisms underlying their rapid effects seem to differ; T(2) and T(3) each induce FAT/CD36 translocation to mitochondria, but only T(2) induces increases in carnitine palmitoyl transferase system activity and in the mitochondrial substrate oxidation rate.

Metabolic basis for thyroid hormone liver preconditioning: upregulation of AMP-activated protein kinase signaling

The liver is a major organ responsible for most functions of cellular metabolism and a mediator between dietary and endogenous sources of energy for extrahepatic tissues. In this context, adenosine-monophosphate- (AMP-) activated protein kinase (AMPK) constitutes an intrahepatic energy sensor regulating physiological energy dynamics by limiting anabolism and stimulating catabolism, thus increasing ATP availability. This is achieved by mechanisms involving direct allosteric activation and reversible phosphorylation of AMPK, in response to signals such as energy status, serum insulin/glucagon ratio, nutritional stresses, pharmacological and natural compounds, and oxidative stress status. Reactive oxygen species (ROS) lead to cellular AMPK activation and downstream signaling under several experimental conditions. Thyroid hormone (L-3,3′,5-triiodothyronine, T₃) administration, a condition that enhances liver ROS generation, triggers the redox upregulation of cytoprotective proteins affording preconditioning against ischemia-reperfusion (IR) liver injury. Data discussed in this work suggest that T₃-induced liver activation of AMPK may be of importance in the promotion of metabolic processes favouring energy supply for the induction and operation of preconditioning mechanisms. These include antioxidant, antiapoptotic, and anti-inflammatory mechanisms, repair or resynthesis of altered biomolecules, induction of the homeostatic acute-phase response, and stimulation of liver cell proliferation, which are required to cope with the damaging processes set in by IR.

Triiodothyronine stimulates glucose transport in bone cells

Thyroid hormones increase energy expenditure and bone turnover in vivo. To study whether 3,3',5-triiodo-l-thyronine (T₃) stimulates the uptake of glucose in osteoblastic cells, PyMS (a cell line derived from rat bone) cells were kept in serum-free culture medium and treated with T₃. We measured [1-¹⁴C]-2-deoxy-D: -glucose (2DG) uptake and looked for expression of the high-affinity glucose transporters GLUT1 and GLUT3 by northern and western analysis. T₃ did not influence the cell number but slightly (1.3-fold) increased the protein content of the cell cultures. 2DG uptake was low in serum-deprived cell cultures and was increased by T₃ (up to 2.5-fold at 1 nmol l⁻¹ after 4 days) in a dose- and time-dependent manner. Triiodothyronine at 1 nmol l⁻¹ increased GLUT1 and GLUT3 abundance in membranes. Therefore, increased glucose uptake induced by T₃ in osteoblasts may be mediated by the known high-affinity glucose transporters GLUT1 and GLUT3.

3,5,3'triiodo-L-thyronine induces SREBP-1 expression by non-genomic actions in human HEP G2 cells

Liver is an important target for thyroid hormone actions. T(3) exerts its effects by two mechanisms: (i) Genomic actions consisting of T(3) link to nuclear receptors that bind responsive elements in the promoter of target genes, (ii) non-genomic actions including integrin αvb3 receptor-mediated MAPK/ERK and PI3K/Akt/mTOR-C1 activation. SREBP-1a, SREBP-1c, and SREBP-2 are transcription factors involved in the regulation of lipogenic genes. We show in Hep G2 cells that T(3) determined a dose- and time-dependent increase in the level of the precursor form of SREBP-1 without affecting SREBP-1 mRNA abundance. T(3) also induced phosphorylation of ERK1/2, Akt and of mTOR-C1 target S6K-P70, and the cytosol-to-membrane translocation of PKC-α. Modulation of SREBP-1 protein level by T(3) was dependent on MAPK/ERK, PI3K/Akt/mTOR-C1 pathway activation since the MEK inhibitor PD98059 or the PI3K inhibitor LY294002 abolished the stimulatory effect of T(3) . Conversely, the effect of T(3) on SREBP-1 level was enhanced by using rapamycin, mTOR-C1 inhibitor. These data suggest a negative control of mTOR-C1 target S6K-P70 on PI3K/Akt pathway. The effect of T(3) on SREBP-1 content increased also by using PKC inhibitors. These inhibitors increased the action of T(3) on Akt phosphorylation suggesting that conventional PKCs may work as negative regulators of the T(3) -dependent SREBP-1 increase. T(3) effects were partially abrogated by tetrac, an inhibitor of the T(3) -αvβ3 receptor interaction and partially evoked by T(3) analog T(3) -agarose. These findings support a model in which T(3) activates intracellular signaling pathways which may be involved in the increment of SREBP-1 level through an IRES-mediated translation mechanism.

3,5-Diiodo-L-thyronine prevents high-fat-diet-induced insulin resistance in rat skeletal muscle through metabolic and structural adaptations

The worldwide prevalence of obesity-associated pathologies, including type 2 diabetes, requires thorough investigation of mechanisms and interventions. Recent studies have highlighted thyroid hormone analogs and derivatives as potential agents able to counteract such pathologies. In this study, in rats receiving a high-fat diet (HFD), we analyzed the effects of a 4-wk daily administration of a naturally occurring iodothyronine, 3,5-diiodo-L-thyronine (T2), on the gastrocnemius muscle metabolic/structural phenotype and insulin signaling. The HFD-induced increases in muscle levels of fatty acid translocase (3-fold; P<0.05) and TGs (2-fold, P<0.05) were prevented by T2 (each; P<0.05 vs. HFD). T2 increased insulin-stimulated Akt phosphorylation levels (∼2.5-fold; P<0.05 vs. HFD). T2 induced these effects while sparing muscle mass and without cardiac hypertrophy. T2 increased the muscle contents of fast/glycolytic fibers (2-fold; P<0.05 vs. HFD) and sarcolemmal glucose transporter 4 (3-fold; P<0.05 vs. HFD). Adipocyte differentiation-related protein was predominantly present within the slow/oxidative fibers in HFD-T2. In T2-treated rats (vs. HFD), glycolytic enzymes and associated components were up-regulated (proteomic analysis, significance limit: 2-fold; P<0.05), as was phosphofructokinase activity (by 1.3-fold; P<0.05), supporting the metabolic shift toward a more glycolytic phenotype. These results highlight T2 as a potential therapeutic approach to the treatment of diet-induced metabolic dysfunctions.

AMP-activated protein kinase controls metabolism and heat production during embryonic development in birds

During embryonic and early juvenile development, endotherms must balance energy allocation between growth and heat production. Failure to either match the ATP demand of growing tissue or produce heat at the correct developmental stage will lead to damage of the organism. We tested the hypothesis that AMP-activated protein kinase (AMPK) is involved in the regulation of energy metabolism and heat production during development in the chicken (Gallus gallus). We show that mRNA concentrations of regulatory and catalytic AMPK subunits, AMPK total protein, and AMPK phosphorylation increase during development [3 days (-3 days) and one day (-1 day) before hatching, and +1 day and +8 days after hatching] in liver, and to a lesser extent in skeletal muscle. Chronic stimulation with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) significantly increases AMPK phosphorylation in skeletal muscle and in liver. This increase was paralleled by significant increases in heat production, glucose utilization, and liver and skeletal muscle mitochondrial capacity (citrate synthase activity). The effects of AMPK are likely to be mediated by inhibition of acetyl CoA carboxylase (ACC) after hatching, when ACC protein concentration increases significantly, and by a significant AMPK-induced increase in PGC-1alpha mRNA concentration (at +1 day), but not in NRF-1 mRNA concentration. AMPK phosphorylation is under the control of thyroid hormone, and AMPK phosphorylation decreases significantly following the induction of hypothyroidism. We propose AMPK as a principal regulatory mechanism during the transition from ectothermy to endothermy in birds, and show that AMPK function in birds is similar to that observed in mammals.

Thyroid hormone receptors are down-regulated in skeletal muscle of patients with non-thyroidal illness syndrome secondary to non-septic shock

AIM

Non-thyroidal illness syndrome (NTIS) is related to changes in thyroid hormone (TH) physiology. Skeletal muscle (SM) plays a major role in metabolism, and TH regulates SM phenotype and metabolism. We aimed to characterize the SM of non-septic shock NTIS patients in terms of: i) expression of genes and proteins involved in TH metabolism and actions; and ii) NFKB's pathway activation, a responsible factor for some of the phenotypic changes in NTIS. We also investigated whether the patient's serum can induce in vitro the effects observed in vivo.

METHODS

Serum samples and SM biopsies from 14 patients with non-septic shock NTIS and 11 controls. Gene and protein expression and NFKB1 activation were analyzed by quantitative PCR and immunoblotting. Human SM cell (HSkMC) cultures to investigate the effects of patient's serum on TH action mediators.

RESULTS

Patients with non-septic shock NTIS showed higher levels of pro-inflammatory cytokines than controls. Expression of TRβ (THRB), TRα1 (THRA), and retinoid X receptor γ (RXRG) was decreased in NTIS patients. RXRA gene expression was higher, but its protein was lower in NTIS than controls, suggesting the existence of a post-transcriptional mechanism that down-regulates protein levels. NFKB1 pathway activation was not different between NTIS and control patients. HSkMC incubated with patient's serum increased TH receptor and RXRG gene expression after 48  h.

CONCLUSIONS

Patients with non-septic shock NTIS showed decreased expression of TH receptors and RXRs, which were not related to increased activation of the NFKB1 pathway. These findings could not be replicated in cultures of HSkMCs incubated in the patient's serum.