Acute administration of 3,5-diiodo-l-thyronine to hypothyroid rats affects bioenergetic parameters in rat skeletal muscle mitochondria

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.

3,5-Diiodothyronine protects against cardiac ischaemia-reperfusion injury in male rats

NEW FINDINGS

What is the central question of this study? 3,5-Diiodothyronine (3,5-T2) administration increases resting metabolic rate, prevents or treats liver steatosis in rodent models, and ameliorates insulin resistance: what are its effects on cardiac electrical and contractile properties and autonomic regulation? What is the main finding and its importance? Chronic 3,5-T2 administration has no adverse effects on cardiac function. Remarkably, 3,5-T2 improves the autonomous control of the rat heart and protects against ischaemia-reperfusion injury.

ABSTRACT

The use of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) to treat metabolic diseases has been hindered by potential adverse effects on liver, lipid metabolism and cardiac electrical properties. It is recognized that 3,5-diiodothyronine (3,5-T2) administration increases resting metabolic rate, prevents or treats liver steatosis in rodent models and ameliorates insulin resistance, suggesting 3,5-T2 as a potential therapeutic tool. However, a comprehensive assessment of cardiac electrical and contractile properties has not been made so far. Three-month-old Wistar rats were daily administered vehicle, 3,5-T2 or 3,5-T2+T4 and no signs of atrial or ventricular arrhythmia were detected in non-anaesthetized rats during 90 days. Cardiac function was preserved as heart rate, left ventricle diameter and shortening fraction in 3,5-T2-treated rats compared to vehicle and 3,5-T2+T4 groups. Power spectral analysis indicated an amelioration of the heart rate variability only in 3,5-T2-treated rats. An increased baroreflex sensitivity at rest was observed in both 3,5-T2-treated groups. Finally, 3,5-T2 Langendorff-perfused hearts presented a significant recovery of left ventricular function and remarkably smaller infarction area after ischaemia-reperfusion injury. In conclusion, chronic 3,5-T2 administration ameliorates tonic cardiac autonomic control and confers cardioprotection against ischaemia-reperfusion injury in healthy male rats.

Absence of Uncoupling Protein-3 at Thermoneutrality Impacts Lipid Handling and Energy Homeostasis in Mice

The role of uncoupling protein-3 (UCP3) in energy and lipid metabolism was investigated. Male wild-type (WT) and UCP3-null (KO) mice that were housed at thermoneutrality (30 °C) were used as the animal model. In KO mice, the ability of skeletal muscle mitochondria to oxidize fatty acids (but not pyruvate or succinate) was reduced. At whole animal level, adult KO mice presented blunted resting metabolic rates, energy expenditure, food intake, and the use of lipids as metabolic substrates. When WT and KO mice were fed with a standard/low-fat diet for 80 days, since weaning, they showed similar weight gain and body composition. Interestingly, KO mice showed lower fat accumulation in visceral adipose tissue and higher ectopic fat accumulation in liver and skeletal muscle. When fed with a high-fat diet for 80 days, since weaning, KO mice showed enhanced energy efficiency and an increased lipid gain (thus leading to a change in body composition between the two genotypes). We conclude that UCP3 plays a role in energy and lipid homeostasis and in preserving lean tissues by lipotoxicity, in mice that were housed at thermoneutrality.

3,5-diiodo-L-thyronine increases de novo lipogenesis in liver from hypothyroid rats by SREBP-1 and ChREBP-mediated transcriptional mechanisms

Hepatic de novo lipogenesis (DNL), the process by which carbohydrates are converted into lipids, is strictly controlled by nutritional and hormonal status. 3,5-Diiodo-L-thyronine (T2), a product of the 3,5,3'-triiodo-L-thyronine (T3) peripheral metabolism, has been shown to mimic some T3 effects on lipid metabolism by a short-term mechanism independent of protein synthesis. Here, we report that T2, administered for 1 week to hypothyroid rats, increases total fatty acid synthesis from acetate in isolated hepatocytes. Studies carried out on liver subcellular fractions demonstrated that T2 not only increases the activity and the expression of acetyl-CoA carboxylase and fatty acid synthase but also of other proteins linked to DNL such as the mitochondrial citrate carrier and the cytosolic ATP citrate lyase. Parallelly, T2 stimulates the activities of enzymes supplying cytosolic NADPH needed for the reductive steps of DNL. With respect to both euthyroid and hypothyroid rats, T2 administration decreases the hepatic mRNA level of SREBP-1, a transcription factor which represents a master regulator of DNL. However, when compared to hypothyroid rats T2 significantly increases, without bringing to the euthyroid value, the content of both mature (nSREBP-1), and precursor (pSREBP-1) forms of the SREBP-1 protein as well as their ratio. Moreover, T2 administration strongly augmented the nuclear content of ChREBP, another crucial transcription factor involved in the regulation of lipogenic genes. Based on these results, we can conclude that in the liver of hypothyroid rats the transcriptional activation by T2 of DNL genes could depend, at least in part, on SREBP-1- and ChREBP-dependent mechanisms. © 2019 IUBMB Life, 2019.

3,5-Diiodo-l-Thyronine Increases Glucose Consumption in Cardiomyoblasts Without Affecting the Contractile Performance in Rat Heart

3,5-diiodo-l-thyronine (T2) is an endogenous derivative of thyroid hormone that has been suggested to regulate energy expenditure, resting metabolic rate and oxygen consumption with a mechanism that involves the activation of mitochondrial function. In this study, we focused on the cardiac effects of T2, which have been poorly investigated so far, by using both in vitro and ex vivo models. As a comparison, the response to T3 and T4 was also determined. Rat cardiomyoblasts (H9c2 cells) were used to determine T2, T3, and T4 uptake by high-performance liquid chromatography-tandem mass spectrometry. In the same experimental model, MTT test, crystal violet staining, and glucose consumption were investigated, using T2 concentrations ranging from 0.1 to 10 µM. To assess cardiac functional effects, isolated working rat hearts were perfused with T2, T3, or T4 in Krebs-Ringer buffer, and the hemodynamic variables were recorded. T2 was taken up by cardiomyoblasts, and in cell lysate T2 levels increased slowly over time, reaching higher concentrations than in the incubation medium. T2 significantly decreased MTT staining at 0.5-10 µM concentration (P < 0.05). Crystal violet staining confirmed a reduction of cell viability only upon treatment with 10 µM T2, while equimolar T3 and T4 did not share this effect. Glucose consumption was also significantly affected as indicated by glucose uptake being increased by 24 or 35% in cells exposed to 0.1 or 1.0 µM T2 (P < 0.05 in both cases). On the contrary, T3 did not affect glucose consumption which, in turn, was significantly reduced by 1 and 10 µM T4 (-24 and -41% vs control, respectively, P < 0.05 and P < 0.01). In the isolated perfused rat heart, 10 µM T2 produced a slight and transient reduction in cardiac output, while T3 and T4 did not produce any hemodynamic effect. Our findings indicate that T2 is taken up by cardiomyoblasts, and at 0.1-1.0 µM concentration it can modulate cardiac energy metabolism by increasing glucose consumption. Some evidence of toxicity and a transient impairment of contractile performance are observed only at 10 µM concentration. These effects appear to be specific for T2, since they are not reproduced by T3 or T4.

3,5-Diiodothyronine: A Novel Thyroid Hormone Metabolite and Potent Modulator of Energy Metabolism

Over 30 years of research has demonstrated that 3,5-diiodo-L-thyronine (3,5-T2), an endogenous metabolite of thyroid hormones, exhibits interesting metabolic activities. In rodent models, exogenously administered 3,5-T2 rapidly increases resting metabolic rate and elicits short-term beneficial hypolipidemic effects; however, very few studies have evaluated the effects of endogenous and exogenous T2 in humans. Further analyses on larger cohorts are needed to determine whether 3,5-T2 is a potent additional modulator of energy metabolism. In addition, while several lines of evidence suggest that 3,5-T2 mainly acts through Thyroid hormone receptors (THRs)- independent ways, with mitochondria as a likely cellular target, THRs-mediated actions have also been described. The detailed cellular and molecular mechanisms through which 3,5-T2 elicits a multiplicity of actions remains unknown. Here, we provide an overview of the most recent literature on 3,5-T2 bioactivity with a particular focus on short-term and long-term effects, describing data obtained through in vivo and in vitro approaches in both mammalian and non-mammalian species.

Direct and rapid effects of 3,5-diiodo-L-thyronine (T2)

A growing number of researchers are focusing their attention on the possibility that thyroid hormone metabolites, particularly 3,5-diiodothyronine (T2), may actively regulate energy metabolism at the cellular, rather than the nuclear, level. Due to their biochemical features, mitochondria have been the focus of research on the thermogenic effects of thyroid hormones. Indeed, mitochondrial activities have been shown to be regulated both directly and indirectly by T2-specific pathways. Herein, we describe the effects of T2 on energy metabolism.

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.

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.

3,5-Diiodo-L-thyronine activates brown adipose tissue thermogenesis in hypothyroid rats

3,5-Diiodo-l-thyronine (T2), a thyroid hormone derivative, is capable of increasing energy expenditure, as well as preventing high fat diet-induced overweight and related metabolic dysfunction. Most studies to date on T2 have been carried out on liver and skeletal muscle. Considering the role of brown adipose tissue (BAT) in energy and metabolic homeostasis, we explored whether T2 could activate BAT thermogenesis. Using euthyroid, hypothyroid, and T2-treated hypothyroid rats (all maintained at thermoneutrality) in morphological and functional studies, we found that hypothyroidism suppresses the maximal oxidative capacity of BAT and thermogenesis, as revealed by reduced mitochondrial content and respiration, enlarged cells and lipid droplets, and increased number of unilocular cells within the tissue. In vivo administration of T2 to hypothyroid rats activated BAT thermogenesis and increased the sympathetic innervation and vascularization of tissue. Likewise, T2 increased BAT oxidative capacity in vitro when added to BAT homogenates from hypothyroid rats. In vivo administration of T2 to hypothyroid rats enhanced mitochondrial respiration. Moreover, UCP1 seems to be a molecular determinant underlying the effect of T2 on mitochondrial thermogenesis. In fact, inhibition of mitochondrial respiration by GDP and its reactivation by fatty acids were greater in mitochondria from T2-treated hypothyroid rats than untreated hypothyroid rats. In vivo administration of T2 led to an increase in PGC-1α protein levels in nuclei (transient) and mitochondria (longer lasting), suggesting a coordinate effect of T2 in these organelles that ultimately promotes net activation of mitochondrial biogenesis and BAT thermogenesis. The effect of T2 on PGC-1α is similar to that elicited by triiodothyronine. As a whole, the data reported here indicate T2 is a thyroid hormone derivative able to activate BAT thermogenesis.

Mitochondrial efficiency and insulin resistance

Insulin resistance, “a relative impairment in the ability of insulin to exert its effects on glucose, protein and lipid metabolism in target tissues,” has many detrimental effects on metabolism and is strongly correlated to deposition of lipids in non-adipose tissues. Mitochondria are the main cellular sites devoted to ATP production and fatty acid oxidation. Therefore, a role for mitochondrial dysfunction in the onset of skeletal muscle insulin resistance has been proposed and many studies have dealt with possible alteration in mitochondrial function in obesity and diabetes, both in humans and animal models. Data reporting evidence of mitochondrial dysfunction in type two diabetes mellitus are numerous, even though the issue that this reduced mitochondrial function is causal in the development of the disease is not yet solved, also because a variety of parameters have been used in the studies carried out on this subject. By assessing the alterations in mitochondrial efficiency as well as the impact of this parameter on metabolic homeostasis of skeletal muscle cells, we have obtained results that allow us to suggest that an increase in mitochondrial efficiency precedes and therefore can contribute to the development of high-fat-induced insulin resistance in skeletal muscle.

Administration of 3,5-diiodothyronine (3,5-T2) causes central hypothyroidism and stimulates thyroid-sensitive tissues

In general, 3,5-diiodothyronine (3,5-T2) increases the resting metabolic rate and oxygen consumption, exerting short-term beneficial metabolic effects on rats subjected to a high-fat diet. Our aim was to evaluate the effects of chronic 3,5-T2 administration on the hypothalamus-pituitary-thyroid axis, body mass gain, adipose tissue mass, and body oxygen consumption in Wistar rats from 3 to 6 months of age. The rats were treated daily with 3,5-T2 (25, 50, or 75 μg/100 g body weight, s.c.) for 90 days between the ages of 3 and 6 months. The administration of 3,5-T2 suppressed thyroid function, reducing not only thyroid iodide uptake but also thyroperoxidase, NADPH oxidase 4 (NOX4), and thyroid type 1 iodothyronine deiodinase (D1 (DIO1)) activities and expression levels, whereas the expression of the TSH receptor and dual oxidase (DUOX) were increased. Serum TSH, 3,3',5-triiodothyronine, and thyroxine were reduced in a 3,5-T2 dose-dependent manner, whereas oxygen consumption increased in these animals, indicating the direct action of 3,5-T2 on this physiological variable. Type 2 deiodinase activity increased in both the hypothalamus and the pituitary, and D1 activities in the liver and kidney were also increased in groups treated with 3,5-T2. Moreover, after 3 months of 3,5-T2 administration, body mass and retroperitoneal fat pad mass were significantly reduced, whereas the heart rate and mass were unchanged. Thus, 3,5-T2 acts as a direct stimulator of energy expenditure and reduces body mass gain; however, TSH suppression may develop secondary to 3,5-T2 administration.

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.

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.

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.

3,5-Diiodo-L-thyronine modulates the expression of genes of lipid metabolism in a rat model of fatty liver

Recent reports demonstrated that 3,5-diiodo-l-thyronine (T(2)) was able to prevent lipid accumulation in the liver of rats fed a high-fat diet (HFD). In this study, we investigated how the rat liver responds to HFD and T(2) treatment by assessing the transcription profiles of some genes involved in the pathways of lipid metabolism: oxidation, storage and secretion. The mRNA levels of the peroxisome proliferator-activated receptors (PPARα, PPARγ and PPARδ), and of their target enzymes acyl-CoA oxidase and stearoyl-CoA desaturase were evaluated by real-time RT-PCR. Moreover, the expression of the adipose triglyceride lipase involved in lipid mobilisation, of the main PAT proteins acting in lipid droplet (LD) turnover, and of apoprotein B (apo B), the major protein component of very low-density lipoproteins (VLDLs) were analysed. Overall, our data demonstrated that T(2) administration to HFD rats counteracts most of the hepatic transcriptional changes that occurred in response to the excess exogenous fat. In particular, our results suggest that T(2) may prevent the pathways leading to lipid storage in LDs, promote the processes of lipid mobilisation from LDs and secretion as VLDL, in addition to the stimulation of pathways of lipid oxidation. In conclusion, our findings might give an insight into the mechanisms underlying the anti-steatotic ability of T(2) and help to define the potential therapeutic role of T(2) for preventing or treating liver steatosis.

3,5-diiodo-L-thyronine increases FoF1-ATP synthase activity and cardiolipin level in liver mitochondria of hypothyroid rats

Short-term effects of 3,5-L-diiodothyronine (T(2)) administration to hypothyroid rats on F(o)F(1)-ATP synthase activity were investigated in liver mitochondria. One hour after T(2) injection, state 4 and state 3 respiration rates were noticeably stimulated in mitochondria subsequently isolated. F(o)F(1)-ATP synthase activity, which was reduced in mitochondria from hypothyroid rats as compared to mitochondria from euthyroid rats, was significantly increased by T(2) administration in both the ATP-synthesis and hydrolysis direction. No change in β-subunit mRNA accumulation and protein amount of the α-β subunit of F(o)F(1)-ATP synthase was found, ruling out a T(2) genomic effect. In T(2)-treated rats, changes in the composition of mitochondrial phospholipids were observed, cardiolipin (CL) showing the greatest alteration. In mitochondria isolated from hypothyroid rats the decrease in the amount of CL was accompanied by an increase in the level of peroxidised CL. T(2) administration to hypothyroid rats enhanced the level of CL and decreased the amount of peroxidised CL in subsequently isolated mitochondria, tending to restore the CL value to the euthyroid level. Minor T(2)-induced changes in mitochondrial fatty acid composition were detected. Overall, the enhanced F(o)F(1)-ATP synthase activity observed following injection of T(2) to hypothyroid rats may be ascribed, at least in part, to an increased level of mitochondrial CL associated with decreased peroxidation of CL.

l-Thyroxine vs. 3,5,3′-triiodo-l-thyronine and cell proliferation: activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase

3,5,3′-Triiodo-l-thyronine (T3), but not l-thyroxine (T4), activated Src kinase and, downstream, phosphatidylinositol 3-kinase (PI3-kinase) by means of an αvβ3 integrin receptor on human glioblastoma U-87 MG cells. Although both T3 and T4 stimulated extracellular signal-regulated kinase (ERK) 1/2, activated ERK1/2 did not contribute to T3-induced Src kinase or PI3-kinase activation, and an inhibitor of PI3-kinase, LY-294002, did not block activation of ERK1/2 by physiological concentrations of T3 and T4. Thus the PI3-kinase, Src kinase, and ERK1/2 signaling cascades are parallel pathways in T3-treated U-87 MG cells. T3 and T4 both caused proliferation of U-87 MG cells; these effects were blocked by the ERK1/2 inhibitor PD-98059 but not by LY-294002. Small-interfering RNA knockdown of PI3-kinase confirmed that PI3-kinase was not involved in the proliferative action of T3 on U-87 MG cells. PI3-kinase-dependent actions of T3 in these cells included shuttling of nuclear thyroid hormone receptor-α (TRα) from cytoplasm to nucleus and accumulation of hypoxia-inducible factor ( HIF)- 1α mRNA; LY-294002 inhibited these actions. Results of studies involving αvβ3 receptor antagonists tetraiodothyroacetic acid (tetrac) and Arg-Gly-Asp (RGD) peptide, together with mathematical modeling of the kinetics of displacement of radiolabeled T3 from the integrin by unlabeled T3 and by unlabeled T4, are consistent with the presence of two iodothyronine receptor domains on the integrin. A model proposes that one site binds T3 exclusively, activates PI3-kinase via Src kinase, and stimulates TRα trafficking and HIF- 1α gene expression. Tetrac and RGD peptide both inhibit T3 action at this site. The second site binds T4 and T3, and, via this receptor, the iodothyronines stimulate ERK1/2-dependent tumor cell proliferation. T3 action here is inhibited by tetrac alone, but the effect of T4 is blocked by both tetrac and the RGD peptide.

3,5-Diiodo-L-thyronine rapidly enhances mitochondrial fatty acid oxidation rate and thermogenesis in rat skeletal muscle: AMP-activated protein kinase involvement

Triiodothyronine regulates energy metabolism and thermogenesis. Among triiodothyronine derivatives, 3,5-diiodo-l-thyronine (T(2)) has been shown to exert marked effects on energy metabolism by acting mainly at the mitochondrial level. Here we investigated the capacity of T(2) to affect both skeletal muscle mitochondrial substrate oxidation and thermogenesis within 1 h after its injection into hypothyroid rats. Administration of T(2) induced an increase in mitochondrial oxidation when palmitoyl-CoA (+104%), palmitoylcarnitine (+80%), or succinate (+30%) was used as substrate, but it had no effect when pyruvate was used. T(2) was able to 1) activate the AMPK-ACC-malonyl-CoA metabolic signaling pathway known to direct lipid partitioning toward oxidation and 2) increase the importing of fatty acids into the mitochondrion. These results suggest that T(2) stimulates mitochondrial fatty acid oxidation by activating several metabolic pathways, such as the fatty acid import/beta-oxidation cycle/FADH(2)-linked respiratory pathways, where fatty acids are imported. T(2) also enhanced skeletal muscle mitochondrial thermogenesis by activating pathways involved in the dissipation of the proton-motive force not associated with ATP synthesis ("proton leak"), the effect being dependent on the presence of free fatty acids inside mitochondria. We conclude that skeletal muscle is a target for T(2), and we propose that, by activating processes able to enhance mitochondrial fatty acid oxidation and thermogenesis, T(2) could play a role in protecting skeletal muscle against excessive intramyocellular lipid storage, possibly allowing it to avoid functional disorders.

Thyroid hormone-induced angiogenesis

A series of reports in the past decade have ascribed pro-angiogenic activity to several thyroid hormone analogues, including L-thyroxine (T(4)), 3,5,3-triiodo-L-thyronine (T(3)) and diiodothyropropionic acid (DITPA). Model systems of angiogenesis have demonstrated that thyroid hormone-induced neovascularization is initiated at a cell surface receptor for the hormone on an integrin. The hormone signal is transduced within the cell by extracellular regulated kinase 1/2 (ERK1/2) into secretion of basic fibroblast growth factor (bFGF) and other vascular growth factors and consequent angiogenesis. Intact animal studies have shown that endogenous thyroid hormone supports blood vessel density in heart and brain and that thyroid hormone administration can induce angiogenesis in ischemic limbs.

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

T3 stimulates metabolic rate in many tissues and induces changes in fuel use. The pathways by which T3 induces metabolic/structural changes related to altered fuel use in skeletal muscle have not been fully clarified. Gastrocnemius muscle (isolated at different time points after a single injection of T3 into hypothyroid rats), displayed rapid inductions of AMP-activated protein kinase (AMPK) phosphorylation (threonine 172; within 6 h) and acetyl-coenzyme A carboxylase phosphorylation (serine 79; within 12 h). As a consequence, increases occurred in mitochondrial fatty acid oxidation and carnitine palmitoyl transferase activity. Concomitantly, T3 stimulated signaling toward increased glycolysis through a rapid increase in Akt/protein kinase B (serine 473) phosphorylation (within 6 h) and a directly related increase in the activity of phosphofructokinase. The kinase specificity of the above effects was verified by treatment with inhibitors of AMPK and Akt activity (compound C and wortmannin, respectively). In contrast, glucose transporter 4 translocation to the membrane (activated by T3 within 6 h) was maintained when either AMPK or Akt activity was inhibited. The metabolic changes were accompanied by a decline in myosin heavy-chain Ib protein [causing a shift toward the fast-twitch (glycolytic) phenotype]. The increases in AMPK and acetyl-coenzyme A carboxylase phosphorylation were transient events, both levels declining from 12 h after the T3 injection, but Akt phosphorylation remained elevated until at least 48h after the injection. These data show that in skeletal muscle, T3 stimulates both fatty acid and glucose metabolism through rapid activations of the associated signaling pathways involving AMPK and Akt/protein kinase B.