Antonia Lanni

ACTION OF THYROID HORMONES AT THE CELLULAR LEVEL : THE MITOCHONDRIAL TARGET

Thyroid hormones exert profound effects on the energy metabolism. An inspection of the early and more recent literature shows that several targets at the cellular level have been identified. Since their effects on the nuclear signalling pathway have already been well‐defined and extensively reviewed, this article focuses on the regulation of mitochondrial activity by thyroid hormones. Mitochondria, by virtue of their biochemical functions, are a natural candidate as a direct target for the calorigenic effects of thyroid hormones. To judge from results coming from various laboratories, it is quite conceivable that mitochondrial activities are regulated both directly and indirectly. Not only triiodo‐L‐thyronine, but also diiodothyronines are active in regulating the energy metabolism. They influence the resting metabolism in rats with 3,5‐diiodo‐L‐thyronine seeming to show a clearer effect.

Thyroid hormone and uncoupling proteins

Thyroid hormone (TH/T3) exerts many of its effects on energy metabolism by affecting gene transcription. However, although this is an important target for T3, only a limited number of T3‐responsive genes have been identified and studied. Among these, the genes for uncoupling proteins (UCPs) have attracted the interest of scientists. Although the role of UCP1 seems quite well established, uncertainty surrounds the physiological function of the recently discovered UCP1 analogs, UCP2 and UCP3. The literature suggests that T3 affects both the expression and the activity of each of these UCPs but further studies are needed to establish whether the mechanisms activated by the hormone are the same. Recently, because of their larger range of expression, much attention has been devoted to UCP2 and UCP3. Most detailed studies on the involvement of these proteins as mediators of the effects of T3 on metabolism have focused on UCP3 because of its expression in skeletal muscle. T3 seems to be unique in having the ability to stimulate the expression and activity of UCP3 and this may be related to the capacity of T3 to activate the integrated biochemical processes linked to UCP activity, such as those related to fatty acids, coenzyme Q and free radicals.

Fuel economy in food-deprived skeletal muscle: signaling pathways and regulatory mechanisms

Energy deprivation poses a tremendous challenge to skeletal muscle. Glucose (ATP) depletion causes muscle fibers to undergo rapid adaptive changes toward the use of fatty acids (instead of glucose) as fuel. Physiological situations involving energy deprivation in skeletal muscle include exercise and fasting. A vast body of evidence is available on the signaling pathways that lead to structural/metabolic changes in muscle during exercise and endurance training. In contrast, only recently has a systematic, overall picture been obtained of the signaling processes (and their kinetics and sequential order) that lead to adaptations of the muscle to the fasting state. It has become clear that the reaction of the organism to food restraint or deprivation involves a rapid signaling process causing skeletal muscles, which generally use glucose as their predominant fuel, to switch to the use of fat as fuel. Efficient sensing of glucose depletion in skeletal muscle guarantees maintained activity in those tissues that rely entirely on glucose (such as the brain). To metabolize fatty acids, skeletal muscle needs to activate complex transcription, translation, and phosphorylation pathways. Only recently has it become clear that these pathways are interrelated and tightly regulated in a rapid, transient manner. Food deprivation may trigger these responses with a timing/intensity that differs among animal species and that may depend on their individual ability to induce structural/metabolic changes that serve to safeguard whole‐body energy homeostasis in the longer term. The increased cellular AMP/ATP ratio induced by food deprivation, which results in activation of AMP‐activated protein kinase (AMPK), initiates a rapid signaling process, resulting in the recruitment of factors mediating the structural/ metabolic shift in skeletal muscle toward this change in fuel usage. These factors include peroxisome prolifera‐tor‐activated receptor (PPAR)γ coactivator‐1α (PGC‐1α), PPARδ, and their target genes, which are involved in the formation of oxidative muscle fibers, mitochon‐drial biogenesis, oxidative phosphorylation, and fatty acid oxidation. Fatty acids, besides being the fuel for mitochondrial oxidation, have been identified as important signaling molecules regulating the transcription and/or activity of the genes or gene products involved in fatty acid metabolism during food deprivation. It is thus becoming increasingly clear that fatty acids determine the economy of their own usage. We discuss the order of events from the onset of food deprivation and their importance.—de Lange P., Moreno, M., Silvestri, E., Lombardi, A., Goglia, F., Lanni A. Fuel economy in food‐deprived skeletal muscle: signaling pathways and regulatory mechanisms. FASEB J. 21, 3431–3441 (2007)

Fenofibrate prevents and reduces body weight gain and adiposity in diet-induced obese rats

Fibrates are hypolipidemic drugs that activate the peroxisome proliferator‐activated receptors. Since fibrates may also increase energy expenditure, we investigated whether fenofibrate (FF) had this effect in diet‐induced obese rats. A 2‐month administration of a high‐fat palatable diet to adult rats increased body weight by 25% and white adipose mass by 163% compared with a standard diet. These effects were prevented by FF, both when administered for the 2 months of high‐fat feeding and when given for only the second month. Consequently, FF‐treated rats had a final body weight and white adipose tissue mass similar to untreated animals on the standard diet. FF also increased resting metabolic rate, hepatic peroxisomal and mitochondrial palmitoyl‐dependent oxygen uptake and mRNA levels of acyl‐CoA oxidase and lipoprotein lipase. Finally, FF lowered mRNA levels of uncoupling protein‐2 and did not affect mitochondrial respiration in skeletal muscle. Therefore, FF seems to act as a weight‐stabilizer mainly through its effect on liver metabolism.

How the thyroid controls metabolism in the rat: different roles for triiodothyronine and diiodothyronines

1 Although the first evidence of a relationship between the thyroid and metabolism was reported in 1895, the mechanism by which thyroid hormones influence resting metabolic rate in whole animals is still poorly understood. This paper reports an attempt to test whether diiodothyronines (T2s) and triiodothyronine (T3) have different roles in the control of resting metabolism (RM). 2 Changes in resting metabolic rate were measured in hypothyroid rats treated acutely (25μg (100 g body weight)−1) either with one of the T2s or with T3. Injection of T3 induced an increase of about 35% in RM that started 25–30 h after the injection and lasted until 5–6 days after the injection, the maximal value being observed at 50–75 h. The injection of T2s evoked a temporally different pattern of response. The increases in RM started 6–12 h after the injection, had almost disappeared after 48 h, and the maximal stimulation was observed at 28–30 h. 3 When actinomycin D (an inhibitor of protein synthesis) and T3 were given together, the stimulation of RM was almost completely abolished. The simultaneous injection of actinomycin D and either of the T2s, on the other hand, did not cause any attenuation of the stimulation seen with the T2s alone. 4 Following chronic treatment (3 weeks) with either T3 or T2s there was a stimulation of organ growth only after the administration of T3. 5 Chronic administration of either T2s or T3 to hypothyroid rats significantly enhanced the oxidative capacity of each of the tissues considered. In the case of T2s the stimulation was almost the same whether it was expressed as an increase in specific activity or total tissue activity. In the case of T3 the increases were, in the main, secondary to the hypertrophic or hyper plastic effect. 6 These results indicate that T2s and T3 exert different effects on RM. The effects of T2s are rapid and possibly mediated by their direct interaction with mitochondria. Those of T3 are slower and more prolonged, and at least partly attributable to a modulation of the cellularity of tissues that are metabolically very active.

Expression of uncoupling protein‐3 and mitochondrial activity in the transition from hypothyroid to hyperthyroid state in rat skeletal muscle

We sought a correlation between rat skeletal muscle triiodothyronine (T3)‐mediated regulation of uncoupling protein‐3 (UCP3) expression and mitochondrial activity. UCP3 mRNA expression increased strongly during the hypothyroid‐hyperthyroid transition. The rank order of mitochondrial State 3 and State 4 respiration rates was hypothyroid

3,5-Diiodo-L-thyronine powerfully reduces adiposity in rats by increasing the burning of fats

The effect of thyroid hormones on metabolism has long supported their potential as drugs to stimulate fat reduction, but the concomitant induction of a thyrotoxic state has greatly limited their use. Recent evidence suggests that 3,5‐diiodo‐L‐thyronine (T2), a naturally occurring iodothyronine, stimulates metabolic rate via mechanisms involving the mitochondrial apparatus. We examined whether this effect would result in reduced energy storage. Here, we show that T2 administration to rats receiving a high‐fat diet (HFD) reduces both adiposity and body weight gain without inducing thyrotoxicity. Rats receiving HFD + T2 showed (when compared with rats receiving HFD alone) a 13% lower body weight, a 42% higher liver fatty acid oxidation rate, ∼50% less fat mass, a complete disappearance of fat from the liver, and significant reductions in the serum triglyceride and cholesterol levels (−52% and −18%, respectively). Thyroid hormones and thyroid‐stimulating hormone (TSH) serum levels were not influenced by T2 administration. The biochemical mechanism underlying the effects of T2 on liver metabolism involves the carnitine palmitoyl‐transferase system and mitochondrial uncoupling. If the results hold true for humans, pharmacological administration of T2 might serve to counteract the problems associated with overweight, such as accumulation of lipids in liver and serum, without inducing thyrotoxicity. However, the results reported here do not exclude deleterious effects of T2 on a longer time scale as well as do not show that T2 acts in the same way in humans.

Induction of UCP2 mRNA by thyroid hormones in rat heart

The possible regulation of the expression of uncoupling protein‐2 (UCP2) mRNA by thyroid hormones in different tissues was examined in rats. Triiodothyronine (T3) was found to produce an organ‐specific enhancement of UCP2 expression in rat tissues. The effect of T3 was markedly observed in heart, whereas a moderate effect was seen in skeletal muscle and no effect in kidney or liver. These results suggest that UCP2 is a protein that may be involved in the nuclear‐mediated effect of T3 on resting metabolic rate in the rat.

Sequential changes in the signal transduction responses of skeletal muscle following food deprivation

Coping with reduced energy sources entails drastic morphological and functional changes in skeletal muscle, but the sequence of events required classification. We found that gastrocnemius muscle from food‐deprived rats shows acute rises in peroxisome proliferator activated receptor (PPAR) γ coactivator (PGC) −1α/PPAR δ nuclear protein and myosin heavy chain (MHC) Ib protein, while type I fibers accumulate and the muscle tissue appears redder. AMP levels, phosphorylation of both AMP‐activated protein kinase (AMPK) and its downstream target acetyl coenzyme A carboxylase (ACC) are induced within 6 h. Rapidly increased MyoD mRNA levels are followed by an increase in uncoupling protein (UCP) 3 (UCP3) transcription. Increased serum fatty acid levels coincide with increases in mitochondrial UCP3 protein levels and fatty acid oxidation. Accompanying this is a decrease in AMPK phophorylation, reversible upon nicotinic acid treatment, indicating that fatty acids may modulate this kinases activity after the metabolic challenges posed by food deprivation.—de Lange, P., Farina, P., Moreno, M., Ragni, M., Lombardi, A., Silvestri, E., Burrone, L., Lanni, A., Goglia, F. Sequential changes in the signal transduction responses of skeletal muscle following food deprivation. FASEB J. 20, E2015–E2025 (2006)

3,5-diiodo-l-thyronine, by modulating mitochondrial functions, reverses hepatic fat accumulation in rats fed a high-fat diet.

BACKGROUND/AIMS Mitochondrial dysfunction is central to the physiopathology of steatosis and/or non-alcoholic fatty liver disease. In this study on rats we investigated whether 3,5-diiodo-l-thyronine (T2), a biologically active iodothyronine, acting at mitochondrial level is able to reverse hepatic steatosis after its induction through a high-fat diet. METHODS Hepatic steatosis was induced by long-term high-fat feeding of rats for six weeks which were then fed the same high-fat diet for the next 4 weeks and were simultaneously treated or not treated with T2. Histological analyses were performed on liver sections (by staining with Sudan black B). In liver mitochondria fatty acid oxidation rate, mitochondrial efficiency (by measuring proton conductance) and mitochondrial oxidative stress (by measuring H(2)O(2) release, aconitase and SOD activity) were detected. RESULTS Stained sections showed that T2 treatment reduced hepatic fatty accumulation induced by a high-fat diet. At the mitochondrial level, the fatty acid oxidation rate and carnitine palmitoyl transferase activity were enhanced by T2 treatment. Moreover, by stimulating mitochondrial uncoupling, T2 caused less efficient utilization of fatty acid substrates and ameliorated mitochondrial oxidative stress. CONCLUSION These data demonstrate that T2, by activating mitochondrial processes, markedly reverses hepatic steatosis in vivo.