Pieter de Lange

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)

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.

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.

Nonthyrotoxic Prevention of Diet-Induced Insulin Resistance by 3,5-Diiodo-L-Thyronine in Rats

OBJECTIVE High-fat diets (HFDs) are known to induce insulin resistance. Previously, we showed that 3,5-diiodothyronine (T2), concomitantly administered to rats on a 4-week HFD, prevented gain in body weight and adipose mass. Here we investigated whether and how T2 prevented HFD-induced insulin resistance. RESEARCH DESIGN AND METHODS We investigated the biochemical targets of T2 related to lipid and glucose homeostasis over time using various techniques, including genomic and proteomic profiling, immunoblotting, transient transfection, and enzyme activity analysis. RESULTS Here we show that, in rats, HFD feeding induced insulin resistance (as expected), whereas T2 administration prevented its onset. T2 did so by rapidly stimulating hepatic fatty acid oxidation, decreasing hepatic triglyceride levels, and improving the serum lipid profile, while at the same time sparing skeletal muscle from fat accumulation. At the mechanistic level, 1) transfection studies show that T2 does not act via thyroid hormone receptor β; 2) AMP-activated protein kinase is not involved in triggering the effects of T2; 3) in HFD rats, T2 rapidly increases hepatic nuclear sirtuin 1 (SIRT1) activity; 4) in an in vitro assay, T2 directly activates SIRT1; and 5) the SIRT1 targets peroxisome proliferator–activated receptor (PPAR)-γ coactivator (PGC-1α) and sterol regulatory element–binding protein (SREBP)-1c are deacetylated with concomitant upregulation of genes involved in mitochondrial biogenesis and downregulation of lipogenic genes, and PPARα/δ-induced genes are upregulated, whereas genes involved in hepatic gluconeogenesis are downregulated. Proteomic analysis of the hepatic protein profile supported these changes. CONCLUSIONS T2, by activating SIRT1, triggers a cascade of events resulting in improvement of the serum lipid profile, prevention of fat accumulation, and, finally, prevention of diet-induced insulin resistance.

Combined cDNA array/RT-PCR analysis of gene expression profile in rat gastrocnemius muscle: relation to its adaptive function in energy metabolism during fasting

We evaluated the effects of fasting on the gene expression profile in rat gastrocnemius muscle using a combined cDNA array and RT‐PCR approach. Of the 1176 distinct rat genes analyzed on the cDNA array, 114 were up‐regulated more than twofold in response to fasting, including all 17 genes related to lipid metabolism present on the membranes and all 10 analyzed components of the proteasome machinery. Only 7 genes were down‐regulated more than twofold. On the basis of our analysis of genes on the cDNA array plus the data from our RT‐PCR assays, the metabolic adaptations shown by rat gastrocnemius muscle during fasting are reflected by i) increased transcription both of myosin heavy chain (MHC) Ib (associated with type I fibers) and of at least three factors involved in the shift toward type I fibers [p27kip1, muscle LIM protein (MLP), cystein rich protein‐2], of which one (MLP) has been shown to enhance the activity of MyoD, which would explain the known increase in the expression of skeletal muscle uncoupling protein‐3 (UCP3); ii) increased lipoprotein lipase (LPL) expression, known to trigger UCP3 transcription, which tends, together with the first point, to underline the suggested role of UCP3 in mitochondrial lipid handling (the variations under the first point and this one have not been observed in mice, indicating a species‐specific regulation of these mechanisms); iii) reduced expression of the muscle‐specific coenzyme Q (CoQ)7 gene, which is necessary for mitochondrial CoQ synthesis, together with an increased expression of mitochondrial adenylate kinase 3, which inactivates the resident key enzyme for CoQ synthesis, 3‐hydroxy‐3‐methylglutaryl CoA reductase (HMGR), the mRNA level for which fell during fasting; and iv) increased transcription of components of the proteasomal pathways involved in protein degradation/turnover.

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.—Moreno, M., Silvestri, E., De Matteis, R., de Lange, P., Lombardi, A., Glinni, D., Senese, R., Cioffi, F., Salzano, A. M., Scaloni, A., Lanni, A., Goglia, F. 3,5‐Diiodo‐L‐thyronine prevents high‐fat diet‐induced insulin resistance in rat skeletal muscle through metabolic and structural adaptations. FASEB J. 25, 3312–3324 (2011). www.fasebj.org

Uncoupling proteins: A complex journey to function discovery

Since their discovery, uncoupling proteins have aroused great interest due to the crucial importance of energy‐dissipating system for cellular physiology. The uncoupling effect and the physiological role of UCP1 (the first‐described uncoupling protein) are well established. However, the reactions catalyzed by UCP1 homologues (UCPs), and their physiological roles are still under debate, with the literature containing contrasting results. Current hypothesis propose several physiological functions for novel UCPs, such as: (i) attenuation of reactive oxygen species production and protection against oxidative damage, (ii) thermogenic function, although UCPs do not generally seem to affect thermogenesis, UCP3 can be thermogenic under certain conditions, (iii) involvement in fatty acid handling and/or transport, although recent experimental evidence argues against the previously hypothesized role for UCPs in the export of fatty acid anions, (iv) fatty acid hydroperoxide export, although this function, due to the paucity of the experimental evidence, remains hypothetical, (v) Ca2+ uptake, although results for and against a role in Ca2+ uptake are still emerging, (vi) a signaling role in pancreatic beta cells, where it attenuates glucose‐induced insulin secretion. From the above, it is evident that more research will be needed to establish universally accepted functions for UCPs.

Fasting, lipid metabolism, and triiodothyronine in rat gastrocnemius muscle: interrelated roles of uncoupling protein 3, mitochondrial thioesterase, and coenzyme Q

We investigated the role of uncoupling protein 3 (UCP3) during fasting and examined the effect of triiodothyronine (T3) administration in such a condition. The possible involvement of mitochondrial thioesterase (MTE I) and the role of putative cofactors, such as coenzyme Q (CoQ), was also examined. Here, we report that fasting induced a more than twofold elevation in the expression and activity of MTE I, and an increase in UCP3 expression, without any associated uncoupling activity. Administration of T3 to fasting rats further up‐regulated UCP3 as well as MTE I expression, markedly enhanced MTE I enzyme activity and prevented the impairment of the uncoupling activity of UCP3 normally seen during fasting. Indeed, T3‐treatment induced an UCP3‐dependent decrease in mitochondrial membrane potential, which was abolished by the addition of either GDP or superoxide dismutase (SOD). T3 administration also prevented the marked decrease of CoQ levels observed in fasting rats and this provides evidence that also, in vivo, CoQ represents an essential cofactor for the UCP3‐mediated uncoupling. The data also show that MTE I and UCP3 are likely involved in the same biochemical mechanism and that UCP3 postulated functions, such as lipid handling and uncoupling, are not mutually exclusive but may coexist in vivo.

UCP3 Translocates Lipid Hydroperoxide and Mediates Lipid Hydroperoxide-dependent Mitochondrial Uncoupling

Although the literature contains many studies on the function of UCP3, its role is still being debated. It has been hypothesized that UCP3 may mediate lipid hydroperoxide (LOOH) translocation across the mitochondrial inner membrane (MIM), thus protecting the mitochondrial matrix from this very aggressive molecule. However, no experiments on mitochondria have provided evidence in support of this hypothesis. Here, using mitochondria isolated from UCP3-null mice and their wild-type littermates, we demonstrate the following. (i) In the absence of free fatty acids, proton conductance did not differ between wild-type and UCP3-null mitochondria. Addition of arachidonic acid (AA) to such mitochondria induced an increase in proton conductance, with wild-type mitochondria showing greater enhancement. In wild-type mitochondria, the uncoupling effect of AA was significantly reduced both when the release of O2̇̄ in the matrix was inhibited and when the formation of LOOH was inhibited. In UCP3-null mitochondria, however, the uncoupling effect of AA was independent of the above mechanisms. (ii) In the presence of AA, wild-type mitochondria released significantly more LOOH compared with UCP3-null mitochondria. This difference was abolished both when UCP3 was inhibited by GDP and under a condition in which there was reduced LOOH formation on the matrix side of the MIM. These data demonstrate that UCP3 is involved both in mediating the translocation of LOOH across the MIM and in LOOH-dependent mitochondrial uncoupling.