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.

3,5-Diiodo-l-thyronine (T2) powerfully reduces adiposity in rats fed a high-fat diet (HFD), stimulating (in the liver) fatty acid oxidation and mitochondrial uncoupling, and strongly counteracting steatosis, a condition commonly associated with diet-induced obesity. Proteomics offer unique possibilities for the investigation of changes in the levels and modifications of proteins. Here, combining 2D-E, mass spectrometry, and blue native (BN) PAGE, we studied how the subcellular hepatic phenotype responds to HFD and T2-treatment. By identifying differentially expressed proteins and analyzing their interrelation [using the Ingenuity Pathway Analysis (IPA) platform], we obtained an integrated view of the phenotypic/metabolic adaptations occurring in the liver proteome during HFD with or without T2-treatment. Interestingly, T2 counteracted several HFD-induced changes, mostly in mitochondria. BN-PAGE and subsequent in-gel activity analysis of OXPHOS complexes revealed a modified profile of individual complexes in HFD mitochondria vs. normal ones. This pattern was re-normalized in mitochondria from T2-treated HFD animals. These data indicate that in HFD rats, the effects of T2 on the liver proteome cause it to resemble that associated with a non-steatotic condition. The identified metabolic pathways (mainly at the mitochondrial level) may be responsible for the beneficial effects of T2 on liver adiposity and metabolism.

Fatty liver or nonalcoholic fatty liver disease (NAFLD), a problem of increasing clinical significance and prevalence worldwide, is associated with increased risk for the development of cirrhosis and hepatocellular carcinoma. Although several therapeutic approaches can be used in the context of NAFLD, dietary and physical activities are still the most frequently used strategies. Some pharmacological agents show promising results although no conclusions can be drawn from recent clinical trials. Thyroid hormones [THs; thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3)] coordinate a diverse array of physiological events during development and lipid/energy homeostasis and have some potentially therapeutic actions which include inducing weight loss, and lowering plasma cholesterol levels and tissue adiposity. The thyroid hormones exert their physiological effects by binding to specific nuclear receptors [thyroid hormone receptors (TR)] of which the TRβ isoform is liver specific and has been considered a putative target for the treatment of dyslipidemia and fatty liver. In view of this, the aim of the review is (1) to provide an overview of the action of T3 on lipid metabolism with implications for liver steatosis and (2) to provide an update on the current knowledge concerning the administration of TRβ selective thyromimetics (GC-1 and MB07811), as well as of 3,5-diiodo-L-thyronine and its novel functional analogue TRC150094 in animal models of overweight and related disorders including primarily fatty liver.

A novel functional iodothyronine analogue, TRC150094, which has a much lower potency toward thyroid hormone receptor (α1/β1) activation than triiodothyronine, has been shown to be effective at reducing adiposity in rats simultaneously receiving a high-fat diet (HFD). Here, by combining metabolic, functional and proteomic analysis, we studied how the hepatic and skeletal muscle phenotypes might respond to TRC150094 treatment in HFD-fed overweight rats. Drug treatment increased both the liver and skeletal muscle mitochondrial oxidative capacities without altering mitochondrial efficiency. Coherently, in terms of individual respiratory in-gel activity, blue-native analysis revealed an increased activity of complex V in the liver and of complexes II and V in tibialis muscle in TCR150094-treated animals. Subsequently, the identification of differentially expressed proteins and the analysis of their interrelations gave an integrated view of the phenotypic/metabolic adaptations occurring in the liver and muscle proteomes during drug treatment. TRC150094 significantly altered the expression of several proteins involved in key liver metabolic pathways, including amino acid and nitrogen metabolism, and fructose and mannose metabolism. The canonical pathways most strongly influenced by TRC150094 in tibialis muscle included glycolysis and gluconeogenesis, amino acid, fructose and mannose metabolism, and cell signaling. The phenotypic/metabolic influence of TRC150094 on the liver and skeletal muscle of HFD-fed overweight rats suggests the potential clinical application of this iodothyronine analogue in ameliorating metabolic risk parameters altered by diet regimens.

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