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.
Rosalba Senese, Federica Cioffi, Pieter de Lange, Fernando Goglia and Antonia Lanni
Elena Grasselli, Adriana Voci, Laura Canesi, Fernando Goglia, Silvia Ravera, Isabella Panfoli, Gabriella Gallo and Laura Vergani
Iodothyronines influence lipid metabolism and energy homeostasis. Previous studies demonstrated that 3,5-l-diiodothyronine (T2), as well as 3,3′,5-l-triiodothyronine (T3), was able to both prevent and reverse hepatic steatosis in rats fed a high-fat diet, and this effect depends on a direct action of iodothyronines on the hepatocyte. However, the involvement of thyroid hormone receptors (TRs) in mediating the lipid-lowering effect of iodothyronines was not elucidated. In this study, we investigated the ability of T2 and T3 to reduce the lipid overloading using the rat hepatoma FaO cells defective for functional TRs. The absence of constitutive mRNA expression of both TR α 1 and TR β 1 in FaO cells was verified by RT-qPCR. To mimic the fatty liver condition, FaO cells were treated with a fatty acid mixture and then exposed to pharmacological doses of T2 or T3 for 24 h. Lipid accumulation, mRNA expression of the peroxisome proliferator-activated receptors (PPAR-α, -γ, -δ) the acyl-CoA oxidase (AOX), and the stearoyl CoA desaturase (SCD1), as well as fuel-stimulated O2 consumption in intact cells, were evaluated. Lipid accumulation was associated with an increase in triacylglycerol content, PPAR γ mRNA expression, and a decrease in PPARδ and SCD1 mRNA expression. The addition of T2 or T3 to lipid-overloaded cells resulted in i) reduction in lipid content; ii) downregulation of PPARα, PPARγ, and AOX expression; iii) increase in PPARδ expression; and iv) stimulation of mitochondrial uncoupling. These data demonstrate, for the first time, that in the hepatocyte, the lipid-lowering actions of both T2 and T3 are not mediated by TRs.
Giuseppe Calamita, Maria Moreno, Domenico Ferri, Elena Silvestri, Patrizia Roberti, Luigi Schiavo, Patrizia Gena, Maria Svelto and Fernando Goglia
The recent identification of aquaporin-8 (AQP8), an aquaporin (AQP) channel permeable to water and ammonia, in the inner membrane (IMM) of rat liver mitochondria suggested a role for such AQP in the hydration state and the metabolic function of mitochondria. Since thyroid hormone triiodothyronine (T3) is known to modulate both the shape and the metabolic activities of liver mitochondria, it was interesting to investigate the expression and distribution of AQP8 as well as the osmotic water permeability of the IMM in liver mitochondria from rats in different thyroid states. By semi-quantitative reverse transcriptase (RT)-PCR, when compared with the euthyroid counterpart, the levels of hepatic AQP8 mRNA significantly increased in the hypothyroid state, whereas they were strongly decreased after administration of T3. A similar pattern was seen at the protein level by immunoblotting mitochondrial membranes. The upregulation of mitochondrial AQP8 in the hypothyroid liver was confirmed by immunogold electron microscopy. Stopped-flow light scattering with IMM vesicles showed no significant differences in terms of osmotic water permeability among the IMMs in the various thyroid states. Overall, our data indicate that the T3 modulation of the AQP8 gene is a rapid downregulation of transcription. Modulation of hepatic AQP8 expression may be relevant to the regulation of mitochondrial metabolism by thyroid hormones.
Elena Grasselli, Adriana Voci, Ilaria Demori, Laura Canesi, Rita De Matteis, Fernando Goglia, Antonia Lanni, Gabriella Gallo and Laura Vergani
Recent reports demonstrated that 3,5-diiodo-l-thyronine (T2) 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 T2 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 T2 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 T2 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 T2 and help to define the potential therapeutic role of T2 for preventing or treating liver steatosis.