Effect of thyroid status on catecholamine stimulation of thyroxine 5'-deiodinase in brown adipocytes

Coupling between Nutrient Availability and Thyroid Hormone Activation

The activity of the thyroid gland is stimulated by food availability via leptin-induced thyrotropin-releasing hormone/thyroid-stimulating hormone expression. Here we show that food availability also stimulates thyroid hormone activation by accelerating the conversion of thyroxine to triiodothyronine via type 2 deiodinase in mouse skeletal muscle and in a cell model transitioning from 0.1 to 10% FBS. The underlying mechanism is transcriptional derepression of DIO2 through the mTORC2 pathway as defined in rictor knockdown cells. In cells kept in 0.1% FBS, there is DIO2 inhibition via FOXO1 binding to the DIO2 promoter. Repression of DIO2 by FOXO1 was confirmed using its specific inhibitor AS1842856 or adenoviral infection of constitutively active FOXO1. ChIP studies indicate that 4 h after 10% FBS-containing medium, FOXO1 binding markedly decreases, and the DIO2 promoter is activated. Studies in the insulin receptor FOXO1 KO mouse indicate that insulin is a key signaling molecule in this process. We conclude that FOXO1 represses DIO2 during fasting and that derepression occurs via nutritional activation of the PI3K-mTORC2-Akt pathway.

Uncoupling protein-3 as a molecular determinant of the action of 3,5,3'-triiodothyronine on energy metabolism

Thyroid hormones are known to stimulate thermogenesis in rodents by exerting a permissive effect on norepinephrine that affects uncoupling protein-1 (UCP1) expression in brown adipose tissue (BAT). The aim of this study was to identify new targets of the thermogenic effects of T3 in tissues other than the BAT, such as skeletal muscle. In beta(1)/beta(2)/beta(3)-adrenoceptor knockout (beta-less) mice, that are dramatically cold intolerant, a normal body temperature was maintained throughout 48 h of cold exposure by T3 administration. In these mice, BAT UCP1 protein expression was not modified either by cold exposure or by T3 administration. To test the possibility that T3 might act via muscle uncoupling protein-3 (UCP3), an UCP3 knockout (KO) model was used. This model exhibited a normal phenotype except that, upon T3 administration, stimulated oxygen consumption of the UCP3KO mice was significantly lower by 6% than that of the wild-type (WT) mice. This difference was observed only during the dark period (between 7.00 p.m. and 7.00 a.m.), i.e. when the mice are the most active at consuming food. Therefore, UCP3 might participate in the correction by T3 of the dramatic cold intolerance of the beta-less mice. These results reactivate the idea that UCP3 might play a role in the control of energy balance.

Type 2 iodothyronine deiodinase is highly expressed in human thyroid

Type 2 iodothyronine deiodinase (D2) is a recently cloned selenodeiodinase thought to provide intracellular 3,5,3' triiodothyronine (T3) to a restricted group of tissues. We report here the presence of D2 mRNA in human thyroid at levels 50-150-fold higher than in placenta. Surprisingly, while type 1 deiodinase (D1) is known to be present in human thyroid, D2 has not been evaluated previously. D2 mRNA was especially high in thyroids from Graves' patients and in follicular adenomas. Stimulated thyroids had higher D2 to D1 mRNA ratios than normal or multinodular glands suggesting differential regulation of D1 and D2 expression. Microsomes from normal, Graves', and TSH-stimulated thyroids contained low Km D2 activity resistant to propylthiouracil (1 mM) or to inactivation by N-bromoacetyl T3, agents which block or inactivate D1. At 2 nM thyroxine (T4), 100 times the physiological-free T4 levels, 60-80% of T4 to T3 conversion in stimulated, but only 27% of that in normal thyroids, is catalyzed by D2. We conclude that intrathyroidal T4 to T3 conversion by D2 may contribute significantly to the relative increase in thyroidal T3 production in patients with Graves' disease, toxic adenomas, and, perhaps, iodine deficiency.

Adrenergic control of induction of type II iodothyronine 5'-deiodinase activity in cultured mouse brown adipocytes

Iodothyronine 5'-deiodinase (5'D) of mouse brown adipocytes differentiated in cell culture was characterized in detail with respect to the adrenergic control of its biosynthesis. The stimulation of 5'D required mRNA and protein synthesis and was dependent on the stage of differentiation of the cells. The maximum induction was observed around confluence (7-day-old cells), in pre- and post-confluent cells the 5'D activity was significantly less induced. The transient responsiveness of brown fat-cells to the stimulatory effect of adrenergic agents was reflected also in the time course of the induction of 5'D by different concentrations of agonists. The maximum response occurred regularly after an 8 h incubation and implicated a rather fast turnover of the induced enzyme. On the basis of the inhibitory effects of cycloheximide and actinomycin D, the half-life of the induced 5'D and its mRNA were estimated to be 1.5 and 3.3 h respectively. The noradrenaline-induced 5'D activity was shown to be that of the type II enzyme, insensitive to propylthiouracil (PTU). The estimated values of its apparent Km for thyroxine, Km for the co-substrate dithiothreitol, and Vmax. in the presence of 1 mM PTU were 2 nM, 2.6 mM, and 0.1 pmol of I-/h per mg of protein respectively. The 5'D activity was effectively induced by forskolin and dibutyryl cyclic AMP, as well as by isoprenaline, noradrenaline and CGP-12177, but not by phenylephrine, cirazoline or oxymetazoline. This indicates that, contrary to previous observations in vivo, stimulation of 5'D in cultured brown fat-cells involves elevated cyclic AMP levels and is mediated predominantly via beta-receptors, particularly via the so-called beta 3-adrenoceptors.