Liraglutide Activates Type 2 Deiodinase and Enhances ß3-Adrenergic-Induced Thermogenesis in Mouse Adipose Tissue.

Aims: Liraglutide is a long-acting glucagon-like peptide 1 (GLP-1) receptor agonist used as an anti-hyperglycemic agent in type 2 diabetes treatment and recently approved for obesity management. Weight loss is attributed to appetite suppression, but therapy may also increase energy expenditure. To further investigate the effect of GLP-1 signaling in thermogenic fat, we assessed adipose tissue oxygen consumption and type 2 deiodinase (D2) activity in mice treated with liraglutide, both basally and after ß3-adrenergic treatment. Methods: Male C57BL/6J mice were randomly assigned to receive liraglutide (400 µg/kg, n=12) or vehicle (n=12). After 16 days, mice in each group were co-treated with the selective ß3-adrenergic agonist CL316,243 (1 mg/kg, n=6) or vehicle (n=6) for 5 days. Adipose tissue depots were assessed for gene and protein expression, oxygen consumption, and D2 activity. Results: Liraglutide increased interscapular brown adipose tissue (iBAT) oxygen consumption and enhanced ß3-adrenergic-induced oxygen consumption in iBAT and inguinal white adipose tissue (ingWAT). These effects were accompanied by upregulation of UCP-1 protein levels in iBAT and ingWAT. Notably, liraglutide increased D2 activity without significantly upregulating its mRNA levels in iBAT and exhibited additive effects to ß3-adrenergic stimulation in inducing D2 activity in ingWAT. Conclusions: Liraglutide exhibits additive effects to those of ß3-adrenergic stimulation in thermogenic fat and increases D2 activity in BAT, implying that it may activate this adipose tissue depot by increasing intracellular thyroid activation, adding to the currently known mechanisms of GLP-1A-induced weight loss.

Regulation of Dio2 gene expression by thyroid hormones in normal and type 1 deiodinase-deficient C3H mice.

The C3H/HeJ mouse presents an inherited type 1 deiodinase (D1) deficiency that results in elevated serum thyroxine (T(4)), whereas TSH and tri-iodothyronine (T(3)) concentrations are normal when compared with those in the C57BL/6J strain. Here, we evaluated the expression of the type 2 (D2), the other T(4)-activating enzyme, in C3H mice. A comparative analysis revealed that D2 mRNA levels in C3H are similar to those in C57 animals. The D2 activity in C3H pituitary and brain are reduced when compared with those in the C57 strain (3.75 +/- 1.08 vs 5.78 +/- 0.33 and 0.17 +/- 0.05 vs 0.26 +/- 0.07 fmol/min per mg protein respectively). However, no differences on D2 activity levels were observed in the brown adipose tissue (BAT) between both strains (0.34 +/- 0.06 vs 0.36 +/- 0.09 fmol/min per mg protein). Experiments using different T(4) doses showed that higher levels of serum T(4) than those of the C3H mouse are required to downregulate D2 activity in this tissue. T(3) administration to euthyroid mice resulted in a two- to four-fold increase on D2 activity in BAT and brain of both strains, despite a marked decrease in BAT D2 transcripts and no changes in brain D2 mRNA levels. The increase in D2 activity was preceded by a decrease in serum T(4) levels, which appears to reduce D2 degradation. Indeed, administration of T(3) plus T(4) abolished the T(3)-induced D2 upregulation. In conclusion, our results demonstrated that D2 activity is mainly regulated at posttranslational level in a tissue-specific manner. These observations further characterize and provide insights into the complex and dual regulatory role of the iodothyronines in D2 regulation.

Dissecting thyroid hormone transport and metabolism in dendritic cells.

We reported thyroid hormone (TH) receptor expression in murine dendritic cells (DCs) and 3,5,3'-triiodothyronine (T3)-dependent stimulation of DC maturation and ability to develop a Th1-type adaptive response. Moreover, an increased DC capacity to promote antigen-specific cytotoxic T-cell activity, exploited in a DC-based antitumor vaccination protocol, was revealed. However, putative effects of the main circulating TH, l-thyroxine (T4) and the mechanisms of TH transport and metabolism at DC level, crucial events for TH action at target cell level, were not known. Herein, we show that T4 did not reproduce those registered T3-dependent effects, finding that may reflect a homoeostatic control to prevent unspecific systemic activation of DCs. Besides, DCs express MCT10 and LAT2 TH transporters, and these cells mainly transport T3 with a favored involvement of MCT10 as its inhibition almost prevented T3 saturable uptake mechanism and reduced T3-induced IL-12 production. In turn, DCs express iodothyronine deiodonases type 2 and 3 (D2, D3) and exhibit both enzymatic activities with a prevalence towards TH inactivation. Moreover, T3 increased MCT10 and LAT2 expression and T3 efflux from DCs but not T3 uptake, whereas it induced a robust induction of D3 with a parallel slight reduction in D2. These findings disclose pivotal events involved in the mechanism of action of THs on DCs, providing valuable tools for manipulating the immunogenic potential of these cells. Furthermore, they broaden the knowledge of the TH mechanism of action at the immune system network.

The rs225017 polymorphism in the 3'UTR of the human DIO2 gene is associated with increased insulin resistance.

The Thr92Ala (rs225014) polymorphism in the type 2 deiodinase (DIO2) gene has been associated with insulin resistance (IR) and decreased enzyme activity in human tissues but kinetic studies failed to detect changes in the mutant enzyme, suggesting that this variant might be a marker of abnormal DIO2 expression. Thus, we aimed to investigate whether other DIO2 polymorphisms, individually or in combination with the Thr92Ala, may contribute to IR. The entire coding-region of DIO2 gene was sequenced in 12 patients with type 2 diabetes mellitus (T2DM). Potentially informative variants were evaluated in 1077 T2DM patients and 516 nondiabetic subjects. IR was evaluated using the homeostasis model assessment (HOMA-IR) index. DIO2 gene sequencing revealed no new mutation but 5 previously described single nucleotide polymorphisms (SNPs). We observed that all T2DM patients displaying high HOMA-IR index (n = 6) were homozygous for the rs225017 (T/A) polymorphism. Further analysis showed that the median fasting plasma insulin and HOMA-IR of T2DM patients carrying the T/T genotype were higher than in patients carrying the A allele (P = 0.013 and P = 0.002, respectively). These associations were magnified in the presence of the Ala92Ala genotype of the Thr92Ala polymorphism. Moreover, the rs225017 and the Thr92Ala polymorphisms were in partial linkage disequilibrium (|D'| = 0.811; r2 = 0.365). In conclusion, the rs225017 polymorphism is associated with greater IR in T2DM and it seems to interact with the Thr92Ala polymorphism in the modulation of IR.

Type 2 deiodinase Thr92Ala polymorphism is associated with disrupted placental activity but not with dysglycemia or adverse gestational outcomes: a genetic association study.

OBJECTIVE: To study whether the D2 Thr92Ala polymorphism-a genetic marker that is associated with reduced thyroid type 2 deiodinase (D2) activity, increased insulin resistance, and risk for type 2 diabetes-is associated with disrupted placental D2 activity and with glycemic control and gestational outcomes. DESIGN: Cross-sectional study. SETTING: Tertiary hospital in Brazil. PATIENT(S): Consecutive singleton-pregnancy patients, 18-45 years old. INTERVENTION(S): Clinical examination and genotyping of the D2 Thr92Ala polymorphism, with placental samples collected and assayed for D2 mRNA and activity. MAIN OUTCOME MEASURE(S): Glucose homeostasis and gestational outcomes. RESULT(S): A total of 294 patients were included in the study. The clinical and laboratory characteristics were similar among the D2 genotypes. No differences were observed in D2 placental mRNA levels, but D2 activity was decreased in patients with the Ala92Ala genotype (0.35 ± 0.15 vs. 1.96 ± 1.02 fmol/mg/min.). Newborn serum thyroid-stimulating hormone levels (TSHneo) did not differ according to maternal D2 Thr92Ala genotype. Also, maternal glucose control, insulin resistance evaluated by the homeostasis model assessment (HOMA-IR), and gestational outcomes did not differ across D2 genotypes. CONCLUSION(S): The D2 Ala92Ala genotype is associated with reduced placental D2 activity but is not associated with dysglycemia, increased insulin resistance, or worse gestational outcomes.