Noncanonical thyroid hormone signaling mediates cardiometabolic effects in vivo.

Thyroid hormone (TH) and TH receptors (TRs) α and ß act by binding to TH response elements (TREs) in regulatory regions of target genes. This nuclear signaling is established as the canonical or type 1 pathway for TH action. Nevertheless, TRs also rapidly activate intracellular second-messenger signaling pathways independently of gene expression (noncanonical or type 3 TR signaling). To test the physiological relevance of noncanonical TR signaling, we generated knockin mice with a mutation in the TR DNA-binding domain that abrogates binding to DNA and leads to complete loss of canonical TH action. We show that several important physiological TH effects are preserved despite the disruption of DNA binding of TRα and TRß, most notably heart rate, body temperature, blood glucose, and triglyceride concentration, all of which were regulated by noncanonical TR signaling. Additionally, we confirm that TRE-binding-defective TRß leads to disruption of the hypothalamic-pituitary-thyroid axis with resistance to TH, while mutation of TRα causes a severe delay in skeletal development, thus demonstrating tissue- and TR isoform-specific canonical signaling. These findings provide in vivo evidence that noncanonical TR signaling exerts physiologically important cardiometabolic effects that are distinct from canonical actions. These data challenge the current paradigm that in vivo physiological TH action is mediated exclusively via regulation of gene transcription at the nuclear level.

Noncanonical Thyroid Hormone Receptor α Action Mediates Arterial Vasodilation.

CONTEXT: Hypothyroidism impairs cardiovascular health and contributes to endothelial dysfunction with reduced vasodilation. How 3,5,3'-triiodothyronine (T3) and its receptors are involved in the regulation of vasomotion is not yet fully understood. In general, thyroid hormone receptors (TRs) either influence gene expression (canonical action) or rapidly activate intracellular signaling pathways (noncanonical action). OBJECTIVE: Here we aimed to characterize the T3 action underlying the mechanism of arterial vasodilation and blood pressure (BP) regulation. METHODS: Mesenteric arteries were isolated from male rats, wild-type (WT) mice, TRα knockout (TRα 0) mice, and from knockin mice with a mutation in the DNA-binding domain (TRα GS). In this mutant, DNA binding and thus canonical action is abrogated while noncanonical signaling is preserved. In a wire myograph system, the isolated vessels were preconstricted with norepinephrine. The response to T3 was measured, and the resulting vasodilation (Δ force [mN]) was normalized to maximum contraction with norepinephrine and expressed as percentage vasodilation after maximal preconstriction with norepinephrine (%NE). Isolated vessels were treated with T3 (1 × 10-15 to 1 × 10-5 mol/L) alone and in combination with the endothelial nitric oxide-synthase (eNOS) inhibitor L-NG-nitroarginine methyl ester (L-NAME) or the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin. The endothelium was removed to determine the contribution of T3 to endothelium-dependent vasodilation. The physiological relevance of T3-induced vasodilation was determined by in vivo arterial BP measurements in male and female mice. RESULTS: T3 treatment induced vasodilation of mesenteric arteries from WT mice within 2 minutes (by 21.5 ±â€…1.7%NE). This effect was absent in arteries from TRα 0 mice (by 5.3 ±â€…0.6%NE, P < .001 vs WT) but preserved in TRα GS arteries (by 17.2 ±â€…1.1%NE, not significant vs WT). Inhibition of either eNOS or PI3K reduced T3-mediated vasodilation from 52.7 ±â€…4.5%NE to 28.5 ±â€…4.1%NE and 22.7 ±â€…2.9%NE, respectively. Removal of the endothelium abolished the T3-mediated vasodilation in rat mesenteric arteries (by 36.7 ±â€…5.4%NE vs 3.5 ±â€…6.2%NE). In vivo, T3 injection led to a rapid decrease of arterial BP in WT (by 13.9 ±â€…1.9 mm Hg) and TRα GS mice (by 12.4 ±â€…1.9 mm Hg), but not in TRα 0 mice (by 4.1 ±â€…1.9 mm Hg). CONCLUSION: These results demonstrate that T3 acting through noncanonical TRα action affects cardiovascular physiology by inducing endothelium-dependent vasodilation within minutes via PI3K and eNOS activation.

Noncanonical Action of Thyroid Hormone Receptors α and ß.

Thyroid hormone (TH) is essential for the regulation of many physiological processes, especially growth, organ development, energy metabolism and cardiovascular effects. TH acts via the TH receptors (TR) α and ß. By binding to thyroid hormone responsive elements (TREs) on the DNA, TRs regulate expression of TH target genes. Thus, TRs are mainly characterized as ligand dependent transcription factors and regulation of gene expression and protein synthesis is considered the canonical mode of TH/TR action. The demonstration that the ligand-bound TRs α and ß also mediate activation of the phosphatidylinositol-3-kinase (PI3K) pathway established noncanonical TH/TR action as an additional mode of TH signaling. Recently, TR mutant mouse models allowed to determine the underlying mode of TH/TR action, either canonical or noncanonical TH/TR signaling, for several physiological TH effects in vivo: Regulation of the hypothalamic-pituitary-thyroid axis requires DNA-binding of TRß, whereas hepatic triglyceride content appears to be regulated by noncanonical TRß signaling. TRα mediated effects in bone development are dependent on DNA-binding, whereas several cardiovascular TRα effects are rapid and independent from DNA-binding. Therefore, noncanonical TH/TR action contributes to the overall effects of TH in physiology.

Fetal Exposure to High Maternal Thyroid Hormone Levels Causes Central Resistance to Thyroid Hormone in Adult Humans and Mice.

Context: Fetuses exposed to the high thyroid hormone (TH) levels of mothers with resistance to thyroid hormone beta (RTH-ß), due to mutations in the THRB gene, have low birth weight and suppressed TSH. Objective: Determine if such exposure to high TH levels in embryonic life has a long-term effect into adulthood. Design: Observations in humans with a parallel design on animals to obtain a preliminary information regarding mechanism. Setting: University research centers. Patients or other participants: Humans and mice with no RTH-ß exposed during intrauterine life to high TH levels from mothers who were euthyroid due to RTH-ß. Controls were humans and mice of the same genotype but born to fathers with RTH-ß and mothers without RTH-ß and thus, with normal serum TH levels. Interventions: TSH responses to stimulation with thyrotropin-releasing hormone (TRH) during adult life in humans and male mice before and after treatment with triiodothyronine (T3). We also measured gene expression in anterior pituitaries, hypothalami, and cerebral cortices of mice. Results: Adult humans and mice without RTH-ß, exposed to high maternal TH in utero, showed persistent central resistance to TH, as evidenced by reduced responses of serum TSH to TRH when treated with T3. In mice, anterior pituitary TSH-ß and deiodinase 3 (D3) mRNAs, but not hypothalamic and cerebral cortex D3, were increased. Conclusions: Adult humans and mice without RTH-ß exposed in utero to high maternal TH levels have persistent central resistance to TH. This is likely mediated by the increased expression of D3 in the anterior pituitary, enhancing local T3 degradation.