Physiological Role and Use of Thyroid Hormone Metabolites - Potential Utility in COVID-19 Patients

Thyroxine and triiodothyronine (T3) are classical thyroid hormones and with relatively well-understood actions. In contrast, the physiological role of thyroid hormone metabolites, also circulating in the blood, is less well characterized. These molecules, namely, reverse triiodothyronine, 3,5-diiodothyronine, 3-iodothyronamine, tetraiodoacetic acid and triiodoacetic acid, mediate both agonistic (thyromimetic) and antagonistic actions additional to the effects of the classical thyroid hormones. Here, we provide an overview of the main factors influencing thyroid hormone action, and then go on to describe the main effects of the metabolites and their potential use in medicine. One section addresses thyroid hormone levels in corona virus disease 19 (COVID-19). It appears that i) the more potently-acting molecules T3 and triiodoacetic acid have shorter half-lives than the less potent antagonists 3-iodothyronamine and tetraiodoacetic acid; ii) reverse T3 and 3,5-diiodothyronine may serve as indicators for metabolic dysregulation and disease, and iii) Nanotetrac may be a promising candidate for treating cancer, and resmetirom and VK2809 for steatohepatitis. Further, the use of L-T3 in the treatment of severely ill COVID-19 patients is critically discussed.

Thyroid: biological actions of 'nonclassical' thyroid hormones

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.

Mechanisms of nongenomic actions of thyroid hormone

The nongenomic actions of thyroid hormone require a plasma membrane receptor or nuclear receptors located in cytoplasm. The plasma membrane receptor is located on integrin alphaVbeta3 at the Arg-Gly-Asp recognition site important to the binding by the integrin of extracellular matrix proteins. l-Thyroxine (T(4)) is bound with greater affinity at this site than 3,5,3'-triiodo-l-thyronine (T(3)). Mitogen-activated protein kinase (MAPK; ERK1/2) transduces the hormone signal into complex cellular/nuclear events including angiogenesis and tumor cell proliferation. Acting at the integrin receptor and without cell entry, thyroid hormone can foster ERK1/2-dependent serine phosphorylation of nuclear thyroid hormone receptor-beta1 (TRbeta1) and de-repress the latter. The integrin receptor also mediates actions of the hormone on intracellular protein trafficking and on plasma membrane ion pumps, including the sodium/protein antiporter. Tetraiodothyroacetic (tetrac) is a T(4) analog that inhibits binding of iodothyronines to the integrin receptor and is a probe for the participation of this receptor in cellular actions of the hormone. Tetrac blocks thyroid hormone effects on angiogenesis and cancer cell proliferation. Acting on a truncated form of nuclear TRalpha1 (TRDeltaalpha1) located in cytoplasm, T(4) and 3,3',5'-triiodothyronine (reverse T(3)), but not T(3), cause conversion of soluble actin to fibrous (F) actin that is important to cell motility, e.g., in cells such as glia and neurons. Normal development of the central nervous system requires such motility. TRbeta1 in cytoplasm mediates action of T(3) on expression of certain genes via phosphatidylinositol 3-kinase (PI 3-K) and the protein kinase B/Akt pathway. PI 3-K and, possibly, cytoplasmic TRbeta1 are involved in stimulation by T(3) of insertion of Na,K-ATPase in the plasma membrane and of increase in activity of this pump. Because ambient thyroid hormone levels are constant in the euthyroid intact organism, these nongenomic hormone actions are likely to be contributors to basal rate-setting of transcription of certain genes and of complex cellular events such as angiogenesis and cancer cell proliferation.

Relationships between thyroid status, tissue oxidative metabolism, and muscle differentiation in bovine fetuses

The temporal relationships between thyroid status and differentiation of liver, heart and different skeletal muscles were examined in 42 bovine fetuses from day 110 to day 260 of development using principal component analysis of the data. Plasma concentrations of reverse-triiodothyronine (rT(3)) and thyroxine (T(4)) increased during development from day 110 to day 210 or 260, respectively, whereas concentration of triiodothyronine (T(3)) and hepatic type-1 5'-deiodinase activity (5'D1) increased from day 180 onwards. On day 260, high T(4) and rT(3) and low T(3) concentrations were observed together with a mature 5'D1 activity. Cytochrome-c oxidase (COX) activity expressed per mg protein increased at day 180 in masseter and near birth in masseter, rectus abdominis and cutaneus trunci muscles (P<0.05). Significant changes in citrate synthase (CS) activity per mg protein were observed between day 110 and day 180 in the liver and between day 210 and day 260 in the liver, the heart and the longissimus thoracis muscle (P<0.05). Muscle contractile differentiation was shown by the disappearance of the fetal myosin heavy chain from day 180 onwards. A positive correlation (r>0.47, P<0.01) was shown between thyroid status parameters (5'D1, concentrations of T(4) and T(3)) and COX activity in muscles known to be oxidative after birth (masseter, rectus abdominis) but not in liver and heart, nor in muscles known to be glycolytic after birth (cutaneus trunci, longissimus thoracis). A similar correlation was found between thyroid parameters and CS activity in liver and masseter. Results indicate that elevation of plasma T(3) concentrations in the last gestational trimester could be involved in the differentiation of oxidative skeletal muscles.

Type 1 deiodinase is stimulated by iodothyronines and involved in thyroid hormone metabolism in human somatomammotroph GX cells

BACKGROUND

Local 5'-deiOdination of l-thyroxine (T4) to the active thyroid hormone, 3,3',5-tri-iodothyronine (T3) via two deiodinase isoenzymes (D1 and D2) has an important role for various T3-dependent functions in the anterior pituitary. However, no evidence has been presented yet for thyroid hormone inactivation via the 5-deiodinase (D3) in anterior pituitary models.

METHODS

Using the human somatomammotroph cell line, GX, we analysed effects of T3 and its 5'-deiodination product, 3,5-di-iodothyronine (3,5-T2), on deiodinase activities, measuring release of iodide-125 (125I-) from phenolic-ring- or tyrosyl-ring-labelled substrates respectively.

RESULTS

T3 and 3,5-T2 rapidly stimulated D1 activity in GX cells in the presence of serum in the culture medium, whereas D2 activity was not detectable under these conditions. However, when the cells were kept under serum-free conditions, specific activity of D2 reached levels similar to those of D1. With tyrosyl-ring labelled 3, 5-[125I]-,3'-T3 as substrate, a significant release of 125I- was observed in GX cell homogenates. This is comparable to the D1 activity of liver membranes, which preferentially catalyses 5'-deiodination, but to some extent also 5-deiodination, at the tyrosyl ring.

CONCLUSIONS

D1 activity of human GX cells is increased by T3 and 3,5-T2. Inactivation of T3 in the anterior pituitary might occur by deiodination at the tyrosyl ring via D1, thus terminating the stimulatory thyroid hormone signal in human somatomammotroph cells.

Triiodothyronine (T3) antagonizes adverse effects of high circulating reverse-T3 (rT3) during hemorrhagic shock

To examine whether triiodothyronine (T3) could counteract the lethal effect of exogenous reverse T3 (rT3) in hemorrhagic shock, 21 anesthetized, heparinized mongrel dogs were given 15 micrograms/kg of rT3 IV. Thirty minutes later, the dogs were bled rapidly into a reservoir to achieve and maintain a mean arterial pressure of 40 mm Hg. After 60 minutes at 40 mm Hg (compensated shock), the reservoir line was clamped for 30 minutes (uncompensated shock). The shed blood was then reinfused over 30 minutes, and the dogs were monitored for an additional 60 minutes. At the start of uncompensated shock, 11 dogs were given at least 15 micrograms/kg of T3 IV, and 10 animals received saline. Before T3 treatment, there were no significant intergroup differences in the measured hemodynamic and blood gas variables. In the untreated group, 8 of 10 dogs (80%) died during uncompensated shock, in comparison to 3 of 11 dogs (27%) that received T3 (P less than 0.01). Long-term survival in the T3 group was 5/11 (45%), significantly higher than that (1/10, 10%) in the untreated group (P less than 0.05). These results, interpreted in relationship to previous studies, suggest that the therapeutic efficacy of T3 in canine hemorrhagic shock may be related to antagonism of adverse effects of endogenous rT3.

The role of 3,3',5'-triiodothyronine in the regulation of type II iodothyronine 5'-deiodinase in the rat cerebral cortex

Type II iodothyronine 5'-deiodinase (5'D-II) activity is the source of 75-80% of the cerebral cortex T3 content in euthyroid rats. The activity of this enzyme is increased in hypothyroidism and can be quickly suppressed by T4 and rT3 by mechanisms involving neither protein synthesis nor nuclear T3 receptors. We have examined the possibility that endogenous cerebrocortical rT3 levels play a physiological role in the regulation of this enzyme. Thyroidectomized rats were injected with graded doses of [125I]rT3, and cortex 5'D-II activity and rT3 content were determined at various times thereafter. Enzyme activity was reduced as early as 10 min after the injection of 0.75 microgram rT3/100 g BW, and 18 h after 25 micrograms/100 g BW remained 60% suppressed. Regardless of the time after the injection, 5'D-II activity was inversely related to the rT3 content in the cortex; nearly complete suppression was observed at 0.5 ng rT3/g tissue, 50% at 80 pg/g, and 20-30% at 30 pg/g, the euthyroid level. After the infusion of 0.75 microgram rT3/100 g, maximal inhibition occurred at 10 min, before the rT3 content reached maximum levels, and the 5'D-II activity started to recover after the rT3 level fell below 300 pg/g tissue. After increasing doses of T4 administered to thyroidectomized rats, serum and cerebrocortical T4 concentrations increased in a dose-dependent manner, but the increment in the latter was steeper than that in the former. Serum rT3 increments were also proportional to the dose of T4, but cerebrocortical rT3 increased to a greater extent, as evidenced by a 3-fold increment in the cerebrocortical rT3 to T4 ratio. With 1.6 microgram T4/100 g BW, cerebrocortical rT3 reached approximately 100 pg/g, about 3 times the euthyroid level, suggesting that at this T4 dose, the rT3 formed from T4 accounts for part of the inhibition of 5'D-II. With the half-maximal suppressive dose of T4, cortex T4 was about 400 pg/g, but rT3 was negligible. We conclude that: suppression of cortex 5'D-II by rT3 is rapid and requires the presence of rT3 in the tissue (i.e. no long-lived mediators); intracortical rT3 is about 5 times more potent than T4 in suppressing this enzyme; the cortex of rT3-5'D-II suppression relationships suggest that the euthyroid levels of cortex rT3 may be significant in the modulation of 5'D-II.(ABSTRACT TRUNCATED AT 400 WORDS)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) as a potent and persistent thyroxine agonist: a mechanistic model for toxicity based on molecular reactivity

TCDD and thyroxine have common molecular reactivity properties which enable them to present a planar face and lateral halogens in interactions with proteins. These molecular properties are consistent with the structure-toxicity relationship for TCDD and related compounds. Biological evidence is discussed including preliminary studies on the effects of TCDD exposure on tadpole growth and development which is consistent with the possible thyroxine-like activity of TCDD. The work suggests the possibility that toxicity is at least in part the expression of potent and persistent thyroid hormone activity (responses induced by TCDD which qualitatively correspond to those mediated by thyroid hormones). A mechanism for toxicity is proposed which involves receptor proteins; the planar aromatic system controls binding to cytosolic proteins and halogen substituents regulate binding to nuclear proteins. This simple model based on molecular reactivity sheds light on the diversified effects of TCDD and related compound toxicity and on certain thyroid hormone action. The model also permits predictions to be made with regard to the toxicity and thyroid hormone activity of untested compounds. In addition, the model suggests a general mechanism for hormone action based on metabolically regulated differential and cooperative protein receptor binding events in cellular compartments which can explain agonism, antagonism and potentiation within the framework of receptor occupancy theory.

["Low triiodothyronine (T3) syndrome": "thyroxine (T4) euthyroidism" evidence (author's transl)]

As first described in serious systemic illnesses isolated decreased T3 plasma concentration was related to impaired peripheral conversion of T4, to T3 with preferential production of reverse T3 (rT3). A "low T3 syndrome" was seen in 47 out of 109 patients with extra-thyroidal diseases. Metabolic state, TSH and TSH responses to TRH were normal despite of low T3 concentration. Euthyroidism seems mainly due to T4 itself in these patients.