Mini-review: Cell surface receptor for thyroid hormone and nongenomic regulation of ion fluxes in excitable cells

The Iodine Rush: Over- or Under-Iodination Risk in the Prophylactic Use of Iodine for Thyroid Blocking in the Event of a Nuclear Disaster

Iodine is an essential element for the production of thyroid hormones (THs). Both deficient and excess iodine intakes may precipitate in adverse thyroidal events. Radioactive iodine (RI) is a common byproduct of nuclear fission processes. During nuclear emergencies RI may be released in a plume, or cloud, contaminating the environment. If inhaled or ingested, it may lead to internal radiation exposure and the uptake of RI mainly by the thyroid gland that absorbs stable iodine (SI) and RI in the same way. A dose of radiation delivered to the thyroid gland is a main risk factor for the thyroid cancer development. The SI prophylaxis helps prevent childhood thyroid cancer. The thyroid gland saturation with prophylactic SI ingestion, reduces the internal exposure of the thyroid by blocking the uptake of RI and inhibiting iodide organification. However, negative impact of inadequate SI intake must be considered. We provide an overview on the recommended iodine intake and the impact of SI and RI on thyroid in children and adolescents, discussing the benefits and adverse effects of the prophylactic SI for thyroid blocking during a nuclear accident. The use of SI for protection against RI may be recommended in cases of radiological or nuclear emergencies, moreover the administration of iodine for prophylactic purposes should be cautious. Benefits and risks should also be considered according to age. Adverse effects from iodine administration cannot be excluded. Precise indications are mandatory to use the iodine for thyroid blocking. Due to this natural adaption mechanism it's possible to tolerate large doses of iodine without clinical effects, however, a prolonged assumption of the iodine when not needed can be dangerous and may precipitate in severe thyroidal and non-thyroidal negative effects.

Deiodinases and the Metabolic Code for Thyroid Hormone Action

Deiodinases modify the biological activity of thyroid hormone (TH) molecules, ie, they may activate thyroxine (T4) to 3,5,3'-triiodothyronine (T3), or they may inactivate T3 to 3,3'-diiodo-L-thyronine (T2) or T4 to reverse triiodothyronine (rT3). Although evidence of deiodination of T4 to T3 has been available since the 1950s, objective evidence of TH metabolism was not established until the 1970s. The modern paradigm considers that the deiodinases not only play a role in the homeostasis of circulating T3, but they also provide dynamic control of TH signaling: cells that express the activating type 2 deiodinase (D2) have enhanced TH signaling due to intracellular build-up of T3; the opposite is seen in cells that express type 3 deiodinase (D3), the inactivating deiodinase. D2 and D3 are expressed in metabolically relevant tissues such as brown adipose tissue, skeletal muscle and liver, and their roles have been investigated using cell, animal, and human models. During development, D2 and D3 expression customize for each tissue/organ the timing and intensity of TH signaling. In adult cells, D2 is induced by cyclic adenosine monophosphate (cAMP), and its expression is invariably associated with enhanced T3 signaling, expression of PGC1 and accelerated energy expenditure. In contrast, D3 expression is induced by hypoxia-inducible factor 1α (HIF-1a), dampening T3 signaling and the metabolic rate. The coordinated expression of these enzymes adjusts TH signaling in a time- and tissue-specific fashion, affecting metabolic pathways in health and disease states.

Thyroid hormones reduce nicotinic receptor mediated currents in SH-SY5Y neuroblastoma cells

BACKGROUND

Thyroid hormones (THs) are crucial for maturation and functioning of mammalian CNS. THs "classical" signaling involves nuclear receptors binding but also their non genomic actions, as rapid modulators of cell activity, are widely recognized. Since THs imbalance affects cognition and the cholinergic system is deeply involved in learning and memory processes we have studied THs effects at the level of the nicotinic acetylcholine receptors (nAchR).

METHODS

We used the patch-clamp technique to analyze T3 and T4 modulation of nicotine (NIC)-mediated current in SH-SY5Y neuroblastoma cells.

RESULTS

Both hormones decreased NIC-evoked current in a dose dependent fashion. The antagonism was reversible, not competitive and not blocked by Tetrac, an integrin αVβ3 receptor antagonist. A similar effect was detected with the endogenous agonist Acetylcholine. THs potencies were higher at 100 μM NIC (IC₅₀ = 4.6 ± 2 μM for T3 and 4.8 ± 2 μM for T4) compared to those measured at 10 μM NIC (IC₅₀ = 10 ± 4 μM for T3 and 8 ± 4 μM for T4). Furthermore, the efficacy of THs reached almost 90% at 100 μM NIC while was about 30 % at 10 μM NIC. THs inhibited nAchR-mediated currents by enhancing receptor desensitization and this effect was more pronounced at high agonist concentrations.

CONCLUSIONS

Our results make light on a new non genomic activity of THs at the level of nAchR. This mechanism of action  of THs  can provide  a new explanation for the cognitive deficits associated with tyroid dysfunction.

Thyroid Hormones in the Brain and Their Impact in Recovery Mechanisms After Stroke

Thyroid hormones are of fundamental importance for brain development and essential factors to warrant brain functions throughout life. Their actions are mediated by binding to specific intracellular and membranous receptors regulating genomic and non-genomic mechanisms in neurons and populations of glial cells, respectively. Among others, mechanisms include the regulation of neuronal plasticity processes, stimulation of angiogenesis and neurogenesis as well modulating the dynamics of cytoskeletal elements and intracellular transport processes. These mechanisms overlap with those that have been identified to enhance recovery of lost neurological functions during the first weeks and months after ischemic stroke. Stimulation of thyroid hormone signaling in the postischemic brain might be a promising therapeutic strategy to foster endogenous mechanisms of repair. Several studies have pointed to a significant association between thyroid hormones and outcome after stroke. With this review, we will provide an overview on functions of thyroid hormones in the healthy brain and summarize their mechanisms of action in the developing and adult brain. Also, we compile the major thyroid-modulated molecular pathways in the pathophysiology of ischemic stroke that can enhance recovery, highlighting thyroid hormones as a potential target for therapeutic intervention.

Naming difficulties after thyroid stimulating hormone suppression therapy in patients with differentiated thyroid carcinoma: a prospective cohort study

BACKGROUND

Thyroid stimulating hormone (TSH) suppression therapy after differentiated thyroid carcinoma surgery causes cognitive impairment. However, data on naming difficulties (anomia)-related specific cognitive impairment are lacking.

METHODS

A prospective cohort study was conducted, in which, patients with differentiated thyroid carcinoma and benign thyroid nodules were given oral L-T4 therapy after surgery, after meeting the criteria of TSH suppression therapy and thyroxine replacement therapy, respectively, the patients were continually given L-T4 therapy for 6 and 12 months, and then, the neuropsychological test was performed.

RESULTS

Of the 255 subjects, 212 cases (83.13%) completed all the tests, including 33 cases in the normal control group (NC group), 110 cases in the TSH suppression therapy group (TS group), and 69 cases in the thyroxine replacement therapy group (TR group). There was no significant difference in background data among the three groups (P > 0.05). The scores of mini-mental state examination, clock drawing test, digit symbol substitution test, personal history, temporal and spatial orientation, digit order relation, visual object recognition, associative learning, and color naming in the TS and TR groups were not significantly different from those in the NC group after 6 and 12 months of L-T4 therapy (P > 0.05); the scores of picture recall, visual recall, comprehension memory, and digit span forward in the TS and TR groups were notably lower than those in the NC group (P < 0.01); the scores of confrontation naming and listing the names in the TS group were significantly lower than those in the NC and TR groups, and the scores decreased with the prolongation of TSH suppression therapy (P < 0.01).

CONCLUSION

TSH suppression therapy after differentiated thyroid carcinoma surgery could lead to short-term memory impairment, attention impairment, word selection anomia, and depression, of which, word selection anomia was aggravated with the prolongation of TSH suppression therapy. Therefore, we suggested that optimal TSH goals for individual patients must balance the potential benefit of TSH suppression therapy with the possible harm from subclinical hyperthyroidism especially in low risk differentiated thyroid carcinoma patients (ClinicalTrials.gov Protocol Registration System: ClinicalTrials.gov ID NCT0266532, Registered on 21 June 2016).

Maternal marginal iodine deficiency delays cerebellar Bergmann glial cell development in rat offspring: Involvement of Notch signaling pathway

During early pregnancy, iodine deficiency (ID) is linked to adverse effects on child motor and psychomotor function. Maternal marginal ID has become a common public health problem. It is unclear whether marginal ID influences the development of the cerebellum or its underlying mechanisms. Thus, the purpose of this study was to determine the effects of marginal ID on the development of cerebellar Bergmann glial cells (BGs) and investigate the activation of the Notch signaling pathway, which is crucial for the development and morphology of BGs. We treated Wistar rats with an ID diet (iodine content 60 ± 1.5 ng/g) supplemented with deionized water containing different concentrations of potassium iodide (KI) (183, 117, and 0 μg/L for the control, marginal ID, and severe ID groups, respectively) during pregnancy and lactation. We explored the morphology of the BGs by Golgi-Cox staining and immunofluorescence and investigated the Notch signaling pathway using western blot. Our results showed that the marginal ID and severe ID groups had decreased cerebellar BG fiber lengths (P < 0.05 and 0.01, respectively) and numbers (P < 0.01 for both) on postnatal day (PN) 7, PN14, and PN21 compared to the control group. Moreover, the data showed that severe ID significantly reduced Dll1, Notch1, RBP-Jκ, and BLBP protein levels at all three time points. Marginal ID slightly reduced the expression of Notch1 on PN7 (P < 0.05) and PN21 (P < 0.01), RBP-Jκ on PN14 (P < 0.01) and PN21 (P < 0.05), and BLBP on PN7 (P < 0.05). There was no significant difference in Dll1 protein levels between the marginal ID and control groups at any time point. Our study suggests that marginal ID leads to mild damage to BG morphogenesis in the cerebellum. The abnormal regulation of the Notch signaling pathway may be involved in the damage to BGs.

Reverse T3 interacts with αvβ3 integrin receptor and restores enzyme activities in the hippocampus of hypothyroid developing rats: Insight on signaling mechanisms

In the present study we provide evidence that 3,3',5'-triiodothyronine (reverse T₃, rT₃) restores neurochemical parameters induced by congenital hypothyroidism in rat hippocampus. Congenital hypothyroidism was induced by adding 0.05% propylthiouracil in the drinking water from gestation day 8 and continually up to lactation day 15. In the in vivo rT₃ exposure, hypothyroid 12-day old pups were daily injected with rT₃ (50 ng/kg body weight) or saline until day 14. In the ex vivo rT₃ treatment, hippocampal slices from 15-day-old hypothyroid pups were incubated for 30 min with or without rT₃ (1 nM). We found that ex vivo and/or in vivo exposure to rT₃ failed in restoring the decreased ¹⁴C-glutamate uptake; however, restored the phosphorylation of glial fibrillary acidic protein (GFAP), ⁴⁵Ca²⁺ influx, aspartate transaminase (AST), glutamine synthetase (GS) and gamma-glutamate transferase (GGT) activities, as well as glutathione (GSH) levels in hypothyroid hippocampus. In addition, rT₃ improved ¹⁴C-2-deoxy-D-glucose uptake and lactate dehydrogenase (LDH) activity. Receptor agonists/antagonists (RGD peptide and AP-5), kinase inhibitors of p38MAPK, ERK1/2, CaMKII, PKA (SB239063, PD98059, KN93 and H89, respectively), L-type voltage-dependent calcium channel blocker (nifedipine) and intracellular calcium chelator (BAPTA-AM) were used to determine the mechanisms of the nongenomic rT₃ action on GGT activity. Using molecular docking analysis, we found rT₃ interaction with αvβ3 integrin receptors, nongenomically activating signaling pathways (PKA, CaMKII, p38MAPK) that restored GGT activity. We provide evidence that rT₃ is an active TH metabolite and our results represent an important contribution to elucidate the nonclassical mechanism of action of this metabolite in hypothyroidism.

Maternal marginal iodine deficiency limits dendritic growth of cerebellar purkinje cells in rat offspring by NF-κB signaling and MAP1B

Iodine deficiency (ID) during early pregnancy had an adverse effect on children's psychomotor and motor function. It is worth noting that maternal marginal ID tends to be a common public health problem. Whether marginal ID potentially had adverse effects on the development of cerebellum and the underlying mechanisms remain unclear. Therefore, our aim was to study the effects of marginal ID on the dendritic growth in filial cerebellar Purkinje cells (PCs) and the underlying mechanism. In the present study, we established Wistar rat models by feeding dam rats with a diet deficient in iodine and deionized water supplemented with potassium iodide. We examined the total dendritic length using immunofluorescence, and Western blot analysis was conducted to investigate the activity of nuclear factor-κB (NF-κB) signaling and microtubule-associated protein 1B (MAP1B). Our results showed that marginal ID reduced the total dendritic length of cerebellar PCs, slightly down-regulated the activity of NF-κB signaling and decreased MAP1B in cerebellar PCs on postnatal day (PN) 7, PN14, and PN21. Our study may support the hypothesis that decreased T₄ induced by marginal ID limits PCs dendritic growth, which may involve in the disturbance of NF-κB signaling and MAP1B on the cerebellum. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1241-1251, 2017.

Thyroid hormone and astroglia: endocrine control of the neural environment

Thyroid hormones (THs) play key roles in brain development and function. The lack of THs during childhood is associated with the impairment of several neuronal connections, cognitive deficits and mental disorders. Several lines of evidence point to astrocytes as TH targets and as mediators of TH action in the central nervous system; however, the mechanisms underlying these events are still not completely known. In this review, we focus on advances in our understanding of the effects of THs on astroglial cells and the impact of these effects on neurone-astrocyte interactions. First, we discuss the signalling pathways involved in TH metabolism and the molecular mechanisms underlying TH receptor function. Then, we discuss data related to the effects of THs on astroglial cells, as well as studies regarding the generation of mutant TH receptor transgenic mice that have contributed to our understanding of TH function in brain development. We argue that astrocytes are key mediators of hormone actions on development of the cerebral cortex and cerebellum and that the identification of the molecules and pathways involved in these events might be important for determining the molecular-level basis of the neural deficits associated with endocrine diseases.

Guidelines for the treatment of hypothyroidism: prepared by the american thyroid association task force on thyroid hormone replacement

BACKGROUND

A number of recent advances in our understanding of thyroid physiology may shed light on why some patients feel unwell while taking levothyroxine monotherapy. The purpose of this task force was to review the goals of levothyroxine therapy, the optimal prescription of conventional levothyroxine therapy, the sources of dissatisfaction with levothyroxine therapy, the evidence on treatment alternatives, and the relevant knowledge gaps. We wished to determine whether there are sufficient new data generated by well-designed studies to provide reason to pursue such therapies and change the current standard of care. This document is intended to inform clinical decision-making on thyroid hormone replacement therapy; it is not a replacement for individualized clinical judgment.

METHODS

Task force members identified 24 questions relevant to the treatment of hypothyroidism. The clinical literature relating to each question was then reviewed. Clinical reviews were supplemented, when relevant, with related mechanistic and bench research literature reviews, performed by our team of translational scientists. Ethics reviews were provided, when relevant, by a bioethicist. The responses to questions were formatted, when possible, in the form of a formal clinical recommendation statement. When responses were not suitable for a formal clinical recommendation, a summary response statement without a formal clinical recommendation was developed. For clinical recommendations, the supporting evidence was appraised, and the strength of each clinical recommendation was assessed, using the American College of Physicians system. The final document was organized so that each topic is introduced with a question, followed by a formal clinical recommendation. Stakeholder input was received at a national meeting, with some subsequent refinement of the clinical questions addressed in the document. Consensus was achieved for all recommendations by the task force.

RESULTS

We reviewed the following therapeutic categories: (i) levothyroxine therapy, (ii) non-levothyroxine-based thyroid hormone therapies, and (iii) use of thyroid hormone analogs. The second category included thyroid extracts, synthetic combination therapy, triiodothyronine therapy, and compounded thyroid hormones.

CONCLUSIONS

We concluded that levothyroxine should remain the standard of care for treating hypothyroidism. We found no consistently strong evidence for the superiority of alternative preparations (e.g., levothyroxine-liothyronine combination therapy, or thyroid extract therapy, or others) over monotherapy with levothyroxine, in improving health outcomes. Some examples of future research needs include the development of superior biomarkers of euthyroidism to supplement thyrotropin measurements, mechanistic research on serum triiodothyronine levels (including effects of age and disease status, relationship with tissue concentrations, as well as potential therapeutic targeting), and long-term outcome clinical trials testing combination therapy or thyroid extracts (including subgroup effects). Additional research is also needed to develop thyroid hormone analogs with a favorable benefit to risk profile.

Restoration of thyroid hormone balance: a game changer in the treatment of heart failure?

The link between low thyroid hormone (TH) function and heart failure is reviewed in the present report. The idea that TH dysfunction may contribute to diseases leading to HF has been discussed for over 60 yr. A growing body of evidence from animal and human studies, particularly in recent years, suggests that TH treatment may improve clinical outcomes. Indeed, if a similar amount of positive information were available for a newly developed heart drug, there is little doubt that large-scale clinical trials would be underway with considerable excitement. THs offer the promise of improving ventricular contraction and relaxation, improving coronary blood flow, and inhibiting atherosclerosis, and new results suggest they may even reduce the incidence of arrhythmias in heart diseases. Are the potential clinical benefits worth the risk of possible overdosing? After so many years, why has this question not been answered? Clearly, the concept has not been disproven. This review explores the body of clinical evidence related to TH dysfunction and heart failure, discuss insights into pathophysiological, cellular, and molecular mechanisms provided by animal research, and discuss what is needed to resolve this long-standing issue in cardiology and move forward.

Triiodothyronine rapidly alters the TSH content and the secretory granules distribution in male rat thyrotrophs by a cytoskeleton rearrangement-independent mechanism

Rapid actions of T3 on TSH synthesis in posttranscriptional steps, such as polyadenylation and translation rate, have already been described. The focus of this paper was to characterize rapid actions of T3 on TSH secretion and the involvement of actin and microtubule cytoskeleton in this process. For that, sham-operated (SO) and thyroidectomized (Tx) rats were subjected to acute or chronic treatment with T3. We observed a disarrangement in microtubule and actin cytoskeletons and an increase in Tshb mRNA levels in Tx rats, whereas the total TSH protein content was reduced in the pituitary gland as a whole, but increased in the secretory granules close to the plasma membrane of thyrotrophs, as well as in the extracellular space. The acute T3 dose promoted a rapid increase and redistribution of TSH secretory granules throughout the cytoplasm, as well as a rearrangement in actin and microtubule cytoskeletons. The T3 chronic treatment outcome reinforces the acute effects observed and, additionally, evinces an increase in the α-tubulin content and a rearrangement in microtubule cytoskeleton. Thus, T3 is able to rapidly suppress TSH secretion and, in parallel, to promote a rearrangement in actin and microtubules assembly throughout the pituitary gland, effects that seem to be independent from each other.

Nongenomic effect of levothyroxine on the synchronous electrical activity of the spinal dorsal horn in the rat

Levothyroxine (T4) has a well-known effect on the central nervous system (CNS). This effect requires hours of latency by genetic pathway. We tested for short latency nongenomic effects of T4 superfusion on the spinal dorsal horn (DH) evaluating lumbar somatosensory evoked potentials in rats. T4 increased N and P wave amplitudes and N wave area under the curve, but reduced P wave duration and N-P interval, suggesting that T4 exerts both excitatory and synchronizing effects on DH interneurons in less than 300 s, thus, providing evidence of nongenomic effects of T4 on DH.

New approaches to thyroid hormones and purinergic signaling

It is known that thyroid hormones influence a wide variety of events at the molecular, cellular, and functional levels. Thyroid hormones (TH) play pivotal roles in growth, cell proliferation, differentiation, apoptosis, development, and metabolic homeostasis via thyroid hormone receptors (TRs) by controlling the expression of TR target genes. Most of these effects result in pathological and physiological events and are already well described in the literature. Even so, many recent studies have been devoted to bringing new information on problems in controlling the synthesis and release of these hormones and to elucidating mechanisms of the action of these hormones unconventionally. The purinergic system was recently linked to thyroid diseases, including enzymes, receptors, and enzyme products related to neurotransmitter release, nociception, behavior, and other vascular systems. Thus, throughout this text we intend to relate the relationship between the TH in physiological and pathological situations with the purinergic signaling.

Integrin participates in the effect of thyroxine on plasma membrane in immature rat testis

BACKGROUND

The secretory activity of Sertoli cells (SC) is dependent on ion channel functions and protein synthesis and is critical to ongoing spermatogenesis. The aim of this study was to investigate the mechanism of action associated with a non-metabolizable amino acid [14C]-MeAIB (alpha-(methyl-amino)isobutyric acid) accumulation stimulated by T4 and the role of the integrin receptor in this event, and also to clarify whether the T4 effect on MeAIB accumulation and on Ca2+ influx culminates in cell secretion.

METHODS

We have studied the rapid and plasma membrane initiated effects of T4 by using 45Ca2+ uptake and [45C]-MeAIB accumulation assays, respectively. Thymidine incorporation into DNA was used to monitor nuclear activity and quinacrine to analyze the secretory activity on SC.

RESULTS

The stimulation of MeAIB accumulation byT4 appears to be mediated by the integrin receptor in the plasma membrane since tetrac and RGD peptide were able to nullify the effect of this hormone. In addition, T4 increases extracellular Ca2+ uptake and Ca2+ from intracellular stocks to enhance nuclear activity, but this genomic action seems not to influence SC secretion mediated by T4. Also, the cytoskeleton and CIC-3 chloride channel contribute to the membrane-associated responses of SC.

CONCLUSIONS

T4 integrin receptor activation ultimately determines the plasma membrane responses on amino acid transport in SC, but it is not involved in calcium influx, cell secretion or the nuclear effect of the hormone.

GENERAL SIGNIFICANCE

The integrin receptor activation by T4 may take a role in plasma membrane processes involved in the male reproductive system.

Is rapid effect of thyroxine on GABAergic IPSCs purely postsynaptic?

BACKGROUND

Thyroid hormones (THs) are well known for their genomic effects but recently several studies revealed their actions as rapid modulators of membrane receptors. In particular, fast thyroxine effect on GABA(A) receptors have been reported. We addressed question whether presynaptic mechanisms can be also involved in modulation of GABAergic transmission by thyroxine.

METHODS

Using patch-clamp technique we examined fast effects of thyroxine (2 μM) on evoked GABAergic postsynaptic currents.

RESULTS

We found that in addition to the inhibitory effect on IPSC amplitude, thyroxine changed IPSC coefficient of variation (CV).

CONCLUSION

This result suggests involvement of a presynaptic mechanism in thyroxine effect on GABAergic transmission.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

Gestational doxorubicin alters fetal thyroid-brain axis

Administration of chemotherapy during pregnancy may represent a big risk factor for the developing brain, therefore we studied whether the transplacental transport of doxorubicin (DOX) may affect the development of neuroendocrine system. DOX (25 mg/kg; 3 times interaperitoneally/week) was given to pregnant rats during whole gestation period. The disturbances in neuroendocrine functions were investigated at gestation day (GD) 15 and 20 by following the maternal and fetal thyroid hormone levels, fetal nucleotides (ATP, ADP, AMP) levels and adenosine triphosphatase (Na(+), K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase) activities in two brain regions, cerebrum and cerebellum. In control group, the levels of maternal and fetal serum thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH), and fetal serum growth hormone (GH) increased from days 15 to 20, whereas in the DOX group, a decrease in maternal and fetal T4, T3 and increase in TSH levels (hypothyroid status) were observed. Also, the levels of fetal GH decreased continuously from GD 15 to 20 with respect to control group. In cerebrum and cerebellum, the levels of fetal nucleotides and the activities of fetal ATPases in control group followed a synchronized course of development. The fetal hypothyroidism due to maternal administration of DOX decreased the levels of nucleotides, ATPases activities, and total adenylate, instead, the adenylate energy charge showed a trend to an increase in both brain regions at all ages tested. These alterations were dose- and age-dependent and this, in turn, may impair the nerve transmission. Finally, DOX may act as neuroendocrine disruptor causing hypothyroidism and fetal brain energetic dysfunction.

Mechanisms of thyroid hormone action

Our understanding of thyroid hormone action has been substantially altered by recent clinical observations of thyroid signaling defects in syndromes of hormone resistance and in a broad range of conditions, including profound mental retardation, obesity, metabolic disorders, and a number of cancers. The mechanism of thyroid hormone action has been informed by these clinical observations as well as by animal models and has influenced the way we view the role of local ligand availability; tissue and cell-specific thyroid hormone transporters, corepressors, and coactivators; thyroid hormone receptor (TR) isoform-specific action; and cross-talk in metabolic regulation and neural development. In some cases, our new understanding has already been translated into therapeutic strategies, especially for treating hyperlipidemia and obesity, and other drugs are in development to treat cardiac disease and cancer and to improve cognitive function.

Thyroid hormone receptor-α and vascular function

Thyroid hormone (TH) treatment exerts beneficial effects on the cardiovascular system: it lowers cholesterol and LDL levels and enhances cardiac contractile function. However, little is known about the effect of TH on vascular function or the functional role of TH receptors (TRs) in the regulation of vascular tone. We have investigated the contribution of TRs to vascular contractility in the heart. Among different TR subtype-specific knockout (KO) mice, vascular contraction was significantly enhanced in coronary arteries isolated from TRα KO compared with wild-type mice, while chronic TH treatment significantly attenuated coronary vascular contraction. We found that TRα is the predominant TR in mouse coronary smooth muscle cells (SMCs). Coronary SMCs isolated from TRα KO mice exhibited a significant decrease in K(+) channel activity, whereas TH treatment increased K(+) channel activity in a dose-dependent manner. These data suggest that TRα in SMCs has prominent effects on regulation of vascular tone and TH treatment helps decrease coronary vascular tone by increasing K(+) channel activity through TRα in SMCs.

Molecular basis for certain neuroprotective effects of thyroid hormone

The pathophysiology of brain damage that is common to ischemia-reperfusion injury and brain trauma include disodered neuronal and glial cell energetics, intracellular acidosis, calcium toxicity, extracellular excitotoxic glutamate accumulation, and dysfunction of the cytoskeleton and endoplasmic reticulum. The principal thyroid hormones, 3,5,3'-triiodo-l-thyronine (T(3)) and l-thyroxine (T(4)), have non-genomic and genomic actions that are relevant to repair of certain features of the pathophysiology of brain damage. The hormone can non-genomically repair intracellular H(+) accumulation by stimulation of the Na(+)/H(+) exchanger and can support desirably low [Ca(2+)](i.c.) by activation of plasma membrane Ca(2+)-ATPase. Thyroid hormone non-genomically stimulates astrocyte glutamate uptake, an action that protects both glial cells and neurons. The hormone supports the integrity of the microfilament cytoskeleton by its effect on actin. Several proteins linked to thyroid hormone action are also neuroprotective. For example, the hormone stimulates expression of the seladin-1 gene whose gene product is anti-apoptotic and is potentially protective in the setting of neurodegeneration. Transthyretin (TTR) is a serum transport protein for T(4) that is important to blood-brain barrier transfer of the hormone and TTR also has been found to be neuroprotective in the setting of ischemia. Finally, the interesting thyronamine derivatives of T(4) have been shown to protect against ischemic brain damage through their ability to induce hypothermia in the intact organism. Thus, thyroid hormone or hormone derivatives have experimental promise as neuroprotective agents.

Membrane-initiated actions of thyroid hormones on the male reproductive system

The presence of specific nuclear receptors to thyroid hormones, described in prepubertal Sertoli cells, implies the existence of an early and critical influence of these hormones on testis development. Although the mechanism of action thyroid hormones has been classically established as a genomic action regulating testis development, our research group has demonstrated that these hormones exert several effects in Sertoli cells lacking nuclear receptor activation. These findings led to the identification of non-classical thyroid hormone binding elements in the plasma membrane of testicular cells. Through binding to these sites, thyroid hormones could exert nongenomic effects, including those on ion fluxes at the plasma membrane, on signal transduction via kinase pathways, on amino acid accumulation, on modulation of extracellular nucleotide levels and on vimentin cytoskeleton. The evidence of the participation of different K(+), Ca(2+) and Cl(-) channels in the mechanism of action of thyroid hormones, characterizes the plasma membrane as an important microenvironment able to coordinate strategic signal transduction pathways in rat testis. The physiological responses of the Sertoli cells to hormones are dependent on continuous cross-talking of different signal transduction pathways. Apparently, the choice of the signaling pathways to be activated after the interaction of the hormone with cell surface binding sites is directly related to the physiological action to be accomplished. Yet, the enormous complexity of the nongenomic actions of thyroid hormones implies that different specific binding sites located on the plasma membrane or in the cytosol are believed to initiate specific cell responses.

Membrane receptor for thyroid hormone: physiologic and pharmacologic implications

Plasma membrane integrin αvβ3 is a cell surface receptor for thyroid hormone at which nongenomic actions are initiated. L-thyroxine (T₄) and 3,3',5-triiodo-L-thyronine (T₃) promote angiogenesis and tumor cell proliferation via the receptor. Tetraiodothyroacetic acid (tetrac), a deaminated T₄ derivative, blocks the nongenomic proliferative and proangiogenic actions of T₄ and T₃. Acting at the integrin independently of T₄ and T₃, tetrac and a novel nanoparticulate formulation of tetrac that acts exclusively at the cell surface have oncologically desirable antiproliferative actions on multiple tumor cell survival pathway genes. These agents also block the angiogenic activity of vascular growth factors. Volume and vascular support of xenografts of human pancreatic, kidney, lung, and breast cancers are downregulated by tetrac formulations. The integrin αvβ3 receptor site for thyroid hormone selectively regulates signal transduction pathways and distinguishes between unmodified tetrac and the nanoparticulate formulation. The receptor also mediates nongenomic thyroid hormone effects on plasma membrane ion transporters and on intracellular protein trafficking.

GC-MS metabolomic analysis reveals significant alterations in cerebellar metabolic physiology in a mouse model of adult onset hypothyroidism

Although adult-onset hypothyroidism (AOH) has been connected to neural activity alterations, including movement, behavioral, and mental dysfunctions, the underlying changes in brain metabolic physiology have not been investigated in a systemic and systematic way. The current knowledge remains fragmented, referring to different experimental setups and recovered from various brain regions. In this study, we developed and applied a gas chromatography-mass spectrometry (GC-MS) metabolomics protocol to obtain a holistic view of the cerebellar metabolic physiology in a Balb/cJ mouse model of prolonged adult-onset hypothyroidism induced by a 64-day treatment with 1% potassium perchlorate in the drinking water of the animals. The high-throughput analysis enabled the correlation between multiple parallel-occurring metabolic phenomena; some have been previously related to AOH, while others implicated new pathways, designating new directions for further research. Specifically, an overall decline in the metabolic activity of the hypothyroid compared to the euthyroid cerebellum was observed, characteristically manifested in energy metabolism, glutamate/glutamine metabolism, osmolytic/antioxidant capacity, and protein/lipid synthesis. These alterations provide strong evidence that the mammalian cerebellum is metabolically responsive to AOH. In light of the cerebellum core functions and its increasingly recognized role in neurocognition, these findings further support the known phenotypic manifestations of AOH into movement and cognitive dysfunctions.

Thyroid hormone inhibits ERK phosphorylation in pressure overload-induced hypertrophied mouse hearts through a receptor-mediated mechanism

Pressure overload-induced cardiac hypertrophy results in a pathological type of hypertrophy with activation of signaling cascades like the extracellular signal-regulated kinase (ERK) pathway, which promotes negative cardiac remodeling and decreased contractile function. In contrast, thyroid hormone mediates a physiological type of hypertrophy resulting in enhanced contractile function. In addition, thyroid hormone action is diminished in pressure overload-induced cardiac hypertrophy. We hypothesized that thyroid hormone status modulates ERK activity and that administration of thyroid hormone could alter the activity of this kinase in cardiac hypertrophy induced by pressure overload. ERK is activated by phosphorylation; accordingly, we investigated phosphorylation of ERK in hearts of control, hypothyroid, and hyperthyroid mice. In addition, the effect of T3 treatment on ERK phosphorylation in hypertrophied hearts from transverse aortic-constricted (TAC) mice was investigated. Results showed that phosphorylated ERK (p-ERK) was decreased by 25% in hyperthyroid mice. In contrast, hypothyroid mice presented increased p-ERK by 80%. TAC mice presented a greater than fourfold increase of p-ERK compared with control mice. Interestingly, T3 administration dramatically canceled TAC-induced ERK phosphorylation (36% lower compared with control). Raf-1 is upstream of the ERK pathway. TAC mice presented a 45% increase in phospho-Raf-1 (Ser338). T3 treatment inhibited this effect of pressure overload and further decreased p-Raf-1 (Ser338) by 37%, compared with control. Overexpression of thyroid hormone receptor-α in cultured cardiomyocytes potentiated the inhibitory effect of T3 on ERK phosphorylation. We concluded that thyroid hormone has an inhibitory effect on the Raf-1/ERK pathway. Furthermore, treatment of TAC mice with T3 inhibited Raf-1/ERK pathway by a thyroid hormone receptor-dependent mechanism.