Propranolol inhibits myocardial infarction-induced brown adipose tissue D2 activation and maintains a low thyroid hormone state in rats

Considering the recognized role of thyroid hormones on the cardiovascular system during health and disease, we hypothesized that type 2 deiodinase (D2) activity, the main activation pathway of thyroxine (T4)-to-triiodothyronine (T3), could be an important site to modulate thyroid hormone status, which would then constitute a possible target for β-adrenergic blocking agents in a myocardial infarction (MI) model induced by left coronary occlusion in rats. Despite a sustained and dramatic fall in serum T4 concentrations (60-70%), the serum T3 concentration fell only transiently in the first week post-infarction (53%) and returned to control levels at 8 and 12 weeks after surgery compared to the Sham group (P<0.05). Brown adipose tissue (BAT) D2 activity (fmol T4·min-1·mg ptn-1) was significantly increased by approximately 77% in the 8th week and approximately 100% in the 12th week in the MI group compared to that of the Sham group (P<0.05). Beta-blocker treatment (0.5 g/L propranolol given in the drinking water) maintained a low T3 state in MI animals, dampening both BAT D2 activity (44% reduction) and serum T3 (66% reduction in serum T3) compared to that of the non-treated MI group 12 weeks after surgery (P<0.05). Propranolol improved cardiac function (assessed by echocardiogram) in the MI group compared to the non-treated MI group by 40 and 57%, 1 and 12 weeks after treatment, respectively (P<0.05). Our data suggested that the beta-adrenergic pathway may contribute to BAT D2 hyperactivity and T3 normalization after MI in rats. Propranolol treatment maintained low T3 state and improved cardiac function additionally.

Effects of magnesium on cytomorphology and enzyme activities in thyroid of rats.

Till date knowledge regarding the effects of high dietary magnesium on thyroid gland is incomprehensive though certain epidemiological studies reported development of thyroid gland dysfunctions in people with chronic exposure to hard water (especially with high magnesium) despite sufficient iodine consumption. The present study is to explore the effects of chronic high dietary magnesium exposure on thyroid morphology and functional status. Male adult albino Wistar strain rats were treated with graded doses of magnesium sulphate (MgSO4; 0.5, 1.0 and 1.5 g %) for 60 days and changes in different thyroid parameters were investigated. Significantly stimulated thyroid peroxidase and Na+–K+-ATPase and altered idothyronine 5/- deiodinase type I activities, enhanced serum thyroxine (T4) (both total and free), total triiodothyronine (T3) and thyroid stimulating hormone with decreased free T3 levels and T3/T4 ratio (T3:T4) along with enlargement of thyroid with associated histopathological changes were observed in the treated groups. The results clearly confirm that chronic high dietary magnesium exposure causes potential thyroid disruption as reported in earlier epidemiological studies.

Expressão das iodotironinas desiodases nas neoplasias tireoidianas / Iodothyronine deiodinases expression in thyroid neoplasias

As iodotironinas desiodases formam uma família de selenoenzimas com propriedades catalíticas distintas que ativam ou inativam os hormônios tireoidianos via desiodação do anel fenólico ou tirosínico da molécula do T4. As desiodases tipo I e II (D1 e D2) são as enzimas responsáveis pela geração do T3 e são amplamente expressas na tireóide normal. A transformação neoplásica benigna ou maligna da glândula tireóide está associada a alterações na expressão dessas isoenzimas, sugerindo um possível papel da D1 e da D2 como marcadores de diferenciação celular. Anormalidades na expressão de ambas enzimas e da desiodase tipo III (D3), inativadora do hormônios tireoidianos, são também encontradas em outras neoplasias humanas. Os mecanismos ou implicações do aumento ou diminuição das desiodases na patogênese neoplásica são pouco compreendidas. No entanto, é importante observar que a expressão anormal da D2 pode ser responsável por um quadro de tireotoxicose em pacientes com metástases de carcinoma folicular de tireóide, enquanto que o aumento da D3 em hemangiomas pode causar hipotireoidismo de difícil tratamento.

The iodothyronine deiodinases constitute a family of selenoenzymes that catalyze the removal of iodine from the outer ring or inner ring of the thyroid hormones. The activating enzymes, deiodinases type I (D1) and type II (D2), are highly expressed in normal thyroid gland. Benign or malignant neoplastic transformation of the thyroid cells is associated with changes on the expression of these enzymes, suggesting that D1 or D2 can be markers of cellular differentiation. Abnormalities on the expression of both enzymes and also of the deiodinase type III (D3), that inactivates thyroid hormones, have been found in other human neoplasias. So far, the mechanism or implications of these findings on tumor pathogenesis are not well understood. Nevertheless, its noteworthy that abnormal expression of D2 can cause thyrotoxicosis in patients with metastasis of follicular thyroid carcinoma and that increased D3 expression in large hemangiomas causes severe hypothyroidism.

The functional role of the CARM1-SNF5 complex and its associated HMT activity in transcriptional activation by thyroid hormone receptor

We have investigated the function and mechanisms of the CARM1-SNF5 complex in T3-dependent transcriptional activation. Using specific small interfering RNAs (siRNA) to knock down coactivators in HeLa alpha2 cells, we found that coactivator associated arginine methyltransferase 1 (CARM1) and SWI/SNF complex component 5 (SNF5) are important for T3-dependent transcriptional activation. The CARM1- SWI/SNF chromatin remodeling complex serves as a mechanism for the rapid reversal of H3-K9 methylation. Importantly, siRNA treatment against CARM1 and/or SNF5 increased the recruitment of HMTase G9a to the type 1 deiodinase (D1) promoter even with T3. Knocking- down either CARM1 or SNF5 also inhibited the down- regulation of histone macroH2A, which is correlated with transcriptional activation. Finally, knocking down CARM1 and SNF5 by siRNA impaired the association of these coactivators to the D1 promoter, suggesting functional importance of CARM1- SNF5 complex in T3-dependent transcriptional activation.

Hypercholesterolemia and tissue-specific differential mRNA expression of type-1 5'-iodothyronine deiodinase under different selenium status in rats

Type-1 5'-iodothyronine deiodinase (5'-DI) is responsible for conversion of T4 to T3. Selenium (Se) is an integral part of this enzyme. Keeping in view the strong association between atherosclerosis and hypothyroidism, the present study examined the behavior of 5'-DI in liver, aorta and thyroid during hypercholesterolemia following different Se status, i.e., Se deficiency (0.02ppm), adequate (0.2ppm) and excess dose (1ppm) in SD male rats. Animals were fed a control or high-cholesterol diet (2%) for 1 and 2 months. 5'-DI activity and mRNA expression was measured by RIA and RT-PCR respectively. In liver and aorta, 5'-DI expression significantly decreased with the Se-deficient and the high-cholesterol diet. The trend was opposite in thyroid, i.e., mRNA expression increased significantly during selenium deficiency and with a high-cholesterol feeding. But with 1ppm Se supplementation, the 5'-DI expression increased in all the three tissues. The present study indicates that hypercholesterolemia along with selenium deficiency is co-responsible for differential regulation of 5'-DI enzyme in thyroidal vs. extrathyroidal tissues. Distinct regulation of 5'-DI in the thyroid reflects the clinical importance of this selenoprotein during hypercholesterolemia as this enzyme is essential for T3 production, which further has a vital role in the maintenance of lipid metabolism.

Evaluation of possible goitrogenic and anti-thyroidal effect of nitrate, a potential environmental pollutant.

Nitrate is a wide spread contaminant of ground and surface water. The source of nitrate in the ground water may be from run off or seepage from fertilized soil, municipal or industrial waste water, land fills, septic system, urban drainage or decaying plants. Human and animal systems are affected severely on nitrate exposure. The study was to investigate the effect of dietary nitrate exposure on the thyroid status along with the state of iodine nutrition. Rats were fed diet containing 3% potassium nitrate (KNO3) for 4 weeks and then thyroid status was evaluated by thyroid gland weight, urinary iodine excretion pattern, thyroid peroxidase (TPO) activity, serum levels of total thyroxine (T4), triiodothyronine (T3) and thyroid stimulating hormone (TSH) concentrations. In nitrate treated animals, the weight of thyroid gland was increased significantly (P<0.001) while thyroid peroxidase activity (P<0.01), serum T4 (P<0.01) and serum T3 levels (P<0.001) were reduced; but serum TSH level was increased (P<0.001) along with slightly elevated iodine excretion level (P<0.001) in comparison to control animals. The overall results indicated the development of a relative state of functional hypothyroidism with enlarged thyroid after nitrate exposure. This study can explain a part for the persistence of residual goitre in the post-salt iodization phase.

La tiroides como modelo de mecanismos moleculares en enfermedades geneticas / The thyroid as a model for molecular mechanisms in genetic diseases

Las enfermedades tiroideas constituyen una heterogénea colección de anormalidades asociadas a mutaciones en los genes responsables en el desarrollo de la tiroides: factor de transcripción tiroideo 1 (TTF-1), factor de transcripción tiroideo 2 (TTF-2) y PAX8, o en uno de los genes que codifican para las proteínas involucradas en la biosíntesis de hormonas tiroideas como tiroglobulina (TG), tiroperoxidasa (TPO),sistema de generación de peróxido de hidrógeno (DUOX2), cotransportdor de Na/I– (NIS), pendrina (PDS), TSH y receptor de TSH. El hipotiroidismo congénito ocurre con una prevalencia de 1 en 4.000 nacidos. Los pacientes coneste síndrome pueden ser divididos en dos grupos: con hipotiroidismo congénito sin bocio (disembriogénesis) o con bocio (dishormonogénesis). El grupo de disembriogénesis, que corresponde al 85% de los casos, resulta de ectopía,agenesia o hipoplasia. En una minoría de estos pacientes, el hipotiroidismo congénito está asociado con mutaciones en los genes TTF-1, TTF-2, PAX-8, TSH o TSHr. La resencia de bocio congénito (15% de los casos) se ha asociado a mutaciones en los genes NIS, TG, TPO, DUOX2 o PDS. El hipotiroidismo congénito por dishormonogénesis es trasmitido en forma autonómica recesiva. Mutaciones somáticas en el TSHr han sido identificadas en adenomas tiroideos hiperfuncionantes. Otra enfermedad tiroidea bien establecida es la resistencia a hormonas tiroideas(RTH). Es un síndrome de reducida respuesta tisular a la acción hormonal causado por mutaciones localizadas en el gen del receptor de hormonas tiroideas (TR). Mutantes de TRinterfieren con la función del receptor normal por un mecanismo de dominancia negativa. En conclusión, la identificación de mutaciones en los genes de expresión tiroidea ha permitido un mayor entendimiento sobre la relación estructura-función de los mismos. La tiroides constituye un excelente modelo para el estudio molecular de las enfermedades genéticas.

Goitrogenic content of Indian cyanogenic plant foods & their in vitro anti-thyroidal activity.

BACKGROUND & OBJECTIVES: Consumption of cyanogenic foods has been considered as one of the etiological factors in certain instances for the persistence of endemic goitre. The present study was undertaken to study the cyanogenic glucosides, glucosinolates and thiocyanate content in edible portion of certain selected plant foods of Indian origin. Further in vitro anti-thyroidal activity using raw, boiled and cooked extracts of these plants with and without excess iodide was also studied. METHODS: Cyanogenic plant foods generally vegetables were collected from different areas of West Bengal and Tripura. Cassava was obtained from Meghalaya and Kerala and their cyanogenic glucosides, glucosinolates and thiocyanate were estimated. Thyroid peroxidase activity (TPO) of human thyroid was assayed from microsomal fraction following I3- from iodide. The anti-TPO activities of the plants were assayed after adding raw, boiled and cooked extracts in the assay medium with and without extra iodide. Relative antithyroidal potency of the plant extracts was also evaluated in terms of the concentration (IC50) necessary to produce 50 per cent inhibition of TPO activity. PTU equivalence of the plant foods was also determined. RESULTS: Cabbage and cauliflower were rich in glucosinolates, bamboo shoot and cassava were rich in cyanogenic glucosides, mustard, turnip and radish were relatively rich in thiocyanate however all the constituents were present in each plant. Boiled extracts showed maximum inhibition of TPO activity followed by cooked and raw extracts. Excess iodide was found relatively effective for raw extract but less effective for boiled and cooked extracts in reversing anti-TPO activity. Inhibition constant (IC50) was found highest with bamboo shoot and least with cabbage. INTERPRETATION & CONCLUSION: Raw, boiled and cooked extracts of the plants showed anti-thyroidal activity in vitro. Excess iodide reversed the anti-TPO activity to same extent but could not neutralise it.

Thyroid peroxidase activity is inhibited by amino acids

Normal in vitro thyroid peroxidase (TPO) iodide oxidation activity was completely inhibited by a hydrolyzed TPO preparation (0.15 mg/ml) or hydrolyzed bovine serum albumin (BSA, 0.2 mg/ml). A pancreatic hydrolysate of casein (trypticase peptone, 0.1 mg/ml) and some amino acids (cysteine, tryptophan and methionine, 50 µM each) also inhibited the TPO iodide oxidation reaction completely, whereas casamino acids (0.1 mg/ml), and tyrosine, phenylalanine and histidine (50 µM each) inhibited the TPO reaction by 54 per cent or less. A pancreatic digest of gelatin (0.1 mg/ml) or any other amino acid (50 µM) tested did not significantly decrease TPO activity. The amino acids that impair iodide oxidation also inhibit the TPO albumin iodination activity. The inhibitory amino acids contain side chains with either sulfur atoms (cysteine and methionine) or aromatic rings (tyrosine, tryptophan, histidine and phenylalanine). Among the amino acids tested, only cysteine affected the TPO guaiacol oxidation reaction, producing a transient inhibition at 25 or 50 µM. The iodide oxidation inhibitory activity of cysteine, methionine and tryptophan was reversed by increasing iodide concentrations from 12 to 18 mM, while no such effect was observed when the cofactor (H2O2) concentration was increased. The inhibitory substances might interfere with the enzyme activity by competing with its normal substrates for their binding sites, binding to the free substrates or reducing their oxidized form.

Effect of thiocyanate induced hypothyroidism on 5'deiodinase activity and T3 receptors in developing rat brain.

Experiments were performed in weaning rats to understand the influence of thiocyanate, an hydrolytic product of glucosinolates present in foods, on the generation of T3 in situ by type II 5'deiodinase and the binding of 125I T3 to specific nuclear receptors in developing brain. Feeding of thiocyanate through gestation and lactation resulted in an increase in type II 5'deiodinase activity in cerebrum, cerebellum and brainstem of the 21 day old pups compared to controls. Hypothyroidism induced by thiocyanate further resulted in augmentation of the maximum binding capacity of receptors in the cerebrum of the weaning pups. Affinity constants for binding of 125I T3 were however, unaltered. Increase in type II 5'deiodinase activity and the number of binding sites point to an adaptive increase in response to thiocyanate induced hypothyroidism to maintain the cellular T3 levels within a narrow limit.

Biologia molecular dos defeitos genéticos de síntese, transporte e açäo periférica dos hormônios da glândula tireóide / Molecular biology of the genetic defects of synthesis, transport and peripheral action of the thyroid gland hormones

Os avanços da biologia molecular e o emprego de técnicas de identificaçäo de porçöes de DNA codificadoras para proteínas importantes na síntese, armazenamento, transporte e açäo periférica dos hormônios tireoideos, permitiram a elucidaçäo, a nivel molecular, da etiologia de várias condiçöes genéticas da tireóide. Em pacientes com defeito na peroxidase tireóidea (TPO) observou-se a presença de polimorfismo (RFLP) com a endonuclease Bgl-I que se segrega, em 2 alelos, com o fenótipo com bócio, hipotireoidismo e teste de perclorato positivo. Em defeitos da tireoglobulina (Tg), observou-se nível quantitativamente baixo de Tg, concomitante com a virtual ausência da RNA mensageiro para a Ig. Por outro lado, a ausência de incorporaçäo de ácido siálico na molécula de Tg, produz proteína com defeito estrutural que impede a síntese adequada de T3 e T4. Tais defeitos da Tg säo igualmente encontrados em animais com bócio congênito (gados bovino e caprino, e camundongos cog/cog). A presença de mutaçäo puntiforme no nucleotídeo 281 do exon 5 do RNA da globulina carreadora de T4 (TBG) leva a expressäo genética de proteína instável, com queda da TBG sérica. O efeito nos receptores nucleares de T3 estäo ligados ao gene codificador (c-erb-A-beta) localizado no cormossoma 3. A presença de polimorfismo EcorRV nos indivíduos, sugere gene mutante codificando proteína que poderia näo fixar T3 ou produzir proteína anômala com atividade bloqueadora da açäo de T3 impedindo a fixaçäo nuclear

A technical improvement for the thyroid peroxidase iodination assay

Thyroid peroxidase (TPO) iodination activity is generally evaluated in vitro by the iodination of poorly iodinated thyroglobulin or bovine serum albumin, followed by separation of protein-bound and inorganic iodide by paper chromatography,. Precipitation of protein-bound iodine by trichloroacetic acid (TCA) was evaluated as an alternative to the time-consuming paper chromatographic separation (PC) in normal rat TPO preparations. The protein-bound iodine estimates as well as the iodination activities determined by these two procedures were significantly correlated (r = 0.95 and 0.98, respectively, P<0.001), and the iodination activities regression line slope (b = 0.97 + or - 0.11) was not different from 1. The protein-bound iodine separation by TCA is simpler and faster, without loss of precision. Thus, it can be a useful alternative step in the thyroid peroxidase iodination assay

Thyroid peroxidase activity in human nodular goiters

1. Thyroid peroxidase (TPO, iodide-oxidation) activity was evaluated in nodular and paranodular tissue samples from 27 patients with nodular goiter (19 "cold" and 8 "hot" nodules), and compared to 11 diffuse toxic goiter and 9 normal thyroid tissue samples. 2. In terms of U/g digitonin solubilized protein, TPO activity was increased in hot nodules (p<0.05), although not as much as in diffuse toxic goiters (p<0.01> 3. The mean TPO activity of tissues paranodular to a cold nodule was not different from that of normal thyroids. 4. Both the highest and the lowest TPO activities were found in cold nodules, but their mean value did not differ from those of their paranodular tissues or normal thyroids. 5. Inter-tissue variability was significantly increased (p<0.01) in cold nodules and in tissues paranoudular to a hot nodule. 6. These data show that heterogeneity both within and among tissues contributes to the wide range of TPO activity detected in nodular goiters