Thyroid function and polyamines. III. Changes in ornithine decarboxylase activity and polyamine contents in the rat thyroid during hyperplasia and involution

Changes in ornithine decarboxylase (ODC) activity and in polyamine contents of the rat thyroid were studied under various experimental conditions. Methylthiouracil (MTU) treatment produced several-fold increases in the thyroid ODC activity and in the content of putrescine, spermidine and spermine within a week. While serum thyrotropin (TSH) levels increased gradually up to 3 weeks, the content of both putrescine and spermidine tended to reach a plateau after 2 weeks of the goitrogen treatment; spermine content continued to increase progressively for 3 weeks. Discontinuance of MTU at 7 days resulted in a rapid decline in the elevated thyroid ODC activity, followed by a diminution of putrescine, spermidine and RNA contents. Thyroidal putrescine, spermidine and RNA responded more sensitively to both introduction and withdrawal of TSH stimulation than thyroidal spermine and DNA. Excess iodide, having no effect on the basal level of thyroid ODC, suppressed the MTU-induced increase in this enzyme activity without affecting circulating TSH, thyroxine (T4) and triiodothyronine (T3) levels. There was a significant negative correlation between the ODC activity and intrathyroidal concentration of iodine in MTU-pretreated rats. Theophylline increased the thyroid weight and ODC activity when given to rats fed with a subeffective dose of MTU. Analyses of serum TSH, T4, T3 and of thyroidal iodine revealed that TSH-induced thyroid ODC activity was suppressed by increased circulating thyroid hormones and/or intrathyroidal iodine. Furthermore, it was suggested that thyroid hormones and excess iodide acted directly on the thyroid to alter polyamine biosynthesis, possibly by changing the responsiveness of the gland to TSH.     

Propranolol has direct antithyroid activity: Inhibition of iodide transport in cultured thyroid follicles

The effect of propranolol on the process of thyroid hormone formation was studied in a physiological culture system. Porcine thyroid follicles were preincubated with propranolol for 24 h. Iodide transport, iodine organification, and de novo thyroid hormone formation were measured by incubating these follicles with the mixture of carrier-free 0·1 μCi Na ¹²⁵I and 50 nM NaI for 2 to 6 h at 37°C. A concentration of propranolol greater than 100 μM inhibited iodide transport in a dose-dependent manner; this inhibition was non-competitive with iodide and independent of thyrotropin (TSH). Reduced iodine organification and thyroid hormone formation was seen with 150 μM propranolol or greater. The inhibitory action of propranolol was not caused by beta-blocking activity, since D -propranolol (devoid of beta-blocking activity) inhibited iodide transport, and other beta-blockers (metoprolol, atenolol, and labetalol) did not inhibit iodide transport. The inhibition of iodide transport was most likely caused by membrane stabilizing activity since quinidine, which possess the same membrane stabilizing activity as propranolol, also inhibited iodide transport. TSH-mediated cAMP generation and Na ⁺K⁺ ATPase activity, membrane functions for iodide transport, were unaffected by propranolol. Our study has shown, for the first time, that propranolol has a direct antithyroid action, namely inhibition of iodide transport in the intact thyroid follicle.     

Catechin induced modulation in the activities of thyroid hormone synthesizing enzymes leading to hypothyroidism

Catechins, the flavonoids found in abundance in green tea, have many beneficial health effects such as antioxidative, anticarcinogenic, anti-inflammatory, antiallergic, and hypotensive properties. However, flavonoids have antithyroid/goitrogenic effect, although less information is available about the effect of pure catechin on thyroid physiology. The present investigation has been undertaken to explore the effect of catechin administration on thyroid physiology in rat model. For the in vivo experiment catechin was injected intraperitoneally (i.p.) at doses of 10, 20 and 30 mg/kg body to male albino rats for 15 and 30 days, respectively, and thyroid activities were evaluated with respect to determination of serum levels of thyroid hormones, thyroid peroxidase, 5′-deiodinase I (5′-DI), and Na⁺, K⁺-ATPase activities that are involved in the synthesis of thyroid hormone. Catechin decreased the activities of thyroid peroxidase and thyroidal 5′-deiodinase I, while Na⁺, K⁺-ATPase activity significantly increased in dose-dependent manner; substantial decrease in serum T3 and T4 levels coupled with significant elevation of serum TSH were also noted. Histological examinations of the thyroid gland revealed marked hypertrophy and/or hyperplasia of the thyroid follicles with depleted colloid content. In in vitro study, short-term exposure of rat thyroid tissue to catechin at the concentrations of 0.10, 0.20, and 0.30 mg/ml leads to decrease in the activities of thyroid peroxidase and 5′-deiodinase I, while the activity of thyroidal Na⁺, K⁺-ATPase remains unaltered even at high concentration of catechin treatment. The present study reinforces the concept that catechin, tea flavonoids possess potent antithyroid activity as evidenced from in vivo and in vitro studies.     

Role of the hypophysis in thyroid regeneration after partial thyroidectomy.

The role of the hypophysis in thyroid regeneration was investigated by measuring the mitotic activity of the thyroid remnant in hemithyroidectomized rats as well as the blood levels of thyroid hormone at various time-intervals after hemithyroidectomy. Mitotic activity underwent a significant increase to reach a peak (a 5- to 8- fold increase) 2 days after hemithyroidectomy. The thyroid hormone level in blood was lower than in controls. Histologically, the thyroid gland showed signs of an elevated rate of functional activity, as indicated by losses of colloid and cell hypertrophy. In a second approach, the mitotic activity of the thyroid remnant was estimated in hypophysectomized and in thyroxine treated rats. Both hypophysectomy and thyroxine injection prevented occurrence of the mitotic peak at 2 days. The regeneration of the thyroid after hemithyroidectomy, as it occurred in the present work, may be explained by a release of thyroid stimulating hormone from the pituitary, brought about by the low level of circulating thyroid hormone, itself resulting from a loss of thyroid tissue.     

Effects of ovine prolactin on iodide metabolism and thyroid activity in the eel

Summary In the eel, ovine prolactin (oPrl) treatment (0.018 IU/day·g body weight), for 8 to 13 days modifies neither iodide absorption from the water nor excretion, extrathyroidal metabolism and plasma level of iodide.Thyroid activity, evaluated by epithelial cell height, radioiodine uptake and absolute iodide uptake is approximately twice that of controls. However, the amounts of total iodine, thyroxine (T₄) and triiodothyronine (T₃) in thyroid are unaltered by oPrl. Therefore, the decrease of plasma T₄ and the increase of plasma T₃, previously observed in oPrl-treated eels, do not result from a preferential thyroidal secretion of T₃, but only from a stimulation of peripheral conversion of T₄ to T₃. Furthermore, the increased thyroid activity probably originates from a decreased feedback inhibition following the fall of circulating T₄ induced by oPrl.Abbreviations oPrl ovine prolactin - T ₄ Thyroxine - T ₃ 3.5.3 triiodothyronine - TRH thyrotropin releasing hormone - TSH thyroid stimulating hormone - PBI protein bound iodine     

Free diiodotyrosine effects on protein iodination and thyroid hormone synthesis catalyzed by thyroid peroxidase.

Free diiosotyrosine exerts two opposite effects on the reactions catalyzed by thyroid peroxidase, thyroglobulin iodination and thyroid hormone formation. 1. Inhibition of thyroglobulin iodination catalyzed by thyroid peroxidase was observed when free diiodotyrosine concentration was higher than 5 muM. This inhibition was competitive, suggesting that free diiodotyrosine interacts with the substrate site(s) of thyroid peroxidase. Free diiodotyrosine also competively inhibited iodide peroxidation to I2. 2. Free diiodotyrosine, when incubated with thyroid peroxidase in the absence of iodide was recovered unmodified; in the presence of iodide an exchange reaction was observed between the iodine atoms present in the diiodotyrosine molecule and iodide present in the medium. Using 14C-labelled diiodotyrosine, 14C-labelled non-iodinated products were also observed, showing that deiodination occurred as a minor degradation pathway. However, no monoiodo[14C]tyrosine or E114C]tyrosine were observed. Exchange reaction between free diiototyrosine and iodide is therefore direct and does not imply deiodination-iodination intermediary steps. Thyroglobulin inhibits diiodotyrosine-iodide exchange and vice versa, again suggesting competition for both reactions. These results support, by a different experimental approach, the two-site model for peroxidase previously described by us in this journal. 3. Free diiodotyrosine when present at a very low concentration, 0.05 muM, exerts a stimulatory effect on throid hormones synthesis. The relationship between diiodotyrosine concentration and thyroid hormone synthesis give an S-shaped curve, suggesting that free diiodotyrosine acts as a regulatory ligand for thyroid peroxidase. Evidence is also presented that free diiodotyrosine is not incorporated into thyroid hormones. Therefore, thyroid peroxidase catalyzes only intra-molecular coupling between iodotyrosine hormonogenic residues. 4. Finally, although no direct proof exists that these free diiodotyrosine effects upon thyroglobulin iodination and thyroid hormone synthesis are physiologically significant, such a possibility deserves further investigation.     

Monoamine oxidase and aspartate aminotransferase in cellular fractions of thyroid gland in rats after hypophysectomy and TSH stimulation

Monoamine oxidase (MAO) and aspartate aminotransferase activities in cellular fractions of thyroid gland in rats after hypophysectomy and TSH treatment were investigated. MAO and aspartate aminotransferase activities in thyroid mitochondria were decreased after hypophysectomy and significantly increased after daily injection of TSH during five days to hypophysectomized rats. In microsomes after hypophysectomy a similar decrease of MAO activity was found but TSH was without effect on this activity. In conclusion: it is evident that MAO and aspartate aminotransferase in thyroid mitochondria the enzymes which could be a source for hydrogen peroxide to catalyze thyroid hormone synthesis are under the regulatory influence of TSH.     

Thyroidal autoregulation: Iodide-induced suppression of thyrotropin-stimulated cyclic AMP production and iodinating activity in thyroid cells

In continuing our study of the thyroidal autoregulation phenomenon, we have investigated the effects of iodide on several thyroidal responses to thyrotropin. Thus, we have found that the 2–4-fold thyrotropin stimulation of protein iodination in beef thyroid cells was reduced about 30% by 4 h of preincubation with 10 μM iodide, and virtually abolished with 50 μM iodide. Similarly the 8-fold thyrotropin stimulation of cyclic AMP accumulation in the cells was reduced about 30% by 3 h of preincubation with 50 μM iodide. It appears therefore that the so-called autoregulation of the thyroid gland does include influences of iodide on the thyrotropin stimulation of cyclic AMP production, iodide transport, and protein iodination which can be demonstrated in vitro in the dispersed thyroid cell system. Two other effects of thyrotropin, namely, the stimulation of [¹⁴C]leucine incorporation into protein and of iodide efflux were not at all affected by treatment with excess iodide, and hence may not be subject to the autoregulatory influence of iodide.     

Additive effects of thyroid hormone,growth hormone and testosterone on deoxyribonucleic acid-dependent ribonucleic acid polymerase in rat-liver nuclei

1. The stimulations of DNA-dependent RNA polymerase in isolated rat-liver nuclei by thyroid hormone, human growth hormone and testosterone are compared. 2. Single or multiple administrations of growth-promoting doses of tri-iodo-l-thyronine, human growth hormone and testosterone stimulate the Mg²⁺-activated RNA-polymerase reaction in nuclei from thyroidectomized, hypophysectomized and castrated rats respectively. The magnitude of stimulation was proportional to the degree of enhancement of liver growth by each hormone. After a single injection, the latent period preceding the stimulation was 1, 2 and 10hr. for growth hormone, testosterone and tri-iodothyronine respectively. The time-course of stimulation of enzyme activity and the synthesis of rapidly labelled nuclear RNA in vivo were also different for each hormone. 3. Growth hormone administration failed to stimulate the Mn²⁺/ammonium sulphate-activated RNA-polymerase reaction. Thyroid hormone and testosterone, however, stimulated it but the effect was less pronounced and occurred several hours later than that observed for the Mg²⁺-activated RNA-polymerase reaction. 4. In combination experiments, hypophysectomized or the thyroidectomized rats were given growth hormone or tri-iodothyronine in a single or repeated doses at levels that produced the maximum stimulation of Mg²⁺-activated RNA-polymerase activity. Taking into account the different latent period for each hormone, a single administration of the second hormone caused an additional stimulation of the enzyme activity. Similar additive effects were observed in thyroidectomized–castrated rats after treatment with tri-iodothyronine and testosterone. The magnitude of the additional stimulation caused by the administration of the second hormone was compatible with the capacity of that hormone to promote liver growth in rats deprived of it. 5. It is concluded that, although these hormones have some similar effects, the regulation of nuclear RNA synthesis may be mediated via different routes for each hormone.     

Glucocorticoid effects on gastric peroxidase activity

The peroxidase activity in rat gastric mucosa is inhibited after administration of glucocorticoids. The synthetic steroid dexamethasone is more potent than the naturally occurring steroids, such as cortisone or corticosterone. Almost complete inhibition of the enzyme occurs after 24 h with a single dose of 100 μg dexamethasone/120 g body weight. Other mitochondrial enzyme activities, like monoamine oxidase, succinic dehydrogenase and Mg²⁺-ATPase, remain unaltered under the same experimental condition. Submaxillary peroxidase and thyroid peroxidase activity are not inhibited by dexamethasone. Gastric peroxidase activity is increased 200–250% on the 6th day after adrenalectomy. This effect is blocked by the administration of dexamethasone. In fact, the enzyme becomes more sensitive to dexamethasone after adrenalectomy, since it is inhibited by more than 90% at the dose of 25 μg/120 g body weight. The inhibition by dexamethasone in normal animals is reversible. The enzyme is also inhibited after the administration of a single dose of ACTH. The apparent K_m of the enzyme for H₂O₂ is not altered after dexamethasone treatment or after adrenalectomy. The increase in enzyme activity following adrenalectomy is not blocked by actinomycin D or by α-amanitin, but is prevented by puromycin or cycloheximide. After administration of dexamethasone, the iodide concentration process in the gastric mucosa is not affected, but the organification of iodide is significantly diminished.     

Stimulation of ornithine decarboxylase synthesis in the rat thyroid

High titer antiserum to hepatic ornithine decarboxylase was prepared by employing enzyme·monospecific antibody complex as the immunizing antigen. This new antiserum preparation was successfully labeled with ¹²⁵I and was found to retain its specific immune properties. Iodinated antiserum was used to precipitate thyroid ornithine decarboxylase induced by a mixture of thyroid stimulating hormone and methyl xanthine in rat thyroids $\text{in vitro}$. ¹²⁵I-labeled antibody incorporation into the enzyme antibody complex after induction $\text{in vitro}$ showed an increase which paralleled the increase in enzymatic activity and thus suggested $\text{de novo}$ synthesis of thyroid enzyme protein.     

The effect of long-term external ionizing radiation on the functional activity of rat thyroid under enhanced potassium iodide consumption

The study was devoted to the effect of long-term (20 days) external ionizing radiation at a dose of 0.5 Gy on the iodide metabolism in the rat thyroid under supplementation of high iodine doses (10 daily KI doses). It was found that the potassium iodide administration partially prevented the effects of a post radiation decrease of serum thyroid hormone levels (the level of T4 was normal and that of T3 was 77.4% of the controls). After the supplementation of 10 daily iodide doses, the rat thyroid tissue showed the most pronounced increase in the levels of total, free and protein-bound iodide compared to the groups of animals consuming normal and elevated KI doses. Pronounced inhibition of thyroid peroxidase activity (3.1-fold) was noted in the same group. The data obtained indicate a radiation-induced activation of iodide uptake during its enhanced supplementation and disturbed iodide enzymatic oxidation and organification.     

Selenoenzyme activities in selenium- and iodine-deficient sheep

This study was conducted to evaluate the effects of single and combined deficiencies of selenium and iodine on selenoenzyme activities in sheep. Twenty-four male lambs were assigned to one of four semisynthetic diets: combined deficient A (Se⁻I), Se-deficient B (Se⁻I⁺), I-deficient C (Se⁺I⁻), and basal diet D (Se⁺I⁺). Thyroid hormones (T₃, T₄), thyroid stimulating hormone (TSH), and inorganic iodine (PII) were determined in plasma. Selenium and glutathione peroxidase activity (GSH-Px) were determined in erythrocytes, and tissue samples, including the thyroid, liver, kidney, and brain, were taken for selenoenzyme analysis. Plasma T₃, T₄, and TSH concentrations were similar in all groups. Type I deiodinase (ID-I) activity in liver and kidney remained unchanged in Se or I deficiency. In contrast, hepatic ID-I activity was increased by 70% in combined Se-I deficiency. Thyroidal cystolic GSH-Px (c-GSH-Px) and phospholipid GSH-Px (ph-GSH-Px) activities remained constant in both Se-deficient groups, whereas thyroidal c-GSH-Px activity increased (57%) in I deficiency. Type II deiodinase (ID-II) activity was not detectable in the cerebrum and cerebellum, whereas cerebellum Type III deiodinase (ID-III) activity was decreased in I deficiency and combined Se-I deficiencies. The results of the present study support a sensitive interaction between Se and I deficiencies in sheep thyroid and brain. Furthermore, the lack of thyroidal ID-I activity, the presservation of the thyroidal antioxidant enzymes, and the increases in hepatic ID-I indicate that a compensatory mechanism(s) works toward retaining plasma T₃ levels, mostly by de novo synthesis of T₃ and peripheral deiodination of T₄ in Se- and I-deficient sheep.     

Differential action of thyroid hormones and chemically related compounds on the activity of UDP-glucuronosyltransferases and cytochrome P-450 isozymes in rat liver

The effect of thyroid hormones and chemically related compounds, on the activity of UDP-glucuronosyltransferases (EC 2.4.1.17) and cytochrome P-450-dependent monooxygenases in rat liver microsomes was investigated. The animals were thyroidectomized and treated with different doses of the drugs for 3 weeks. Opposite effects were observed depending on the isoenzyme of UDP-glucuronosyltransferase considered. While 3,3′,5-triiodo-l-thyronine, 3,3′,5-triiodothyroacetic acid, 3,3′,5-triiodothyropropionic acid, isopropyldiiodothyronine and l- and d-thryoxine strongly increased 4-nitrophenol glucuronidation in a dose-dependent fashion, they decreased markedly bilirubin glucuronidation. However, the activity toward nopol, a monoterpenoid alcohol, was not significantly changed regardless of which compound or dose was used. Variation of UDP-glucuronosyltransferase observed with 4-nitrophenol and bilirubin was related to the thyromimetic effect of the drugs estimated from the increase in α-glycerophosphate dehydrogenase. Thyronine and 3,5-diiodo-l-tyrosine, which did not enhance this activity, also failed to affect glucuronidation. Variations in UDP-glucuronosyltransferase activity were more likely due to changes in protein expression rather than changes in enzyme latency, since lipid organization of the microsomal membrane, as estimated from the mean anisotropy of 1,6-diphenyl-1,3,5-hexatriene by fluorescence polarization was not significantly modified by the drug administration. Although some of the drugs could significantly decrease the triacylglycerol and cholesterol contents in plasma, all failed to affect lauric acid hydroxylation. The activities of catalase, palmitoyl-CoA dehydrogenase (CN^? insensitive) and carnitine acetyltransferase in the fraction enriched in peroxisomes were also not significantly affected by treatment with the thyroid hormone LT₃. In contrast, the activity of 7-ethoxycoumarine O-deethylase was increased by large doses of thyronine and by 3,3′,5′-triiodothyropropionic acid. The concentration of total cytochrome P-450 was decreased in a dose-dependent fashion by all the compounds used, except thyronine. Finally, significant correlations were observed between glucuronidation of bilirubin and 4-nitrophenol and the content in cytochrome P-450. This suggests a possible coordinate regulation of the two processes, which depends on the physicochemical characteristics of the thyroid hormones and related compounds.     

Brain biogenic amines and altered thyroid function.

Evidence has been presented that alterations in thyroidal status produce marked changes in the metabolism of several biogenic amines in developing brain. Neonatal hypothyroidism induced either by ¹³¹I or by an anti-thyroid agent, methimazole, markedly decreased the concentrations of norepinephrine, dopamine and 5-hydroxytryptamine and the activity of their rate-limiting enzymes, tyrosine hydroxylase and tryptophan hydroxylase. However, the levels of 5-hydroxyindoleacetic acid, the chief metabolite of 5-hydroxytryptamine were elevated in several regions of the brain. Whereas thyroid deficiency in early life produced no appreciable change in whole brain monoamine oxidase activity, it was increased in mid brain and decreased in the hypothalamus. Brain acetylcholine levels were significantly elevated and the activity of acetylcholinesterase remained unchanged in rats made hypothyroid at 1 day of age. Delaying thyroidectomy for 20 days after birth produced less appreciable changes in norepinephrine and 5-hydroxytryptamine metabolism. Thyroid deficiency suppressed the ontogenesis of behavioural arousal and spontaneous locomotor activity. The administration of L-triiodothyronine to hypothyroid animals in early life restored the metabolism of various neurohumors virtually to the normal limits. However, when the replacement therapy was postponed until adulthood, L-triiodothyronine failed to produce any restorative effects, suggesting that a critical period exists in early life during which thyroid hormone must be present to permit normal developmental pattern of central amines. Data also have been obtained demonstrating that neonatal hyperthyroidism induced by daily administration of L-triiodothyronine results in an increased turnover of norepinephrine and 5-hydroxytryptamine. These amine changes were accompanied by a marked rise in the spontaneous locomotor activity in hyperthyroid rats. Finally, chronic treatment with lithium, an antimanic drug, also known to suppress thyroid hormone production, significantly decreased not only the spontaneous locomotor activity, but also changes in the turnover of 5-hydroxytryptamine and norepinephrine in neonatally hyperthyroid rats.     

An improved assay method for thyroid peroxidase applicable for a few milligrams of abnormal human thyroid tissues

A peroxidase assay method (Mini assay method) which is applicable for a minute amount (as small as a few mg) of thyroid tissue was developed, employing guaiacol or iodide as the second substrate. This method is a modification of the previous one (Ordinary assay method): the volume of the reaction mixture was reduced to about one-tenth with prior solubilization of the enzyme. The correlation between the Mini assay and Ordinary assay methods, and between the guaiacol and iodide assays by both methods were satisfactorily good, but the iodine content of thyroglobulin was found to be not directly correlated to the peroxidase activities. Protein-based specific activities of peroxidase from normal human thyroid tissue were about 0.030 guaiacol units/mg protein and 0.0066 iodide units/mg protein, which were slightly higher than those of porcine thyroid tissue. The Mini assay method developed in the present study was used for the determination of peroxidase activity in a small amount (1-8 mg) of thyroid tissue obtained by means of a needle biopsy from patients with thyroid disorders. One specimen (goitrous cretinism) showed no peroxidase activity in both the guaiacol and iodide assays, and three specimens (two chronic thyroiditis, one familial nontoxic goiter) possessed no ability to catalyze the oxidation of iodide in spite of the high reactivity towards guaiacol, suggesting the presence of an abnormal peroxidase in these tissues.     

PCB153 disrupts thyroid hormone homeostasis by affecting its biosynthesis, biotransformation, feedback regulation, and metabolism

PCB153, one of the 3 dominant congeners in the food chain, causes the disruption of the endocrine system in humans and animals. In order to elucidate the effects of PCB153 on the biosynthesis, biotransformation, regulation, metabolism, and transport of thyroid hormones (THs), Sprague-Dawley (SD) rats were dosed with PCB153 intraperitoneally (i.p.) at 0, 4, 16 and 32 mg/kg/day for 5 consecutive days and sacrificed 24 h after the last dose. Results showed that after treatment with PCB153, serum total thyroxine (TT4), total triiodothyronine (TT3), and thyrotropin releasing hormone (TRH) decreased, whereas serum thyroid stimulating hormone (TSH) concentration did not alter. The serum sodium iodide symporter (NIS), thyroid peroxidase (TPO), and thyroglobulin (Tg) levels decreased. The mRNA expressions of type 2 and 3 deiodinases (D2 and D3) reduced, but the type 1 deiodinase (D1) showed no significant change. The TSH receptor (TSHr) and TRH receptor (TRHr) levels declined. PCB153 induced hepatic enzymes, and the UDPGTs, CYP2B1, and CYP3A1 mRNA levels were significantly elevated. Taken together, the observed results from the present study indicated that PCB153 disrupted thyroid hormone homeostasis through influencing synthesis-associated proteins (NIS, TPO and Tg), deiodinases, receptors (TSHr and TRHr), and hepatic enzymes, and the decrease of D3 expression might be the compensatory response of body.