Rapid alteration in circulating free thyroxine modulates pituitary type II 5' deiodinase and basal thyrotropin secretion in the rat

Sustained Pituitary T3 Production Explains the T4-mediated TSH Feedback Mechanism

The regulation of thyroid activity and thyroid hormone (TH) secretion is based on feedback mechanisms that involve the anterior pituitary TSH and medial basal hypothalamus TSH-releasing hormone. Plasma T3 levels can be "sensed" directly by the anterior pituitary and medial basal hypothalamus; plasma T4 levels require local conversion of T4 to T3, which is mediated by the type 2 deiodinase (D2). To study D2-mediated T4 to T3 conversion and T3 production in the anterior pituitary gland, we used mouse pituitary explants incubated with 125I-T4 for 48 hours to measure T3 production at different concentrations of free T4. The results were compared with cultures of D1- or D2-expressing cells, as well as freshly isolated mouse tissue. These studies revealed a unique regulation of the D2 pathway in the anterior pituitary gland, distinct from that observed in nonpituitary tissues. In the anterior pituitary, increasing T4 levels reduced D2 activity slightly but caused a direct increase in T3 production. However, the same changes in T4 levels decreased T3 production in human HSkM cells and murine C2C12 cells (both skeletal muscle) and mouse bone marrow tissue, which reached zero at 50 pM free T4. In contrast, the increase in T4 levels caused the pig kidney LLC-PK1 cells and kidney fragments to proportionally increase T3 production. These findings have important implications for both physiology and clinical practice because they clarify the mechanism by which fluctuations in plasma T4 levels are transduced in the anterior pituitary gland to mediate the TSH feedback mechanism.

Perinatal exposure to insecticide fipronil: effects on the reproductive system in male rats

Fipronil is an insecticide widely used in agriculture, veterinary medicine and public health that has recently been listed as a potential endocrine disrupter. In the present study we evaluated the effects of perinatal exposure to fipronil during the period of sexual brain differentiation and its later repercussions on reproductive parameters in male rats. Pregnant rats were exposed (via gavage) to fipronil (0.03, 0.3 or 3 mg kg–1) from Gestational Day 15 until Postnatal Day 7. Fipronil exposure did not compromise the onset of puberty. In adulthood, there was no effect on organ weight or sperm production. Furthermore, there were no adverse effects on the number of Sertoli cells per seminiferous tubule, testicular and epididymal histomorphometry or histopathology or expression patterns of androgen receptor in the testis. Similarly, no changes were observed in the sexual behaviour or hormone levels. However, in rats exposed to fipronil, changes in sperm motility were observed, with a decrease in motile spermatozoa and an increase in non-mobile spermatozoa, which can compromise sperm quality in these rats. Perinatal exposure to fipronil has long-term effects on sperm parameters, and the epididymis can be a target organ. Additional studies should be undertaken to identify the mechanisms by which fipronil affects sperm motility.

The effects of intra-hippocampal L-thyroxine infusion on long-term potentiation and long-term depression: A possible role for the αvβ3 integrin receptor

Although the effects of long-term experimental dysthyroidism on long-term potentiation (LTP) and long-term depression (LTD) have been documented, the relationship between LTP/LTD and acute administration of L-thyroxine (T4) has not been described. Here, we investigated the effects of intra-hippocampal administration of T4 on synaptic plasticity in the dentate gyrus of the hippocampal formation. After a 15-minute baseline recording, LTP and LTD were induced by application of high- and low-frequency stimulation protocols, respectively. Infusions of saline or T4 and tetraiodothyroacetic acid (tetrac), a T4 analog that inhibits binding of iodothyronines to the integrin αvβ3 receptor, either alone or together, were made during the stimulation protocols. The averages of the excitatory postsynaptic potential (EPSP) slopes and population spike (PS) amplitudes, between 55 to 60 minutes, were used as a measure of the LTP/LTD magnitude and were analyzed by two-way univariate ANOVA with T4 and tetrac as between-subjects factors. The input-output curves of the infusion groups were comparable to each other, as shown by the non significant interaction observed between stimulus intensity and infused drug. The magnitude of the LTP in T4-infused rats was significantly lower as compared to saline-infused rats. Both the PS amplitude and the EPSP slope were depressed more markedly with T4 infusion than with saline, tetrac, and T4 + tetrac infusion. Data of this study provide in vivo evidence that T4 can promote LTD over LTP via the integrin αvβ3 receptor, and that the effect of endogenous T4 on this receptor can be suppressed by tetrac in the hippocampus. © 2016 Wiley Periodicals, Inc.

Effect of a combined iodine and selenium supplementation on I and Se status of cows and their calves

Iodine (I) and selenium (Se) deficiencies are commonly reported in cattle, however, there are also studies regarding a very high iodine supply. The aim of the study was to determine the long-term effect of I and Se supplementation on non-pregnant cows, pregnant cows and their calves. The hypothalamus pituitary axis was investigated (TSH, T4, T3 assays) during a TRH challenge on non-pregnant cows. Twenty-four cows, half of them pregnant, were assigned into 2 diet-groups, one group with a low I (0.45 ppm) and Se (0.15 ppm) diet (LISe), the other with a high I (5.45 ppm) and Se (0.45 ppm) diet (HISe), for a period of 120 days. Nutritional (plasma iodide, urinary I, plasma Se, I content in colostrum and foetal fluids) and functional (thyrotropin, thyroid hormones, glutathione-peroxidase activity in erythrocytes) markers of I and Se status were assayed in dams at regular intervals for 120 days and in their calves at birth. A TRH challenge was performed on 8 non-pregnant cows at day 110 of the trial. At the end of the study, I and Se nutritional markers were higher in dams in the HISe group, compared to the LISe group, except for plasma Se. At birth, I nutritional markers in calves in the HISe group were higher compared to the LISe group. Reactivity of the pituitary-thyroid-axis was not influenced by I and Se supplementation. I and Se supplementation is efficient in improving newborn status.

Feedback regulation of thyrotropin-releasing hormone gene expression by thyroid hormone in the hypothalamic paraventricular nucleus

Hypothyroidism caused by chemical or surgical thyroidectomy or hypophysectomy causes a substantial increase in the content of thyrotropin-releasing hormone (TRH) mRNA and proTRH exclusively in cells of the medial and periventricular paravocellular subdivisions of the hypothalamic paraventricular nucleus (PVN). This response may be important to raise the anterior pituitary thyrostat to promote increased secretion of thyroid-stimulating hormone (TSH) and to induce the secretion of a more biologically active TSH. The increase in TRH mRNA can be obliterated by stereotaxic implants of hormonally active L-triiodothyronine (T3) placed into the anterior hypothalamus but not by implants of the hormonally inactive 3,5'-diiodo-L-thyronine (T2); we therefore suggested that T3 has a direct action on TRH-containing cells of the PVN. Ablation of brainstem catecholaminergic projection fields to the PVN (known to stimulate TRH secretion) has no effect on TRH mRNA expression; beta 1 thyroid hormone receptor mRNA is present in extracts of the PVN. Euthyroid levels of serum T3 in hypothyroid animals achieved via intraperitoneally implanted osmotic minipumps are not associated with a return of PVN levels of TRH mRNA to normal unless circulating T3 levels are raised into the hyperthyroid range (1.7 times normal). This requirement is similar to that needed to normalize nuclear thyroid hormone receptor levels in the anterior pituitary of hypothyroid animals, suggesting that in addition to circulating T3 monodeiodination of T4 to T3 within the brain must also contribute to feedback inhibition of TRH mRNA. As Type II deiodinase activity is absent or very low in the PVN and does not rise with hypothyroidism, we propose that an alternative source for T4 monodeiodination exists within the central nervous system.

Atypical expression of type 2 iodothyronine deiodinase in thyrotrophs explains the thyroxine-mediated pituitary thyrotropin feedback mechanism

T(4), the main product of thyroid secretion, is a critical signal in plasma that mediates the TSH-negative feedback mechanism. As a prohormone, T(4) must be converted to T(3) to acquire biological activity; thus, type 2 iodothyronine deiodinase (D2) is expected to play a critical role in this feedback mechanism. However, the mechanistic details of this pathway are still missing because, counterintuitively, D2 activity is rapidly lost in the presence of T(4) by a ubiquitin-proteasomal mechanism. In the present study, we demonstrate that D2 and TSH are coexpressed in rat pituitary thyrotrophs and that hypothyroidism increases D2 expression in these cells. Studies using two murine-derived thyrotroph cells, TtT-97 and TalphaT1, demonstrate high expression of D2 in thyrotrophs and confirm its sensitivity to negative regulation by T(4)-induced proteasomal degradation of this enzyme. Despite this, expression of the Dio2 gene in TalphaT1 cells is higher than their T(4)-induced D2 ubiquitinating capacity. As a result, D2 activity and net T(3) production in these cells are sustained, even at free T(4) concentrations that are severalfold above the physiological range. In this system, free T(4) concentrations and net D2-mediated T(3) production correlated negatively with TSHbeta gene expression. These results resolve the apparent paradox between the homeostatic regulation of D2 and its role in mediating the critical mechanism by which T(4) triggers the TSH-negative feedback.

Selenium and thyroid function in infants, children and adolescents

Selenium is an integral component of the enzymes glutathione peroxidase (GPx) and iodothyronine deiodinases. Although selenium nutrition could conceivably affect thyroid function in infants, children and adolescents, available data suggest that the effect of selenium deficiency on thyroid function is relatively modest. In patients with isolated selenium deficiency (such as patients with phenylketonuria receiving a low-protein diet), peripheral thyroid hormone metabolism is impaired but there are no changes in thyrotropin (TSH) or clinical signs of hypothyroidism, suggesting that these patients are euthyroid. Selenium supplementation may be advisable to optimize tissue GPx activity and prevent potential oxidative stress damage. In areas where combined selenium and iodine deficiencies are present (such as endemic goiter areas in Central Africa), selenium deficiency may be responsible for the destruction of the thyroid gland in myxoedematous cretins but may also play a protective role by mitigating fetal hypothyroidism. In these areas, selenium supplementation should only be advocated at the same time or after iodine supplementation. In patients with absent or decreased production of thyroid hormones and who rely solely on deiodination of exogenous L-thyroxine for generation of the active triiodothyronine (such as patients with congenital hypothyroidism), selenium supplementation may optimize thyroid hormone feedback at the pituitary level and decrease stimulation of the residual thyroid tissue.

Cloning, expression, and functional characterization of the substrate binding subunit of rat type II iodothyronine 5'-deiodinase

Type II iodothyronine 5'-deiodinase catalyzes the bioactivation of thyroid hormone in the brain. In astrocytes, this approximately 200-kDa, membrane-bound enzyme is composed of at least one p29 subunit, an approximately 60-kDa, cAMP-induced activation protein, and one or more unidentified catalytic subunit(s). Recently, an artificial type II-like selenodeiodinase was engineered by fusing two independent cDNAs together; however, no native type II selenodeiodinase polypeptide is translated in the brain or brown adipose tissue of rats. These data suggest that the native type II 5'-deiodinase in rat brain is unrelated to this artificial selenoprotein. In this report, we describe the cloning of the 29-kDa subunit (p29) of type II 5'-deiodinase from a lambdazapII cDNA library prepared from cAMP-induced astrocytes. The 3.3-kilobase (kb) cDNA encodes an approximately 30-kDa, 277-amino acid long, hydrophobic protein lacking selenocysteine. Northern blot analysis showed that a 3.5-kb p29 mRNA was present in tissues showing type II 5'-deiodinase activity such as brain and cAMP-stimulated astrocytes. Domain-specific, anti-p29 antibodies specifically immunoprecipitated enzyme activity. Overexpression of exogenous p29 or a green fluorescence protein (GFP)-tagged p29 fusion protein led to a >100-fold increase in deiodinating activity in cAMP-stimulated astrocytes, and the increased activity was specifically immunoprecipitated by anti-GFP antibodies. Steady-state reaction kinetics of the enzyme in GFP-tagged p29-expressing astrocytes are identical to those of the native enzyme in brain. Direct injection of replication-deficient Ad5-p29(GFP) virus particles into the cerebral cortex of neonatal rats leads to a approximately 2-fold increase in brain type II 5'-deiodinating activity. These data show 1) that the 3.3-kb p29 cDNA encodes an essential subunit of rat type II iodothyronine 5'-deiodinase and 2) identify the first non-selenocysteine containing subunit of the deiodinase family of enzymes.

Effect of 3,5-diiodo-L-thyronine on thyroid stimulating hormone and growth hormone serum levels in hypothyroid rats

We have investigated the biological effects of physiological doses of 3,5-diiodo-L-thyronine (3,5-T2) and 3,3'-diiodo-L-thyronine (3,3'-T2) (at doses from 2.5 to 10 microg/100 g BW) on serum TSH and GH levels in rats made hypothyroid by propylthiouracil and iopanoic acid administration. In such animals deiodinase activities were inhibited and thyroid hormones serum levels strongly reduced. The effects of T2s were compared with those elicited by 3,5,3'-triiodo-L-thyronine (T3) (2.5 microg/100 g BW).The serum TSH level was much greater in hypothyroid rats than in euthyroid ones. T3 administration suppressed TSH by 88% compared to control (i.e, the level in hypothyroid rats); it thus reached a value not significantly different from that seen in the euthyroid rats. 3,5-T2 produced a similar effect, suppressing the TSH level by about 75% compared to control; it thus reached values not significantly different from those of the euthyroid and T3-treated rats. By contrast, 3,3'-T2 had no effect on TSH, whatever the dose. The serum GH level was much lower in hypothyroid rats than in euthyroid ones. T3 administration increased the GH level by about 5-fold, restoring it to the value seen in euthyroid rats. 3,5-T2-treated hypothyroid rats, at all the doses used (from 2.5 to 10 microg/100 g BW), showed increased serum GH levels: at a dose of 10 microg/100 g BW the level reached a value about 5-fold higher than that in hypothyroid rats. This value was not significantly different from those of euthyroid and T3-treated rats. 3,3'-T2 did not affect GH levels whatever the dose. Thus, 3,5-T2 (but not 3,3'-T2) seems to mimic the effects of T3 on serum TSH and GH levels in rats.

Rapid stimulation of type I 5'-deiodinase in rat pituitaries by 3,3',5-triiodo-L-thyronine

The negative feedback control of thyrotropin production by the anterior pituitary involves local 5'-deiodination of L-thyroxine (L-T4) to the active thyroid hormone 3,3',5-triiodo-L-thyronine (T3) by two 5'-deiodinase isozymes which are distinctly regulated by thyroid hormone. T3 rapidly increased steady-state mRNA levels and activity of type I iodothyronine-5'-deiodinase (5'DI) in rat anterior pituitary and in reaggregate cultures of anterior pituitaries. Type II 5'-deiodinase activity determined in parallel with 3,3',5-triiodo-L-thyronine (rT3) as substrate was markedly lower than that of 5'DI. 5'DI mRNA levels and activity were higher in anterior pituitaries of female compared to male rats. Neither gender differences nor T3 stimulation of 5'DI activity were found in the posterior part. These data demonstrate a T3 dependent expression of 5'DI in euthyroid anterior pituitary and suggest that this isozyme serves a major function within the complex network of thyroid hormone homeostasis.

The thyroid gland is a major source of circulating T3 in the rat

In rats, the respective contribution of the thyroid and peripheral tissues to the pool of T3 remains unclear. Most, if not all, of the circulating T3 produced by extrathyroidal sources is generated by 5'-deiodination of T4, catalyzed by the selenoenzyme, type I iodothyronine 5'-deiodinase (5'D-I). 5'D-I in the liver and kidney is almost completely lost in selenium deficiency, resulting in a marked decrease in T4 deiodination and an increase in circulating T4 levels. Surprisingly, circulating T3 levels are only marginally decreased by selenium deficiency. In this study, we used selenium deficiency and thyroidectomy to determine the relative contribution of thyroidal and extrathyroidal sources to the total body pool of T3. Despite maintaining normal serum T4 concentrations in thyroidectomized rats by T4 replacement, serum T3 concentrations remained 55% lower than those seen in intact rats. In intact rats, restricting selenium intake had no effect on circulating T3 concentrations. Decreasing 5'D-I activity in the liver and kidney by > 90% by restricting selenium intake resulted in a further 20% decrease in serum T3 concentrations in the thyroidectomized, T4 replaced rats, suggesting that peripheral T4 to T3 conversion in these tissues generates approximately 20% of the circulating T3 concentrations. While dietary selenium restriction markedly decreased intrahepatic selenium content (> 95%), intrathyroidal selenium content decreased by only 27%. Further, thyroid 5'D-I activity actually increased 25% in the selenium deficient rats, suggesting the continued synthesis of this selenoenzyme over selenoproteins in other tissues in selenium deficiency. These data demonstrate that the thyroid is the major source of T3 in the rat and suggest that intrathyroidal T4 to T3 conversion may account for most of the T3 released by the thyroid.