Paula Bargi-Souza

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.

Hypothyroidism in adult male rats alters posttranscriptional mechanisms of luteinizing hormone biosynthesis.

BACKGROUND Studies in men are not consistent regarding the effects of thyroid hormone on the production of gonadotropins. In hypothyroidism consequent to diverse causes, an increase or no change in serum luteinizing hormone (LH) have been reported. The attempt to explain the mechanisms involved in this pathology using rats as an experimental model also seems to repeat this divergence, since hypothyroidism has been shown to induce hypogonadotropic hypogonadism, a hypergonadotropic state, or not to affect the basal levels of LH. Notably, the promoter region of the gene encoding the Lh beta subunit and GnRH (gonadotropin-releasing factor) does not contain a thyroid responsive element. Therefore, we investigated the hypothesis that, in male rats, posttranscriptional mechanisms of LH synthesis are altered in hypothyroidism. We also attempted to determine if hypothyroidism directly affects testicular function in male rats. METHODS Male Wistar rats, 60 days old, were thyroidectomized or sham-operated. After 20 days, they were decapitated, and the pituitaries were collected and analyzed for Lh mRNA, LH content, poly(A) tail length, and polysome profile. The testes were collected and analyzed for Lh receptor mRNA, LH receptor content, and histology using morphometric analyses. The testis, epididymis, seminal vesicle, and ventral prostate were weighed, and serum concentrations of LH, testosterone, thyrotropin (TSH), and triiodothyronine (T3) were measured. RESULTS Hypothyroidism was associated, in the pituitary, with an increase in Lh mRNA expression, a reduction in Lh mRNA poly(A) tail length, a reduction in the number of LH transcripts associated with polysomes. Pituitary LH was decreased but serum LH was increased from 102 to 543 pg/mL. Despite this, serum testosterone concentrations were decreased from 1.8 to 0.25 ng/mL. A decreased germinative epithelium height of the testes and a reduced weight of androgen-responsive tissues were observed (ventral prostrate: 74 vs. 23 mg/100 g body weight [BW]; seminal vesicle undrained: 280 vs. 70 mg/100 g BW; and seminal vesicle drained: 190 vs. 60 mg/100 g BW). CONCLUSIONS Hypothyroidism in adult male rats has dual effects on the pituitary testicular axis. It alters posttranscriptional mechanisms of LH synthesis and probably has a direct effect on testicular function. However, these data suggest the possibility that reduced LH bioactivity may account in part for impaired testicular function.

Repercussions of hypo and hyperthyroidism on the heart circadian clock

ABSTRACT Myocardial gene expression and metabolism fluctuate over the course of the day in association with changes in energy supply and demand. Time-of-day-dependent oscillations in myocardial processes have been linked to the intrinsic cardiomyocyte circadian clock. Triiodothyronine (T3) is an important modulator of heart metabolism and function. Recently, our group has reported time-of-day-dependent rhythms in cardiac T3 sensitivity, as well as, T3-mediated acute alterations on core clock components. Hypo and hyperthyroidism are the second most prevalent endocrine disease worldwide. Considering the importance of the cardiomyocyte circadian clock and T3 to cardiac physiology, the aim of this study was to investigate the consequences of chronic hypo and hyperthyroidism on 24-h rhythms of circadian clock genes in the heart. Hypo and hyperthyroidism was induced in rats by thyroidectomy (Tx) and i.p. injections of supraphysiological dose of T3, respectively. Here we report alterations in mRNA levels of the major core clock components (Bmal1, Per2, Nr1d1, and Rora) for both experimental conditions (with the exception of Per2 during hyperthyroid condition). Oscillations in mRNA levels of key glucose and fatty-acid metabolism genes known to be clock controlled (Pdk4, Ucp3, Acot1, and Cd36) were equally affected by the experimental conditions, especially during the hypothyroid state. These findings suggest that chronic alterations in thyroid status significantly impacts 24-h rhythms in circadian clock and metabolic genes in the heart. Whether these perturbations contribute toward the pathogenesis of cardiac dysfunction associated with hypo and hyperthyroidism requires further elucidation.

Beneficial effects of thyroid hormone on adipose inflammation and insulin sensitivity of obese Wistar rats

Thyroid hormones play an important role in glucose metabolism and there is evidence of increased prevalence of thyroid dysfunction in obese and diabetic patients. This study aimed at evaluating the thyroid function and the effects of the triiodothyronine (T3) treatment on glycemia control, insulin sensitivity and subclinical inflammation in cafeteria‐diet‐induced obesity in rats. Obesity was induced in male Wistar rats by offering a cafeteria diet and a subset of the obese rats was treated with T3 (1.5 μg per 100 g of body weight) for a 28‐day period. The pituitary‐thyroid axis was evaluated by molecular and biochemical parameters. Cytokine content was measured in the serum as well as in the mesenteric and epididymal white adipose tissue. Obese rats exhibited impairment of glycemia control, increased content of inflammatory cytokines in mesenteric white adipose tissue, decreased serum thyrotropin (TSH) concentration and increased sodium/iodide symporter (NIS) and TSH receptor (TSHR) protein content in thyroid gland. T3 treatment improved insulin sensitivity, glucose tolerance, and reduced inflammatory cytokine content in mesenteric white adipose tissue. In the thyroid gland NIS, TSHR, and thyroperoxidase (TPO) content were reduced while thyroglobulin (TG) content was increased by T3. The thyrotrophic response to negative feedback exerted by T3 was preserved in obese rats. The present data reinforce the beneficial effects of T3 treatment of obese rats on the improvement of insulin sensitivity and on the negative modulation of inflammatory cytokine expression in adipose tissue. Moreover, we have evidenced that the pituitary‐thyroid axis is affected in obese rats, as illustrated by the impaired TSH secretion.

Posttranscriptional actions of triiodothyronine on Tshb expression in TαT1 cells: New insights into molecular mechanisms of negative feedback

Rapid actions of triiodothyronine (T3) on thyrotropin (TSH) synthesis and secretion have been described in hypothyroid male rats. However, the molecular mechanisms remain unknown. TαT1 cells, a thyrotroph cell line, was used herein to characterize the possible non-genomic actions of T3 on the expression of alpha (Cga) and Tshb genes, and the posttranscriptional processing and translation of both transcripts. The involvement of αVβ3 integrin was also assessed. T3 quickly reduced Tshb mRNA content, poly(A) tail length and its association with ribosomes. The effect of T3 on Tshb gene expression was detected even in the presence of a transcription inhibitor. The decrease in Tshb mRNA content and polyadenylation depend on T3 interaction with αVβ3 integrin, while T3 reduced Cga mRNA content by transcriptional action. The translational rate of both transcripts was reduced by a mechanism, which does not depend on T3-αVβ3 integrin interaction. Results indicate that, in parallel with the inhibitory transcriptional action in Cga and Tshb gene expression, T3 rapidly triggers additional posttranscriptional mechanisms, reducing the TSH synthesis. These non-genomic actions partially depend on T3-αVβ3 integrin interaction at the plasma membrane of thyrotrophs and add new insights to the molecular mechanisms involved in T3 negative feedback loop.

High-fat diet affects gut nutrients transporters in hypo and hyperthyroid mice by PPAR-a independent mechanism

Aims: High fat diet consumes and thyroid hormones (THs) disorders may affect nutrients metabolism, but their impact on the absorptive epithelium, the first place of nutrients access, remains unknown. Our aim was to evaluate the intestinal morphology and nutrients transporters content in mice fed standard (LFD) or high fat (HFD) diets in hypo or hyperthyroidism‐induced condition. Material and methods: C57BL/6 male mice fed LFD or HFD diets for 12 weeks, followed by saline, PTU (antithyroid drug) or T3 treatment up to 30 days. The mice were euthanized and proximal intestine was removed to study GLUT2, GLUT5, PEPT1, FAT‐CD36, FATP4, NPC1L1 and NHE3 distribution by Western blotting. Since PPAR‐a is activated by fatty acids, which is abundant in the HFD, we also evaluated whether PPAR‐a affects nutrients transporters. Thus, mice were treated with fenofibrate, a PPAR‐a agonist. Key findings: HFD decreased GLUT2, PEPT1, FAT‐CD6 and NPC1L1, but increased NHE3, while GLUT5 and FATP4 remained unaltered. THs did not alter distribution of nutrients transporters neither in LFD nor in HFD groups, but they increased villi length and depth crypt in LFD and HFD, respectively. Fenofibrate did not affect content of nutrients transporters, excluding PPAR‐a involvement on the HFD‐induced changes. Significance: We assume that chronic HFD consumption reduced most of the nutrients transporters content in the small intestine of mice, which might limit the entrance of nutrients and gain weight. Since NHE3 promotes sodium absorption, and it was increased in HFD group, this finding could contribute to explain the hypertension observed in obesity.

Triiodothyronine differentially modulates the LH and FSH synthesis and secretion in male rats

Hypothyroidism and thyrotoxicosis produce adverse effects in male reproduction by unknown mechanisms. We investigated whether triiodothyronine (T3) modulates luteinizing hormone (LH) and follicle stimulating hormone (FSH) synthesis/secretion, by inducing different thyroid states. In hypothyroidism, the content of Lhb and Fshb mRNAs was increased, while their association to ribosomes and the protein content were reduced and the serum LH and FSH concentrations were augmented and decreased, respectively. Thyrotoxicosis reduced Lhb mRNA and LH serum concentration, and increased Lhb mRNA translational rate. The Fshb mRNA content and its association to ribosomes were also increased, whereas FSH serum concentrations were comparable to euthyroid levels. Acute T3 treatment decreased the total content of Lhb and Fshb mRNAs, and increased their association to ribosomes, as well as the LHB and FSHB contents in secretory granules. This study shows that T3 acts on gonadotrophs, resulting in direct effects on LH and FSH synthesis/secretion of male rats, suggesting that some reproductive disorders observed in men may be associated with thyroid hormone imbalances.