Immunohistochemical localization of nuclear 3,5,3'-triiodothyronine receptor proteins in rat tissues studied with antiserum against C-ERB A/T3 receptor

Thyroid hormone action in the developing testis: intergenerational epigenetics

Male fertility involves the successful transmission of the genetic code to the next generation. It requires appropriately timed cellular processes during testis development, adequate support of spermatogenesis by hormonal cues from the reproductive axis and cellular cross-talk between germ and somatic cells. In addition to being the vessel of the father’s genome, increasing evidence shows that the mature sperm carries valuable epigenetic information – the epigenome – that, after fecundation, influences the development of the next generation, affecting biological traits and disease susceptibility. The epigenome of the germ line is susceptible to environmental factors, including exogenous chemicals and diet, but it is also affected by endogenous molecules and pathophysiological conditions. Factors affecting testis development and the epigenetic information of the germ line are critical for fertility and of relevance to the non-genetic but heritable component in the etiology of complex conditions. Thyroid hormones are one of those factors and their action, when untimely, produces profound effects on the developing testis, affecting spermatogenesis, steroidogenesis, testis size, reproductive hormones and fertility. Altered thyroid hormone states can also change the epigenetic information of the male germ line, with phenotypic consequences for future generations. In the context of past literature concerning the consequences of altered thyroid hormone action for testis development, here we review recent findings about the pathophysiological roles of the principal determinants of testicular thyroid hormone action. We also discuss limited work on the effects of thyroid hormone on the male germ line epigenome and the implications for the intergenerational transmission of phenotypes via epigenetic mechanisms.

Stimulating effect of thyroid hormones in peripheral nerve regeneration: research history and future direction toward clinical therapy

Injury to peripheral nerves is often observed in the clinic and severe injuries may cause loss of motor and sensory functions. Despite extensive investigation, testing various surgical repair techniques and neurotrophic molecules, at present, a satisfactory method to ensuring successful recovery does not exist. For successful molecular therapy in nerve regeneration, it is essential to improve the intrinsic ability of neurons to survive and to increase the speed of axonal outgrowth. Also to induce Schwann cell phenotypical changes to prepare the local environment favorable for axonal regeneration and myelination. Therefore, any molecule that regulates gene expression of both neurons and Schwann cells could play a crucial role in peripheral nerve regeneration. Clinical and experimental studies have reported that thyroid hormones are essential for the normal development and function of the nervous system, so they could be candidates for nervous system regeneration. This review provides an overview of studies devoted to testing the effect of thyroid hormones on peripheral nerve regeneration. Also it emphasizes the importance of combining biodegradable tubes with local administration of triiodothyronine for future clinical therapy of human severe injured nerves. We highlight that the local and single administration of triiodothyronine within biodegradable nerve guide improves significantly the regeneration of severed peripheral nerves, and accelerates functional recovering. This technique provides a serious step towards future clinical application of triiodothyronine in human severe injured nerves. The possible regulatory mechanism by which triiodothyronine stimulates peripheral nerve regeneration is a rapid action on both axotomized neurons and Schwann cells.

Expression of dominant-negative thyroid hormone receptor alpha1 in Leydig and Sertoli cells demonstrates no additional defect compared with expression in Sertoli cells only

BACKGROUND

In the testis, thyroid hormone (T3) regulates the number of gametes produced through its action on Sertoli cell proliferation. However, the role of T3 in the regulation of steroidogenesis is still controversial.

METHODS

The TRαAMI knock-in allele allows the generation of transgenic mice expressing a dominant-negative TRα1 (thyroid receptor α1) isoform restricted to specific target cells after Cre-loxP recombination. Here, we introduced this mutant allele in both Sertoli and Leydig cells using a novel aromatase-iCre (ARO-iCre) line that expresses Cre recombinase under control of the human Cyp19(IIa)/aromatase promoter.

FINDINGS

We showed that loxP recombination induced by this ARO-iCre is restricted to male and female gonads, and is effective in Sertoli and Leydig cells, but not in germ cells. We compared this model with the previous introduction of TRαAMI specifically in Sertoli cells in order to investigate T3 regulation of steroidogenesis. We demonstrated that TRαAMI-ARO males exhibited increased testis weight, increased sperm reserve in adulthood correlated to an increased proliferative index at P3 in vivo, and a loss of T3-response in vitro. Nevertheless, TRαAMI-ARO males showed normal fertility. This phenotype is similar to TRαAMI-SC males. Importantly, plasma testosterone and luteinizing hormone levels, as well as mRNA levels of steroidogenesis enzymes StAR, Cyp11a1 and Cyp17a1 were not affected in TRαAMI-ARO.

CONCLUSIONS/SIGNIFICANCE

We concluded that the presence of a mutant TRαAMI allele in both Leydig and Sertoli cells does not accentuate the phenotype in comparison with its presence in Sertoli cells only. This suggests that direct T3 regulation of steroidogenesis through TRα1 is moderate in Leydig cells, and that Sertoli cells are the main target of T3 action in the testis.

Chronic hypothyroidism only marginally affects adult-type Leydig cell regeneration after EDS administration

Chronic prenatally induced dietary hypothyroidism delays adult-type Leydig cell development, but does not block this process. Using a chemical model to induce hypothyroidism, it was suggested that development of a new population of Leydig cells was completely inhibited following the addition of the cytotoxic compound ethane-1,2-dimethyl sulphonate (EDS). In this study, we used a dietary approach to induce hypothyroidism and reinvestigated the regeneration of the Leydig cell population following EDS administration. Eighty-four day old euthyroid and chronically hypothyroid rats received an injection of EDS and were killed directly before or at regular intervals up to 77 days after EDS. In some control and hypothyroid animals, the first progenitor-type Leydig cells were observed at day 12 after EDS. At day 16, Leydig cell progenitors were present in all rats. The percentage of proliferating Leydig cells peaked in the euthyroid animals at day 21 after EDS. In the hypothyroid testis such a peak was not observed, although the percentage of proliferating regenerating Leydig cells was significantly higher from days 35 to 56 compared with the controls. This suggested that the wave of Leydig cell proliferation was delayed in the hypothyroid animals as compared with the euthyroid controls. On the day of EDS injection, the Leydig/Sertoli cell ratio was 37% lower in the hypothyroid rats compared with the controls. The Leydig/Sertoli cell ratio remained lower in the EDS-treated hypothyroid animals compared with the controls at all time points investigated. At day 77 after EDS, the Leydig cell population had returned to its pre-treatment size in both groups. Plasma testosterone production was reduced to below detectable levels immediately after EDS injection, and started to increase again on day 16, reaching pre-treatment values on day 21 in both groups. Taken together, severely reduced thyroid hormone levels did not block the regeneration of the adult-type Leydig cell population following EDS, as has been suggested previously.

Prenatal induced chronic dietary hypothyroidism delays but does not block adult-type Leydig cell development

Transient hypothyroidism induced by propyl-2-thiouracyl blocks postpartum Leydig cell development. In the present study, the effects of chronic hypothyroidism on the formation of this adult-type Leydig cell population were investigated, using a more physiological approach. Before mating, dams were put on a diet consisting of an iodide-poor feed supplemented with a low dose of perchlorate and, with their offspring, were kept on this diet until death. In the pups at day 12 postpartum, plasma thyroid-stimulating hormone levels were increased by 20-fold, whereas thyroxine and free tri-iodothyronine levels were severely depressed, confirming a hypothyroid condition. Adult-type progenitor Leydig cell formation and proliferation were reduced by 40-60% on days 16 and 28 postpartum. This was followed by increased Leydig cell proliferation at later ages, suggesting a possible slower developmental onset of the adult-type Leydig cell population under hypothyroid conditions. Testosterone levels were increased 2- to 10-fold in the hypothyroid animals between days 21 and 42 postpartum compared with the age-matched controls. Combined with the decreased presence of 5alpha-reductase, this implicates a lower production capacity of 5alpha-reduced androgens. In 84-day-old rats, after correction for body weight-to-testis weight ratio, plasma insulin-like factor-3 levels were 35% lower in the hypothyroid animals, suggestive of a reduced Leydig cell population. This is confirmed by a 37% reduction in the Sertoli cell-to-Leydig cell ratio in hypothyroid rats. In conclusion, we show that dietary-induced hypothyroidism delays but, unlike propyl-2-thiouracyl, does not block the development of the adult-type Leydig cell population.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Antithyroid drugs inhibit thyroid hormone receptor-mediated transcription

CONTEXT

Methimazole (MMI) and propylthiouracil (PTU) are widely used as antithyroid drugs (ATDs) for the treatment of Graves' disease. Both MMI and PTU reduce thyroid hormone levels by several mechanisms, including inhibition of thyroid hormone synthesis and secretion. In addition, PTU decreases 5'-deiodination of T(4) in peripheral tissues. ATDs may also interfere with T(3) binding to nuclear thyroid hormone receptors (TRs). However, the effect of ATDs on the transcriptional activities of T(3) mediated by TRs has not been studied.

OBJECTIVE

The present study was undertaken to determine whether ATDs have an effect on the gene transcription regulated by T(3) and TRs in vitro.

METHODS

Transient gene expression experiments and GH secretion assays were performed. To elucidate possible mechanisms of the antagonistic action of ATDs, the interaction between TR and nuclear cofactors was examined.

RESULTS

In the transient gene expression experiments, both MMI and PTU significantly suppressed transcriptional activities mediated by the TR and T(3) in a dose-dependent manner. In mammalian two-hybrid assays, both drugs recruited one of the nuclear corepressors, nuclear receptor corepressor, to the TR in the absence of T(3). In addition, PTU dissociated nuclear coactivators, such as steroid receptor coactivator-1 and glucocorticoid receptor interacting protein-1, from the TR in the presence of T(3). Finally, MMI decreased the GH release that was stimulated by T(3).

CONCLUSIONS

ATDs inhibit T(3) action by recruitment of transcriptional corepressors and/or dissociation of coactivators. This is the first report to show that ATDs can modulate T(3) action at the transcriptional level.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

Thyroid hormones: their role in testicular steroidogenesis

Thyroid hormones are important for growth and development of many tissues. Altered thyroid hormone status causes testicular abnormalities. For instance, juvenile hypothyroidism/neonatal transient hypothyroidism induces macroorchidism, increases testicular cell number (Sertoli, Leydig, and germ cells) and daily sperm production. Triiodothyronine (T3) receptors have been identified in sperm, developing germ cells, Sertoli, Leydig, and peritubular cells. T3 stimulates Sertoli cell lactate secretion as well as mRNA expression of inhibin-alpha, androgen receptor, IGF-I, and IGFBP-4. It also inhibits Sertoli cell mRNA expression of Müllerian inhibiting substance (MIS), aromatase, estradiol receptor, and androgen binding protein (ABP) and ABP secretion. T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. It stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. Although the role of T3 on sperm, germ, and peritubular cells has not yet been completely studied, it is clear that T3 directly regulates Sertoli and Leydig cell functions. Further studies are required to elucidate the direct effect of T3 on sperm, germ, and peritubular cells.

Adverse effects of neonatal hypothyroidism on Wistar rat spermatogenesis

The effect of neonatal hypothyroidism on spermatogenesis was studied in Wistar rats of different age groups. Hypothyroidism was induced in newborn male rats from day one postpartum up to day 60 postpartum by daily administration of 0.05% methimazole (MMI) to the nursing mothers or directly through drinking water. The animals were killed at days 10, 15, 30, 40, and 60 postpartum, blood plasma was collected, and testes, epididymides, prostates, and seminal vesicles were separated and weighed. Testes were fixed in formalin for histological studies. Plasma testosterone (T), estradiol (E2), and sex hormone binding globulin (SHBG) were measured by radioimmunoassay. Hypothyroidism significantly reduced seminiferous tubule and lumen diameter. Control rats showed active spermatogenesis whereas in hypothyroid rats, the proliferation and differentiation of germ cells were arrested and their number was decreased. Plasma T, E2, and SHBG levels were significantly decreased at all ages for hypothyroid rats. The absolute weight of testes was decreased irrespective of age (except day 10 postpartum), however ventral, dorsolateral prostate, and epididymis weights were decreased at 30, 40, and 60 days postpartum. Coagulating gland weight was decreased in all age groups of hypothyroid rats. Hypothyroid rats of day 40 and 60 postpartum showed a decrease in absolute seminal vesicle weight. Relative testicular weights of hypothyroid rats decreased by postpartum day 15, 30, 40, and 60 whereas the opposite effect was observed by postpartum day 10. Relative organ weights were increased in epididymides (day 15 and 30 postpartum), seminal vesicles (day 30 and 40 postpartum), and dorsolateral prostates (day 15, 30, and 40 postpartum) and decreased in 10 and 60 day old hypothyroid rat. Ventral prostate relative weight was decreased in 40 and 60 day old rats. Th coagulating gland weight was decreased in 10, 15, and 60 days postpartum and an opposite effect was observed in 30 and 40 days hypothyroid rats. The present study clearly indicates that hypothyroidism adversely affects spermatogenesis; it also indicates that thyroid hormones are essential for normal spermatogenesis. Their effect may either be direct or indirect.

Thyroid hormone export varies among primary cells and appears to differ from hormone uptake

We characterized T3 efflux in primary cultures of cells derived from human placenta, neonatal rat cardiac myocytes, and rat inner medullary collecting ducts (IMCD). The T3 efflux rate was highest in placenta cells, followed by ventriculocytes, atriocytes, and IMCD cells. Verapamil reversibly blocked [125I]T3 efflux in these cells in a manner that correlated with their T3 efflux rate. Thus, verapamil inhibition of [125I]T3 efflux in placenta cells led to a 432% increase in the [125I]T3 content compared with 33% increase in IMCD cells. Several unlabeled iodothyronines, but not TRIAC, differentially blocked [125I]T3 efflux such as (T4 > T3 > rT3 = D-T3 > D-T4) in placenta cells and (T4 > rT3 = D-T4 = T3 > D-T3) in ventriculocytes, suggesting tissue-specific differences in the carriers/transporters responsible for T3 efflux. This hypothesis draws further support from the fact that D-T3 inhibited [125I]T3 efflux in placenta cells, but not in ventriculocytes. TRIAC did not affect T3 efflux in ventriculocytes or placenta cells, but it greatly inhibited [125I]T3 uptake in these cells, suggesting that [125I]T3 uptake and efflux mechanisms are distinct and appear to be mediated by distinct carrier/transporter proteins. Collectively, these data suggest that differences in thyroid hormone transport in target cells may provide an important mechanism for regulating hormone action in a tissue-specific fashion.

Severe and mild neonatal hypothyroidism mediate opposite effects on Leydig cells of rats

The effects of neonatal hypothyroidism on the number of Leydig cells were studied in neonatal Wistar rats. Moderate or severe hypothyroidism were induced during neonatal life by giving different amounts of methimazole (MMI; 0.05% or 0.1%) in the drinking water of pregnant and lactating dams. Rats were sacrificed on day 21 of postnatal life. Severely hypothyroid rats had approximately 45-fold higher serum thyrotropin (TSH) values and demonstrated approximately a 65% decrease in testes weight (p < 0.05) and the number of Leydig cells. However, in moderately hypothyroid rats, in which serum TSH was only approximately 16-fold higher, testicular weight was normal and the number of Leydig cells almost doubled (p < 0.01). There were no significant differences between the serum-free testosterone levels of the moderately and severely hypothyroid rats versus controls. Serum levels of 3alpha-androstanediol glucuronide, although decreased to less than 10% in severely hypothyroid rats (p < 0.01), were not changed by mild hypothyroidism. The number of Sertoli cells was increased in moderately hypothyroid rats versus controls (p < 0.05) and even further increased in severely hypothyroid rats (p < 0.05). We conclude that (1) severe neonatal hypothyroidism impairs the development and function of the testes and (2) moderate neonatal hypothyroidism stimulates the proliferation of Leydig cells.

Differential effect of hyperthyroidism on rat epididymal glycosidases

The impact of hyperthyroidism on epididymal glycosidases was studied in albino rats. Hyperthyroidism was induced in Wistar rats aged 30 days by daily injection of T4 (25 microg/100 g body weight/day intramuscularly) for 30 or 60 days; control rats were injected with vehicle (alkaline saline, pH 7.8). One set of hyperthyroid rats was reverted to euthyroid status by withdrawing T4 treatment after 30 days of hyperthyroidism. To asses the direct effect of thyroid hormone on epididymal hexosaminidases, caput, corpus and cauda tissues were stimulated with 25, 50 or 100 ng/mL T3 for 24 h, after an initial culture of 24 h. The activity of beta-glucosidase decreased in caput, corpus and cauda epididymis of hyperthyroid rats. beta-Galactosidase activity increased in the caput epididymis irrespective of the duration of hyperthyroidism. While a similar decrease occurred in the corpus and cauda epididymis in the 30 day hyperthyroid group, an opposite trend was observed in 60 day hyperthyroid rats. Caput beta-N-acetylglucosaminidase activities increased at both time points, whereas activity decreased in the corpus and cauda in 30 day, but increased in 60 day hyperthyroid rats. Hyperthyroidism consistently increased caput and corpus beta-N-acetylgalactosaminidase activity irrespective of the duration. Cauda epididymal beta-N-acetylgalactosaminidase activity was decreased in 30 day and increased in 60 day hyperthyroid rats. Hyperthyroidism induced changes in caput beta-galactosidase, beta-N-acetylgalactosaminidases, corpus beta-N-acetylglucosaminidase and cauda beta-N-acetylgalactosaminidase which were irreversible while the remaining actvities were brought back to normal when T4 treatment was withdrawn. In vitro studies showed that T3 stimulates epididymal hexosaminidases (beta-N-acetylglucosaminidase and beta-N-acetylgalactosaminidase) irrespective of the dose. These data suggest that thyroid hormones have a specific and direct influence on glycosidases in specific regions of the epididymis.

Effects of thyroid hormone on Leydig cell regeneration in the adult rat following ethane dimethane sulphonate treatment

We tested the effects of thyroid hormone on Leydig cell (LC) regeneration in the adult rat testis after ethane dimethyl sulphonate (EDS) treatment. Ninety-day-old, thyroid-intact (n = 96) and thyroidectomized (n = 5) male Sprague-Dawley rats were injected intraperitoneally (single injection) with EDS (75 mg/kg) to destroy LC. Thyroid-intact, EDS-treated rats were equally divided into three groups (n = 32 per group) and treated as follows: control (saline-injected), hypothyroid (provided 0.1% propyl thiouracil in drinking water), and hyperthyroid (received daily subcutaneous injections of tri-iodothyronine, 100 microg/kg). Testing was done at Days 2, 7, 14, and 21 for thyroid-intact rats and at Day 21 for thyroidectomized rats after the EDS treatment. Leydig cells were absent in control and hyperthyroid rats at Days 2, 7, and 14; in hypothyroid rats at all ages; and in thyroidectomized rats at Day 21. The LC number per testis in hyperthyroid rats was twice as those of controls at Day 21. 3beta-Hydroxysteroid dehydrogenase (LC marker) immunocytochemistry results agreed with these findings. Mesenchymal cell number per testis was similar in the three treatment groups of thyroid-intact rats on Days 2 and 7, but it was different on Days 14 and 21. The highest number was in the hypothyroid rats, and the lowest was in the hyperthyroid rats. Serum testosterone levels could be measured in control rats only on Day 21, were undetectable in hypothyroid rats at all stages, and were detected in hyperthyroid rats on Days 14 and 21. These levels in hyperthyroid rats were twofold greater than those of controls on Day 21. Serum androstenedione levels could be measured only in the hyperthyroid rats on Day 21. Testosterone and androstenedione levels in the incubation media showed similar patterns to those in serum, but with larger values. These findings indicate that hypothyroidism inhibits LC regeneration and hyperthyroidism results in accelerated differentiation of more mesenchymal cells into LC following the EDS treatment. The observations of the EDS-treated, thyroidectomized rats confirmed that the findings in hypothyroid rats were, indeed, due to the deficiency of thyroid hormone.

Developmental expression of thyroid hormone receptors in the rat testis

Sertoli cell proliferation in the rat is completed by Days 15-20 postnatally. Thyroid hormones appear to regulate the duration of Sertoli cell proliferation, affecting adult Sertoli cell number and hence the capacity of the testis to produce sperm. In the present study, a combination of immunohistochemistry, immunoblot analysis, and reverse transcription-polymerase chain reaction was used to demonstrate the expression pattern of thyroid hormone receptors (TR) in the juvenile and adult rat testis. The results indicated that TRalpha1 was expressed in proliferating Sertoli cell nuclei, its expression decreasing coincident with the cessation of proliferation. TRalpha2, TRalpha3, and TRbeta1 mRNAs were expressed at low levels during development; however, the corresponding protein was not detected by immunoblot analysis. In addition, TRalpha1 was found to be expressed in germ cells from intermediate spermatogonia to mid-cycle pachytene spermatocytes. Immunohistochemistry also demonstrated TR expression in a subset of interstitial cells. The demonstration of TR expression in germ cells undergoing spermatogenic differentiation suggests a possible role for thyroid hormones in the adult testis.

Molecular mechanisms of thyroid hormone-stimulated steroidogenesis in mouse leydig tumor cells. Involvement of the steroidogenic acute regulatory (StAR) protein

Using a mouse Leydig tumor cell line, we explored the mechanisms involved in thyroid hormone-induced steroidogenic acute regulatory (StAR) protein gene expression, and steroidogenesis. Triiodothyronine (T3) induced a approximately 3.6-fold increase in the steady-state level of StAR mRNA which paralleled with those of the acute steroid response ( approximately 4.0-fold), as monitored by quantitative reverse transcriptase-polymerase chain reaction assay and progesterone production, respectively. The T3-stimulated progesterone production was effectively inhibited by actinomycin-D or cycloheximide, indicating the requirement of on-going mRNA and protein synthesis. T3 displayed the highest affinity of [125I]iodo-T3 binding and was most potent in stimulating StAR mRNA expression. In accordance, T3 significantly increased testosterone production in primary cultures of adult mouse Leydig cells. The T3 and human chorionic gonadotropin (hCG) effects on StAR expression were similar in magnitude and additive. Cells expressing steroidogenic factor 1 (SF-1) showed marginal elevation of StAR expression, but coordinately increased T3-induced StAR mRNA expression and progesterone levels. In contrast, overexpression of DAX-1 markedly diminished the SF-1 mRNA expression, and concomitantly abolished T3-mediated responses. Noteworthy, T3 augmented the SF-1 mRNA expression while inhibition of the latter by DAX-1 strongly impaired T3 action. Northern hybridization analysis revealed four StAR transcripts which increased 3-6-fold following T3 stimulation. These observations clearly identified a regulatory cascade of thyroid hormone-stimulated StAR expression and steroidogenesis that provides novel insight into the importance of a thyroid-gonadal connection in the hormonal control of Leydig cell steroidogenesis.

Starvation-induced decrease in the maximal binding capacity for triiodothyronine of the thyroid hormone receptor is due to a decrease in the receptor protein

Biological responses to thyroid hormones are mediated by the nuclear thyroid hormone receptor (TR). Alterations in the maximal triiodothyronine (T3)-binding capacity (Cmax) of TR measured using a ligand binding assay have been reported under some pathophysiological conditions. Northern blot analysis has indicated that TR mRNA concentrations do not necessarily correlate with Cmax levels. For example, although the decrease in Cmax in rat liver induced by prolonged fasting is well established, TR mRNA concentrations have been reported to be constant. In the present study, we examined starvation-induced changes in TR by Western blot with anti-TR(alpha 1 + beta)antiserum and by Scatchard plot analysis. Starvation of rats for 72 hours decreased Cmax in the liver to 72.5% of control levels. The 47- and 55-kd TR proteins detected in hepatic nuclear extract by Western blotting also decreased to 64% and 66% of control values, respectively. The starvation-induced changes in Cmax and TR protein levels paralleled the change in total hepatic nuclear protein concentration. These results suggest that the decrease in T3-binding activity of the TR is due to a reduction of the TR protein itself.

Analysis of the functional state of T3 nuclear receptors expressed in thyroid cells

T3 nuclear receptors (TR) are present in thyroid cells. We have analyzed the ability of thyroid TR to function as transcriptional regulators. Studies were performed on pig thyrocytes in primary culture. Messenger RNA corresponding to TR alpha 1, alpha 2 and beta were detected in pig thyrocytes by RT-PCR and Northern blot; the alpha 2 mRNA was more abundant than the alpha 1 mRNA. Thyrocytes were transiently transfected with different plasmids containing the CAT (chloramphenicol acetyl transferase) gene placed under the control of different promoters (delta MTV, TK or delta SV40) and bearing a thyroid hormone response element, TREp or TRE DR + 4. It was found that TSH induced a concentration-dependent increase of the transfection efficiency, an effect reproduced by (Bu)2cAMP and Forskolin. Cells transfected with either delta MTV-, TK- or delta SV40-TREp-CAT expressed similar basal CAT activities. Addition of T3 produced a 3-fold increase of CAT activity expressed from each of these vectors. In contrast, CAT activity expressed from a vector containing the TRE DR + 4 was decreased by about 50% by T3. Thus, TREp and TRE DR + 4 gave distinct responses. These data demonstrate that TR physiologically expressed in thyroid cells can act as transcriptional regulators in a T3-dependent manner. This finding directly substantiates the concept of autocrine regulatory actions of thyroid hormones.

Purification of putative thyroid hormone receptor from the ovarian nuclei of fresh water perch, Anabas testudineus

Perch ovarian putative T3 (3,5,3'-triiodo-L-thyroxine) receptor was purified to 580-fold by extracting the ovarian nuclear preparation with 0.4 M KCl, gel filtration on Sephadex G-25, DEAE-Sephacel chromatography and FPLC Superose 6 chromatography. To monitor the T3 binding protein at each purification step, aliquots from each peak protein fractions were incubated with (125)I-T3 (0.16 pmol to 3.2 nmol) in the absence or presence of 500-fold excess of unlabelled T3. Maximum binding capacity (Bmax) obtained from the Scatchard plot analysis was estimated to determine the extent of purification at each step. Purified putative T3 receptors showed a single band in polyacrylamide gel electrophoresis (PAGE) indicating homogeneity of the putative receptor protein. The molecular weight of the putative T3 receptor protein, as determined on a FPLC Superose 6 column, was 50 kD. Treatment of putative T3 receptor protein with β-mercaptoethanol followed by SDS-PAGE resulted in two subunits of 26 and 31 kD. Purification increased the specific activity of the receptor, but did not alter its affinity. Analogue specificity of the purified receptor corresponded to that of the crude nuclear preparation. Triiodothyroacetic acid (Triac) and T3 equally competed in inhibiting radiolabelled T3 binding while thyroxine (T4) was a poor competitor. T3 receptor antiserum crossreacted with the receptor protein. (125)I-labelled receptor protein binding with its antiserum was inhibited by increasing logarithmic concentrations of unlabelled receptor. In contrast to earlier reports on hepatic T3 receptor, which is a monomer, present investigation demonstrated T3 binding protein in the perch ovary to be a heterodimer held together by disulphide bond.

Characterization of thyroid hormone effect on the visual system of the adult rat

To characterize the effect of thyroid hormone on the central nervous system in adult rats, we recorded evoked potentials by photic (VEP) and electrical stimulation in normal, thyroidectomized (Tx) and T3- or T4-supplemented (Tx-T3, Tx-T4) male adult rats under pentobarbital anesthesia. The latencies of VEPs recorded in the visual cortex (VC) and lateral geniculate nucleus (LGN), and the b-waves of the electroretinograms (ERG) of these three groups were compared. A significant prolongation of the latency of VEP by photic stimulation (flash light intensity 2J, 0.2 Hz) in the VC was observed 14 to 28 days after thyroidectomy (27.9 +/- 0.4 vs. 33.4 +/- 0.43 msec, p < 0.01). Similar prolongations were also observed when those of the VEP in the LGN and the b-wave of the ERG were compared to those of normal rats (22.4 +/- 0.37 vs. 27.3 +/- 0.41 msec, p < 0.01; 21.8 +/- 0.21 vs. 25.3 +/- 0.41 msec, p < 0.01, respectively). The prolonged latencies observed in the VC, LGN, and ERG-b-wave were partially normalized 24 hr after T3 supplement (50 micrograms/kg, sc) and fairly restored to normal levels by 48 hr after T4 injection (100 micrograms/kg sc). The VEP latencies in the LGN and VC showed a positive correlation with the b-wave latency of the ERG in these three groups.(ABSTRACT TRUNCATED AT 250 WORDS)

A point mutation of the T3 receptor beta 1 gene in a kindred of generalized resistance to thyroid hormone

Mutations of the thyroid hormone receptor (TR) beta 1 gene have recently been detected in several unrelated families with generalized resistance to thyroid hormone (GRTH). We now report a novel point mutation in the TR beta 1 gene in a case of a Korean-Japanese kindred. The intracellular localization and the amount of TR proteins were considered to be normal by the immunocytochemical study of cultured skin fibroblasts from the patients using anti-T3 receptor antibody. The cDNA of the T3-binding domain of the TR beta 1 gene, synthesized from the total RNA of the patients' fibroblasts, was amplified by the polymerase chain reaction, and was sequenced. A point mutation, A to G, in one allele at 1612 resulting in an amino acid substitution from lysine 438 to glutamic acid was detected. The same mutation was identified in one allele in each of the affected members. In vitro translation products of the mutant TR beta 1 gene showed decreased T3-binding activity. These data suggest that a TR mutation is predominantly responsible for GRTH, irrespective of ethnic background.

Loss of thyroid hormone receptor activity in primary cultured rat hepatocytes is reversed by 2-mercaptoethanol

In primary cultures of rat hepatocytes, specific thyroid-hormone-binding activity diminished with time and was hardly detectable at 24 h. In accordance with the loss of 3,5,3'-tri-iodothyronine (T3) binding, responses to the hormone disappeared, as indicated by low induction of the thyroid-hormone-responsive gene S14. In contrast, thyroid hormone receptor proteins were present, as determined by immunostaining with a specific antibody against the receptor. Thus the loss of T3 binding was due to receptor inactivation. After various attempts to restore the T3-binding activity, we found that 2-mercaptoethanol, a reducing agent, when added to the culture medium restored the hormone binding activity in a dose- and time-dependent manner. The observed kinetics and experiments using cycloheximide suggested that mercaptoethanol prevented inactivation of the newly synthesized receptors. Oxidoreductive conditions within cells may have a role in determining the level of activity of thyroid hormone receptors.

Site-specific anti-c-erb A antibodies recognizing native thyroid hormone receptors: their use to detect the expression and localization of alpha and beta c-erb A proteins in rat liver

The cell-specific expression and tissue distribution of c-erbA proteins alpha and beta is still unknown. To address this problem, we prepared anti-peptide antibodies directed against epitopes of human (h) c-erbA, specific for the alpha or beta form of thyroid hormone receptors. The cDNAs coding for h c-erbA beta 1, alpha 1 and alpha 2 were transcribed and the mRNAs were translated in vitro in the presence of 35S-methionine, and then their reactivity with the antisera was evaluated. The antiserum anti-beta 62-81 immunoprecipitated only the beta 1 receptor. The antiserum anti-alpha 144-162 determined precipitation of both alpha 1 and alpha 2 proteins but not of the beta 1 receptor. Anti-alpha 2 431-451 produced a selective precipitation of alpha 2, and had no effect on alpha 1 or beta 1 receptor. In order to study the interaction of the antibodies with native T3 receptor we evaluated the binding of antibodies to rat liver T3 receptors by Sephacryl S300 chromatography: both antisera anti-beta 62-81 and anti-alpha 144-162 caused a partial shift of the labeled T3-receptor complex to a higher molecular form, while the antibody directed against c-erbA alpha 2 did not produce any significant shift. The anti-peptide antibodies were then immunopurified by affinity chromatography and used to immunolocalize the different forms of c-erb A proteins in adult and fetal rat liver, by a sensitive immunohistochemical technique. All 3 antibodies stained mainly the nuclei of the majority of adult liver cells. No staining was detectable when the original antiserum was deprived of anti-peptide antibodies by running through the affinity columns or when the antibodies were pre-absorbed with the homologous peptide. No significant staining was present in the liver from rat fetus.