Neuroanatomical pathways for thyroid hormone feedback in the human hypothalamus.

CONTEXT: Recent findings point to an increasing number of hypothalamic proteins involved in the central regulation of thyroid hormone feedback. The functional neuroanatomy of these proteins in the human hypothalamus is largely unknown at present. OBJECTIVE: The aim of this study was to report the distribution of type II and type III deiodinase (D2 and D3) as well as the recently identified T(3) transporter, monocarboxylate transporter 8 (MCT8), in the human hypothalamus. DESIGN: The study included enzyme activity assays, immunocytochemical studies, and mRNA in situ hybridizations in postmortem human hypothalamus (n = 9). RESULTS: D2 immunoreactivity is prominent in glial cells of the infundibular nucleus/median eminence, blood vessels, and cells lining the third ventricle. By contrast, both D3 and MCT8 are expressed by neurons of the paraventricular (PVN), supraoptic, and infundibular nucleus (IFN). In support of these immunocytochemical data, D2 and D3 enzyme activities are detectable in the mediobasal human hypothalamus. Combined D2, D3, MCT8, and thyroid hormone receptor immunohistochemistry and TRH mRNA in situ hybridization clearly showed that D3, MCT8, and thyroid hormone receptor isoforms are all expressed in TRH neurons of the PVN, whereas D2 is not. CONCLUSIONS AND IMPLICATIONS: Based on these findings, we propose three possible routes for thyroid hormone feedback on TRH neurons in the human PVN: 1) local thyroid hormone uptake from the vascular compartment within the PVN, 2) thyroid hormone uptake from the cerebrospinal fluid in the third ventricle followed by transport to TRH neurons in the PVN or IFN neurons projecting to TRH neurons in the PVN, and 3) thyroid hormone sensing in the IFN of the mediobasal hypothalamus by neurons projecting to TRH neurons in the PVN.

Dietary soy protein isolate and isoflavones modulate hepatic thyroid hormone receptors in rats.

Thyroid hormone receptors (TRs) are regulators of many genes involved in cholesterol and lipid metabolism. The purpose of this study was to examine the effect of soy protein isolate (SPI) and isoflavones on hepatic TRs in rats. In Expt. 1, Sprague-Dawley rats were fed diets containing either casein or alcohol-washed SPI with or without isoflavone supplementation (5-1250 mg/kg diet) for 70, 190, and 310 d. The offspring (F1) were fed the same diets as their parents (F0). In Expt. 2, Sprague-Dawley rats were fed diets containing casein or casein plus isoflavones (50-400 mg/kg diet) for 120 d. The mRNA and protein contents of the hepatic TRs were measured by semiquantitative RT-PCR and Western blot, respectively. TRalpha1, TRalpha2, and TRbeta2 contents were not affected by SPI. However, the content of the 52-kDa TRbeta1 protein, the major isoform present in the liver, was markedly increased by dietary SPI in both sexes of F0 and F1 compared with casein. The supplemental isoflavones had no effect on TRbeta1, whereas the high doses of isoflavones (250 and 1250 mg/kg diet) reduced the hepatic TRalpha1 protein content in F1 male rats on d 28. SPI had no effect on total T3 and T4 levels. However, higher dose of supplemental isoflavones markedly increased T4 level in female rats. Overall, this study demonstrates for the first time that SPI upregulates hepatic TRbeta1 expression, and that isoflavones reduce the hepatic TRalpha1 level in young male rats. The SPI-induced TRbeta1 may play a role in mediating the hypocholesterolemic and lipid-lowering actions of soy protein.

Type 1 iodothyronine deiodinase in the house musk shrew (Suncus murinus, Insectivora: Soricidae): cloning and characterization of complementary DNA, unique tissue distribution and regulation by T(3).

The house musk shrew Suncus murinus (Insectivora: Soricidae) has been reported as having low thyroxine to 3,3'5-triiodothyronine (T(3)) converting activity in liver and kidney homogenates and was assumed to be type 1 iodothyronine deiodinase (D1)-deficient. To study whether this is due to structural abnormality of shrew D1, we cloned the cDNA and characterized the enzyme. The deduced amino acid sequence of shrew D1 was found to be highly homologous to other known D1s and the enzyme itself to have similar catalytic activity. However, unlike in other species, the D1 activity was detected only in liver. Moreover, the D1 activity in liver of the shrew was less than half of that in rat liver and its expression was not up-regulated by T(3). In contrast, a very high activity of D2 was demonstrated in brain and brown adipose tissue. The present study also revealed that the serum level of T(3) in the shrew was in the same range as these in other mammals. These results suggest that D2 contributes to the production and maintenance of T(3) levels in the house musk shrew.

Deletion of the thyroid hormone beta1 receptor increases basal and triiodothyronine-induced growth hormone messenger ribonucleic acid in GH3 cells.

The conserved diversity, restricted distribution, and differential regulation of the thyroid hormone receptor (TR) isoforms raise the possibility of isoform-specific functions. We have addressed the roles of individual TRs in GH gene expression in GH3 cells by using an isoform-specific antisense RNA to delete TRbeta1. An antisense RNA vector, directed against the isoform-specific coding sequence of the parent TRbeta1 complementary DNA, was constructed. Stable transfected GH3-derived cell lines expressing this construct were established. Appropriate control cell lines were established in parallel. Depletion of TRbeta1 in cells expressing the antisense construct was confirmed at both the messenger RNA and protein levels. Total TR expression was maintained in these cells by a reciprocal increase in TRbeta2 levels. This perturbation of the TR population was associated with a 10.5-fold increase in basal and a 5.0-fold increase in T3-stimulated GH gene expression, but no increase in total T3 binding of nuclear extracts. In transient cotransfection experiments, there were no differences between control cells and those expressing the antisense construct in either basal or T3-stimulated expression of reporters containing a variety of thyroid hormone response elements. Depletion of TRbeta1 in GH3 cells results in a reciprocal increase in TRbeta2. These changes are associated with increased basal and T3-stimulated GH gene expression, which are not due to a nonspecific enhancement of basal or hormone-stimulated transcription. We demonstrate that TRbeta1 is not required for T3 induction of the GH gene in GH3 cells and that TRbeta1 and TRbeta2 are not equivalent in their effects on basal repression of the GH promoter. The data illustrate the potential for isoform- and promoter-specific dissociation of the repression and activation properties of the TRs.

In situ hybridization histochemistry of c-erbA alpha 2 mRNA in the hypothalamus and its surrounding structures in the adult male rat.

In order to detect the localization of c-erbA alpha 2 mRNA in the hypothalamus and its surrounding structures, in situ hybridization histochemistry was performed in the adult male rat. Sections through the hypothalamus were hybridized with a [3H]-labeled RNA probe complementary to c-erbA alpha 2 mRNA and then coated with photographic emulsion. Autoradiograms were then developed and localization of the hybridization signal was detected under a light microscope. Hybridization signal was detected throughout the hypothalamic and extrahypothalamic structures such as the paraventricular hypothalamic nucleus (PVN), ventromedial nucleus (VMN), arcuate nucleus (Arc), amygdala (Am), habenula (Hb), paraventricular thalamic nucleus (PVT) and hippocampus (Hipp). No hybridization signal was detected in the section which was hybridized with the sense probe. These results not only confirm previous findings in the PVN, Hb, PVT and Hipp, but also extend the finding to the VMN, Arc and Am, in which cells expressing c-erbA alpha 2 primary transcript were concentrated. These new findings may provide an important aid to the understanding of the roles of thyroid hormone in the function of the central nervous system.