TH Action Research Articles

Promoter-specific regulation of the brain-derived neurotropic factor gene by thyroid hormone in the developing rat cerebellum.

Thyroid hormone (TH) plays a critical role in normal cerebellar development. However, the molecular mechanisms of TH action in the developing cerebellum are not fully understood. This action could be exerted in part through brain-derived neurotropic factor (BDNF), as cerebellar BDNF messenger RNA (mRNA) expression is lower, and replacement of BDNF partially reverses the abnormal neurogenesis in the hypothyroid rat. The rat BDNF gene consists of four noncoding exons (exons I-IV), each of which is linked to a different promoter, and a protein-coding exon (exon V). To study promoter-specific regulation of the BDNF gene by TH, ribonuclease protection assay of each exon mRNA was performed using total developing rat cerebellar RNA. During cerebellar development, all exon mRNAs were detected, but with different expression patterns; among noncoding exon mRNAs, exon II mRNA was the most abundant. Daily TH replacement induced a 3-fold increase in exon II mRNA on postnatal day (P) 15. On P30, exon II mRNA was still much greater in the TH-replaced animal. Exon I mRNA was detected on P2 and P7. However, in contrast to exon II mRNA, TH treatment suppressed the expression of exon I mRNA on P2. Exon III and IV mRNAs were not detected on P2 and P7, but small amounts were observed starting on P15 in TH-replaced animals. They were not detected by P30 in hypothyroid animals. In contrast, in the cerebral cortex, although all exons are differentially regulated during development, the expression of each mRNA was not significantly altered by TH. These results indicate that TH regulates BDNF gene expression in a promoter-, developmental stage-, and brain region-specific manner, which may play an important role in region- and stage-specific regulation of brain development by TH.

Iodine metabolism and thyroid physiology: current concepts.

Iodine plays a central role in thyroid physiology, being both a major constituent of thyroid hormones (THS) and a regulator of thyroid gland function. This review concerns those aspects of thyroid physiology in which significant advances have been made in recent years. We have known for decades that the thyroid gland concentrates iodine (I-) against an electrochemical gradient by a carrier-mediated mechanism driven by ATP. A similar I- uptake mechanism is found in other organs, including salivary glands, stomach, choroid plexus, and mammary glands, but only in the thyroid does TSH regulate the process. This past year saw a major advance with the cloning of the thyroid I- transporter. This development opens the way to an elucidation of the regulation of I- transport in the normal gland and in thyroid neoplasms that lack this property ("cold" nodules). All of the subsequent steps in TH biosynthesis, from oxidation and organification of iodide to the secretion of T4 and T3 into the circulation, are stimulated by TSH and inhibited by excess iodine. Recently, some of the regulatory mechanisms have been clarified. The function of the major TH-binding proteins in plasma is to maintain an equilibrium between extracellular and cellular hormone pools. Transthyretin, the principal T4-binding protein in cerebrospinal fluid, may play a similar role in the central nervous system. Although it generally is agreed that cellular uptake of TH is a function of the unbound (free) form of the hormone, there is evidence that certain TH-binding plasma proteins (i.e., apolipoproteins) may serve specific transport functions. The intracellular concentration of T3, the active TH, is determined by the rates of cellular uptake of T4 and T3, the rates of metabolic transformation, including conversion of T4 to T3, and the rate of T3 efflux. The latter has been assumed to be a passive process. However, recent studies by our group in San Francisco have shown that T3 is transported out of cells by a specific, saturable, verapamil-inhibitable mechanism. This T3 efflux system is widespread among cells from many tissues, and, at least in liver, modulates intracellular and nuclear concentration of the hormone and thereby influences TH action.

Myocardial alpha-thrombin receptor activation induces hypertrophy and increases atrial natriuretic factor gene expression.

The protease, alpha-thrombin (alpha Th), affects myocardial cell contractility, a feature common among agents that induce hypertrophy. However, it is not known whether cardiac myocytes possess alpha Th receptors (alpha Th-R), or if long term treatment with alpha Th can enhance growth and gene expression. In the present study primary neonatal rat ventricular myocytes expressed a 3.6-kilobase mRNA species that hybridized with a rat alpha Th-R-specific probe. After 48 h, alpha Th induced hypertrophy, sarcomeric organization, and enhanced atrial natriuretic factor (ANF) expression, all of which were blocked by the alpha Th-selective protease inhibitor, D-Phe-Pro-Arg-chloromethyl ketone. The alpha Th-R agonist peptide, SFLLRNPND, was a potent activator of ANF expression, however, the non-agonist, FLLRNPND, was inactive. Transfection experiments showed the enhancement of ANF expression by alpha Th to be transcriptional. The abilities of alpha Th to induce myocyte hypertrophy and to augment ANF transcription and peptide production were inhibited by the protein kinase C inhibitor, chelerythrine, and by the tyrosine kinase inhibitor, tyrphostin. Thus, myocardial cell alpha Th-Rs are stimulated by the specific proteolytic actions of alpha Th, and pathways involving both protein kinase C and protein tyrosine kinases are required for subsequent hypertrophy and ANF expression. Further, these findings suggest a new role for extracellular proteases as regulators of myocardial cell gene expression and growth.

Ontogeny and subcellular localization of 3β-hydroxysteroid dehydrogenase (3β-HSD) in the human and rat adrenal, ovary and testis

Primates are unique in having adrenals that secrete large amounts of the precursor sex steroids (PSS) dehydroepiandrosterone (DHEA) and especially DHEA-sulfate. The adrenal PSS require tha ction of 3β-hydroxysteroid dehydrogenase/5-ene-4 ene isomerase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), 5α-reductase and/or aromatase to form the androgen dihydrotestosterone (DHT) or the estrogens 17β-estradiol and androst-5-ene-diol. Knowing the crucial role of 3β-HSD and 17β-HSD in sex steroid biosynthesis both in classical as well as in peripheral steroidogenic tissues, we have concentrated our efforts on the elucidation of the molecular structure of these enzyme families. We have thus characterized two types of human 3β-HSD cDNA clones and their corresponding genes which encode deduced proteins of 371 and 372 amino acids and share 93.5% homology. Human type I 3β-HSD is the almost exclusive mRNA species expressed in the placenta and skin, while human type II is the predominant mRNA species in the adrenals, ovaries and testes. We have also recently elucidated the structure of three types of rat 3β-HSD cDNAs which all encode a 372 amino acid protein. The predicted rat type I and II 3β-HSD proteins expressed adrenals, gonads and adipose tissue share 94% homology while they share 80% similarity with the liver-specific type III 3β-HSD. Transient expression of human type I and II as well as rat type I and II 3β-HSD cDNAs in HeLa human cervical carcinoma cells reveals that 3β-ol dehydrogenase and 5-ene-4-ene isomerase activities reside within a single protein and that these cDNAs encode functional 3β-HSD proteins. The expressed rat type III protein possesses a unique property catalyzing selectively the reduction of 3β-androstane 5α-steroids such as DHT. Furthermore, we have also demonstrated by site-directed mutagenesis that the lower activity of expressed rat type II compared to rat type I 3β-HSD protein is due to a change of four amino acid residues potentially involved in a membrane-spanning domain. In parallel, we have characterized the complete nucleotide sequence of human 17β-HSD cDNA clones encoding a 327 amino acid protein as well as two in tandem 17β-HSD genes. Two major 17β-HSD mRNA species have been detected in several tissues due to a tissue-specific alternative site of initiation of transcription. It is quite clear that our attention should be focused on intracrinology, in order to better understand the physiological mechanisms controlling local steroid formation.

The effect of thorium substitution in CeCu2Si2 and CeNi2Si2

Abstract Resistivity, L III absorption, and X-ray diffraction have been made on several alloys of the form Ce 1- x Th x Cu 2 Si 2 and the related system Ce 1- x Th x Ni 2 Si 2 . In the Cu-based compounds the transport measurements indicate that the Kondo effect is substantially quenched with increasing Th substitution. In both the Cu and Ni based alloys L III absorption measurements of Ce show a decrease of the Ce valence with Th substitution. These results are in accord with the action of Th substitution in other Ce intermetallic compounds, however, they stand in contrast to the action of Th in elemental Ce.

Comparison of Radiolysis Products of Thymine and Thymidine with E.S.R. Results

Radicals determined by e.s.r. spectrometry of irradiated thymine or thymidine and radiolytic products generated under tha ction of gamma rays in aerated aqueous solutions have been compared. This comparison lies mainly in the fact that a radical R gives rapidly the corresponding peroxide ROOH. The authors have isolated and characterized twenty peroxides, i.e., the four isomers cis (-), cis (+), trans (-), trans(+) of 6-hydroperoxy-5-hydroxy-5,6-dihydrothymidine; the four isomers cis (-), cis (+), trans (-), trans (+) of 5-hydroperoxy-6-hydroxy-5,6-dihydrothymidine; 5-hydroperoxy-2-deoxyuridin;cis and trans 6-hydroperoxy-5-hydroxy-5,6-dihydrothymine; cis and trans 5-hydroperoxy-6-hydroxy-5,6-dihydrothymine; 5-hydroperoxymethyl-uracil; 5-hydroperoxy-5,6-dihydrothymine;cis and trans 6-hydroperoxy-5,6-dihydrothymine; 5-hydroperoxy-5-methyl barbituric acid; 5-hydroperoxy-5-methyl hydantoin; trans 5,6-dihydroperoxy-5,6-dihydrothymine. Most of thethymine and thymidine radicals hypothesized or described in the literature were correlated to these peroxides. However, the presence of certain peroxides could not be explained by recognized radicals. Taking advantage of this fact, the existence of new thymine or thymidine radicals so far unknown can be predicted.

The mechanism of anion permeation in thorium-treated gallbladder.

Polyvalent cations (such as Ca+2, La+3, and Th+4) and low pH shift the permeability of some biological membranes from preferentially cation-permeable towards preferentially anion-permeable. The action of Th+4 on permselectivity, and the anion permeation mechanism it produces, have been studied in rabbit gallbladder epithelium. The effect involves both a suppression of cation conductance and an increase of anion conductance. Most residual cation conductance is in a free-solution shunt. The anion permeability sequence, SCN−>ClO 4 − >NO 3 − >I−>Br−≧ClO 3 − >Cl−>BrO 3 − , is neither the lyotropic series nor a radius sequence but is in accord with considerations of equilibrium selectivity. Conductance increases linearly with bathing-solution salt concentrations from 10mm up to at least 400mm. The cation-to-anion permeability ratio shows little change with concentration up to at least 300mm. The current-voltage relation is linear in symmetrical solutions and in the presence of a single-salt concentration gradient up to at least 570 mV. Dilution potentials and biionic potentials yield similar permeability ratios among anions. These permeability properties resemble (surprisingly) those of a membrane with fixed neutral sites rather than those of a classical ion-exchanger.

Potentiometric determination of plutonium in the presence of uranium

A modification of Waterbury-Metz's method for the determination of plutonium has been proposed. Perchloric acid was used for the oxidation to Pu (VI) in the method, which, however, was dangerous for handling in plastic gloveboxes. Corpel applied argentic peroxide, but it was unstable.It is found by the authors that ammonium persulfate and silver nitrate is an effective oxidizer which is safely handled in plastic gloveboxes. Sulfuric acid (3 M) is conveniently used as an eluting reagent in the anionexchange technique, when separation of plutonium from metallic impurities is necessary.Interfering action of Pb and Th for the plutonium determination is pointed out in addition to the known interfering elements, Cr, Mn, V and Pt.

9. Physiological Action of Th??ine

9. Physiological Action of Th??ine