Placental 5-deiodinase activity and fetal thyroid hormone economy are unaffected by selenium deficiency in the rat

Sodium selenite supplementation during pregnancy and lactation promotes anxiolysis and improves mnemonic performance in wistar rats' offspring

Selenium is a micronutrient which is part of selenoprotein molecules and participates in a vast number of physiological roles and, among them,we have fetal and neonatal development. Therefore, the aimof this studywas to evaluate possible behavioral changes in offspring of female rats supplemented during pregnancy and lactation with sodium selenite. To address that, we treated two groups of female rats by saline or sodium selenite at a dose of 1mg/kg through oral route and performed neurochemical and behavioral tests. In the offspring, the thyroid profile and hippocampal neurochemistrywere evaluated. Behavioral testswere performed in pups both during childhood and adulthood. We found out that selenium (Se) supplementation increased serum levels of triiodothyronine (25%, p b 0.001) and thyroxine (18%, p b 0.05) and promoted a tryptophan hydroxylase 2 (TPH 2) expression decrease (17%, p b 0.01) and tyrosine hydroxylase (TH) expression increase (202%, p b 0.01) in the hippocampus. The cholinesterase activity was decreased (28%, p b 0.01) in Se supplemented rats, suggesting a neurochemical modulation in the hippocampal activity. During childhood, the Sesupplemented offspring had a reduction in anxiety-like behavior both in elevated plus maze test and in light–dark box test. In adulthood, Se-treated pups had an increase in the locomotor activity (36%, p b 0.05) and in rearing episodes (77%, p b 0.001) in the open field test, while in the elevated plus maze test they also exhibited an increase in the time spent in the open arms (243%, p b 0.01). For the object recognition test, Se-treated offspring showed increase in the absolute (230.16%, p b 0.05) and relative index discrimination (234%, p b 0.05). These results demonstrate that maternal supplementation by sodium selenite promoted psychobiological changes both during childhood and adulthood. Therefore, the behavioral profile observed possibly can be explained by neurochemical changes induced by thyroid hormones during the critical period of the central nervous system ontogeny.

Na+/I- symporter and type 3 iodothyronine deiodinase gene expression in amniotic membrane and placenta and its relationship to maternal thyroid hormones

Placental type 3 iodothyronine deiodinase (D3) potentially protects the fetus from the elevated maternal thyroid hormones. Na(+)/I(-) symporter (NIS) is a plasma membrane glycoprotein, which mediates active iodide uptake. Our objectives were to establish the distribution of NIS and D3 gene expressions in the placenta and the amniotic membrane and to investigate the relationship between placental D3 and NIS gene expressions and maternal iodine, selenium, and thyroid hormone status. Thyroid hormones, urinary iodine concentration (UIC), and selenium levels were measured in 49 healthy term pregnant women. NIS and D3 gene expressions were studied with the total mRNA RT-PCR method in tissues from maternal placenta (n = 49), fetal placenta (n = 9), and amniotic membrane (n = 9). NIS and D3 gene expressions were shown in the fetal and maternal sides of the placenta and amniotic membrane. Mean blood selenium level was 66 ± 26.5 μg/l, and median UIC was 143 μg/l. We could not demonstrate any statistically significant relationship of spot UIC and blood selenium with NIS and D3 expression (p > 0.05). Positive correlations were found between NIS and thyroxine-binding globulin (TBG) (r = 0.3, p = 0.042) and between D3 and preoperative glucose levels (r = 0.4, p = 0.006). D3 and NIS genes are expressed in term placenta and amniotic membrane; thus, in addition to placenta, amniotic membrane contributes to regulation of maternofetal iodine and thyroid hormone transmission. Further studies are needed to clarify the relationship between maternal glucose levels and placental D3 expression and between TBG and placental NIS expression.

Effect of methyl iodide on deiodinase activity

Methyl iodide (MeI) has been proposed as an alternative for methyl bromide in pre-plant soil fumigation applications that does not affect stratospheric ozone. Preliminary studies in rabbits noted fetal resorptions if the pregnant does were exposed to MeI during a critical period during gestation. In addition, abnormalities in thyroid hormone parameters were also observed in animals exposed to MeI. Since monodeiodination is the major metabolic pathway of the thyroid hormones, we examined the effect of MeI on deiodinase activity as a possible etiology for the alteration in thyroid hormone parameters and ultimate fetal demise. In vitro studies using tissue microsomes and cell culture showed that MeI has no effect on type I 5'-deiodinase (D1) or type II 5'-deiodinase (D2) at physiologically relevant concentrations. At high concentrations (>10 mM,>10,000 ppm), MeI caused a nonspecific inactivation of D1 and D2. Analysis of D1 and D2 activity in rats exposed by inhalation to increasing concentrations of MeI showed a significant decrease in enzyme activity at 100 ppm, while brain type III 5'-deiodinase (D3) was unaffected by MeI at the exposures studied. While the drop in D1 can be explained by the induction of a hypothyroid state in the exposed rats, there is no clear explanation for the fall in D2 levels. In the rabbit studies, there was a significant decrease in kidney D1 in the adult rabbits exposed to 20 ppm MeI. However, there was no effect on liver D1, brain D2, or placental D3 in the MeI-exposed rabbits. Similarly, there was no effect of MeI on fetal D1 or D2 activity. The lack of a significant direct effect of MeI on deiodinase activity and the absence of a change in placental or fetal deiodinase activity make it unlikely that alterations in deiodinase activity plays a role in the fetal resorptions in the MeI-exposed rabbits.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Selenium and the control of thyroid hormone metabolism

Thyroid hormone synthesis, metabolism and action require adequate availability of the essential trace elements iodine and selenium, which affect homeostasis of thyroid hormone-dependent metabolic pathways. The three selenocysteine-containing iodothyronine deiodinases constitute a novel gene family. Selenium is retained and deiodinase expression is maintained at almost normal levels in the thyroid gland, the brain and several other endocrine tissues during selenium deficiency, thus guaranteeing adequate local and systemic levels of the active thyroid hormone T(3). Due to their low tissue concentrations and their mRNA SECIS elements deiodinases rank high in the cellular and tissue-specific hierarchy of selenium distribution among various selenoproteins. While systemic selenium status and expression of abundant selenoproteins (glutathione peroxidase or selenoprotein P) is already impaired in patients with cancer, disturbed gastrointestinal resorption, unbalanced nutrition or patients requiring intensive care treatment, selenium-dependent deiodinase function might still be adequate. However, disease-associated alterations in proinflammatory cytokines, growth factors, hormones and pharmaceuticals modulate deiodinase isoenzyme expression independent from altered selenium status and might thus pretend causal relationships between systemic selenium status and altered thyroid hormone metabolism. Limited or inadequate supply of both trace elements, iodine and selenium, leads to complex rearrangements of thyroid hormone metabolism enabling adaptation to unfavorable conditions.

The effects of gestational arsenic exposure and dietary selenium deficiency on selenium and selenoenzymes in maternal and fetal tissues in mice

Although toxicological and metabolic interactions of arsenic (As) and selenium (Se) have been suggested by epidemiolgical literatures, the past experimental studies mostly focused on acute, high-dose interaction, leaving the long-term, low-dose interaction unexplored. In the present study pregnant mice, fed either Se-deficient or adequate (0 or 5 micromol Se/kg diet, respectively) diet, were given oral gavage of sodium arsenite (0 or 58 micromol/kg per day; chosen as less than half of the fetotoxic dose in this protocol) from gestational day (GD) 7-16. The levels of As and Se as well as five selenoenzymes (glutathione peroxidase (GPx), thioredoxin reductase (TRxR), and type-I, -II and -III iodothyronine deiodinases (DI-I, -II and -III) were examined on GD17 in the tissues of dams and of fetus. The Se-deficient mice showed significantly enhanced accumulation of As compared to the Se-adequate mice in maternal liver (increased by 48%) and fetal brain (by 31%). Although no direct evidence of the enhanced toxicity in the Se-deficient group was obtained, the As exposure affected the levels of Se and selenoenzymes, an effect which was more discernible in Se-deficient group. Although most of theses changes were mild or moderate, the DI-II activity in Se-deficient fetal brain showed a drastic four-fold increase by As exposure, suggesting a possible disturbance of thyroid hormone environment in the fetus. These data suggested that apparently non-toxic, in utero dose of As, showed enhanced accumulation when combined with Se-deficiency and could affect the metabolism/kinetics of Se in fetal brain, which might result in developmental toxicity in mice.

Selenium in pregnancy: is selenium an active defective ion against environmental chemical stress?

Transportation of selenium from mother to fetus and its possible effects on mother's zinc, copper, cadmium, and mercury levels were studied together during the first trimester and at term in 216 mothers. Mothers came from three geographical places with different selenium intakes. The role of selenium as a biomarker for the vital function was estimated by studying the associations between tissue or blood selenium content and placental cytochrome P450 enzyme activities and the newborn's birth weight. Regardless of the selenium intake of the mothers, higher concentrations were found in the cord blood than in mother's blood reflecting active transportation of selenium to the fetus. Active smoking was associated with higher placental selenium concentrations like it is associated with higher placental zinc concentrations. When the cadmium concentrations were high in placenta, as in smokers, the transfer of selenium from blood to placenta was increased, decreasing the selenium levels in blood. On the other hand, the high selenium concentrations in blood were connected to lower cadmium concentrations in placenta also in nonsmokers. Selenium had correlations with copper and zinc. ECOD activity in placental tissue, mercury in mothers' hair, mothers' age, and selenium concentrations in cord blood and placental selenium all seem to have connections with xenobiotic-metabolizing enzymes linked effects among mothers. These data suggest that selenium has an active role in the mother's defense systems against the toxicity of environmental pollutants and the constituents of cigarette smoke.

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.

Effects of selenium deficiency on tissue selenium content, deiodinase activity, and thyroid hormone economy in the rat during development

The iodothyronine deiodinases, D1, D2, and D3, all contain selenium (Se) in the form of selenocysteine at their active sites, and they play crucial roles in determining the circulating and intracellular levels of the active thyroid hormone (TH), T3. However, not only are serum T3 levels normal in Se-deficient rats but phenotypic and reproductive abnormalities are minimal, and it has been suggested that regulatory mechanisms exist to conserve Se in critical tissues. The present study was designed to determine, in rats: 1) whether the effects of Se-deficiency are greater in the fetus and neonate than in the adult; 2) whether there are tissues other than brain and thyroid in which deiodinase activities are maintained; 3) whether the maintenance of deiodinase activity in a specific tissue is associated with a concomitant preservation of Se level in that tissue; and 4) whether TH economy and general health is maintained over several generations. The tissues studied included liver, cerebrum, thyroid, pituitary, skin, brown adipose tissue, uterus, ovary, testis, placenta, and the implantation site (uterus plus contents) at E9. The results have revealed that, with the exception of liver, skin, and nonpregnant uterus, all of the tissues studied maintained substantial deiodinase activity (>50%) during prolonged Se-deficiency. Second, although the ability of a tissue to maintain deiodinase activity in the face of dietary Se deprivation was associated in some tissues with a concomitant local preservation of Se concentration, this was not the case for all tissues. Only when Se levels were decreased by more than 80% was deiodinase activity markedly decreased. Third, the effects of Se-deficiency were no greater in the fetus than in the adult; and fourth, at the level of Se-deficiency employed in this study, TH economy and general health were successfully maintained over six generations of Se-deficient rats. How Se levels are maintained in specific tissues, whether Se is sequestered in specific cells of a tissue or organ during dietary Se deprivation, and the precise mechanisms by which plasma T3 levels are maintained in Se-deficient animals remain unanswered. Further insights may be gained by using diets that are even lower in Se than those that were used herein and/or by conducting studies using radioactive forms of Se and thyroid hormones.

In utero methylmercury exposure differentially affects the activities of selenoenzymes in the fetal mouse brain

Pregnant ICR mice were subcutaneously injected with 0,5, or 3x3 mg Hg/kg of methylmercury (MeHg) on days 12,13, and 14(G12-14) of gestation and were sacrificed on G17. Activity of selenoenzymes, including glutathione peroxidase (GPx) and 5'- or 5-iodothyronine deiodinases (5'-DI, 5-DI), was determined in fetal brain and placenta. MeHg did not affect the concentration of Se in these tissues, while it significantly inhibited the activity of GPx in the fetal brain and placenta, but not in the maternal brain. Although the levels of thyroid hormones in the maternal and fetal plasma were not affected by MeHg, 5-DI decreased and 5'-DI increased in the fetal brain, as if they had responded to hypothyroidism. Because the level of T4 in the fetal plasma was not affected by MeHg, these changes in enzymatic activities may result in a harmful excess of T3 in the fetal brain. In addition, 5-DI activity was increased in the placenta of MeHg-treated mice. These effects of prenatal MeHg exposure on fetal and placental DIs differed from those of dietary-induced Se deficiency, where the activities of DIs were decreased or not affected. Further evaluation of the effect of MeHg on selenoenzymes, especially 5-DIs, is warranted.

Deficiency of selenium enhances the K+-induced release of dopamine in the striatum of mice

To determine whether a selenium (Se) deficiency in the brain leads to a functional change in dopaminergic transmission in the striatum, in vivo microdialysis was conducted in mice fed a low-Se diet. After 11-13 weeks of the diet regimen, the activity of glutathione peroxidase (GPx) in the Se-deficient brain was reduced to 60% of the control brain. A high K+ perfusion (100 mM) increased the level of dopamine in the dialysate to 67 +/- 16 times the basal level; the increase was significantly greater than that observed in the control group (28 +/- 4 times). Such a between-group difference was not observed after 4-5 weeks of the Se-diet. These results indicated that prolonged Se deficiency altered the function of striatal dopaminergic neurons in mice. A possible contribution of enhanced oxidative stress due to the reduced GPx activity is discussed.

The deiodinase family of selenoproteins

The realization some forty years ago that several iodothyronine compounds are present in the circulation suggested that deiodination occurs in various tissues. Subsequently, deiodination was indeed documented in in vivo studies. Later, using in vitro assay techniques, three deiodinase processes, termed types 1, 2 and 3, were defined that differed in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization and sensitivity to inhibitors. Although purification of the deiodinase enzymes has continued to be problematic, recent molecular cloning studies have identified cDNAs for these three deiodinase isoforms from multiple species. These cDNAs have provided important insights into the structural characteristics of this family of enzymes. Foremost among the structural features has been the demonstration that all three deiodinase isoforms contain at their active site the uncommon amino acid selenocysteine which is of critical importance to their catalytic activity. The availability of cDNAs for these enzymes provides important reagents for pursuing additional studies aimed at defining their biochemical features and roles in thyroid hormone economy.

Type 3 lodothyronine deiodinase: cloning, in vitro expression, and functional analysis of the placental selenoenzyme

Type 3 iodothyronine deiodinase (D3) catalyzes the conversion of T4 and T3 to inactive metabolites. It is highly expressed in placenta and thus can regulate circulating fetal thyroid hormone concentrations throughout gestation. We have cloned and expressed a 2.1-kb human placental D3 cDNA which encodes a 32-kD protein with a Km of 1.2 nM for 5 deiodination of T3 and 340 nM for 5' deiodination of reverse T3. The reaction requires DTT and is not inhibited by 6n-propylthiouracil. We quantitated transiently expressed D3 by specifically labeling the protein with bromoacetyl [125I]T3. The Kcat/Km ratio for 5 deiodination of T3 was over 1,000-fold that for 5' deiodination of reverse T3. Human D3 is a selenoenzyme as evidenced by (a) the presence of an in frame UGA codon at position 144, (b) the synthesis of a 32-kD 75Se-labeled protein in D3 cDNA transfected cells, and (c) the presence of a selenocysteine insertion sequence element in the 3' untranslated region of the mRNA which is required for its expression. The D3 selenocysteine insertion sequence element is more potent than that in the type 1 deiodinase or glutathione peroxidase gene, suggesting a high priority for selenocysteine incorporation into this enzyme. The conservation of this enzyme from Xenopus laevis tadpoles to humans implies an essential role for regulation of thyroid hormone inactivation during embryological development.

Cloning and expression of a cDNA for a mammalian type III iodothyronine deiodinase

The type III iodothyronine deiodinase metabolizes the active thyroid hormones thyroxine and 3,5,3'-triiodothyronine to inactive compounds. Recently, we have characterized a Xenopus laevis cDNA (XL-15) that encodes a selenoprotein with type III deiodinase activity (St. Germain, D.L., Schwartzman, R., Croteau, W., Kanamori, A., Wang, Z., Brown, D.D., and Galton, V.A. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 7767-7771). Using the XL-15 as a probe, we screened a rat neonatal skin cDNA library. Among the clones isolated was one (rNS43-1) which contained a 2.1-kilobase pair cDNA insert that manifested significant homology to both the XL-15 and the G21 rat type I deiodinase cDNAs, including the presence of an in-frame TGA codon. Expression studies demonstrated that the rNS43-1 cDNA encodes a protein with 5-, but not 5'-, deiodinase activity that is resistant to inhibition by propylthiouracil and aurothioglucose. Northern analysis demonstrated a pattern of tissue expression in the rat consistent with that of the type III deiodinase and site directed mutagenesis confirmed that the TGA triplet codes for selenocysteine. We conclude that the rNS43-1 cDNA encodes the rat type III deiodinase and that the types I and III deiodinases present in amphibians and mammals constitute a family of conserved selenoproteins important in the metabolism of thyroid hormones.

Investigation of type I and type III iodothyronine deiodinases in rat tissues using N-bromoacetyl-iodothyronine affinity labels

In the present study the hypothesis was tested that N-bromoacetyl-3,3',5-[125I]triiodothyronine (BrAc[125I]T3) is a useful affinity label for both type I and type III iodothyronine deiodinases (ID-I and ID-III). Therefore, the microsomal fractions of various rat tissues were tested for ID-I and ID-III activities, and microsomal proteins were labeled with BrAc[125I]T3 and analyzed by SDS-PAGE. In agreement with previous observations, high ID-I activities were found in liver, kidney and thyroid, and high ID-III activities in brain, in particular fetal brain, and placenta. SDS-PAGE of BrAc[125I]T3-labeled microsomes showed a prominent radioactive approximately 27 kDa protein (p27) in liver, kidney and thyroid, which was previously identified as ID-I, and a approximately 32 kDa protein (p32) in brain, in particular fetal brain, and placenta. A good correlation was found between the affinity labeling of p32 and the inactivation of ID-III by BrAcT3, suggesting that p32 represents ID-III or a subunit thereof. After treatment of microsomes with 0.05% deoxycholate or carbonate buffer (pH 11.5) p32 was still labeled by BrAc[125I]T3, indicating that p32 is a transmembrane protein. Although 3,3',5'-triiodothyronine (rT3) is not a substrate for ID-III, p32 was readily labeled with BrAc[125I]rT3. Labeling of p32 in rat brain microsomes by BrAc[125I]rT3 was not affected by addition of 100 microM unlabeled thyroxine (T4) or T3, whereas deiodination of [125I]T3 by ID-III was inhibited by 91 and 96% in the presence of 1 microM T4 and T3, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)