Tri-iodothyronine upregulates adiponutrin mRNA expression in rat and human adipocytes.

Adiponutrin (PNPLA3) is expressed in adipose tissue. Although its precise function is unknown, some data suggest a dual role in lipid homeostasis. We have investigated the influence of thyroid hormone (TH) on PNPLA3 mRNA, in rat and human cultured white adipocytes and in rat white adipose tissue (WAT). Pnpla3 mRNA increased during differentiation of rat adipocytes in an insulin-dependent manner. Tri-iodothyronine further increased Pnpla3 expression at any day during differentiation and its effects were time and dose-dependent. The Pnpla3 mRNA half-life was stabilized by tri-iodothyronine, but a transcriptional component was also observed. Pnpla3 mRNA decreased in WAT of hypothyroid rats and was partially restored by treatment with TH. Taqman analysis showed that tri-iodothyronine also increased human PNPLA3 expression in cultured subcutaneous adipocytes from obese patients. In conclusion, PNPLA3 mRNA expression is upregulated by tri-iodothyronine in adipocytes in vitro, in humans and rats, and in vivo in rat WAT.

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis.

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.

Ontogenesis of thyroid function and interactions with maternal function.

Fetal and neonatal development of thyroid function involves the embryogenesis, differentiation and maturation of the thyroid gland, of the hypothalamic-pituitary-thyroid axis and of the systems controlling thyroid hormone metabolism. We focus here on aspects related to neurodevelopment. Throughout gestation, thyroxine (T4) transferred from the mother, present in embryonic fluids by 4 weeks, protects the fetal brain. Free T4 (FT4) in fetal fluids increases rapidly, approaching adult levels by midgestation, in concentrations that are determined by the maternal serum T4. T3 remains very low throughout pregnancy. In the cerebral cortex T3, generated from T4, reaches adult values by midgestation and is partly bound to specific nuclear receptor isoforms. The iodothyronine deiodinases are important for the spatial and temporal presence of T3 in different fetal brain areas. After onset of fetal thyroid secretion at midgestation, maternal transfer of T4 continues to contribute importantly to fetal serum T4, protecting neurodevelopment until birth. In rats, even a transient period of maternal hypothyroxinemia disrupts neurodevelopment irreversibly, supporting epidemiological evidence for its negative role in human neurodevelopment. The prompt treatment of maternal hypothyroidism or hypothyroxinemia should mitigate negative effects on neurodevelopment. Neurodevelopmental deficits of preterm infants might also result from an untimely interruption of the maternal transfer of T4 [Morreale de Escobar et al: J Clin Endocrinol Metab 2000;85:3975-3987; Best Pract Res Clin Endocrinol Metab 2004;18:225-248; Eur J Endocrinol 2004;151(suppl 3):U25-U37].

The role of type I and type II 5' deiodinases on hexachlorobenzene-induced alteration of the hormonal thyroid status.

Treatment of male Wistar rats with hexachlorobenzene (HCB) (1000 mg/kg b.w.) for 3-30 days decreases circulating levels of thyroxine (T4) but does not affect triiodothyronine (T3). Time courses were determined for 5' deiodinase type I (5' D-I) activity in thyroid, liver, and kidney and 5' deiodinase type II (5' D-II) activity in brown adipose tissue (BAT) to test the possibility that increased deiodinase activity might contribute to the maintenance of the serum T3 level. Specific 5' D-I activity was increased in the thyroid at 21 days and thereafter. No significant changes were observed in the liver, however, total 5' D-I activity in this tissue was increased at 30 days of treatment as a consequence of liver weight enhancement. HCB decreased kidney 5' D-I activity after 15 days, and BAT 5' D-II activity after 21 days of treatment. Total body 5' D-I activity was significantly increased by 30 days of HCB-treatment. HCB increased the activity of hepatic T4 uridine diphosphoglucuronosyl transferase (UDPGT) in a time-dependent manner, without changes in T3 UDPGT. We propose that increased T4 to T3 conversion in the thyroid and in the greatly enlarged liver may account for the maintenance of serum T3 concentration in hypothyroxinemic HCB-treated rats.

Potent thermogenic action of triiodothyroacetic acid in brown adipocytes.

Triiodothyroacetic acid (TRIAC) is a triiodothyronine (T3) metabolite with high affinity for T3 nuclear receptors. We compared the thermogenic action of TRIAC versus T3 in brown adipocytes, by studying target genes known to mediate thermogenic action: uncoupling protein 1 (UCP-1), a marker of brown adipocytes, and type II-5'deiodinase (D2), which provides the T3 required for thermogenesis. TRIAC is 10-50 times more potent than T3 at increasing the adrenergic induction of UCP-1 mRNA and D2 activities. TRIAC action on UCP-1 is exerted at the transcriptional level. In the presence of an adrenergic stimulus, TRIAC is also more potent than T3, inducing lipoprotein lipase mRNA and 5 deiodinase (D3) activity and mRNA. Maximal effects occur at very low concentrations (0.2 nM). The greater potency of TRIAC is not due to preferential cellular or nuclear uptake. Therefore, TRIAC is a potent thermogenic agent that might increase energy expenditure and regulate T3 production in brown adipocytes.

Age-dependent adaptation of the liver thyroid status and recovery of serum levels and hepatic insulin-like growth factor-I expression in neonatal and adult diabetic rats.

The effect of treatment with thyroxine (T(4)) on the hepatic deiodinase (5'D-I) activity and triiodothyronine (T(3)) content and on insulin-like growth factor-I (IGF-I) secretion and mRNA hepatic expression were studied in neonatal and adult diabetic (D) rats and compared with 4 thyroidectomized (Tx) groups: neonatal and adult Tx rats treated or not with T(4). Serum T(3) and T(4) decreased by 92% in both Tx populations and by 80% to 70% in D adults according to the severity of diabetes: -70 mg/kg body weight (BW) (D(70)) or 50 mg/kg BW (D(50)) of streptozotocin (STZ) injected, whereas only a 30% to 33% decrease was found in D neonates. A similar decrease of liver 5'D-I activity and T(3) concentrations was found in neonatal and adult Tx rats, whereas a significant reduction in those parameters was observed only in adult diabetics, either D(70) or D(50), but not in D neonates. Serum levels and liver mRNA expression of IGF-I determined by ribonuclease protection assay, plasma and pituitary growth hormone (GH), plasma insulin, and glycemia were also measured in both D populations. A decrease in circulating IGF-I, previously reported for Tx adult rats, was also found in both D populations. T(4) treatment recovered IGF-I and liver T(3) in both Tx groups and D neonates, but not in D adults. These results show an age-dependent adaptation of the liver thyroid economy in diabetes, as hepatic 5'D-I does not respond to diabetes in neonates and IGF-I is insensitive to T(4) treatment in adult diabetics and suggest a positive correlation between hepatic T(3) content and IGF-I expression in conditions of diabetes and Tx.

The expression of the sodium/iodide symporter is up-regulated in the thyroid of fetuses of iodine-deficient rats.

Is the fetal thyroid already capable to increase its iodide uptake in response to iodine deficiency? To answer this question, we analyzed the expression of the Na(+)/I(-) symporter and several other genes in the thyroid of rat fetuses at 21 d of gestation from control mothers presenting a mild or more severe iodine deficiency. Female rats were placed on a low iodine diet, not supplemented, or supplemented with iodide or perchlorate for 3 months. The maternal and fetal thyroidal iodide uptake was measured 24 h after injection of 10 microCi Na (125)I into the dams. The absolute iodide uptake of the maternal thyroid was unchanged in a low iodine diet, not supplemented, compared with one supplemented with iodide. In contrast, the fetal thyroid absolute iodide uptake of a low iodine diet, not supplemented, and one supplemented with perchlorate was decreased by 70% and 95% compared with that supplemented with iodide. Na(+)/I(-) symporter mRNA was detected in the fetal thyroid of supplemented with iodide and increased about 2- and 4- fold in the thyroid of fetuses from a low iodine diet, not supplemented, and one supplemented with perchlorate, respectively. Na(+)/I(-) symporter expression was induced in the fetal side of the placenta in both a low iodine diet, not supplemented, and one supplemented with perchlorate; in contrast, Na(+)/I(-) symporter mRNA was never detected in the maternal side of the placenta. Fetal thyroid thyroglobulin and type I deiodinase mRNA contents were only significantly increased with a diet supplemented with perchlorate. Glucose transporter 4 mRNA was decreased in the fetal thyroid of both a low iodine diet, not supplemented, and one supplemented with perchlorate compared with one supplemented with iodide. In conclusion, although the up-regulation of Na(+)/I(-) symporter expression in fetal thyroid and placenta in the low iodine diet, not supplemented group did not lead to restoration of a normal absolute iodide uptake, our data show that all adaptive and/or defending mechanisms against iodine deficiency are already present in the fetus.

Influence of type II 5' deiodinase on TSH content in diabetic rats.

The influence of hypothalamic and pituitary type II 5'deiodinase (5'D-II) activities and T3 content on pituitary TSH content was investigated in streptozotocin (STZ)-induced diabetic rats (D). The results show, first, that hypothalamic and pituitary 5'D-II activities were lower in neonatal D rats versus control (C) rats, and the normal developmental pattern was altered. Secondly, when D and C rats were thyroidectomized (Tx) at 25 days of age (D+Tx, C+Tx), pituitary and hypothalamic 5'D-II activities increased ten days later in both populations vs. intact rats, but the percentage of increase was smaller in D+Tx than in C+Tx. The hypothalamic T3 to T4 ratios were also decreased in D+Tx animals (0.38) as compared to C+Tx rats (1.64). The hypothalamic T3 content was reduced by 30% in D as compared to C rats and by 80% in D+Tx as compared to C+Tx rats, showing a defect in hypothalamic T4 deiodination. Pituitary TSH content increased after Tx in D+Tx, but not in C+Tx. These results in diabetic rats indicate that the hypothalamic and pituitary 5'D-II activity and hypothalamic T3 content are affected by diabetes and play a role in the regulation of pituitary TSH content.

Is neuropsychological development related to maternal hypothyroidism or to maternal hypothyroxinemia?

Several recent publications have drawn attention to the role of the thyroid hormone status of the mother on the future neuropsychological development of the child. The screening of pregnant women for clinical or subclinical hypothyroidism based on second trimester elevated maternal TSH values has been proposed. Here, we have summarized present epidemiological and experimental evidence strongly suggesting that conditions resulting in first trimester hypothyroxinemia (a low for gestational age circulating maternal free T4, whether or not TSH is increased) pose an increased risk for poor neuropsychological development of the fetus. This would be a consequence of decreased availability of maternal T4 to the developing brain, its only source of thyroid hormone during the first trimester; T4 is the required substrate for the ontogenically regulated generation of T3 in the amounts needed for optimal development in different brain structures, both temporally and spatially. Normal maternal T3 concentrations do not seem to prevent the potential damage of a low supply of T4, although they might prevent an increase in circulating TSH and detection of the hypothyroxinemia if only TSH is measured. Hypothyroxinemia seems to be much more frequent in pregnant women than either clinical or subclinical hypothyroidism and autoimmune thyroid disease, especially in regions where the iodine intake of the pregnant woman is inadequate to meet her increased needs for T4. It is proposed that the screening of pregnant women for thyroid disorders should include the determination of free T4 as soon as possible during the first trimester as a major test, because hypothyroxinemia has been related to poor developmental outcome, irrespective of the presence of high titers of thyroid autoantibodies or elevated serum TSH. The frequency with which this may occur is probably 150 times or more that of congenital hypothyroidism, for which successful screening programs have been instituted in many countries.

Triiodothyronine amplifies the adrenergic stimulation of uncoupling protein expression in rat brown adipocytes.

Uncoupling protein (UCP), the mitochondrial protein specific to brown adipose tissue, is activated transcriptionally in response to cold and adrenergic agents. We studied the role of triiodothyronine (T(3)) on the adrenergic stimulation of UCP mRNA expression by use of primary cultures of rat brown adipocytes. Basal UCP mRNA levels are undetectable. Norepinephrine (NE) increases UCP mRNA during differentiation, not during proliferation. In hypothyroid conditions, UCP mRNA response to NE is almost absent. The presence of T(3) (0.2-20 nM) greatly increases the adrenergic response (30-fold). The sensitivity of UCP mRNA responses to NE is potentiated approximately 100-fold by the presence of T(3). The effect is proportional to the dose and time of preexposure to T(3). The increases obtained with NE and T(3) are prevented by actinomycin and cycloheximide. T(3) greatly stabilizes UCP mRNA transcripts. The effects of thyroxine and retinoic acid are weaker than those of T(3). In conclusion, in cultured rat brown adipocytes, T(3) is required and both synergizes with NE to increase UCP mRNA and stabilizes its mRNA transcripts.

Norepinephrine, tri-iodothyronine and insulin upregulate glyceraldehyde-3-phosphate dehydrogenase mRNA during Brown adipocyte differentiation.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene expression was studied in differentiating brown adipocytes. Northern blot analysis showed that GAPDH mRNA levels increased during differentiation of precursor cells into mature adipocytes, mainly in the initial stages of the differentiation process. Insulin, tri-iodothyronine (T(3)) and norepinephrine, the main regulators of brown adipose tissue function, upregulated GAPDH mRNA levels, whereas retinoic acid inhibited them. The effect of insulin was present on all culture days examined, was time- and dose-dependent, and was exerted through its own receptors, as demonstrated by comparing insulin and insulin-like growth factor (IGF)-I and -II potencies in this system. Using the transcriptional inhibitor, actinomycin D, we demonstrated that T(3), and to a lesser extent insulin, stabilized GAPDH mRNA. Experiments with cycloheximide indicated that both hormones require de novo protein synthesis to achieve their effects. Using cAMP analogs, we showed that the effect of norepinephrine is probably exerted through this second messenger. Co-operation was elucidated between norepinephrine- and insulin-mediated induction of GAPDH mRNA levels. In summary, we have demonstrated that GAPDH mRNA is subjected to multifactorial regulation in differentiating brown adipocytes that includes differentiation of precursor cells and the lipogenic/lipolytic regulators of the tissue.

Tissue-specific patterns of changes in 3,5,3'-triiodo-L-thyronine concentrations in thyroidectomized rats infused with increasing doses of the hormone. Which are the regulatory mechanisms?

We have measured 3,5,3'triiodothyronine (T3) in 12 tissues from thyroidectomized (Tx) rats infused with increasing doses of T3, and related them to their corresponding plasma levels. Young adult Wistar rats were surgically Tx. After 4 weeks, the animals were infused with placebo or T3 (0.25, 0.50, 0.75, 1.00 or 2.00 microg/100 g body weight/day). Placebo-infused intact rats served as euthyroid controls. Plasma and samples of cerebral cortex, cerebellum, brown adipose tissue (BAT), pituitary, liver, heart, lung, kidney, spleen, skeletal muscle, ovary and adrenal were obtained after 12-13 days of infusion. We determined plasma T3 and thyrotropin (TSH), and tissue T3 and thyroxine (T4), the latter being virtually undetectable. Results were compared with the relationships between tissue and plasma T3 in Tx rats on T4 infusions. Most tissues presented changes which paralleled those in plasma T3, irrespective of its source (infusion of T3, or generation from infused T4). However, at similar plasma T3 concentrations, cerebral cortex, cerebellum and BAT (containing type II 5' iodothyronine deiodinase (DII) activity), reached much lower T3 levels in the T3-infused Tx rats, than in Tx rats on T4, and required elevated plasma T3 levels for normal tissue T3. In these tissues, and in the pituitary, T3 concentrations were always lower than expected from plasma T3 levels. On the contrary, the lung and ovary of the T3-infused Tx rats contained more T3 than expected from plasma T3. Unexpectedly, both the ovary and adrenal attained higher tissue T3 concentrations in Tx rats on T3 than on T4 at comparable plasma T3 levels. In conclusion, the patterns of changes of the concentrations of T3 as a function of increasing plasma T3 are not only tissue-specific when T4 is provided, but also when circulating T3 is the only source of this iodothyronine. Further studies are needed to identify the mechanisms involved in the regulation of tissue T3 concentrations.

Thyroid hormones in human tumoral and normal nervous tissues.

We have studied T4 and T3 concentrations, DNA and protein concentrations and 5' and 5 deiodinases in samples of brain tumors obtained at surgery from 49 patients, and, in most cases, also from surrounding normal tissue. T4 concentrations in normal cortical tissue (6.19+/-0.45 ng/g) were lower than in white matter, but the difference disappeared when referred to the DNA content (2.26+/-0.27 ng/mg DNA). No other differences were found between cortical and white matter, or among cortical lobes. T4 in normal tissue was higher than previously reported, mostly from autopsy samples, whereas T3 (0.99+/-0.07 ng/g) was similar. 5'D-I activity was negligible as compared to 5'D-II (8.11+/-1.09 fmol/h/mg protein). When expressed in relation to the different DNA contents of normal vs. tumoral tissue, 5'D-II activities were the same for both. 5D activity was highly variable in the tumoral tissue, with negligible activities in meningiomas and pituitary adenomas. When referred to the DNA content, T4 and 5'D-II were the same, but T3 concentrations were lower in the tumor (0.24+/-0.03 ng/mg DNA) as compared to normal (0.35+/-0.04 ng/mg DNA) tissue samples. Whether or not this decrease of T3 affects the expression of T3-sensitive processes remains to be studied.

Immunohistochemical and morphometric studies of the fetal pancreas in diabetic pregnant rats. Effects of insulin administration.

BACKGROUND: Maternal diabetes influences fetal pancreas development. As there are some controversial reports, we studied the morphometric changes of the fetal insular pancreas and insulin immunostain of beta cells as well as the proliferative activity of insular cells in 21-day-old fetuses from control, diabetic, and insulin-treated diabetic pregnant rats. METHODS: Streptozotocin was injected into 7-day-pregnant rats (controls were not injected). Some rats were either left untreated (diabetic) or injected with insulin. Animals were killed at 21 days of gestation. Fetal pancreas were fixed in toto for the morphometry and immunohistochemistry studies using anti-insulin, anti-Ki-67 and anti-proliferating cell nuclear antigen (PCNA) antibodies. RESULTS: Diabetic status was determined by measuring maternal and fetal serum glucose and insulin levels. The morphometric studies showed hyperplasia of the diabetic fetal insular tissue which had not been normalized by insulin therapy. Diabetes caused an increase of both insulin-positive and insulin-negative cells. The increase in insulin-positive cells was not corrected by insulin treatment, although the number of non-beta cells became normal. The nuclear area in beta cells increased in diabetic rats but was not corrected by insulin. The cytoplasmic area decreased in diabetic rats and was normalized by insulin administration. Diabetes increased the expression of the nuclear antigen Ki-67 in fetal insular pancreas, and insulin treatment returned it to the normal state. CONCLUSIONS: Maternal diabetes leads to hyperstimulation of fetal beta cells, with increased proliferative activity. Insulin administration to the dams corrects some of the changes observed.

Iodothyronine deiodinase activities in fetal rat tissues at several levels of iodine deficiency: a role for the skin in 3,5,3'-triiodothyronine economy?

Iodothyronine deiodinases, types I, II, and III (D1, D2, and D3) activities were measured in tissues of fetal rats, at 18 and 21 days of gestation, at several levels of iodine deficiency (ID): mild ID diet (MID) and moderately severe ID, MID + 0.005% perchlorate (MID+P). D2 was present in fetal skin, increased between days 18 and 21, and also in MID and MID+P. In skin, D3 increased during ID at day 18, whereas there was a decrease at day 21. Skin T4 decreased in MID and MID+P, showing an inverse relationship with D2. Skin T3 decreased at day 18 in MID and MID+P but increased at day 21, probably because of the increased D2 and decreased D3, maintaining T3 concentrations. No effect of ID was observed on hepatic D1. D2 increased in brain and brown adipose tissue at day 21 in MID+P. No changes were found in maternal placental D2 and D3, but D2 and D3 increased in the fetal placenta at day 18 in MID+P. A higher level of D2 is present in fetal skin than in the brain. As the activity is increased, in even mild ID (and already at 18 days) it can be concluded that skin D2 is likely to be of considerable physiological importance, at least for fetal thyroid hormone economy, by contributing to the intracellular T3 content of the skin and, possibly, to the plasma T3.

Transcriptional activation of type III inner ring deiodinase by growth factors in cultured rat brown adipocytes.

The activity of the type III inner ring deiodinase (DIII), which converts T4 and T3 to inactive metabolites, is induced by serum and growth factors in primary cultures of rat brown adipocytes. The contribution of pretranslational mechanisms to this increase in DIII activity was examined in the present studies. DIII mRNA is undetectable in differentiated brown adipocytes when cultured in serum-free medium. However, exposure to epidermal growth factor (EGF), acidic or basic fibroblast growth factors (aFGF or bFGF) increase DIII transcript levels. Lesser inductions are found with platelet-derived growth factor, and insulin-like growth factor I has no effect. Maximal induction of DIII mRNA is obtained after 9 h of exposure to EGF, bFGF, or aFGF at a concentration of 10 ng/ml. The increase in DIII mRNA in response to aFGF, bFGF, and EGF requires gene transcription and protein synthesis, as the inductive effect on mRNA is completely blocked by actinomycin D or cycloheximide. The DIII mRNA half-life is 4 h when stimulated with bFGF and increases to 12 h when 10% serum, EGF, or aFGF is present. In conclusion, EGF, aFGF, and bFGF increase DIII mRNA expression in differentiated brown adipocytes. This effect appears to be exerted at the level of both enhanced transcription and mRNA stabilization.

Norepinephrine potentiates the mitogenic effect of growth factors in quiescent brown preadipocytes: relationship with uncoupling protein messenger ribonucleic acid expression.

Rat brown preadipocytes cultured in low serum conditions increase DNA synthesis and proliferate in response to serum and a variety of growth factors and hormones. Epidermal growth factor, platelet-derived growth factor, and acidic and basic fibroblast growth factors stimulate DNA synthesis in a dose-dependent manner and induce at least a 5-fold increase in [3H]thymidine incorporation after 40 h of exposure. The physiological activator of brown adipose tissue, norepinephrine, has a low mitogenic effect per se, but increases DNA synthesis stimulation exerted by serum, epidermal growth factor, basic fibroblast growth factor, and the neuropeptide vasopressin. The addition of vasopressin plus norepinephrine greatly potentiates the mitogenic effect of growth factors to levels comparable to the effect of 10% serum. Preadipocytes cultured in the presence of these mitogen combinations (growth factor, vasopressin, and norepinephrine) express a differentiation marker, the uncoupling protein. Thus, our results show 1) that a variety of growth factors and hormones induce DNA synthesis in a synergistic fashion in brown preadipocytes in primary culture; and 2) there is evidence for a role of norepinephrine in the regulation of brown adipocyte proliferation, potentiating the action of serum and mitogens, besides its role in uncoupling protein messenger RNA expression.

The type 2 iodothyronine deiodinase is expressed primarily in glial cells in the neonatal rat brain.

Thyroid hormone plays an essential role in mammalian brain maturation and function, in large part by regulating the expression of specific neuronal genes. In this tissue, the type 2 deiodinase (D2) appears to be essential for providing adequate levels of the active thyroid hormone 3,5,3'-triiodothyronine (T3) during the developmental period. We have studied the regional and cellular localization of D2 mRNA in the brain of 15-day-old neonatal rats. D2 is expressed in the cerebral cortex, olfactory bulb, hippocampus, caudate, thalamus, hypothalamus, and cerebellum and was absent from the white matter. At the cellular level, D2 is expressed predominantly, if not exclusively, in astrocytes and in the tanycytes lining the third ventricle and present in the median eminence. These results suggest a close metabolic coupling between subsets of glial cells and neurons, whereby thyroxine is taken up from the blood and/or cerebrospinal fluid by astrocytes and tanycytes, is deiodinated to T3, and then is released for utilization by neurons.

Regulation of iodothyronine deiodinase activity as studied in thyroidectomized rats infused with thyroxine or triiodothyronine.

To provide new insights into the in vivo regulation of iodothyronine deiodinases in the different tissues of the rat, we have evaluated the effects on these enzymatic activities of T4 or T3 infusions into thyroidectomized rats. Thyroidectomized rats were infused with placebo, T4, or T3. Placebo-infused intact rats served as euthyroid controls. Plasma and samples of cerebral cortex, brown adipose tissue, pituitary, liver, and lung were obtained after 12-13 days of infusion. Plasma TSH, plasma and tissue T4 and T3, and iodothyronine deiodinase activities were determined. Type II 5'-deiodinase (DII) was increased in cortex, brown adipose tissue, and pituitary from animals infused with placebo. DII activity returned to normal only with T4 infusion, remaining elevated in the animals infused with T3 alone despite normal tissue T3 concentrations. Cortex type III 5-deiodinase was only increased when hyperthyroidism was induced by infusion of T3. Liver type I 5'-deiodinase (DI) paralleled the changes in plasma and tissue T3 regardless of whether T4 or T3 was infused. On the contrary, the increase in lung DI, proportional to the increases in plasma and tissue T3, was higher when T4 was infused. As a rule, the tissues with DII presented a tighter homeostasis in their T3 concentrations than the tissues with DI. In conclusion, the regulation of iodothyronine deiodinases depends on the hormone infused into the thyroidectomized animals and on the tissue in which the deiodinase is studied, demonstrating the existence of tissue-specific regulation of its thyroid hormone concentrations.

Maternal nonthyroidal illness and fetal thyroid hormone status, as studied in the streptozotocin-induced diabetes mellitus rat model.

We have used the streptozotocin-induced diabetes mellitus pregnant rat as a model of maternal nonthyroidal illness. We measured the effects of different degrees of diabetes mellitus on maternal body weight, the outcome of pregnancy, circulating glucose, insulin, T4, T3, rT3, and TSH in mother and fetus, T4 and T3 in maternal and fetal tissues, and iodothyronine deiodinases in liver, lung, and brain. All of the changes in thyroid hormone status typical of nonthyroidal illnesses were observed in the mothers and were related to the degree of the metabolic imbalances. Most were controlled with a daily insulin dose of 0.5 U/100 g BW. Normalization of maternal placental T4, however, required higher insulin doses than in other maternal tissues. The number and body weight of the fetuses, their pituitary GH contents, and their thyroid hormone status were severely affected. The total extrathyroidal T4 and T3 pools decreased to one third of normal fetal values. T4 and T3 concentrations in the fetal brain were lower than normal, and the expected increase in type II 5'deiodinase activity was not observed. The low cerebral T3 only improved with adequate insulin treatment of the dams. It is concluded that maternal diabetes mellitus, and possibly other nonthyroidal illnesses that impair the availability of intracellular energy stores, may affect fetal brain T3 when thyroid hormones are essential for normal development.