Is the Intrinsic Genomic Activity of Thyroxine Relevant In Vivo? Effects on Gene Expression in Primary Cerebrocortical and Neuroblastoma Cells

BACKGROUND

The possibility that the intrinsic genomic activity of thyroxine (T4) is of physiological relevance has been frequently hypothesized. It might explain gene expression patterns in the brain found in type 2-deiodinase (Dio2)-deficient mice. These mice display normal expression of most thyroid hormone-dependent genes, despite decreased brain triiodothyronine (T3).

METHODS

The relative effects of T4 and T3 on gene expression were analyzed in mouse neuro-2a (N2a) cells stably expressing the thyroid hormone receptor α1, and in primary mouse cerebrocortical cells enriched in astrocytes or in neurons. Cortical cells were derived from Dio2-deficient mice to prevent conversion of T4 to T3. T4 and T3 were measured in the media at the beginning and end of incubation, and T4 and T3 antibodies were used to block T4 and T3 action.

RESULTS

In all cell types, T4 had intrinsic genomic activity. In N2a cells, T4 activity was higher on negative regulation (1/5th of T3 activity) than on positive regulation (1/40th of T3 activity). T4 activity on positive regulation was dependent on the cell context, and was higher in primary cells than in N2a cells.

CONCLUSION

T4 has intrinsic genomic activity. Positive regulation depends on the cell context, and primary cells appear much more sensitive than neuroblastoma cells. In all cells, negative regulation is more sensitive to T4 than positive regulation. These properties may explain the mostly normal gene expression in the brain of Dio2-deficient mice.

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.

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.

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.

Maternal diabetes mellitus, a rat model for nonthyroidal illness: correction of hypothyroxinemia with thyroxine treatment does not improve fetal thyroid hormone status

Maintenance of normal maternal thyroxinemia prevents severe triiodothyronine (T3) deficiency of the fetus with primary thyroid failure (1). We have studied whether thyroxine (T4) would also protect the fetal brain when maternal hypothyroxinemia is caused by nonthyroidal illnesses. We have used the streptozotocin-induced diabetes mellitus pregnant rat as a model of maternal nonthyroidal illness. We measured the effects of diabetes mellitus, and of correction of the ensuing maternal hypothyroxinemia with T4 as compared to insulin, on maternal body weight, the outcome of pregnancy, glucose, insulin, T4, T3, reverse T3, and thyrotropin levels in the maternal and fetal circulation, as well as T4 and T3 concentrations in tissues, and iodothyronine deiodinases in liver, lung, and brain. The diabetic mothers showed changes in thyroid hormone status typical of nonthyroidal illnesses. Thyroid hormone status of the fetuses was severely affected: the total T4 and T3 pools decreased to one-third of normal values. T4 and T3 concentrations in the fetal brain were lower than normal and the expected increase in 5'-deiodinase activity was not observed. Although insulin treatment avoided or mitigated these changes, the low cerebral T3 did not improve with T4 treatment of the maternal hypothyroxinemia. Several findings indicated that treatment of the severely ill dams with T4 was actually harmful for the outcome of pregnancy. These negative effects were observed without the expected increase in the maternal or fetal T3 pools.

Displacement of T4 from transthyretin by the synthetic flavonoid EMD 21388 results in increased production of T3 from T4 in rat dams and fetuses

EMD 21388 displaces T4 from circulating transthyretin, is a potent in vitro inhibitor of outer-ring deiodination (5'D) of T4 and affects thyroid hormone secretion. To study its extrathyroidal effects on the thyroid hormone status of pregnant dams and their fetuses, we treated the dams with methimazole and infused them with T4 and with either 2.5 mg EMD 21388/rat per day [(EMD(+)], or placebo solution [EMD(-)]. EMD reduced total T4 and T3 in the maternal circulation, but free T4 increased and free T3 decreased. The total amount of T3 generated from T4 in the maternal compartment increased. Placental T3 also increased in EMD(+) animals, T4 remaining the same. EMD also reached the fetal circulation. The total fetal extrathyroidal T4 pool decreased to half that of EMD(-) fetuses, whereas T3 increased 1.8-fold, thus mitigating fetal T3 deficiency, especially in the lung. Thus, if the maternal supply of T4 is kept constant, EMD mitigates the T3 deficiency of many tissues of the hypothyroid fetus. Most of the effects of this dose of EMD could result from the displacement of T4 from circulating transthyretin. Liver 5'D-I activity did not decrease, but actually increased by 40% in dams and fetuses. The enhanced transfer of T4 into tissues would also increase the amount of substrate available to 5'D-I, leading to an increased amount of T3 in maternal and fetal pools. This had not been anticipated from the changes in circulating T4 and T3, whether maternal or fetal, total or free.

Regulation of uncoupling protein messenger ribonucleic acid and 5'-deiodinase activity by thyroid hormones in fetal brown adipose tissue

We studied the regulation of type II 5'-deiodinase (5'D-II) activity and uncoupling protein (UCP) messenger RNA (mRNA) by thyroid hormones in fetal brown adipose tissue (BAT). Fetuses were obtained from hypothyroid pregnant rats infused with increasing doses of T4 or T3. Infusion of T4 into hypothyroid pregnant rats increased T4 and T3 concentrations and inhibited 5'D-II activity in fetal BAT. In contrast, infusion of T3 increased BAT 5'D-II activities at low, normal, or high BAT T3 concentrations. The relationship between thyroid hormone concentrations in fetal BAT and plasma showed that BAT T3 concentrations are relatively stable, increasing less than 2-fold over a wide range of circulating T4 (3-fold) or T3 (8-fold) concentrations. Most T3 in fetal BAT are locally derived from T4 and not from plasma T3. UCP mRNA expression decreased to 30% of control values in hypothyroid fetuses. UCP mRNA levels were restored to normal in parallel with BAT T3 concentrations after the infusion of either T4 or T3. UCP mRNA levels correlate well with BAT T3 concentrations. Supraphysiological doses of T4 did not further increase either BAT T3 or UCP mRNA levels. T3 might regulate basal UCP mRNA expression during fetal life.

Presence and mRNA expression of T3 receptors in differentiating rat brown adipocytes

Nuclear T3 binding and T3 receptors (TR) expression were studied in brown adipocytes differentiated in culture from precursor cells. High affinity T3 receptors were found. During adipocyte differentiation maximal binding capacity (MBC) was doubled (up to 763 fmol T3/mg DNA), and an apparent decrease in receptor affinity was also observed (due to a decrease in the association rate constant). A very high disappearance rate of T3 was found in the cellular and nuclear fractions under binding assay conditions (serum-free medium). MBC increased 30% under hypothyroid conditions, but was not affected by physiological doses of T3 or retinoic acid. TR beta 1, TR alpha 1 and c-erbA-alpha 2 mRNAs were detected in brown adipocytes. During differentiation TR beta 1 decreased to 30%. Long exposure to T3 increased 2-fold TR beta 1 and decreased TR alpha 1 levels, when using insulin-depleted medium. Short exposure (4 h) to 10 nM insulin reduced both TR beta 1 and TR alpha mRNAs species.

T3 potentiates the adrenergic stimulation of type II 5'-deiodinase activity in cultured rat brown adipocytes

Iodothyronine type II 5'-deiodinase (5'D-II) activities were studied in cultures of rat brown adipocytes. In the presence of serum, the adrenergically stimulated 5'D-II activities were very low. In the absence of serum, adenosine 3',5'-cyclic monophosphate (cAMP) analogues stimulated 5'D-II activity. Thyroxine (T4) inhibited these increases. Norepinephrine slightly increased 5'D-II activity in hypothyroid conditions, but 3,5,3'-triiodothyronine (T3) strongly potentiated the adrenergic stimulation of 5'D-II (20-fold). T3 amplification of the adrenergic stimulation was via beta-adrenergic receptors, specifically mimicked by beta3-agonists, but it was not observed using cAMP analogues. The stimulatory effect of T3 predominated over the inhibitory action of T4, increased with exposure to T3, and required de novo protein synthesis. The half-life of 5'D-II was 30 min, suggesting that stabilization of 5'D-II did not occur. The effect was only observed in differentiated adipocytes. Retinoic acid has similar although smaller effects than T3. In conclusion, the presence of T3 is required and strongly potentiates the noradrenergic stimulation of 5'D-II activity in rat brown adipocytes.

Only the combined treatment with thyroxine and triiodothyronine ensures euthyroidism in all tissues of the thyroidectomized rat

We have recently shown that it is not possible to restore euthyroidism completely in all tissues of thyroidectomized rats infused with T4 alone. The present study was undertaken to determine whether this is achieved when T3 is added to the continuous sc infusion of T4. Thyroidectomized rats were infused with placebo or T4 (0.80 and 0.90 microgram/100 g BW.day), alone or in combination with T3 (0.10, 0.15, or 0.20 microgram/100 g BW.day). Placebo-infused intact rats served as euthyroid controls. Plasma and 12 tissues were obtained after 12 days of infusion. Plasma TSH and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine deiodinase activities were assayed using cerebral cortex, pituitary, brown adipose tissue, liver, and lung. Circulating and tissue T4 levels were normal in all the groups infused with thyroid hormones. On the contrary, T3 in plasma and most tissues and plasma TSH only reached normal levels when T3 was added to the T4 infusion. The combination of 0.9 microgram T4 and 0.15 microgram T3/100 g BW.day resulted in normal T4 and T3 concentrations in plasma and all tissues as well as normal circulating TSH and normal or near-normal 5'-deiodinase activities. Combined replacement therapy with T4 and T3 (in proportions similar to those secreted by the normal rat thyroid) completely restored euthyroidism in thyroidectomized rats at much lower doses of T4 than those needed to normalize T3 in most tissues when T4 alone was used. If pertinent to man, these results might well justify a change in the current therapy for hypothyroidism.

Effects of thyroid hormone on the androgenic expression of KAP gene in mouse kidney

The kidney androgen-regulated protein (KAP) gene exhibits a cell-specific hormonal regulation of its expression in the epithelial cells of proximal tubules of mouse kidney, where T3 is required for constitutive expression in the straight segments and androgens for expression in the convoluted ones. By using different models of hypothyroidism, we demonstrate that maximal androgen-mediated induction of the gene depends on thyroid hormone as well. This constitutes a specific event, since vitamin D3 cannot mimic the effects of T3, albeit their remarkable functional relationship. It is also shown that while congenital hypothyroid hyt/hyt male mice, exposed to maternal T3 in the gestational period, exhibit diminished but existent androgen-dependent cortical responses, mice exposed to goitrogens during gestation and postnatally are unable to express the gene even at postnatal day ninety. Impairment of KAP cortical expression in hypothyroid animals does not correlate with lower levels of androgens or androgen receptor expression.

Postnatal selective suppression of lipoprotein lipase gene expression in brown adipose tissue (relative to the expression of the gene for the uncoupling protein) is not due to adrenergic insensitivity: a possible specific inhibitory effect of colostrum

The levels of mRNA coding for the uncoupling protein (UCP) and for lipoprotein lipase (LPL) were monitored in the brown adipose tissue of newborn rat pups. At 5 h after birth, the mRNA levels of UCP and LPL were high in pups exposed singly to 28 degrees C and low in pups kept singly at thermoneutrality (36 degrees C); in pups staying with the dam, the UCP mRNA levels were intermediate. However, the LPL mRNA levels were lower in pups staying with the dam than in pups at 36 degrees C, implying that factors additional to environmental temperature influenced LPL gene expression. Injection of noradrenaline into pups at thermoneutrality (36 degrees C) led to increases in UCP and LPL gene expression, but noradrenaline injections had no further effect in cold-exposed pups. The adrenergic effects were mediated via beta-adrenergic receptors. The cold-induced increases in both UCP and LPL gene expression were abolished by the beta-adrenergic antagonist propranolol. Thus differences in adrenergic responsiveness could not explain the differential expression of the UCP and LPL genes observed in pups staying with the dam. The presence of a physiological suppressor was examined by feeding single pups at 28 degrees C with different foods: nothing, water, Intralipid, cow's milk, rat milk and rat colostrum. None of these agents led to suppression of UCP gene expression, but colostrum led to a selective suppression of LPL gene expression. It was concluded that the genes for UCP and LPL were responsive to adrenergic stimuli immediately after birth, and it is suggested that a component of rat colostrum can selectively suppress LPL gene expression.

Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats

We have studied whether, or not, tissue-specific regulatory mechanisms provide normal 3,5,3'-triiodothyronine (T3) concentrations simultaneously in all tissues of a hypothyroid animal receiving thyroxine (T4), an assumption implicit in the replacement therapy of hypothyroid patients with T4 alone. Thyroidectomized rats were infused with placebo or 1 of 10 T4 doses (0.2-8.0 micrograms per 100 grams of body weight per day). Placebo-infused intact rats served as controls. Plasma and 10 tissues were obtained after 12-13 d of infusion. Plasma thyrotropin and plasma and tissue T4 and T3 were determined by RIA. Iodothyronine-deiodinase activities were assayed using cerebral cortex, liver, and lung. No single dose of T4 was able to restore normal plasma thyrotropin, T4 and T3, as well as T4 and T3 in all tissues, or at least to restore T3 simultaneously in plasma and all tissues. Moreover, in most tissues, the dose of T4 needed to ensure normal T3 levels resulted in supraphysiological T4 concentrations. Notable exceptions were the cortex, brown adipose tissue, and cerebellum, which maintained T3 homeostasis over a wide range of plasma T4 and T3 levels. Deiodinase activities explained some, but not all, of the tissue-specific and dose related changes in tissue T3 concentrations. In conclusion, euthyroidism is not restored in plasma and all tissues of thyroidectomized rats on T4 alone. These results may well be pertinent to patients on T4 replacement therapy.

Presence of growth factors-induced type III iodothyronine 5-deiodinase in cultured rat brown adipocytes

We found low T3 concentrations in rat brown adipocytes differentiated in vitro. This might be due to the high metabolic rate of T3, possibly caused by elevated type III iodothyronine 5-deiodinase activity (5DIII), induced by serum growth factors. We tested the ability of several growth factors to induce 5DIII activity. Epidermal growth factor and basic and acidic fibroblast growth factors produced a strong induction of 5DIII activity (25, 45-, and 50-fold, respectively). This process required gene transcription and de novo protein synthesis. The half-life of 5DIII was approximately 3 h. Heparin was required for full acidic fibroblast growth factor activity. Platelet-derived growth factor, vasopressin, and insulin-like growth factor-I induced lower 5DIII activities (3- to 6-fold). Vasopressin amplified basic fibroblast growth factor and epidermal growth factor inductions when used at submaximal doses. We found a Km of 22.5 nM using T3 as substrate. Although brown adipose tissue has undetectable 5DIII activities in vivo, the present data explain the low T3 concentrations found in cultured rat brown adipocytes and suggest that growth factors, by stimulating 5DIII, may lead to low T3 concentrations and indirectly inhibit the expression of some genes regulated by T3, e.g. the rat uncoupling protein.

Effects of thyroid hormone deiodination on regulation of thyroid axis in undernourished rats

The possible influence of hypothalamic and pituitary 5'-deiodinase II (5'-D-II) activity and 3,5,3'-triiodothyronine (T3) content on the modulation of thyroid-stimulating hormone (TSH) synthesis was studied. 1) Alterations in 5'-D-II activity and hypothalamic and pituitary T3 content produced by undernutrition were observed in fetal (21 days) and neonatal rats vs. controls. 2) After thyroidectomy, plasma TSH increased in both populations, undernourished and control, but pituitary TSH increased only in the former and not in the latter. The results obtained by giving small doses of thyroxine (T4; 0.5 micrograms/100 g body wt) to intact and thyroidectomized rats suggest a lower inhibitory effect by T4 on the pituitary in undernourished than in control rats. Although hypothalamic and pituitary 5'-D-II activity increased in both groups after thyroidectomy, the percentage increase was lower in undernourished vs. control rats, resulting in lower overall T3 content in tissues from undernourished animals. These studies on thyroid axis regulation show the in vivo regulation of TSH synthesis by hypothalamic and pituitary 5'-D-II activity and T3 content.

Thyroid hormone controls the cell-specific expression of the kidney androgen-regulated protein gene in S3 mouse kidney cells

The kidney androgen-regulated protein (KAP) gene is expressed in epithelial cells of proximal convoluted tubules of mouse kidney. Although TSH proved to be necessary for the constitutive expression of the gene in the outer stripe of the outer medulla, androgens are responsible for expression in cortical segments of the proximal tubules. We have used the congenital thyroid hormone (TH)-deficient hyt/hyt mouse to demonstrate that TH, and not TSH, is responsible for the constitutive expression of the gene in the mouse kidney. Although the androgen-dependent cortical response is partially impaired in hypothyroid mice, the expression can be fully restored after the administration of TH or pharmacological doses of testosterone, suggesting some cooperativity between TH and androgens in promoting cortical KAP gene expression. Results in hyt/hyt mice after treatment with retinoic acid, alone or in combination with TH, demonstrated that this regulator does not have any effect on the regulation of the KAP gene in mouse kidney and that induction of the gene by T3 does not require heterodimerization of TR with retinoic acid-related receptors. By using immunocytochemical analysis and specific antibodies against alpha- and beta-TH receptors we have determined the presence of both types of receptors in all segments of the proximal tubules.

Thyroid hormones in tissues from fetal and adult rats

Concentrations of T4 and T3 were recently measured in rat fetal tissues, and the reported values were found to be more than 10-fold higher than those found by us. The differences have been explained by the assumption that previous analytical procedures, neither avoid deiodination during autopsy of the animals or during extraction and purification, because phloretin [(3'),4',4,6-(tetra)trihydroxyaurone], a potent inhibitor of 5'-iodothyronine deiodinase activity in vitro, had not been used to prevent such problems. We here show that perfusion with phloretin during autopsy does not affect 5'-iodothyronine activity or T4 and T3 concentrations in liver, kidney, or brain. Evidence is also provided that the addition of phloretin during the homogenization process is superfluous, as the use of 80% ethanol and 0.02 M NaOH for this step results in undetectable deiodinase activity. Data are presented showing that during the final sample drying, no losses or degradation of T4 and T3 occur, confirming the adequacy of the individual recovery corrections using radiolabeled iodothyronines as internal tracers. We also present quantitative information on the intralaboratory variability of the T4 and T3 concentrations found in tissues from normal fetuses and their mothers as well as in adult males and nonpregnant females. Results are comparable to those obtained by others using entirely different analytical procedures.

Malic enzyme gene expression in differentiating brown adipocytes: regulation by insulin and triiodothyronine

Primary cultures of brown adipocytes were used to investigate the regulation of malic enzyme (ME) gene expression by insulin and T3. No ME gene expression was detected in undifferentiated preadipocytes. The levels of ME mRNA increased slightly during cell differentiation. Physiological doses of insulin or T3 increased ME gene expression, which reached a maximum after 24 h, on whichever culture day they were added. The effects of insulin and T3 were at the transcriptional level, as measured by run-on assays. Both hormones also increased the stabilization of the transcripts and required ongoing protein synthesis to exert their effects. A comparison of the potencies of insulin and insulin-like growth factor-I and -II (IGF-I and -II) in this system indicated that induction by insulin is mediated via its own receptor. The effects of insulin and T3 were independent of the extracellular glucose concentration, but were additive to that of glucose. Moreover, insulin and T3 act additively to increase ME gene expression, suggesting that they interact either at the transcription level or that of mRNA stabilization.

Effects of iodine deficiency on thyroid hormone metabolism and the brain in fetal rats: the role of the maternal transfer of thyroxin

Thyroid hormones, thyroxin (T4) and 3,5,3'-triiodothyronine (T3), of maternal origin, are available to the mammalian embryo early in development. However, after the onset of fetal thyroid function, they are of both fetal and maternal origin. Maternal T4 has a protective effect on the fetal brain in cases of congenital hypothyroidism. In severe iodine deficiency, maternal T4 is low, although T3 is normal; the developing embryo is markedly T4-deficient; and T3 deficiency increases with gestational age. In contrast to mechanisms in the hypothyroid fetus from a normal mother, the low T4 of the iodine-deficient mother prevents any protective effects on the fetal brain. Thyroid hormone deficiency of the iodine-deficient fetus, including the brain, is more severe and prolonged than it is in the cases of maternal or fetal thyroid failures. These findings may help to explain the relationship between severe maternal hypothyroxinemia and the severe central nervous system damage of the neurological endemic cretin.

The rat placenta and the transfer of thyroid hormones from the mother to the fetus. Effects of maternal thyroid status

We have studied the effects of maternal thyroid status on the effectiveness of the rat placenta near term as a barrier for the transfer of T4 and T3 to the fetus. Dams were given methimazole to minimize the fetal contribution to the T4 and T3 pools, so that the iodothyronines found in the conceptus are ultimately of maternal origin. The dams were infused with saline, or with T4 or T3 at doses ranging from 2.3-27.8 nmol T4 and from 0.77-20.7 nmol T3/100 g BW per day. A group of normal pregnant dams (C) was included. At 21 days of gestation T4, T3, and rT3 were measured by RIA in maternal and fetal plasma, and in maternal and fetal sides of the placenta. The total fetal extrathyroidal T4 and T3 pools were also determined. The dose-related changes in T4, T3, and rT3 levels in the placenta confirm the presence of both inner and outer ring iodothyronine deiodinase activities, and suggest increasing accumulation of the iodothyronines. Despite this, fetal extrathyroidal T4 and T3 increase progressively in T4-infused groups as a function of maternal circulating T4 levels. Fetal extrathyroidal T3 increases progressively in T3-infused groups as a function of maternal plasma T3. There was no evidence that the net maternal contribution of T4 or T3 would be proportionally less when the maternal pools became very high. It was concluded that the rat placenta is only a limited barrier for the transfer of T4 and T3 to the fetus.

Thyroid hormones and 5'-deiodinase activity in neonatal undernourished rats

Undernutrition was induced in rats submitted to food restriction from the fetal stage, and malnutrition was continued after birth until 70 days of life. Body weight was decreased to less than 50%. Plasma T4 and T3 and pituitary TSH content were determined between 8-70 days of life. In control rats, plasma T4 and T3 reached a maximum at 14 and 35 days of life, respectively, and TSH pituitary content at 45 days of life. In undernourished rats, after 8 days of life, plasma T4 and T3 and pituitary TSH content were decreased to about 50% or less, and the pattern of sequential changes observed in control rats was absent or modified. T4 and T3 concentrations were measured in heart, liver, and brain in the fetus (22 days old) and 8, 14, and 23 days after birth, as well as liver and brain 5'-deiodinases (5'D). Hepatic 5'D type I was always decreased in undernourished rats from 8-70 days after birth. Liver and heart T4 and T3 concentrations were decreased in 14-day-old undernourished rats as well as brain T3. Brain 5'D type II was decreased at 8 and 14 days, and total brain 5'D activities at 8 days. These changes occurred during the critical period for brain development (7th to 20th day) during which most processes of myelination take place and T3 brain normal levels are required.

Outer ring iodothyronine deiodinases and thyroid hormone economy: responses to iodine deficiency in the rat fetus and neonate

Female rats were fed a low iodine diet (LID) or the same diet supplemented with KI (IOD) and mated. Plasma TSH, T4 and T3 in thyroid, plasma, and tissues, and 5'-deiodinase activities (5'D) were measured in maternal, fetal, and neonatal samples. Plasma T4 was markedly reduced in LID dams, TSH was increased, and T3 was normal. Placental T4 was decreased to 10%, and placental T3 to 50%. In LID fetuses there was a complete depletion of both extrathyroidal and intrathyroidal stores of T4 and T3. The thyroid responded with increased synthesis and secretion of T3 over T4, as assessed from the T3 to T4 ratios. Near birth, brain T4 and T3 concentrations were only 6.7% and 12% of those in IOD fetuses, despite a marked increase in brain 5'D-II and a T4-sparing decrease in liver and lung 5'D-I. Brown adipose tissue 5'D-II increased 7-fold, and brown adipose tissue T4 and T3 concentrations were only decreased by 50%. After birth, the availability of iodine improved somewhat through maternal milk, and the thyroidal and extrathyroidal pools of T4 and T3 increased, although they remained much lower than those in IOD pups. Brain 5'D-II markedly increased in LID pups, and this together with an increase in plasma and brain T4 ensured almost normal brain T3 during the suckling period. The thyroidal secretion of T3 over T4 continued to be increased in LID pups during the suckling period and appeared to be related to their high circulating TSH levels. Both LID fetuses and newborns can respond to iodine deficiency as adults rats, but the fetus is more sensitive to LID because of its dependence on maternal T4. The success of the adaptative mechanisms in protecting the brain from severe T3 deficiency depends on the supply of iodine, the limiting factor for the synthesis of T4.

Maternal hypothyroidism and fetal chondro-osseous development in rats

The bone development of rat fetuses from female Wistar rats thyroidectomized prior to mating was studied, between 16 and 21 days of gestation. The parameters studied in each rat fetus were fetal weight, number of fetuses per litter, radiological data and frequency of appearance of ossification centers, diaphyseal radiological length of long bones, longitudinal ratio between epiphysis and diaphysis of long bones on cleaned and stained limbs, and volumetric tibial studies on the same limbs. Our results show a delay of fetal chondro-osseous development and a delay of weight increase in fetuses from thyroidectomized rats. After onset of fetal thyroid function, the fetal development begins a catch-up growth, which is not sufficient to completely overcome development delay prior to birth.

Thyroid hormones and 5'-deiodinase in the rat fetus late in gestation: effects of maternal hypothyroidism

Having previously observed that T4 and T3 levels in fetal rat brain and brown adipose tissue are clearly higher than expected from their low circulating levels, we have now studied thyroid hormone concentrations and 5'-deiodinase activities (5'D) in several other rat fetal tissues during the last 6 days of gestation (dg), namely 17-22 dg. This period comprises the onset of fetal thyroid activity. Total thyroidal T4 and T3 contents increased 100- and 400-fold, respectively; T4 concentrations increased 8- to 10-fold in plasma, carcass, lung, and liver, and T3 increased 4.5- to 9-fold, except in plasma and liver, where T3 levels increased less than 2-fold in plasma and 3-fold in liver. During this developmental period 5'D activity increased 5- and 10-fold in fetal liver and lung, respectively. In fetuses from hypothyroid [thyroidectomized (T)] dams, body weight was lower than in fetuses from normal dams. Total thyroidal T4 and T3 contents were initially the same, but decreased markedly in fetuses from T dams by the end of gestation. At the earliest fetal ages studied (17-18 dg) T4 and T3 concentrations were lower in carcass, liver, lung, and brain, although near term there were no consistent differences between the fetal tissues from T and control dams, probably because of compensatory stimulation of thyroidal secretion. Liver 5'D was decreased by 50% throughout gestation, and lung 5'D activities were lower by the end of gestation. Thyroid hormones in placentas from T dams were very low, but increased by the end of gestation because of the contribution by the fetal thyroid. Present results describe the ontogenic profiles for thyroid hormone concentrations and 5'D activities during late fetal development; active regulatory mechanisms are already present at this age. It has been frequently stated that rat fetuses near term are deficient in thyroid hormones, and that their thyroid hormone economy is independent of maternal thyroid status, but present results show that near term, T4 and T3 concentrations in several tissues reach levels that are 50% or more of those described for adult animals, and that fetal thyroid function is influenced by maternal hypothyroidism.

Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3'-triiodothyronine in the protection of the fetal brain

To study the protective effects of maternal thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in congenital hypothyroidism, we gave pregnant rats methimazole (MMI), an antithyroid drug that crosses the placenta, and infused them with three different doses of T4 or T3. The concentrations of both T4 and T3 were determined in maternal and fetal plasma and tissues (obtained near term) by specific RIAs. Several thyroid hormone-dependent biological end-points were also measured. MMI treatment resulted in marked fetal T4 and T3 deficiency. Infusion of T4 into the mothers increased both these pools in a dose-dependent fashion. There was a preferential increase of T3 in the fetal brain. Thus, with a T4 dose maintaining maternal euthyroidism, fetal brain T3 reached normal values, although fetal plasma T4 was 40% of normal and plasma TSH was high. The infusion of T3 pool into the mothers increased the total fetal extrathyroidal T3 pool in a dose-dependent fashion. The fetal T4 pools were not increased, however, and this deprived the fetal brain (and possibly the pituitary) of local generation of T3 from T4. As a consequence, fetal brain T3 deficiency was not mitigated even when dams were infused with a toxic dose of T3. The results show that (a) there is a preferential protection of the brain of the hypothyroid fetus from T3 deficiency; (b) maternal T4, but not T3, plays a crucial role in this protection, and (c) any condition which lowers maternal T4 (including treatment with T3) is potentially harmful for the brain of a hypothyroid fetus. Recent confirmation of transplacental passage of T4 in women at term suggests that present results are relevant for human fetuses with impairment of thyroid function. Finding signs of hypothyroidism at birth does not necessarily mean that the brain was unprotected in utero, provided maternal T4 is normal. It is crucial to realize that maintainance of maternal "euthyroidism" is not sufficient, as despite hypothyroxinemia, the mothers may be clinically euthyroid if their T3 levels are normal.

Thyroid hormones and 5'-deiodinase in rat brown adipose tissue during fetal life

Brown adipose tissue (BAT) iodothyronine 5'-deiodinase (5'D) activities are very high during fetal life but decrease 10-fold a few hours before birth. Accordingly, BAT 3,5,3'-triiodothyronine (T3) concentrations are also very high. The temporal patterns of changes in BAT 5'-D and fetal plasma insulin are similar (and differ from the pattern for catecholamines) but are not superimposable. A causal role for insulin in the activation of fetal BAT 5'-D is therefore not supported by the data. Maternal thyroidectomy leads to a decrease in the total and relative weight of fetal BAT and to a 30-50% increase in BAT 5'-D activities; BAT thyroid hormone concentrations are essentially unchanged. Fetal hypothyroidism was induced by giving methimazole and resulted in a marked decrease of BAT thyroxine (T4) and T3 concentrations. This treatment increased BAT 5'-D activity only on day 21 of gestation, but no effect was observed on day 20. The fetal 5'-D response to thyroid hormones infused into the methimazole-treated dams was studied at 21 days of gestation. The increase in BAT 5'-D induced by methimazole treatment was prevented by T4 infused into control dams but not by T3. In fetuses from thyroidectomized dams, the pattern of 5'-D regulation by thyroid hormones was impaired. It is suggested that the high concentrations of thyroid hormones present in fetal BAT might participate in the general maturation and development of fetal BAT.

Postnatal recruitment of brown adipose tissue is induced by the cold stress experienced by the pups. An analysis of mRNA levels for thermogenin and lipoprotein lipase

In order to investigate the postnatal recruitment process, gene expression in the brown adipose tissue of rat pups was followed during the first 20 h of life. In normal pups, the level of mRNA coding for the uncoupling protein thermogenin increased markedly but gradually within the first 24 h. Lipoprotein lipase and actin mRNA levels were relatively low and remained constant. In pups exposed to thermoneutral temperature (35 degrees C) for the first 12 h after birth, no increase in thermogenin mRNA or lipoprotein lipase mRNA was observed, whereas in pups exposed to 28 degrees C a clear increase in both thermogenin and lipoprotein lipase mRNA levels was found. Actin mRNA levels were not affected by the environmental temperature under these circumstances. It was concluded that the postnatal recruitment in brown adipose tissue is a consequence of the cold stress experienced by the newborn pups. Thus, postnatal recruitment is not ontogenically predetermined.

Generalized deficiency of 3,5,3'-triiodo-L-thyronine (T3) in tissues from rats on a low iodine intake, despite normal circulating T3 levels

Rats fed a low iodine diet have decreased total and nuclear T3 concentrations in the liver and brain, as compared with rats supplemented with iodine, possibly because of the very low plasma and tissue T4 pools in low-iodine diet rats, leading to decreased intracellular generation of T3 in those tissues. If so, T3 levels should not decrease in heart and skeletal muscle, as plasma T3 is normal in low-iodine diet rats and these two tissues derive their intracellular T3 directly from plasma T3. We have studied this point in male rats fed a low-iodine diet, a low-iodine diet + iodine, and the stock diet. As in previous studies, low-iodine rats had very low plasma T4 and high plasma TSH levels, plasma T3 levels being normal. Liver T3 decreased, and so did the brain T3 levels despite a compensatory increase in type II 5' iodothyronine deiodinase activity. Contrary to expectations, T3 concentrations were lower in the heart and skeletal muscle of low-iodine diet rats. Attempts to clarify the possible mechanism(s) involved have been unsuccessful so far. The present results show that, despite normal plasma T3, a deficiency of T3 occurs in more tissues of rats on a low iodine intake than previously assumed. If the present results are pertinent to inhabitants from areas with severe iodine deficiency, it would appear that they might suffer from a generalized tissue T3 deficiency (and hypothyroidism?), even if overt clinical signs are not usually present.

Comparison of maternal to fetal transfer of 3,5,3'-triiodothyronine versus thyroxine in rats, as assessed from 3,5,3'-triiodothyronine levels in fetal tissues

Thyroxine (T4) is transferred from the mother to the hypothyroid rat fetus late in gestation, mitigating T4 and T3 deficiency in fetal tissues, the brain included. We have now compared the effects of maternal infusion with T3. Normal and thyroidectomized rats were started on methimazole (MMI) on the 14th day of gestation, given alone, or together with a constant infusion of 0.45 micrograms (0.69 nmol) T3 or of 1.8 microgram (2.3 nmol) T4/100 g per day. Maternal and fetal samples were obtained at the 21st day of gestation. The doses of T3 and T4 were biologically equivalent for the dams, as assessed from maternal plasma and tissue T3, and plasma TSH levels. MMI blocked the fetal thyroid; T4 and T3 levels were low in all fetal tissues, and fetal plasma TSH was high. Maternal infusion with T4 mitigated both T4 and T3 deficiency in all fetal tissues, the brain included, and decreased fetal plasma TSH. In contrast, infusion of T3 normalized fetal plasma T3 and increased the T3 levels in several tissues, but not in the brain. Neither did it decrease the high fetal plasma TSH levels. The results show that when the fetus is hypothyroid, T3 crosses the rat placenta at the end of gestation, but does not affect all tissues to the same degree. In contrast to the effects of maternal T4, maternal T3 does not alleviate the T3 deficiency of the brain or, presumably, of the thyrotrope. Thus, end-points of thyroid hormone action related to TSH release should not be used to measure transfer of maternal T3 to the fetal compartment. Moreover, T4 should be given, and not T3 to protect the hypothyroid fetal brain.

Developmental changes in rat brain 5'-deiodinase and thyroid hormones during the fetal period: the effects of fetal hypothyroidism and maternal thyroid hormones

We have studied the ontogenesis of 5'-deiodinase (5'D) activity in rat brain during fetal life, its capacity to respond to maternal or fetal hypothyroidism, and its regulation by maternal thyroid hormones. Type II 5'D (5' D-II) activity increases 4-fold during the period studied (17 to 22 days of gestation), mainly between days 19 and 21. Fetal brain T4 concentrations increase in parallel with fetal plasma T4, whereas fetal brain T3 concentrations increase 18 times (days 17-21), six times more than would have been expected from the small increase in fetal plasma T3 levels. Maternal thyroidectomy did not affect 5'D-II activity or thyroid hormone concentrations in fetal brain (except brain T4 at 18 days of gestation). Fetal hypothyroidism, induced by giving a goitrogen (methimazole) to the mothers, depleted all fetal tissues studied, including the fetal thyroid, from thyroid hormones. By 19 days of gestation, the fetal brain was able to respond to hypothyroidism with a 3- to 5-fold increase in 5'D-II activity. Earlier onset of treatment with methimazole led to 2- to 3-fold increases in 5'D already at 17 and 18 days of gestation, showing that when fetal thyroid secretion starts the fetal brain 5'D-II is able to respond to hypothyroidism. Replacement of methimazole-treated mothers with physiological doses of T4, given by constant infusion, increased T4 and T3 concentrations in fetal brain, and inhibited fetal, as well as maternal, brain 5'D-II activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Iodothyronine 5'-deiodinase activity and thyroid hormone content in brown adipose tissue during the breeding cycle of the rat

Brown adipose tissue iodothyronine 5'-deiodinase activity is significantly lower in 17-day pregnant rats compared with virgin controls and remains low during late pregnancy and lactation. It fully recovers with abrupt weaning, but only partially with spontaneous weaning. Even though this profile of changes is remarkably in step with the known pattern of modifications in brown fat thermogenesis during the breeding cycle, the lowered iodothyronine 5'-deiodinase activity appearing between days 15 and 17 of pregnancy occurs earlier than the reduction in brown adipose tissue thermogenesis. Brown fat 3,3',5-tri-iodothyronine content is also reduced in late pregnant, early and mid-lactating rats, most probably as a consequence of the lowered 5'-deiodination of thyroxine in situ. Acute insulin treatment increases brown fat iodothyronine 5'-deiodinase activity in virgin animals as well as in late-pregnant and lactating rats, despite the lowered basal enzyme activity levels in the latter groups. Thus an impaired response to insulin in brown fat does not appear to be a factor leading to the lowered iodothyronine 5'-deiodinase activity during late pregnancy and lactation.

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue during development

Iodothyronine 5'-deiodinase activity in rat brown adipose tissue has a characteristic pattern of developmental changes that is completely different from that of the liver. Fetal brown fat exhibits an extremely high iodothyronine 5'-deiodinase activity that is approx. 10-fold that in adult rats. Even though brown fat iodothyronine 5'-deiodinase activity falls suddenly at birth, there is a new peak in the activity around days 5-7 of life, whereas it remains very low afterwards. Just after birth, brown adipose tissue iodothyronine 5'-deiodinase activity is already capable of stimulation by noradrenaline. The postnatal peak in brown fat iodothyronine 5'-deiodinase correlates with the known increase in the thermogenic activity of the tissue in the neonatal rat, thus reinforcing the suggestion that local 3',3,5-triiodothyronine generation could be an important event related to thermogenesis in brown adipose tissue. However, the high fetal activity was only slightly related to the thermogenic activity of brown fat. Moreover, the increased iodothyronine 5'-deiodinase activity of brown adipose tissue during fetal and neonatal life suggests a substantial contribution by brown fat in the overall extrathyroidal 3',3,5-triiodothyronine production in these physiological periods.

Cerebral hypothyroidism in rats with adult-onset iodine deficiency

Rats fed chronically a low iodine diet may have low serum T4 and high circulating TSH, despite normal serum T3. As the brain depends to a great extent on intracellular generation of T3 from T4 for its total and nuclear T3, we have carried out two experiments to determine whether the brain of iodine-deficient rats may become hypothyroid, despite normal serum T3 levels. In both experiments we confirmed previous data, showing that the pituitary and liver of iodine-deficient rats with very low plasma T4 levels are hypothyroid as compared to those of animals receiving the same diet supplemented with KI, though not as markedly as animals which had undetectable circulating levels of both T4 and T3 as a consequence of chronic ingestion of KC1O-4, or of surgical thyroidectomy. We have further found that the nuclear T3 content was decreased in the brain of iodine-deficient rats, as compared with the animals on the iodine-supplemented diet. The nuclear to plasma ratios of labeled T3 showed that the uptake of this hormone into liver and brain nuclei is not decreased in the iodine-deficient rats as compared with those on the iodine-supplemented diet. This finding indicates that the decreased liver and brain nuclear T3 contents of iodine-deficient rats are likely to be a consequence of the marked reduction of their T4 pool, leading to decreased amounts of intracellularly generated T3. The number of spines on shafts of pyramidal neurons from the visual cortex of iodine-deficient rats was lower than that of rats fed the same diet supplemented with KI. Their distributions along the shaft were also not the same. Such changes might well be an index of cerebral hypothyroidism, as they are similar to those found after thyroidectomy of adult rats. It is concluded from the present findings that normal circulating T3 levels may not be sufficient to maintain brain euthyroidism in rats fed a diet iodine deficient enough to result in very low circulating T4 levels.

Decreased T4 to T3 conversion in tissues of streptozotocin-diabetic rats

The effect of experimental diabetes on T4 to T3 conversion, T3-deiodination, and the pituitary response to a dose of T4 and T3 was studied. Pituitary GH and plasma TSH were determined as a measure of the biological response to thyroid hormones. Thyroidectomized rats, 5 days after injection with saline or streptozotocin (thyroidectomized-control (Th.C) and thyroidectomized-diabetic (Th.D) rats, respectively) received an ip dose of T4 + [125I]T4 or T3 + [125I]T3. Rats from each group were sacrificed at varying intervals after thyroid hormone injection. Th.D rats had hyperglycaemia, glycosuria, and a body weight of about 80% of that of Th.C rats. The concentrations of [125I]T4 and [125I]T3 were measured in several tissues after ethanol extraction, separation by thin-layer chromatography, and identification with markers. Plasma TSH and pituitary GH were measured by specific RIAs. Diabetes decreased the conversion of T4 to T3 in several tissues, including the pituitary, but did not affect the deiodination of T3. The decrease in pituitary T3 content after a dose of T4 was accompanied by a diminution of the biological effect of the T4 dose on pituitary GH. Since diabetes also interferes with this biological response to a T3 dose, it seems likely that the reduced biological effect of thyroid hormones on pituitary GH may be related to an alteration in the somatotrophin T3 receptors, or in post-receptor events. Moreover, the data indicate that although T3 generation in the pituitary was reduced, the same dose of T4 had a greater inhibitory effect on TSH secretion in Th.D rats than in Th.C rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Analytical artifacts in radioimmunoassay of L-thyroxin in human milk

Previous results are contradictory regarding the concentration of thyroxin in human milk. Using a sensitive radioimmunoassay, we have found a lack of parallelism between the standard curve for thyroxin and the curve for serial dilutions of whole human milk, skimmed milk, or ethanol extracts of milk. Nonspecific binding also indicated the presence of analytical artifacts. Thus we have separated thyroxin from other milk components by means of a strongly basic Bio-Rad anion-exchange resin with quaternary ammonium exchange groups attached to a styrene divinyl benzene copolymer lattice, radioimmunoassaying the fractions eluted with an equivolume mixture of acetic acid and water. Parallelism with the standard curve was good, and results were the same whether or not the resin eluate was further purified by paper chromatography. The range of thyroxin concentration in 21 samples of human milk was 0.29-2.00 micrograms/L (mean 0.71, SD 0.40, microgram/L). Such concentrations are unlikely to afford protection to the developing brain of a breast-fed athyreotic baby, as previously claimed.

Analytical artifacts in radioimmunoassay of L-thyroxin in human milk

Previous results are contradictory regarding the concentration of thyroxin in human milk. Using a sensitive radioimmunoassay, we have found a lack of parallelism between the standard curve for thyroxin and the curve for serial dilutions of whole human milk, skimmed milk, or ethanol extracts of milk. Nonspecific binding also indicated the presence of analytical artifacts. Thus we have separated thyroxin from other milk components by means of a strongly basic Bio-Rad anion-exchange resin with quaternary ammonium exchange groups attached to a styrene divinyl benzene copolymer lattice, radioimmunoassaying the fractions eluted with an equivolume mixture of acetic acid and water. Parallelism with the standard curve was good, and results were the same whether or not the resin eluate was further purified by paper chromatography. The range of thyroxin concentration in 21 samples of human milk was 0.29-2.00 micrograms/L (mean 0.71, SD 0.40, microgram/L). Such concentrations are unlikely to afford protection to the developing brain of a breast-fed athyreotic baby, as previously claimed.

Thyroxine radioimmunoassay for population surveys using dried blood: modifications of a highly sensitive method

A highly sensitive radioimmunoassay for the determination of thyroxine in blood spotted on filter paper is described. The radioimmunoassay developed by Weeke and Orskov for determination of serum thyroxine was simplified and shortened, and its cost was decreased, for use in programmes for mass screening of thyroid function. The method permits the use of small (2 mm diameter) dried blood dots, thus avoiding interference by binding proteins. Polyethylene glycol 6000 is used for the final separation of antibody bound labelled thyroxine, instead of the expensive and time-consuming wick chromatography used in the reference method. Serum thyroxine concentrations obtained using dried blood by the modified procedure correlate well with those obtained using serum by the reference method. The distribution of serum thyroxine concentration for a population of 2134 babies is given.