Ontogenesis of iodothyronine-5'-deiodinase. Induction of 5'-deiodinating activity by insulin, glucocorticoid, and thyroxine in cultured fetal mouse liver

Short communication: quantitative comparison of iodothyronine deiodinase I and II mRNA expression in ovine tissues

Iodothyronine deiodinases I and II (DIO1 and DIO2) remove iodine from T4 to convert it to a more biologically active T3. The relative contribution of different tissue deiodinases to the establishment of a euthyroid state in sheep is not known. The objective of this study was to quantitate the amounts of transcription of DIO1 and DIO2 deiodinases in different ovine tissues. Using RT-qPCR, we found that DIO1 deiodinase is transcribed in skeletal muscle, kidney, and heart, more than thyroid, in diaphragm in quantities very similar to thyroid, and in liver, spleen, lung, and mammary gland lower than thyroid. We also found that the level of DIO2 transcription in all other tissues was lower than that in thyroid. In clinical settings, measurement of DIO1 and DIO2 expression in a given tissue may provide important clues on the intensity of selenium deficiency and its effects on the metabolism of thyroid hormones.

3-Iodothyronamine: a modulator of the hypothalamus-pancreas-thyroid axes in mice

BACKGROUND AND PURPOSE Preclinical pharmacology of 3-iodothyronamine (T1AM), an endogenous derivative of thyroid hormones, indicates that it is a rapid modulator of rodent metabolism and behaviour. Since T1AM undergoes rapid enzymatic degradation, particularly by MAO, we hypothesized that the effects of T1AM might be altered by inhibition of MAO. EXPERIMENTAL APPROACH We investigated the effects of injecting T1AM (i.c.v.) on (i) feeding behaviour, hyperglycaemia and plasma levels of thyroid hormones and (ii) T1AM systemic bioavailability, in overnight fasted mice, under control conditions and after pretreatment with the MAO inhibitor clorgyline. T1AM (1.3, 6.6, 13, 20 and 26 µg·kg(-1) ) or vehicle were injected i.c.v. in fasted male mice not pretreated or pretreated i.p. with clorgyline (2.5 mg·kg(-1) ). Glycaemia was measured by a glucorefractometer, plasma triiodothyronine (fT3) by a chemiluminescent immunometric assay, c-fos activation immunohistochemically and plasma T1AM by HPLC coupled to tandem-MS. KEY RESULTS T1AM, 1.3 µg·kg(-1) , produced a hypophagic effect (-24% vs. control) and reduced c-fos activation. This dose showed systemic bioavailability (0.12% of injected dose), raised plasma glucose levels and reduced peripheral insulin sensitivity (-33% vs. control) and plasma fT3 levels. These effects were not linearly related to the dose injected. Clorgyline pretreatment strongly increased the systemic bioavailability of T1AM and prevented the hyperglycaemia and reduction in fT3 induced by T1AM. CONCLUSIONS AND IMPLICATIONS T1AM induces central and peripheral effects including hyperglycaemia and a reduction in plasma fT3 levels in fasted mice. These effects critically depend on the concentration of T1AM or its metabolites in target organs.

Mitochondrial energy metabolism in a model of undernutrition induced by dexamethasone

The present investigation was undertaken to evaluate whether mitochondrial energy metabolism is altered in a model of malnutrition induced by dexamethasone (DEX) treatment (1·5mg/kg per d for 5d). The gastrocnemius and liver mitochondria were isolated from DEX-treated, pair-fed (PF) and control (CON) rats. Body weight was reduced significantly more in the DEX-treated group (−16%) than in the PF group (−9%). DEX treatment increased liver mass (+59%v.PF, +23%v. CON) and decreased gastrocnemius mass. Moreover, in DEX-treated rats, liver mitochondria had an increased rate of non-phosphorylative O2consumption with all substrates (approximately +42%). There was no difference in enzymatic complex activities in liver mitochondria between rat groups. Collectively, these results suggest an increased proton leak and/or redox slipping in the liver mitochondria of DEX-treated rats. In addition, DEX decreased the thermodynamic coupling and efficiency of oxidative phosphorylation. We therefore suggest that this increase in the proton leak and/or redox slip in the liver is responsible for the decrease in the thermodynamic efficiency of energy conversion. In contrast, none of the variables of energy metabolism determined in gastrocnemius mitochondria was altered by DEX treatment. Therefore, it appears that DEX specifically affects mitochondrial energy metabolism in the liver.

Developmental control of iodothyronine deiodinases by cortisol in the ovine fetus and placenta near term

Preterm infants have low serum T4 and T3 levels, which may partly explain the immaturity of their tissues. Deiodinase enzymes are important in determining the bioavailability of thyroid hormones: deiodinases D1 and D2 convert T4 to T3, whereas deiodinase D3 inactivates T3 and produces rT3 from T4. In human and ovine fetuses, plasma T3 rises near term in association with the prepartum cortisol surge. This study investigated the developmental effects of cortisol and T3 on tissue deiodinases and plasma thyroid hormones in fetal sheep during late gestation. Plasma cortisol and T3 concentrations in utero were manipulated by exogenous hormone infusion and fetal adrenalectomy. Between 130 and 144 d of gestation (term 145+/-2 d), maturational increments in plasma cortisol and T3, and D1 (hepatic, renal, perirenal adipose tissue) and D3 (cerebral), and decrements in renal and placental D3 activities were abolished by fetal adrenalectomy. Between 125 and 130 d, iv cortisol infusion raised hepatic, renal, and perirenal adipose tissue D1 and reduced renal and placental D3 activities. Infusion with T3 alone increased hepatic D1 and decreased renal D3 activities. Therefore, in the sheep fetus, the prepartum cortisol surge induces tissue-specific changes in deiodinase activity that, by promoting production and suppressing clearance of T3, may be responsible for the rise in plasma T3 concentration near term. Some of the maturational effects of cortisol on deiodinase activity may be mediated by T3.

Regulation of type II deiodinase expression by EGF and glucocorticoid in HC11 mouse mammary epithelium

Thyroid hormones are important for mammary gland growth and development. The iodothyronine deiodinases play a key role in thyroid hormone metabolism. We have showed that type II 5'-deiodinase (5'D2) activity and mRNA are present in the mouse mammary gland and that their levels are reduced in the lactating gland. To investigate the regulatory mechanism of mouse 5'D2 gene (mdio2) expression in mammary epithelium, we employed the HC11 cell line, which is derived from mouse mammary epithelial cells and retains the ability to express differentiated function. HC11 cells were treated with combinations of insulin, glucocorticoid (GC, dexamethasone), prolactin, and epidermal growth factor (EGF), and 5'D2 activity and the D2-to-GAPDH mRNA ratio were measured by (125)I(-) release from (125)I-labeled thyroxine and semiquantitative RT-PCR, respectively. EGF increased both 5'D2 activity and mRNA levels about twofold. GC reduced both 5'D2 activity and mRNA in a dose-dependent manner, and their levels were decreased to approximately one-tenth and one-fifth, respectively, of control levels. These data demonstrated that mdio2 expression in HC11 cells is upregulated by EGF mainly at the pretranslational level and downregulated by GC at both pre- and posttranslational levels. Furthermore, we showed that GC reduced the promoter activity of the 627- bp 5'-upstream region of the mdio2/luciferase chimeric reporter gene, suggesting that GC exerts its effect, at least in part, at the transcriptional level.

Glucocorticoids, thyroid hormones, and iodothyronine deiodinases in embryonic saltwater crocodiles

We investigated the relationship between glucocorticoids, thyroid hormones, and outer ring and inner ring deiodinases (ORD and IRD) during embryonic development in the saltwater crocodile (Crocodylus porosus). We treated the embryos with the synthetic glucocorticoid dexamethasone (Dex), 3,3',5-triiodothyronine (T(3)), and a combination of these two hormones (Dex + T(3)). The effects of these treatments were specific in different tissues and at different stages of development and also brought about changes in plasma concentrations of free thyroid hormones and corticosterone. Administration of Dex to crocodile eggs resulted in a decrease in 3,3',5,5'-tetraiodothyronine (T(4)) ORD activities in liver and kidney microsomes, and a decrease in the high-K(m) rT(3) ORD activity in kidney microsomes, on day 60 of incubation. Dex treatment increased the T(4) ORD activity in liver microsomes, but not kidney microsomes, on day 75 of incubation. Dex administration decreased T(3) IRD activity in liver microsomes. However, this decrease did not change plasma-free T(3) concentrations, which suggests that free thyroid hormone levels are likely to be tightly regulated during development.

Congenital hypothyroid Pax8(-/-) mutant mice can be rescued by inactivating the TRalpha gene

Mice devoid of all TRs are viable, whereas Pax8(-/-) mice, which lack the follicular cells producing T4 and T3 in the thyroid gland, die during the first weeks of postnatal life. A precise comparison between the two types of mutants reveals that their phenotypes are similar, but the defects in spleen, bone, and small intestine are more pronounced in Pax8(-/-) mice. This is interpreted as the result of a negative effect of the unliganded TR on thyroid hormone target genes expression in the Pax8(-/-) mutants. Pax8/TRalpha compound mutants can survive to adulthood, and the expression of target genes is partially restored. This demonstrates the importance of TRalpha aporeceptor activity in several aspects of postnatal development.

Hepatic monitoring of essential amino acid availability may regulate IGF-I activity, thermogenesis, and fatty acid oxidation/synthesis

Diets that are low in certain essential amino acids (EAAs), whether owing to low protein content or poor protein quality, tend to down-regulate systemic IGF-I activity, boost thermogenesis, and suppress hepatic capacity for lipogenesis, while promoting hepatic fatty acid oxidation. It is proposed that for each EAA there is a regulatory protein in hepatocytes whose activity is repressed by adequate levels of its EAA; if one (or more) of these regulatory proteins is active, it serves as a signal of EAA deficiency which then mediates the aforementioned effects on IGF-I activity, thermogenesis, and hepatic fatty acid metabolism. Mechanisms which monitor EAA availability likewise play a role in appetite regulation, thus accounting for the fact that spontaneous calorie consumption tends to be lower on high-protein diets. Diets low in protein quantity or quality may decrease insulin secretion, an effect which should contribute to their impact on IGF-I activity and lipid metabolism. The fact that vegans ingest diets that tend to be relatively low in certain EAAs may play a key role in their characteristic leanness and their decreased risk for diabetes, coronary disease, and cancer.

A comparison of thyroid function in DBA/2J and C57BL/6J mice

DBA/2J mice exhibit audiogenic seizure susceptibility (AGSS) and lower electroshock seizure thresholds compared with C57BL/6J mice. Thyroid function, including thyroxine (T4), 3,5,3'-triiodothyronine (T3), and thyrotropin (TSH) concentrations, T4/T3 ratio, and iodide uptake, of DBA and C57 mice were compared. Thyroid function was also assessed in relation to AGSS and severity in DBA mice. DBA mice have a larger thyroidal pool of iodide due to increased iodide uptake and possibly decreased release, but not to an increased organification rate. This increased iodide uptake exists until about 40 days of age. DBA mice also have a decreased radiochloride space and increased thyroid weight, indicative of enhanced TSH activity. The DBA mice show high T4 and TSH concentrations and a high T4/T3 ratio between the ages of 20 and 40 days. Beginning at 40 days of age the DBA mice have high T4, TSH, and T3 concentrations leading to a T4/T3 ratio approximating the C57 ratio. At any age, DBA mice demonstrating clonic/tonic seizures in response to auditory stimulation have hormone concentrations similar to their 21-day-old counterparts with seizures. Mice that show decreased response to auditory stimulation have hormone concentrations similar to the older age group. The increased thyroid activity of DBA mice is the result of enhanced TSH secretion. The increased TSH production is due to adaptations corresponding to the different age and AGSS. A decreased conversion of T4 to T3 by 3,3,5'-monodeiodinase, is responsible for the increase in TSH due to loss of T3 negative feedback on the anterior pituitary gland. By 40 days of age, the Type I 5'-deiodinase matures whereas the brain deiodinase activity remains subnormal.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sauna and glucose intake on TSH and thyroid hormone levels in plasma of euthyroid subjects

The effect of sauna on thyroid function parameters and its modification by glucose was studied in young euthyroid male volunteers. A 30-minute stay in sauna resulted in an increase in plasma TSH; the response was exaggerated if glycemia had been increased by oral glucose intake at the beginning of the experiment. Plasma rT3 also increased in sauna, this response was, however, blunted by the higher glycemia. TSH response to sauna was definitely present in young men (aged 20 to 25) and absent in middle-aged ones (50 to 55). To explore the mechanism of the effect of increased glycemia, TRH tests were performed and dopamine infusions were administered with and without glucose pretreatment. Increased glycemia did not affect TSH and T3 response to TRH in young volunteers; however, 90 minutes after the administration, plasma rT3 levels were significantly lower in glucose pretreated subjects than in those receiving TRH injections after water pretreatment. Simultaneous infusion of glucose prevented the inhibitory effect of dopamine infusion on plasma TSH. It was concluded that glucose directly modulates the effect of sauna on plasma TSH at a suprapituitary level, while the inhibiting effect of glucose on plasma rT3 response to sauna and TRH is probably mediated by the insulin effect on thyroid hormone metabolism.