Liver D1 Activity Research Articles

Relationship between serum thyroid hormone and IL-1β levels and postmortem tissue deiodinase activity in critically ill patients

Objective: This study investigated the relationship between serum thyroid hormones and IL-1β levels and postmortem tissue deiodinase activity in critically ill patients. Methods: Serum thyroid hormones and IL-1β were measured on the 5th, 15th, and last day of 80 critically ill patients. Forty of these patients were non-survived, and liver and skeletal muscle were harvested to analyze deiodinase activity. Results: Serum TSH, T4, and T3 were decreased, and rT3 and IL-1β were increased in non-survivors. From day 5 to the last day, serum TSH, T4, and T3 levels increased, and rT3 and IL-1β levels decreased with time in survivors, while serum TSH, T4, and T3 levels decreased or remained unchanged, and rT3 and IL-1β increased in non-survivors. On the last day, liver D1 activity was in a negative correlation with serum rT3 and IL-1β, while liver and skeletal muscle D3 activities were in a positive correlation. Conclusion: Serum thyroid hormones and IL-1β are correlated with postmortem deiodinase activity in critically ill patients.

Adaptive Thermogenesis Driving Catch-Up Fat Is Associated With Increased Muscle Type 3 and Decreased Hepatic Type 1 Iodothyronine Deiodinase Activities: A Functional and Proteomic Study.

Refeeding after caloric restriction induces weight regain and a disproportionate recovering of fat mass rather than lean mass (catch-up fat) that, in humans, associates with higher risks to develop chronic dysmetabolism. Studies in a well-established rat model of semistarvation-refeeding have reported that catch-up fat associates with hyperinsulinemia, glucose redistribution from skeletal muscle to white adipose tissue and suppressed adaptive thermogenesis sustaining a high efficiency for fat deposition. The skeletal muscle of catch-up fat animals exhibits reduced insulin-stimulated glucose utilization, mitochondrial dysfunction, delayed in vivo contraction-relaxation kinetics, increased proportion of slow fibers and altered local thyroid hormone metabolism, with suggestions of a role for iodothyronine deiodinases. To obtain novel insights into the skeletal muscle response during catch-up fat in this rat model, the functional proteomes of tibialis anterior and soleus muscles, harvested after 2 weeks of caloric restriction and 1 week of refeeding, were studied. Furthermore, to assess the implication of thyroid hormone metabolism in catch-up fat, circulatory thyroid hormones as well as liver type 1 (D1) and liver and skeletal muscle type 3 (D3) iodothyronine deiodinase activities were evaluated. The proteomic profiling of both skeletal muscles indicated catch-up fat-induced alterations, reflecting metabolic and contractile adjustments in soleus muscle and changes in glucose utilization and oxidative stress in tibialis anterior muscle. In response to caloric restriction, D3 activity increased in both liver and skeletal muscle, and persisted only in skeletal muscle upon refeeding. In parallel, liver D1 activity decreased during caloric restriction, and persisted during catch-up fat at a time-point when circulating levels of T4, T3 and rT3 were all restored to those of controls. Thus, during catch-up fat, a local hypothyroidism may occur in liver and skeletal muscle despite systemic euthyroidism. The resulting reduced tissue thyroid hormone bioavailability, likely D1- and D3-dependent in liver and skeletal muscle, respectively, may be part of the adaptive thermogenesis sustaining catch-up fat. These results open new perspectives in understanding the metabolic processes associated with the high efficiency of body fat recovery after caloric restriction, revealing new implications for iodothyronine deiodinases as putative biological brakes contributing in suppressed thermogenesis driving catch-up fat during weight regain.

Thyroid hormones and deiodinase activities in plasma and tissues from East Greenland polar bears (Ursus maritimus) during winter season

Thyroid hormones (THs) are essential for metabolism and thermoregulation in arctic animals. Still, there is a lack of deeper basic knowledge regarding the regulation and functioning of THs in the environmental physiology of polar bears (Ursus maritimus). This is necessary in order to understand the true impact and consequences of the combination of stressors such as depletion of sea ice and endocrine-disrupting contaminants for the polar bear species. As a first step to gain insight into TH physiology in polar bears, TH concentrations in liver, kidney and muscle in East Greenland polar bears sampled February–March 2011 were analysed and their associations with circulating levels of THs were investigated. In addition, type 1 deiodinase (D1) activities in liver, kidney and muscle and type 2 deiodinase (D2) activities in muscle were analysed. Concentrations of 3,5,3′,5′-tetraiodothyronine (T4) were highest in plasma, followed by liver, kidney and muscle, whereas concentrations of 3,5,3′-triiodothyronine (T3) were highest in kidney followed by liver, plasma and muscle. D1 activities in the tissues varied in the order liver > kidney ≫ muscle, while D2 activity was only analysed in muscle. There were significant positive relationships between T4 in plasma and liver as well as between T4 in plasma and D1 activity in liver. This implies that liver is an important non-thyroidal organ for deiodination of T4, thus providing T3 to the plasma pool, in polar bears. The polar bears with the lowest body condition also had the lowest concentrations of free T3 in plasma and D2 activities in muscle.

Environmental heat stress modulates thyroid status and its response to repeated endotoxin challenge in steers

The objective of this study was to evaluate in cattle, the effects of acute exposure to a heat stress (HS) environment on the status of the pituitary (thyrotropin, TSH)–thyroid (thyroxine, T4)–peripheral tissue T4 deiodination (type 1 5’-deiodinase [D1]; triiodothyronine [T3]; reverse-triiodothyronine [rT3]) axis, and the further response of this pituitary–thyroid–peripheral tissue axis (PTTA) to perturbation caused by the induction of the proinflammatory innate immune state provoked by the administration of gram-negative bacteria endotoxin (lipopolysaccharide [LPS]). Ten steers (318 ± 49 kg body weight) housed in controlled environment chambers were subjected to either a thermoneutral (TN: constant 19°C) or HS temperature conditions (cyclical daily temperatures: 32.2°C–40.0°C) for a total period of 9 d. To minimize the effects of altered plane of nutrition due to HS, steers in TN were pair-fed to animals in HS conditions. Steers received 2 LPS challenges 3 d apart (LPS1 and LPS2; 0.2 μg/kg body weight, intravenously, Escherichia coli 055:B5) with the first challenge administered on day 4 relative to the start of the environmental conditioning. Jugular blood samples were collected at 0, 1, 2, 4, 7, and 24 h relative to the start of each LPS challenge. Plasma TSH, T4, T3, and rT3 were measured by radioimmunoassay. Liver D1 activity was measured in biopsy samples collected before the LPS1 (0 h) and 24 h after LPS2. Before the start of LPS1, HS decreased (P < 0.01 vs TN) plasma TSH (40%), T4 (45.4%), and T3 (25.9%), but did not affect rT3 concentrations. In TN steers, the LPS1 challenge decreased (P < 0.01 vs 0 h) plasma concentrations of TSH between 1 and 7 h and T4 and T3 at 7 and 24 h. In HS steers, plasma TSH concentrations were decreased at 2 h only (P < 0.05), whereas plasma T3 was decreased at 7 and 24 h (P < 0.01). Whereas plasma T4 concentrations were already depressed in HS steers at 0 h, LPS1 did not further affect the levels. Plasma rT3 concentrations were increased in all steers at 4, 7, and 24 h after LPS1 (P < 0.01). The patterns of concentration change of T4, T3, and rT3 during LPS2 mirrored those observed in LPS1; the responses in plasma TSH were of smaller magnitude than those incurred after LPS1. The LPS challenges reduced (P < 0.01) hepatic activity of D1 in all animals but no differences were observed between steers subjected to TN or HS environment. The data are consistent with the concept that acute exposure of cattle to a HS environment results in the depression of the pituitary and thyroid components of the PTTA, whereas a normal capacity to generate T3 from T4 in the liver is preserved. The data also suggest that LPS challenge further suppresses all components of the PTTA including liver T3 generation, and these PTTA perturbations are more pronounced in steers that encounter a HS exposure.

Thyroid hormone transport in developing brain

To discuss the recent advances on thyroid hormone transport in the brain. A special attention is paid to the X-linked thyroid hormone cell transport (THCT) defect (also known as the Allan-Herndon-Dudley syndrome), caused by mutations of the specific thyroid hormone transporter MCT8 gene. MCT8 is involved in thyroid hormone transport in the brain. MRI of patients with THCT defect showed myelination delays, probably related to impaired thyroid hormone action on oligodendrocytes. MCT8 is also expressed in the thyroid and has an important role in thyroid hormone secretion. The altered circulating concentrations of thyroid hormone in the patients are partly because of impaired secretion and altered peripheral metabolism. Increased deiodinase activity is important in the pathophysiology of the syndrome. High D1 activity in liver and kidney increases T4 and rT3 deiodination, and contributes to the increased serum T3. High D2 activity in the brain contributes to compensate the deficient T3 transport by increasing local T3 production. Patients with suspected X-linked leukoencephalopathy should be screened for MCT8 gene mutations. Research on the brain pathophysiology of the THCT defect should focus on the specific role of Mct8 on oligodendrocytes and myelination.

Social stress-induced hypothyroidism is attenuated by antidepressant treatment in rats

Although serotonergic system has been classically implicated in mood modulation, there has been relatively little study on the relationship between this system and thyroid hormones (TH) economy in stress models. When TH are studied, the effects of stress on thyroid function seems to be complex and depend on the kind and time of stress which counts for the elusiveness of mechanisms underlying changes in TH economy. Herein, we hypothesized that serum TH are affected in a time-dependent fashion after repeated social stressful stimuli and serotonergic system is implicated in these changes. Therefore, we aimed to investigate the possible alterations in thyroid hormone economy and type 1 (D1) and type 2 (D2) deiodinase activity in a model of social defeat stress. Thereafter, we tested the responsiveness of these changes to fluoxetine treatment. Both short (STS) and a long-term (LTS) stress were performed. Blood samples were drawn just before and 1 (STS) or 4 and 8 weeks (LTS) after the beginning of stress to assess serum T4, T3 and corticosterone. Deiodinases activity was assessed at the end of each protocol. Stress-induced behavior studied in open field arena and hypercorticosteronemia were mainly observed in LTS (week 4). Stress-induced behavior was associated to hypothyroidism which occurred before, since week 1 in stressed group. Serum TH was restored to control levels in week 8, when behavior changes were not observed anymore, and was mainly associated with high brown adipose tissue D2 activity since thyroid and liver D1 activity were low or normal in the STS and LTS respectively in stressed rats compared to control. Antidepressant study revealed that fluoxetine treatment (10mg/kg po during four weeks) fully reversed stress-induced behavior and normalized serum T4, but not T3 levels and hypercorticosteronemia in stressed group compared to control. The current work adds new concepts concerning TH metabolism changes induced by social stress and suggests that serotonergic system impairment may take part in the key events which ultimately lead to hypothyroxinemia and behavioral changes induced by chronic social defeat.This article is part of a Special Issue entitled ‘Anxiety and Depression’.

Maternal Flaxseed Diet During Lactation Programs Thyroid Hormones Metabolism and Action in the Male Adult Offspring in Rats

Flaxseed has several benefits for health such as improvement in lipid profile; and since thyroid hormones increases cholesterol biliary excretion, we decide to evaluate the programming effect of maternal flaxseed diet during lactation upon thyroid hormone metabolism and action in the adult offspring in rats. At birth, lactating rats were divided into: flaxseed dams (F) - diet with 25% of flaxseed - and controls dams (C). F and C pups received normal diet after weaning and male offspring were sacrificed at 21 and 180 days old. We evaluated serum T3, T4, and TSH; type 1 and 2 deiodinase activities (D1 and D2) in the liver, thyroid, brown adipose tissue (BAT), and pituitary; thyroid hormone receptor (TRβ1) expression and mitochondrial glycerophosphate-dehydrogenase activity (GPDm) in the liver. F offspring showed lower T3 levels at weaning (-30%, p<0.05) probably caused by lower liver D1 activity (-32%, p<0.05) and higher TSH levels (+84.6%, p<0.05) characterizing a profile of hypothyroidism. At 180 days old, F offspring had lower T4 and thyroid D1 and D2 activities (-28.3%, -18.5%, and -44.2%, respectively, p<0.05) and higher BAT D2 activity (+34.5%, p<0.05). We suggest that adult F animals present an inappropriate TSH action on the thyroid, since thyroid deiodinase was lower. Serum T3 was normal probably due to a higher BAT D2 activity and may reflect the tissue T3 concentration because liver D1, TRβ1, and GPDm were normal. Thus, maternal flaxseed diet during lactation may affect the thyroid hormones metabolism in a long-term.

Thyroid Hormone Receptor α Modulates Lipopolysaccharide-Induced Changes in Peripheral Thyroid Hormone Metabolism

Acute inflammation is characterized by low serum T(3) and T(4) levels accompanied by changes in liver type 1 deiodinase (D1), liver D3, muscle D2, and muscle D3 expression. It is unknown at present whether thyroid hormone receptor alpha (TRalpha) plays a role in altered peripheral thyroid hormone metabolism during acute illness in vivo. We induced acute illness in TRalpha-deficient (TRalpha(0/0)) mice by administration of a sublethal dose of LPS. Compared with wild-type, TRalpha(0/0) mice have lower basal serum T(4) and lower liver D1 activity and muscle D3 mRNA expression, whereas liver D3 activity is higher. These changes are gender specific. The inflammatory response to LPS was similar in WT and TRalpha(0/0) mice. The decrease in serum thyroid hormones and liver D1 was attenuated in TRalpha(0/0) mice, whereas the LPS induced fall in liver D3 mRNA was more pronounced in TRalpha(0/0) mice. Muscle D2 mRNA increased similarly in both strains, whereas muscle D3 mRNA decreased less pronounced in TRalpha(0/0) mice. We conclude that alterations in peripheral thyroid hormone metabolism induced by LPS administration are partly regulated via TRalpha.

Impaired Bacterial Clearance in Type 3 Deiodinase-Deficient Mice Infected with Streptococcus pneumoniae

The activation of type 3 deiodinase (D3) has been postulated to play a role in the reduction of thyroid hormone levels during illness. Using a mouse model of acute bacterial infection, we have recently demonstrated marked D3 immunostaining in neutrophils infiltrating infected organs. These observations suggest a possible additional role for this enzyme in the innate immune response. To further assess the role of D3 in the response to acute bacterial infection, we used null D3 [D3 knockout (D3KO)] and wild type (WT) mice and infected them with Streptococcus pneumoniae. Marked reductions in serum thyroid hormone levels were observed both in D3KO and WT mice. Infection resulted also in a decrease in liver D1 activity in WT, but not in infected D3KO mice. Upon infection, pulmonary neutrophilic influx (measured by myeloperoxidase levels) and IL-6 and TNF concentrations increased equally in D3KO and WT mice, and histological examination of infected mice showed similar pulmonary inflammation in both strains. However, D3KO animals demonstrated significantly higher bacterial load in blood, lung, and spleen compared with WT mice. We conclude that 1) D3 is not required to generate the systemic manifestations of the nonthyroidal illness syndrome in this model; 2) the lack of D3 does not affect the extent of pulmonary inflammation; and 3) bacterial outgrowth in blood, spleen, and lung of D3KO mice is significantly higher than in WT mice. Our results suggest a protective role for D3 in the defense against acute bacterial infection, probably by reinforcing the microbial killing capacity of neutrophils.

Low replacement doses of thyroxine during food restriction restores type 1 deiodinase activity in rats and promotes body protein loss

During food restriction, decreased basal metabolic rate secondary to reduced serum thyroid hormones levels contributes to weight loss resistance. Thyroxine (T(4)) and 3,3',5-tri-iodothyronine (T(3)) administration during caloric restriction produce deleterious side effects; however, the administration of physiological doses of T(4) during food restriction has never been evaluated. The aim of this study was to analyze the effects of low replacement doses of T(4) in Wistar rats subjected to 40% food restriction. Food restriction for 30 days led to significantly reduced liver type 1 deiodinase activity, serum TSH, leptin, T(4), T(3), metabolic rate, and body mass. The significant reduction in hepatic deiodinase activity found during food restriction was normalized in a dose-dependent manner by T(4) replacement, showing that decreased type 1 deiodinase (D1) activity is secondary to decreased serum thyroid hormone levels during caloric restriction. The lowest replacement dose of T(4) did not normalize resting metabolic rate, but was able to potentiate the effects of food restriction on carcass fat loss and did not spare body protein. The highest dose of T(4) produced a normalization of daily oxygen consumption and determined a significant reduction in both carcass fat and protein content. Our results show that serum T(4) normalization during food restriction restores serum T(3) and liver D1 activity, while body protein is not spared. Thus, decreased serum T(4) during caloric restriction corresponds to a protective mechanism to avoid body protein loss, highlighting the importance of other strategies to reduce body mass without lean mass loss.

Cold exposure restores the decrease in leptin receptors (OB-Rb) caused by neonatal leptin treatment in 30-day-old rats

We had previously shown that neonatal leptin treatment programs thyroid function in adulthood. As both thyroid hormones (TH) and leptin increased thermogenesis, it was interesting to evaluate the effect of cold exposure on the thyroid function of neonate rats treated with leptin. Pups were divided into two groups: Lep, injected with leptin (8 mug/100 g/BW, s.c.) for the first 10 days of lactation and control (C), injected with saline. When they were 30 days old, the groups were subdivided into two subgroups: LepC and CC, which were exposed to 8 degrees C for 12 h and compared with C and Lep groups, maintained at 25 +/- 1 degrees C. Serum leptin, TH, and TSH were measured by RIA. Type I liver deiodinase (D1) and mitochondrial alpha-glycerol-3-phosphate dehydrogenase (mGPD) activities were assayed by the release of (125)I from (125)I-reverse and colorimetric method respectively. Leptin receptor (OB-Rb) was evaluated by western blot. Lep group had hyperleptinemia (+22%) and lower free tri-iodothyronine (FT(3); -33%). Cold exposure increased TH both in LepC and CC groups compared with respective controls free thyroxine (FT(4):+63 and +39%; FT(3):+75 and +40%). Liver D1 activity was lower in Lep (-22%) and increased with cold exposure (LepC +51% and CC +22%). The mGPD activity was lower in Lep (-34%) and increased (fourfold) when this group is cold exposed. Hypothalamic and thyroidal OB-Rb receptors were lower in Lep group (-47 and -36% respectively) and they were restored to normal levels after cold exposure. Leptin-programmed rats had higher TH response after cold exposure. OB-Rb had a fast response to cold exposure normalizing the lower levels observed in the leptin-programmed animals and may contribute to the higher TH cold responses.

Chronic Leptin Treatment Inhibits Liver Mitochondrial α-Glycerol-β-phosphate Dehydrogenase in Euthyroid Rats

Leptin modulates the hypothalamus-pituitary-thyroid axis and peripheral metabolism of thyroid hormones (THs). We have studied the effect of acute and chronic leptin treatment upon liver mitochondrial glycerol phosphate dehydrogenase activity (mGPD), whose expression and activity are TH dependent. We performed 2 experiments: 1) acute leptin treatment - LepA: adult rats received a single leptin injection (8 microg/100 g BW); 2) chronic leptin treatment - LepC: adult rats received leptin (8 microg/100 g BW) daily, for 6 days. In both experiments, control groups were saline-treated. All rats were sacrificed 2 hours after the last dose. Liver mGPD activity was determined by colorimetric method. Liver D1 activity was measured by the release of (125)I from (125)I-rT3. Serum hormones were measured by RIA. LepA rats showed higher serum thyroid stimulating hormone (TSH) (+ 64%, p<0.05), free T4 (+ 34%, p<0.05), free T3 (+ 64%, p<0.05), and liver D1 activity (+ 85%, p<0.05), but no change in mGPD activity. Since THs increase mGPD activity, the unchanged level in the acute experiment is suggestive of an inhibitory role of leptin. LepC rats presented lower mGPD activity (-1.7-fold, p<0.05) and higher liver D1 activity (+ 32%, p<0.05), but no alteration in serum TSH and free THs. Our results show that leptin downregulates mGPD activity, mainly when hyperleptinemia is chronic.

Sexual dimorphism in thyroid function and type 1 iodothyronine deiodinase activity in pre-pubertal and adult rats

Iodothyronine deiodinase activities are regulated by sex steroids; however, the mechanisms underlying the reported sexual dimorphism are poorly defined. In the present report, we aimed to investigate whether type 1 deiodinase (D1) sexual dimorphism exists early in sexual development by studying pre-pubertal male (Pm) and female (Pf) rats, as well as adult controls (C) and gonadectomized male and females rats. Adult male Wistar rats were studied 21 days after orchiectomy (Tex), and adult females were studied 21 days after ovariectomy (Ovx), and after estradiol benzoate (Eb) replacement. Serum total triiodothyronine (T3) was higher in pre-pubertal (P) rats than in the matching adults, with no difference between genders, although in adult males T3 was significantly lower than in females. There were no sex or age differences in serum total T4. Serum TSH in pre-pubertal (P) rats was within the adult female range, and both were significantly lower than in adult males. D1 activity in liver was greater in Pm than in Pf. In adult females, liver D1 activity was lower, while in adult males it was higher than in P rats. The same pattern of D1 activity was found in kidney. In thyroid and pituitary, D1 activity was similar in Pm, Pf, and adult females, which were all significantly lower than in the adult male. There were no differences in serum T3 and T4 between C and Tex males, but serum TSH was significantly decreased in Tex rats. Hepatic and renal D1 activities were lower in Tex than in C, but no changes were detected in thyroid and pituitary. In Ovx females, T3 was significantly lower than in the C group. Serum T4 was significantly decreased by estradiol replacement therapy in Ovx rats, in both doses used, whereas TSH was unchanged. Eb replacement increased liver and thyroid D1 activity, but in the kidney, only the highest estradiol dose promoted a significant D1 increase. In conclusion, in males, hepatic and renal D1 activity appears to be significantly influenced by gonadal hormones, in contrast to females, in which only exogenous Eb treatment stimulated D1 activity. The comparison between pre-pubertal and adult rats suggests that serum T3 is not the main regulator of D1 activity, and other factors, besides T3 and gonadal hormones, can modulate D1 activity during murine maturation.

Serum 3,3′,5′-Triiodothyronine (rT3) and 3,5,3′-Triiodothyronine/rT3Are Prognostic Markers in Critically Ill Patients and Are Associated with Postmortem Tissue Deiodinase Activities

Critical illness is associated with reduced TSH and thyroid hormone secretion, and with changes in peripheral thyroid hormone metabolism, resulting in low serum T3 and high rT3. In 451 critically ill patients who received intensive care for more than 5 d, serum thyroid parameters were determined on d 1, 5, 15, and last day (LD). All patients had been randomized for intensive or conventional insulin treatment. Seventy-one patients died, and postmortem liver and skeletal muscle biopsies were obtained from 50 of them for analysis of deiodinase (D1-3) activities. Insulin treatment did not affect thyroid parameters. On d 1, rT3 was higher and T3/rT3 was lower in nonsurvivors as compared with survivors (P = 0.001). Odds ratio for survival of the highest vs. the lowest quartile was 0.3 for rT3 and 2.9 for T3/rT3. TSH, T4, and T3 were lower in nonsurvivors from d 5 until LD (P < 0.001). TSH, T4, T3, and T3/rT3 increased over time in survivors, but decreased or remained unaltered in nonsurvivors. Liver D1 activity was positively correlated with LD serum T3/rT3 (R = 0.83, P < 0.001) and negatively correlated with rT3 (R = -0.69, P < 0.001). Both liver and skeletal muscle D3 activity were positively correlated with LD serum rT3 (R = 0.32, P = 0.02 and R = 0.31, P = 0.03). In critically ill patients who required more than 5 d of intensive care, rT3 and T3/rT3 were already prognostic for survival on d 1. On d 5, T4, T3, but also TSH levels are higher in patients who will survive. Serum rT3 and T3/rT3 were correlated with postmortem tissue deiodinase activities.

Iodothyronine deiodinases and the control of plasma and tissue thyroid hormone levels in hyperthyroid tilapia (Oreochromis niloticus)

Thyroid status is one of the most potent regulators of peripheral thyroid hormone metabolism in vertebrates. Despite this, the few papers that have been published concerning the role of thyroid hormones in the regulation of thyroid function in fish often offer conflicting data. We therefore set out to investigate the effects of tetraiodothyronine (thyroxine) (T4) or tri-iodothyronine (T3) supplementation (48 p.p.m.) via the food on plasma and tissue thyroid hormone levels as well as iodothyronine deiodinase (D) activities in the Nile tilapia (Oreochromis niloticus). T4 supplementation did not induce a hyperthyroid state and subsequently had no effects on the thyroid hormone parameters measured, with the liver as the sole notable exception. In T4-fed tilapias, the hepatic T4 levels increased substantially, and this was accompanied by an increase in in vitro type I deiodinase (D1) activity. Although the lack of effect of T4 supplementation could be partially explained by an inefficient uptake of T4 from the gut, our current data suggest that also the increased conversion of T4 into reverse (r)T3 by the D1 present in the liver plays an important role in this respect. In addition, T3 supplementation increased plasma T3 and decreased plasma T4 concentrations. T3 levels were also increased in the liver, brain, kidney, gill and white muscle, but without affecting local T4 concentrations. However, this increase in T3 availability remained without effect on D1 activity in liver and kidney. This observation, together with the 6-n-propylthiouracyl (PTU) insensitivity of the D1 enzyme in fish, sets the D1 in teleost fish clearly apart from its mammalian and avian counterparts. The changes in hepatic deiodinases confirm the role of the liver as an important T3-regulating tissue. However, the very short plasma half-life of exogenously administered T3 implies the existence of an efficient T3 clearing/degradation mechanism other than deiodination.

Characteristics and thyroid state-dependent regulation of iodothyronine deiodinases in pigs.

Three iodothyronine deiodinases (D1, D2, and D3) regulate local and systemic availability of thyroid hormone. D1 and D2 activate the prohormone T4 to the thyromimetic T3, and D3 inactivates T4 and T3 to rT3 and 3,3'-diiodothyronine, respectively. The expression of the three deiodinases is tightly regulated with regard to developmental stage and cell type to provide fine tuning of T3 supply to target cells. Most studies regarding distribution and regulation of deiodinases have been carried out in rodents. However, in different respects, rodents do not seem to be the optimal experimental model for human thyroid hormone physiology. For instance, D2 expression has been observed in human thyroid and skeletal muscle but not in these tissues in rodents. In this study, we have explored the pig as an alternative model. Porcine D1, D2, and D3 were cloned by RT-PCR, and their catalytic properties were shown to be virtually identical to those reported for human and rodent deiodinases. The tissue distribution of deiodinases was studied in normal pigs and in pigs made hypothyroid by methimazole treatment or in pigs made hyperthyroid by T4 treatment. D1 activity in liver and kidney was increased in T4-treated pigs. D2 activities in cerebrum and pituitary were decreased after T4 treatment and strongly increased after methimazole treatment. Remarkably, D2 activity in thyroid and skeletal muscle was induced in hypothyroid pigs. Significant expression of D3 was observed in cerebrum and was positively regulated by thyroid state. In conclusion, the pig appears to be a valuable model for human thyroid hormone physiology. The expression of D2 activity in thyroid and skeletal muscle is of particular interest for studies on the importance of this enzyme in (hypothyroid) humans.

Increased 5'-iodothyronine deiodinase activity is a maternal adaptive mechanism in response to protein restriction during lactation

We have shown that protein restriction during lactation is associated with higher levels of serum and milk tri-iodothyronine (T(3)) with lower serum thyroxine (T(4)), suggesting an increased T(4) to T(3) conversion. To investigate this hypothesis, the activity of type 1 (D1) and/or type 2 (D2) iodothyronine deiodinases was evaluated on days 4, 12 and 21 of lactation in several tIssues of dams fed an 8% protein-restricted (PR) diet and controls fed a 23% protein diet. Serum TSH, T(3) and T(4) were measured by radioimmunoassay. Deiodinase activity was determined by the release of (125)I from (125)I-reverse T(3), under specific conditions for D1 or D2. PR dams had a transitory reduction in liver D1 activity (P<0.05) on day 12, and a small increase in thyroid D1 on day 12 followed by a small decrease on day 21. However, thyroid D2 activity was higher than controls (P<0.05) during the whole of the lactation period. Mammary gland D1 and D2 activities were lower on day 4 of lactation in PR dams (P<0.05), and D2 was higher on day 21 (P<0.05). Potentially, a lower conversion of T(3) to di-iodothyronine in the mammary glands of PR dams at the beginning of lactation may serve to provide more T(3) through the milk. Brown adipose tIssue (BAT) D2 activity was higher (P<0.05) in PR dams during all periods of lactation. PR dams showed higher skeletal muscle D1 activity only at the end of lactation, but no changes in D2 activity. Higher pituitary D1 and D2 activities in the PR group (P<0.05) at the end of lactation could have contributed to the lower serum TSH. These data suggest that the higher thyroid and BAT D2 activity during the whole of lactation and skeletal muscle D1 activity at the end of lactation may contribute to the higher serum T(3) in PR dams.

Type 1 iodothyronine deiodinase in the house musk shrew ( Suncus murinus, Insectivora: Soricidae): cloning and characterization of complementary DNA, unique tissue distribution and regulation by T 3

The house musk shrew Suncus murinus (Insectivora: Soricidae) has been reported as having low thyroxine to 3,3 ′5-triiodothyronine (T 3) converting activity in liver and kidney homogenates and was assumed to be type 1 iodothyronine deiodinase (D1)-deficient. To study whether this is due to structural abnormality of shrew D1, we cloned the cDNA and characterized the enzyme. The deduced amino acid sequence of shrew D1 was found to be highly homologous to other known D1s and the enzyme itself to have similar catalytic activity. However, unlike in other species, the D1 activity was detected only in liver. Moreover, the D1 activity in liver of the shrew was less than half of that in rat liver and its expression was not up-regulated by T 3. In contrast, a very high activity of D2 was demonstrated in brain and brown adipose tissue. The present study also revealed that the serum level of T 3 in the shrew was in the same range as these in other mammals. These results suggest that D2 contributes to the production and maintenance of T 3 levels in the house musk shrew.

Effects of proinflammatory cytokines on anterior pituitary 5'-deiodinase type I and type II.

Cytokines are locally produced in the anterior pituitary and act through para-/autocrine mechanisms to modulate cell growth and hormone production. 5'-Deiodinases type I (D1) and type II (D2) are also expressed in the anterior pituitary and play an integrative role in the regulation of hormone production and pituitary feedback. D1 activity is known to be regulated by proinflammatory cytokines in liver and thyroid. Therefore, we examined the effects of IL-1 beta, IL-6 and TNF alpha on 5'-deiodinase activities in reaggregates of rat anterior pituitaries and rat somatomammotroph GH3 cells cultured alone, or in a bicameral culture system together with the murine folliculo-stellate (FS) cell line TtT/Gf. In reaggregate cultures of rat anterior pituitaries IL-1 beta stimulated D1 and D2 dose-dependently and D2 activity was increased by TNF alpha. When GH3 cells were cocultured with TtT/Gf cells, D2 activities were 2.3-fold lower than in GH3 cells cultured alone. TNF alpha (50 ng/ml) and IL-6 (100 U/ml) stimulated D2 in GH3 cells when the cells were cultured alone and treated with these cytokines for 24 h. When TtT/Gf cells in the coculture model were treated with IL-1 beta, TNF alpha and IL-6, no effect on D1 or D2 activities in GH3 cells was observed. In male, adult rats a single LPS injection (i.p.) stimulated D2 and D1 activities in the anterior pituitary, and decreased liver D1 activities and serum TSH levels. In vitro, LPS stimulation of the coculture model of GH3 and FS cells also increased D1 activity. Electrophoretic mobility shift assays (EMSAs) revealed that IL-1 beta and TNF alpha activate the transcription factor NF kappa B in reaggregates of rat anterior pituitaries and in TtT/Gf cells cultured alone or cocultured with GH3 cells. Taken together, these findings imply that in anterior pituitary cells 5'-deiodinase activities are stimulated by locally produced cytokines in a para-/autocrine manner but cell types other than FS cells seem to mediate some of the effects.

Extrathyroidal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on thyroid hormone turnover in male Sprague-Dawley rats.

Treatment of rats with different polyhalogenated aromatic hydrocarbons strongly decreases plasma T4, with little or no decrease in plasma T3. The extrathyroidal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thyroid hormone turnover were studied by i.p. administration of a single dose of 10 microg TCDD/kg BW or vehicle (corn oil) to euthyroid (Eu) rats, thyroidectomized (Tx) rats, and Tx rats infused with 1 microg T4 (Tx+T4) or 0.4 microg T3 (Tx+T3)/100 g BW x day by osmotic minipumps. Tx rats showed decreased plasma T4 and T3 and increased plasma TSH levels, decreased hepatic type I deiodinase (D1) and malic enzyme activities, and increased brain type II deiodinase (D2) activities. All parameters were largely restored to Eu levels in Tx+T4 rats and, except for plasma T4 and brain D2 activity, in Tx+T3 rats, validating the thyroid hormone-replaced Tx rats as models to study the peripheral effects of TCDD. Three days after TCDD administration, plasma T4 and free T4 levels were significantly reduced in Eu and Tx+T4 rats, and plasma T3 was significantly reduced in Tx+T3, but not in Eu or Tx+T4 rats. Plasma TSH was not affected by TCDD in any group. Hepatic T4 UDP-glucuronyltransferase (UGT) activity was induced approximately 5-fold by TCDD, whereas T3 UGT activity was only increased by about 20% (P = NS) in the different groups. TCDD produced an insignificant decrease in liver D1 activity in Tx rats and an insignificant increase in brain D2 activity in Tx rats and hormone-replaced Tx rats. Hepatic malic enzyme activity was significantly increased by TCDD in all groups, except Tx rats. These results strongly suggest that the thyroid hormone-decreasing effects of TCDD are predominantly extrathyroidal and mediated by the marked induction of hepatic T4 UGT activity.