Thyroid hormone regulation of malic enzyme synthesis. Dual tissue-specific control

Six1 promotes skeletal muscle thyroid hormone response through regulation of the MCT10 transporter

BACKGROUND

The Six1 transcription factor is implicated in controlling the development of several tissue types, notably skeletal muscle. Six1 also contributes to muscle metabolism and its activity is associated with the fast-twitch, glycolytic phenotype. Six1 regulates the expression of certain genes of the fast muscle program by directly stimulating their transcription or indirectly acting through a long non-coding RNA. We hypothesized that additional mechanisms of action of Six1 might be at play.

METHODS

A combined analysis of gene expression profiling and genome-wide location analysis data was performed. Results were validated using in vivo RNA interference loss-of-function assays followed by measurement of gene expression by RT-PCR and transcriptional reporter assays.

RESULTS

The Slc16a10 gene, encoding the thyroid hormone transmembrane transporter MCT10, was identified as a gene with a transcriptional enhancer directly bound by Six1 and requiring Six1 activity for full expression in adult mouse tibialis anterior, a predominantly fast-twitch muscle. Of the various thyroid hormone transporters, MCT10 mRNA was found to be the most abundant in skeletal muscle, and to have a stronger expression in fast-twitch compared to slow-twitch muscle groups. Loss-of-function of MCT10 in the tibialis anterior recapitulated the effect of Six1 on the expression of fast-twitch muscle genes and led to lower activity of a thyroid hormone receptor-dependent reporter gene.

CONCLUSIONS

These results shed light on the molecular mechanisms controlling the tissue expression profile of MCT10 and identify modulation of the thyroid hormone signaling pathway as an additional mechanism by which Six1 influences skeletal muscle metabolism.

Pathogenesis of hypothyroidism-induced NAFLD is driven by intra- and extrahepatic mechanisms

Hypothyroidism, a metabolic disease characterized by low thyroid hormone (TH) and high thyroid-stimulating hormone (TSH) levels in the serum, is strongly associated with nonalcoholic fatty liver disease (NAFLD). Hypothyroidism-induced NAFLD has generally been attributed to reduced TH signaling in the liver with a consequent decrease in lipid utilization. Here, we found that mildly hypothyroid mice develop NAFLD without down-regulation of hepatic TH signaling or decreased hepatic lipid utilization. NAFLD was induced by impaired suppression of adipose tissue lipolysis due to decreased insulin secretion and to a reduced response of adipose tissue itself to insulin. This condition leads to increased shuttling of fatty acids (FAs) to the liver, where they are esterified and accumulated as triglycerides. Lipid accumulation in the liver induces hepatic insulin resistance, which leads to impaired suppression of endogenous glucose production after feeding. Hepatic insulin resistance, synergistically with lowered insulin secretion, increases serum glucose levels, which stimulates de novo lipogenesis (DNL) in the liver. Up-regulation of DNL also contributes to NAFLD. In contrast, severely hypothyroid mice show down-regulation of TH signaling in their livers and profound suppression of adipose tissue lipolysis, which decreases delivery of FAs to the liver. The resulting lack of substrates for triglyceride esterification protects severely hypothyroid mice against NAFLD. Our findings demonstrate that NAFLD occurs when TH levels are mildly reduced, but, paradoxically, not when they are severely reduced. Our results show that the pathogenesis of hypothyroidism-induced NAFLD is both intra- and extrahepatic; they also reveal key metabolic differences between mild and severe hypothyroidism.

Trichoplax adhaerens reveals a network of nuclear receptors sensitive to 9-cis-retinoic acid at the base of metazoan evolution

Trichoplax adhaerens, the only known species of Placozoa is likely to be closely related to an early metazoan that preceded branching of Cnidaria and Bilateria. This animal species is surprisingly well adapted to free life in the World Ocean inhabiting tidal costal zones of oceans and seas with warm to moderate temperatures and shallow waters. The genome of T. adhaerens (sp. Grell) includes four nuclear receptors, namely orthologue of RXR (NR2B), HNF4 (NR2A), COUP-TF (NR2F) and ERR (NR3B) that show a high degree of similarity with human orthologues. In the case of RXR, the sequence identity to human RXR alpha reaches 81% in the DNA binding domain and 70% in the ligand binding domain. We show that T. adhaerens RXR (TaRXR) binds 9-cis retinoic acid (9-cis-RA) with high affinity, as well as high specificity and that exposure of T. adhaerens to 9-cis-RA regulates the expression of the putative T. adhaerens orthologue of vertebrate L-malate-NADP⁺ oxidoreductase (EC 1.1.1.40) which in vertebrates is regulated by a heterodimer of RXR and thyroid hormone receptor. Treatment by 9-cis-RA alters the relative expression profile of T. adhaerens nuclear receptors, suggesting the existence of natural ligands. Keeping with this, algal food composition has a profound effect on T. adhaerens growth and appearance. We show that nanomolar concentrations of 9-cis-RA interfere with T. adhaerens growth response to specific algal food and causes growth arrest. Our results uncover an endocrine-like network of nuclear receptors sensitive to 9-cis-RA in T. adhaerens and support the existence of a ligand-sensitive network of nuclear receptors at the base of metazoan evolution.

An extremely high dietary iodide supply forestalls severe hypothyroidism in Na+/I- symporter (NIS) knockout mice

The sodium/iodide symporter (NIS) mediates active iodide (I⁻) accumulation in the thyroid, the first step in thyroid hormone (TH) biosynthesis. Mutations in the SLC5A5 gene encoding NIS that result in a non-functional protein lead to congenital hypothyroidism due to I⁻ transport defect (ITD). ITD is a rare autosomal disorder that, if not treated promptly in infancy, can cause mental retardation, as the TH decrease results in improper development of the nervous system. However, in some patients, hypothyroidism has been ameliorated by unusually large amounts of dietary I⁻. Here we report the first NIS knockout (KO) mouse model, obtained by targeting exons 6 and 7 of the Slc5a5 gene. In NIS KO mice, in the thyroid, stomach, and salivary gland, NIS is absent, and hence there is no active accumulation of the NIS substrate pertechnetate (^99mTcO₄⁻). NIS KO mice showed undetectable serum T₄ and very low serum T₃ levels when fed a diet supplying the minimum I⁻ requirement for rodents. These hypothyroid mice displayed oxidative stress in the thyroid, but not in the brown adipose tissue or liver. Feeding the mice a high-I⁻ diet partially rescued TH biosynthesis, demonstrating that, at high I⁻ concentrations, I⁻ enters the thyroid through routes other than NIS.

Fat metabolism is regulated by altered gene expression of lipogenic enzymes and regulatory factors in liver and adipose tissue but not in semimembranosus muscle of pigs during the fattening period

It has been shown previously that lipid metabolism is regulated by fatty acids (FA) and that thyroid hormones are important regulators of energy metabolism. The effects of weight, dietary fat level and dietary FA profile on thyroid hormone levels and expression of lipogenic genes and tissue FA composition were studied. Sixty-one crossbred gilts weighing 62 ± 5.2 kg BW average were either slaughtered at the beginning of the trial (n = 5) or fed one of seven diets (n = 8 pigs per diet): a semi-synthetic diet formulated to contain a very low level of fat (NF) and six diets based on barley-soybean meal supplemented with approximately 10% fat of different origin and slaughtered at 100 kg BW. The supplemental fats were tallow, high-oleic sunflower oil, sunflower oil (SFO), linseed oil, fat blend (55% tallow, 35% sunflower oil, 10% linseed oil) and fish oil blend (40% fish oil, 60% linseed oil). In general, the dietary FA profiles altered the FA composition of liver, semimembranosus muscle and adipose tissues. Pigs fed the NF diet had the highest free and total triiodothyronine (T3) values followed by pigs fed SFO. Total T3 levels were higher in pigs at 60 kg than in pigs at 100 kg. Correlations between thyroid hormones and genes encoding enzymes of fat synthesis in adipose tissue (acetyl CoA carboxylase (ACACA), fatty acid synthase and stearoyl CoA desaturase (SCD)) and the large differences in expression of lipogenic genes at different weights (60 and 100 kg BW), suggest a role for thyroid hormones and for T3, in particular, in regulating whole animal fat metabolism, with effects brought about by altered expression of lipogenic genes. Liver sterol receptor element binding protein-1 (SREBP1) mRNA content was affected by dietary treatment (P < 0.001) and was correlated with ACACA and SCD, whereas adipose tissue SREBP1 was not correlated with the mRNA abundance of any lipogenic enzyme. Weight and tissue factors showed greater influence on mRNA abundance of genes related with lipid metabolism than diet and tissue FA composition. In the pig, FA synthesis appear to be of greater magnitude in adipose tissue than in the liver as suggested by the higher expression of lipogenic genes in adipose tissue.

Selective upregulation of lipid metabolism in skeletal muscle of foraging juvenile king penguins: an integrative study

The passage from shore to marine life of juvenile penguins represents a major energetic challenge to fuel intense and prolonged demands for thermoregulation and locomotion. Some functional changes developed at this crucial step were investigated by comparing pre-fledging king penguins with sea-acclimatized (SA) juveniles (Aptenodytes patagonicus). Transcriptomic analysis of pectoralis muscle biopsies revealed that most genes encoding proteins involved in lipid transport or catabolism were upregulated, while genes involved in carbohydrate metabolism were mostly downregulated in SA birds. Determination of muscle enzymatic activities showed no changes in enzymes involved in the glycolytic pathway, but increased 3-hydroxyacyl-CoA dehydrogenase, an enzyme of the β-oxidation pathway. The respiratory rates of isolated muscle mitochondria were much higher with a substrate arising from lipid metabolism (palmitoyl-L-carnitine) in SA juveniles than in terrestrial controls, while no difference emerged with a substrate arising from carbohydrate metabolism (pyruvate). In vivo, perfusion of a lipid emulsion induced a fourfold larger thermogenic effect in SA than in control juveniles. The present integrative study shows that fuel selection towards lipid oxidation characterizes penguin acclimatization to marine life. Such acclimatization may involve thyroid hormones through their nuclear beta receptor and nuclear coactivators.

Beyond low plasma T3: local thyroid hormone metabolism during inflammation and infection

Decreased serum thyroid hormone concentrations in severely ill patients were first reported in the 1970s, but the functional meaning of the observed changes in thyroid hormone levels, together known as nonthyroidal illness syndrome (NTIS), remains enigmatic. Although the common view was that NTIS results in overall down-regulation of metabolism in order to save energy, recent work has shown a more complex picture. NTIS comprises marked variation in transcriptional and translational activity of genes involved in thyroid hormone metabolism, ranging from inhibition to activation, dependent on the organ or tissue studied. Illness-induced changes in each of these organs appear to be very different during acute or chronic inflammation, adding an additional level of complexity. Organ- and timing-specific changes in the activity of thyroid hormone deiodinating enzymes (deiodinase types 1, 2, and 3) highlight deiodinases as proactive players in the response to illness, whereas the granulocyte is a novel and potentially important cell type involved in NTIS during bacterial infection. Although acute NTIS can be seen as an adaptive response to support the immune response, NTIS may turn disadvantageous when critical illness enters a chronic phase necessitating prolonged life support. For instance, changes in thyroid hormone metabolism in muscle during critical illness may be relevant for the pathogenesis of myopathy associated with prolonged ventilator dependence. This review focuses on NTIS as a timing-related and organ-specific response to illness, occurring independently from the decrease in serum thyroid hormone levels and potentially relevant for disease progression.

Effects of short term triiodothyronine administration to broiler chickens fed methimazole

The purposes of these experiments were to determine possible relationships among certain indices of lipid metabolism and specific gene expression in chickens (Gallus gallus) fed methimazole (MMI) and the subsequent effects of providing supplemental T3 to relieve the effects of MMI. Male, broiler chickens growing from 14 to 28 days of age were fed diets containing 18% crude protein and either 0 or 1 g MMI/kg of diet. At 28 days, birds received 18% crude protein diets containing either 0 or 1 mg triiodothyronine (T3)/kg. Birds were sampled at 0, 1, 2 & 4 days post relief from MMI or at 0, 3, 6, 9, 24 & 48 h. Measurements taken in the first experiment included in vitro lipogenesis (IVL), malic enzyme (ME), isocitrate dehydrogenase (ICDNADP), aspartate aminotransferase (AST) enzyme activities and the expression of the genes for ME, fatty acid synthase (FAS) and acetyl coenzyme carboxylase (ACC), ICD and AST. The same enzyme activities and gene expressions were assayed over the intervals mentioned above. In vitro lipogenesis was eliminated due to constraints imposed by sampling times. Gene expression was estimated with real time RT-PCR assays. Dietary MMI decreased IVL and ME at 28 days of age. T3 supplementation for 1 day restored both IVL and ME. Continuing T3 replenishment decreased IVL without affecting ME activity. Although MMI decreased ME gene expression, there was only a transitory relationship between enzyme activity and gene expression when apparent thyroid function was restored with exogenous T3. Metabolic changes in response to feeding T3 occurred within a short period, suggesting that changes in intermediary metabolism preceded morphological changes. Furthermore, the thyroid state of the animal will determine responses to exogenous T3.

Responses of chickens subjected to thyroid hormone depletion–repletion

The purpose of this experiment was to determine the relationship between lipid metabolism and the expression of specific genes in chickens fed methimazole to produce hypothyroidism. Male, broiler chickens growing from 14 to 28 days of age were fed diets containing 18% crude protein and either 0 or 1 g methimazole per kg of diet. At 28 days, these two groups were further subdivided into groups receiving 18% crude protein diets containing either 0 or 1 mg triiodothyronine (T(3)) per kg. Birds were sampled at intervals from 0 to 120 h. Measurements taken included in vitro lipogenesis (IVL), malic enzyme (ME), isocitrate dehydrogenase (ICD-NADP), aspartate aminotransferase (AAT) activities and the expression of the genes for ME, fatty acid synthase (FAS), NADP-ICD, AAT and acetyl coenzyme carboxylase (ACC). Gene expression was estimated with real time RT-PCR assays. Expression rates were noted as C(t)'s. Dietary methimazole decreased IVL and ME at 28 days of age. T(3) and supplementation for 1 day restored both IVL and ME. Paradoxically, continuing T(3) replenishment for a longer period decreased IVL without affecting ME activity. Although methimazole decreased ME gene expression, there was only a transitory relationship between enzyme activity and gene expression when plasma T(3) was replenished with exogenous T(3). These data explain the apparent dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. Most metabolic changes in response to feeding T(3) occurred within a short period of time, suggesting that changes in intermediary metabolism preceded morphological changes. Furthermore, the thyroid state of the animal will determine responses to exogenous T(3).

Protein and enery relationships in the broiler chicken 12. Dietary protein and triidothyronine (T3) effects on the response of broilers to isoproterenol and cyclic adenosine monophosphate in vitro

Indian River male broiler chickens (7-d-old) were fed on diets containing 120, 210 or 300 g crude protein/kg + 0 or 1 mg triiodothyronine (T3)/kg diet (Expt 1) and 120, 150, 180 or 210 g crude protein/kg +0 or 1 mg T3/kg diet (Expt 2) to determine the effects of crude protein level and T3on growth and metabolism. Body composition of chickens was determined by a combination of dissection of muscle and abdominal fat pads, and chemical extraction (Expt 1).In vitrolipogenesis (IVL) was determined in both experiments by incubating liver explants for 2 h at 37° in the presence of 10-4M-dibutyryl cyclic AMP (cAMP) or 10-5M isoproterenol (ISO) and 10-2M-[2-14C]acetate. Acetate incorporation into total lipid was an indication of IVL. Activity ratios for each of these additions relative to control (?cAMP?ISO) were calculated to ascertain basal ν. inhibited rates of IVL. The relative muscle mass was increased by increasing crude protein from 120 to 210 g/kg diet but not from 210 to 300 g/kg diet. Dietary T3decreased total body lipid regardless of the dietary crude-protein level. Increasing dietary crude protein decreased (P<0.05) basal IVL (?cAMP? ISO) but not IVL (+ cAMP). Dietary T3decreased basal IVL in birds fed on the diets containing 120 and 210 g crude protein/kg but had little effect on the two inhibited states of lipogenesis (+ cAMP or +ISO). The component of lipogenesis sensitive toin vitroinhibition is also the component under dietary control.

Methimazole, thyroid hormone replacement, and lipogenic enzyme gene expression in broilers

The purpose of this experiment was to determine the possible relationship between certain indices of lipid metabolism and specific gene expression in chickens fed methimazole to simulate hypothyroidism. Male broiler chickens (Gallus gallus) growing from 7 to 28 days of age were fed diets containing 18% crude protein and either 0 or 1 g methimazole per kilogram of diet. At 28 days, these two groups were further subdivided into groups receiving 18% crude protein diets containing either 0 or 1 mg triiodothyronine (T3) per kilogram. Birds were sampled at 28, 30, and 33 days. Measurements taken included in vitro lipogenesis (IVL), malic enzyme (ME) activity, isocitrate dehydrogenase, aspartate amino transferase, and the expression of the genes for ME, fatty acid synthase (FAS), and acetyl coenzyme carboxylase (ACC). Hypothyroidism decreased IVL and ME at 28 days of age; however, T3 supplementation for 2 days restored both IVL and ME. Paradoxically, continuing T3 replenishment for an additional 3 days decreased IVL but did not decrease ME activity. In contrast, supplemental T3 decreased IVL in euthyroid birds, regardless of the dosing interval, but had no effect on ME activity. Although methimazole decreased ME gene expression, there was only a transitory relationship between enzyme activity and gene expression when plasma T3 was restored with exogenous T3. These data may help to explain some of the apparent reported dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. In addition, most metabolic changes in response to feeding T3 occurred within 2 to 5 days, suggesting that changes in intermediary metabolism preceded morphological changes. In conclusion, the thyroid state of the animal will determine responses to exogenous T3.

Thyroid hormone stimulates myoglobin gene expression in rat cardiac muscle

T3 increases the heart activity, O2 consumption and the reactive O2 species production. Myoglobin (Mb) is highly expressed in the heart, where it facilitates O2 diffusion, mitochondrial respiration, and scavenges reactive O2 species. Here we investigate, by dose-response (0.3-100 microg/100 g BW, i.p., 5 days) and time-course studies (100 microg/100 g BW, i.v., from 0.5 to 24h), whether T3 affects the Mb mRNA and protein expression in atrium (A) and ventricle (V), by Northern and Western blot. We show that the Mb gene is controlled by T3 in A and V, as indicated by Mb mRNA and protein content decrease in thyroidectomized (Tx) rats, and restoration by T3 treatment. In the A, the different doses of T3 induced the Mb mRNA and protein recovery to the euthyroid levels; in the time-course study, this occurred only with the protein levels. In the V, T3 progressively increased the Mb mRNA above the euthyroid levels at a dose of 25 microg/100g BW; higher doses decreased it to the euthyroid levels. Mb protein increased only to the euthyroid levels at all T3 doses injected. The time-course study showed a progressive increase in the ventricular Mb mRNA and protein, which exceeded the euthyroid levels from 6 to 24h, and at 2 and 6 h of the T3 treatment, respectively. We conclude that heart Mb gene expression is influenced by thyroid status.

Thyroid hormone stimulates myoglobin expression in soleus and extensorum digitalis longus muscles of rats: concomitant alterations in the activities of Krebs cycle oxidative enzymes

Myoglobin (Mb) gene expression, Citrate Synthase (CS) and Succinate Dehydrogenase (SDH) activities of Soleus (S) and Extensorum Digitalis Longus (EDL) muscles were studied in intact, thyroidectomized and T3-treated (25 microg/100g, BW, ip, 15 days) rats. The fiber type composition of S muscle was also evaluated and used as control of the T3-induced effects. In the S muscle, the T3 treatment increased the Mb mRNA and protein expression, as well as the CS and SDH activity. These changes occurred parallel to the expected increase in type II (fast) and decrease in type I (slow)-fibers in S muscle. In the hypothyroid state, the Mb mRNA was decreased, while the Mb expression and CS activity tended to decrease. In contrast the SDH activity was increased, probably due to the enhanced motor activity that occurs as a short-term response to the hypothermia induced by hypothyroidism. In the EDL, the alterations were milder than those in S muscle in both thyroid states. These findings show that Mb gene expression is induced by T3. This is concomitant with the enhancement of Krebs Cycle enzyme activities and provides additional evidence that thyroid hormone increases the aerobic potential of skeletal muscles, as well as the speed of muscle contraction.

Increased sensitivity to thyroid hormone in mice with complete deficiency of thyroid hormone receptor

Only three of the four thyroid hormone receptor (TR) isoforms, alpha1, beta1, and beta2, bind thyroid hormone (TH) and are considered to be true TRs. TRalpha2 binds to TH response elements on DNA, but its role in vivo is still unknown. We produced mice completely deficient in TRalpha (TRalpha(o/o)) that maintain normal serum thyroid-stimulating hormone (TSH) concentration despite low serum thyroxine (T(4)), suggesting increased sensitivity to TH. We therefore examined the effects of TH (L-3,3',5-triiodothyronine, L-T3) given to TH-deprived and to intact TRalpha(o/o) mice. Controls were wild-type (WT) mice of the same strain and mice resistant to TH due to deficiency in TRbeta (TRbeta(-/-)). In liver, T3 produced significantly greater responses in TRalpha(o/o) and smaller responses in TRbeta(-/-) as compared with WT mice. In contrast, cardiac responses to L-T3 were absent or reduced in TRalpha(o/o), whereas they were similar in WT and TRbeta(-/-) mice, supporting the notion that TRalpha1 is the dominant TH-dependent TR isoform in heart. 5-Triiodothyronine (L-T3) given to intact mice produced a greater suppression of serum T(4) in TRalpha(o/o) than it did in WT mice and reduced by a greater amount the TSH response to TSH-releasing hormone. This is an in vivo demonstration that a TR deficiency can enhance sensitivity to TH. This effect is likely due to the abrogation of the constitutive "silencing" effect of TRalpha2 in tissues expressing the TRbeta isoforms.

Hexachlorobenzene, a dioxin-type compound, increases malic enzyme gene transcription through a mechanism involving the thyroid hormone response element

Hexachlorobenzene (HCB) is a dioxin-type chemical that acts mainly through the aryl hydrocarbon receptor. Chronic exposure of rats to HCB increases the activity of malic enzyme (ME). In this report, we show that this increase is correlated with an induction of ME messenger RNA (mRNA) levels, with the maximal HCB effect achieved after 9 days of intoxication. This effect is specific for ME, as other liver enzymes, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenol pyruvate carboxykinase, and mitochondrial alpha-glycerol-3-phosphate dehydrogenase, are not affected by HCB. The induction of ME mRNA levels is accompanied by an increase in ME promoter activity, as demonstrated by transient transfection experiments performed in rat hepatoma H35 cells. In an attempt to identify the cis-regulatory elements responsible for the HCB effect, different promoter deletions and mutations were used. The results obtained localize the responsive region between positions -315 and -177. This region does not contain either consensus xenobiotic response or activating protein-1 elements, the two main mediators of dioxin compounds described to date. In contrast, a thyroid hormone response element (TRE) is located between -281 to -261. Deletions and mutations of the TRE element do not respond to HCB, demonstrating that this element mediates the response of this dioxin-type compound. As ME gene expression is regulated mainly by thyroid hormones, we next investigated the role of T3 receptor (T3R) in the ME gene transcriptional induction mediated by HCB. Using Scatchard analysis, we show that neither T3R binding features for its ligand nor alpha1 or beta1T3R mRNA levels are changed with the toxic. In gel shift assays, however, we observed that protein/DNA complexes formed on TRE from the ME promoter were induced by HCB. Using an oligonucleotide with a mutation that eliminates the TRE function, we demonstrate a loss of the induced protein/DNA complexes. Together, these data suggest that the dioxin-type compound HCB increases ME gene transcription by modulating the levels of still unidentified nuclear proteins that bind to the TRE element of the ME promoter.

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

Thyroid hormone action on liver, heart, and energy expenditure in thyroid hormone receptor beta-deficient mice

Thyroid hormone (TH) responsive genes can be both positively and negatively regulated by TH through receptors (TR) alpha and beta expressed in most body tissues. However, their relative roles in the regulation of specific gene expression remain unknown. The TR beta knockout mouse, which lacks both TR beta1 and TR beta2 isoforms, provides a model to examine the role of these receptors in mediating TH action. TR beta deficient (TR beta-/-) mice that show no compensatory increase in TR alpha, and wild-type (TR beta+/+) mice of the same strain were deprived of TH by feeding them a low iodine diet containing propylthiouracil, and were then treated with supraphysiological doses of L-T3 (0.5, 5.5, and 25 microg/day/mouse). TH deprivation alone increased the serum cholesterol concentration by 25% in TR beta+/+ mice and reduced it paradoxically by 23% in TR beta-/- mice. TH deprivation reduced the serum alkaline phosphatase (AP) concentration by 31% in TR beta+/+ mice but showed no change in the TR beta-/- mice. Treatment with L-T3 (0.5 to 25 microg/mouse/day) caused a 57% decrease in serum cholesterol and a 231% increase in serum AP in the TR beta+/+ mice. The TR beta-/- mice were resistant to the L-T3 induced changes in serum cholesterol and showed increase in AP only with the highest L-T3 dose. Basal heart rate (HR) in TR beta-/- mice was higher than that of TR beta+/+ mice by 11%. HR and energy expenditure (EE) in both TR beta+/+ and TR beta-/- mice showed similar decreases (49 and 46%) and increases (49 and 41%) in response to TH deprivation and L-T3 treatment, respectively. The effect of TH on the accumulation of messenger RNA (mRNA) of TH regulated liver genes was also examined. TH deprivation down regulated spot 14 (S14) mRNA and showed no change in malic enzyme (ME) mRNA in both TR beta+/+ and TR beta-/- mice. In contrast treatment with L-T3 produced an increase in S14 and ME but no change in TR beta-/- mice. From these results, it can be concluded that regulation of HR and EE are independent of TR beta. With the exception of serum cholesterol concentration and liver ME mRNA accumulation, all other markers of TH action examined during TH deprivation exhibited the expected responses in the absence of TR beta. Thus, as previously shown for serum TSH, TR beta is not absolutely necessary for some changes typical of hypothyroidism to occur. In contrast, except for HR and EE, the full manifestation of TH-mediated action required the presence of TR beta.

Thyroid hormone and cardiovascular disease

Thyroid hormone directly affects the heart and peripheral vascular system. The hormone can increase myocardial inotropy and heart rate and dilate peripheral arteries to increase cardiac output. An excessive deficiency of thyroid hormone can cause cardiovascular disease and aggravate many preexisting conditions. In severe systemic illness and after major surgical procedures changes in thyroid function can occur, leading to the "euthyroid sick syndrome." Patients will have normal or decreased levels of T4, decreased free and total T3, and usually normal levels of thyroid stimulating hormone. This syndrome may be an adaptive response to systemic illness that usually will revert to normal without hormone supplementation as the illness subsides. Recently, however, many investigators have explored the benefits of thyroid hormone supplementation in those diseases associated with euthyroid sick syndrome. Thyroid hormone's effects on the cardiovascular system make it an attractive therapy for those patients with impaired hemodynamics and low T3. Thyroid hormone has also been considered a treatment for patients with congestive heart failure, for patients undergoing cardiopulmonary bypass and heart transplantation, and for patients with hyperlipidemia. At present there is no evidence suggesting a favorable treatment outcome using thyroid hormone supplementation for any systemic condition except in those patients with documented hypothyroidism.

Polarity and specific sequence requirements of peroxisome proliferator-activated receptor (PPAR)/retinoid X receptor heterodimer binding to DNA. A functional analysis of the malic enzyme gene PPAR response element

The malic enzyme (ME) gene is a target for both thyroid hormone receptors and peroxisome proliferator-activated receptors (PPAR). Within the ME promoter, two direct repeat (DR)-1-like elements, MEp and MEd, have been identified as putative PPAR response elements (PPRE). We demonstrate that only MEp and not MEd is able to bind PPAR/retinoid X receptor (RXR) heterodimers and mediate peroxisome proliferator signaling. Taking advantage of the close sequence resemblance of MEp and MEd, we have identified crucial determinants of a PPRE. Using reciprocal mutation analyses of these two elements, we show the preference for adenine as the spacing nucleotide between the two half-sites of the PPRE and demonstrate the importance of the two first bases flanking the core DR1 in 5'. This latter feature of the PPRE lead us to consider the polarity of the PPAR/RXR heterodimer bound to its cognate element. We demonstrate that, in contrast to the polarity of RXR/TR and RXR/RAR bound to DR4 and DR5 elements respectively, PPAR binds to the 5' extended half-site of the response element, while RXR occupies the 3' half-site. Consistent with this polarity is our finding that formation and binding of the PPAR/RXR heterodimer requires an intact hinge T region in RXR while its integrity is not required for binding of the RXR/TR heterodimer to a DR4.

Identification of a nuclear protein from rat developing brain as heterodimerization partner with thyroid hormone receptor-beta

Thyroid hormone receptors (TR) are ligand-activated transcription factors that modulate the expression of certain target genes in a developmental and tissue-specific manner. These specificities are determined by the tissue distribution of the TR isoforms alpha1 and beta1, the structure of the thyroid hormone response element (TRE) bound by the receptor, and heterodimerization partners. Among these, retinoid X receptors (RXR) have been recognized as the principal partners for TR. The present work reports the identification of a novel nuclear protein from 19-day-old embryonic rat brain that displays a distinct interaction pattern with TR isoforms at the level of the TRE of two genes known to be differentially expressed and regulated by thyroid hormone (T3): the ubiquitous malic enzyme and the brain-specific myelin basic protein. Electrophoretic gel mobility shift assays demonstrate that only TRbeta1 forms a specific complex with the rat brain nuclear factor on the myelin basic protein-TRE, but not on the malic enzyme-TRE. Thus, the interaction is selectively determined by both the receptor isoform and the structure of the TRE. The expression of this brain nuclear factor is restricted to the perinatal period, when myelination is sensitive to T3. Gel supershift assays with RXR-specific antibodies indicate that this factor is not one of the known RXR isoforms. However, it is most likely a new member of the RXR subfamily because it could be supershifted with an antibody raised against the highly conserved DNA-binding domain of RXRs.

Alterations of cardiac alpha 1-adrenoceptor subtypes in hypothyroid rats

1. Alterations in the cardiac alpha 1-adrenoceptor and its subtypes in hypothyroid rats were studied by radioligand binding assays and reverse transcription-polymerase chain reaction (RT-PCR). Hypothyroidism was created by feeding rats with 0.2% 2-thiouracil solution instead of daily drinking water for 20 days. 2. The density of cardiac alpha 1-adrenoceptors (Bmax) was increased from 67.5 +/- 4.3 fmol/mg in control rats to 81.1 +/- 7.2 fmol/mg (P < 0.05) in hypothyroid rats. 3. Compared with control rats, in hypothyroid rats the percentages of high-affinity sites for (+)-niguldipine and 5-methylurapidil were increased from 13.8 +/- 5.6 and 31.9 +/- 6.3%, respectively, to 24.9 +/- 7.3 and 45.5 +/- 2.4%, respectively (both P < 0.05), while those for BMY7378 were decreased from 37.2 +/- 8.9 to 23.8 +/- 8.4% (P < 0.05), respectively. The percentage of high-affinity sites for WB4101 was not significantly different in control and hypothyroid rats (43.3 +/- 9.1 and 39.4 +/- 3.6%, respectively). 4. Reverse transcription-PCR experiments revealed that the steady state levels of mRNA for alpha 1A- and alpha 1B-adrenoceptors were increased, while those for alpha 1D-adrenoceptor were decreased in the hearts of hypothyroid rats. 5. The concentration-contraction response curves for noradrenaline in the presence of a beta-adrenoceptor antagonist in control and hypothyroid rats showed that the maximal response was reduced from 344 +/- 58 to 200 +/- 23 mg, respectively (P < 0.05). 6. The data suggest that in hypothyroid rats the total number of cardiac alpha 1-adrenoceptors is increased. The change is subtype-selective, with levels of alpha 1A- and alpha 1B-adrenoceptors being increased and levels of alpha 1D-adrenoceptors being reduced. Furthermore, the positive inotropic response mediated by alpha 1-adrenoceptors is reduced in hypothyroid rats.

Characterization of a low affinity thyroid hormone receptor binding site within the rat GLUT4 gene promoter

Previous studies have demonstrated that thyroid hormone (T3) stimulates insulin-responsive glucose transporter (GLUT4) transcription and protein expression in rat skeletal muscle. The aim of the present study was to define a putative thyroid hormone response element (TRE) within the rat GLUT4 promoter and thus perhaps determine whether T3 acts directly to augment skeletal muscle GLUT4 transcription. To this end, electrophoretic mobility shift analyses were performed to analyze thyroid hormone receptor (TR) binding to a previously characterized 281-bp T3-responsive region of the rat GLUT4 promoter. Indeed, within this region, a TR-binding site of the standard DR + 4 TRE variety was located between bases -457/ -426 and was shown to posses a specific affinity for in vitro translated TRs. Interestingly, however, the GLUT4 TR-binding site demonstrated a significantly lower affinity compared to a consensus DR + 4 TRE, and only bound TRs appreciatively in the form of high affinity heterodimers, in this case with the cis-retinoic acid receptor. In conclusion, these data demonstrated the presence of a specific TR-binding site within a T3-responsive region of the rat GLUT4 promoter and thus support the supposition that thyroid hormone acts directly to stimulate GLUT4 transcription in rat skeletal muscle. Moreover, characterization of a novel TR-binding site with low affinity suggests an additional mechanism by which the intrinsic activity and responsiveness of thyroid hormone regulated genes may be modulated.

Hypothyroidism affects the expression of the beta-F1-ATPase gene and limits mitochondrial proliferation in rat liver at all stages of development

In order to analyze the role of thyroid hormones in mitochondrial biogenesis, we have studied the expression pattern of the beta subunit of the mitochondrial ATP-synthase complex in liver and in isolated mitochondria during postnatal development of hypothyroid rats. Chemically induced hypothyroidism promoted a significant reduction in body and liver masses at all stages of development. Furthermore, plasma 3,5,3'-triiodo-L-thyronine (T3) and 3,5,3',5'-tetraiodo-L-thyronine (T4) concentrations were significantly reduced in hypothyroid animals when compared to euthyroid animals. Remarkably, steady-state beta-F1-ATPase mRNA levels in livers of hypothyroid animals showed an approximately 50% reduction when compared to age-matched euthyroid rats at all stages of development. The relative amounts of beta-F1-ATPase protein determined in isolated mitochondria of 1-day-old and adult hypothyroid animals were similar to those determined in mitochondria of age-matched euthyroids, indicating that hypothyroidism does not affect organelle differentiation in the liver of suckling and adult rats. In contrast, the relative amount of beta-F1-ATPase protein in liver homogenates varied (0-30% reduction) due to the hypothyroid condition during development. These findings suggest the existence of compensatory mechanisms operating at the translational and/or post-translational levels which promote proliferation of mitochondria in the hypothyroid liver. However, when the liver mass was considered, hypothyroidism significantly reduced overall mitochondrial proliferation in rat liver. Interestingly, the effects of thyroid hormones on the biogenesis of the ATP synthase complex at latter stages of development provide an example in which the hypothyroid condition limits the expression of the nuclear-encoded gene with no apparent effect on the expression of the mitochondrial-encoded genes (ATP synthase subunits 6-8).

Tissue-specific regulation of lipogenic mRNAs by thyroid hormone

We have previously shown that triiodothyronine (T3) regulates rat fatty acid synthesis in a tissue specific manner. Here, we determined the effects of thyroid state on mRNAs encoding the lipogenic enzymes, acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS). S14 mRNA, a sequence tightly associated with lipogenesis, was also measured. Levels of the three mRNA were 9-13-fold higher in hyper- than hypothyroid liver. Limited expression in kidney and heart was also increased by thyroid hormone. In brown adipose tissue, highest levels were recorded in hypothyroid animals. Thyroid state did not affect expression in lung and brain. All these changes are consistent with those previously measured in fatty acid synthesis. In white adipose tissue, mRNA expression was increased by hyperthyroidism. This increase may not be reflected in fatty acid synthesis, since we recently showed lipogenesis to be reduced under these circumstances. All three mRNAs responded rapidly to T3 in liver, but more slowly in kidney and fat. Thus, T3 regulates lipogenesis by altering levels of ACC and FAS mRNAs. S14 mRNA changes in parallel.

Thyroid hormones promote transcriptional activation of the nuclear gene coding for mitochondrial beta-F1-ATPase in rat liver

Thyroid hormones acutely regulate gene expression of the beta-catalytic subunit of the mitochondrial F1-ATPase complex in the liver of hypothyroid rat neonates at either a transcriptional and/or post-transcriptional level [(1990) J. Biol. Chem. 265, 9090-9097]. Administration at birth of various thyroid hormone doses to hypothyroid newborn rats promote a rapid (1 h) increase in liver steady-state amounts of both beta-F1-ATPase protein and mRNA. Induction of the beta-F1-ATPase mRNA is coincident with an elevation in gene transcription detected using nascent RNA chains synthesized by isolated nuclei. These results suggest that thyroid hormones induction of postnatal mitochondrial differentiation in the liver of hypothyroid rat neonates is mostly triggered by transcriptional regulation of beta-F1-ATPase gene.

The thyroid and the heart

Cardiovascular manifestations are a frequent finding in hyperthyroid and hypothyroid states. In this review, potential mechanisms by which thyroid hormones may exert their cardiovascular effects and pathophysiological consequences of such effects are briefly discussed. Two major concepts have emerged about how thyroid hormones exert their cardiovascular effects. First, there is increasing evidence that thyroid hormones exert direct effects on the myocardium, which are mediated by stimulation of specific nuclear receptors, which in turn leads to specific mRNAs production. Furthermore, there is some evidence that thyroid hormones may also activate extranuclear sites and may directly alter plasma membrane function. Second, thyroid hormones interact with the sympathetic nervous system by altering responsiveness to sympathetic stimulation presumably by modulating adrenergic receptor function and/or density. Pathophysiological consequences of such direct and indirect thyroid hormone effects include increased myocardial contractility and relaxation that may be related to stimulation by T3 of specific myocardial enzymes. However, when left ventricular hypertrophy occurs in association with hyperthyroidism, it may be related to either direct thyroid hormone-induced stimulation of myocardial protein synthesis or to thyrotoxicosis-induced increases in cardiac work load. Although hyperthyroidism generally has little or no effect on mean arterial blood pressure, hypothyroidism is often associated with increases in diastolic blood pressure that are reversible after hormone substitution and may be mediated in part by sympathetic activation. Moreover, there is increasing evidence that thyroid hormones have direct chronotropic effect on the heart that are independent of the sympathetic nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

Ascites in broilers. 2. Disturbances in the hormonal regulation of metabolic rate and fat metabolism

Effects of different broiler stocks, ambient temperatures (Ta), dietary energy content (AME), and dietary levels of unsaturated fat on plasma thyroid and growth hormone concentrations and energy metabolism were studied. An experiment with a 2 x 2 x 2 x 2 factorial split-plot arrangement of treatments with 96 groups of 12 male broilers each was performed. Blood samples were taken at 3, 4, and 5 wk of age. Energy metabolism parameters were determined over an entire period from 1 to 5 wk of age. Chickens from a line selected for fast growth rate and low feed conversion ratio but also more sensitive to heart failure syndrome (HFS) and ascites (Line SS) than commercial birds (Line BC) exhibited the greatest responses to experimental factors. Differences in levels of plasma thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3), and growth hormone (GH) between stocks at different ages were highly dependent on Ta and dietary fat content. Differences in heat production per metabolic weight, percentage of retained fat energy in retained energy, and efficiency of AME intake for retained energy between stocks corresponded to differences in hormone levels. High-fat diets (polyunsaturated fatty acids) inhibited the extra thyroidal conversion of T4 to T3 in both stocks. Differences between stocks in T3 and rT3 levels in plasma indicated that BC birds (in contrast to SS birds) were better able to compensate for an inhibited T4 conversion to T3 by producing more T4. Overall results suggest that the occurrence of HFS and ascites in SS birds could be initiated independently by different factors. These factors might be a limited thyroid hormone production and a lower capacity for oxygen consumption. An inverse relationship between T3 and GH levels found in particular combinations of experimental factors, together with changes in fat deposition, support published concepts about the positive effects of T3 on lipogenesis and GH on lipolysis.

Triiodothyronine has diverse and multiple stimulating effects on expression of the major myelin protein genes

If the importance of triiodothyronine (T3) on brain development including myelinogenesis has long been recognized, its mechanism of action at the gene level is still not fully elucidated. We studied the effect of T3 on the expression of myelin protein genes in aggregating brain cell cultures. T3 increases the concentrations of mRNA transcribed from the following four myelin protein genes: myelin basic protein (Mbp), myelin-associated glycoprotein (Mag), proteolipid protein (Plp), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (Cnp). T3 is not only a triggering signal for oligodendrocyte differentiation, but it has continuous stimulatory effects on myelin gene expression. Transcription in isolated nuclei experiments shows that T3 increases Mag and Cnp transcription rates. After inhibiting transcription with actinomycin D, we measured the half-lives of specific mRNAs. Our results show that T3 increases the stability of mRNA for myelin basic protein, and probably proteolipid protein. In vitro translation followed by myelin basic protein-specific immunoprecipitation showed a direct stimulatory effect of T3 on myelin basic protein mRNA translation. Moreover, this stimulation was higher when the mRNA was already stabilized in culture, indicating that stabilization is achieved through mRNA structural modifications. These results demonstrate the diverse and multiple mechanisms of T3 stimulation of myelin protein genes.

Transcriptional stimulation by thyroid hormone of a cytosolic thyroid hormone binding protein which is homologous to a subunit of pyruvate kinase M1

We have recently shown that the monomer of rat pituitary pyruvate kinase subtype M1 (p58-M1) is a cytosolic binding protein for 3,3',5-triiodo-L-thyronine (T3). To understand the role p58-M1 plays in thyroid hormone action, we examined the regulation of p58-M1 by T3 in GH3 cells. Expression of p58-M1 was evaluated by metabolically labeling GH3 cells cultured in regular medium, thyroid hormone depleted medium (Td medium), or Td medium supplemented with T3 (Td + T3 medium) followed by immunoprecipitation. T3 stimulates the expression of p58-M1 by 2-fold. Analysis by pulse-chase experiments indicates that the increased expression is not due to the increase of stability of p58-M1. Northern analysis of mRNA prepared from cells cultured in regular, Td, or Td + T3 medium demonstrates that T3 increases the accumulation of cytoplasmic mRNA by 2-fold. Nuclei from cells cultured in the three conditions were prepared, and the rates of synthesis of nascent nuclear RNA were compared by an in vitro transcription assay. Addition of T3 stimulates the rate of transcription by 2-fold. The parallel and identical magnitude in the increase of transcription rate and the accumulation of mRNA indicates that T3 stimulates the synthesis of p58-M1 by increasing the transcriptional activity of its gene.

Thyroid hormone and androgen regulation of nerve growth factor gene expression in the mouse submandibular gland

The nerve growth factor (NGF) content of the mouse submandibular gland (SMG) is under hormonal control and is modulated by both thyroid hormones (TH) and androgens. The sexual dimorphism of the gland is well documented. In the adult male mouse, the SMG contains 10 times more NGF compared to the female. Conversely, castration of male mice reduces the SMG NGF levels to those found in control females. In order to determine the locus at which androgens and TH exert their effect on NGF gene expression in the SMG, steady-state NGF mRNA levels were determined. Daily treatment of adult female mice with TH for 1 week increased NGF mRNA levels 6-fold. Androgen treatment produced a 20-fold increase in SMG NGF mRNA, which was comparable to levels detected in the control adult male SMG. The effect of TH on NGF mRNA levels was time-dependent and coincided with the increase in NGF protein concentrations. At 48 h after a single TH injection, NGF mRNA levels (measured in SMG total RNA) increased 2-4-fold, while heteronuclear (hn) RNA levels were increased 1.5-2-fold. The NGF gene transcription rate was determined by run-on assay following TH treatment. A small but significant 2-fold induction by TH of NGF gene transcription was found at 24-48 h. Cytoplasmic RNA prepared from the same SMGs used in the run-on experiments was tested by S1 nuclease protection; NGF cytoplasmic RNA was increased 7-fold in the SMGs of females treated with TH 48 h previously. These results demonstrate that the effect of TH on NGF gene expression is due in part to an induction of NGF gene transcription. The discrepancies observed between transcription rate and mRNA levels suggest that the major effect of TH is at the post-transcriptional level, possibly mRNA stabilization. The time required to observe an induction of TH on NGF gene transcription is suggestive of an indirect effect, possibly through the induction by TH of another protein which in turn activates the NGF gene.

Cellular binding proteins of thyroid hormones

Cellular binding proteins of thyroid hormones are present in the cell nucleus, cytosol, cell membrane, and mitochondria. While nuclear binding is proven to mediate hormone action, the exact roles of the other binding sites remain to be established. Nuclear receptor associates with DNA, core histone, and nuclear matrix and preferentially distributes in transcriptionally active chromatin due to interaction with H1 histone. Of particular importance is the binding of nuclear receptor to specific DNA sequences of target genes, termed thyroid-responsive elements. The binding is stabilized by non-receptor nuclear protein. Upon binding thyroid hormone, nuclear receptor is activated through alterations in the steric configuration, leading to changes in the rate of transcription of the target genes. Multiple nuclear receptor forms exist with likely distinct functional roles. Cytosolic thyroid hormone binding proteins are also heterogeneous. One form is under the control of cell metabolism (NADP and NADPH) and it may have a role in transport of the hormone to mitochondria and nucleus. Membrane-linked thyroid hormone binding proteins may have dual functional roles: one is to mediate hormone action and the other is to support active uptake of hormones by cells. Mitochondrial function may be regulated by thyroid hormone through mitochondrial binding sites in cooperation with nuclear receptor-mediated pathway. Further studies are required to elucidate the exact functional roles of non nuclear thyroid hormone binding proteins.

Metamorphosis of the amphibian eye

For many metamorphosing amphibians, the visual system must remain functional as the animal changes from an aquatic to a terrestrial habitat. Thyroid hormone, the trigger for metamorphosis, brings about changes at all levels of the animal, and profoundly alters the visual system, from cellular changes within the eye to new central connections subserving the binocular vision that develops during metamorphosis in some species. I will survey the alterations in the visual system in the metamorphosis of several Amphibian groups, and consider the role of thyroid hormone in bringing about these transformations through action at the molecular level.

Triiodothyronine inhibits transcription from the human growth hormone promoter

Three DNA constructs, the natural human growth hormone gene (hGH-hGH) its 500 bp promoter linked to the chloramphenicol acetyl transferase reporter gene (hGH-CAT), and its structural part linked to the herpes virus thymidine kinase promoter (TK-hGH) were introduced into rat pituitary GC cells by DEAE-dextran transfection. Transient expression was followed as a function of triiodothyronine (T3) concentration. The hGH-CAT expression was specifically inhibited by T3 following a typical dose-response curve while hGH-GH gene expression was not significantly modified. The transient expression of TK-hGH increased as a function of T3 concentration. These results indicate that T3 exerts two opposite effects on hGH gene expression. First, it down-regulates expression by acting on the promoter; second, it up-regulates expression by acting on the structural part of the gene. These action could be due to regulation of transcription and mRNA stabilization, respectively.

Biochemical basis of thyroid hormone action in the heart

Thyroid hormone-induced changes in cardiac function have been recognized for over 150 years; however, the biochemical basis of triiodothyronine (T3) action in the heart has been intensely investigated only during the last two decades. T3-induced changes in cardiac function can result from direct or indirect T3 effects. Direct T3 effects result from T3 action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T3 effects, which occur independent of nuclear T3 receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T3 effects are mediated by the binding of T3 to specific nuclear receptor proteins, which results in increased transcription of T3-responsive cardiac genes. The T3 receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. The c-erb A protein is the cellular homologue of the viral erythroblastosis A (v-erb A) protein, which causes red cell leukemia in chickens. Currently, three T3-binding isoforms of the c-erb protein and two non-T3-binding nuclear proteins that exert positive and negative effects on T3-responsive cardiac genes have been identified. T3 increases the heart transcription of the myosin heavy chain (MHC) alpha gene and decreases the transcription of the MHC beta gene, leading to an increase of myosin V1 and a decrease in myosin V3 isoenzymes. Myosin V1, which is composed of two MHC alpha, has a higher myosin ATPase activity than myosin V3, which contains two MHC beta. The globular head of myosin V1, with its higher ATPase activity, leads to a more rapid movement of the globular head of myosin along the thin filament, resulting in an increased velocity of contraction. T3 also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum (SR). This T3 effect results from T3-induced increases in the level of the mRNA coding for the SR calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the SR. Overall, thyroid hormone leads to an increase in ATP consumption in the heart. In addition, less chemical energy of ATP is used for contractile purposes and more of it goes toward heat production, which causes a decreased efficiency of the contractile process in the hyperthyroid heart.

Hormonal regulation of malic enzyme expression in primary cultures of foetal brown adipocytes

Insulin increased malic enzyme activity and protein content in foetal brown adipocyte primary cultures through a two fold increase in the rates of enzyme synthesis. Conversely, noradrenaline prevented the induction caused by insulin on enzyme expression through an accelerate rate of malic enzyme degradation. Tri-iodothyronine had no effect on the expression of malic enzyme in cultured foetal brown adipocytes.

Unesterified long-chain fatty acids inhibit thyroid hormone binding to the nuclear receptor. Solubilized receptor and the receptor in cultured cells

Unesterified long-chain fatty acids strongly inhibited thyroid hormone (T3) binding to nuclear receptors extracted from rat liver, kidney, spleen, brain, testis and heart. Oleic acid was the most potent inhibitor, attaining 50% inhibition at 2.8 microM. Oleic acid similarly inhibited the partially purified receptor and enhanced dissociation of the preformed T3-receptor complex. The fatty acid acted in a soluble form and in a competitive manner for the T3-binding sites, thereby reducing the affinity of the receptor for T3. The affinity of the receptor for oleic acid (Ki) was 1.0 microM. In HTC rat hepatoma cells in culture, fatty acids added to the medium reached the nucleus and inhibited nuclear T3 binding; oleic acid being the most potent. T3 binding of the cells was reversibly restored in fresh medium free of added fatty acids. Oleic acid did not affect all the T3-binding sites in the HTC cells: one form (80%) was inhibited and the other was not and these two forms were commonly present in all rat tissues examined. Thus, fatty acids inhibited the solubilized nuclear receptor as well as a class of nuclear T3-binding sites in cells in culture.

Retinoic acid blockade of imidazole-induced tyrosinase expression in B16 melanoma cultures: similar effects of the active retinoid and triiodothyronine

The effect of retinoic acid on the induction of tyrosinase (EC 1:14.18.1) by imidazole was determined in cultured B16/C3 melanoma cells. Retinoic acid could block the induction of enzyme activity within 3 hours of addition to the culture medium at a physiological concentration (10nM). The blockade was similar to that of 3,3',5-L-triiodothyronine (T3) already reported. mRNA hybridizable to a tyrosinase DNA probe was induced by imidazole while retinoic acid and T3 blocked that increase. These observations suggest that retinoic acid can mimic the action of T3 in B16 melanoma cells in culture.