Effect of thyroid hormones on acid cholesterol ester hydrolase activity in rat liver,heart and epididymal fat pads

The regulation of acid cholesterol ester hydrolase activity by thyroid hormones was studied in subcellular fractions from rat liver, heart, and epididymal fat pads; hydrolase activity was determined at pH 5 with a glycerol-dispersed cholesterol oleate substrate preparation. Acid cholesterol ester hydrolase activity was decreased in liver preparations from thyroidectomized rats relative to activity in livers from euthyroid control rats. Administration of triidothyronine to either euthyroid or hypothyroid (thyroidectomized) rats resulted in an increase in acid cholesterol ester hydrolase activity in liver preparations. Similar effects of thyroidectomy and the administration of triiodothyronine on acid cholesterol ester hydrolase activity were observed with fat pad preparations. In contrast, no effect of thyroid hormones was observed on acid cholesterol ester hydrolase activity in heart. These results suggest that thyroid hormones may regulate the catabolism of serum lipoproteins, in part, by alterations in lysosomal acid cholesterol ester hydrolase activity in liver and epididymal fat pads.     

Effect of thyroid hormones on 5'-nucleotidase of isolated rat fat cells.

5'-Nucleotidase was measured in isolated fat cells from normal, hypothyroid and hyperthyroid rats. This was done to find out whether thyroid hormones had an effect on the production of adenosine by the fat cell. The results showed that 5'-nucleotidase is modified when the rats received injections of 3,3',5-triiodo-L-thyronine (T3). There was no change in the enzyme in hypothyroidism or when T3 was added to incubation of cells.     

Effects of thyroid state on the expression of hepatic thyroid hormone transporters in rats

Liver uptake of thyroxine (T4) is mediated by transporters and is rate limiting for hepatic 3,3',5-triiodothyronine (T3) production. We investigated whether hepatic mRNA for T4 transporters is regulated by thyroid state using Xenopus laevis oocytes as an expression system. Because X. laevis oocytes show high endogenous uptake of T4, T4 sulfamate (T4NS) was used as an alternative ligand for the hepatic T4 transporters. Oocytes were injected with 23 ng liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats, and after 3-4 days uptake was determined by incubation of injected and uninjected oocytes for 1 h at 25 degrees C or for 4 h at 18 degrees C with 10 nM [125I]T4NS. Expression of type I deiodinase (D1), which is regulated by thyroid state, was studied in the oocytes as an internal control. Uptake of T4NS showed similar approximately fourfold increases after injection of liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats. A similar lack of effect of thyroid state was observed using reverse T3 as ligand. In contrast, D1 activity induced by liver mRNA from hyperthyroid and hypothyroid rats in the oocytes was 2.4-fold higher and 2.7-fold lower, respectively, compared with euthyroid rats. Studies have shown that uptake of iodothyronines in rat liver is mediated in part by several organic anion transporters, such as the Na+/taurocholate-cotransporting polypeptide (rNTCP) and the Na-independent organic anion-transporting polypeptide (rOATP1). Therefore, the effects of thyroid state on rNTCP, rOATP1, and D1 mRNA levels in rat liver were also determined. Northern analysis showed no differences in rNTCP or rOATP1 mRNA levels between hyperthyroid and hypothyroid rats, whereas D1 mRNA levels varied widely as expected. These results suggest little effect of thyroid state on the levels of mRNA coding for T4 transporters in rat liver, including rNTCP and rOATP1. However, they do not exclude regulation of hepatic T4 transporters by thyroid hormone at the translational and posttranslational level.     

Acute effects of leptin on 5'-deiodinases are modulated by thyroid state of fed rats.

Leptin has been shown to modulate deiodinase type 1 (D1) and type 2 (D2) enzymes responsible for thyroxine (T4) to triiodothyronine (T3) conversion. Previously, it was demonstrated that a single injection of leptin in euthyroid fed rats rapidly increased liver, pituitary, and thyroid D1 activity, and simultaneously decreased brown adipose tissue (BAT) and hypothalamic D2 activity. We have now examined D1 and D2 activities, two hours after a single subcutaneous injection of leptin (8 microg/100 g BW) into hypo- and hyperthyroid rats. In hypothyroid rats, leptin did not modify pituitary, liver and thyroid D1, and thyroid D2 activity, while pituitary D2 was decreased by 41% (p<0.05) and hypothalamic D2 showed a 1.5-fold increase. In hyperthyroid rats, thyroid and pituitary D1, and pituitary and hypothalamic D2 were not affected by leptin injection, while liver D1 showed a 42% decrease (p<0.05). BAT D2 was decreased by leptin injection both in hypo- and hyperthyroid states (42 and 48% reduction, p<0.001). Serum TH and TSH showed the expected variations of hypo- and hyperthyroid state, and leptin had no effect. Serum insulin was lower in hypothyroid than in hyperthyroid rats and remained unchanged after leptin. Therefore, acute effects of leptin on D1 and D2 activity, expect for BAT D2, were abolished or modified by altered thyroid state, in a tissue-specific manner, showing an IN VIVO interplay of thyroid hormones and leptin in deiodinase regulation.     

Influence of hyper‐ and hypothyroidism on lipid peroxidation,unsaturation of phospholipids,glutathione system and oxidative damage to nuclear and mitochondrial DNA in mice skeletal muscle

While the biochemical literature on free radical metabolism is extensive, there is little information on the endocrine control of tissue oxidative stress, and in the case of thyroid hormones it is mainly limited to liver tissue and to short-term effects on a few selected biochemical parameters. In this investigation, chronic hypothyroidism and hyperthyroidism were successfully induced in mice, and various oxidative-stress-related parameters were studied in skeletal muscle. In vivo and in vitro lipid peroxidation significantly increased in hyperthyroidism and did not change in the hypothyroid state. The fatty acid composition of the major phospholipid classes was affected by thyroid hormones, leading to a significant decrease in total fatty acid unsaturation both in hypothyroid and hyperthyroid muscle in phosphatidylcholine and phosphatidylethanolamine fractions. In cardiolipin, however, the double bond content significantly increased as a function of thyroid status, leading to a 2.7 fold increase in the peroxidizability index from euthyroid to hyperthyroid muscle. Cardiolipin content was also directly and significantly related to thyroid state across the three groups. Glutathione system was not modified by thyroid state. The oxidative damage marker 8-oxo-7,8-dihydro-2'-deoxyguanosine did not change in mitochondrial DNA, and decreased in genomic DNA both in hypothyroid and hyperthyroid muscle. The results indicate that chronic alterations in thyroid status specially affect oxidative damage to lipids in skeletal muscle, with a probably stronger effect on mitochondrial membranes, whereas the cytosolic redox potential and DNA are better protected possibly due to homeostatic compensatory reactions on the long-term.     

Poly(ADP-ribose) polymerase is affected early by thyroid state during liver regeneration in rats

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme involved in DNA synthesis, DNA repair, and cell replication and transformation, also plays a role in the early steps of liver regeneration induced by partial hepatectomy (PH). PARP and DNA topoisomerase I (Topo I) activities and de novo DNA synthesis were studied during liver regeneration in rats with altered thyroid state. Hepatic PARP activity, evaluated as [(32)P]NAD incorporated into isolated liver nuclei, was inhibited in hyperthyroid rats and increased in hypothyroid animals. In both euthyroid and hyperthyroid rats PARP activity was rapidly stimulated, peaking 6 h after PH. In hypothyroid animals, an early decrease in activity was found, at a minimum of 6 h after PH, followed by an early onset of DNA synthesis. An inverse relationship between PARP and Topo I activities was a shared feature among euthyroid, hypothyroid, and hyperthyroid rats. Together these data show that, in replicating hepatocytes, thyroid hormones exert a regulatory role on PARP activity, which reflects the control of a number of nuclear proteins involved in DNA metabolism.     

Thyroid status affects rat liver regeneration after partial hepatectomy by regulating cell cycle and apoptosis.

In rats, various growth factors and hormones, as well as partial hepatectomy (PH) are able to trigger the proliferative response of hepatocytes. Although recent evidence highlights the important role of thyroid hormones and thyroid status in regulating the growth of liver cells in vitro and in vivo models, the mechanism involved in the pro-proliferative effects of thyroid hormones is still unclear. Here we have investigated how in rats made hypo- and hyperthyroid after prolonged treatment respectively with propylthiouracil (PTU) and triiodothyronine (T3), the thyroid status affects liver regeneration after PH by regulating cell cycle and apoptosis proteins. Our results show that both in control and partially hepatectomized animals hyperthyroidism increases the cyclin D1, E and A levels and the activity of cyclin-cdk complexes, and decreases the levels of cdk inhibitors such as p16 and p27. On the contrary hypothyroidism induces a down-regulation of the activity of cyclin cdk complexes decreasing cyclin levels. Thyroid hormones control also p53 and p73, two proteins involved in apoptosis and growth arrest which are induced by PH. In particular, hypothyroidism increases and T3 treatment decreases p73 levels. The analysis of the phosphorylated forms of p42/44 and p38 MAPK revealed that they are induced during hepatic regeneration in euthyroid and hyperthyroid rats whereas they are negatively regulated in hypothyroid rats. In conclusion our data demonstrate that thyroid status can affects liver regeneration, altering the expression and the activity of the proteins involved in the control of cell cycle and growth arrest.     

Mitochondrial respiration in muscle and liver from cold-acclimated hypothyroid rats.

The effects of long-term cold exposure on muscle and liver mitochondrial oxygen consumption in hypothyroid and normal rats were examined. Thyroid ablation was performed after 8-wk acclimation to 4 degrees C. Hypothyroid and normal controls remained in the cold for an additional 8 wk. At the end of 16-wk cold exposure, all hypothyroid rats were alive and normothermic and had normal body weight. At ambient temperature (24 degrees C), thyroid ablation induced a 65% fall in muscle mitochondrial oxygen consumption, which was reversed by thyroxine but not by norepinephrine administration. After cold acclimation was reached, suppression of thyroid function reduced muscle mitochondrial respiration by 30%, but the hypothyroid values remained about threefold higher than those in hypothyroid muscle in the warm. Blockade of beta- and alpha1-adrenergic receptors in both hypothyroid and normal rats produced hypothermia in vivo and a fall in muscle, liver, and brown adipose tissue mitochondria respiration in vitro. In normal rats, cold acclimation enhanced muscle respiration by 35%, in liver 18%, and in brown adipose tissue 450% over values in the warm. The results demonstrate that thyroid hormones, in the presence of norepinephrine, are major determinants of thermogenic activity in muscle and liver of cold-acclimated rats. After thyroid ablation, cold-induced nonshivering thermogenesis replaced 3,5,3'-triiodothyronine-induced thermogenesis, and normal body temperature was maintained.     

Effect of thyroid state on cyclic AMP-mediated induction of hepatic phosphoenolpyruvate carboxykinase.

Hepatic phosphoenolpyruvate carboxykinase (PEPCK) is significantly increased in the hyperthyroid starved rat, and moderately decreased in the hypothyroid starved rat. As tri-iodothyronine by itself has only a small and sustained effect on the induction of this enzyme, as was previously shown in the isolated perfused organ, the effect of hypo- and hyper-thyroidism on the increase in cytosolic PEPCK provoked by dibutyryl cyclic AMP (Bt2cAMP) was investigated in vivo and in the isolated perfused liver. Compared with euthyroid fed controls, in hypothyroid fed rats Bt2cAMP provoked in 2 h only a small increase in translatable mRNA coding for PEPCK. In contrast, in hyperthyroid animals PEPCK mRNA as measured by translation in vitro was already increased in the fed state, and further enhanced by Bt2cAMP injection to values as in euthyroid controls. Under all thyroid states a close correlation between PEPCK mRNA activity and PEPCK synthesis was observed. In the isolated perfused liver from the hyperthyroid fed rat, the increase in PEPCK provoked by Bt2cAMP or Bt2cAMP + isobutylmethylxanthine was considerably enhanced compared with those obtained in livers of hypothyroid rats. Also, adrenaline provoked a stimulated induction of PEPCK in hyperthyroid rats compared with hypothyroid rats. To summarize, our data indicate that the primary action of thyroid hormones on the synthesis of hepatic cytosolic PEPCK is to accelerate the cyclic AMP- or adrenaline-induction of the enzyme, acting primarily at a pretranslational level.     

Thyroid-thymus interactions during development and aging

A good body of experimental and clinical evidences suggests that bidirectional interactions do exist between the neuroendocrine system and the thymus activity. In particular, thymic endocrine activity seems to be strongly influenced by neuroendocrine signals. In this context, studies performed in hyper- and hypothyroid subjects and in the low triiodothyronine (T3) syndrome, which affects premature infants, have clearly shown that thyroid hormones and in particular T3 physiologically modulate thymic peptide secretion. In vitro experiments, with thymic whole-organ cultures, have demonstrated that thyroid hormones exert their action on the epithelial cells of the thymus deputed to synthesize and secrete thymic peptides and that such an effect does not seem to depend on the known permissive action of thyroid hormones.     

Influence of thyroid condition on 5'-nucleotidase activity of the plasma membrane in the rat liver

The effect of thyroid hormones on the Kinetic properties of plasmatic membrane 5'-nucleotidase in rat liver is studied. The Vmax and Km which increased significatively a week after thyroidectomy were reported to the normal values by the chronic administration of T3. The decrease in Vmax and Km observed five weeks after thyroid ablation may be attributed to secondary effects occurring with time.     

Effects of oestradiol, testosterone and medroxyprogesterone on subcellular fraction marker enzyme activities from rat liver and brain

The following enzymes have been studied (subcellular fractions are shown between parentheses): NAG and beta-glucuronidase (lysosomes); SDH (mitochondrial); glucose-6-phosphatase (endoplasmic reticulum); 5'-nucleotidase and (Na+, K+)Mg2+ ATPase (plasma membranes). Alterations on their activities were observed after subcutaneous injection of sex hormones, compared with controls. NAG activity from liver was always significantly decreased in lysosomal and microsomal fractions after the hormonal treatment. In the same conditions, NAG from brain was always increased. beta-Glucuronidase behaves like NAG in brain; in liver it was not modified by testosterone and it was slightly increased in lysosomal fraction after oestradiol treatment. SDH activity was not modified in mitochondrial fractions from liver, but this activity was always significantly increased in brain. Glucose-6-phosphatase activity was always significantly decreased in microsomal fractions from liver. It was increased in brain after oestradiol and testosterone injection, but medroxyprogesterone treatment caused a decreased activity. 5'-Nucleotidase and (Na+, K+)Mg2+ ATPase from brain were significantly increased in microsomal fractions by oestradiol and testosterone. Medroxyprogesterone, however, caused an increase in ATPase, but did not affect 5'-nucleotidase. Both activities in liver were decreased by oestradiol and increased by testosterone, but medroxyprogesterone caused (Na+, K+)Mg2+ ATPase to rise and 5'-nucleotidase to fall.     

Effect of thyroid state on susceptibility to oxidants and swelling of mitochondria from rat tissues

The effects of the thyroid state on oxidative damage, antioxidant capacity, susceptibility to in vitro oxidative stress and Ca(2+)-induced permeabilization of mitochondria from rat tissues (liver, heart, and gastrocnemious muscle) were examined. Hypothyroidism was induced by administering methimazole in drinking water for 15 d. Hyperthyroidism was elicited by a 10 d treatment of hypothyroid rats with triiodothyronine (10 micro g/100 g body weight). Mitochondrial levels of hydroperoxides and protein-bound carbonyls significantly decreased in hypothyroid tissues and were reported above euthroid values in hypothyroid rats after T(3) treatment. Mitochondrial vitamin E levels were not affected by changes of animal thyroid state. Mitochondrial Coenzyme Q9 levels decreased in liver and heart from hypothyroid rats and increased in all hyperthyroid tissues, while Coenzyme Q10 levels decreased in hypothyroid liver and increased in all hyperthyroid tissues. The antioxidant capacity of mitochondria was not significantly different in hypothyroid and euthyroid tissues, whereas it decreased in the hyperthyroid ones. Susceptibility to in vitro oxidative challenge decreased in mitochondria from hypothyroid tissues and increased in mitochondria from hyperthyroid tissues, while susceptibility to Ca(2+)-induced swelling decreased only in hypothyroid liver mitochondria and increased in mitochondria from all hyperthyroid tissues. The tissue-dependence of the mitochondrial susceptibility to stressful conditions in altered thyroid states can be explained by different thyroid hormone-induced changes in mitochondrial ROS production and relative amounts of mitochondrial hemoproteins and antioxidants. We suggest that susceptibilities to oxidants and Ca(2+)-induced swelling may have important implications for the thyroid hormone regulation of the turnover of proteins and whole mitochondria, respectively.     

Effects of hypothyroidism induced by 6-n-propylthiouracil and its reversal by T3 on rat heart superoxide dismutase, catalase and lipid peroxidation

The present study critically evaluates the effects of hypothyroid and hyperthyroid states on lipid peroxidation and two enzymes of active oxygen metabolism, namely superoxide dismutase (SOD) and catalase (CAT) in the rat heart mitochondrial and post-mitochondrial fractions. Lipid peroxidation, an index of oxidative stress, was elevated in the heart tissue in hypothyroid state but reduced upon T3 supplementation. Hyperthyroidism registered increased SOD activity in post-mitochondrial fraction. Mitochondrial SOD activity was reduced in hypothyroid state, which was further reduced by T3 administration. In contrast, different thyroid states had no effect on catalase activity in the mitochondrial fraction. The hypothyroid state however, significantly augmented catalase activity in post-mitochondrial fraction. The results suggest that the antioxidant defence status of cardiac tissue is well modulated by thyroid hormone.     

Oxidative stress by inorganic arsenic: modulation by thyroid hormones in rat

Arsenic toxicity is attributed mainly to lipid peroxidation and oxidative stress. We therefore studied the modulatory effects of thyroid hormones on arsenic toxicity in rat on lipid peroxidation and oxidative stress. Thyroid hormones, through a mechanism unknown at present, inhibit arsenic accumulation in liver and kidney. Mobilization of arsenic apparently diminishes lipid peroxidation and improves reduced glutathione status, two biochemical demands of combating arsenic toxicity. Results are discussed in reference to the effect of thyroid hormones on microsomal metabolism of arsenic. Arsenic is less toxic in hyperthyroid than in hypothyroid rats. A physiological antagonism between arsenic and thyroxine is discussed.     

Effect of hypo- and hyperthyroidism on the function and metabolism of macrophages in rats

The effect of thyroid hormones on monocyte migration, phagocytic capacity and hydrogen peroxide production by macrophages and the effect of these hormones on glutamine and glucose metabolism was investigated. The experiments were performed on resident, thioglycollate- and BCG-stimulated cells from hypo- and hyperthyroid rats. High plasma levels of thyroid hormones suppressed the migration of monocytes and hydrogen peroxide production, whereas hypothyuroidism did not affect cell migration but rasied the phagocytic capacity and the hydrogen peroxide production. Hyperthyroidism increased the activities of glutaminase and hexokinase and the rates of decarboxylation of [U-¹⁴C]-glutamine and [U-¹⁴C]-glucose in inflammatory and activated cells. Hypothyroidism stimulated glucose metabolism and had only a slight effect on glutaminolysis. The activity of the TCA cycle was, however, diminished in the presence of high plasma levels of thyroid hormones and enhanced by the hypothyroid state. These findings suggest that the functional changes observed are more likely to be related to the activity of the TCA cycle rather than to glutaminolysis and glycolysis.     

Effects of glucagon and 3,5,3'-triiodo-L-thyronine on oxidative phosphorylation of thyroidectomized rat liver mitochondria

In normal or thyroidectomized rat liver mitochondria, glucagon produced fast but transient stimulation of respiration rates in state 3 and state 4 whatever the substrates. Stimulation reached its maximum 20 to 30 minutes after glucagon injection. However, the effects of glucagon are less marked after removal of the thyroid gland, since the increases observed in the oxygen consumption and basal metabolic rates were only half those shown in normal rats. The activating effects of triiodothyronine and glucagon on the ADP phosphorylation rates were found to be additive. Pretreatment with cycloheximide blocked the activation induced by glucagon but not that induced by triiodothyronine. Both hormones therefore stimulate oxidative phosphorylation but by different mechanisms. Thyroidectomy did not alter the early rise in glycaemia observed in response to glucagon. It may therefore be assumed that the hypothyroid rat's sensitivity to glucagon is not directly connected with the change in cAMP metabolism.