Cyclic nucleotide phosphodiesterases and thyroid hormones

Urinary polycyclic aromatic hydrocarbons and childhood obesity: NHANES (2001-2006)

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are known carcinogens and suspected endocrine disruptors. Prenatal exposure to PAHs has been associated with obesity in early childhood.

OBJECTIVE

We examined the association of urinary PAH metabolites with adiposity outcomes [body mass index (BMI) z-score, waist circumference (WC), and rate of obesity] in children and adolescents.

METHODS

We performed whole-sample analyses of 3,189 individuals 6-19 years of age who participated in the 2001-2006 National Health and Nutrition Examination Survey. We performed multivariate linear and logistic regression to analyze the association of BMI z-score, WC, and obesity with concentrations of single urinary PAH compounds and the sum of PAHs. Furthermore, the analyses were stratified by developmental stage [i.e., children (6-11 years) and adolescents (12-19 years)].

RESULTS

BMI z-score, WC, and obesity were positively associated with the molecular mass sum of the PAHs and the total sum of naphthalene metabolites. Most associations increased monotonically with increasing quartiles of exposure among children 6-11 years of age, whereas dose-response trends were less consistent for adolescents (12-19 years of age). Neither total PAHs nor total naphthalene metabolites were associated with overweight in either age group, and there was little evidence of associations between the outcomes and individual PAHs.

CONCLUSIONS

Total urinary PAH metabolites and naphthalene metabolites were associated with higher BMI, WC, and obesity in children 6-11 years of age, with positive but less consistent associations among adolescents.

CITATION

Scinicariello F, Buser MC. 2014. Urinary polycyclic aromatic hydrocarbons and childhood obesity: NHANES (2001-2006). Environ Health Perspect 122:299-303; http://dx.doi.org/10.1289/ehp.1307234.

Endocrine disrupting chemicals and the developmental programming of adipogenesis and obesity

Obesity and related disorders are a burgeoning public health epidemic, particularly in the U.S. Currently 34% of the U.S. population is clinically obese (BMI > 30) and 68% are overweight (BMI > 25), more than double the worldwide average and 10-fold higher than Japan and South Korea. Obesity occurs when energy intake exceeds energy expenditure; however, individuals vary widely in their propensity to gain weight and accrue fat mass, even at identical levels of excess caloric input. Clinical, epidemiological, and biological studies show that obesity is largely programmed during early life, including the intrauterine period. The environmental obesogen hypothesis holds that prenatal or early life exposure to certain endocrine disrupting chemicals can predispose exposed individuals to increased fat mass and obesity. Obesogen exposure can alter the epigenome of multipotent stromal stem cells, biasing them toward the adipocyte lineage at the expense of bone. Hence, humans exposed to obesogens during early life might have an altered stem cell compartment, which is preprogrammed toward an adipogenic fate. This results in a higher steady state number of adipocytes and potentially a lifelong struggle to maintain a healthy weight, which can be exacerbated by societal influences that promote poor diet and inadequate exercise. This review focuses on the developmental origins of the adipocyte, the relationship between adipocyte number and obesity, and how obesogenic chemicals may interfere with the highly efficient homeostatic mechanisms regulating adipocyte number and energy balance.

Signalling mechanisms regulating lipolysis

Adipose tissue plays an important role providing energy to other tissues and functioning as an energy reserve organ. The energy supply is produced by triglycerides stored in a large vacuole representing approximately 95% of adipocyte volume. In the fasting period, triglyceride hydrolysis produces glycerol and free fatty acids which are important oxidative fuels for other tissues such as liver, skeletal muscle, kidney and myocardium. Hormone-sensitive lipase (HSL) is the enzyme that hydrolyzes intracellular triacylglycerol and diacylglycerol, and is one of the key molecules controlling lipolysis. Hormones and physiological factors such as dieting, physical exercise and ageing regulate intensively the release of glycerol and free fatty acids from adipocytes. One of the best known mechanisms that activate lipolysis in the adipocyte is the cAMP dependent pathway. cAMP production is modulated by hormone receptors coupled to Gs/Gi family of GTP binding proteins, such as beta-adrenergic receptors, whereas cAMP degradation is controlled by modulation of phosphodiesterase activity, increased by insulin receptor signalling. cAMP activates PKA which activates HSL by promoting its phosphorylation. Hormonal control of lipolysis can also be achieved by receptors coupled G proteins of the Gq family, through molecular mechanisms that involve PKC and MAPK, which are currently under investigation. cGMP and PKG have also been found to activate lipolysis in adipocytes. In this review we have compiled data from literature reporting both the classical and the alternative mechanisms of lipolysis.

Regulation of beta 1- and beta 3-adrenergic agonist-stimulated lipolytic response in hyperthyroid and hypothyroid rat white adipocytes

1. This study examined the effects of thyroid status on the lipolytic responses of rat white adipocytes to beta-adrenoceptor (beta-AR) stimulation. The beta 1- and beta 3-AR mRNAs and proteins were measured by Northern and saturation analyses, respectively. Glycerol production and adenyl cyclase (AC) activity induced by various non-selective and selective beta 1/beta 3-AR agonists and drugs which act distal to the receptor in the signalling cascade were measured in cells from untreated, triiodothyronine (T3)-treated and thyroidectomized rats. 2. The beta 3-AR density was enhanced (72%) by T3-treatment and reduced (50%) by introduction of a hypothyroid state while beta 1-AR number remained unaffected. The beta 1- and beta 3-AR density was correlated with the specific mRNA level in all thyroid status. 3. The lipolytic responses to isoprenaline, noradrenaline (beta 1/beta 3/beta 3-AR agonists) and BRL 37344 (beta 3-AR agonist) were potentiated by 48, 58 and 48%, respectively in hyperthyroidism and reduced by about 80% in hypothyroidism. 4. T3-treatment increased the maximal lipolytic response to the partial beta 3-AR (CGP 12177) and beta 1-AR (xamoterol) agonists by 234 and 260%, respectively, increasing their efficacy (intrinsic activity: 0.95 versus 0.43 and 1.02 versus 0.42). The maximal AC response to these agonists was increased by 84 and 58%, respectively, without changing their efficacy. 5. In the hypothyroid state, the maximal lipolytic and AC responses were decreased with CGP (0.17 +/- 0.03 versus 0.41 +/- 0.08 mumol glycerol/10(6) adipocytes; 0.048 +/- 0.005 versus 0.114 +/- 0.006 pmol cyclic AMP min-1 mg-1) but not changed with xamoterol. 6. The changes in lipolytic responses to postreceptor-acting agents (forskolin, enprofylline and dibutenyl cyclic AMP, (Bu)2cAMP) suggest the modifications on receptor coupling and phosphodiesterase levels in both thyroid states. 7. Thyroid status affects lipolysis by modifying beta 3-AR density and postreceptor events without changes in the beta 1-AR functionality.

Endothelial membrane potential regulates production of both nitric oxide and superoxide--a fundamental determinant of vascular health

There is recent evidence that the membrane potential of vascular endothelium regulates not only nitric oxide (NO) synthesis, but also superoxide generation, such that hyperpolarization stimulates NO production while suppressing that of superoxide. Given that NO works in a variety of ways to inhibit atherothrombotic disease and hypertension, whereas superoxide not only vetoes the benefits of NO but also disrupts endothelial metabolism and promotes LDL oxidation through its oxidant activity, it is thus evident that endothelium membrane potential is a crucial determinant of cardiovascular risk. Membrane polarization can be enhanced by measures which increase the synthesis or availability of the Na+-K+-ATPase, moderately enhance serum K+ and increase the conductance of membrane K+ channels. Such measures may include high-K+/low-Na+ natural diets, insulin sensitizing modalities, 'euthyroid replacement therapy' and ACE inhibitors. Epidemiological correlations of insulin resistance with hypertension and cardiovascular risk may reflect the low membrane potential of insulin-resistant vascular endothelium. Adjunctive measures for suppressing the generation or half-life of endothelial superoxide are suggested.

Local and systemic alterations in cyclic 3',5' AMP phosphodiesterase activity in relation to tail regeneration under hypothyroidism and T4 replacement in the lizard, Mabuya carinata

To establish the relationship between thyroid hormone and cyclic Adenosine monophosphate (cAMP) during lacertilian tail regeneration, cAMP phosphodiesterase, the hydrolytic enzyme of cAMP, was assayed in the tail regenerate, liver, and skeletal muscle of control (group A), chemically thyroidectomized (group B), and thyroidectomized and T4-replaced (group C) animals during various periods of tail regeneration. Enzyme activity was elevated in all three tissues of group B animals. Animals of group C showed an intermediate level of enzyme activity between controls (group A) and experimental animals (group B). These observations suggest a possible regulatory role of thyroxine in maintaining optimum levels of phosphodiesterase. The retardation in regeneration observable in the hypothyroid group of animals may be correlated with low levels of tissue cAMP. However, the operation of other influencing factors on phosphodiesterase during regeneration can be surmised from the observed tendency to exhibit similar patterns of phase-specific modulations in enzyme activity. Our observations are discussed in terms of phase-specific involvement of cAMP in regeneration, as well as its role in other metabolic aspects and the possible mode of indirect control exerted by thyroxine on lacertilian tail regeneration.

Triiodothyronine regulates beta 3-adrenoceptor expression in 3T3-F442A differentiating adipocytes

The effect of thyroid hormones on the beta 3-adrenoceptor expression was studied in the preadipose 3T3-F442A cell line. As assessed by molecular and pharmacological analyses, triiodothyronine addition to differentiating 3T3-F442A cells caused a 2.3-fold increase in beta 3-adrenoceptor mRNA levels, which was correlated with a parallel induction of beta 3-adrenoceptor number and of beta 3-adrenoceptor coupling to the adenylate cyclase system. Nuclear transcription experiments showed that triiodothyronine did not significantly alter the transcription rate of the beta 3-adrenoceptor gene. By contrast, the hormone increased by 36% the half-life of beta 3-adrenoceptor mRNA. Triiodothyronine exhibited a discrete effect on beta 3-adrenoceptor expression when added to mature 3T3-F442A adipocytes. This study indicates that thyroid hormones exert a differentiation-dependent and post-transcriptional regulation of beta 3-adrenoceptor expression in adipocytes.

Central insulin may up-regulate thyroid activity by suppressing neuropeptide Y release in the paraventricular nucleus

Down-regulation of thyroid activity during underfeeding or diabetes - and upregulation during overfeeding - have not been adequately explained. Experimental findings suggest that hypothalamic secretion of thyrotropin releasing hormone (TRH) is modulated by feeding status; neuropeptide Y may be a key mediator of this modulation. I propose that insulin, acting centrally as a signal of carbohydrate availability, promotes TRH secretion by inhibiting release of neuropeptide Y in the paraventricular nucleus. This mechanism may contribute to the weight loss reported during administration of certain insulin-sensitizing agents, and observed during low-fat diets.

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.

Endothelin mechanisms in altered thyroid states in the rat

Endothelin (ET) and its receptor characteristics were studied in hyper- and hypo-thyroid states in the rats. Hyperthyroidism was induced by daily administration of thyroxine (0.1 mg/kg i.p.) for 8 weeks, while hypothyrodism was induced by daily administration of methimazole (10 mg/kg i.p.) for 8 weeks. The chronic administration of thyroxine to rats decreased their rate of gain of body weight, increased serum T3 and T4 concentration, blood pressure and heart rate. The chronic administration of methimazole decreased the rate of gain of body weight, serum T3 and T4 concentration, blood pressure and heart rate as compared to vehicle-treated control. Plasma ET-1 levels were found to be similar in control and methimazole-treated rats, while the levels were found to be significantly (P < 0.002) increased in thyroxine-treated rats as compared to control rats. Binding studies showed that [125I]ET-1 bound to a single, high affinity binding site in the cerebral cortex, hypothalamus and pituitary. The density (Bmax) and the affinity (Kd) of [125I]ET-1 binding in the cerebral cortex and hypothalamus were found to be similar in control, methimazole- and thyroxine-treated rats. The pituitary of thyroxine-treated rats showed a decrease in the binding (34.3% decrease in the density) of [125I]ET-1 as compared to control rats. No difference was observed in the binding of [125I]ET-1 to pituitary membranes from control and methimazole-treated rats. Competition studies showed that the IC50 and Ki values of ET-3 for [125]ET-1 binding were about 8 to 11 times higher than ET-1 in cerebral cortex, hypothalamus and pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thyroid hormone status on the expression of the mRNAs of components of the lipolytic regulatory cascade in brown adipose tissue

1. The levels of mRNAs for RII beta and G beta were about 50% lower in brown adipose tissue (BAT) from hyperthyroid than from hypothyroid rats. 2. Treatment of hypothyroid rats with T3 resulted in a 50% decrease in mRNAs for RII beta and G beta in BAT occurring by 12 hr after treatment. 3. The levels of mRNAs for hormone-sensitive lipase, G alpha s and C alpha in BAT were unchanged by thyroid hormone status. 4. The results suggest that thyroid hormone may be involved in negative regulation of the expression of RII beta and G beta at the transcriptional level in BAT.

The effect of triiodothyronine on glucose utilization in adipocytes from obese rats

1. In the presence of insulin, 10(-5) M 3,3',5-triiodothyronine (T3) treatment for 1/2 hr decreased fatty acid synthesis 35% only in adipocytes from lean rats, whereas at 10(-11) M through 10(-7) M T3 the obese adipocytes had nearly a 20% increase in fatty acid synthesis. 2. A 2 hr pretreatment of adipocytes with 10(-9) and 10(-7) M T3 decreased insulin-stimulated fatty acid synthesis by nearly 20% in both lean and obese adipocytes. 3. In the absence of insulin, the 2 hr pretreatment with 10(-9) M T3 resulted in a 45% increase in lean adipocyte fatty acid synthesis, though the obese adipocytes required at least 10(-7) M T3 for 2 hr to increase the non-insulin-stimulated fatty acid synthesis by 50%. 4. At 10(-9) M T3 concentrations non-insulin-stimulated fatty acid synthesis was increased by 200% in lean adipose tissue explants, but obese adipose explants were not significantly affected under these conditions. 5. The addition of 10(-9) M T3 plus insulin to the explant media decreased fatty acid synthesis by 35% in both the lean and obese tissues. 6. The results also imply that the low T3 status of the obese rat may be contributory to the elevated fatty acid synthesis observed in obese adipocytes.

Insulin control of cyclic AMP phosphodiesterase

Cyclic AMP phosphodiesterase (PDE) is an enzyme involved in cellular homeostasis of cyclic AMP. It exists as multiple isozymes in cells, but only the high affinity, membrane-bound isozyme is sensitive to hormonal modulation. Several isozymes or isoforms of the low Km PDE have been detected. Data suggest that several mechanisms exist for hormonal modulation of PDE. Activity of the low Km PDE species may be modulated by phosphorylation/dephosphorylation, phospholipid substrate concentration, insulin second messenger, cyclic GMP, guanine nucleotide binding proteins, calmodulin, or aggregation/disaggregation of monomeric forms. Modulation of PDE isoforms by different hormones may be through different regulatory components or mechanisms.

Influence of thyroid status on responses of rat isolated pulmonary artery, vas deferens and trachea to smooth muscle relaxant drugs

1 Responses to relaxant drugs have been examined on isolated KCl-contracted smooth muscle preparations from rats in which thyroid status was changed by prior treatment with either thyroxine (T4) for 1 week (preparations of pulmonary artery, trachea and vas deferens) or methimazole for 10-12 weeks (pulmonary artery preparations). 2 On pulmonary artery preparations, T4 treatment caused a significant increase in the magnitude of the relaxant responses to noradrenaline and isoprenaline but not those to adrenaline. The potency of noradrenaline was increased 5.6 fold but that of isoprenaline and adrenaline was unchanged. This resulted in a change in the relative potencies from adrenaline greater than noradrenaline (controls) to noradrenaline = adrenaline (T4-treated). Methimazole treatment caused a significant reduction in the magnitude of the responses to noradrenaline and in its potency (2.8 fold). Isoprenaline and procaterol were unaffected. 3 On pulmonary artery preparations, T4 treatment did not affect the magnitude of the responses to forskolin, sodium nitrite or isobutylmethylxanthine (IBMX) or their potency. In vitro treatment with the monoamine oxidase (MAO) inhibitors, iproniazid or pargyline, did not potentiate responses to either noradrenaline or isoprenaline. Therefore, it was concluded that the T4-induced changes in the magnitude of the responses to noradrenaline and isoprenaline and in the potency of noradrenaline were unlikely to be due to reduced activity of cyclic nucleotide phosphodiesterase(s) or MAO. 4 On preparations of vas deferens and trachea, T4 treatment had no effect on the magnitude of the responses to noradrenaline, isoprenaline, adrenaline or procaterol. 5 We concluded that, on pulmonary artery T4 treatment of rats increased, while methimazole treatment reduced, the magnitude of the responses to, and/or the potency of, the beta-adrenoceptor agonists, noradrenaline and isoprenaline, by a mechanism which is specifically associated with the beta-adrenoceptors, and which is probably selective for the beta-subtype. T4 treatment caused no change in responses of vas deferens to beta-adrenoceptor agonists. On trachea the only change was a small increase in the potency of noradrenaline. The differences in the effects of T4 treatment on beta-adrenoceptormediated responses of rat pulmonary artery, vas deferens and trachea may be due to the differences in the beta-adrenoceptor populations of these three tissue types and/or differences in the effects of thyroid hormones on vascular compared with non-vascular smooth muscle.

Effect of N6-(L-2-phenylisopropyl)adenosine and insulin on cAMP metabolism in 3T3-L1 adipocytes

The antilipolytic effect of N6-(L-2-phenylisopropyl)-adenosine (PIA), an adenosine analogue thought to act via cell surface receptors, was investigated in 3T3-L1 adipocytes. PIA (1 microM) was as effective as 1 nM insulin in reducing lipolysis stimulated by 1 nM isoproterenol and more effective than insulin at higher isoproterenol concentrations. In intact adipocytes, PIA reduced isoproterenol-induced cyclic AMP (cAMP) accumulation and increased particulate cAMP phosphodiesterase. In particulate preparations PIA suppressed isoproterenol stimulation of adenylate cyclase. PIA was more effective than 5'-N-ethylcarboxamide adenosine (NECA) or adenosine in inhibiting adenylate cyclase and activating phosphodiesterase. In intact adipocytes, two agents with so-called "insulin-like" activities, i.e., anti-insulin receptor antibodies and wheat germ agglutinin (WGA), also increased particulate cAMP phosphodiesterase. Pertussis toxin, which inhibits stimulation of the particulate cAMP phosphodiesterase by insulin (but not by isoproterenol), also inhibited the effects of PIA, anti-insulin receptor antibodies, and WGA. In 3T3-L1 cells, PIA appears to inhibit lipolysis by inhibiting adenylate cyclase and stimulating phosphodiesterase; these effects of PIA, as well as those of anti-insulin receptor antibodies and WGA on phosphodiesterase, may be mediated via guanyl nucleotide-binding proteins.

Pertussis toxin effects on adenylate cyclase activity, cyclic AMP accumulation and lipolysis in adipocytes from hypothyroid, euthyroid and hyperthyroid rats

Adipocytes from hypothyroid rats have a decreased responsiveness to agents that activate adenylate cyclase, whereas cells from hyperthyroid rats have an increased responsiveness as compared to the controls. This is reflected in cyclic AMP accumulation as well as lipolysis. Administration of pertussis toxin to rats or its in vitro addition to adipocytes increased basal lipolysis and cyclic AMP accumulation as well as the response to norepinephrine or forskolin. The effects of thyroid status was not abolished by toxin treatment. Pertussis toxin-catalyzed ADP ribosylation of Ni was increased in adipocyte membranes from hypothyroid rats as compared to those from euthyroid rats. However, no change in sensitivity to N6-(phenylisopropyl)adenosine was observed. The data suggest that the amount of Ni might not be rate-limiting for the inhibitory action of adenosine. A consistent decrease in maximal lipolysis was observed in freshly isolated adipocytes from hypothyroid animals as compared to those from the controls. Such defective maximal lipolysis was not corrected by adenosine deaminase or in vivo administration of pertussis toxin. The relationship between cyclic AMP levels and lipolysis suggests that in fat cells from hypothyroid rats either the cyclic AMP-dependent protein kinase or the lipase activity itself may limit maximal lipolysis. There appears to be multiple effects of thyroid status on lipolysis involving factors other than those affecting adenylate cyclase activation.

Thyroid hormones and fat cell phosphorylation

The phosphorylation of cytosolic and plasma membrane proteins was studied in isolated fat cells from euthyroid and thyroidectomized rats. The analysis, by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, of subcellular fractions of 32P-labelled fat cells revealed the presence of 10-12 phosphoprotein bands in the cytosol. The washed plasma membrane fraction contained 4 major phosphoproteins with estimated molecular weights of 70-67, 60, 42-40 and 26-22 kDa. Two-dimensional analysis of the 32P-labelled phosphoproteins showed that their isoelectric points were between 6.3 and 4.1. The profiles and the isoelectric points were similar in fat cells from euthyroid and thyroidectomized rats. The state of hypothyroidism did not affect the basal phosphorylation of fat cell proteins of the cytosolic or plasma membrane fractions. The incubation of fat cells from euthyroid rats in the presence of isoproterenol or dibutyryl adenosine cyclic monophosphate led to (a) an increase in the 32P labelling of cytosolic proteins which may be subunits of acetyl CoA carboxylase, ATP citrate lyase, hormone-sensitive lipase and other proteins, with apparent molecular weights between 50 and 42 kDa, and (b) an increase in the 32P labelling of plasma membrane proteins of 26-22 kDa. In the case of fat cells from hypothyroid rats, the dibutyryl adenosine cyclic monophosphate increased the 32P labelling of plasma membrane proteins, whereas in the presence of isoproterenol these reactions did not occur. These results show that thyroid hormones control the 32P labelling of proteins of the cytosol and plasma membrane fractions of rat fat cells and therefore, at least in some cases, the lipolytic and lipogenic pathways.

Inhibitory effect of thyroid hormones on pituitary cyclic amp phosphodiesterase activity in vitro

Thyroid hormones effectively inhibited partially purified cyclic AMP phosphodiesterase from the anterior and posterior lobes of bovine pituitary gland competitively with the substrate, but thyronine, diiodotyrosine, monoiodotyrosine, thyrotropin, thyrotropin releasing hormone, dexamethasone, and luteinizing hormone releasing hormone did not inhibit this enzyme activity. Ki values were found to be 1.0 and 2.5 microM for thyroxine, and 8.0 and 13.5 microM for triiodothyronine in the anterior and posterior lobes, respectively.

Thyroid hormone action on intermediary metabolism. Part II: Lipid metabolism in hypo- and hyperthyroidism

Despite their enhanced endogenous de novo cholesterol synthesis, hyperthyroid patients exhibit decreased total and low-density lipoprotein (LDL) cholesterol levels in the serum because of a concomitant increase in LDL catabolism, cholesterol excretion by bile and a reduced enterohepatic bile acid circulation. Hypothyroidism exhibits a reduction (1) in the synthesis of cholesterol and (2) in LDL catabolism, whereas cholesterol reabsorption is unchanged or even enhanced. In addition, obese hypothyroid patients showed an increased cholesterol synthesis which is independent of thyroid hormones and which contributes to the observed LDL cholesterolaemia. Thyroid hormones per se have only a minor influence on plasma triglyceride (TG) levels, but they induce an acceleration of TG turnover and chylomicron clearance rate. In addition, the hepatic lipogenic capacity is increased in hyperthyroidism and reduced in hypothyroidism. However, hepatic total and very low-density lipoprotein (VLDL) triglyceride output is decreased by thyroid hormones due to a reduced re-esterification and a simultaneously increased oxidation of newly synthesized fatty acids. Hypothyroid livers, by contrast, reveal an increased VLDL secretion. Despite their reduced lipogenesis, obese hypothyroidism is often accompanied by a hypertriglyceridaemia type III. The simultaneous stimulation of the synthesis of fatty acids, which are still in part converted to TG, and the degradation of TG contributes to the enhanced thermogenesis in hyperthyroid patients. The concentration and turnover of free fatty acids (FFA) are increased in hyperthyroidism, resulting from a thyroid hormone-induced increase in: (1) lipolysis, explained by an increased adipose tissue sensitivity for lipolytic hormones; and (2) oxidation of fatty acids to CO2 as well as to ketone bodies (KB).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyroid hormone deficiency on lipolysis in chicken fat cells

Effect of lack of thyroid hormones on lipolysis in chicken fat cells was studied. Isolated fat cells from hypothyroid chickens in contrast to normal animals, have an impaired ability to give lipolytic response to glucagon. However activation of triglyceride lipolysis was induced to the same level by theophylline in hypothyroid and normal chickens.

Plasma and urine cyclic nucleotide levels in patients with hyperthyroidism and hypothyroidism

Plasma levels and 24-h urinary excretion of cyclic AMP and cyclic GMP were measured in 18 patients with hyperthyroidism, 7 patients with hypothyroidism and 25 normal subjects. Mean plasma and urinary levels of both cyclic AMP and cyclic GMP were significantly positive correlations between the serum thyroid hormone levels and plasma and urinary cyclic nucleotide concentrations were also found, suggesting that the elevated extracellular cyclic nucleotide levels in hyperthyroidism are probably a consequence of increased secretion of thyroid hormones. In the hypothyroid patients the extracellular cyclic nucleotide concentrations did not differ significantly from those of the normal subjects.

Dependence of beta-adrenergic responsiveness on thyroid state of male rats

The dose of thyroxine (0-150 micrograms/kg body weight) administered subcutaneously daily to surgically thyroidectomized male rats was correlated significantly with their metabolic activity as assessed by rate of oxygen consumption and colonic temperature. There was a significant dose-dependent increase in total serum thyroxine, triiodothyronine and reverse-triiodothyronine concentrations with increasing doses of thyroxine administered. The cardiostimulatory response to administration of isoprenaline (8 micrograms/kg body weight s.c.) was also correlated directly with the dose of thyroxine administered. The concentration of cardiac beta-adrenoceptors in these animals was correlated significantly with total serum thyroxine and triiodothyronine concentrations, basal heart rate, and the chronotropic response to administration of isoprenaline.

Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats

In brown adipose tissue of the rat, chemically or surgically induced hypothyroidism caused the following effects. A large decrease of the magnitude of the metabolic response to electrical nerve stimulation. The deactivation half-time of the response was reduced to 70% of the control value, with no change in catechol O-methyltransferase activity. Pre-incubation of tissues with norepinephrine, 10(-5) M, increased the response to subsequent nerve stimulation almost to that of the controls. The catecholamine analogue dose-response curves were shifted to the right. The shift was very pronounced for isoproterenol (K50 426 nM versus 2 nM), somewhat less marked for norepinephrine (7373 nM versus 194) and very slight for phenylephrine (2803 nM versus 1649); there was almost no change in Emax values. An increase of octanoate oxidative capacity. A decrease of the capacity of the stereoselective binding of (-)-[3H]dihydroalprenolol of the high-affinity (Kd 2.0 nM) sites to a fourth of the control value and an increase by a factor of 2.9 of the Kd of the low-affinity binding sites. This decrease of binding to the beta-receptors was not sufficient quantitatively to explain the decrease in the metabolic response, suggesting the existence of an additional defective reaction which could occur between the binding to the beta-receptors and the activation of the triglyceride lipase. These results show that the sharp decrease of the metabolic response of brown adipose tissue to nerve stimulation has multiple causes. The findings are discussed in the context of the drastic decrease of cold resistance in hypothyroid rats.

Thyroid hormone binding to plasma membrane preparations: studies in different thyroid states and tissues

Two orders of high-affinity saturable binding sites for L-T4 and L-T3 were evidenced in purified plasma membrane preparations from rat liver (apparent equilibrium dissociation constant KD for T4 congruent to 0.6 and 23 nM, for T3 congruent to 9 and 237 nM) and kidney (KD for T4 congruent to 4 and 127 nM; for T3 congruent to 15 and 270 nM). Differences of statistical significance were only found for the higher affinity T4 binding site. In contrast, no saturable T4 or T3 binding could be detected in spleen plasma membranes. Testis plasma membranes exhibited 2 sets of T4 binding sites but with a lower affinity than in liver and kidney (KD congruent to 28 and 286 nM), and only one set of T3 binding sites (KD congruent to 266 nM). A good correlation was found between the plasmalemma T4 and T3 binding properties of a tissue and its ability to respond to and/or metabolize thyroid hormones. T4 and T3 binding was also examined in liver plasma membranes of rats under various thyroid status; no difference could be detected in either KD or total capacity for both sets of T4 and T3 binding sites when comparing normal with hyper- or hypothyroid rats. The distribution of plasmalemma high-affinity specific T4 and T3 binding sites in different tissues suggests that these sites are involved in hormone action, or in the transport of these hormones within the cell.

Regulation of lipogenesis in adipocytes. Independent effects of thyroid hormones, cyclic AMP and insulin on the uptake of deoxy-D-glucose

Thyroidectomy is known to enhance fat cell phosphodiesterase activity; as a result, the response to lipolytic hormones is markedly reduced. Thyroidectomy also stimulates overall lipogenesis and the uptake of glucose: the present experiments investigated whether there was a correlation between cyclic AMP and glucose uptake. The parameter measured was the transport and phosphorylation (uptake) of deoxy-D-glucose in the presence of two modifiers of the cyclic AMP pool: phosphodiesterase inhibitors and the analogue, dibutyryl cyclic AMP. The inhibition by methylxanthines and dibutyryl cyclic AMP of deoxy-D-glucose uptake observed, was the same in fat cells from normal and thyroidectomized rats: the latter nonetheless still maintained their enhanced glucose uptake. It was therefore concluded that thyroid hormones and cyclic AMP control this step by different, separate pathways. Insulin, well known for its lipogenic effect, enhanced deoxy-D-glucose uptake in fat cells from both normal and thyroidectomized rats to the same extent (about 40%). An additive effect of thyroidectomy and insulin on glucose uptake was thus demonstrated. These results imply that glucose uptake in the adipocyte is controlled by at least three factors: thyroid hormones, cyclic AMP and insulin, each of which can act independently. Maximum glucose uptake is achieved in the presence of a combination of low concentrations of cyclic AMP, of insulin, and in the absence of thyroid hormones.

Cyclic AMP and lipogenesis in fat cells from thyroidectomized rats

Thyroid hormones regulate lipid metabolism by affecting lipogenesis as well as lipolysis. The present paper discusses the way thyroidectomy induced an enhancement in lipogenesis in rat fat cells. The doubling in the conversion of glucose to CO2 and fatty acids seen after thyroidectomy was found to be due to a modification in the actual pathway of glucose metabolism: there was a preferential stimulation of the conversion of glucose to CO2 by the pentose cycle (utilisation of [1-14C]glucose) while the production of fatty acids and glyceride-glycerol proceeded, respectively, much more, or only slightly more, via the pathway of [6-14C]glucose metabolism. Studies employing the phosphodiesterase inhibitor MIX, or the cyclic AMP analogue, DBcAMP showed that the lipogenic process depends on cyclic AMP. As the stimulatory effect of thyroidectomy was not abolished, however, lipogenesis must be under the independent control of both cyclic AMP and absence of thyroid hormones. Insulin, a further mediator of lipogenesis was found to further enhance the already preexisting high conversion of glucose to CO2 in fat cells from thyroidectomized rats. It is concluded that at least three factors modify lipogenesis: thyroidectomy, cyclic AMP and insulin; each achieving its effect in an independent manner.

Glucose metabolism by isolated fat cells from sloth

Glucose metabolism by sloth fat cells with and without addition of insulin was investigated. The data were compared to the results obtained with rat fat cells incubated under the same experimental conditions. Sloth fat cells showed a very low glucose oxidation to 14CO2 and incorporation into total lipids. The glucose incorporated into lipids is mainly in the glyceride-glycerol moiety. Addition of insulin did not produce an increase on glucose oxidation and a slight increase in the incorporation into total lipids was observed. Since it has been reported that sloths have a very low rate on thyroxine secretion, the results are discussed in relation to data in the literature on carbohydrate and lipid metabolism in hypothyroid animals.

Adaptive responses of enzymes of carbohydrate and lipid metabolism to dietary alteration in genetically obese Zucker rats (fa/fa)

1. Measurements have been made of the activities of enzymes of the glycolytic route, the pentose phosphate pathway, the tricarboxylic acid cycle and lipogenesis in liver and adipose tissue from genetically obese (fa/fa) rats and their lean litter mates (fa/ --). The effect of food restriction for a period of three weeks on the enzyme profile of liver and adipose tissue of the obese rat was also studied. 2. The most striking increases in enzyme activity in livers from obese rats were: (a) among enzymes of lipogenesis; ATP-citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase, malate dehydrogenase (decarboxylating) and cytoplasmic glycerolphosphate dehydrogenase; (b) within the pentose phosphate pathway; glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase; (c) within the glycolytic pathway; glucokinase, pyruvate kinase and lactate dehydrogenase. All of these enzymes showed a significant increase in activity on the basis of U/g liver and U/mg DNA. In adipose tissue all the enzymes of lipogenesis, of the glycolytic route, of the oxidative segment of the pentose phosphate pathway and of the tricarboxylic acid cycle were increased when expressed as U/2 fat pads or as U/mg DNA. 3. The restriction of the food intake of obese rats to that consumed by their lean litter mates for periods of three weeks did not produce the expected adaptive decrease in enzymes of lipogenesis; in adipose tissue, only ATP-citrate lyase and malate dehydrogenase (decarboxylating) showed a marked decrease; no significant change was found in adipose tissue or liver of the activities of acetyl-CoA carboxylase and fatty acid synthetase, when expressed on a cell basis (U/mg DNA). The non-oxidative enzymes of the pentose phosphate pathway and enzymes involved in glycerogenesis (pyruvate carboxylase, malate dehydrogenase and phosphoenolpyruvate carboxykinase) all increased in adipose tissue from limit-fed obese rats. 4. The rate of conversion of specifically labelled glucose to (14C)O2 and 14C-labelled lipid by pieces of adipose tissue and by liver slices was also measured. Insulin caused an increase in the conversion of (1-14C)glucose to (14C)O2 and 14C-labelled lipid in obese rats fed ad libitum, limit-fed rats and in their lean litter mates. 5. The results are discussed in relation to the raised insulin and hypothyroid state of the obese rat. The effect of this altered hormonal status on the activity of cyclic nucleotide phosphodiesterases and cellular levels of adenosine 3' :5'-monophosphate and guanosine 3' :5'-monophosphate and guanosine 3' :5'-monophosphate in relation to the obese syndrome is considered.

Hormone action at the membrane level. VIII. Adrenergic receptors in rat heart and adipocytes and their modulation by thyroxine

The regulation of adrenergic receptors in rat heart was measured in rats made hyperthyroid by injection with thyroxine and made hypothyroid by addition of propylthiouracil to the drinking water. Hyperthyroid rats display cardiac hypertrophy and a decrease in epididymal fat pad weight. The maximal beta-receptor level of ventricular membranes, as determined by (-)-[3H]dihydroalprenolol binding, was increased 60% by thyroxine treatment and decreased about 30% by propylthiouracil treatment. The affinity of the beta receptor was unchanged after thyroxine or propylthiouracil treatment. The maximal activity of the isoproterenol-stimulated adenylate cyclase (EC 4.6.1.1) varied with thyroid state in a manner parallel to the increase in beta-adrenergic binding sites. Thyroxine treatment also increases by 2-fold the beta receptors in isolated rat fat cells. Propylthiouracil treatment lowered the level of alpha receptors in heart by 30% as measured by [3H]dihydroergocryptine binding, but increased the affinity about 2.5-fold. The highest level of alpha receptors was seen in control hearts. These studies indicate that thyroxine may control the turnover of beta-adrenergic receptors in heart and fat cells and regulate physiological responses in these tissues via a hormone-hormone interplay system. Thyroxine treatment reduced the activity of the membrane-bound Mg2+-ATPase (EC 3.6.1.3) and 5'-mononucleotidase (EC 3.1.3.5) but appears to increase the activity of the (Na+ + K+)ATPase (EC 3.6.1.4).

Adrenergic modulation of insulin secretion in vivo dependent on thyroid states

Insulin secretory responses via adrenergic mechanisms were studied in vivo with hyperthyroid rats prepared by daily injections with thyroxine and with rats rendered hypothyroid by the addition of methylthiouracil in the drinking water. Isoproterenol, a beta-adrenergic agent, caused hyperinsulinemia in hyperthyroid rats more markedly than in euthyroid rats, but failed to induce hyperinsulinemia in hypothyroid rats. The isoproterenol-induced hyperinsulinemia was abolished by a beta-adrenergic receptor blocker in hyperthyroid as well as in euthyroid rats. The glucose-induced hyperinsulinemia was enhanced in hyperthyroid, was not essentially affected in euthyroid, and was inhibited in hypothyroid rats, by an alpha- and beta-adrenergic agent such as epinephrine. It is concluded that the relative function of alpha- to beta-adrenergic receptors responsible for the pancreatic secretion of insulin is dependent on the thyroid state; beta-adrenergic actions are predominant over alpha-actions in hyperthyroidism and vice versa in hypothyroidism.