Effects of hypo- and hyper-thyroidism on in vivo lipogenesis in fed and fasted rats

Both 3,5-Diiodo-L-Thyronine and 3,5,3'-Triiodo-L-Thyronine Prevent Short-term Hepatic Lipid Accumulation via Distinct Mechanisms in Rats Being Fed a High-Fat Diet

3,3′,5-triiodo-L-thyronine (T3) improves hepatic lipid accumulation by increasing lipid catabolism but it also increases lipogenesis, which at first glance appears contradictory. Recent studies have shown that 3,5-diiodothyronine (T2), a natural thyroid hormone derivative, also has the capacity to stimulate hepatic lipid catabolism, however, little is known about its possible effects on lipogenic gene expression. Because genes classically involved in hepatic lipogenesis such as SPOT14, acetyl-CoA-carboxylase (ACC), and fatty acid synthase (FAS) contain thyroid hormone response elements (TREs), we studied their transcriptional regulation, focusing on TRE-mediated effects of T3 compared to T2 in rats receiving high-fat diet (HFD) for 1 week. HFD rats showed a marked lipid accumulation in the liver, which was significantly reduced upon simultaneous administration of either T3 or T2 with the diet. When administered to HFD rats, T2, in contrast with T3, markedly downregulated the expression of the above-mentioned genes. T2 downregulated expression of the transcription factors carbohydrate-response element-binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c) involved in activation of transcription of these genes, which explains the suppressed expression of their target genes involved in lipogenesis. T3, however, did not repress expression of the TRE-containing ChREBP gene but repressed SREBP-1c expression. Despite suppression of SREBP-1c expression by T3 (which can be explained by the presence of nTRE in its promoter), the target genes were not suppressed, but normalized to HFD reference levels or even upregulated (ACC), partly due to the presence of TREs on the promoters of these genes and partly to the lack of suppression of ChREBP. Thus, T2 and T3 probably act by different molecular mechanisms to achieve inhibition of hepatic lipid accumulation.

Principles of endocrinology

The endocrine system plays a major role in human survival. Endocrine glands secrete chemical messengers or hormones that affect every tissue of the body, including the periodontium, during the life of the individual. As the endocrine system influences a broad assortment of biological activities necessary for life, a general understanding of the principal components and functions of this system is essential. A fundamental assessment of hormone structure, mechanism of action and hormone transport, as well as influence on homeostasis is reviewed. A concise evaluation of the functions of the central endocrine glands, the functions of the major peripheral endocrine glands (other than gonadal tissues) and the known relationships of these hormones to the periodontium is examined.

Hepatic regulation of fatty acid synthase by insulin and T3: evidence for T3 genomic and nongenomic actions

Fatty acid synthase (FAS) is a key enzyme of hepatic lipogenesis responsible for the synthesis of long-chain saturated fatty acids. This enzyme is mainly regulated at the transcriptional level by nutrients and hormones. In particular, glucose, insulin, and T(3) increase FAS activity, whereas glucagon and saturated and polyunsaturated fatty acids decrease it. In the present study we show that, in liver, T(3) and insulin were able to activate FAS enzymatic activity, mRNA expression, and gene transcription. We localized the T(3) response element (TRE) that mediates the T(3) genomic effect, on the FAS promoter between -741 and -696 bp that mediates the T(3) genomic effect. We show that both T(3) and insulin regulate FAS transcription via this sequence. The TRE binds a TR/RXR heterodimer even in the absence of hormone, and this binding is increased in response to T(3) and/or insulin treatment. The use of H7, a serine/threonine kinase inhibitor, reveals that a phosphorylation mechanism is implicated in the transcriptional regulation of FAS in response to both hormones. Specifically, we show that T(3) is able to modulate FAS transcription via a nongenomic action targeting the TRE through the activation of a PI 3-kinase-ERK1/2-MAPK-dependent pathway. Insulin also targets the TRE sequence, probably via the activation of two parallel pathways: Ras/ERK1/2 MAPK and PI 3-kinase/Akt. Finally, our data suggest that the nongenomic actions of T(3) and insulin are probably common to several TREs, as we observed similar effects on a classical DR4 consensus sequence.

Thyroid hormone regulates the acetyl-CoA carboxylase PI promoter

The acetyl-CoA carboxylase-alpha gene has two promoters, PI and PII. A variety of mRNA products result from this gene, depending on promoter usage and splicing events. We have investigated thyroid hormone regulation of acetyl-CoA carboxylase-alpha gene expression, using the reverse-transcription polymerase chain reaction with PI- or PII-specific primers. RNA was extracted from a range of tissues taken from hypo-, eu-, or hyperthyroid rats. PII-generated products were found in all tissues examined at similar levels and were not affected by thyroid state. Products derived from PI were also widely found but with more variable levels of expression. PI mRNAs were reduced in hypo- and elevated in hyperthyroid livers. In brown adipose tissue, more PI products were found in hypothyroid animals. Thus, thyroid hormone regulates the activity of the acetyl-CoA carboxylase PI promoter to influence fatty acid synthesis in a tissue-specific manner.

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.

Effects of tri-iodothyronine administration on the disposal of oral [1-14C]triolein, lipoprotein lipase activity and lipogenesis in the rat during lactation and on removal of the litter

The effect of tri-iodothyronine (T3) administration on the utilization of dietary [14C]lipid by the mammary gland and adipose tissue of lactating and litter-removed rats was studied. (1) After an oral load of [1-14C]triolein, the lactating rats treated with T3 (50 micrograms/100 g body wt.) over 24 h showed an increase in 14CO2 production and a decrease in the total [14C]lipid transferred through the mammary gland that was paralleled by a decrease in tissue lipoprotein lipase (LPL) activity. (2) T3 administration decreased plasma prolactin in the lactating rats. Prolactin replacement in T3-treated rats restored LPL activity in the mammary gland, but did not increase the amount of dietary [14C]lipid transferred to the milk. (3) Chronic T3 administration (4 days) to lactating rats did not affect pup growth or the lipogenic rate in the mammary gland. (4) The administration of T3 to litter-removed rats inhibited the increase of LPL activity in white adipose tissue and decreased the accumulation of dietary [14C]lipid. This decrease was accompanied by increased 14CO2 production and [14C]lipid accumulation in skeletal muscle and heart. (5) It is concluded that hyperthyroidism depresses LPL activity in mammary gland and white adipose tissue, but not in muscle. The increased accumulation of [14C]lipid in muscle and increased production of 14CO2 in lactating and in litter-removed rats treated with T3 is in part due to the decreased total LPL in mammary gland and adipose tissue respectively, which are therefore less able to compete with muscle for the available plasma triacylglycerols.

Effects of triiodothyronine administration on dietary [14C]triolein partitioning between deposition in adipose tissue and oxidation to [14C]CO2 in ad libitum-fed or food-restricted rats

Refeeding a chow meal containing [1-14C]triolein to food-restricted rats results in increased accumulation of [14C]lipid in carcass and epididymal adipose tissue and lower oxidation to [14C]CO2 compared to ad libitum-fed rats (Biochem. J. 285, 773-778, 1992). In the present experiments the effects of treatment with triiodothyronine (T3) for three days on lipid accumulation in refed food-restricted rats has been examined. T3 decreased accumulation of [14C]lipid in carcass and epididymal adipose tissue (32 and 77%, respectively) of food-restricted rats on refeeding the chow-[1-14C]triolein meal. This decreased accumulation of [14C]lipid was accompanied by increased [14C]CO2 production (77%) and decreased heparin-elutable lipoprotein lipase activity in the epididymal fat pad (90%) and subcutaneous adipose tissue (80%). Accumulation of [14C]lipid in the latter did not decrease significantly. In contrast, T3 treatment of ad libitum-fed rats increased [14C]lipid deposition in carcass (44%) and in subcutaneous adipose tissue (240%) on refeeding, when compared to untreated ad libitum rats. Lipoprotein lipase activity in the two adipose tissue depots of the refed ad libitum+T3 rats, however, decreased. Thus, the effects of T3 on [14C]lipid deposition are adipose-tissue-depot-specific and depend on the previous dietary intake (over 14 days) of the rat. T3-treatment increased the lipoprotein lipase activity released from perfused hearts to a similar extent in both food-restricted and ad libitum-fed rats compared to the corresponding untreated groups. The rates of lipogenesis in-vivo in liver, epididymal and subcutaneous adipose tissue of food-restricted rats refed chow were not altered by T3. It is concluded that the increased deposition of dietary lipid in the food-restricted rat can be partially reversed by treatment with T3, suggesting that the low-T3 state associated with this condition may be in part responsible.

Relationships between fatty acid synthesis and lipid secretion in the isolated perfused rat liver: effects of hyperthyroidism, glucose and oleate

Various studies on the effects of thyroid status on hepatic fatty acid synthesis have produced conflicting results. Several variables (e.g., plasma free fatty acid and glucose concentrations) are altered simultaneously by thyroid status and can affect fatty acid synthesis. To evaluate the effects of these variables, hepatic fatty acid synthesis (lipogenesis) was studied in isolated perfused livers from normal and triiodothyronine-treated rats. Livers were perfused with media containing either 5.5 or 25 mM glucose without fatty acid, or 5.5 mM glucose and 0.7 mM oleate. Rates of lipogenesis were determined by measurement of incorporation of 3H2O into fatty acids. Lipogenesis in livers from hyperthyroid animals exceeded that of controls, when perfused with 5.5 mM glucose with or without oleate. Perfusion with 25 mM glucose increased lipogenesis in both euthyroid and hyperthyroid groups to the same level, abolishing this difference between them. Perfusion with oleate reduced rates of lipogenesis by livers from euthyroid and hyperthyroid rats to a similar extent, but stimulated secretion of radioactive fatty acid in phospholipid and free fatty acid fractions. Oleate increased ketogenesis by livers from normal and triiodothyronine-treated rats, with higher rates of ketogenesis in the triiodothyronine-treated group. When oleate was omitted, ketogenesis in the presence of 5.5 mM glucose by the hyperthyroid group was similar to that of euthyroid controls, while ketogenesis was decreased in the hyperthyroid group relative to controls when perfused with 25 mM glucose. About 30% of the radioactivity incorporated into the total fatty acid of both groups was recovered in palmitate, with the remainder in longer chain saturated and unsaturated fatty acids. In both euthyroid and hyperthyroid groups, the ratio of triacylglycerol:phospholipid fatty acid radioactivity was not only less than predicted (based on synthetic rates of PL and TG) but also was decreased in perfusions with exogenous oleate compared to perfusions without oleate. In perfusions with oleate, both groups incorporated twice as much radioactivity into phospholipid as into triacylglycerol. The data suggest the following concepts: while hepatic fatty acid synthesis and oxidation are increased simultaneously in the hyperthyroid state, de novo synthesized fatty acids seem to be poorer substrates for oxidation than are exogenous fatty acids, and are preferentially incorporated into phospholipid, while exogenous fatty acids are better substrates for oxidation and esterification to triacylglycerol. The preferential utilization of de novo synthesized fatty acid for phospholipid synthesis may be an important physiologic adaptation insuring a constant source of fatty acid for membrane synthesis.

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.

Metabolic response to starvation at late gestation in chronically ethanol-treated and pair-fed undernourished rats

To study the role of undernourishment in the negative effects of ethanol during pregnancy and to determine whether maternal ethanol intake modifies metabolic response to starvation at late gestation, female rats receiving ethanol in their drinking water before and during pregnancy (ethanol group) were compared with animals that received the same amount of solid diet as the ethanol group rats (pair-fed group) and with normal rats fed ad libitum (control group). All animals were killed on the 21st day of gestation, either in the fed state or after 24-hours fasting. The body weight of ethanol rats was lower than that of controls but higher than that of pair-fed rats. When compared with controls, ethanol and pair-fed rats had reduced fetal body weights, whereas fetal body length was reduced only in the former. In the fed state, blood glucose concentration was lower in the ethanol and pair-fed rats and fetuses than in controls. Twenty-four-hour starvation caused a reduction in this parameter only in control and ethanol mothers. In the fed state, maternal liver glycogen concentration was lower in ethanol and higher in pair-fed mothers than in controls. Blood beta-hydroxybutyrate levels were higher in ethanol-treated mothers than in the others, and 24-hour starvation increased this parameter in ethanol and control rats to a greater extent than in the pair-fed ones. Liver triacylglyceride concentration was higher in ethanol-treated mothers than in the other two groups, and starvation caused this concentration to increase in ethanol and control groups but not in the pair-fed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct effect of triiodothyronine on glucose utilization in adipocytes from obese and nonobese Zucker rats

The direct effects of L-T3 (triiodothyronine) on glucose utilization, 2-deoxyglucose uptake and O2 consumption were evaluated in vitro in isolated adipocytes from 6-week-old obese and nonobese Zucker rats. Adipocytes treated for 30 min with L-T3 had a 20-25% decrease in glucose conversion to CO2, fatty acids and glyceride-glycerol and uptake of 2-deoxyglucose. Incubation of adipocytes with either D-T3 or rT3 (reverse T3) did not decrease utilization of glucose. T3 also decreased the insulin-stimulated fatty acid synthesis from glucose in both obese and nonobese adipocytes. Adipocyte O2 consumption was decreased by T3 in media that contained glucose but not in media that contained either acetate or no exogenous substrate. These data indicate that T3 has a direct, immediate and specific action on adipocyte glucose utilization and may act to modulate adipose lipogenesis and lipolysis.

Thyroid-epididymal relationship. I. Influence of hypothyroidism on epididymal lipids

The influence of thyroidectomy on the lipid composition of caput and cauda epididymides have been studied. The analysis was conducted in epididymal tissues free from fluids and sperm. A general tendency towards accumulation of epididymal lipids was observed in hypothyroid rats. Hypothyroidism also brought about a differential regional response, which may be age-related. The existence of a relationship between triacylglycerols, phospholipids and diacylglycerols has been suggested. Since immediate thyroxine replacement to thyroidectomised rats maintained epididymal lipids at control levels, it is concluded that hypothyroidism has a definite influence on the epididymal lipid composition.

Liver glucose, glycogen and lipid synthesis in fed and 24-hour fasted rats soon after a pulse of [3-14C]pyruvate through the portal vein

1. A pulse of [3-14C]pyruvate was given to rats through the portal vein and blood was collected at brief intervals from the inferior cava vein at the level of the suprahepatic veins. 2. In 24 hr fasted rats, the appearance of [14C]glucose in blood and blood glucose specific radioactivity were higher than in fed animals from the first minute after delivery of the tracer. At this time total radioactivity did not differ between the two groups. 3. After 5 and 20 min. liver radioactivity present in glycogen and glyceride glycerol was enhanced while in fatty acids it was reduced in fasted as compared with fed animals. 4. It is proposed that, in the fasted state, both glycogen and glyceride glycerol synthesis are predominantly gluconeogenic processes.

Triiodothyronine-dietary interrelationships in the modulation of brown adipose tissue and liver lipogenesis in the rat

The influence of diet and triiodothyronine on "in vivo" lipogenesis has been compared in liver and brown adipose tissue of the rat at weaning and in adult life. At both ages studied, hyperthyroidism increases lipogenesis only in the liver. However brown adipose tissue accumulates lipid (probably derived from white adipose tissue) thus helping to explain previous observations of low rates of lipogenesis in this tissue during the late suckling period when thermogenesis is high and plasma triiodothyronine levels rise. Suppression of lipogenesis by dietary polyunsaturated fat in both tissues at both ages studied confirms previous studies on adult animals. Malic enzyme and glucose-6-phosphate dehydrogenase activities correlate well with changes in lipogenesis in brown adipose tissue but not in liver and the hepatic enzymes exhibit a diminished response to dietary and hormonal factors with age.

Regulation of net triacylglycerol synthesis by metabolic substrates in isolated rat liver cells

Triacylglcerol metabolism was studied in isolated rat hepatocytes using a fully enzymatic method to masure cellular glyceride-glycerol. With this method 40 and 50 MUmol triacylglycerol were found per g cellular protein in liver cells from fed and starved rats, respectively, comparable to values obtained after organic solvent extraction and alkaline hydrolysis of neutral lipids. Carbohydrate refeeding of animals increased triacylglycerol levels in hepatocytes to 80 mumol. Upon incubation without fatty acids a 15% decrease in cellular triacylglycerol was found in 60 min. When 1mM oleate or palmitate were added cellular triacylglycerol increased. The rates of net triacylglycerol increased. The rates of net triacylglycerol synthesis were not significantly different with oleate and palmitate. Starvation reduced the rates from both fatty acids, whereas carbohydrate refeeding led to a marked increase in net triacylglycerol synthesis. Besides 20mM glucose, 5mM L-lactate and 5mM fructose stimulated triacylglycerol synthesis from fatty acids. THe stimulatory effect of lactate was higher in hepatocytes from starved animals, so that the differences in triacylglycerol synthesis between liver cells from fed and starved rats were abolished. Fatty acids taken up and not recovered in newly formed triacylglycerol were released as ketone bodies. When radioactive lectate was offered to cells from starved rats, label incorporated into neutral lipids was exclusively recovered from the glycerol moiety of triacylglycerols. 5mM ethanol which alone increased fatty acid esterification, reduced the stimulatory effect of lactate but increased the effect of fructose on net triacylglycerol formation. These findings indicate that esterification rate in liver cells from starved rats can be limited by availability of alpha-glycerophosphate, which is provided by glyceroneogenesis. The possible physiological significance of these findings is discussed with regard to liver cell heterogeneity and nutritional adaptation of liver triacylglycerol formation.