Thyromimetic mode of action of peroxisome proliferators: activation of 'malic' enzyme gene transcription

Toxicological evaluation of ammonium perfluorobutyrate in rats: twenty-eight-day and ninety-day oral gavage studies

Sequential 28-day and 90-day oral toxicity studies were performed in male and female rats with ammonium perfluorobutyrate (NH(4)(+)PFBA) at doses up to 150 and 30mg/kg-d, respectively. Ammonium perfluorooctanoate was used as a comparator at a dose of 30mg/kg-d in the 28-day study. Female rats were unaffected by NH(4)(+)PFBA. Effects in males included: increased liver weight, slight to minimal hepatocellular hypertrophy; decreased serum total cholesterol; and reduced serum thyroxin with no change in serum thyrotropin. During recovery, liver weight, histological, and cholesterol effects were resolved. Results of RT-qPCR were consistent with increased transcriptional expression of the xenosensor nuclear receptors PPARα and CAR as well as the thyroid receptor, and decreased expression of Cyp1A1 (Ah receptor-regulated). No observable adverse effect levels (NOAELs) were 6 and >150mg/kg-d for male and female rats in the 28-day study and 6 and >30mg/kg-d in the 90-dat study, respectively.

Thyroid hormone and myocardial mitochondrial biogenesis

Mitochondria have been central in the development of some of the most important ideas in modern biology. Since the discovery that mitochondria have its own DNA and specific mutations and deletions were found in association with neuromuscular and heart diseases, as well as in aging, an extraordinary number of publications have followed, and the term mitochondrial medicine was coined. Recently, it has been found that thyroid hormone (TH) stimulates cardiac mitochondrial biogenesis increasing myocardial mitochondrial mass, mitochondrial respiration, oxidative phosphorylation (OXPHOS), enzyme activities, mitochondrial protein synthesis (by stimulation in a T3-dependent manner), cytochrome, phospholipid and mtDNA content. Also, TH therapy may modulate cardiac mitochondrial protein-import apparatus. To identify the sequence of events, molecules and signaling pathways that is activated by TH affecting mitochondrial structure, biogenesis and function further research is warranted.

Comparative study of sesame lignans (sesamin, episesamin and sesamolin) affecting gene expression profile and fatty acid oxidation in rat liver

The impact of sesamin, episesamin and sesamolin (sesame lignans) on hepatic gene expression profiles was compared with a DNA microarray. Male Sprague-Dawley rats were fed experimental diets containing 0.2% sesamin, episesamin or sesamolin, and a control diet free of lignans for 15 d. Compared to a lignan-free diet, a diet containing sesamin, episesamin and sesamolin caused 1.5- and 2-fold changes in the expression of 128 and 40, 526 and 152, and 516 and 140 genes, respectively. The lignans modified not only the mRNA levels of many enzymes involved in hepatic fatty acid oxidation, but also those of proteins involved in the transportation of fatty acids into hepatocytes and their organelles, and regulating hepatic concentrations of carnitine, CoA and malonyl-CoA. It is apparent that sesame lignans stimulate hepatic fatty acid oxidation by affecting the gene expression of various proteins regulating hepatic fatty acid metabolism. We also observed that lignans modified the gene expression of various proteins involved in hepatic lipogenesis, cholesterogenesis and glucose metabolism. The changes were generally greater with episesamin and sesamolin than with sesamin. In terms of the amounts accumulated in serum and the liver, the lignans ranked in the order sesamolin, episesamin and sesamin. The differences in bio-availability among these lignans appear to be important to their divergent physiological activities.

Bioenergetic remodeling of heart mitochondria by thyroid hormone

Changes in thyroid status are associated with profound alterations in biochemical and physiological functioning of cardiac muscle impacting metabolic rate, contractility and structural hypertrophy. Using an in vivo model of chronic treatment with thyroid hormone (T4, 0.3 mg/kg/day), we evaluated how mitochondria are regulated in response to T4, and assessed the relationship of T4-induced mitochondrial biogenesis and bioenergetics to overall cardiac hypertrophy. The role of thyroid hormone in cardiac bioenergetic remodeling was addressed in rats treated with T4 for 5, 10 and 15 days. Over that time, myocardial oxygen consumption substantially increased as did cardiac hypertrophy. Myocardial levels of mitochondrial enzyme activities, mitochondrial DNA (mtDNA), specific proteins and transcript were assessed. Activity levels of respiratory complexes I-V and citrate synthase significantly increased with 15 but not with 5 or 10-day T4 treatment. Myocardial levels of mtDNA, mitochondrial proteins (e.g. cytochrome c, cytochrome b, ATPase subunits, MnSOD) and the global transcription factor PPARalpha were significantly elevated with 15-day T4. Transcript analysis revealed increased expression of transcription factors and cofactors involved in mitochondrial biogenesis including PPARalpha, mtTFA, ErbAalpha and PGC-1alpha. Our findings indicate parallel increases in myocardial mitochondrial bioenergetic capacity, oxygen consumption and markers of mitochondrial biogenesis with 15-day T4; these changes were not present with 10-day T4 even with significant cardiac hypertrophy. The marked, parallel increases in PPARalpha levels suggest its potential involvement in mediating myocardial-specific remodeling of mitochondria in response to T4.

Protein profiling of mouse livers with peroxisome proliferator-activated receptor alpha activation

Peroxisome proliferator-activated receptor alpha (PPARalpha) is important in the induction of cell-specific pleiotropic responses, including the development of liver tumors, when it is chronically activated by structurally diverse synthetic ligands such as Wy-14,643 or by unmetabolized endogenous ligands resulting from the disruption of the gene encoding acyl coenzyme A (CoA) oxidase (AOX). Alterations in gene expression patterns in livers with PPARalpha activation were delineated by using a proteomic approach to analyze liver proteins of Wy-14,643-treated and AOX(-/-) mice. We identified 46 differentially expressed proteins in mouse livers with PPARalpha activation. Up-regulated proteins, including acetyl-CoA acetyltransferase, farnesyl pyrophosphate synthase, and carnitine O-octanoyltransferase, are involved in fatty acid metabolism, whereas down-regulated proteins, including ketohexokinase, formiminotransferase-cyclodeaminase, fructose-bisphosphatase aldolase B, sarcosine dehydrogenase, and cysteine sulfinic acid decarboxylase, are involved in carbohydrate and amino acid metabolism. Among stress response and xenobiotic metabolism proteins, selenium-binding protein 2 and catalase showed a dramatic approximately 18-fold decrease in expression and a modest approximately 6-fold increase in expression, respectively. In addition, glycine N-methyltransferase, pyrophosphate phosphohydrolase, and protein phosphatase 1D were down-regulated with PPARalpha activation. These observations establish proteomic profiles reflecting a common and predictable pattern of differential protein expression in livers with PPARalpha activation. We conclude that livers with PPARalpha activation are transcriptionally geared towards fatty acid combustion.

Investigation of potential mechanisms regulating protein expression of hepatic pyruvate dehydrogenase kinase isoforms 2 and 4 by fatty acids and thyroid hormone

Liver contains two pyruvate dehydrogenase kinases (PDKs), namely PDK2 and PDK4, which regulate glucose oxidation through inhibitory phosphorylation of the pyruvate dehydrogenase complex (PDC). Starvation increases hepatic PDK2 and PDK4 protein expression, the latter occurring, in part, via a mechanism involving peroxisome proliferator-activated receptor-alpha (PPARalpha). High-fat feeding and hyperthyroidism, which increase circulating lipid supply, enhance hepatic PDK2 protein expression, but these increases are insufficient to account for observed increases in hepatic PDK activity. Enhanced expression of PDK4, but not PDK2, occurs in part via a mechanism involving PPAR-alpha. Heterodimerization partners for retinoid X receptors (RXRs) include PPARalpha and thyroid-hormone receptors (TRs). We therefore investigated the responses of hepatic PDK protein expression to high-fat feeding and hyperthyroidism in relation to hepatic lipid delivery and disposal. High-fat feeding increased hepatic PDK2, but not PDK4, protein expression whereas hyperthyroidism increased both hepatic PDK2 and PDK4 protein expression. Both manipulations decreased the sensitivity of hepatic carnitine palmitoyltransferase I (CPT I) to suppression by malonyl-CoA, but only hyperthyrodism elevated plasma fatty acid and ketone-body concentrations and CPT I maximal activity. Administration of the selective PPAR-alpha activator WY14,643 significantly increased PDK4 protein to a similar extent in both control and high-fat-fed rats, but WY14,643 treatment and hyperthyroidism did not have additive effects on hepatic PDK4 protein expression. PPARalpha activation did not influence hepatic PDK2 protein expression in euthyroid rats, suggesting that up-regulation of PDK2 by hyperthyroidism does not involve PPARalpha, but attenuated the effect of hyperthyroidism to increase hepatic PDK2 expression. The results indicate that hepatic PDK4 up-regulation can be achieved by heterodimerization of either PPARalpha or TR with the RXR receptor and that effects of PPARalpha activation on hepatic PDK2 and PDK4 expression favour a switch towards preferential expression of PDK4.

Inhibition of endogenous thyroid hormone receptor-beta and peroxisome proliferator-activated receptor-alpha activities by humic acid in a human-derived liver cell line

Humic acid (HA), know to be ubiquitous in the natural environment, is present in almost all soil, surface water, and plants. Earlier studies indicate that HA can affect thyroid economy via binding with iodide, inhibiting both thyroid peroxidase and hepatic 5'-deiodinase in rodents. However, the effect of HA, a peroxisome proliferator in rodents, on thyroid hormone receptor (TR) and peroxisome proliferator-activated receptor (PPAR) in human cells has not yet been examined. In this study, we demonstrate that the malic enzyme activity and the transcriptional activities of endogenous TR and PPAR were inhibited after treatment with HA in human hepatocyte Chang liver cell line. Although the protein expression levels of TR-beta, PPAR-alpha and retinoid X receptor-alpha (RXRalpha) were not changed significantly by HA treatment, both the binding abilities of endogenous TR-beta on thyroid hormone response element (TRE) and PPAR-alpha on the PPAR response element (PPRE) were inhibited by HA treatment. The study of the subcellular distribution of HA, relying on the inherent HA fluorescence, showed that HA distributed in the intracellular compartments including cytoplasm and nucleus. The 50% binding inhibition values (CI(50)) of HA on ME-TRE (malic enzyme gene-TRE) and ACOX-PPRE (acylCoA oxidase gene-PPRE) were 19.31 and 19.94 microg/mL, respectively. These results suggest that HA-induced endemic goiter may link in part to the disruption of TRbeta and PPARalpha function in human Chang liver cells. This model may be useful in the investigation of environmental goitrogens.

Sesamin, a sesame lignan, decreases fatty acid synthesis in rat liver accompanying the down-regulation of sterol regulatory element binding protein-1

The effect of sesamin, one of the most abundant lignans in sesame seed, on hepatic fatty acid synthesis was examined in rats. Rats were fed experimental diets containing varying amounts (0, 0.1 and 0.2% for Exp. 1 and 0, 0.2 and 0.4% for Exp. 2, respectively) of sesamin for 15 days. The activity and gene expression of enzymes involved in fatty acid synthesis including acetyl-CoA carboxylase, fatty acid synthase, ATP-citrate lyase and glucose-6-phosphate dehydrogenase decreased as the dietary level of sesamin increased in Exp. 1 and in rats fed the 0.2% sesamin diet they were approximately one-half those in animals fed a sesamin-free diet. In Exp. 2, the 0.2% sesamin diet lowered these parameters to one-half the level for a sesamin-free diet, but no further reduction was seen in animals fed the 0.4% sesamin diet. Dietary sesamin dose-dependently decreased the sterol regulatory element binding protein-1 (SREBP-1) mRNA level, and the value in rats fed a 0.4% sesamin diet was approximately one-half that in those fed a sesamin-free diet. The protein content of the membrane-bound precursor form of SREBP-1 decreased as dietary sesamin increased and was 37% lower in rats fed the 0.4% sesamin diet than in those fed a sesamin-free diet. Dietary sesamin exerted a more marked influence on the protein content of the mature nuclear form of SREBP-1. Diets containing 0.2 and 0.4% sesamin lowered the amount of mature SREBP-1 protein to less than one-fifth of that in the animals fed a sesamin-free diet. It was suggested that the dietary sesamin-dependent decrease in lipogenic enzyme gene expression is due to the suppression of the gene expression of SREBP-1 as well as the proteolysis of the membrane-bound precursor form of this transcriptional factor to generate the mature form.

Effect of sesame seeds rich in sesamin and sesamolin on fatty acid oxidation in rat liver

Activities of enzymes involved in hepatic fatty acid oxidation and synthesis among rats fed sesame (Sesamum indicum L.) differing in lignan content (sesamin and sesamolin) were compared. Sesame seeds rich in lignans from two lines, 0730 and 0732, lines established in this laborary, and those from a conventional cultivar (Masekin) were employed. Seeds from the 0730 and 0732 lines contained sesamin and sesamolin at amounts twice those from Masekin. Sesame seeds were added at levels of 200 g/kg to the experimental diets. Sesame increased both the hepatic mitochondrial and the peroxisomal fatty acid oxidation rate. Increases were greater with sesame rich in lignans than with Maskin. Noticeably, peroxisomal activity levels were >3 times higher in rats fed diets containing sesame seeds from the 0730 and 0732 lines than in those fed a control diet without sesame. The diet containing Masekin seed caused only a 50% increase in the value, however. Diets containing seeds from the 0730 and 0732 lines, compared to the control and Masekin diets, also significantly increased the activity of hepatic fatty acid oxidation enzymes including acyl-CoA oxidase, carnitine palmitoyltranferase, 3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase. In contrast, diets containing sesame lowered the activity of enzymes involved in fatty acid synthesis including fatty acid synthase, glucose-6-phosphate dehydrogenase, ATP-citrate lyase, and pyruvate kinase. No significant differences in enzyme activities were, however, seen among diets containing sesame from Masekin cultivar and lines 0730 and 0732. Serum triacylglycerol concentrations were lower in rats fed diets containing sesame from lines 0730 and 0732 than in those fed the control or Masekin diet. It is apparent that sesame rich in lignans more profoundly affects hepatic fatty acid oxidation and serum triacylglycerol levels. Therefore, consumption of sesame rich in lignans results in physiological activity to alter lipid metabolism in a potentially beneficial manner.

Role of thyroid hormones in hepatic effects of peroxisome proliferators

Peroxisome proliferators are endocrine disrupting chemicals that cause liver tumors in rodents but not humans. Although the receptor that mediates key hepatic effects, the peroxisome proliferator-activated receptor alpha (PPAR-alpha), and its endogenous ligands have been identified, the mechanism whereby these commonly used chemicals cause liver tumors in rodents has yet to be elucidated. Species differences in PPAR-alpha and DNA response elements may explain some of the variability in response upon exposure to peroxisome proliferators. The possibility that thyroid-modulating effects of peroxisome proliferators may contribute to the hepatic effects of peroxisome proliferators has yet to be fully explored. When the potent peroxisome proliferator, WY-14,643, was given to hypothyroid rats, there was a blunting of the hepatomegaly and hepatocyte proliferative responses seen in thyroid-intact animals. Acyl-CoA oxidase activity was unaltered by changes in thyroid hormone status. In addition, preliminary evidence indicates that peroxisome proliferators increased hepatic thyroid receptor (TRalpha1) expression, but TRalpha1 levels in liver tumors were similar to those in unexposed animals. Significant differences between humans and rodents with respect to thyroid hormone physiology and metabolism, in conjunction with the results of these studies, may be indicative of yet another mechanism to explain differential sensitivity to hepatic effects of peroxisome proliferators.

New molecular aspects of regulation of mitochondrial activity by fenofibrate and fasting

Fenofibrate and fasting are known to regulate several genes involved in lipid metabolism in a similar way. In this study measuring several mitochondrial enzyme activities, we demonstrate that, in contrast to citrate synthase and complex II, cytochrome c oxidase (COX) is a specific target of these two treatments. In mouse liver organelles, Western blot experiments indicated that mitochondrial levels of p43, a mitochondrial T3 receptor, and mitochondrial peroxisome proliferator activated receptor (mt-PPAR), previously described as a dimeric partner of p43 in the organelle, are increased by both fenofibrate and fasting. In addition, in PPAR alpha-deficient mice, this influence was abolished for mt-PPAR but not for p43, whereas the increase in COX activity was not altered. These data indicate that: (1) PPAR alpha is involved in specific regulation of mt-PPAR expression by both treatments; (2) fenofibrate and fasting regulate the mitochondrial levels of p43 and thus affect the efficiency of the direct T3 mitochondrial pathway.

Peroxisome proliferator-activated receptor alpha-mediated pathways are altered in hepatocyte-specific retinoid X receptor alpha-deficient mice

Retinoid x receptor alpha (RXRalpha) serves as an active partner of peroxisome proliferator-activated receptor (PPARalpha). In order to dissect the functional role of RXRalpha and PPARalpha in PPARalpha-mediated pathways, the hepatocyte RXRalpha-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRalpha and PPARalpha deficiency are different in several aspects. At the basal untreated level, RXRalpha deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARalpha-null mice. Fasting-induced PPARalpha activation was drastically prevented in the absence of hepatocyte RXRalpha. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRalpha. Hepatocyte RXRalpha deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRalpha-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRalpha. These data suggested that in comparison to PPARalpha, hepatocyte RXRalpha has its unique role in lipid homeostasis and that the effect of RXRalpha, -beta, and -gamma is redundant in certain aspects.

A 45 kDa protein related to PPARgamma2, induced by peroxisome proliferators, is located in the mitochondrial matrix

Besides their involvement in the control of nuclear gene expression by activating several peroxisome proliferator-activated receptors (PPARs), peroxisome proliferators influence mitochondrial activity. By analogy with the previous characterization of a mitochondrial T3 receptor (p43), we searched for the presence of a peroxisome proliferator target in the organelle. Using several antisera raised against different domains of PPARs, we demonstrated by Western blotting, immunoprecipitation and electron microscopy experiments, that a 45 kDa protein related to PPARgamma2 (mt-PPAR) is located in the matrix of rat liver mitochondria. In addition, we found that the amounts of mt-PPAR are increased by clofibrate treatment. Moreover, in EMSA experiments mt-PPAR bound to a DR2 sequence located in the mitochondrial D-loop, by forming a complex with p43. Last, studies of tissue-specific expression indicated that mt-PPAR is detected in mitochondria of all tissues tested except the brain in amounts positively related to p43 abundance. Besides their involvement in the control of nuclear gene expression by activating several peroxisome proliferator-activated receptors (PPARs), peroxisome proliferators influence mitochondrial activity. By analogy with the previous characterization of a mitochondrial T3 receptor (p43), we searched for the presence of a peroxisome proliferator target in the organelle. Using several antisera raised against different domains of PPARs, we demonstrated by Western blotting, immunoprecipitation and electron microscopy experiments, that a 45 kDa protein related to PPARgamma2 (mt-PPAR) is located in the matrix of rat liver mitochondria. In addition, we found that the amounts of mt-PPAR are increased by clofibrate treatment. Moreover, in EMSA experiments mt-PPAR bound to a DR2 sequence located in the mitochondrial D-loop, by forming a complex with p43. Last, studies of tissue-specific expression indicated that mt-PPAR is detected in mitochondria of all tissues tested except the brain in amounts positively related to p43 abundance.

Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipose tissue were investigated in rats fasted for 15 h under basal and calorigenic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG). About two-thirds of FFA reesterification are carried out before FFA release into plasma, whereas the rest consists of plasma FFA extracted by adipose tissue. Thirty-six percent of the fasting lipolytic flux is accounted for by oxidation of plasma FFA, whereas only a minor fraction is channeled into hepatic very low density lipoprotein-triglycerides (VLDL-TG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calorigenesis induced by treatment with beta, beta'-tetramethylhexadecanedioic acid (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxidation, respectively, maintained by a 1.5-fold increase in adipose fat lipolysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible under both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic demands.

Voluntary sucrose ingestion, like corticosterone replacement, prevents the metabolic deficits of adrenalectomy

We tested whether corticosterone replacement causes increased sucrose drinking in adrenalectomized (ADX) rats compared to sham-ADX (sham) rats. ADX rats given high doses of corticosterone drank as much sucrose as sham rats, whereas at three lower doses of corticosterone, drinking was similar between groups and was only approximately 40% of that ingested by shams. Compared to sham rats, ADX rats drinking saline, or saline and saccharin, gain weight more slowly, contain less white adipose tissue, and have higher sympathetic outflow as assessed by uncoupling protein content in brown adipose tissue. Allowing sucrose as well as saline to drink restored all of these variables to normal in ADX rats with no- or low-corticosterone. All endpoints from sucrose-drinking ADX rats with no-or low-corticosterone were indistinguishable from those in water-drinking shams. By contrast, sucrose-drinking ADX rats that were given high doses of corticosterone exhibited the usual catabolic effects of corticosterone on body weight gain and, unlike sucrose-drinking shams, were obese. We conclude that (i) high corticosterone stimulates the potability of sucrose and inhibits sympathetic stimulation of uncoupling protein; (ii) sucrose, without corticosterone, normalizes metabolic deficits in ADX rats probably through actions mediated both peripherally and by the central nervous system; and (iii) ADX rats have a distinct sucrose appetite.

Peroxisome proliferators as adjuvants for the reverse-electron-transport therapy of obesity: an explanation for the large increase in metabolic rate of MEDICA 16-treated rats

The efficacy of reverse-electron-transport therapy of obesity should be promoted by agents which up-regulate hepatocyte enzymes that are potentially rate-limiting for mitochondrial fatty acid oxidation and electron shuttles. Peroxisome proliferator drugs, including the fibrates used to treat hyperlipidemia, may be useful in this regard, as they induce malic enzyme, the mitochondrial glycerol-3-phosphate dehydrogenase, and carnitine palmitoyl transferase I in rodent hepatocytes. An agent of this class, MEDICA 16, has the additional property of potently inhibiting both citrate lyase and acetyl-CoA carboxylase. As a result, methyl-substituted diacarboxylic acids (MEDICA) 16 can be expected to disinhibit hepatic fatty acid oxidation while up-regulating electron shuttle mechanisms, and thus should stimulate reverse electron transport. This may explain the remarkable 40% increase in basal metabolic rate observed in normal rats ingesting MEDICA 16--an effect not associated with any compensatory increase in food intake. Relative to controls, the MEDICA 16-treated rats achieved a 50% reduction in body fat and a modest increase in lean mass, such that weight and growth were not changed. In other rodent strains, MEDICA 16 has prevented obesity diabetes and atherogenesis. However, whether MEDICA 16 and other peroxisome proliferator drugs will have clinical utility in reverse-electron-transport therapy may hinge on their ability to induce key enzymes in human hepatocytes; cell culture studies to evaluate this are required.

Sesamin, a sesame lignan, is a potent inducer of hepatic fatty acid oxidation in the rat

The effects of sesamin, one of the most abundant lignans in sesame seed, on hepatic fatty acid oxidation were examined in rats that were fed experimental diets containing various amounts (0%, 0.1%, 0.2%, and 0.5%) of sesamin (a 1:1 mixture of sesamin and episesamin) for 15 days. Dietary sesamin dose-dependently increased both mitochondrial and peroxisomal palmitoyl-coenzyme A (CoA) oxidation rates. Mitochondrial activity almost doubled in rats on the 0.5% sesamin diet. Peroxisomal activity increased more than 10-fold in rats fed a 0.5% sesamin diet in relation to rats on the sesamin-free diet. Dietary sesamin greatly increased the hepatic activity of fatty acid oxidation enzymes, including carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increased the activity of 2,4-dienoyl-CoA reductase and delta3,delta2-enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for beta-oxidation of unsaturated fatty acids dose-dependently. Examination of hepatic mRNA levels using specific cDNA probes showed a sesamin-induced increase in the gene expression of mitochondrial and peroxisomal fatty acid oxidation enzymes. Among these various enzymes, peroxisomal acyl-CoA oxidase and bifunctional enzyme gene expression were affected most by dietary sesamin (15- and 50-fold increase by the 0.5% dietary level). Sesamin-induced alterations in the activity and gene expression of carnitine palmitoyltransferase I and acyl-CoA oxidase were in parallel with changes in the mitochondrial and peroxisomal palmitoyl-CoA oxidation rate, respectively. In contrast, dietary sesamin decreased the hepatic activity and mRNA abundance of fatty acid synthase and pyruvate kinase, the lipogenic enzymes. However, this lignan increased the activity and gene expression of malic enzyme, another lipogenic enzyme. An alteration in hepatic fatty acid metabolism may therefore account for the serum lipid-lowering effect of sesamin in the rat.

Tissue-specific effect of clofibrate on rat lipogenic enzyme gene expression

Fibrate derivatives are commonly used to treat hyperlipidaemia; however, the mechanism of the antilipidaemic action of these drugs is still unknown. The effect of clofibrate (fibrate derivative) administration for 14 days on lipogenesis and on malic enzyme (EC 1.1.1.40) and fatty acid synthase (EC 2.3.1.85) gene expression in brown and white adipose tissues and in the liver was examined in rats. The rate of brown adipose tissue lipogenesis in the clofibrate-treated animals was significantly lower than that of the control rats. The rate of liver and white adipose tissue lipogenesis was not affected significantly by clofibrate. In brown adipose tissue, the drug treatment resulted in a depression of fatty acid synthase and malic enzyme mRNA levels. The fatty acid synthase mRNA level did not change significantly in the liver, whereas the malic enzyme mRNA level increased approximately 6-fold in this organ after clofibrate treatment. The malic enzyme mRNA level in white adipose tissue increased about 2-fold, while the fatty acid synthase mRNA level was unchanged after clofibrate feeding. The results presented in this paper provide further evidence that the hypolipidaemia caused by treatment of rats with clofibrate cannot be related to the inhibition of fatty acid synthesis in the liver and white adipose tissue. These data also indicate that clofibrate exhibits tissue specificity.

The nuclear receptors peroxisome proliferator-activated receptor alpha and Rev-erbalpha mediate the species-specific regulation of apolipoprotein A-I expression by fibrates

Fibrates are widely used hypolipidemic drugs which activate the nuclear peroxisome proliferator-activated receptor (PPAR) alpha and thereby alter the transcription of genes controlling lipoprotein metabolism. Fibrates influence plasma high density lipoprotein and its major protein, apolipoprotein (apo) A-I, in an opposite manner in man (increase) versus rodents (decrease). In the present study we studied the molecular mechanisms of this species-specific regulation of apoA-I expression by fibrates. In primary rat and human hepatocytes fenofibric acid, respectively, decreased and increased apoA-I mRNA levels. The absence of induction of rat apoA-I gene expression by fibrates is due to 3 nucleotide differences between the rat and the human apoA-I promoter A site, rendering a positive PPAR-response element in the human apoA-I promoter nonfunctional in rats. In contrast, rat, but not human, apoA-I transcription is repressed by the nuclear receptor Rev-erbalpha, which binds to a negative response element adjacent to the TATA box of the rat apoA-I promoter. In rats fibrates increase liver Rev-erbalpha mRNA levels >10-fold. In conclusion, the opposite regulation of rat and human apoA-I gene expression by fibrates is linked to differences in cis-elements in their respective promoters leading to repression by Rev-erbalpha of rat apoA-I and activation by PPARalpha of human apoA-I. Finally, Rev-erbalpha is identified as a novel fibrate target gene, suggesting a role for this nuclear receptor in lipid and lipoprotein metabolism.

Effects of environmental synthetic chemicals on thyroid function

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

Fenofibrate modifies transaminase gene expression via a peroxisome proliferator activated receptor alpha-dependent pathway

Fibrates modify the expression of genes implicated in lipoprotein and fatty acid metabolism via the peroxisome proliferator-activated receptor alpha(PPARalpha), leading to reductions in serum triglycerides and cholesterol. The expression of certain genes regulated by PPARalpha have been shown to be modified in a species dependent manner. Aspartate aminotransferase (AspAT or GOT) and alanine aminotransferase (AlaAT or GPT) are enzymes involved in intermediate metabolism in all cells and in hepatic gluconeogenesis. These enzymes are also widely used as serum markers of possible tissue damage. This study investigated whether fenofibrate could modify the expression of liver AspAT and/or AlaAT and thus possibly alter transaminase levels independently of a cytotoxic effect. In human Hep G2 cells, fenofibrate increased cytosolic AspAT (cAspAT) activity by 40% and AlaAT activity by 100%, as well as both mRNAs. Nuclear run on assays showed that this effect was, at least in part, transcriptional. Increases in mRNA were also observed in human hepatocyte cultures at concentrations of the drug attained in patients. In C57BL/6 mice, fenofibrate decreased cAspAT and cAlaAT mRNA, while these effects were abolished in PPARalpha knock-out mice. In conclusion, fenofibrate has been shown to modify cAspAT and AlaAT gene expression in a species and PPARalpha dependent manner. This is the first demonstration that cAspAT and AlaAT activities may be pharmacologically altered, independently of a toxic phenomenon.

Mitochondria uncoupling by a long chain fatty acyl analogue

Mitochondria uncoupling by fatty acids in vivo is still questionable, being confounded by their dual role as substrates for oxidation and as putative genuine uncouplers of oxidative phosphorylation. To dissociate between substrate and the uncoupling activity of fatty acids in oxidative phosphorylation, the uncoupling effect was studied here using a nonmetabolizable long chain fatty acyl analogue. beta,beta'-Methyl-substituted hexadecane alpha,omega-dioic acid (MEDICA 16) is reported here to induce in freshly isolated liver cells a saturable oligomycin-insensitive decrease in mitochondrial proton motive force with a concomitant increase in cellular respiration. Similarly, MEDICA 16 induced a saturable decrease in membrane potential, proton gradient, and proton motive force in isolated liver and heart mitochondria accompanied by an increase in mitochondrial respiration. Uncoupling by MEDICA 16 in isolated mitochondria was partially suppressed by added atractyloside. Hence, fatty acids may act as genuine uncouplers of cellular oxidative phosphorylation by interacting with specific mitochondrial proteins, including the adenine nucleotide translocase.

Direct effects of leptin on brown and white adipose tissue

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

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.

Early bioenergetic changes in hepatocarcinogenesis: preneoplastic phenotypes mimic responses to insulin and thyroid hormone

Biochemical and molecular biological approaches in situ have provided compelling evidence for early bioenergetic changes in hepatocarcinogenesis. Hepatocellular neoplasms regularly develop from preneoplastic foci of altered hepatocytes, irrespective of whether they are caused by chemicals, radiation, viruses, or transgenic oncogenes. Two striking early metabolic aberrations were discovered: (1) a focal excessive storage of glycogen (glycogenosis) leading via various intermediate stages to neoplasms, the malignant phenotype of which is poor in glycogen but rich in ribosomes (basophilic), and (2) an accumulation of mitochondria in so-called oncocytes and amphophilic cells, giving rise to well-differentiated neoplasms. The metabolic pattern of human and experimentally induced focal hepatic glycogenosis mimics the phenotype of hepatocytes exposed to insulin. The conversion of the highly differentiated glycogenotic hepatocytes to the poorly differentiated cancer cells is usually associated with a reduction in gluconeogenesis, an activation of the pentose phosphate pathway and glycolysis, and an ever increasing cell proliferation. The metabolic pattern of preneoplastic amphophilic cell populations has only been studied to a limited extent. The few available data suggest that thyromimetic effects of peroxisomal proliferators and hepadnaviral infection may be responsible for the emergence of the amphophilic cell lineage of hepatocarcinogenesis. The actions of both insulin and thyroid hormone are mediated by intracellular signal transduction. It is, thus, conceivable that the early changes in energy metabolism during hepatocarcinogenesis are the consequence of alterations in the complex network of signal transduction pathways, which may be caused by genetic as well as epigenetic primary lesions, and elicit adaptive metabolic changes eventually resulting in the malignant neoplastic phenotype.