Identification of thyroid hormone response elements in the human fatty acid synthase promoter

Understanding the Roles of Selenium on Thyroid Hormone-Induced Thermogenesis in Adipose Tissue

Brown adipose tissue (BAT) and white adipose tissue (WAT) are known to regulate lipid metabolism. A lower amount of BAT compared to WAT, along with adipose tissue dysfunction, can result in obesity. Studies have shown that selenium supplementation protects against adipocyte dysfunction, decreases WAT triglycerides, and increases BAT triiodothyronine (T3). In this review, we discuss the relationship between selenium and lipid metabolism regulation through selenoprotein deiodinases and the role of deiodinases and thyroid hormones in the induction of adipose tissue thermogenesis. Upon 22 studies included in our review, we found that studies investigating the relationship between selenium and deiodinases demonstrated that selenium supplementation affects the iodothyronine deiodinase 2 (DIO2) protein and the expression of its associated gene, DIO2, proportionally. However, its effect on DIO1 is inconsistent while its effect on DIO3 activity is not detected. Studies have shown that the activity of deiodinases especially DIO2 protein and DIO2 gene expression is increased along with other browning markers upon white adipose tissue browning induction. Studies showed that thermogenesis is stimulated by the thyroid hormone T3 as its activity is correlated to the expression of other thermogenesis markers. A proposed mechanism of thermogenesis induction in selenium supplementation is by autophagy control. However, more studies are needed to establish the role of T3 and autophagy in adipose tissue thermogenesis, especially, since some studies have shown that thermogenesis can function even when T3 activity is lacking and studies related to autophagy in adipose tissue thermogenesis have contradictory results.

Thyroid Hormone Signaling and the Liver

Thyroid hormone (TH) plays a critical role in maintaining metabolic homeostasis throughout life. It is well known that the liver and thyroid are intimately linked, with TH playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Indeed, patients with hypothyroidism have abnormal lipid panels with higher levels of low-density lipoprotein levels, triglycerides (triacylglycerol; TAG), and apolipoprotein B levels. Even in euthyroid patients, lower serum-free thyroxine levels are associated with higher total cholesterol levels, LDL, and TAG levels. In addition to abnormal serum lipids, the risk of nonalcoholic fatty liver disease (NAFLD) increases with lower free thyroxine levels. As free thyroxine rises, the risk of NAFLD is reduced. This has led to numerous animal studies and clinical trials investigating TH analogs and TH receptor agonists as potential therapies for NAFLD and hyperlipidemia. Thus, TH plays an important role in maintaining hepatic homeostasis, and this continues to be an important area of study. A review of TH action and TH actions on the liver will be presented here.

A Transgenic Mouse Model for Detection of Tissue-Specific Thyroid Hormone Action

Thyroid hormone (TH) is present in the systemic circulation and thus should affect all cells similarly in the body. However, tissues have a complex machinery that allows tissue-specific optimization of local TH action that calls for the assessment of TH action in a tissue-specific manner. Here, we report the creation of a TH action indicator (THAI) mouse model to study tissue-specific TH action. The model uses a firefly luciferase reporter readout in the context of an intact transcriptional apparatus and all elements of TH metabolism and transport and signaling. The THAI mouse allows the assessment of the changes of TH signaling in tissue samples or in live animals using bioluminescence, both in hypothyroidism and hyperthyroidism. Beyond pharmacologically manipulated TH levels, the THAI mouse is sufficiently sensitive to detect deiodinase-mediated changes of TH action in the interscapular brown adipose tissue (BAT) that preserves thermal homeostasis during cold stress. The model revealed that in contrast to the cold-induced changes of TH action in the BAT, the TH action in this tissue, at room temperature, is independent of noradrenergic signaling. Our data demonstrate that the THAI mouse can also be used to test TH receptor isoform-specific TH action. Thus, THAI mouse constitutes a unique model to study tissue-specific TH action within a physiological/pathophysiological context and test the performance of thyromimetics. In conclusion, THAI mouse provides an in vivo model to assess a high degree of tissue specificity of TH signaling, allowing alteration of tissue function in health and disease, independently of changes in circulating levels of TH.

Triiodothyronine enhances accumulation of intracellular lipids in adipocytes through thyroid hormone receptor α via direct and indirect mechanisms

Triiodothyronine (T3) enhanced the expression of adipogenic and lipogenic genes with elevation of the intracellular lipids through thyroid hormone receptor (TR) α in mouse 3T3-L1 cells. However, the transcription of the SREBP-1c and HSL genes was decreased by T3. Such T3-mediated alterations were negated by TRα siRNA. Chromatin immunoprecipitation assay showed that the binding of TRα to the TR-responsive element (TRE) of the FAS promoter was elevated by T3. In contrast, the ability of TRα to bind to the TRE of the SREBP-1c promoter was decreased by T3. In addition, the binding of SREBP-1c to the SRE of the HSL promoter was lowered by T3. These results indicate that T3 increased the accumulation of intracellular lipids by enhancing the expression of the FAS gene through direct binding of TRα to the FAS promoter and simultaneously lowered the amount of lipolysis via reduced binding of T3-decreased SREBP-1c to the HSL promoter.

The thyroid hormone-αvβ3 integrin axis in ovarian cancer: regulation of gene transcription and MAPK-dependent proliferation

Ovarian carcinoma is the fifth common cause of cancer death in women, despite advanced therapeutic approaches. αvβ3 integrin, a plasma membrane receptor, binds thyroid hormones (L-thyroxine, T4; 3,5,3'-triiodo-L-thyronine, T3) and is overexpressed in ovarian cancer. We have demonstrated selective binding of fluorescently labeled hormones to αvβ3-positive ovarian cancer cells but not to integrin-negative cells. Physiologically relevant T3 (1 nM) and T4 (100 nM) concentrations in OVCAR-3 (high αvβ3) and A2780 (low αvβ3) cells promoted αv and β3 transcription in association with basal integrin levels. This transcription was effectively blocked by RGD (Arg-Gly-Asp) peptide and neutralizing αvβ3 antibodies, excluding T3-induced β3 messenger RNA, suggesting subspecialization of T3 and T4 binding to the integrin receptor pocket. We have provided support for extracellular regulated kinase (ERK)-mediated transcriptional regulation of the αv monomer by T3 and of β3 monomer by both hormones and documented a rapid (30-120 min) and dose-dependent (0.1-1000 nM) ERK activation. OVCAR-3 cells and αvβ3-deficient HEK293 cells treated with αvβ3 blockers confirmed the requirement for an intact thyroid hormone-integrin interaction in ERK activation. In addition, novel data indicated that T4, but not T3, controls integrin's outside-in signaling by phosphorylating tyrosine 759 in the β3 subunit. Both hormones induced cell proliferation (cell counts), survival (Annexin-PI), viability (WST-1) and significantly reduced the expression of genes that inhibit cell cycle (p21, p16), promote mitochondrial apoptosis (Nix, PUMA) and tumor suppression (GDF-15, IGFBP-6), particularly in cells with high integrin expression. At last, we have confirmed that hypothyroid environment attenuated ovarian cancer growth using a novel experimental platform that exploited paired euthyroid and severe hypothyroid serum samples from human subjects. To conclude, our data define a critical role for thyroid hormones as potent αvβ3-ligands, driving ovarian cancer cell proliferation and suggest that disruption of this axis may present a novel treatment strategy in this aggressive disease.

Thyroid hormone and retinoid X receptor function and expression during sea lamprey (Petromyzon marinus) metamorphosis

Sea lampreys (Petromyzon marinus) are members of the ancient class Agnatha and undergo a metamorphosis that transforms blind, sedentary, filter-feeding larvae into free-swimming, parasitic juveniles. Thyroid hormones (THs) appear to be important for lamprey metamorphosis, however, serum TH concentrations are elevated in the larval phase, decline rapidly during early metamorphosis and remain low until metamorphosis is complete; these TH fluctuations are contrary to those of other metamorphosing vertebrates. Moreover, thyroid hormone synthesis inhibitors (goitrogens) induce precocious metamorphosis and exogenous TH treatments disrupt natural metamorphosis in P. marinus. Given that THs exert their effects by binding to TH nuclear receptors (TRs) that often act as heterodimers with retinoid X receptors (RXRs), we cloned and characterized these receptors from P. marinus and examined their expression during metamorphosis. Two TRs (PmTR1 and PmTR2) and three RXRs (PmRXRs) were isolated from P. marinus cDNA. Phylogenetic analyses group the PmTRs together on a branch prior to the gnathostome TRα/β split. The three RXRs also group together, but our data indicated that these transcripts are most likely either allelic variants of the same gene locus, or the products of a lamprey-specific duplication event. Importantly, these P. marinus receptors more closely resemble vertebrate as opposed to invertebrate chordate receptors. Functional analysis revealed that PmTR1 and PmTR2 can activate transcription of TH-responsive genes when treated with nanomolar concentrations of TH and they have distinct pharmacological profiles reminiscent of vertebrate TRβ and TRα, respectively. Also similar to other metamorphosing vertebrates, expression patterns of the PmTRs during lamprey metamorphosis suggest that PmTR1 has a dynamic, tissue-specific expression pattern that correlates with tissue morphogenesis and biochemical changes and PmTR2 has a more uniform expression pattern. This TR expression data suggests that THs, either directly or via a metabolite, may function to positively modulate changes at the tissue or organ levels during lamprey metamorphosis. Collectively the results presented herein support the hypothesis that THs have a dual functional role in the lamprey life cycle whereby high levels promote larval feeding, growth and lipogenesis and low levels promote metamorphosis.

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

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

Influence of neonatal hypothyroidism on hepatic gene expression and lipid metabolism in adulthood

Thyroid hormones are required for normal growth and development in mammals. Congenital-neonatal hypothyroidism (CH) has a profound impact on physiology, but its specific influence in liver is less understood. Here, we studied how CH influences the liver gene expression program in adulthood. Pregnant rats were given the antithyroid drug methimazole (MMI) from GD12 until PND30 to induce CH in male offspring. Growth defects due to CH were evident as reductions in body weight and tail length from the second week of life. Once the MMI treatment was discontinued, the feed efficiency increased in CH, and this was accompanied by significant catch-up growth. On PND80, significant reductions in body mass, tail length, and circulating IGF-I levels remained in CH rats. Conversely, the mRNA levels of known GH target genes were significantly upregulated. The serum levels of thyroid hormones, cholesterol, and triglycerides showed no significant differences. In contrast, CH rats showed significant changes in the expression of hepatic genes involved in lipid metabolism, including an increased transcription of PPARα and a reduced expression of genes involved in fatty acid and cholesterol uptake, cellular sterol efflux, triglyceride assembly, bile acid synthesis, and lipogenesis. These changes were associated with a decrease of intrahepatic lipids. Finally, CH rats responded to the onset of hypothyroidism in adulthood with a reduction of serum fatty acids and hepatic cholesteryl esters and to T3 replacement with an enhanced activation of malic enzyme. In summary, we provide in vivo evidence that neonatal hypothyroidism influences the hepatic transcriptional program and tissue sensitivity to hormone treatment in adulthood. This highlights the critical role that a euthyroid state during development plays on normal liver physiology in adulthood.

Genetic evidence that thyroid hormone is indispensable for prepubertal insulin-like growth factor-I expression and bone acquisition in mice

Understanding how bone growth is regulated by hormonal and mechanical factors during early growth periods is important for optimizing the attainment of peak bone mass to prevent or postpone the occurrence of fragility fractures later in life. Using genetic mouse models that are deficient in thyroid hormone (TH) (Tshr(-/-) and Duox2(-/-)), growth hormone (GH) (Ghrhr(lit/lit)), or both (Tshr(-/-); Ghrhr(lit/lit)), we demonstrate that there is an important period prior to puberty when the effects of GH are surprisingly small and TH plays a critical role in the regulation of skeletal growth. Daily administration of T3/T4 during days 5 to 14, the time when serum levels of T3 increase rapidly in mice, rescued the skeletal deficit in TH-deficient mice but not in mice lacking both TH and GH. However, treatment of double-mutant mice with both GH and T3/T4 rescued the bone density deficit. Increased body fat in the TH-deficient as well as TH/GH double-mutant mice was rescued by T3/T4 treatment during days 5 to 14. In vitro studies in osteoblasts revealed that T3 in the presence of TH receptor (TR) α1 bound to a TH response element in intron 1 of the IGF-I gene to stimulate transcription. In vivo studies using TRα and TRβ knockout mice revealed evidence for differential regulation of insulin-like growth factor (IGF)-I expression by the two receptors. Furthermore, blockade of IGF-I action partially inhibited the biological effects of TH, thus suggesting that both IGF-I-dependent and IGF-I-independent mechanisms contribute to TH effects on prepubertal bone acquisition.

Effects of farnesoid X receptor on the expression of the fatty acid synthetase and hepatic lipase

The farnesoid X receptor (FXR) is a nuclear receptor that regulates gene expression in response to bile acids (BAs). FXR plays an important role in the homeostasis of bile acid, cholesterol, lipoprotein and triglyceride. In this report, we identified fatty acid synthase (FAS) and hepatic lipase (HL) genes as novel target genes of FXR. Human hepatoma HepG2 cells were treated with chenodeoxycholic acid, the natural FXR ligand, and the messenger RNA and protein levels of FAS and HL were determined by RT-PCR and Western blot analysis, respectively. Chenodeoxycholic acid (CDCA) down-regulated the expression of FAS and HL genes in a dose and time-dependent manner in human hepatoma HepG2 cells. In addition, treatment of mice with CDCA significantly decreased the expression of FAS and HL in mouse liver and the activity of HL. These results demonstrated that FAS and HL might be FXR-regulated genes in liver cells. In view of the role of FAS and HL in lipogenesis and plasma lipoprotein metabolism, our results further support the central role of FXR in the homeostasis of fatty acid and lipid.

Subcutaneous fat shows higher thyroid hormone receptor-alpha1 gene expression than omental fat

The aims of this work were to evaluate thyroid hormone receptor-alpha (TR alpha), TR alpha 1, and TR alpha 2 mRNA gene expression and TR alpha 1:TR alpha 2 ratio, identified as candidate factors for explaining regional differences between human adipose tissue depots. TR alpha, TR alpha 1, and TR alpha 2 mRNA levels, and the gene expressions of arginine-serine-rich, splicing factor 2 (SF2), heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1), heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1), and Spot 14 (S14) were evaluated in 76 paired adipose tissue samples obtained from a population of 38 women who varied widely in terms of obesity and body fat distribution. Gene expression for these factors was also studied in stromal-vascular cells (SVCs) and mature adipocytes (MAs) from eight paired fat depots. TR alpha gene and TR alpha 1 mRNA expression were increased 1.46-fold (P = 0.006) and 1.80-fold (P < 0.0001), respectively, in subcutaneous (SC) vs. visceral fat. These differences in gene expression levels were most significant in the obese group, in which the TR alpha 1:TR alpha 2 ratio was 2.24-fold (P < 0.0001) higher in SC vs. visceral fat. S14 gene expression was also increased by 2.42-fold (P < 0.0001) and correlated significantly with TR alpha and TR alpha 1 gene expression and with the TR alpha 1:TR alpha 2 ratio. In agreement with these findings, hnRNP A1:SF2 ratio was decreased by 1.39-fold (P = 0.001). TR alpha and S14 levels were 2.1-fold (P < 0.0001) and 112.4-fold (P < 0.0001), respectively, higher in MAs than in SVCs from both fat depots. In summary, genes for TR-alpha, their upstream regulators, and downstream effectors were differentially expressed in SC vs. omental (OM) adipose tissue. Our findings suggest that TR alpha1 could contribute to SC adipose tissue expandability in obese subjects.

A key role for orphan nuclear receptor liver receptor homologue-1 in activation of fatty acid synthase promoter by liver X receptor

Liver X receptor (LXR) activates fatty acid synthase (FAS) gene expression through binding to a DR-4 element in the promoter. We show that a distinct nuclear receptor half-site 21 bases downstream of the DR-4 element is also critical for the response of FAS to LXR but is not involved in LXR binding to DNA. This half-site specifically binds liver receptor homologue-1 (LRH-1) in vitro and in vivo, and we show LRH-1 is required for maximal LXR responsiveness of the endogenous FAS gene as well as from promoter reporter constructs. We also demonstrate that LRH-1 stimulation of the FAS LXR response is blocked by the addition of small heterodimer partner (SHP) and that FAS mRNA is overexpressed in SHP knock-out animals, providing evidence that FAS is an in vivo target of SHP repression. Taken together, these findings identify the first direct lipogenic gene target of LRH-1/SHP repression and provide a mechanistic explanation for bile acid repression of FAS and lipogenesis recently reported by others.

Tissue-specific regulation of acetyl-CoA carboxylase gene expression by dietary soya protein isolate in rats

We have recently reported that intake of soya protein isolate (SPI) inhibited the DNA-binding activities of hepatic thyroid hormone receptor (TR). The genes for acetyl-CoA carboxylase (ACC), a rate-limiting enzyme in fatty acid synthesis, contain the thyroid hormone response element in their promoters and are regulated by TR. The present study has examined the effect oflong-term feeding of SPI and soya isoflavones (ISF) on the gene expression and protein phosphorylation of different ACC isoforms in different tissues and plasma triacylglycerol (TAG) levels in rats. Sprague-Dawley female rats were fed diets containing 20% casein or alcohol-washed SPI with or without supplemental ISF for 70, 190 and 310d. SPI intake significantly reduced plasma TAG concentrations compared with casein, whereas supplemental ISF had no effect. Hepatic ACCα and ACCβ mRNA abundance and protein content were markedly lowerin the rats fed SPI than in those fed casein. The protein contents of ACCα in the kidneyand ACCβ, the predominant isoform in the heart and kidney, were unchanged by dietary SPI.The ratios of phospho-ACCα/ACCα and phospho-ACCβ/ACCβ were not differentamong dietary groups in all tissues measured. The present study demonstrates that ingestion ofSPI decreases plasma TAG level and down-regulates ACCα and ACCβ gene expression in the liver but not in the heart and kidney. The results indicate that the effect of SPI is tissue-specific and that alteration of ACC gene expression rather than phosphorylation status may play a major role in the regulation of ACC activities by soya proteins.

Coordinated control of bile acids and lipogenesis through FXR-dependent regulation of fatty acid synthase

We discovered a nuclear receptor element in the FAS promoter consisting of an inverted repeat spaced by one nucleotide (IR-1) and located 21 bases downstream of a direct repeat sequenced by 4 nucleotides (DR-4) oxysterol liver X receptor response element. An IR-1 is present in promoters of several genes of bile acid and lipid homeostasis and binds farnesoid X receptor/retinoid X receptor (FXR/RXR) heterodimers to mediate bile acid-dependent transcription. We show that FXR/RXRalpha specifically binds to the FAS IR-1 and that the FAS promoter is activated approximately 10-fold by the addition of a synthetic FXR agonist in transient transfection assays. We also demonstrate that endogenous FXR binds directly to the murine FAS promoter in the hepatic genome using a tissue-based chromatin immunoprecipitation procedure. Furthermore, we show that feeding wild-type mice a chow diet supplemented with the natural FXR agonist chenodeoxycholic acid results in a significant induction of FAS mRNA expression. Thus, we have identified a novel IR-1 in the FAS promoter and demonstrate that it mediates FXR/bile acid regulation of the FAS gene. These findings provide the first evidence for direct regulation of lipogenesis by bile acids and also provide a mechanistic rationale for previously unexplained observations regarding bile acid control of FAS expression.

The Lipoprotein Lipase Inhibitor ANGPTL3 Is Negatively Regulated by Thyroid Hormone

Whereas the role of thyroid hormone is clearly established in the regulation of cholesterol homeostasis, its involvement in the control of serum triglyceride (TG) levels remains largely debated. Angiopoietin-like proteins 3 and 4 have recently been characterized as potent lipoprotein lipase inhibitors and therefore as important components of plasma triglyceride homeostasis. In the present study, the role of thyroid hormone in the regulation of both ANGPTL4 and ANGPTL3 gene expression was investigated. In vivo studies revealed that thyroid hormone down-regulates ANGPTL3 but not ANGPTL4 gene expression in hypothyroid rats. Using thyroid hormone receptor (TR)-deficient mice, we show that thyroid hormone regulates ANGPTL3 gene expression in a TRbeta-dependent manner. Transfection studies revealed that this inhibition occurs at the transcriptional level in a DNA binding-independent fashion and requires the proximal (-171 to +66) region of the ANGPTL3 gene promoter. Moreover, site-directed mutagenesis experiments indicate that the HNF1 site within this proximal region mediates this TRbeta-dependent repression. Finally, co-transfection studies and electrophoretic mobility shift assays suggest that TRbeta antagonizes the HNF1alpha signaling pathway by inhibiting its transcriptional activity without interfering with its DNA-binding capacity. Taken together, our results lead to the identification of ANGPTL3 as a novel TRbeta target gene and provide a new potential mechanism to explain the hypotriglyceridemic properties of TRbeta agonists in vivo.

KGF induces lipogenic genes through a PI3K and JNK/SREBP-1 pathway in H292 cells

Lipid synthesis is required for cell growth and is subject to pharmacologic regulation. Keratinocyte growth factor (KGF) stimulates proliferation and lipogenesis in H292 cells, a pulmonary epithelial cancer cell line, but the signaling pathways are not known. KGF stimulated the expression of the transcription factors sterol-regulatory element binding protein-1 (SREBP-1), CCAAT/enhancer binding protein alpha (C/EBPalpha), and C/EBPdelta and two key enzymes involved in lipogenesis, FAS and stearoyl coenzyme A desaturase-1 (SCD-1). We found that KGF induced rapid activation of Akt, p70 S6K, JNK, and extracellular signal-regulated (ERK). Induction of SREBP-1, SCD-1, and FAS by KGF was inhibited by the JNK inhibitor SP600125 and the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 but not by the ERK inhibitor PD98059. Using FAS and SCD-1-luciferase promoter constructs, we observed that KGF stimulated the transcription of these promoters and that exogenous cholesterol inhibited the induction. Mutation of the SREBP-1 binding site in the SCD-1 promoter abolished the effect of KGF on SCD-1 transcription. In addition, overexpression of active SREBP-1 directly stimulated SCD-1 and FAS. Conversely, adenovirus-mediated overexpression of a dominant negative form of SREBP-1 inhibited the KGF effect on FAS and SCD-1 expression. In summary, we conclude that KGF requires both PI3K and JNK signaling pathways to induce SREBP-1, which in turn induces SCD-1 and FAS expression in H292 cells.

Effect of thyroxine on abnormal pancreatic proteomes of the hypothyroid rdw rat

A mutation in the thyroglobulin (Tg) gene is the primary cause of hereditary dwarfism and hypothyroidism in the rdw rat. Despite the Tg mutation that causes a Tg shortage, rdw rats survive. The present study examines the influences of this condition on the pancreatic proteome. Normal control (group 1; n = 19) and rdw rats that did not receive L-thyroxine (T4) (group 2; n = 27) were sacrificed from 4 to 56 weeks after birth. The rdw rats were supplemented either with daily intraperitoneal injections of T4 from 3 to 28 days after birth (group 3; n = 4) or with normal thyroid tissues grafted at 4 weeks of age (group 4; n = 3). Groups 3 and 4 were sacrificed 12 weeks after birth. Pancreatic proteomes analyzed by two-dimensional gel electrophoresis showed that levels of at least four pancreatic proteins were higher in group 2 than in group 1, and that those of four were lower. Cluster decomposition and principal component analysis of the eight protein contents showed that groups 1 and 2 were separated into two clusters and that pancreatic proteomes of group 4 were better normalized than those of group 3. Injecting T4 into group 3 was temporarily effective, whereas the thyroid graft to group 4 provided a continuous positive effect, which concurred with the increased body weight of the other two groups of rdw rats that received grafts of normal thyroids.

Soy protein isolate increases hepatic thyroid hormone receptor content and inhibits its binding to target genes in rats

Our previous studies showed that intake of 20% alcohol-washed soy protein isolate (SPI) significantly increased hepatic thyroid hormone receptor (TR) beta1 protein content in rats. However, whether SPI influences the binding ability of TR to its target genes is unknown. The purpose of this study was to examine the effect of increasing amounts of dietary SPI on hepatic TRbeta1 content and the binding of TR to thyroid hormone response element (TRE) in rats. Sprague-Dawley rats (28 d old) were fed diets containing casein (20%) with or without isoflavone supplementation (50 mg/kg diet) or alcohol-washed SPI (5, 10, or 20%) for 90 d. The hepatic TRbeta1 protein content was measured by Western blot, and the binding ability of TR to DNA was examined by electrophoretic mobility shift assay. Consumption of the 20% SPI diet increased pancreatic relative weight and decreased spleen relative weight. Intake of SPI markedly elevated TRbeta1 content in both male and female rats compared with a casein-based control diet. The increase in TRbeta1 in females was much higher than that in males. Interestingly, the binding abilities of TR to DNA were significantly inhibited by increasing amounts of dietary SPI in female rats. In conclusion, this study shows for the first time that dietary SPI increases hepatic TRbeta1 protein content and inhibits the binding of TR to target genes. Modulation of hepatic TRbeta1, a key regulator of gene expression involved in lipid metabolism, by SPI may be a novel mechanism by which soy components lower blood lipid level and exert their hypocholesterolemic actions.

Thyroid hormone regulates the hypotriglyceridemic gene APOA5

The apolipoprotein AV gene (APOA5) is a key determinant of plasma triglyceride levels, a major risk factor for coronary artery disease and a biomarker for the metabolic syndrome. Since thyroid hormones influence very low density lipoprotein triglyceride metabolism and clinical studies have demonstrated an inverse correlation between thyroid status and plasma triglyceride levels, we examined whether APOA5 is regulated by thyroid hormone. Here we report that 3,5,3'-triiodo-L-thyronine (T3) and a synthetic thyroid receptor beta (TRbeta) ligand increase APOA5 mRNA and protein levels in hepatocytes. Our data revealed that T3-activated TR directly regulates APOA5 promoter through a functional direct repeat separated by four nucleotides (DR4). Interestingly, we show that upstream stimulatory factor 1, a transcription factor associated with familial combined hyperlipidemia and elevated triglyceride levels in humans, and upstream stimulatory factor 2 cooperate with TR, resulting in a synergistic activation of APOA5 promoter in a ligand-dependent manner via an adjacent E-box motif. In rats, we observed that apoAV levels declines with thyroid hormone depletion but returned to normal levels upon T3 administration. In addition, treatments with a TRbeta-selective agonist increased apoAV and diminished triglyceride levels. The identification of APOA5 as a T3 target gene provides a new potential mechanism whereby thyroid hormones can influence triglyceride homeostasis. Additionally, these data suggest that TRbeta may be a potential pharmacological target for the treatment of hypertriglyceridemia.

Multi-tissue gene-expression analysis in a mouse model of thyroid hormone resistance

BACKGROUND

Resistance to thyroid hormone (RTH) is caused by mutations of the thyroid hormone receptor beta (TRbeta) gene. To understand the transcriptional program underlying TRbeta mutant-induced phenotypic expression of RTH, cDNA microarrays were used to profile the expression of 11,500 genes in a mouse model of human RTH.

RESULTS

We analyzed transcript levels in cerebellum, heart and white adipose tissue from a knock-in mouse (TRbetaPV/PV mouse) that harbors a human mutation (referred to as PV) and faithfully reproduces human RTH. Because TRbetaPV/PV mice have elevated thyroid hormone (T3), to define T3-responsive genes in the context of normal TRbeta, we also analyzed T3 effects in hyperthyroid wild-type gender-matched littermates. Microarray analysis revealed 163 genes responsive to T3 treatment and 187 genes differentially expressed between TRbetaPV/PV mice and wild-type littermates. Both the magnitude and gene make-up of the transcriptional response varied widely across tissues and conditions. We identified genes modulated in T3-dependent PV-independent, T3- and PV-dependent, and T3-independent PV-dependent pathways that illuminated the biological consequences of PV action in vivo. Most T3-responsive genes that were dysregulated in the heart and white adipose tissue of TRbetaPV/PV mice were repressed in T3-treated wild-type mice and upregulated in TRbetaPV/PV mice, suggesting the inappropriate activation of T3-suppressed genes in RTH.

CONCLUSIONS

Comprehensive multi-tissue gene-expression analysis uncovered complex multiple signaling pathways that mediate the molecular actions of TRbeta mutants in vivo. In particular, the T3-independent mutant-dependent genomic response unveiled the contribution of a novel 'change-of-function' of TRbeta mutants to the pathogenesis of RTH. Thus, the molecular actions of TRbeta mutants are more complex than previously envisioned.

Dietary soy protein isolate and isoflavones modulate hepatic thyroid hormone receptors in rats

Thyroid hormone receptors (TRs) are regulators of many genes involved in cholesterol and lipid metabolism. The purpose of this study was to examine the effect of soy protein isolate (SPI) and isoflavones on hepatic TRs in rats. In Expt. 1, Sprague-Dawley rats were fed diets containing either casein or alcohol-washed SPI with or without isoflavone supplementation (5-1250 mg/kg diet) for 70, 190, and 310 d. The offspring (F1) were fed the same diets as their parents (F0). In Expt. 2, Sprague-Dawley rats were fed diets containing casein or casein plus isoflavones (50-400 mg/kg diet) for 120 d. The mRNA and protein contents of the hepatic TRs were measured by semiquantitative RT-PCR and Western blot, respectively. TRalpha1, TRalpha2, and TRbeta2 contents were not affected by SPI. However, the content of the 52-kDa TRbeta1 protein, the major isoform present in the liver, was markedly increased by dietary SPI in both sexes of F0 and F1 compared with casein. The supplemental isoflavones had no effect on TRbeta1, whereas the high doses of isoflavones (250 and 1250 mg/kg diet) reduced the hepatic TRalpha1 protein content in F1 male rats on d 28. SPI had no effect on total T3 and T4 levels. However, higher dose of supplemental isoflavones markedly increased T4 level in female rats. Overall, this study demonstrates for the first time that SPI upregulates hepatic TRbeta1 expression, and that isoflavones reduce the hepatic TRalpha1 level in young male rats. The SPI-induced TRbeta1 may play a role in mediating the hypocholesterolemic and lipid-lowering actions of soy protein.

Human acetyl-CoA carboxylase 1 gene: presence of three promoters and heterogeneity at the 5'-untranslated mRNA region

Acetyl-CoA carboxylase 1 (ACC1) catalyzes the formation of malonyl-CoA, the C2 donor for de novo synthesis of long-chain fatty acids. We have identified 64 exons, including 7 alternatively spliced minor exons (1A, 1B, 1C, 3, 5A', 5A, and 5B) in human ACC1 gene ( approximately 330 kb). The gene is regulated by three promoters (PI, PII, and PIII), which are located upstream of exons 1, 2, and 5A, respectively. PI is a constitutive promoter and has no homology with the PI sequences of other mammalian ACC1. PII is regulated by various hormones. PIII is expressed in a tissue-specific manner. The presence of several alternatively spliced exons does not alter the translation of the 265-kDa ACC1 protein starting from an ATG present in exon 5. Translation of PIII transcripts from exon 5A generates a 259-kDa isoform in which the N-terminal 75 aa of 265-kDa ACC1 are replaced with a new sequence of 17 aa. Interestingly, the inclusion of exon 5B between 5A and 6 in PIII transcripts would yield a third 257-kDa isoform, which is translated from an ATG in exon 6. However, the presence of exon 5B in PI and PII transcripts leads to an in-frame stop codon that results in an ACC1-related 77-aa peptide. The presence of alternatively spliced exons and three isoforms of ACC1 could contribute to overall ACC1 activity either by influencing the mRNA stability and translational efficiency or by increasing the stability and specific activity of the ACC1 protein, respectively.

Paradoxical triiodothyronine suppression of S14 transcription in permanent hepatic cell lines

Both in vivo and in primary rat hepatocyte culture, carbohydrate and triiodothyronine (T(3)) rapidly induce transcription of the rat S14 gene. To determine if regulation of this gene by T(3) is similar in human liver cells, we transfected the S14 upstream region into HepG2 cells. We chose this cell line because many others have used this cell line to study the effect of thyroid hormone on hepatic gene expression. We found that changing media glucose concentration did not affect S14 transcription. Furthermore, addition of T(3) to HepG2 cells caused a marked reduction of rat S14 transcription. This paradoxical reduction was dependent on cotransfection of the T(3) receptor. We obtained similar results in the other human hepatoma cell lines, HuH-7 and Hep3B. The paradoxical response was not limited to human cells. We found a similar response in the nonmalignant permanent mouse liver cell line, AML-12. This paradoxical response was specific to the S14 gene because transfection of all the cell lines with a CAT or luciferase reporter driven by a mouse mammary tumor virus promoter containing 1 or 4 copies of a palindromic thyroid hormone response element (TRE) showed marked induction by T(3). Our results show that T(3) abnormally regulates the S14 gene in proliferating liver cell lines of diverse origins. This paradoxical regulation by T(3) is caused by an interaction between T(3) and the thyroid hormone receptor. The factors that lead to this paradoxical response are not active in primary hepatocytes and normal intact liver.

SREBP-1 integrates the actions of thyroid hormone, insulin, cAMP, and medium-chain fatty acids on ACCalpha transcription in hepatocytes

In chick embryo hepatocytes, activation of acetyl-CoA carboxylase-alpha (ACCalpha) transcription by 3,5,3'-triiodothyronine (T3) is mediated by a cis-acting regulatory unit (-101 to -71 bp) that binds the nuclear T3 receptor (TR) and sterol regulatory element-binding protein-1 (SREBP-1). SREBP-1 directly interacts with TR on the ACCalpha gene to enhance T3-induced transcription. Here, we show that treating hepatocytes with T3 or insulin stimulates a 4-fold increase in the concentration of the mature, active form of SREBP-1. When T3 and insulin are added together, a 7-fold increase in the mature SREBP-1 concentration is observed. Time course studies indicate that the T3-induced increase in mature SREBP-1 abundance is closely associated with changes in ACCalpha transcription and that the mechanism mediating the effect of T3 on mature SREBP-1 is distinct from that mediating the effect of insulin. Transfection analyses indicate that inhibition of ACCalpha transcription by cAMP or hexanoate is mediated by ACCalpha sequences between -101 and -71 bp. Treatment with cAMP or hexanoate suppresses the increase in mature SREBP-1 abundance caused by T3 and insulin. These results establish a new interaction between the SREBP-1 and TR signaling pathways and provide evidence that SREBP-1 plays an active role in mediating the effects of T3, insulin, cAMP, and hexanoate on ACCalpha transcription.

Thyroid Hormone Stimulates Renin Gene Expression Through the Thyroid Hormone Response Element

-We previously reported that thyroid hormone stimulates renin synthesis in vivo and in vitro. Here, we analyzed the 5'-flanking sequence of the human renin gene for promoter activity responsive to thyroid hormone using Calu-6 cells, which secrete renin endogenously and express thyroid hormone receptor-ss. The luciferase reporter gene was cloned together with 5'-flanking portions of the human renin gene of various lengths into the pGL3-Basic vector. Luciferase activity assays were performed using the Dual Luciferase Reporter Assay System. 3,3',5-Triiodo-L-thyronine stimulated the promoter activity of pGL3-Basic-1111/+12 and pGL3-Basic-1298/+12 by 2.3+/-0.1- and 1.7+/-0.1-fold, respectively. Shorter constructs (pGL3-Basic-144/+12, pGL3-Basic-226/+12, pGL3-Basic-452/+12, and pGL3-Basic-953/+12) were not stimulated by thyroid hormone. These results suggest that there is a possible thyroid hormone response element (5'-AGG TCA GGT CAc aat GTT CCT-3') between nucleotides -1111 and -953. In 3 constructs with site-directed mutations in this sequence, basal promoter activities were significantly increased, whereas promoter activation by thyroid hormone was abolished. Electrophoretic mobility shift assays showed that the -1111/-953 DNA fragment of the intact human renin gene was bound to nuclear proteins of Calu-6 cells; however, none of the 3 mutant probes were bound to any nuclear proteins. These results suggest that thyroid hormone stimulates the promoter activity of the human renin gene through thyroid hormone response element-dependent mechanisms in Calu-6 cells.

Regulation of the rat liver carnitine palmitoyltransferase I gene transcription by thyroid hormone

L-CPT I isotype is the main locus of control for liver LCFA oxidation. T3 levels have been described as controlling L-CPT I gene expression, and in this paper we demonstrate that rat liver CPT I promoter responds to T3. Using deleted reporter constructs we located the thyroid hormone-responsive element between -2935 and -2918, consisting of a DR4. This response is mediated by the binding of the thyroid to this sequence as a monomer, homodimer, or heterodimer with RXR.

The activity of fatty acid synthase of epidermal keratinocytes is regulated in the lower stratum spinousum and the stratum basale by local inflammation rather than by circulating hormones

The epidermal keratinocytes produce and secrete lipids to maintain the water barrier of the epidermis. To clarify the regulation of epidermal lipid synthesis, we investigated the hormonal effect on the activity of fatty acid synthase (FAS) of the keratinocytes, and the expression of FAS in the human skin. In cultured keratinocytes, the FAS activity, assayed by measuring the oxidation of NADPH, was slightly increased by hydrocortisone or testosterone, but not influenced by thyroid hormone, estrogen, progesterone or insulin. In immunohistochemical study of normal human epidermis, FAS was expressed strongly in the stratum granulosum and moderately in the uppermost layer of the stratum spinousum (SS), suggesting that fatty acid synthesis may increase during normal epidermal differentiation. In inflammatory disorders, such as psoriasis, lichen planus, and atopic dermatitis, FAS was also expressed in the lower SS and the stratum basale (SB), resulting in strong staining in the whole layers of the epidermis. Remarkable increase of FAS expression was only observed in the lower SS and the SB. Therefore, the activity of FAS in the epidermis may be regulated in the lower SS and the SB by local inflammation rather than by circulating hormones. In other components of the skin, FAS was strongly expressed not only in adipose tissue and sebaceous glands, which are known as active sites of lipid synthesis, but also in sweat glands, suggesting that the sweat glands can synthesize abundant fatty acids de novo.

Sterol regulation of human fatty acid synthase promoter I requires nuclear factor-Y- and Sp-1-binding sites

To understand cholesterol-mediated regulation of human fatty acid synthase promoter I, we tested various 5'-deletion constructs of promoter I-luciferase reporter gene constructs in HepG2 cells. The reporter gene constructs that contained only the Sp-1-binding site (nucleotides -82 to -74) and the two tandem sterol regulatory elements (SREs; nucleotides -63 to -46) did not respond to cholesterol. Only the reporter gene constructs containing a nuclear factor-Y (NF-Y) sequence, the CCAAT sequence (nucleotides -90 to -86), an Sp-1 sequence, and the two tandem SREs responded to cholesterol. The NF-Y-binding site, therefore, is essential for cholesterol response. Mutating the SREs or the NF-Y site and inserting 4 bp between the Sp-1- and NF-Y-binding sites both resulted in a minimal cholesterol response of the reporter genes. Electrophoretic mobility-shift assays using anti-SRE-binding protein (SREBP) and anti-NF-Ya antibodies confirmed that these SREs and the NF-Y site bind the respective factors. We also identified a second Sp-1 site located between nucleotides -40 and -30 that can substitute for the mutated Sp-1 site located between nucleotides -82 and -74. The reporter gene expression of the wild-type promoter and the Sp-1 site (nucleotides -82 to -74) mutant promoter was similar when SREBP1a [the N-terminal domain of SREBP (amino acids 1-520)] was constitutively overexpressed, suggesting that Sp-1 recruits SREBP to the SREs. Under the same conditions, an NF-Y site mutation resulted in significant loss of reporter gene expression, suggesting that NF-Y is required to activate the cholesterol response.

Identification of a novel enhancer sequence in the distal promoter of the rat fatty acid synthase gene

The proximal promoter and first intron of the fatty acid synthase (FAS) gene contains response sequences for insulin and glucose, but the 2- to 3-fold increase in FAS promoter activity attributable to these sequences falls short of the 20- to 30-fold induction in hepatic FAS gene transcription observed in fasted-refed rats. Using DNase I hypersensitivity site (HSS) mapping, two new liver specific sites were localized to the regions of: -8600 to -8500 (HSS 1) and -7300 to -7000 (HSS 2). DNase sensitivity of the -7300 to -7000 region was increased when fasted rats were refed glucose. When rat hepatocytes were transfected with a CAT construct that linked the region of -9700 and -4606 with the insulin response region located between -265 to +65, FAS promoter activity was induced 15-fold. This increase required the presence of both insulin and glucocorticoids. Deleting HSS 2 abolished the 15-fold induction in FAS promoter activity, but removing HSS 1 was without effect. Apparently the in vivo regulation of hepatic FAS gene transcription requires response elements located in the region of -7300 to -7000 and -265 to +65.

Microsomal long-chain acyl-CoA thioesterase (carboxylesterase ES-4) is regulated by thyroxine

Long chain acyl-CoA thioesterase activity is mainly located in microsomes after subcellular fractionation of liver from untreated rats. The physiological function and regulation of expression of this activity is not known. In the present study we have investigated the effect of thyroxine on expression of carboxylesterase ES-4, the major acyl-CoA thioesterase of liver microsomes. Thyroidectomy of rats decreased the palmitoyl-CoA thioesterase activity to about 25% of normal activity. This decrease was accompanied by similar decreases at the protein and mRNA levels (31% and 57%, respectively, of controls). Treatment with thyroxine completely reversed the effect of thyroidectomy and resulted in elevated levels in both thyroidectomized and control rats. For reasons of comparison we also studied the possibility that ES-10 and ES-2, two other members of the same gene family, are affected by thyroxine. ES-10 was not changed at the protein or mRNA level by any of the treatments, while ES-2 expression in liver was decreased by thyroxine treatment. The data shows that changes in activity and expression of ES-4 correlate to thyroxine status in the rat suggesting a physiological regulatory role by this hormone. Since thyroxine regulates the expression of lipogenic enzymes, these results are consistent with a function for this microsomal acyl-CoA thioesterase in fatty acid synthesis and/or secretion, rather than in oxidative degradation of fatty acids.