The CCAAT box binding factor, NF-Y, is required for thyroid hormone regulation of rat liver S14 gene transcription

Regulation of the SOX3 gene expression by retinoid receptors

Sox3/SOX3 gene is considered to be one of the earliest neural markers in vertebrates. Despite the mounting evidence that Sox3/SOX3 is one of the key players in the development of the nervous system, limited data are available regarding the transcriptional regulation of its expression. This review is focused on the retinoic acid induced regulation of SOX3 gene expression, with particular emphasis on the involvement of retinoid receptors. Experiments with human embryonal carcinoma cells identified two response elements involved in retinoic acid/retinoid X receptor-dependent activation of the SOX3 gene expression: distal atypical retinoic acid-response element, consisting of two unique G-rich boxes separated by 49 bp, and proximal element comprising DR-3-like motif, composed of two imperfect hexameric half-sites. Importantly, the retinoic acid-induced SOX3 gene expression could be significantly down-regulated by a synthetic antagonist of retinoid receptors. This cell model provides a solid base for further studies on mechanism(s) underlying regulation of expression of SOX3 gene, which could improve the understanding of molecular signals that induce neurogenesis in the stem/progenitor cells both during development and in adulthood.

A novel mechanism involving coordinated regulation of nuclear levels and acetylation of NF-YA and Bcl6 activates RGS4 transcription

Neuronally enriched RGS4 plays a critical role attenuating G protein signaling in brain, although the mechanisms regulating RGS4 expression are unknown. Here we describe a novel mechanism for transcriptional activation of RGS4 in neuron-like PC6 cells, where RGS4 is markedly induced during confluence-induced growth arrest. Transcriptional activation of RGS4 in confluent PC6 cells was accompanied by impaired G(i/o)-dependent MAPK activation. In the human RGS4 gene promoter, we identified three phylogenetically conserved cis-elements: an inverted CCAAT box element (ICE), a cAMP response element, and a B-cell lymphoma 6 (Bcl6)-binding site. The ICE and the cAMP response element mediate activation, and the Bcl6 site mediates repression of RGS4 transcription. Activation of RGS4 transcription in confluent PC6 cells is accompanied by increases in NF-YA and C/EBPβ and decreases in Bcl6 levels in the nucleus. Increases in NF-YA and C/EBPβ lead to their increased binding to the RGS4 promoter in vivo, and dominant negative forms of these proteins repressed RGS4 promoter activity. Acetylation of NF-YA and Bcl6 were increased in postconfluent cells. Trichostatin A stimulation of RGS4 promoter activity, accompanied by increased binding of NF-YA and decreased binding of Bcl6 to the promoter, was abolished by mutation of the ICE and enhanced by mutation of the Bcl6 site. These findings demonstrate a dynamic and coordinated regulation of nuclear levels and acetylation status of trans-acting factors critical in determining the off/on state of the RGS4 promoter.

The CCAAT box binding transcription factor, nuclear factor-Y (NF-Y) regulates transcription of human aldo-keto reductase 1C1 (AKR1C1) gene

Dihydrodiol dehydrogenases are a family of aldo-keto reductases (AKR1Cs) involved in the metabolism of steroid hormones and xenobiotics. Herein, we have cloned and characterized the proximal promoter region of the human AKR1C1 gene. The 5' flanking proximal promoter region of the AKR1C1 gene consists of a TATA box and an inverted CCAAT binding site. Deletion analysis of the 5' flanking, approximately 3.0 kb region of the human AKR1C1 gene identified the region between -128 and -88 as the minimal proximal promoter essential for basal transcription of AKR1C1 in human ovarian (2008 and 2008/C13*), lung (H23 and A549) and liver carcinoma (HepG2) cells. Site-directed mutagenesis studies indicated that the transcription factor binding sites for NF-Y/CEBP were involved in controlling the basal transcription of AKR1C1 in all the cancer cells studied. Electrophoretic mobility shift (EMSAs) and gel-supershift assays demonstrated that the transcription factor NF-Y preferentially binds to the inverted CCAAT box at (-109)ATTGG(-105) of the AKR1C1 gene. Chromatin immunoprecipitation (ChIP) analysis confirmed the in vivo association between NF-Y and human AKR1C1 gene promoter in human ovarian, lung and liver carcinoma cells. Ectopic expression of NF-Ys increased the AKR1C1 gene transcription, whereas expression of a dominant-negative NF-YA or suppression of NF-YA decreased the AKR1C1 gene transcription. A 2-fold increase in AKR1C1 transcription was observed specifically in cisplatin-treated 2008 cells that were CCAAT box-dependent. These results indicate that the NF-Y regulates the basal transcription of AKR1C1 in human ovarian, lung and liver carcinoma cells and the cisplatin-induced transcription in human ovarian carcinoma cells.

In vivo identification of promoter elements and transcription factors mediating activation of hepatic HMG-CoA reductase by T3

The promoter elements and transcription factors necessary for triiodothyronine (T3) induction of hepatic HMG-CoA reductase (HMGR) were investigated by transfecting rat livers with wild type and mutant HMGR promoter-luciferase constructs using in vivo electroporation. Mutations in the sterol response element (SRE), nuclear factor-y (NF-Y) site, and the newly identified upstream transcription factor-2 (USF-2) site essentially abolished the T3 response. Chromatin immunoprecipitation (ChIP) analysis demonstrated that T(3) treatment caused a 4-fold increase in in vivo binding of USF-2 to the HMGR promoter. Co-transfection of the wild type HMGR promoter with siRNAs to USF-2, SREBP-2, or NF-Y nearly abolished the T3 induction, as measured by promoter activity. These data provide in vivo evidence for functional roles for USF-2, SREBP-2, and NF-Y in mediating the T3-induction of hepatic HMGR transcription.

Genomic structure, transcriptional control, and tissue distribution ofHERG1andKCNQ1genes

The long QT syndrome genes human ether-a-go-go-related gene ( HERG1) and voltage-gated K+channel, KQT-like subfamily, member 1, gene ( KCNQ1), encoding K+channels critical to the repolarization rate and repolarization reserve in cardiac cells, and thereby the likelihood of arrhythmias, are both composed of two isoforms: HERG1a and HERG1b and KCNQ1a and KCNQ1b, respectively. Expression of these genes is dynamic, depending on the differentiation status and disease states. We identified their core promoter regions and transcription start sites. Our data suggest that HERG1a and HERG1b, and KCNQ1a and KCNQ1b, represent independent transcripts instead of being alternatively spliced variants of the same gene, for they each have their own transcription start sites and their own promoter regions. We obtained data pointing to the potential role of stimulating protein 1 (Sp1) in the transactivation of these genes. We compared expression profiling of these genes across a variety of human tissues. Consistent with the general lack of cis elements for cardiac-specific transcription factors and the presence of multiple sites for ubiquitous Sp1 sites in the core promoter regions of HERG1a/HERG1b and KCNQ1a/KCNQ1b genes, the transcripts demonstrated widespread distribution across a variety of human tissues. We further revealed that the mRNA levels of all HERG1 and KCNQ1 isoforms were asymmetrically distributed within the heart, being more abundant in the right atria and ventricles relative to the left atria and ventricles. These findings open up an opportunity for studying interventricular gradients of slow and rapid delayed rectifier K+current and of cardiac repolarization as well. Our study might help us understand the molecular mechanisms for arrhythmias since heterogeneity of ion channel activities is an important substrate for arrhythmogenesis.

CCAAT/enhancer binding protein and nuclear factor-Y regulate adiponectin gene expression in adipose tissue

Adiponectin is one of the adipokines secreted by adipocytes and regulates energy homeostasis associated with insulin sensitivity, suggesting a possibility of nutritional regulation of adiponectin gene expression. In this study, we showed that the transcription of adiponectin gene was induced 4-6 h after refeeding of mice. Also, differentiated 3T3-L1 adipocytes that were treated with high glucose expressed significantly increased adiponectin mRNA. Promoter analysis using nuclear extracts from white adipose tissue revealed that CCAAT/enhancer binding protein (C/EBP) and nuclear factor-Y (NF-Y) bound on the -117/-73 region of the adiponectin promoter. This region was critical for the activity of the adiponectin promoter as the deletion or mutation of this region markedly diminished the promoter activity to a basal level. Furthermore, the C/EBP binding increased in both refed animal and high glucose-treated 3T3-L1 adipocytes in an electrophoretic mobility shift assay, suggesting that C/EBP is responsible for the dietary response of the adiponectin gene expression. Chromatin immunoprecipitation studies demonstrated the binding of C/EBP and NF-Y in both mouse and differentiated 3T3-L1 adipocytes and also that C/EBP binding increased in response to high glucose. These findings demonstrated that C/EBP and NF-Y are critical for the regulation of the adiponectin expression in response to nutrients and in the course of adipocyte differentiation.

The role of liver X receptor-alpha in the fatty acid regulation of hepatic gene expression

Liver X receptors (LXR) alpha and beta play an important role in regulating the expression of genes involved in hepatic bile and fatty acid synthesis, glucose metabolism, as well as sterol efflux. Studies with human embryonic kidney 293 cells indicate that unsaturated fatty acids interfere with oxysterols binding to LXR and antagonize oxysterol-induced LXRalpha activity. In this report, we evaluated the effects of unsaturated fatty acids on LXR-regulated hepatic gene expression. The LXR agonist, T1317, induced mRNAs encoding sterol regulatory element-binding protein 1c (SREBP-1c) and two SREBP-1c-regulated lipogenic genes, e.g. fatty-acid synthase and the S14 protein in primary hepatocytes. Treatment of hepatocytes with eicosapentaenoic acid (20:5n-3) suppressed these mRNAs in the absence and presence of T1317. The cis-regulatory elements targeted by T1317 were not required for fatty-acid suppression of FAS or S14 promoter activity. In contrast to SREBP-1-regulated lipogenic genes, 20:5n-3 had no effect on the T1317 induction of ABCG5 or ABCG8 in the rat hepatoma cell line, FTO-2B. These two genes require LXR but not SREBP-1c for their expression. Feeding rats a diet supplemented with fish oil suppressed hepatic SREBP-1c-regulated genes and induced PPARalpha-regulated genes but had no effect on the LXR-regulated transcripts, CYP7A1, ABCG5, or ABCG8. Transfection studies, using either full-length hLXRalpha or a chimera containing only the LXRalpha ligand binding domain, indicate that a wide array of unsaturated fatty acids had little effect on LXRalpha activity in primary hepatocytes or FTO-2B. These studies suggest that LXRalpha is not a target for unsaturated fatty acid regulation in primary rat hepatocytes or in liver. Thus, oxysterol/LXR-mediated regulation of transcripts involved in bile acid synthesis or sterol efflux appear insensitive to dietary unsaturated fatty acids. The unsaturated fatty acid suppression of SREBP-1 and its targeted lipogenic genes is independent of LXRalpha

A thyroid hormone response unit formed between the promoter and first intron of the carnitine palmitoyltransferase-Ialpha gene mediates the liver-specific induction by thyroid hormone

Carnitine palmitoyltransferase-I (CPT-I) catalyzes the rate-controlling step of fatty acid oxidation. CPT-I converts long-chain fatty acyl-CoAs to acylcarnitines for translocation across the mitochondrial membrane. The mRNA levels and enzyme activity of the liver isoform, CPT-Ialpha, are greatly increased in the liver of hyperthyroid animals. Thyroid hormone (T3) stimulates CPT-Ialpha transcription far more robustly in the liver than in non-hepatic tissues. We have shown that the thyroid hormone receptor (TR) binds to a thyroid hormone response element (TRE) located in the CPT-Ialpha promoter. In addition, elements in the first intron participate in the T3 induction of CPT-Ialpha gene expression, but the CPT-Ialpha intron alone cannot confer a T3 response. We found that deletion of sequences in the first intron between +653 and +744 decreased the T3 induction of CPT-Ialpha. Upstream stimulatory factor (USF) and CCAAT enhancer binding proteins (C/EBPs) bind to elements within this region, and these factors are required for the T3 response. The binding of TR and C/EBP to the CPT-Ialpha gene in vivo was shown by the chromatin immunoprecipitation assay. We determined that TR can physically interact with USF-1, USF-2, and C/EBPalpha. Transgenic mice were created that carry CPT-Ialpha-luciferase transgenes with or without the first intron of the CPT-Ialpha gene. In these mouse lines, the first intron is required for T3 induction as well as high levels of hepatic expression. Our data indicate that the T3 stimulates CPT-Ialpha gene expression in the liver through a T3 response unit consisting of the TRE in the promoter and additional factors, C/EBP and USF, bound in the first intron.

Functional interaction between sterol regulatory element-binding protein-1c, nuclear factor Y, and 3,5,3'-triiodothyronine nuclear receptors

Sterol regulatory element binding protein-1c (SREBP-1c) is a key hepatic transcription factor involved in lipogenic gene expression. In an effort to understand the role SREBP-1c plays in lipogenic gene transcription, we have examined the functional interaction between SREBP-1c, nuclear factor Y, 3,5,3'-triiodothyronine (T(3)) receptors, and co-activators using the S14 gene promoter as a model. T(3), glucose, and insulin rapidly induce S14 gene transcription in rat liver and in primary hepatocytes. Linker scanning analyses of the S14 promoter showed that an SRE at -139/-131 base pairs (bp) binding SREBP-1c and a Y-box at -104/-99 bp binding NF-Y are indispensable for both T(3)- and SREBP-1c-mediated induction of S14 promoter activity in rat primary hepatocytes. T(3) and glucose/insulin induce S14 gene transcription through separate enhancers. Enhancer substitution studies reveal a preferential interaction between SREBP-1c.NF-Y and the T(3) regulatory region (-2.8/-2.5 kb) binding thyroid hormone receptor/RXR heterodimers. Elevating hepatocellular levels of specific co-activators (CBP, p/CAF, or GCN5) induced S14 promoter activity 2-3-fold, while SREBP-1c induced promoter activity 10-fold. The combination of these treatments induced S14 promoter activity (20-35-fold). However, this additive effect was lost when the T(3) regulatory region was deleted. Based on these results, we suggest that the SREBP-1c.NF-Y complex facilitates the interaction between co-activators that are recruited to distal hormone-regulated enhancers and the general transcription machinery that binds the S14 proximal promoter.

The homeodomain proteins PBX and MEIS1 are accessory factors that enhance thyroid hormone regulation of the malic enzyme gene in hepatocytes

Triiodothyronine (T3) stimulates a robust increase (>40-fold) in transcription of the malic enzyme gene in chick embryo hepatocytes. Previous work has shown that optimal T3 regulation of malic enzyme transcription is dependent on the presence of an accessory element (designated as region E) that immediately flanks a cluster of five T3 response elements in the malic enzyme gene. Here, we have analyzed the binding of nuclear proteins to region E and investigated the mechanism by which region E enhances T3 responsiveness. In nuclear extracts from hepatocytes, region E binds heterodimeric complexes consisting of the homeodomain proteins PBX and MEIS1. Region E contains four consecutive PBX/MEIS1 half-sites. PBX-MEIS1 heterodimers bind the first and second half-sites, the third and fourth half-sites, and the first and fourth half-sites. The configuration conferring the greatest increase in T3 responsiveness consists of the first and fourth half-sites that are separated by 7 nucleotides. Stimulation of T3 response element functions by region E does not require the presence of additional malic enzyme sequences. In pull-down experiments, PBX1a and PBX1b specifically bind the nuclear T3 receptor-alpha, and this interaction is enhanced by the presence of T3. A T3 receptor-alpha region containing the DNA binding domain plus flanking sequences (amino acids 21-157) is necessary and sufficient for binding to PBX1a and PBX1b. These results indicate that PBX-MEIS1 complexes interact with nuclear T3 receptors to enhance T3 regulation of malic enzyme transcription in hepatocytes.

Target gene search for the metal-responsive transcription factor MTF-1

Activation of genes by heavy metals, notably zinc, cadmium and copper, depends on MTF-1, a unique zinc finger transcription factor conserved from insects to human. Knockout of MTF-1 in the mouse results in embryonic lethality due to liver decay, while knockout of its best characterized target genes, the stress-inducible metallothionein genes I and II, is viable, suggesting additional target genes of MTF-1. Here we report on a multi-pronged search for potential target genes of MTF-1, including microarray screening, SABRE selective amplification, a computer search for MREs (DNA-binding sites of MTF-1) and transfection of reporter genes driven by candidate gene promoters. Some new candidate target genes emerged, including those encoding alpha-fetoprotein, the liver-enriched transcription factor C/EBPalpha and tear lipocalin/von Ebner's gland protein, all of which have a role in toxicity/the cell stress response. In contrast, expression of other cell stress-associated genes, such as those for superoxide dismutases, thioredoxin and heat shock proteins, do not appear to be affected by loss of MTF-1. Our experiments have also exposed some problems with target gene searches. First, finding the optimal time window for detecting MTF-1 target genes in a lethal phenotype of rapid liver decay proved problematical: 12.5-day-old mouse embryos (stage E12.5) yielded hardly any differentially expressed genes, whereas at stage 13.0 reduced expression of secretory liver proteins probably reflected the onset of liver decay, i.e. a secondary effect. Likewise, up-regulation of some proliferation-associated genes may also just reflect responses to the concomitant loss of hepatocytes. Another sobering finding concerns gamma-glutamylcysteine synthetase(hc) (gamma-GCS(hc)), which controls synthesis of the antioxidant glutathione and which was previously suggested to be a target gene contributing to the lethal phenotype in MTF-1 knockout mice. gamma-GCS(hc) mRNA is reduced at the onset of liver decay but MTF-1 null mutant embryos manage to maintain a very high glutathione level until shortly before that stage, perhaps in an attempt to compensate for low expression of metallothioneins, which also have a role as antioxidants.

Thyroid hormone regulates carnitine palmitoyltransferase Ialpha gene expression through elements in the promoter and first intron

Carnitine palmitoyltransferase I (CPT-I) catalyzes the transfer of long chain fatty acyl groups from CoA to carnitine for translocation across the mitochondrial inner membrane. CPT-Ialpha is a key regulatory enzyme in the oxidation of fatty acids in the liver. CPT-Ialpha is expressed in all tissues except skeletal muscle and adipose tissue, which express CPT-Ibeta. Expression of CPT-Ialpha mRNA and enzyme activity are elevated in the liver in hyperthyroidism, fasting, and diabetes. CPT-Ialpha mRNA abundance is increased 40-fold in the liver of hyperthyroid compared with hypothyroid rats. Here, we examine the mechanisms by which thyroid hormone (T3) stimulates CPT-Ialpha gene expression. Four potential T3 response elements (TRE), which contain direct repeats separated by four nucleotides, are located 3000-4000 base pairs 5' to the start site of transcription in the CPT-Ialpha gene. However, only one of these elements functions as a TRE. This TRE binds the T3 receptor as well as other nuclear proteins. Surprisingly, the first intron of the CPT-Ialpha gene is required for the T3 induction of CPT-Ialpha expression, but this region of the gene does not contain a TRE. In addition, we show that CPT-Ialpha is induced by T3 in cell lines of hepatic origin but not in nonhepatic cell lines.

Dependence of rat spot14 promoter activity on NF-Y binding to the inverted CCAAT-element at -100

Electrophoretic mobility shift assay (EMSA) and in vitro transcription/translation show that NF-Y binds to the inverted CCAAT-element in the promoter of the rodent spot14 gene. The NF-Y-binding sequence has been shown to be responsible for basal activity in H4IIE. Given the similar role found for the inverted CCAAT-element in the promoter of the FAS gene, NF-Y may have an important function in the control of lipogenesis.

Molecular characterization of the mouse Fas ligand promoter in airway epithelial cells

Constitutively expressed Fas ligand in several distinct epithelial cell types appears to protect tissues by inducing apoptosis of Fas(+) immune cells during inflammatory reactions. To study the transcriptional regulation of Fas ligand gene in airway epithelial cells, a 618-bp 5'-flanking region of mouse Fas ligand gene was cloned, sequenced, and the transcriptional start site was determined by using 5'-RACE. Deletion analysis, gel mobility shift assays and site-directed mutagenesis indicated that a CCAAT box located -214 bp upstream from the transcription start site served as a major positive regulatory cis-element in an airway epithelial cell line. This element was not required for constitutive Fas ligand expression in Sertoli cells. Furthermore, the activity of the site did not involve the NF-Y protein complex or c/EBP protein family. UV-cross linking proteins to this element indicated that a approximately 23-kDa transcription factor bound to the Fas ligand promoter CCAAT box and, thus, likely plays an important role in the regulation of Fas ligand expression in airway epithelial cells.

Characterization of proximal transcription regulatory elements in the rat phospholamban promoter

Phospholamban is a major regulator of cardiac diastole, with alterations in expression associated with modified cardiac relaxation. To study transcriptional regulation of phospholamban expression, we made reporter constructs that expressed luciferase under control of putative promoter sequences from the rat phospholamban gene. When transfected into neonatal rat cardiomyocytes, constructs containing at least 159 nucleotides preceding the transcription start site were equally active, while truncation to -66/+64 removed all promoter activity. Constructs were more active in cardiomyocytes than in HeLa cells (which do not express phospholamban), but did not show absolute cell-type specificity of expression. Addition of sequences upstream to -4032, all of the intron (7.4 kb), or 3'UTR sequences (0. 8 kb) did not enhance cell-specific expression. To focus on the basal promoter region (-159/-66), a series of deletion constructs were made that identified a novel 35 bp region (-159/-125; Phospholamban Promoter Element 1, PPE1) required for promoter activity in cardiomyocytes. Site-specific mutations identified nucleotides -150/-133 as containing most of the promoter-enhancing activity. While the rat PPE1 is highly conserved (>70%) in four other mammalian phospholamban genes, it does not contain previously characterized regulatory elements. In cardiomyocytes the PPE1 sequence markedly enhanced activity of the SV40 early promoter. A conserved CCAAT element (-83/-79) was also required for promoter activity in both cardiomyocytes and HeLa cells. Exonuclease III footprinting identified protein/DNA interactions in both the extended CCAAT box and PPE1 domains. Gel shift studies identified the CCAAT elements as binding CBF/NF-Y.

Sterol response element-binding protein 1c (SREBP1c) is involved in the polyunsaturated fatty acid suppression of hepatic S14 gene transcription

Polyunsaturated fatty acids (PUFA) suppress hepatic lipogenic gene transcription through a peroxisome proliferator activated receptor alpha (PPARalpha)- and cyclooxygenase-independent mechanism. Recently, the sterol response element-binding protein 1 (SREBP1) was implicated in the nutrient control of lipogenic gene expression. In this report, we have assessed the role SREBP1 plays in the PUFA control of three hepatic genes, fatty acid synthase, L-pyruvate kinase (LPK), and the S14 protein (S14). PUFA suppressed both the hepatic mRNA(SREBP1) through a PPARalpha-independent mechanism as well as SREBP1c nuclear content (nSREBP1c, 65 kDa). Co-transfection of primary hepatocytes revealed a differential sensitivity of the fatty acid synthase, S14, and LPK promoters to nSREBP1c overexpression. Of the three promoters examined, LPK was the least sensitive to overexpressed nSREBP1c. Promoter deletion and gel shift analyses of the S14 promoter localized a functional SREBP1c cis-regulatory element to an E-box-like sequence ((-139)TCGCCTGAT(-131)) within the S14 PUFA response region. Although overexpression of nSREBP1c significantly reduced PUFA inhibition of S14CAT, overexpression of other factors that induced S14CAT activity, such as steroid receptor co-activator 1 or retinoid X receptor alpha, had no effect on S14CAT PUFA sensitivity. These results suggest that PUFA regulates hepatic nSREBP1c, a factor that functionally interacts with the S14 PUFA response region. PUFA regulation of nSREBP1c may account for the PUFA-mediated suppression of hepatic S14 gene transcription.

Regulation of gene expression by dietary fat

Dietary fat is an important macronutrient for the growth and development of all organisms. In addition to its role as an energy source and its effects on membrane lipid composition, dietary fat has profound effects on gene expression, leading to changes in metabolism, growth, and cell differentiation. The effects of dietary fat on gene expression reflect an adaptive response to changes in the quantity and type of fat ingested. Specific fatty acid-regulated transcription factors have been identified in bacteria, amphibians, and mammals. In mammals, these factors include peroxisome proliferator-activated receptors (PPAR alpha, -beta, and -gamma), HNF4 alpha, NF kappa B, and SREBP1c. These factors are regulated by (a) direct binding of fatty acids, fatty acyl-coenzyme A, or oxidized fatty acids; (b) oxidized fatty acid (eicosanoid) regulation of G-protein-linked cell surface receptors and activation of signaling cascades targeting the nucleus; or (c) oxidized fatty acid regulation of intracellular calcium levels, which affect cell signaling cascades targeting the nucleus. At the cellular level, the physiological response to fatty acids will depend on (a) the quantity, chemistry, and duration of the fat ingested; (b) cell-specific fatty acid metabolism (oxidative pathways, kinetics, and competing reactions); (c) cellular abundance of specific nuclear and membrane receptors; and (d) involvement of specific transcription factors in gene expression. These mechanisms are involved in the control of carbohydrate and lipid metabolism, cell differentiation and growth, and cytokine, adhesion molecule, and eicosanoid production. The effects of fatty acids on the genome provide new insight into how dietary fat might play a role in health and disease.

Dietary polyunsaturated fatty acids and hepatic gene expression

Dietary polyunsaturated fatty acids (PUFA) have profound effects on hepatic gene transcription leading to significant changes in lipid metabolism. PUFA rapidly suppress transcription of genes encoding specific lipogenic and glycolytic enzymes and induce genes encoding specific peroxisomal and cytochrome P450 (CYP) enzymes. Using the peroxisome proliferator-activated receptor alpha (PPAR alpha)-null mouse, we showed that dietary PUFA induction of acyl CoA oxidase (AOX) and CYP4A2 require PPAR alpha. However, PPAR alpha is not required for the PUFA-mediated suppression of fatty acid synthase (FAS), S14, or L-pyruvate kinase (L-PK). Studies in primary rat hepatocytes and cultured 3T3-L1 adipocytes showed that metabolites of 20:4n-6, like prostaglandin E2 (PGE2), suppress mRNA encoding FAS, S14, and L-PK through a Gi/Go-coupled signal transduction cascade. In contrast to adipocytes, 20:4n-6-mediated suppression of lipogenic gene expression in hepatic parenchymal cells does not require cyclooxygenase. Transfection analysis of S14CAT fusion genes in primary hepatocytes shows that peroxisome proliferator-activated PPAR alpha acts on the thyroid hormone response elements (-2.8/-2.5 kb). In contrast, both PGE2 and 20:4n-6 regulate factors that act on the proximal promoter (-150/-80 bp) region, respectively. In conclusion, PUFA affects hepatic gene transcription through at least three distinct mechanisms: (i) a PPAR-dependent pathway, (ii) a prostanoid pathway, and (iii) a PPAR and prostanoid-independent pathway. PUFA regulation of hepatic lipid metabolism involves an integration of these multiple pathways.

Multiple mechanisms for polyunsaturated fatty acid regulation of hepatic gene transcription

Dietary polyunsaturated fatty acids (PUFA) have profound effects on hepatic gene transcription leading to significant changes in lipid metabolism. Highly unsaturated n-3 PUFA suppress the transcription of genes encoding specific lipogenic enzymes and induce the expression of genes encoding specific enzymes involved in peroxisomal and microsomal fatty acid oxidation. Our studies have shown that fatty acid effects on hepatic gene expression may involve at least three distinct pathways. One pathway involves peroxisome proliferator-activated receptor (PPARalpha), a fatty acid activated nuclear receptor. PPARalpha is required for the PUFA induction of mRNAs encoding enzymes involved in fatty acid oxidation. However, PPARalpha is not required for PUFA suppression of mRNAs encoding proteins involved in lipogenesis. A second pathway involves prostanoids. In cultured 3T3-L1 adipocytes, cyclooxygenase derived 20:4 n-6 metabolites, like PGE2, suppress mRNAs encoding proteins involved in lipogenesis. However, in hepatic parenchymal cells, 20:4 n-6 suppression of lipogenic gene expression does not require a cyclooxygenase. Nevertheless, PGE2 and PGF2alpha suppress hepatic lipogenic gene expression. 20:4 n-6 cyclooxygenase products can arise from non-parenchymal cells and through a paracrine control process act on a G-protein linked receptor signaling cascade to suppress lipogenic gene expression. The fact that n-3 and n-6 PUFA suppression of lipogenic gene expression does not require PPARalpha or cyclooxygenase activity indicates the presence of a third pathway for the control of hepatic gene transcription. These studies indicate that the pleiotropic effects of PUFA on hepatic lipid metabolism cannot be attributed to a single regulatory mechanism.

FIRE3 in the promoter of the rat fatty acid synthase (FAS) gene binds the ubiquitous transcription factors CBF and USF but does not mediate an insulin response in a rat hepatoma cell line

Several putative insulin-responsive elements (IRE) in the fatty acid synthase (FAS) promoter have been identified and shown to be functional in adipocytes and hepatocytes. Here we report on the insulin-responsiveness in the rat hepatoma cell line H4IIE of four cis-elements in the FAS promoter: the FAS insulin-responsive elements, FIRE2 and FIRE3; the inverted CCAAT element, ICE; and the insulin/glucose-binding element, designated hepatic FIRE element, hFIRE, originally identified in rat hepatocytes. Using electrophoretic mobility shift assay (EMSA) competition experiments together with supershifts and in vitro transcription/translation we show that FIRE3 (-68/-58) binds not only the upstream stimulatory factors USF-1/USF-2 but also the CCAAT-binding factor CBF, also known as the nuclear factor Y, NF-Y. The putative IRE FIRE2, which shows sequence similarity to FIRE3, is located between -267 and -249. Gel retardation experiments indicate that USF-1 and USF-2 also bind to this element, which contains an imperfect E-box motif. Using the same approach we have shown that hFIRE binds the stimulatory proteins Sp1 and Sp3 in addition to CBF. Transient transfection experiments using FAS promoter constructs deleted for FIRE2 and FIRE3 demonstrate that neither of these elements mediates the insulin response of the FAS promoter in the rat hepatoma cell line H4IIE, however, ICE at -103/-87 is responsible for mediating the effect of the insulin antagonist cAMP. The hFIRE element located at -57/-34, in spite of its role in the glucose/insulin response in primary rat hepatocytes, is apparently not involved in the insulin regulation of the rat FAS promoter in H4IIE cells. The fact that the topology of the promoters of the FAS genes in rat, human, goose and chicken is conserved regarding CBF-binding sites and USF-binding sites implies an important role for these ubiquitously expressed transcription factors in the regulation of the FAS promoter.

Role of CCAAT enhancer-binding protein beta in the thyroid hormone and cAMP induction of phosphoenolpyruvate carboxykinase gene transcription

Transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK) is stimulated by thyroid hormone (T3) and cAMP. Two DNA elements in the PEPCK promoter are required for T3 responsiveness including a thyroid hormone response element and a binding site called P3(I) for the CCAAT enhancer-binding protein (C/EBP). Both the alpha and beta isoforms of C/EBP are highly expressed in the liver. C/EBPalpha contributes to the liver-specific expression and cAMP responsiveness of the PEPCK gene. In this study, we examined the ability of C/EBPbeta when bound to the P3(I) site to regulate PEPCK gene expression. We report that C/EBPbeta can stimulate basal expression and participate in the induction of PEPCK gene transcription by T3 and cAMP. The cAMP-responsive element-binding protein and AP1 proteins that contribute to the induction by cAMP are not involved in the stimulation by T3. A small region of the transactivation domain of C/EBPbeta is sufficient for the stimulation of basal expression and cAMP responsiveness. Our results suggest that C/EBPalpha and C/EBPbeta are functionally interchangeable when bound to the P3(I) site of the PEPCK promoter.

"Spot 14" protein: a metabolic integrator in normal and neoplastic cells

"Spot 14" (S14) was originally identified as a mRNA from rat liver that responded rapidly to thyroid hormone, and has now been shown to play a key role in the tissue-specific regulation of lipid metabolism. In addition to its responsiveness to thyroid hormone, S14 gene transcription is controlled by dietary substrates, such as glucose and polyunsaturated fatty acids, and by fuel-related hormones including insulin and glucagon. The S14 protein forms homodimers via a carboxyl-terminal "zipper" domain. The protein is located primarily in the cell nucleus, and its expression in liver is limited to the perivenous portion of the hepatic lobule, the site of fatty acid synthesis. S14 protein is critical for the induction of key enzymes involved in the switching of hepatic metabolism from the fasted to the fed state. S14 antisense oligonucleotides inhibit both the intracellular production of lipids and their export as very low-density lipoprotein (VLDL) particles. S14 acts at the level of transcription to regulate expression of genes encoding key metabolic enzymes, including those required for long-chain fatty acid synthesis. The human S14 gene is located at 11q13.5, a region that is amplified in a subset of aggressive breast cancers. S14 mRNA is expressed in most breast cancer-derived cell lines, and the protein is found in the nuclei of two thirds of human breast cancer specimens, but not in normal nonlactating mammary glands. S14 expression in breast tumors is highly concordant with overabundance of a key lipogenic enzyme. This indicates the association of S14 with enhanced tumor lipogenesis, an established marker of poor prognosis. In addition to the utility of S14 as a model system for elucidation of the mechanism of thyroid hormone action, studies of its regulation and function have provided insights into tissue-specific metabolic control by hormones and dietary substrates in both normal and neoplastic tissues.

Regulation of the action of steroid/thyroid hormone receptors by medium-chain fatty acids

Triiodothyronine (T3) causes a 30-fold increase in transcription of the malic enzyme gene in chick embryo hepatocytes; medium-chain fatty acids (MCFAs) inhibit this increase. T3 action is mediated by T3 receptors (TRs) that bind to T3 response elements (T3REs) in this gene's 5'-flanking DNA. In transiently transfected hepatocytes, fragments of 5'-flanking DNA of the malic enzyme gene or artificial T3REs that conferred T3 stimulation also conferred MCFA inhibition to linked reporter genes. Thus, MCFA inhibition may be mediated through cis-acting T3REs and trans-acting TRs, distinguishing MCFA action from that of other fatty acids which act through unique sequence elements. Using binding assays and overexpression of TR, we showed that MCFAs inhibited the transactivating but not the silencing function of TR and did not alter binding of T3 to TR or of TR to T3RE. The C-terminal ligand-binding domain of TR was sufficient to confer stimulation by T3, but not inhibition by MCFA. Inhibition of transactivation by MCFA was specific: ligand-stimulated transcription from T3 or estrogen response elements was inhibited, but that from glucocorticoid or cyclic AMP response elements was not. We propose that MCFAs or metabolites thereof influence the activity of a factor(s) that interacts with the T3 and estrogen receptors to inhibit ligand-stimulated transcription.

STAT 5 and NF-Y are involved in expression and growth hormone-mediated sexually dimorphic regulation of cytochrome P450 3A10/lithocholic acid 6beta-hydroxylase

The level of expression of a number of sexually differentiated liver proteins is primarily determined by plasma growth hormone (GH). Adult males have a pulsatile profile of GH release, while females have a relatively steady-state pattern of GH release. An important subset of these sexually differentiated hepatic proteins is certain cytochrome P450s (P450s). CYP3A10/6beta-hydroxylase is a male-specific P450 that catalyzes 6beta-hydroxylation of lithocholic acid, and the pattern of GH secretion is directly responsible for male-specific expression of this gene. The DNA element involved in GH-mediated regulation of CYP3A10/6beta-hydroxylase promoter activity binds a member of the STAT (signal transducers and activators of transcription) family of proteins. In this study we functionally demonstrate that two members of the STAT family, STAT 5a and STAT 5b, mediate GH-dependent regulation of CYP3A10/6beta-hydroxylase promoter activity. Furthermore, a neighboring DNA element binds NF-Y, a transcription factor involved in maintaining high levels of transcription of many genes and known to functionally interact with other factors. In the CYP3A10/6beta-hydroxylase gene, NF-Y also modulates binding of STAT 5, thereby modulating GH-mediated activation of its transcription.