DNase I hypersensitivity sites and nuclear protein binding on the fatty acid synthase gene: identification of an element with properties similar to known glucose-responsive elements

Training down-regulates fatty acid synthase and body fat in obese Zucker rats

INTRODUCTION

The purpose of this study was to investigate whether chronic exercise training attenuates fatty acid synthase, the rate-limiting enzyme for hepatic lipogenesis, and the accumulation of body fat by using obese Zucker rats (OZR) as a model.

METHODS

Female obese Zucker (fa/fa) rats (O, N = 16) and their lean litter mates (L, N = 16) were randomly divided into a trained (T) and untrained (U) group. T was performed on a treadmill for 2 h.d-1, 5 d.wk-1, 10 wk with running speed and grade adjusted to elicit similar workloads. All rats were meal-fed a high-cornstarch diet for 4 h.d-1 and killed 8 h after the initiation of the last meal and 27 h after the last T session, in the resting state.

RESULTS

O rats exhibited twofold higher FAS activity and sixfold higher FAS mRNA abundance in the liver than L rats (P < 0.05), accompanied by a severe hyperinsulinemia (P < 0.05) but normal glucagon and glucose levels. FAS activity, but not mRNA level, was decreased by 18% with T in O rats (P < 0.05). T decreased percent body fat in both O and L rats (P < 0.05), and increased lean body mass in O rats (P < 0.05). Hepatic fatty acid profile showed higher 16:0, 16:1, and 18:1 concentrations in O rats, whereas 18:0, 18:2, and 20:4 were lower (P < 0.05). Training increased 20:4 in both O and L rats (P < 0.08). Nuclear protein binding to the insulin response sequence (IRS/A) and carbohydrate response element (ChoRE) on FAS gene promoter was decreased, whereas inverted CAATT box element (ICE) binding was increased in O versus L rats (P < 0.05). Training did not affect the binding of these gene sequences.

CONCLUSION

De novo lipogenesis was greatly enhanced in OZR. Endurance training decreased body fat, which is partly explained by a decreased FAS activity. However, FAS down-regulation was not due to altered nuclear protein binding to FAS gene.

Exercise down-regulates hepatic fatty acid synthase in streptozotocin-treated rats

An acute bout of prolonged exercise has been shown to decrease hepatic fatty acid synthase (FAS) mRNA and activity induced by high carbohydrate diets. The purpose of the current study was to examine the role of insulin in this exercise down-regulation of FAS. Sixty-four male Wistar rats were randomly divided into normal and streptozotocin (STZ)-treated diabetic groups. After being starved for 48 h and refed a high cornstarch (C) or fructose (F) diet for 10 h, one half of each group of rats was killed after an acute bout of prolonged exercise (E), while the other half of the group was killed in the rested state. STZ treatment suppressed plasma insulin and elevated plasma glucagon levels along with a severe hyperglycemia. FAS mRNA levels decreased by 60% (P < 0.05) with STZ treatment but were 250% higher in F-fed versus C-fed rats. E abolished F-induced FAS mRNA levels in both normal and STZ rats and decreased plasma glucose concentration in STZ rats (P < 0.05). F-fed normal rats showed twofold higher hepatic FAS activity than did C-fed normal rats and this dietary induction was abolished by STZ (P < 0.05). FAS activity in normal rats was not affect by E and was increased with E in STZ rats. Nuclear protein binding to the insulin response sequence was not affected by STZ or diet and increased with E (P < 0.05). Carbohydrate response element binding was greater with F- versus C-feeding (P < 0.05) but unaffected by E. E enhanced inverted CCAAT-box element binding regardless of diet and STZ. We conclude that although insulin status had a great influence on FAS gene expression, E-induced down-regulation of FAS mRNA was not mediated by altered insulin response sequence binding but primarily by increased inverted CCAAT-box element binding to the FAS promoter and/or decreased concentration of carbohydrate metabolites.

A glucose-responsive transcription factor that regulates carbohydrate metabolism in the liver

Carbohydrates mediate their conversion to triglycerides in the liver by promoting both rapid posttranslational activation of rate-limiting glycolytic and lipogenic enzymes and transcriptional induction of the genes encoding many of these same enzymes. The mechanism by which elevated carbohydrate levels affect transcription of these genes remains unknown. Here we report the purification and identification of a transcription factor that recognizes the carbohydrate response element (ChRE) within the promoter of the L-type pyruvate kinase (LPK) gene. The DNA-binding activity of this ChRE-binding protein (ChREBP) in rat livers is specifically induced by a high carbohydrate diet. ChREBP's DNA-binding specificity in vitro precisely correlates with promoter activity in vivo. Furthermore, forced ChREBP overexpression in primary hepatocytes activates transcription from the L-type Pyruvate kinase promoter in response to high glucose levels. The DNA-binding activity of ChREBP can be modulated in vitro by means of changes in its phosphorylation state, suggesting a possible mode of glucose-responsive regulation. ChREBP is likely critical for the optimal long-term storage of excess carbohydrates as fats, and may contribute to the imbalance between nutrient utilization and storage characteristic of obesity.

Involvement of a unique carbohydrate-responsive factor in the glucose regulation of rat liver fatty-acid synthase gene transcription

Refeeding carbohydrate to fasted rats induces the transcription of genes encoding enzymes of fatty acid biosynthesis, e.g. fatty-acid synthase (FAS). Part of this transcriptional induction is mediated by insulin. An insulin response element has been described for the fatty-acid synthase gene region of -600 to +65, but the 2-3-fold increase in fatty-acid synthase promoter activity attributable to this region is small compared with the 20-30-fold induction in fatty-acid synthase gene transcription observed in fasted rats refed carbohydrate. We have previously reported that the fatty-acid synthase gene region between -7382 and -6970 was essential for achieving high in vivo rates of gene transcription. The studies of the current report demonstrate that the region of -7382 to -6970 of the fatty-acid synthase gene contains a carbohydrate response element (CHO-RE(FAS)) with a palindrome sequence (CATGTGn(5)GGCGTG) that is nearly identical to the CHO-RE of the l-type pyruvate kinase and S(14) genes. The glucose responsiveness imparted by CHO-RE(FAS) was independent of insulin. Moreover, CHO-RE(FAS) conferred glucose responsiveness to a heterologous promoter (i.e. l-type pyruvate kinase). Electrophoretic mobility shift assays demonstrated that CHO-RE(FAS) readily bound a unique hepatic ChoRF and that CHO-RE(FAS) competed with the CHO-RE of the l-type pyruvate kinase and S(14) genes for ChoRF binding. In vivo footprinting revealed that fasting reduced and refeeding increased ChoRF binding to CHO-RE(FAS). Thus, carbohydrate responsiveness of rat liver fatty-acid synthase appears to require both insulin and glucose signaling pathways. More importantly, a unique hepatic ChoRF has now been shown to recognize glucose responsive sequences that are common to three different genes: fatty-acid synthase, l-type pyruvate kinase, and S(14).

Mutational analysis of the glucose regulatory element in the promoter of the glucagon receptor gene

Recently, we identified a glucose regulatory element in the promoter of the rat glucagon receptor gene. The effect of glucose is centered on a highly palindromic sequence of 19 nucleotides that we called the G box (Portois et al., 1999, J. Biol. Chem. 274: 8181-8190). This sequence contains two E boxes. Recently, we investigated the role of each individual E box, as well as the contribution of the sequences located upstream and downstream from this G box. (1) Mutation of nucleotides "CA" to "GT" in the first E box (position -543 to -542) suppressed the activation of the CAT reporter gene by glucose. In contrast, mutation of the nucleotides "CA" to "GT" in the second E box (position -534 to -533) had no effect on this glucose activation. (2) Deletion of a sequence upstream from the G box (nucleotides -579 to -555) suppressed the activation by glucose, whereas deletion of a sequence located downstream from the G box (nucleotides -501 to -443) had no effect on this parameter. (3) Subcloning of a small promoter fragment of only 49 nucleotides (-560 to -512) into the pCat5 plasmid conferred to transfected cells sensitivity to glucose in terms of CAT activity. Consequently, all transactivation factors required for this glucose effect must act via this short gene fragment.

Role of Sp1 and Sp3 in the Transcriptional Regulation of the Rat Fatty Acid Synthase Gene

Inspection of the 5' region of the sequence of the rat fatty acid synthase (FAS) gene revealed a high GC content between -900 and +500, implying several binding sites for members of the Sp1 family of transcription factors. Using SL2 and H4IIE cells in conjunction with FAS promoter/luciferase constructs either successively deleted or containing defined deletions we characterized six GC boxes--GC-I to GC-VI--located between -557 and -83 and discovered a seventh, GC-VII, in the first intron. In vitro DNAse I-footprinting, electrophoretic mobility shift assays, and the yeast one-hybrid system indicated that Sp1 as well as Sp3 interacts with GC-I to GC-VII. Each of the GC boxes conferred Sp1-dependent transcription on the FAS-Mini promoter and in the case of GC-I, Sp1, and Sp3 exert an additive effect on FAS promoter activity.

Two 5'-regions are required for nutritional and insulin regulation of the fatty-acid synthase promoter in transgenic mice

We previously reported that 2.1 kilobase pairs of the 5'-flanking sequence are sufficient for tissue-specific and hormonal/metabolic regulation of the fatty-acid synthase (FAS) gene in transgenic mice. We also demonstrated that the -65 E-box is required for insulin regulation of the FAS promoter using 3T3-L1 adipocytes in culture. To further define sequences required for FAS gene expression, we generated transgenic mice carrying from -644, -444, -278, and -131 to +67 base pairs of the rat FAS 5'-flanking sequence fused to the chloramphenicol acetyltransferase (CAT) reporter gene. Similar to the expression observed with -2100-FAS-CAT transgenic mice, transgenic mice harboring -644-FAS-CAT and -444-FAS-CAT expressed high levels of CAT mRNA only in lipogenic tissues (liver and adipose tissue) in a manner identical to the endogenous FAS mRNA. In contrast, -278-FAS-CAT and -131-FAS-CAT transgenic mice did not show appreciable CAT expression in any of the tissues examined. When previously fasted mice were refed a high carbohydrate, fat-free diet, CAT mRNA expression in transgenic mice harboring -644-FAS-CAT and -444-FAS-CAT was induced dramatically in liver and adipose tissue. The induction was virtually identical to that observed in -2100-FAS-CAT transgenic mice and to the endogenous FAS mRNA. In contrast, -278-FAS-CAT transgenic mice showed induction by feeding, but at a much lower magnitude in both liver and adipose tissue. The -131-FAS-CAT transgenic mice did not show any CAT expression either when fasted or refed a high carbohydrate diet. To study further the effect of insulin, we made these transgenic mice insulin-deficient by streptozotocin treatment. Insulin administration to the streptozotocin-diabetic mice increased CAT mRNA levels driven by the -644 FAS and -444 FAS promoters in liver and adipose tissue, paralleling the endogenous FAS mRNA levels. In the case of -278-FAS-CAT, the induction observed was at a much lower magnitude, and deletion to -131 base pairs did not show any increase in CAT expression by insulin. This study demonstrates that the sequence requirement for FAS gene regulation employing an in vitro culture system does not reflect the in vivo situation and that two 5'-flanking regions are required for proper nutritional and insulin regulation of the FAS gene. Cotransfection of the upstream stimulatory factor and various FAS promoter-luciferase constructs as well as in vitro binding studies suggest a function for the upstream stimulatory factor at both the -65 and -332 E-box sequences.

Glucose regulation of the expression of the glucagon receptor gene

The glucagon receptor gene is a member of a gene family, the expression of which is strongly upregulated by glucose. This review deals with the structure of both the glucagon receptor gene and its promoter. Attention is focused on the glucose regulatory element that we discovered in the promoter of this gene. Regulation by glucose of genes implicated in glucose homeostasis represents one mechanism contributing to the control of fuel utilization. Its deficiency or imbalance could potentially lead to or participate in pathological situations such as diabetes mellitus. On the other hand, the regulatory element of the glucagon receptor gene promoter could be used as a tool for the glucose-regulated expression of other genes. Indeed, an analysis of the glucagon receptor gene promoter demonstrated that only a short fragment of the genomic DNA, easy to subclone, contains all required elements for activation by glucose. Its potential use for gene therapy is also considered, therefore, in this report.

Exercise attenuates nuclear protein binding to gene regulatory sequences of hepatic fatty acid synthase

The effect of an acute bout of exhaustive exercise on hepatic fatty acid synthase (FAS) gene expression was examined in rats. Female Sprague-Dawley rats (age 8 wk) were fasted for 48 h (F, n = 6), or fasted, refed a high-fructose diet for 6 h, and killed at rest (R, n = 6) or killed after running on a treadmill at 27 m/min and 5% grade for 88 +/- 7 min (E, n = 6). Gel mobility shift assay indicated that R rats had twofold higher liver nuclear protein binding to oligonucleotides corresponding to the insulin responsive sequence (-71/-50) and carbohydrate response element (+283/+303) on the FAS promoter, compared with F rats. Exercise severely attenuated this binding in liver nuclear extracts to the levels seen in F rats. Competition and supershift experiments revealed that the bound protein complexes contained the upstream stimulatory factors. Nuclear run-on experiment revealed a 49-fold increase in transcription rate of the FAS gene in R vs. F rats, whereas exercise suppressed the transcription rate. FAS mRNA abundance and FAS enzyme activity were dramatically increased with refeeding but were unaltered by exercise. The results reveal that dietary induction of hepatic FAS is stimulated by increased nuclear protein binding to insulin responsive sequence and carbohydrate response element, whereas exhaustive exercise attenuates the binding, which may precede downregulation of FAS mRNA and enzyme synthesis reported in our previous work (M. A. Griffiths, R. Fiebig, M. T. Gore, D. H. Baker, K. Esser, L. Oscai, and L. L. Ji. J. Nutr. 126, 1959-1971, 1996).

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.

Glucose and amino acids interact with hormones to control expression of insulin-like growth factor-I and growth hormone receptor mRNA in cultured pig hepatocytes

Nutrients and hormones are major determinants of animal growth, but the mechanisms of how nutrients influence the growth process are still unclear. A primary pig hepatocyte culture system was used to investigate possible direct effects of glucose and individual amino acids on the expression of growth hormone receptor (GHR) and insulin-like growth factor-I (IGF-I) mRNA. The removal of glucose from the culture medium for 40 h resulted in significant reductions (to 45% of control, P = 0.013) in the expression of GHR in the presence of growth hormone (GH), dexamethasone (DEX) and tri-iodothyronine (T3). The decrease in GHR expression with removal of glucose from the culture medium resulted in a decreased response in class 1 (22% of control, P = 0.011) and 2 (5% of control P = 0. 068) transcripts of IGF-I to any GH added. The effects of glucose on GHR and IGF-I expression were dose-dependent, appearing to plateau at approximately 1-2 g/L (P = 0.031, for quadratic trend). Removal of arginine, proline, threonine, tryptophan or valine inhibited the stimulation of IGF-I expression that was induced by the combination of T3, DEX and GH (to 15, 6, 11, 16 and 16% of control, respectively, P < 0.05), with significant decreases in GHR expression also observed in some cases. The stimulatory effect of some of these amino acids (arginine, proline, threonine and tryptophan) was dose-dependent for expression of class 1 transcripts of IGF-I (P = 0. 041, 0.022, 0.016 and 0.097, respectively, for linear trends), but there was no effect on GHR or class 2 transcripts of IGF-I. Whether the observed effects of nutrients on mRNA levels are due to direct effects on gene transcription or differences in mRNA stability remains to be established. Energy, in the form of glucose, appears to control GHR expression, interacting with the effects of glucocorticoids and thyroid hormones, whereas protein, in the form of certain individual amino acids, appears to control GH-stimulated IGF-I expression.

Glucose consumption by rats decreases cytochrome P450 enzyme activity by altering hepatic lipids

Although glucose is a ubiquitous nutrient, increased consumption of glucose decreases the metabolism of numerous drugs in humans and animals. To understand the mechanisms involved that cause decreased drug metabolism in rats that consume glucose in their water, enzyme activity and expression as well as determining the contribution of the lipids toward decreasing in vitro metabolic activity were investigated. Enzyme assays of hepatic CYP1A2, 2C6, 2C11 and 3A2 showed significant decreases in activity from glucose-treated rats compared to control. While immunodetection of CYP1A1, 2B1/2, 2C11, and 3A1/2 showed no significant difference in protein expression. Hepatic fatty acid synthase activity increased in the glucose-treated rats compared to controls. Studies with glucose-treated microsomal lipids reconstituted with microsomal proteins from control rats caused a significant decrease in benzyloxyresorufin O-dealkylase activity. The results presented here support the hypothesis that the activities of cytochrome P450 proteins are altered by modulating their catalytic activity as a result of the lipid environment rather than changing the level of expression of the individual enzymes.

Identification of a glucose response element in the promoter of the rat glucagon receptor gene

We cloned the 5' upstream region of the rat glucagon receptor gene, demonstrating that the 5' noncoding domain of the glucagon receptor mRNA contained two untranslated exons of 131 and 166 nucleotides (nt), respectively, separated by two introns of 0.6 and 3.2 kilobase pairs. We also observed an alternative splicing involving the 166-base pair exon. Cloning of up to 2 kilobase pairs of the newly identified genomic domain and transfection of various constructs driving a reporter gene, in pancreatic islet cell line INS-1, uncovered a strong glucose regulation of the promoter activity of plasmids containing up to nucleotide -868, or more, upstream from the transcriptional start point. This promoter activity displayed threshold-like behavior, with low activity of the promoter below 5 mM glucose, and maximal activation as of 10 mM glucose. This glucose regulation was mapped to a highly palindromic 19-nucleotide region between nt -545 and -527. Indeed, deletion or mutation of this sequence abolished the glucose regulation. This domain contained two palindromic "E-boxes" CACGTG and CAGCTG separated by 3 nt, a feature similar to the "L4 box" found in the pyruvate kinase L gene promoter. This is the first description of a G protein-coupled receptor gene promoter regulated by glucose.

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.

Essential role in vivo of upstream stimulatory factors for a normal dietary response of the fatty acid synthase gene in the liver

In the liver, transcription of several genes encoding lipogenic and glycolytic enzymes, in particular the gene for fatty acid synthase (FAS), is known to be stimulated by dietary carbohydrates. The molecular dissection of the FAS promoter pointed out the critical role of an E box motif, located at position -65 with respect to the start site of transcription, in mediating the glucose- and insulin-dependent regulation of the gene. Upstream stimulatory factors (USF1 and USF2) and sterol response element binding protein 1 (SREBP1) were shown to be able to interact in vitro with this E box. However, to date, the relative contributions of USFs and SREBP1 ex vivo remain controversial. To gain insight into the specific roles of these factors in vivo, we have analyzed the glucose responsiveness of hepatic FAS gene expression in USF1 and USF2 knock-out mice. In both types of mouse lines, defective in either USF1 or USF2, induction of the FAS gene by refeeding a carbohydrate-rich diet was severely delayed, whereas expression of SREBP1 was almost normal and insulin response unchanged. Therefore, USF transactivators, and especially USF1/USF2 heterodimers, seem to be essential to sustain the dietary induction of the FAS gene in the liver.

A novel factor binding to the glucose response elements of liver pyruvate kinase and fatty acid synthase genes

Transcription of the liver type pyruvate kinase and lipogenesis enzyme genes is induced by high carbohydrate in liver. We have found a novel protein factor in rat liver nuclei that binds to the glucose response element (CACGTG motifs) of the pyruvate kinase gene (Liu, Z. , Thompson, K. S., and Towle, H. C. (1993) J. Biol. Chem. 268, 12787-12795) and the "insulin response element" of fatty acid synthase gene. The amounts of this DNA-binding protein, termed "glucose response element binding protein" (GRBP) in the nuclear extract, were increased in liver by a high carbohydrate diet and decreased by starvation, high fat, and high protein diet. GRBP also occurs in cytosols of liver and is dependent on carbohydrate. Both the nuclear and the cytosolic GRBP showed similar properties, except the former was more resistant to thermal inactivation than the latter. Kinetics of glucose activation of the cytosolic GRBP in a primary culture of hepatocytes indicated that a half-maximum activation was achieved after 6 h, and glucose concentration required for the maximum activation of the GRBP was approximately 12 mM. Dibutyryl-cAMP, okadaic acid, and forskolin inhibited glucose activation of both GRBP and liver pyruvate kinase transcription. These results suggested that GRBP may be a factor that recognizes the glucose response motif site and may be involved in mediating carbohydrate response of the pyruvate kinase gene.

Regulation of mammalian pyruvate dehydrogenase alpha subunit gene expression by glucose in HepG2 cells

We report the effect of glucose on the expression of the gene encoding the pyruvate dehydrogenase (E1) alpha subunit (E1alpha) in human hepatoma (HepG2) cells. Total pyruvate dehydrogenase complex activity as well as the levels of protein and mRNA of the E1alpha subunit were significantly increased in HepG2 cells cultured in medium containing 16.7 mM glucose compared with 1.0 mM glucose for a period of 4 weeks. The level of E1alpha mRNA was elevated approx. 2-fold in HepG2 cells cultured for 24 h in medium containing 16.7 mM glucose compared with 1 mM glucose. This effect was specific to glucose and independent of insulin. Nuclear run-on assays and promoter analysis indicate that the glucose-induced increases in the levels of E1alpha mRNA in HepG2 cells are due to increased transcription of the human E1alpha (PDHA1) gene. Mutational analysis of the E1alpha promoter region has identified two regions, from -78 to -73 bp (CCCCTG) and from -8 to -3 bp (GCGGTG), that are responsible for the effect of glucose on promoter activity; the former exhibits a larger effect. These two sequences represent new variations of the carbohydrate-response element that has been identified in other genes. The stimulation of E1alpha promoter activity by glucose was abolished by okadaic acid at 100 nM but not at 5 nM, suggesting that glucose-mediated regulation of pyruvate dehydrogenase complex E1alpha gene transcription involves a phosphorylation/dephosphorylation mechanism, possibly involving protein phosphatase-1.

Nutritional and hormonal regulation of enzymes in fat synthesis: studies of fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase gene transcription

The activities of critical enzymes in fatty acid and triacylglycerol biosynthesis are tightly controlled by different nutritional, hormonal, and developmental conditions. Feeding previously fasted animals high-carbohydrate, low-fat diets causes a dramatic induction of enzymes-such as fatty acid synthase (FAS) and mitochondrial glycerol-3-phosphate acyltransferase (GPAT)-involved in fatty acid and triacylglycerol synthesis. During fasting and refeeding, transcription of these two enzymes is coordinately regulated by nutrients and hormones, such as glucose, insulin, glucagon, glucocorticoids, and thyroid hormone. Insulin stimulates transcription of the FAS and mitochondrial GPAT genes, and glucagon antagonizes the insulin effect through the cis-acting elements within the promoters and their bound trans-acting factors. This review discusses advances made in the understanding of the transcriptional regulation of FAS and mitochondrial GPAT genes, with emphasis on elucidation of the mechanisms by which multiple nutrients and hormones achieve their effects.

Glucose regulation of gene expression

Regulation of gene expression by nutrients in mammals is an important mechanism allowing them to adapt to the nutritional environment. In-vivo and in-vitro experiments have demonstrated that the transcription of genes coding for lipogenic and glycolytic enzymes in liver and/or adipose tissue is upregulated by glucose. In order for glucose to act as a gene inducer, it must be metabolized. Recent evidence suggests that glucose-6-phosphate is the signal metabolite in the liver. DNA glucose response elements have been characterized and they have in common the presence of two sequences 5'-CACGTG-3' separated by five nucleotides, which bind in vitro a transcription factor of the basic domain, helix-loop-helix, leucine zipper family called USF/MLTF. Experiments concerning the potential role of USF/MLTF in the glucose response have led to opposite results, suggesting that USF/MLTF might not be the only factor involved. Finally, the glucose effect involves a kinase/phosphatase system. The kinase could be the AMP-activated protein kinase, the mammalian analogue of a yeast kinase, or SNF 1 which is important for the derepression of glucose-inhibited genes.

Regulation of hormone-sensitive lipase expression by glucose in 3T3-F442A adipocytes

Hormone-sensitive lipase (HSL) catalyzes the rate-limiting step in adipocyte lipolysis. We have studied the effect of glucose and long chain fatty acids on the HSL mRNA content of 3T3-F442A adipocytes. Exposure of the cells for 32 h to a medium without glucose led to a decrease by a factor of 2.5-3 in the HSL mRNA without a change in C/EBP alpha mRNA or triglyceride content of the adipocytes. The reduction in HSL mRNA level was accompanied by a decrease in HSL total activity. The effect of glucose exposure was observed after 24 h of exposure and was reversible. Treatment of the adipocytes with oleate did not affect HSL expression. These data show that glucose modulates HSL gene expression and hence could participate in the regulation of adipose tissue lipolysis.

Carbon flux via the pentose phosphate pathway regulates the hepatic expression of the glucose-6-phosphatase and phosphoenolpyruvate carboxykinase genes in conscious rats

Hepatic gene expression of P-enolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc-6-Pase) is regulated in response to changes in the availability of substrates, in particular glucose (Glc; Massillon, D., Barzilai, N., Chen, W., Hu, M., and Rossetti, L. (1996) J. Biol. Chem. 271, 9871-9874). We investigated the mechanism(s) in conscious rats. Hyperglycemia per se caused a rapid and marked increase in Glc-6-Pase mRNA abundance and protein levels. By contrast, hyperglycemia decreased the abundance of PEPCK mRNA. Importantly, inhibition of glucokinase activity by glucosamine infusion blunted both the stimulation of Glc-6-Pase and the inhibition of PEPCK gene expression by Glc, suggesting that an intrahepatic signal (metabolite) generated by the metabolism of glucose at or beyond Glc-6-P was responsible for the regulatory effect of Glc. The effect of Glc on the L-type pyruvate kinase gene is mediated by xylulose-5-P (Doiron, B., Cuif, M., Chen, R., and Kahn, A. (1996) J. Biol. Chem. 271, 5321-5324). Thus, we next investigated whether an isolated increase in the hepatic concentration of this metabolite can also reproduce the effects of Glc on Glc-6-Pase and PEPCK gene expression in vivo. Xylitol, which is directly converted to xylulose-5-P in the liver, was infused to raise the hepatic concentration of xylulose-5-P by approximately 3-fold. Xylitol infusion did not alter the levels of Glc-6-P and of fructose-2,6-biphosphate. However, it replicated the effects of hyperglycemia on Glc-6-Pase and PEPCK gene expression and resulted in a 75% increase in the in vivo flux through Glc-6-Pase (total glucose output).

Characterization of the promoter of human adipocyte hormone-sensitive lipase

Hormone-sensitive lipase (HSL) catalyses the rate-limiting step of adipose tissue lipolysis. The human HSL gene is composed of nine exons encoding the adipocyte form and a testis-specific coding exon. Northern blot analyses showed that human adipocytes express a 2.8 kb HSL mRNA, suggesting the presence of a short (20-150 bp) 5' untranslated region (5'-UTR). A single 5'-UTR of approx. 70 nt was detected in RNase H mapping experiments. Two 5'-UTRs of 70 and 170 nt respectively were obtained by rapid amplification of cDNA ends and cDNA library screenings. RNase protection experiments, with probes derived from the two products, showed that human adipocyte HSL mRNA contains only the 70 nt product. Primer extension analysis mapped the transcriptional start site 74 nt upstream of the start codon. In HT29, a human cell line expressing HSL, the presence of the short or the long 5'-UTR is mutually exclusive. The short and long 5'-UTR exons were located 1.5 and approx. 13 kb respectively upstream of the first coding exon. Various portions of the 5'-flanking region upstream of the short product exon were linked to the luciferase gene and transfected into cells that express HSL (HT29 cells and rat adipocytes) and do not express HSL (HeLa cells). High luciferase activity was found for constructs containing the sequence between nt -2400 and -86, but not for shorter constructs. An analysis of 14 kb of genomic sequence revealed the presence of five DNase I hypersensitive sites associated with active gene transcription. Three of the sites are located in the vicinity of the transcriptional start site and could be linked to the minimal promoter activity. Two of the sites are located downstream of the exon containing the start codon, suggesting the presence of intronic regulatory elements.

Effects of increased glucokinase gene copy number on glucose homeostasis and hepatic glucose metabolism

The relationship between glucokinase (GK) gene copy number and glucose homeostasis was studied in transgenic mice with additional copies of the entire GK gene locus (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, 22564-22569). The plasma glucose concentration was reduced by 25 +/- 3% and 37 +/- 4% in mice with one or two extra copies of the gene locus, respectively. The basis for the hypoglycemic phenotype was determined using metabolic tracer techniques in chronically cannulated, conscious mice with one extra GK gene copy. Under basal conditions (6-h fasted) transgenic mice had a lower blood glucose concentration (-12 +/- 1%) and a slightly higher glucose turnover rate (+8 +/- 3%), resulting in a significantly higher glucose clearance rate (+21 +/- 2%). Plasma insulin levels were not different, suggesting that increased glucose clearance was due to augmented hepatic, not islet, GK gene expression. Under hyperglycemic clamp conditions the transgenic mice had glucose turnover and clearance rates similar to the controls, but showed a lower plasma insulin response (-48 +/- 5%). Net hepatic glycogen synthesis was markedly elevated (+360%), whereas skeletal muscle glycogen synthesis was decreased (-40%). These results indicate that increased GK gene dosage leads to increased hepatic glucose metabolism and, consequently, a lower plasma glucose concentration. Increased insulin secretion was not observed, even though the transgene is expressed in islets, because hypoglycemia causes a down-regulation in islet GK content (Niswender, K. D., Postic, C., Jetton, T. L., Bennett, B. D., Piston, D. W., Efrat, S., and Magnuson, M. A. (1997) J. Biol. Chem. 272, in press).

Glucose response elements in a gene that codes for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

We have shown previously that rat hepatoma FTO-2B cells express two mRNAs, called F (fetal) and L (liver), from distinct promoters of the same gene coding for 6-phosphofructo-2-kinase (PFK-2). This enzyme catalyzes the synthesis of fructose 2,6-bisphosphate, an allosteric stimulator of glycolysis. We have now found that glucose, as well as lactate and pyruvate, increases the concentration of the F and L mRNAs. The effect of glucose was mimicked by xylitol, a precursor of xylulose 5-phosphate, and hence of intermediates of the pentose phosphate and glycolytic pathways, and was inhibited by okadaic acid, an inhibitor of protein phosphatases. Transfection experiments showed that the F promoter region is a target of the glucose effect, with glucose stimulating F promoter activity in a way probably similar to mitogens. Another region of the gene, located between the F and L promoters, also behaved as a glucose-sensitive element. This region corresponds to a cluster of DNase I-hypersensitive sites that were induced in chromatin following glucose treatment. The sequence organization of this region is very similar to the functional architecture of the glucose-sensitive insulin gene promoter. We propose a model for the concerted regulation by glucose metabolites of three pathways that lead to increased PFK-2 activity.

Cooperative binding of NF-Y and Sp1 at the DNase I-hypersensitive site, fatty acid synthase insulin-responsive element 1, located at -500 in the rat fatty acid synthase promoter

In vitro DNase I footprint analysis of the rat fatty acid synthase (FAS) promoter from -568 to -468 revealed four protein binding sites: A, B, and C boxes and the FAS insulin-responsive element 1 (FIRE1). As demonstrated by gel mobility shift analysis and supershift experiments, FIRE1, located between -516 and -498, is responsible for binding NF-Y. The C box located downstream of FIRE1 was shown by in vitro footprinting to be a Sp1 binding site, and furthermore, competition with Sp1 also abolished FIRE1 binding. Since the half-life of the Sp1.NF-Y.DNA complex is significantly longer than the half-lives of the Sp1.DNA or NF-Y.DNA complexes, the two transcription factors are deemed to bind cooperatively in the FAS promoter at -500. It is unusual that NF-Y binds at this distance from the start site of transcription. NF-Y binding sites are found in the promoters of at least three other FAS genes, viz. goose, chicken, and man. A second NF-Y binding site is located in the FAS promoter at the more usual position of -103 to -87, and it too has a neighboring Sp1 site. CTF/NF-1 competes for proteins binding to the B box. The A box binds Sp1 and contains a 12/13 match of the inverted repeat sequence responsible for binding the nuclear factor EF-C/RFX-1 in the enhancer regions of hepatitis B virus and the major histocompatibility complex class II antigen promoter. The same relative positions of NF-Y and Sp1 binding sites in the promoters of FAS genes of goose, rat, chicken, and man emphasize the involvement of these transcription factors in the diet and hormonal regulation of FAS.

Regulatory elements in the first intron of the rat fatty acid synthase gene

Sequence elements have been identified within the 1.2 kb-long first intron of the fatty acid synthase (FAS) gene that mediate both positive and negative effects on transcription. The negative regulatory element, when positioned downstream of either the FAS or simian virus 40 promoter, down-regulates the expression of a coupled reporter gene in an orientation-dependent manner. Sequences mediating this effect have been mapped, by deletion mutagenesis, to two regions approximately within nucleotides +405 to +768 and +924 to +1083. Both regions contain sequence elements that are strongly protected from DNase I digestion by nuclear extracts prepared from liver, but not by those prepared from spleen. The results of run-on assays performed with nuclei derived from tissues that express FAS at either high or low levels indicate that the different rates of transcription of the endogenous FAS gene result from differences in the extent of initiation, so it is unlikely that the negative effect is caused by transcriptional pausing in the first intron. The positive element maps to nt +292 to +297 and corresponds to an authentic binding site for upstream stimulatory factor (USF). This USF-binding element can up-regulate transcription from a heterologous promoter in a position- and orientation-independent manner. However, in the context of the entire FAS first intron, the effect of the USF-binding site is masked unless the effect of the negative elements is ablated by mutagenesis. These results suggest that the dominant negative element of the first intron may play a role in determining the tissue-specific expression of the FAS gene.

Regulation of human renin secretion and gene transcription in Calu-6 cells

Calu-6 cells were characterized for studying the transcriptional regulation of the human renin gene. Analysis of cis-acting elements of the renin promoter showed the highest activity within the first 582 bp in serum-free conditions and of the 892 bp in the presence of serum. cAMP activates renin mRNA synthesis parallel to renin production (20-fold increase) as well renin promoter activity (2-fold). cAMP response element and the (-77 to -67) element are both necessary for activation of the renin promoter but do not act independently. Functional analysis of Intron A revealed the presence of a silencer specific to renin-producing cells.

Mechanisms by which carbohydrates regulate expression of genes for glycolytic and lipogenic enzymes

Regulation of gene expression by nutrients is an important mechanism in the adaptation of mammals to their nutritional environment. This is especially true for enzymes involved in the storage of energy, such as the lipogenic and glycolytic enzymes in liver and adipose tissue. Transcription of the genes for lipogenic and glycolytic enzymes is stimulated by glucose in adipose tissue, liver, and pancreatic beta-cells. Several lines of evidence suggest that glucose must be metabolized to glucose-6-phosphate to stimulate gene transcription. In adipose tissue, insulin increases the expression of lipogenic enzymes indirectly by stimulating glucose uptake. In the liver, insulin also acts indirectly by stimulating the expression of glucokinase and, hence, by increasing glucose metabolism. Glucose response elements have been characterized for the L-pyruvate kinase and S14 genes. They have in common the presence of a sequence 5'-CACGTG-3', which binds a transcription factor called USF (upstream stimulatory factor). Another glucose response element, which uses a transcription factor named Sp1, has been characterized in the gene for the acetyl-coenzyme A carboxylase. The mechanisms linking glucose-6-phosphate to the glucose-responsive transcription complex are largely unknown.

Characterization of the chicken fatty acid synthase gene 5' part and promoter region

Fatty acid synthase activity has been shown to be regulated mainly at the transcriptional level under both dietary and hormonal influences. As a first step towards elucidating the factors involved, we isolated and characterized chicken genomic clones encompassing the 5' part of the chicken fatty acid synthase gene and its flanking region. The entire region of the cloned DNA spans 30 kb, and the first three exons of the gene were mapped to a 6.3-kb genomic fragment. The transcription initiation site was determined after subcloning the cDNA which encodes the 5' end of the mRNA. The first exon, which was 129 bp long, was located approximately 5.3 kb upstream of the second exon, which contained the start codon. In the 5' flanking region, putative TATA and CAAT boxes were located 30 and 92 bp, respectively, upstream of the transcription initiation site. The 5' flanking region contained numerous sequences corresponding to consensus binding sites for transcription factors. Various lengths of flanking sequences extending up to 1028 bp upstream of the transcription initiation site and containing 100 bp of the first exon were linked to the bacterial chloramphenicol acetyltransferase gene; in this study, these constructs were analyzed in transient transfection assays in human hepatoma cells. The proximal 125-bp sequence upstream of the transcription start site was shown to be a basal promoter. The cloning and characterization of the chicken fatty-acid synthase gene provides some further insight into the regulation of fatty acid synthesis in birds as compared to mammals.

Transcriptional glucose signaling through the glucose response element is mediated by the pentose phosphate pathway

Glucose catabolism induces the expression of the L-type pyruvate kinase (L-PK) gene through the glucose response element (GIRE). The metabolic pathway used by glucose after its phosphorylation to glucose 6-phosphate by glucokinase to induce L-PK gene expression in hepatocytes remains unknown. The sugar alcohol xylitol is metabolized to xylulose 5-phosphate, an intermediate of the nonoxidative branch of the pentose phosphate pathway. In this study, we demonstrated that xylitol at low concentration (O.5 mM) induced the expression of the L-PK/CAT construct in glucose-responsive mhAT3F hepatoma cells at the same level as 20 mM glucose, while it did not affect intracellular concentration of glucose 6-phosphate significantly. The effect of xylitol on the induction of the L-PK gene expression was noncumulative with that of glucose since 20 mM glucose plus 5 mM xylitol induced the expression of the L-PK/CAT construct similarly to 20 mM glucose alone. In hepatocytes in primary culture, 5 mM xylitol induced accumulation of the L-PK mRNA even in the absence of insulin. Furthermore, the response to xylitol as well as glucose required the presence of a functional GIRE. It can be assumed from these results that glucose induces the expression of the L-PK gene through the nonoxidative branch of the pentose phosphate pathway. The effect of xylitol at low concentration suggests that the glucose signal to the transcriptional machinery is mediated by xylulose 5-phosphate.

Regulation of lipogenic enzyme expression by glucose in liver and adipose tissue: a review of the potential cellular and molecular mechanisms

Regulation of gene expression by nutrients is an important part of the mechanisms allowing mammals to adapt to their nutritional environment. This is especially true for enzymes involved in the storage of energy such as the lipogenic and glycolytic enzymes in the liver and adipose tissue. We review in the present paper the cellular and molecular mechanisms involved in the regulation of glycolytic and lipogenic enzyme gene expression by glucose. In vivo and in vitro experiments have demonstrated that FAS and ACC gene expression is upregulated by glucose in adipose tissue, FAS, ACC and L-PK expression in the liver and ACC and L-PK expression in a pancreatic beta-cell line. This regulation involves the stimulation of their transcription. In order for glucose to act as a gene inducer, it must be metabolized. In adipose tissue, insulin increases indirectly the expression of FAS and ACC by stimulating glucose metabolism through its well-known effect on glucose transport. In the liver, the action of insulin is also indirect by allowing the expression of glucokinase and hence by increasing glucose metabolism. In the liver, fructose has a potentiating effect on the stimulation of gene expression by glucose through its stimulatory effect on glucokinase activity. Several evidences suggest that glucose-6-phosphate is the signal metabolite: (i) the effect of glucose is mimicked by 2-deoxyglucose (a glucose analogue whose metabolism stops after its phosphorylation by hexokinase) in adipose tissue and beta-cell line but not in the liver in which 2-deoxyglucose-6-phosphate does not accumulate, (ii) intracellular glucose-6-phosphate concentration varies in parallel with ACC, FAS and L-PK mRNA concentrations in liver, adipose tissue and beta-cell line, (iii) in vivo, the kinetics of hexose-phosphate fits with the time-related pattern of gene induction. Glucose response elements have been characterized on three genes, L-PK, S14 (a gene which codes for a protein of unknown function but which is directly related to lipogenesis) and FAS. These glucose response elements have all in common the presence of a sequence 5'-CACGTG-3' which binds a transcription factor of the basic domain, helix-loop-helix, leucine zipper family called USF/MLTF, although the organization of the overall glucose response element probably differs from one gene to another. The mechanisms linking glucose-6-phosphate to the glucose responsive transcription complex are presently largely unknown.

Hormonal and nutritional control of the fatty acid synthase promoter in transgenic mice

To study the molecular basis of tissue-specific and hormonally regulated expression of the fatty acid synthase (FAS) gene in vivo, we generated lines of transgenic mice carrying 2.1 kilobases of the 5'-flanking region (-2100 to +67) of the rat FAS gene fused to a chloramphenicol acetyltransferase (CAT) reporter gene. This reporter gene construct was strongly expressed in tissues that normally express high levels of FAS mRNA, which include liver and white adipose tissues. In contrast, CAT reporter activity was not detected in appreciable levels in lung, heart, kidney, and muscle tissues, which do not normally show significant levels of FAS activity. The relative levels of the CAT mRNA driven by the rat FAS promoter in various tissues of the transgenic animals approximated those of the endogenous mouse FAS mRNA. We also examined the hormonal and nutritional regulation of the FAS(2.1)-CAT reporter gene in transgenic mice. CAT activity was increased in both liver and white adipose tissue when fasted animals were refed a high carbohydrate, fat-free diet. These changes in CAT activity and CAT mRNA levels occurred in parallel to the changes in endogenous mouse FAS mRNA levels. On the other hand, fasting/refeeding did not change CAT activity appreciably in other tissues, such as muscle and brown adipose tissue. Administration of dibutyryl cAMP at the start of refeeding prevented an increase in CAT activity in liver. However, the cAMP effect was tissue-specific as cAMP treatment did not bring about change in CAT activity in adipose tissue. Next, to examine the effect of insulin, we made the transgenic mice insulin-deficient by streptozotocin treatment. Insulin treatment of the streptozotocin-diabetic mice increased both the CAT activity and CAT mRNA levels driven by the rat FAS promoter in liver and white adipose tissue. These changes in CAT expression by insulin paralleled those in endogenous FAS mRNA levels. Administration of glucocorticoids increased CAT activity in all tissues examined: liver, white and brown adipose tissues, lung, heart, and spleen. Overall, the first 2.1 kilobases of the 5'-flanking region of the rat FAS gene appear to contain sequence elements necessary to confer tissue-specific and hormonally regulated expression characteristic of the endogenous FAS gene.