Insulin rapidly induces rat liver S14 gene transcription

A Nine-Strain Bacterial Consortium Improves Portal Hypertension and Insulin Signaling and Delays NAFLD Progression In Vivo

The gut microbiome has a recognized role in Non-alcoholic fatty liver disease (NAFLD) and associated comorbidities such as Type-2 diabetes and obesity. Stool transplantation has been shown to improve disease by restoring endothelial function and insulin signaling. However, more patient-friendly treatments are required. The present study aimed to test the effect of a defined bacterial consortium of nine gut commensal strains in two in vivo rodent models of Non-alcoholic steatohepatitis (NASH): a rat model of NASH and portal hypertension (PHT), and the Stelic animal (mouse) model (STAM™). In both studies the consortium was administered orally q.d. after disease induction. In the NASH rats, the consortium was administered for 2 weeks and compared to stool transplant. In the STAM™ study administration was performed for 4 weeks, and the effects compared to vehicle or Telmisartan at the stage of NASH/early fibrosis. A second group of animals was followed for another 3 weeks to assess later-stage fibrosis. In the NASH rats, an improvement in PHT and endothelial function was observed. Gut microbial compositional changes also revealed that the consortium achieved a more defined and richer replacement of the gut microbiome than stool transplantation. Moreover, liver transcriptomics suggested a beneficial modulation of pro-fibrogenic pathways. An improvement in liver fibrosis was then confirmed in the STAM™ study. In this study, the bacterial consortium improved the NAFLD activity score, consistent with a decrease in steatosis and ballooning. Serum cytokeratin-18 levels were also reduced. Therefore, administration of a specific bacterial consortium of defined composition can ameliorate NASH, PHT, and fibrosis, and delay disease progression.

The serum level of a novel lipogenic protein Spot 14 was reduced in metabolic syndrome

Spot 14 (S14) protein is primarily expressed in adipogenic tissues. Compared to wild type, S14 knockout mice had better resistance to diet-induced obesity and glucose tolerance. However, the association between serum S14 level and metabolic variables in humans has never been investigated. The objectives of this study were to evaluate the associations between serum S14 concentrations with components of metabolic syndrome (MetS). A total of 327 subjects were recruited in this cross-sectional study and categorized by presence of MetS. The mean serum levels of S14 were significantly lower in subjects with MetS than those without (87.1±26.3 μg/L vs. 107.3±40.2 μg/L, p<0.001). In addition, the subjects with central obesity, low high density lipoprotein-C (HDL-C) or hypertriglyceridemia also had significantly lower S14 levels in comparison to those without. Adjusted with age and sex, diagnosis of MetS (β = -0.227, p<0.001), central obesity (β = -0.176, p = 0.001), low HDL-C (β = -0.149, p = 0.005), and high triglyceride (TG) (β = -0.198, p<0.001) were negatively associated with log transformation of serum S14 levels (logS14). With 25% logS14 increased, the risk of MetS (OR 0.65, 95% CI, 0.51-0.82, p<0.001), central obesity (OR 0.72, 95% CI, 0.58-0.89, p = 0.002), low HDL-C (OR 0.76, 95% CI, 0.61-0.95, p = 0.015) or high TG (OR 0.65, 95% CI, 0.51-0.83, p = 0.001) was reduced with a dose response trend. Our analysis revealed that patients with MetS had lower serum S14 levels than those without. Negative associations existed between MetS, central obesity, high TG, low HDL-C and logS14.

Serum Spot 14 concentration is negatively associated with thyroid-stimulating hormone level

Spot 14 (S14) is a protein involved in fatty acid synthesis and was shown to be induced by thyroid hormone in rat liver. However, the presence of S14 in human serum and its relations with thyroid function status have not been investigated.The objectives of this study were to compare serum S14 concentrations in patients with hyperthyroidism or euthyroidism and to evaluate the associations between serum S14 and free thyroxine (fT4) or thyroid-stimulating hormone (TSH) levels.We set up an immunoassay for human serum S14 concentrations and compared its levels between hyperthyroid and euthyroid subjects. Twenty-six hyperthyroid patients and 29 euthyroid individuals were recruited. Data of all patients were pooled for the analysis of the associations between the levels of S14 and fT4, TSH, or quartile of TSH.The hyperthyroid patients had significantly higher serum S14 levels than the euthyroid subjects (median [Q1, Q3]: 975 [669, 1612] ng/mL vs 436 [347, 638] ng/mL, P < 0.001). In univariate linear regression, the log-transformed S14 level (logS14) was positively associated with fT4 but negatively associated with creatinine (Cre), total cholesterol (T-C), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and TSH. The positive associations between logS14 and fT4 and the negative associations between logS14 and Cre, TG, T-C, or TSH remained significant after adjustment with sex and age. These associations were prominent in females but not in males. The logS14 levels were negatively associated with the TSH levels grouped by quartile (ß = -0.3020, P < 0.001). The association between logS14 and TSH quartile persisted after adjustment with sex and age (ß = -0.2828, P = 0.001). In stepwise multivariate regression analysis, only TSH grouped by quartile remained significantly associated with logS14 level.We developed an ELISA to measure serum S14 levels in human. Female patients with hyperthyroidism had higher serum S14 levels than the female subjects with euthyroidism. The serum logS14 concentrations were negatively associated with TSH levels. Changes of serum S14 level in the whole thyroid function spectrum deserve further investigation.

Modulation of gene expression by 3-iodothyronamine: genetic evidence for a lipolytic pattern

3-Iodothyronamine (T1AM) is an endogenous biogenic amine, structurally related to thyroid hormone, which is regarded as a novel chemical messenger. The molecular mechanisms underlying T1AM effects are not known, but it is possible to envisage changes in gene expression, since delayed and long-lasting phenotypic effects have been reported, particularly with regard to the modulation of lipid metabolism and body weight. To test this hypothesis we analysed gene expression profiles in adipose tissue and liver of eight rats chronically treated with T1AM (10 mg/Kg twice a day for five days) as compared with eight untreated rats. In vivo T1AM administration produced significant transcriptional effects, since 378 genes were differentially expressed in adipose tissue, and 114 in liver. The reported changes in gene expression are expected to stimulate lipolysis and beta-oxidation, while inhibiting adipogenesis. T1AM also influenced the expression of several genes linked to lipoprotein metabolism suggesting that it may play an important role in the regulation of cholesterol homeostasis. No effect on the expression of genes linked to toxicity was observed. The assay of tissue T1AM showed that in treated animals its endogenous concentration increased by about one order of magnitude, without significant changes in tissue thyroid hormone concentration. Therefore, the effects that we observed might have physiological or pathophysiological importance. Our results provide the basis for the reported effectiveness of T1AM as a lipolytic agent and gain importance in view of a possible clinical use of T1AM in obesity and/or dyslipidaemia.

Thyroid hormone responsive protein Spot14 enhances catalysis of fatty acid synthase in lactating mammary epithelium

Thyroid hormone responsive protein Spot 14 has been consistently associated with de novo fatty acid synthesis activity in multiple tissues, including the lactating mammary gland, which synthesizes large quantities of medium chain fatty acids (MCFAs) exclusively via FASN. However, the molecular function of Spot14 remains undefined during lactation. Spot14-null mice produce milk deficient in total triglyceride and de novo MCFA that does not sustain optimal neonatal growth. The lactation defect was rescued by provision of a high fat diet to the lactating dam. Transgenic mice overexpressing Spot14 in mammary epithelium produced total milk fat equivalent to controls, but with significantly greater MCFA. Spot14-null dams have no diminution of metabolic gene expression, enzyme protein levels, or intermediate metabolites that accounts for impaired de novo MCFA. When [(13)C] fatty acid products were quantified in vitro using crude cytosolic lysates, native FASN activity was 1.6-fold greater in control relative to Spot14-null lysates, and add back of Spot14 partially restored activity. Recombinant FASN catalysis increased 1.4-fold and C = 14:0 yield was enhanced 4-fold in vitro following addition of Spot14. These findings implicate Spot14 as a direct protein enhancer of FASN catalysis in the mammary gland during lactation when maximal MCFA production is needed.

Thyroid hormone responsive Spot 14 increases during differentiation of human adipocytes and its expression is down-regulated in obese subjects

CONTEXT

Very limited information is available regarding the function of human thyroid hormone responsive Spot 14 (human S14, hS14) in adipogenesis and human adiposity.

OBJECTIVE

To evaluate hS14 levels during differentiation of human pre-adipocytes, in human fat depots and isolated fat cells.

DESIGN

This was a cross-sectional study.

SUBJECTS

A total of 161 omental (OM) and 87 subcutaneous (SC) adipose tissue samples obtained during elective surgical procedures from a population who varied widely in terms of obesity.

MEASUREMENTS

hS14 gene expression and protein levels during adipogenesis were assessed by RT-PCR, western blot, and using an automated confocal imaging approach.

RESULTS

hS14 gene expression levels were decreased in OM adipose tissue from overweight (-42.0%) and obese subjects (-56.5%) compared with lean subjects (P<0.05 and P<0.0001, respectively). hS14 mRNA (but not hS14-related) was inversely associated with obesity measures such as body mass index (P=0.001), percent fat mass (P=0.001), waist-to-hip ratio (P=0.020), and systolic blood pressure (P=0.031). hS14 gene expression and protein levels were up-regulated at the early stages of differentiation of human pre-adipocytes as well as for 3T3-L1 cells. That observation was most prominent in those individual cells exhibiting the more marked differentiation features. hS14 gene expression levels increased by approximately 45 000-fold in mature adipocytes. Increased hS14 levels were also found in stromal-vascular cells/pre-adipocytes (3.8-fold, P<0.05) and in adipose tissue samples (1.9-fold, P<0.0001) from SC compared with OM fat depots.

CONCLUSIONS

These results suggest that hS14 is involved in human adipogenesis, but inversely related to obesity and OM fat accumulation.

Activation of TR4 orphan nuclear receptor gene promoter by cAMP/PKA and C/EBP signaling

In earlier studies, we had suggested that the fasting signal induces TR4 orphan nuclear receptor expression in vivo. The detailed mechanism(s), however, remain unclear. In this study, we found that cAMP/PKA, the mediator of fasting and glucagon signals, could induce TR4 gene expression that in turn modulates gluconeogenesis. Mechanistic dissection by in vitro studies in hepatocytes demonstrated that cAMP/PKA might trigger C/EBP alpha and beta binding to the selective cAMP response element, which is located at the TR4 promoter, thus inducing TR4 transcription. We also demonstrated that the binding activity of C/EBPs to the TR4 promoter is increased in response to cAMP treatment. Together, our data identified a new signaling pathway from the fasting signal --> cAMP/PKA --> C/EBP alpha and beta --> TR4 --> gluconeogenesis in hepatocytes; and suggested that TR4 could be an important regulator to control glucose homeostasis. The identification of activator(s)/inhibitor(s) or ligand(s) of TR4 may provide us an alternative way to control gluconeogenesis.

Glucose-responsive gene expression system for gene therapy

Regulation of gene expression by glucose is an important mechanism for mammals in adapting to their nutritional environment. Glucose, the primary fuel for most cells, modulates gene expression that is crucial in the cellular adaptation to glycemic variation. Transcription of the genes for insulin and glycolytic and lipogenic enzymes is stimulated by glucose in pancreatic beta-cells and liver. Recent findings further support the key role of the carbohydrate-responsive element binding protein in the regulation of glycolytic and lipogenic genes by glucose and dietary carbohydrates. Herein, we review the transcriptional regulation of glucose-responsive genes, and recent advances in the gene therapy using glucose-responsive gene expression for diabetes.

S14: insights from knockout mice

Spot 14 (S14) is a protein whose mRNA is rapidly up-regulated by lipogenic stimuli including thyroid hormone and a high-carbohydrate diet. Previous investigation into the role of S14 suggested that it is involved in de novo lipogenesis. Knockout of the gene in mice has given further support to this hypothesis. The lack of S14 in different tissues resulted in varying phenotypic effects. In the lactating mammary gland, levels of lipogenesis, specifically the production of medium chain fatty acids, were decreased, whereas hepatic lipogenesis was not decreased. In fact, hepatic lipogenesis was increased, and the increase may be due to compensation by a paralog of S14 called S14-R. S14-R is expressed in the liver but not the mammary gland. Importantly, S14 knockout mice did not have reduced levels of lipogenic enzymes, implying that it does not affect the transcriptional rate of those enzymes. Instead, S14 may act in the cytoplasm to affect lipogenesis.

Regulation of hepatic fatty acid elongase and desaturase expression in diabetes and obesity

Fatty acid elongases and desaturases play an important role in hepatic and whole body lipid composition. We examined the role that key transcription factors played in the control of hepatic elongase and desaturase expression. Studies with peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice establish that PPARalpha was required for WY14643-mediated induction of fatty acid elongase-5 (Elovl-5), Elovl-6, and all three desaturases [Delta(5) desaturase (Delta(5)D), Delta(6)D, and Delta(9)D]. Increased nuclear sterol-regulatory element binding protein-1 (SREBP-1) correlated with enhanced expression of Elovl-6, Delta(5)D, Delta(6)D, and Delta(9)D. Only Delta(9)D was also regulated independently by liver X receptor (LXR) agonist. Glucose induction of l-type pyruvate kinase, Delta(9)D, and Elovl-6 expression required the carbohydrate-regulatory element binding protein/MAX-like factor X (ChREBP/MLX) heterodimer. Suppression of Elovl-6 and Delta(9)D expression in livers of streptozotocin-induced diabetic rats and high fat-fed glucose-intolerant mice correlated with low levels of nuclear SREBP-1. In leptin-deficient obese mice (Lep(ob/ob)), increased SREBP-1 and MLX nuclear content correlated with the induction of Elovl-5, Elovl-6, and Delta(9)D expression and the massive accumulation of monounsaturated fatty acids (18:1,n-7 and 18:1,n-9) in neutral lipids. Diabetes- and obesity-induced changes in hepatic lipid composition correlated with changes in elongase and desaturase expression. In conclusion, these studies establish a role for PPARalpha, LXR, SREBP-1, ChREBP, and MLX in the control of hepatic fatty acid elongase and desaturase expression and lipid composition.

Selective proteolytic processing of rat hepatic sterol regulatory element binding protein-1 (SREBP-1) and SREBP-2 during postnatal development

Sterol regulatory element-binding protein-1c (SREBP-1c) plays a major role in hepatic lipogenic gene expression. In adult animals, insulin and oxysterols induce SREBP-1c gene transcription, whereas polyunsaturated fatty acids suppress the nuclear content of SREBP-1c through pre-translational regulatory mechanisms. A decline in nuclear SREBP-1 is associated with suppression of hepatic lipogenesis. In contrast to adult rats, hepatic lipogenesis in preweaned neonatal rats is low. Ingestion of milk fat by the neonate may contribute to low hepatic lipogenesis. In this report, we tested the hypothesis that low lipogenic gene expression prior to weaning correlates with low mRNA(SREBP-1c), as well as low precursor and nuclear forms of SREBP-1. In contrast to expectations, levels of mRNA(SREBP-1c) and the 125-kDa SREBP-1 precursor in livers of preweaned rats was comparable with adult levels. Despite high levels of SREBP-1 precursor, mature (65 kDa) SREBP-1 was not detected in rat liver nuclei prior to 18 days postpartum. Weaning rats at 21 days postpartum was accompanied by a rise in nuclear SREBP-1 levels as well as increased lipogenic gene expression. In contrast, SREBP-2 was present in rat liver nuclei, and its target gene, HMG-CoA reductase, was expressed above adult levels prior to weaning. These studies indicate that, prior to weaning, SREBP-2 but not SREBP-1 is proteolytically processed to the mature form. As such, SREBP-2-regulated genes are active. Failure of SREBP-1 to be processed to the mature form <18 days postpartum correlates with low hepatic lipogenic gene expression. This mechanism differs from the hormonal and fatty acid-mediated pre-translational control of SREBP-1c in adult liver.

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.

Spot 14 protein interacts and co-operates with chicken ovalbumin upstream promoter-transcription factor 1 in the transcription of the L-type pyruvate kinase gene through a specificity protein 1 (Sp1) binding site

In hepatocytes, the amount of the Spot 14 (S14) protein is closely related to the full expression of enzymes involved in the glycolytic and lipogenic pathways. In the present study we address the role played by this protein in the control of transcription of the L-type pyruvate kinase (L-PK) gene in primary hepatocytes. We show that human S14, which by itself does not bind to the L-PK promoter, physically interacts with the human chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1) and induces the switch of this factor from a repressor to an activator. However, the enhancing activity of S14 and COUP-TF1 depends on the presence of a proximal GC-rich box (the L0 element) that specifically binds nuclear proteins from the livers of rats fed a glucose-rich diet. Moreover, the L0 element, which strongly binds dephosphorylated specificity protein 1 (Sp1), loses all affinity when this factor is phosphorylated by cAMP-dependent protein kinase. Mutations that affect binding of Sp1 and nuclear proteins to the L0 box also decrease basal transcription and impair glucose responsiveness of the promoter. These results therefore shed light on the mechanism by which the S14 protein, whose concentration rapidly rises after glucose intake, contributes to the full activity of the L-PK promoter.

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).

Effects of diabetes and insulin on hepatic delta6 desaturase gene expression

It has been recognized that rat liver microsomal Delta6 desaturation activity is defective in experimental diabetes, a fact that may be reverted by means of insulin treatment. In the present study, we used streptozotocin-induced diabetic rats in order to determine the regulatory role of insulin on the expression of hepatic Delta6 desaturase gene. The abundance of hepatic Delta6 desaturase mRNA in the diabetic rats is sevenfold lower than in the control. Insulin administration to diabetic rats induces Delta6 desaturase mRNA eightfold within 24 h. The effect of insulin on the Delta6 desaturase mRNA was inhibited 70% with dibutyryl-cAMP and theophylline administration and 90% by cycloheximide administration. Therefore, our data demonstrate that the activity of hepatic Delta6 desaturase in response to insulin is, at least in part, regulated by pretranslational events that require the synthesis of an unknown protein(s).

Glucose and insulin function through two distinct transcription factors to stimulate expression of lipogenic enzyme genes in liver

Transcription of a number of genes involved in lipogenesis is stimulated by dietary carbohydrate in the mammalian liver. Both insulin and increased glucose metabolism have been proposed to be initiating signals for this process, but the pathways by which these effectors act to alter transcription have not been resolved. We have previously defined by electrophoretic mobility shift assay a factor in nuclear extracts from rat liver, designated the carbohydrate-responsive factor (Cho- RF), that binds to liver-type pyruvate kinase and S(14) promoters at sites critical for regulation by carbohydrate. The sterol regulatory element binding protein-1c (SREBP-1c) has also emerged as a major transcription factor involved in this nutritional response. In this study, we examined the relationship between SREBP-1c and ChoRF in lipogenic gene induction. The two factors were found to possess distinct DNA binding specificities both in vitro and in hepatocytes. Reporter constructs containing binding sites for ChoRF were responsive to glucose but not directly to insulin. On the other hand, reporter constructs with an SREBP-1c site responded directly to insulin. The S(14) gene possesses binding sites for both ChoRF and SREBP, and both sites were found to be functionally important for the response of this promoter to glucose and insulin in hepatocytes. Consequently, we propose that SREBP-1c and ChoRF are independent transcription factors that mediate signals generated by insulin and glucose, respectively. For many lipogenic enzyme genes, these two factors may provide an integrated signaling system to support the overall nutritional response to dietary carbohydrate.

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.

Hormonal regulation of stearoyl coenzyme-A desaturase 1 activity and gene expression in primary cultures of chicken hepatocytes

Previous studies have provided evidence for the important role of liver stearoyl-CoA desaturase (SCD) in excessive adiposity in the chicken and suggest that the difference in SCD activity between fat and lean chickens could be explained by a difference in SCD1 gene expression. In the present study, the regulation of SCD1 gene expression was analyzed as the result of insulin and glucagon action, using primary cultures of 6-week-old chicken hepatocytes. Insulin increased SCD1 activity and mRNA levels, whereas glucagon decreased dramatically both the enzyme activity and the mRNA levels. Nuclear run-on transcription assays and mRNA stability investigations demonstrated that insulin and glucagon effects on SCD1 gene expression was primarily transcriptional. Furthermore, the results indicated that the glucagon-mediated inhibition of SCD1 gene transcription was more potent than just counteracting the insulin-mediated effect. These data represent the first demonstration that the glucagon effect on the SCD1 gene expression is primarily transcriptional. Moreover, among hepatic genes involved in lipid metabolism in chicken, SCD1 is the first gene shown to be regulated at the transcriptional level by insulin, in the absence of triiodothyronine. These data point out the potency of the growing chicken hepatocyte culture model in contrast with the embryonic cell culture model as regards the investigations of the insulin effect on gene expression.

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.

"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.

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

Triiodothyronine (T3) activates rat liver S14 gene transcription through T3 receptors (TRbeta) binding distal thyroid hormone response elements located between -2.8 and -2.5 kilobase pairs upstream from the transcription start site. Previous studies suggested that proximal promoter elements located between -220 to -80 base pairs upstream from the 5' end of the S14 gene were involved in hormone activation of the S14 gene. This report identifies an inverted CCAAT box (or Y box) at -104ATTGG-100 as a core cis-regulatory element. Gel shift studies using rat liver nuclear proteins show that at least three CCAAT-binding factors interact with this region as follows: NF-Y and c/EBP-related proteins formed major complexes, whereas NF-1/CTF forms a minor complex in gel shift assay. Mutation of the Y box indicated that loss of NF-Y binding, but not c/EBP or NF-1, correlated closely with a decline in basal activity and a loss of T3-mediated transactivation. Substitution of the S14 Y box in reporter genes with elements binding only NF-Y elevated basal activity and T3-mediated transactivation, whereas substitution with elements binding c/EBP-related proteins or SP1 displayed low basal activity and T3-mediated transactivation. These studies indicate that NF-Y and TRbeta functionally interact to confer T3 control to the S14 gene.

Regulation of the expression of lipogenic enzyme genes by carbohydrate

Diets high in simple carbohydrates and low in fats lead in the mammalian liver to induction of a set of enzymes involved in lipogenesis. This induction occurs, in part, through transcriptional mechanisms that lead to elevated levels of the mRNA for these enzymes. For most of the lipogenic enzymes, an increase in glucose metabolism is required to trigger the transcriptional response. The intracellular mediator of this signaling pathway is unknown, although evidence suggests either glucose-6-phosphate or xylulose-5-phosphate. Studies to map the regulatory sequences of lipogenic enzyme genes involved in the transcriptional response have been performed for the L-type pyruvate kinase, S14, and acetyl-coenzyme A carboxylase genes. These studies have identified the DNA sequences necessary to link the signal generated by carbohydrate metabolism to specific nuclear transcription factors.

Polyunsaturated fatty acids inhibit hepatic stearoyl-CoA desaturase-1 gene in diabetic mice

Insulin and dietary fructose independently induce stearoyl-CoA desaturase 1 (SCD1) gene expression in diabetic mouse liver. In the present study, we again used diabetic mice and supplemented a high fructose diet with polyunsaturated fatty acids (PUFA) to determine the selective repression of SCD1 gene expression by dietary PUFA, as previously shown in normal mice. We saw dramatic repression of SCD1 mRNA expression, with trilinolenin at 3% and triarachidonin at 1% supplementation. We also observed significant repression of insulin-induced SCD1 mRNA upon supplementation of the noninducing starch diet with PUFA. In conclusion, our data demonstrate that PUFA negatively regulate hepatic gene expression through an insulin-independent mechanism.

Expression of S14 protein gene in rat liver in response to partial hepatectomy, and its regulation with T3 and sucrose

In rat liver a significant decrease in the expression of the protein S14 gene was observed 4 h after partial hepatectomy compared to sham operation performed after overnight fasting. After 24 h a decrease was recorded after partial hepatectomy or sham operation and this low level persisted for 120 h. A single i.p. dose of 3,5,3,'-L-triiodothyronine (10 micrograms/100 g body wt), a potent inducer of gene S14, reversed the inhibition of gene expression at each time interval studied in both the partially hepatectomized and sham-operated rats. Sucrose administration to rats 24 h after partial hepatectomy or to sham-operated rats restored the induction of hepatic mRNA S14 gene expression in approx. 60% of controls as recorded after 4 and 24 h. These data suggest that the expression of gene S14 was altered in the liver after partial hepatectomy or sham operation by food restriction, and no compensatory effect of the increased lipid metabolism was found in rat remnant liver. Our findings indicate that thyroid hormone and sucrose administration exert similar effects on hepatic expression of the protein S14 gene both in the regenerating liver and in the intact liver of sham-operated animals. In spite of the increased lipid metabolism of the liver after resection, sensitivity of mRNA S14 expression to triiodothyronine and sucrose was unchanged and its regulation was operative in a positive manner. These results also suggest that the treatment of a regenerating liver containing both the parent and the daughter hepatocytes with sucrose and/or triiodothyronine resulted in a fully restored response.

Polyunsaturated fatty acids inhibit S14 gene transcription in rat liver and cultured hepatocytes

Polyunsaturated fatty acids (PUFAs) have been shown to have significant effects on hepatic lipogenic gene expression. The S14 gene has been used as a model to examine the effects of PUFAs on hepatic lipogenic gene expression. In vivo studies showed that feeding rats a high carbohydrate diet containing menhaden oil rapidly (within hours) and significantly (> or = 50%) attenuates hepatic S14 gene transcription and S14 mRNA abundance. The suppressive effect of menhaden oil was both gene and tissue specific. The effect of PUFAs on expression of the S14 mRNA and a transfected S14 fusion gene (i.e., S14CAT4.3) was examined in cultured hepatocytes in the presence of triiodothyronine (T3), insulin, dexamethasone, and albumin under serum-free conditions. Whereas T3 stimulated both S14 mRNA (> 40-fold) and S14CAT4.3 (> 100-fold), eicosapentaenoic acid (C20:5 omega 3) significantly attenuated (> or = 80%) both S14 mRNA and S14CAT activity in a dose-dependent fashion. The effects of C20:5 on hepatocyte gene expression were both gene and fatty acid specific. Deletion analysis of transfected S14CAT fusion genes indicated that the S14 thyroid hormone response element (at -2.5 to -2.9 kb) was not sensitive to C20:5 control. The cis-linked PUFA response elements were localized to a region within the S14 proximal promoter (at -80 to -220 bp). This region also contains cis-acting elements that potentiate T3 activation of S14 gene transcription. These studies suggest that C20:5 (or its metabolites) regulates factors within the S14 proximal promoter region that are important for T3 activation of S14 gene transcription.

Retinoic acid and dexamethasone interact to regulate S14 gene transcription in 3T3-F442A adipocytes

We have examined the effects of retinoic acid (RA) on S14 gene expression in 3T3-F442A preadipocytes and adipocytes. RA induced mRNAS14 14-fold in adipocytes, but had no effect on S14 gene expression in preadipocytes. Northern analysis of retinoic acid receptor (RAR) isoforms revealed the presence of RAR alpha, but not RAR beta, in both preadipocytes and adipocytes. These results suggest that adipocyte-specific factors expressed during differentiation may be required for RA control of S14 gene expression in cultured adipocytes. When compared to dexamethasone (DEX), RA was a weak inducer of S14 gene expression. However, when added together, RA and DEX acted synergistically to induce S14 gene expression. Since changes in S14 gene transcription paralleled changes in mRNAS14 levels, the principal target for RA and DEX action on S14 gene expression was at the level of gene transcription. In the presence of DEX (0.1 nM), RA effects on S14 gene expression were dose-dependent (ED50 = 5 nM) and rapid (within 4 h). In contrast to S14, RA inhibited glycerol 3-phosphate dehydrogenase (GPD) gene transcription and mRNAGDP abundance by 75%. The effects of RA on S14 and GPD gene transcription in cultured adipocytes suggest that RA action may extend beyond growth and differentiation to include effects on lipid metabolism.

Identification of functional cis-acting elements within the rat liver S14 promoter

The structure of DNAase I hypersensitive site 1 (Hss-1), located adjacent to the 5' end of the rat liver S14 gene, is regulated by tissue-specific factors, and its formation correlates with the transcriptional activation of the S14 gene. We propose that tissue-specific trans-acting factors interacting with key cis-linked elements within this site function in the initiation of S14 gene transcription. To examine this hypothesis we used DNAase I footprint, gel shift and in vitro transcriptional analyses to identify cis-linked elements that function in the control of S14 gene transcription. Binding of rat liver nuclear proteins to the S14 promoter (from -8 to -464 bp) produced four DNAase I footprints (designated A-D). Gel shift studies showed that DNA-protein binding was tissue- and sequence-specific, differentially heat-sensitive, and abolished by proteinase K. The function of the four cis-acting elements was assessed by using an in vitro transcription initiation assay in which the S14 promoter was fused to a reporter gene (G-free cassette). Deletion studies showed that nuclear factors binding to regions A (-48 to -63 bp), B (-88 to -113 bp) and D (-286 to -310 bp) enhanced the rate of initiation of transcription, while proteins binding to region C (-227 to -244 bp) suppressed the rate of initiation of transcription. Based on oligonucleotide competition studies, we suggest that hepatic NF-1 (or a related protein) binding to the A region enhances the rate of initiation of S14 gene transcription. Since trans-acting factors interacting with regions B and D are found in liver but not in spleen or kidney, we suggest that the proteins interacting with these regions may be involved in the tissue-specific augmentation of S14 gene transcription.

Beta-adrenergic-cyclic AMP signalling pathway modulates cell function at the transcriptional level in 3T3-F442A adipocytes

We studied the role of cAMP in the regulation of the expression of the adipsin gene and of some other adipose-specific genes including lipoprotein lipase (LPL), glycerophosphate dehydrogenase (G3PDH), and adipocyte P2 (aP2) in 3T3-F442A adipocytes. Northern blot analysis of isoproterenol (10(-6) M)-, forskolin (10(-5) M)- or 8-bromo-cAMP (10(-3) M)-treated adipocytes showed that the steady-state levels of adipsin mRNA were strongly reduced in a time-dependent and reversible manner. The concentration of isoproterenol giving a half-maximal effect in the down-regulation of the adipsin message was approximately 5 x 10(-8) M. Similarly, cell treatment by forskolin elicited a down-regulation of LPL and G3PDH mRNA levels but did not alter aP2 mRNA level. As determined by nuclear run-on assays, the rate of transcription of adipsin, LPL and G3PDH in isoproterenol-treated adipocytes was respectively 3, 3, and 2 times lower than in control adipocytes. These results indicate (1) that cAMP plays a dominant antilipogenic role in the fat cell through the transcriptional down-regulation of the expression of two major genes involved in triglyceride biosynthesis; (2) that cAMP does not reverse the adipocyte character; (3) hence, that cAMP suppresses adipsin expression at the transcriptional level, providing additional support for the role of adipsin protein in adipocyte metabolism.

Regulation of gene expression by insulin

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