Arachidonic acid suppression of fatty acid synthase gene expression in cultured rat hepatocytes

Interrelated effects of dihomo-γ-linolenic and arachidonic acids, and sesamin on hepatic fatty acid synthesis and oxidation in rats

Interrelated effects of dihomo-γ-linolenic acid (DGLA) and arachidonic acid (ARA), and sesamin, a sesame lignan, on hepatic fatty acid synthesis and oxidation were examined in rats. Rats were fed experimental diets supplemented with 0 or 2 g/kg sesamin (1:1 mixture of sesamin and episesamin), containing 100 g/kg of maize oil or fungal oil rich in DGLA or ARA for 16 d. Among the groups fed sesamin-free diets, oils rich in DGLA or ARA, especially the latter, compared with maize oil strongly reduced the activity and mRNA levels of various lipogenic enzymes. Sesamin, irrespective of the type of fat, reduced the parameters of lipogenic enzymes except for malic enzyme. The type of dietary fat was rather irrelevant in affecting hepatic fatty acid oxidation among rats fed the sesamin-free diets. Sesamin increased the activities of enzymes involved in fatty acid oxidation in all groups of rats given different fats. The extent of the increase depended on the dietary fat type, and the values became much higher with a diet containing sesamin and oil rich in ARA in combination than with a diet containing lignan and maize oil. Analyses of mRNA levels revealed that the combination of sesamin and oil rich in ARA compared with the combination of lignan and maize oil markedly increased the gene expression of various peroxisomal fatty acid oxidation enzymes but not mitochondrial enzymes. The enhancement of sesamin action on hepatic fatty acid oxidation was also confirmed with oil rich in DGLA but to a lesser extent.

Hepatocyte nuclear factor-4alpha contributes to carbohydrate-induced transcriptional activation of hepatic fatty acid synthase

Refeeding a carbohydrate-rich meal after a fast produces a co-ordinated induction of key glycolytic and lipogenic genes in the liver. The transcriptional response is mediated by insulin and increased glucose oxidation, and both signals are necessary for optimal induction of FAS (fatty acid synthase). The glucose-regulated component of FAS promoter activation is mediated in part by ChREBP [ChoRE (carbohydrate response element)-binding protein], which binds to a ChoRE between -7300 and -7000 base-pairs in a carbohydrate-dependent manner. Using in vivo footprinting with nuclei from fasted and refed rats, we identify an imperfect DR-1 (direct repeat-1) element between -7110 and -7090 bp that is protected upon carbohydrate refeeding. Electrophoretic mobility-shift assays establish that this DR-1 element binds HNF-4alpha (hepatocyte nuclear factor 4alpha), and chromatin immunoprecipitation establishes that HNF-4alpha binding to this site is increased approx. 3-fold by glucose refeeding. HNF-4alpha transactivates reporter constructs containing the distal FAS promoter in a DR-1-dependent manner, and this DR-1 is required for full glucose induction of the FAS promoter in primary hepatocytes. In addition, a 3-fold knockdown of hepatocyte HNF-4alpha by small interfering RNA produces a corresponding decrease in FAS gene induction by glucose. Co-immunoprecipitation experiments demonstrate a physical interaction between HNF-4alpha and ChREBP in primary hepatocytes, further supporting an important complementary role for HNF-4alpha in glucose-induced activation of FAS transcription. Taken together, these observations establish for the first time that HNF-4alpha functions in vivo through a DR-1 element in the distal FAS promoter to enhance gene transcription following refeeding of glucose to fasted rats. The findings support the broader view that HNF-4alpha is an integral component of the hepatic nutrient sensing system that co-ordinates transcriptional responses to transitions between nutritional states.

Fatty acid regulation of gene transcription

Dietary fat has a dual role in human physiology: a) it functions as a source of energy and structural components for cells; b) it functions as a regulator of gene expression that impacts lipid, carbohydrate, and protein metabolism, as well as cell growth and differentiation. Fatty acid effects on gene expression are cell-specific and influenced by fatty acid structure and metabolism. Fatty acids interact with the genome through several mechanisms. They regulate the activity or nuclear abundance of several transcription factors, including PPAR, LXR, HNF-4, NFkappaB, and SREBP. Fatty acids or their metabolites bind directly to specific transcription factors to regulate gene transcription. Alternatively, fatty acids indirectly act on gene expression through their effects on a) specific enzyme-mediated pathways, such as cyclooxygenase, lipoxygenase, protein kinase C, or sphingomyelinase signal transduction pathways; or b) pathways that involve changes in membrane lipid/lipid raft composition that affect G-protein receptor or tyrosine kinase-linked receptor signaling. Further definition of these fatty acid-regulated pathways will provide insight into the role dietary fat plays in human health and the onset and progression of several chronic diseases, like coronary artery disease and atherosclerosis, dyslipidemia and inflammation, obesity and diabetes, cancer, major depressive disorders, and schizophrenia.

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).

Modulation of liver microsomal monooxygenase system by dietary n-6/n-3 ratios in rat hepatocarcinogenesis

This study was designed to determine the effects of dietary n-3/n-6 fatty acid ratios on preneoplastic foci and the microsomal monooxygenase system in rat hepatocarcinogenesis. Male Sprague-Dawley rats were fed four kinds of diets containing 15% (wt/wt) fat with different n-6/n-3 ratios: low ratio (> or = 1.0) with tuna oil, low ratio (> or = 1.0) with perilla oil, moderate ratio (< or = 4.0), and high ratio (< or = 10.0). Hepatocarcinogenesis was induced by diethylnitrosamine and partial hepatectomy. The moderate ratio diet decreased significantly the area and number of placental glutathione S-transferase-positive foci compared with the high ratio diet and low ratio diet with perilla oil. The fatty acid composition of microsomal membrane varied extensively, reflecting the dietary n-6/n-3 ratios. Liver microsomal lipid peroxidation was significantly decreased in the group fed the low ratio diet with tuna oil compared with the moderate and high ratio groups. Glucose-6-phosphatase activity, which reflects membrane stability, was significantly higher in the low ratio groups than in the high ratio group. The monooxygenase activities were increased significantly in the moderate ratio group compared with the high ratio group. These results suggest that a moderate n-6/n-3 ratio (< or = 4.0) may be the most effective in decreasing preneoplastic foci by elevating the monooxygenase activities and n-3 fatty acids in fish oil may have a protective effect by lowering the lipid peroxidation and stabilizing the microsomal membrane during rat hepatocarcinogenesis.

Incorporation and distribution of saturated and unsaturated fatty acids into nuclear lipids of hepatic cells

Liver nuclear incorporation of stearic (18:0), linoleic (18:2n-6), and arachidonic (20:4n-6) acids was studied by incubation in vitro of the [1-14C] fatty acids with nuclei, with or without the cytosol fraction at different times. The [1-14C] fatty acids were incorporated into the nuclei as free fatty acids in the following order: 18:0 > 20:4n-6 >> 18:2n-6, and esterified into nuclear lipids by an acyl-CoA pathway. All [1-14C] fatty acids were esterified mainly to phospholipids and triacylglycerols and in a minor proportion to diacylglycerols. Only [1-14C]18:2n-6-CoA was incorporated into cholesterol esters. The incorporation was not modified by cytosol addition. The incorporation of 20:4n-6 into nuclear phosphatidylcholine (PC) pools was also studied by incubation of liver nuclei in vitro with [1-14C]20:4n-6-CoA, and nuclear labeled PC molecular species were determined. From the 15 PC nuclear molecular species determined, five were labeled with [1-14C]20:4n-6-CoA: 18:0-20:4, 16:0-20:4, 18:1-20:4, 18:2-20:4, and 20:4-20:4. The highest specific radioactivity was found in 20:4-20:4 PC, which is a minor species. In conclusion, liver cell nuclei possess the necessary enzymes to incorporate exogenous saturated and unsaturated fatty acids into lipids by an acyl-CoA pathway, showing specificity for each fatty acid. Liver cell nuclei also utilize exogenous 20:4n-6-CoA to synthesize the major molecular species of PC with 20:4n-6 at the sn-2 position. However, the most actively synthesized is 20:4-20:4 PC, which is a quantitatively minor component. The labeling pattern of 20:4-20:4 PC would indicate that this molecular species is synthesized mainly by the de novo pathway.

Regulation of the fatty acid synthase promoter by insulin

Expression of critical enzymes in fatty acid and fat biosynthesis is tightly controlled by nutritional and hormonal stimuli. The expression of fatty acid synthase, which catalyzes all reactions for synthesis of palmitate from acetyl-CoA and malonyl-CoA, and of mitochondrial glycerol-3-phosphate acyltransferase, which catalyzes the first acylation step in glycerophospholipid synthesis, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet after fasting, causes a dramatic increase in the transcription of these genes. Insulin secretion is increased during feeding and has a positive effect on expression. By using adipocytes in culture and transgenic mice that express the reporter gene driven by the fatty acid synthase promoter, the cis-acting sequence that mediates insulin regulation of the fatty acid synthase promoter was defined. Upstream stimulatory factors (USF) that bind to the -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid symthase gene. Sterol regulatory element binding protein (SREBP)-1 may be also involved in induction of these genes during feeding. Using specific inhibitors and expressing various signaling molecules, we found that insulin regulation of the fatty acid synthase promoter is mediated by the phosphatidylinositol (PI)3-kinase signaling pathway and that protein kinase B/akt is a downstream effector.

Dietary fat and protein interactions in the broiler

An experiment was conducted to study the interrelationships between dietary fat and protein levels in the regulation of lipid metabolism in the broiler chicken. Birds were fed diets containing 300, 600, or 1,200 kcal ME from fat (corn oil) with either 124 or 190 g CP/kg. Two additional experimental diets contained 234 or 285 g CP and 300 kcal ME from fat. Regardless of fat level, birds fed the diets containing 124 g CP/kg weighed less and were less efficient than birds fed diets containing 190 g CP/kg. The diet containing 600 kcal as fat decreased lipogenesis and malic enzyme activity (P < 0.05) in birds fed the diet containing 190 g CP/kg diet, but not in birds fed the diet containing 124 g CP/kg. Birds fed the latter level of protein required at least 1,200 kcal as fat to express any significant decrease in lipogenesis or malic enzyme activity (P < 0.05). Dietary fat did not affect plasma levels of triiodothyronine (T3), thyroxine (T4), or insulin-like growth factor-I (IGF-I). Feeding diets containing 124 g CP/kg resulted in decreased plasma T4 and IGF-I and elevated T3 (P < 0.05). Increasing dietary protein (compared to increasing dietary fat) increased body weights, IGF-I, T4 and decreased lipogenesis, malic enzyme activity, and T3. Both of these regimens involve decreasing dietary carbohydrate at equal rates, but results differed. Although replacement of dietary carbohydrates with either fat or protein reduce precursors for fat synthesis, both energy sources have additional unique effects on metabolism. Dietary protein levels modulate metabolic effects of dietary fat.

Cloning, expression, and nutritional regulation of the mammalian Delta-6 desaturase

Arachidonic acid (20:4(n-6)) and docosahexaenoic acid (22:6(n-3)) have a variety of physiological functions that include being the major component of membrane phospholipid in brain and retina, substrates for eicosanoid production, and regulators of nuclear transcription factors. The rate-limiting step in the production of 20:4(n-6) and 22:6(n-3) is the desaturation of 18:2(n-6) and 18:3(n-3) by Delta-6 desaturase. In this report, we describe the cloning, characterization, and expression of a mammalian Delta-6 desaturase. The open reading frames for mouse and human Delta-6 desaturase each encode a 444-amino acid peptide, and the two peptides share an 87% amino acid homology. The amino acid sequence predicts that the peptide contains two membrane-spanning domains as well as a cytochrome b5-like domain that is characteristic of nonmammalian Delta-6 desaturases. Expression of the open reading frame in rat hepatocytes and Chinese hamster ovary cells instilled in these cells the ability to convert 18:2(n-6) and 18:3(n-3) to their respective products, 18:3(n-6) and 18:4(n-3). When mice were fed a diet containing 10% fat, hepatic enzymatic activity and mRNA abundance for hepatic Delta-6 desaturase in mice fed corn oil were 70 and 50% lower than in mice fed triolein. Finally, Northern analysis revealed that the brain contained an amount of Delta-6 desaturase mRNA that was several times greater than that found in other tissues including the liver, lung, heart, and skeletal muscle. The RNA abundance data indicate that prior conclusions regarding the low level of Delta-6 desaturase expression in nonhepatic tissues may need to be reevaluated.

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.

Regulation of fat synthesis and adipose differentiation

Adipocytes have highly specialized function of accumulating fat as stored energy that can be used during periods of food deprivation. The process of fat synthesis and development of adipose tissue are under hormonal and nutritional control. This review first describes transcription of the two critical enzymes involved in fat synthesis, fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet, subsequent to fasting causes dramatic increase in transcription of these genes. Insulin secretion is increased during feeding, having a positive effect, whereas cAMP, which mediates the effect of glucagon which increases during fasting, has a negative effect on transcription of these genes. Using adipocytes in culture and in transgenic mice that express liciferase driven by the fatty acid synthase promoter, cis-acting and trans-acting factors that may mediate the transcriptional regulation were examined. Upstream stimulatory factors (USFs) that bind to -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid synthase gene. This review next describes how pref-1 is a novel inhibitor of adipose differentiation and is a plasma membrane protein containing six EGF-repeats in the extracellular domain. Pref-1 is highly expressed in 3T3-L1 preadipocytes, but is not detectable in mature fat cells. Down regulation of pref-1 is required for adipose differentiation, and constitutive expression of pref-1 inhibits adipogenesis. Moreover, the ectodomain of pref-1 is cleaved to generate a biologically active 50 kDa soluble form. There are four major forms of membrane pref-1 resulting from alternate splicing, but two of the forms with a larger deletion do not produce biologically active soluble form, indicating that alternate splicing determines the range of action, juxtacrine or paracrine, of the pref-1.

Polyunsaturated fatty acids regulate lipogenic and peroxisomal gene expression by independent mechanisms

Polyunsaturated fatty acids of the (n-6) and (n-3) families uniquely coordinate hepatic lipid synthesis and oxidation by suppressing the transcription of hepatic genes encoding lipogenic and glycolytic enzymes while concomitantly inducing the activity of enzymes in mitochondrial and peroxisomal fatty acid oxidation. Recently a group of fatty acid activated nuclear transcription factors termed peroxisome proliferator activated receptors (PPARs) were cloned. The discovery of PPARs led us to hypothesize that polyunsaturated fatty acids coordinately modulated the transcription of lipogenic and oxidative genes via a PPAR mediated process. Rats and mice were fed a potent PPAR activator, 5,8,11,14-eicosatetraynoic acid (ETYA), to ascertain if the expression of hepatic fatty acid synthase and peroxisomal acyl-CoA oxidase were coordinately suppressed and induced in response to PPAR activation. Expectedly, ETYA increased peroxisomal acyl-CoA oxidase mRNA abundance, but PPAR activation neither suppressed fatty acid synthase transcription nor reduced the level of fatty acid synthase mRNA. In fact, ETYA prevented the suppression of hepatic fatty acid synthase expression that characteristically results from feeding corn oil. Fatty acid composition analyses indicated that ETYA interfered with 18:2 (n-6) conversion to 20:4 (n-6). Thus, it appears that PPAR is not the sole factor responsible for the coordinate regulation of lipid synthesis and oxidation by polyunsaturated fatty acids. In addition, our data indicate that the active polyenoic fatty acid responsible for the regulation of gene transcription must undergo delta-6 desaturation.

Selective suppression of endothelial cell activation by arachidonic acid

Endothelial cell (EC) activation plays a key role in inflammation, thrombosis and organ rejection. Normally, EC are in a quiescent state in which their function is to prevent coagulation and thrombosis, and to participate in the regulation of leukocyte migration from the bloodstream into the tissue. Upon activation with cytokines or other stimuli, EC up-regulate a number of genes, including E-selectin (ELAM-1), intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, interleukin (IL)-1, IL-8, tissue factor (TF), plasminogen activator inhibitor-1 (PAI-1), MCP-1 (monocyte chemoattractant protein-1) and endothelial cell inducible gene (ECI-6). Arachidonic acid (AA) is produced by several cell types, including EC, and acts on various cells. We report here that AA inhibits the up-regulation of some, but not all genes that are induced with EC activation in a dose-dependent manner. AA suppresses TNF-alpha, IL-1 alpha, LPS or PMA-induced E-selectin expression, as well as mRNA accumulation of E-selectin, ICAM-1 and IL-8 stimulated by TNF-alpha. The inhibition appears to be at the level of transcription. At the same time under the same conditions AA does not, repress mRNA accumulation for PAI-1, ECI-6, MCP-1 and VCAM-1. We suggest that the induced expression of AA with EC activation may result in a negative feedback loop regulating further activation.

Polyunsaturated fatty acid regulation of hepatic gene transcription

Polyunsaturated fatty acids (PUFA) of the n-6 and n-3 families inhibit transcription of a number of hepatic lipogenic and glycolytic genes, e.g. fatty acid synthase. In contrast, saturated and monounsaturated fatty acids exert no suppressive action on lipogenic gene expression. The unique PUFA regulation of gene expression extends beyond the liver to include genes such as adipocyte glucose transporter-4, lymphocyte stearoyl-CoA desaturase 2, and interleukins. Some of the transcriptional effects of PUFA appear to be mediated by eicosanoids, but PUFA suppression of lipogenic and glycolytic genes is independent of eicosanoid synthesis and appears to involve a nuclear mechanism directly modified by PUFA. With the recent cloning of a fatty acid-activated nuclear factor termed peroxisome-proliferator-activated receptor (PPAR) has come the suggestion that PPAR may be the PUFA response factor. This review, however, presents several lines of evidence that indicate that the PPAR and n-6 and n-3 PUFA regulation of lipogenic and glycolytic gene transcription involve separate and independent mechanisms. Thus PPAR appears not to be the PUFA response factor.

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.

Evidence that dietary arachidonic acid increases circulating triglycerides

Male Syrian hamsters and male CD-1 mice were fed diets supplemented with ethyl esters of oleic, linoleic, arachidonic, and eicosapentaenoic acids (1.1-5%, w/w) for 3-4 wk. Plasma and serum triglycerides were significantly higher in the arachidonic acid-supplemented animals compared to those in the other supplementation groups. Changes in serum insulin and glucose levels did not appear to be related to the changes in circulating triglycerides observed in the arachidonic acid-supplemented group. These data indicate that dietary arachidonic acid elevates circulating triglyceride levels compared to other unsaturated fatty acids in hamsters and mice by unknown mechanisms.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Abnormal gene expression and regulation in the liver of jvs mice with systemic carnitine deficiency

Carnitine-deficient jvs mice expressed reduced levels of a group of genes which are preferentially expressed in the liver, including urea cycle enzyme genes (Biochim. Biophys. Acta 1138, 167-171, 1992). The expression of alpha-fetoprotein and aldolase A was elevated, indicating that the liver of jvs mice is undifferentiated or dedifferentiated (FEBS Lett. 311, 63-66, 1992). Studies of the hormone signal transduction pathway showed that serum cortisol and plasma glucagon levels of jvs mice were 2 and 3 times higher, respectively, than those of normal mice, and that the hormone binding activity of glucocorticoid receptor (GR) in the cytosol of jvs liver was 50% of normal mice, which reflected the amount of receptor protein in the cytosol. On the other hand, GR protein accumulated in the nuclear fraction in jvs mice. Exogenously administrated dexamethasone induced carbamoyl phosphate synthetase (CPS) and tyrosine aminotransferase (TAT) mRNAs in jvs mice, indicating that CPS and TAT genes in jvs mice are responsive to induction by glucocorticoid and cAMP. Analysis of transacting factors by gel retardation assay revealed that HNF-1, COUP-TF and SP-1 were detected at almost the same level in the hepatic nuclear fraction of jvs mice as in normal littermates, and C/EBP and CREB were a little higher in jvs mice, suggesting that these factors are probably not targets of jvs mutation causing abnormal gene expression in the liver. On the other hand, AP-1 binding activity was much higher in jvs mice from an early age, preceding the abnormal expression of urea cycle enzyme, and carnitine administration normalized AP-1 binding activity. We suggest that elevated AP-1 binding induced by carnitine deficiency is closely connected with the abnormal gene expression in the liver.

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.

Developmental changes in mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene expression in rat liver, intestine and kidney

The tissue-specific expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene was studied in 15-day-old suckling rats. The mRNA and protein were present in liver, intestine and kidney, but were absent from brain, heart, skeletal muscles, brown and white adipose tissues. Kidney-cortex mitochondria from suckling rats were able to produce low amounts of ketone bodies from oleate. Hepatic, intestinal and renal HMG-CoA synthase mRNA levels increased slowly during foetal life and markedly after birth. The postnatal increase in liver HMG-CoA synthase mRNA could be due to the increase in plasma glucagon levels, since it rapidly induced the accumulation of HMG-CoA synthase mRNA in cultured foetal hepatocytes. Hepatic, intestinal and renal HMG-CoA synthase mRNA levels remained elevated throughout the suckling period or in rats weaned on to a high-fat carbohydrate-free diet (HF), but decreased by 50% in the liver and totally disappeared from the intestine and the kidney of rats weaned on to a high-carbohydrate low-fat diet (HC). When HC-weaned rats were fed on a HF-diet for a week, HMG-CoA synthase mRNA was re-induced in the intestine and the kidney. The role of hormones and nutrients in the regulation of HMG-CoA synthase gene expression is discussed.

Inhibition of fatty acid synthesis in rat hepatocytes by exogenous polyunsaturated fatty acids is caused by lipid peroxidation

Rat hepatocyte long-term cultures were utilized to investigate the impact of different polyunsaturated fatty acids (PUFA) on the insulin-induced de novo fatty acid synthesis in vitro. The addition of 0.5 mM albumin-complexed oleic, linoleic, columbinic, arachidonic, eicosapentaenoic or docosahexaenoic acid resulted in a marked suppression of fatty acid synthesis. By evaluation of cell viability (determined as the leakage of lactate dehydrogenase (LDH) it turned out, that the antioxidant used (50 microM alpha-tocopherol phosphate) had a low antioxidant activity, resulting in cytotoxic effects by the peroxidized PUFA. Arachidonic acid and eicosapentaenoic acid showed a dose- and time-dependent cytotoxicity. Two other antioxidants: 50 microM alpha-tocopherol acid succinate and 1 microM N,N'-diphenyl-1,4-phenylenediamine, both proved more efficient than alpha-tocopherol phosphate. There was a significant correlation between LDH-leakage and inhibition of fatty acid synthesis. Lipid peroxidation, measured as thiobarbituric acid-reactive substances, also showed a significant correlation with the degree of inhibition of fatty acid synthesis. Furthermore, PUFA had no inhibitory effect on fatty acid synthesis when peroxidation was minimized by the use of proper antioxidants. These data indicate that PUFA in vitro inhibit the insulin-induced de novo fatty acid synthesis in hepatocytes from starved rats, due to cytotoxic effects caused by lipid peroxidation.

Arachidonic acid regulates unsaturated fatty acid synthesis in lymphocytes by inhibiting stearoyl-CoA desaturase gene expression

This work was based upon the observation that a reduction in the level of serum, provided to murine lymphocytes in culture, augmented endogenous unsaturated fatty acid (UFA) synthesis. Since the phospholipids of BW5147 cells grown in 1% serum were especially deficient in arachidonic acid (20:4), and given the findings of previous workers, we suspected that the availability of exogenous 20:4 in serum might correlate with the squelching of UFA synthesis. Indeed, after a 5 h exposure to 4-28 microM 20:4, the 20:4 content of BW5147 cell phospholipids increased from 1% to 15% of the total fatty acids with a coincident reduction in 18:1 synthesis to approx. 30% of starting values. Subsequent studies were done to define the mechanism by which 20:4 down-regulates 18:1 synthesis. The results indicated that 20:4 inhibited endogenous 18:1 synthesis by reducing stearoyl-CoA desaturase (SCD) enzyme activity. Moreover, as determined by Northern blot analyses, the inhibitory effect of 20:4 on stearoyl-CoA desaturase activity coincided with decreased stearoyl-CoA desaturase mRNA levels. Exposure of BW5147 cells to either 20:4, actinomycin D, or both, resulted in a temporal decay of stearoyl-CoA desaturase mRNAs with half-lives ranging from 4.0 h to 4.4 h. Such a similarity in decay times implied that 20:4 regulates stearoyl-CoA desaturase expression by inhibiting transcription. This was confirmed by nuclear run-on studies in which 20:4 was found to inhibit transcription of nascent stearoyl-CoA desaturase mRNA. Collectively, these findings implicate 20:4 as an important regulator of stearoyl-CoA desaturase gene expression, and hence UFA synthesis, in lymphoid cells.