The synthesis and hydrolysis of long-chain fatty acyl-coenzyme A thioesters by soluble and microsomal fractions from the brain of the developing rat.
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1. The specific activities of long-chain fatty acid-CoA ligase (EC6.2.1.3) and of long-chain fatty acyl-CoA hydrolase (EC3.1.2.2) were measured in soluble and microsomal fractions from rat brain. 2. In the presence of either palmitic acid or stearic acid, the specific activity of the ligase increased during development; the specific activity of this enzyme with arachidic acid or behenic acid was considerably lower. 3. The specific activities of palmitoyl-CoA hydrolase and of stearoyl-CoA hydrolase in the microsomal fraction decreased markedly (75%) between 6 and 20 days after birth; by contrast, the corresponding specific activities in the soluble fraction showed no decline. 4. Stearoyl-CoA hydrolase in the microsomal fraction is inhibited (99%) by bovine serum albumin; this is in contrast with the microsomal fatty acid-chain-elongation system, which is stimulated 3.9-fold by albumin. Inhibition of stearoyl-CoA hydrolase does not stimulate stearoyl-CoA chain elongation. Therefore it does not appear likely that the decline in the specific activity of hydrolase during myelogenesis is responsible for the increased rate of fatty acid chain elongation. 5. It is suggested that the decline in specific activity of the microsomal hydrolase and to a lesser extent the increase in the specific activity of the ligase is directly related to the increased demand for long-chain acyl-CoA esters during myelogenesis as substrates in the biosynthesis of myelin lipids. (+info)
Transcriptional regulation of fatty acid synthase gene by insulin/glucose, polyunsaturated fatty acid and leptin in hepatocytes and adipocytes in normal and genetically obese rats.
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Transcriptional regulation of the fatty acid synthase (FAS) gene by insulin/glucose, polyunsaturated fatty acids and leptin was investigated in hepatocytes and adipocytes of Wistar fatty rats and their lean littermates. The sequence spanning nucleotides -57 to -35 of FAS gene, which is responsive to insulin/glucose stimulation [Fukuda, H., Iritani, N. & Noguchi, T. (1997) FEBS Lett. 406, 243-248], was linked to a reporter gene containing a heterologous promoter and transfected into rat hepatocytes or adipocytes. The activity of the reporter, chloramphenicol acetyltransferase, in the presence of glucose alone was similar in the primary cultured cells from the lean and obese rats. In the presence of insulin/glucose, however, chloramphenicol acetyltransferase activity was markedly increased in hepatocytes of lean rats, but was not significantly increased in those of obese rats. The stimulation by insulin/glucose was reduced in arachidonic acid-treated cells of lean rats. Similarly, the stimulation by insulin/glucose was reduced in leptin-treated cells and in cells from lean rats containing an expression vector encoding leptin. However, neither polyunsaturated fatty acids nor leptin-treated cells from obese rats responded to insulin-stimulation. The same effects were observed at endogenous FAS mRNA and enzyme levels. Similar results were seen in adipocytes, although the stimulation and suppression were much smaller than in hepatocytes. The insulin-binding capacities of the receptors of liver and adipose tissue were reduced in the presence of leptin or polyunsaturated fatty acids. Leptin and polyunsaturated fatty acids appeared to suppress the insulin stimulation of FAS transcription by reducing the insulin-binding capacities of receptors. Leptin converged on the insulin/glucose response element of FAS gene and suppressed the transcription. (+info)
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.
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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. (+info)
Dibromopropanone cross-linking of the phosphopantetheine and active-site cysteine thiols of the animal fatty acid synthase can occur both inter- and intrasubunit. Reevaluation of the side-by-side, antiparallel subunit model.
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The objective of this study was to test a new model for the homodimeric animal FAS which implies that the condensation reaction can be catalyzed by the amino-terminal beta-ketoacyl synthase domain in cooperation with the penultimate carboxyl-terminal acyl carrier protein domain of either subunit. Treatment of animal fatty acid synthase dimers with dibromopropanone generates three new molecular species with decreased electrophoretic mobilities; none of these species are formed by fatty acid synthase mutant dimers lacking either the active-site cysteine of the beta-ketoacyl synthase domain (C161A) or the phosphopantetheine thiol of the acyl carrier protein domain (S2151A). A double affinity-labeling strategy was used to isolate dimers that carried one or both mutations on one or both subunits; the heterodimers were treated with dibromopropanone and analyzed by a combination of sodium dodecyl sulfate/polyacrylamide gel electrophoresis, Western blotting, gel filtration, and matrix-assisted laser desorption mass spectrometry. Thus the two slowest moving of these species, which accounted for 45 and 15% of the total, were identified as doubly and singly cross-linked dimers, respectively, whereas the fastest moving species, which accounted for 35% of the total, was identified as originating from internally cross-linked subunits. These results show that the two polypeptides of the fatty acid synthase are oriented such that head-to-tail contacts are formed both between and within subunits, and provide the first structural evidence in support of the new model. (+info)
Induction of lipogenesis during differentiation in a "preadipocyte" cell line.
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3T3-L1 fibroblasts differentiate in culture into cells having adipocyte character. This transition is accompanied by a 40- to 50-fold rise in the incorporation of [14C]acetate into triglyceride. The increase in lipogenic rate is exactly parallel to a coordinate rise in the activities of the key enzymes of the fatty acid biosynthetic pathway (ATP-citrate lyase, acetyl-CoA carboxylase, and fatty acid synthetase). Immunological studies indicate that the elevated acetyl-CoA carboxylase activity is the product of an increased cellular enzyme level. (+info)
Arachidonic acid and PGE2 regulation of hepatic lipogenic gene expression.
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N-6 polyunsaturated fatty acids (PUFA) suppress hepatic and adipocyte de novo lipogenesis by inhibiting the transcription of genes encoding key lipogenic proteins. In cultured 3T3-L1 adipocytes, arachidonic acid (20:4,n-6) suppression of lipogenic gene expression requires cyclooxygenase (COX) activity. In this study, we found no evidence to support a role for COX-1 or -2 in the 20:4,n-6 inhibition of hepatocyte lipogenic gene expression. In contrast to L1 preadipocytes, adipocytes and rat liver, RT-PCR and Western analyses did not detect COX-1 or COX-2 expression in cultured primary hepatocytes. Moreover, the COX inhibitor, flurbiprofen, did not affect the 20:4,n-6 regulation of lipogenic gene expression in primary hepatocytes. Despite the absence of COX-1 and -2 expression in primary hepatocytes, prostaglandins (PGE2 and PGF2alpha) suppressed fatty acid synthase, l-pyruvate kinase, and the S14 protein mRNA, while having no effect on acyl-CoA oxidase or CYP4A2 mRNA. Using PGE2 receptor agonist, the PGE2 effect on lipogenic gene expression was linked to EP3 receptors. PGE2 inhibited S14CAT activity in transfected primary hepatocytes and targeted the S14 PUFA-response region located -220 to -80 bp upstream from the transcription start site. Taken together, these studies show that COX-1 and COX-2 do not contribute to the n-6 PUFA suppression of hepatocyte lipogenic gene expression. However, cyclooxygenase products from non-parenchymal cells can act on parenchymal cells through a paracrine process and mimic the effects of n-6 PUFA on lipogenic gene expression. (+info)
Novel form of lipolysis induced by leptin.
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Hyperleptinemia causes disappearance of body fat without a rise in free fatty acids (FFA) or ketones, suggesting that leptin can deplete adipocytes of fat without releasing FFA. To test this, we measured FFA and glycerol released from adipocytes obtained from normal lean Zucker diabetic fatty rats (+/+) and incubated for 0, 3, 6, or 24 h in either 20 ng/ml recombinant leptin or 100 nM norepinephrine (NE). Whereas NE increased both FFA and glycerol release from adipocytes of +/+ rats, leptin increased glycerol release in +/+ adipocytes without a parallel increase in FFA release. In adipocytes of obese Zucker diabetic fatty rats (fa/fa) with defective leptin receptors, NE increased both FFA and glycerol release, but leptin had no effect on either. Leptin significantly lowered the mRNA of leptin and fatty acid synthase of adipocytes (FAS) (p < 0.05), and up-regulated the mRNA of peroxisome proliferator-activated receptor (PPAR)-alpha, carnitine palmitoyl transferase-1, (CPT-1), and acyl CoA oxidase (ACO) (p < 0.05). NE (100 nM) also lowered leptin mRNA (p < 0.05) but did not affect FAS, PPARalpha, ACO, or CPT-1 expression. We conclude that in normal adipocytes leptin directly decreases FAS expression, increases PPARalpha and the enzymes of FFA oxidation, and stimulates a novel form of lipolysis in which glycerol is released without a proportional release of FFA. (+info)
Polyunsaturated fatty acids inhibit fatty acid synthase and spot-14-protein gene expression in cultured rat hepatocytes by a peroxidative mechanism.
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In vivo, polyunsaturated fatty acids (PUFA) inhibit the expression of hepatic genes related to the lipogenic process such as fatty acid synthase and spot-14-protein (S14) genes. In vitro studies have suggested that this was a direct transcriptional effect of PUFA. In hepatocytes, the inhibition of the lipogenic rate by PUFA is not specific, but is linked to a cytotoxic effect due to peroxidative mechanisms. We have investigated whether peroxidation could also explain the inhibitory effect of PUFA on gene expression. Rat hepatocytes were cultured for 24 h with mono-unsaturated or PUFA. PUFA inhibited the expression of fatty acid synthase and S14 genes, and this inhibition was directly related to the number of unsaturations. However, the beta-actin and albumin mRNA concentrations were also affected by the most unsaturated fatty acids, suggesting a non-specific effect of PUFA on gene expression. Measurement of lactate dehydrogenase released into the medium indicated a cytotoxicity of PUFA. This was associated with their peroxidation as evaluated by the presence of thiobarbituric acid-reactive substances in the culture medium. The addition of high concentrations of antioxidants abolished lipid peroxidation and lactate dehydrogenase leakage and completely reversed the inhibitory effect of PUFA on gene expression. This suggests (i) that the results obtained previously in cultured hepatocytes in the presence of low concentrations of antioxidants must be interpretated cautiously and (ii) that in vivo, the inhibitory effect of PUFA on lipogenesis-related genes could be indirect through hormonal or metabolic changes or that their effect on gene expression is somehow linked to peroxidative mechanisms. (+info)