Synthesis and degradation of the lipogenic enzymes of rat liver

ACLY promotes gastric tumorigenesis and accelerates peritoneal metastasis of gastric cancer regulated by HIF-1A

Mounting evidence indicates the potential involvement of ATP-citrate lyase (ACLY) in the modulation of various cancer types. Nevertheless, the precise biological significance of ACLY in gastric cancer (GC) remains elusive. This study sought to elucidate the biological function of ACLY and uncover its influence on peritoneal metastasis in GC. The expression of ACLY was assessed using both real-time quantitative PCR and western blot techniques. To investigate the impact of ACLY on the proliferation of gastric cancer (GC) cells, colony formation and 5-ethynyl-2'-deoxyuridine (EdU) assays were performed. The migratory and invasive abilities of GC were evaluated using wound healing and transwell assays. Additionally, a bioinformatics analysis was employed to predict the correlation between ACLY and HIF-1A. This interaction was subsequently confirmed through a chromatin immunoprecipitation (ChIP) assay. ACLY exhibited upregulation in gastric cancer (GC) as well as in peritoneal metastasis. Its overexpression was found to facilitate the proliferation and metastasis of GC cells in both in vitro and in vivo experiments. Moreover, ACLY was observed to play a role in promoting angiogenesis and epithelial-mesenchymal transition (EMT). Notably, under hypoxic conditions, HIF-1A levels were elevated, thereby acting as a transcription factor to upregulate ACLY expression. Under the regulatory influence of HIF-1A, ACLY exerts a significant impact on the progression of gastric cancer, thereby facilitating peritoneal metastasis.

ACLY facilitates colon cancer cell metastasis by CTNNB1

Background

Colon cancer is the second leading cancer worldwide. Recurrent disease and chemotherapeutic drug resistance are very common in the advanced stage of colon cancer. ATP-citrate lyase (ACLY), the first-step rate-controlling enzyme in lipid synthesis, is elevated in colon cancer. However, it remains unclear about the exact role of ACLY in the development of colon cancer metastasis.

Methods

To evaluate the role of ACLY in colon cancer metastasis, we performed cell migration and invasion assays in two ACLY-deficient colon cancer cell lines. Colon cancer mouse model is used to examine ACLY’s effects on colon metastasis potentials in vivo. We analyzed the correlation between ACLY and CTNNB1 protein in 78 colon cancer patients by Pearson correlation. To finally explore the relationship of ACLY and CTNNB1, we used western blots, migration and invasion assays to confirm that ACLY may regulate metastasis by CTNNB1.

Results

Our data showed that the abilities of cell migration and invasion were attenuated in ACLY-deficient HCT116 and RKO cell lines. Furthermore, we describe the mechanism of ACLY in promoting colon cancer metastasis in vitro and in vivo. ACLY could stabilize CTNNB1 (beta-catenin 1) protein by interacting, and the complex might promote CTNNB1 translocation through cytoplasm to nucleus, subsequently promote the CTNNB1 transcriptional activity and migration and invasion abilities of colon cancer cells. Immunohistochemical analysis of 78 colon cancer patients showed that the high expression levels of ACLY and CTNNB1 protein was positively correlated with metastasis of colon cancer.

Conclusions

These results shed new light on the molecular mechanism underlying colon cancer metastasis, which might help in improving therapeutic efficacy.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1391-9) contains supplementary material, which is available to authorized users.

Targeting ATP-Citrate Lyase in Hyperlipidemia and Metabolic Disorders

Chronic overnutrition and a sedentary lifestyle promote imbalances in metabolism, often manifesting as risk factors for life-threating diseases such as atherosclerotic cardiovascular disease (ASCVD) and nonalcoholic fatty liver disease (NAFLD). Nucleocytosolic acetyl-coenzyme A (CoA) has emerged as a central signaling node used to coordinate metabolic adaptations in response to a changing nutritional status. ATP-citrate lyase (ACL) is the enzyme primarily responsible for the production of extramitochondrial acetyl-CoA and is thus strategically positioned at the intersection of nutrient catabolism and lipid biosynthesis. Here, we discuss recent findings from preclinical studies, as well as Mendelian and clinical randomized trials, demonstrating the importance of ACL activity in metabolism, and supporting its inhibition as a potential therapeutic approach to treating ASCVD, NAFLD, and other metabolic disorders.

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.

Influence of virgin coconut oil-enriched diet on the transcriptional regulation of fatty acid synthesis and oxidation in rats - a comparative study

The present study was carried out to evaluate the effects of virgin coconut oil (VCO) compared with copra oil, olive oil and sunflower-seed oil on the synthesis and oxidation of fatty acids and the molecular regulation of fatty acid metabolism in normal rats. Male Sprague-Dawley rats were fed the test oils at 8 % for 45 d along with a synthetic diet. Dietary supplementation of VCO decreased tissue lipid levels and reduced the activity of the enzymes involved in lipogenesis, namely acyl CoA carboxylase and fatty acid synthase (FAS) (P< 0·05). Moreover, VCO significantly (P< 0·05) reduced the de novo synthesis of fatty acids by down-regulating the mRNA expression of FAS and its transcription factor, sterol regulatory element-binding protein-1c, compared with the other oils. VCO significantly (P< 0·05) increased the mitochondrial and peroxisomal β-oxidation of fatty acids, which was evident from the increased activities of carnitine palmitoyl transferase I, acyl CoA oxidase and the enzymes involved in mitochondrial β-oxidation; this was accomplished by up-regulating the mRNA expression of PPARα and its target genes involved in fatty acid oxidation. In conclusion, the present results confirmed that supplementation of VCO has beneficial effects on lipid parameters by reducing lipogenesis and enhancing the rate of fatty acid catabolism; this effect was mediated at least in part via PPARα-dependent pathways. Thus, dietary VCO reduces the risk for CHD by beneficially modulating the synthesis and degradation of fatty acids.

Effects of dietary fatty acids on lipogenesis and lipid traits in muscle, adipose tissue and liver of growing rabbits

The effects of fat source on lipogenesis and lipid traits of longissimus muscle, liver and perirenal adipose tissue, were studied in 48 New Zealand White rabbits, slaughtered at 11 or 15 weeks of age. Rabbits were offered diets with 20 g added fat per kg, containing either medium-chain (COC: coconut oil), saturated and monounsaturated (PAL: palm oil) or polyunsaturated (SUN: sunflower oil) fatty acids as major components. Diets did not affect growth performance, dressing proportion and tissue weights. Intramuscular lipid content was lower for COC than for SUN and PAL (e.g. 10 v. 13 and 12 glkg, at 11 weeks, respectively, P < 0·05), whereas lipid content was unaffected by diet in liver and perirenal fat. In muscle, the fat source did not influence the activities of acetyl-CoA-carboxylase (CBX), malic enzyme (ME) and glucose-6-phosphodehydrogenase (G6PDH). In liver, activities ofGSPDH and ME were depressed from the SUN diet, as compared with the COC or PAL diets. The diet-induced variations in enzyme activities in perirenal fat were lower than in the liver and were not significant. Medium-chain fatty acids were found only in tissue lipids of animals given COC. The ratio of polyunsaturated to saturated fatty acids decreased in the order SUN > PAL > COC in muscle and perirenal fat. Thus, polyunsaturated fatty acids exert an inhibition of G6PDH and ME activities specifically in liver. Compared with COC, the addition of SUN to the basal diet increased total lipids and polyunsaturated fatty acids contents in the longissimus lumborum muscle, which might improve the organoleptic and dietetic qualities of rabbit meat.

Hypolipidemic effect of soluble fiber isolated from seeds of Cassia tora Linn. in rats fed a high-cholesterol diet

Soluble fibers isolated from the seeds of Cassia tora Linn. (SFC) have attracted considerable attention in recent years due to their phenomenal rheological behavior. In this study were investigated the effects of SFC on lipid metabolism. Male Sprague-Dawley rats were fed one of three experimental diets, a normal diet, a high-cholesterol diet, or a high-cholesterol diet with 5% SFC, for 5 weeks. The serum concentration of total cholesterol in rats fed SFC was 27% lower (p < 0.05) compared to that of the control group, but the serum high-density lipoprotein cholesterol level was increased in the SFC group. Liver total cholesterol and triglyceride levels were reduced significantly (p < 0.05) in rats fed the SFC diet. In addition, fecal bile acid and lipid excretion was significantly increased by SFC consumption. These results indicate that SFC enhances fecal lipid excretion and may cause a reduction in serum and hepatic lipid concentrations in rats.

Direct interaction between USF and SREBP-1c mediates synergistic activation of the fatty-acid synthase promoter

To understand the molecular mechanisms underlying transcriptional activation of fatty-acid synthase (FAS), we examined the relationship between upstream stimulatory factor (USF) and SREBP-1c, two transcription factors that we have shown previously to be critical for FAS induction by feeding/insulin. Here, by using a combination of tandem affinity purification and coimmunoprecipitation, we demonstrate, for the first time, that USF and SREBP-1 interact in vitro and in vivo. Glutathione S-transferase pulldown experiments with various USF and sterol regulatory element-binding protein (SREBP) deletion constructs indicate that the basic helix-loop-helix domain of USF interacts directly with the basic helix-loop-helix and an N-terminal region of SREBP-1c. Furthermore, cotransfection of USF and SREBP-1c with an FAS promoter-luciferase reporter construct in Drosophila SL2 cells results in highly synergistic activation of the FAS promoter. We also show similar cooperative activation of the mitochondrial glycerol-3-phosphate acyltransferase promoter by USF and SREBP-1c. Chromatin immunoprecipitation analysis of mouse liver demonstrates that USF binds constitutively to the mitochondrial glycerol 3-phosphate acyltransferase promoter during fasting/refeeding in vivo, whereas binding of SREBP-1 is observed only during refeeding, in a manner identical to that of the FAS promoter. In addition, we show that the synergy we have observed depends on the activation domains of both proteins and that mutated USF or SREBP lacking the N-terminal activation domain could inhibit the transactivation of the other. Closely positioned E-boxes and sterol regulatory elements found in the promoters of several lipogenic genes suggest a common mechanism of induction by feeding/insulin.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

The Spot 14 protein is required for de novo lipid synthesis in the lactating mammary gland

We generated a Spot 14 null mouse to assess the role of Spot 14 in de novo lipid synthesis and report the Spot 14 null mouse exhibits a phenotype in the lactating mammary gland. Spot 14 null pups nursed by Spot 14 null dams gain significantly less weight than wild-type pups nursed by wild-type dams. In contrast, Spot 14 null pups nursed by heterozygous dams show similar weight gain to wild-type littermates. We found the triglyceride content in Spot 14 null milk is significantly reduced. We demonstrate this reduction is the direct result of decreased de novo lipid synthesis in lactating mammary glands, corroborated by a marked reduction of medium-chain fatty acids in the triglyceride pool. Importantly, the reduced lipogenic rate is not associated with significant changes in the activities or mRNA of key lipogenic enzymes. Finally, we report the expression of a Spot 14-related gene in liver and adipose tissue, which is absent in the lactating mammary gland. We suggest that expression of both the Spot 14 and Spot 14-related proteins is required for maximum efficiency of de novo lipid synthesis in vivo and that these proteins impart a novel mechanism regulating de novo lipogenesis.

The roles of sterol regulatory element-binding proteins in the transactivation of the rat ATP citrate-lyase promoter

ATP citrate-lyase (ACL) is a key enzyme supplying acetyl-CoA for fatty acid and cholesterol synthesis. Its expression is drastically up-regulated when an animal is fed a low fat, high carbohydrate diet after prolonged fasting. In this report, we describe the role of sterol regulatory element-binding proteins (SREBPs) in the transactivation of the rat ACL promoter. ACL promoter activity was markedly stimulated by the overexpression of SREBP-1a and, to a lesser extent, by SREBP-2 in Alexander human hepatoma cells. The promoter elements responsive to SREBPs were located within the 55-base pair sequences from -114 to -60. The gel mobility shift assay revealed four SREBP-1a binding sites in this region. Of these four elements, the -102/-94 region, immediately upstream of the inverted Y-box, and the -70/-61 region, just adjacent to Sp1 binding site, played critical roles in SREBPs-mediated stimulation. The mutation in the inverted Y-box and the coexpression of dominant negative nuclear factor-Y (NF-Y) significantly attenuated the transactivation by SREBP-1a, suggesting that NF-Y binding is a prerequisite for SREBPs to activate the ACL promoter. However, the multiple Sp1 binding sites did not affect the transactivation of the ACL promoter by SREBPs. The binding affinity of SREBP-1a to SREs of the ACL promoter also was much higher than that of SREBP-2. The transactivation potencies of the chimeric SREBPs, of which the activation domains (70 amino acids of the amino terminus) were derived from the different species of their carboxyl-terminal region, were similar to those of SREBPs corresponding to their carboxyl termini. Therefore, it is suggested that the carboxyl-terminal portions of SREBPs containing DNA binding domains are important in determining their transactivation potencies to a certain promoter.

Polychlorinated biphenyls affect the activities of gluconeogenic and lipogenic enzymes in rat liver: is there an interference with regulatory hormone actions?

1. The effects of dietary polychlorinated biphenyls (PCBs) (30-2000 ppm) on activities of gluconeogenic (phosphoenolpyruvate carboxykinase-PEPCK, and fructose 1,6-bisphosphatase-FdPase) and lipogenic enzymes (fatty acid synthase-FAS, ATP citrate lyase-ACL, malic enzyme-ME, glucose 6-phosphate dehydrogenase-G6PDH, and 6-phosphogluconate dehydrogenase-PGDH) were studied in livers of the female Sprague-Dawley and Wistar rat. 2. PCB amounts accumulating in the liver reflected the extent of dietary exposure. The Wistar strain was more sensitive to PCBs than the Sprague-Dawley strain. Of the Clophentype PCBs those containing 60 and 64% chlorine displayed the most pronounced effects. 3. Activities of gluconeogenic enzymes (PEPCK and FdPase) were dose-dependently decreased by PCBs, PEPCK being considerably more sensitive. This decrease was also found under conditions where the activity of PEPCK was induced (administration of adrenalin, glucagon or cAMP, feeding high protein diets, starvation). 4. Activities of lipogenic enzymes were induced by PCBs. The increase was much greater with ME, G6PDH and PGDH (up to 10-fold) than with FAS and ACL (approximately 2-fold). PCB effects were dose-dependent, but transient. 5. In cultured hepatocytes basal activities of lipogenic enzymes were induced by PCBs in the absence of hormones. With saturating levels of insulin or triiodothyronine, enzyme activities were also induced, but addition of PCBs resulted in an additive effect. 6. These results suggest that in the female rat PCBs can mimic the actions of certain hormones by affecting either hormone levels, hormone receptor systems or regulatory systems.

Developmental changes in lipogenic enzymes in muscle compared to liver and extramuscular adipose tissues in the rabbit (Oryctolagus cuniculus)

The developmental changes in the activities of acetyl-CoA-carboxylase, malic enzyme and glucose-6-phosphate dehydrogenase were compared in longissimus muscle, liver and adipose tissues in growing rabbits. Activities of lipogenic enzymes were low in muscle, as compared to the other tissues studied. The lipogenic activities in longissimus muscle increased with age. This increase was well correlated with the age-related increase in intramuscular lipid content, suggesting that intramuscular adipose tissue results from in situ lipid synthesis. During growth, each tissue displayed a specific developmental pattern for lipogenic enzyme activities. In liver and adipose tissues, the three lipogenic enzyme activities first increased and subsequently decreased, during the postweaning period. In the muscle, no such decrease was observed, suggesting that intramuscular adipose tissue develops later than the other tissues tested. Throughout postnatal period, the ratio of malic enzyme to glucose-6-phosphate dehydrogenase was reversed in muscle compared to other fat sites (5 vs 0.04). Further studies are necessary to determine the role of malic enzyme in rabbit intramuscular lipid metabolism.

Time-dependent effects of insulin on lipid synthesis in cultured fetal rat hepatocytes: a comparison between lipogenesis and glycogenesis

The lipogenic effect of insulin was studied in 18-day-old fetal rat hepatocytes after 2 to 3 days of culture in the presence of glucocorticoids when an acute stimulatory effect of insulin on glycogenesis was present. The rate of [1-14C]-acetate incorporation into lipids measured for 4 hours was much higher than with [U-14C]-glucose (30 v 3.8 nmol/h/mg protein). The stimulatory effect of insulin on lipid labeling remained weak (1.2-fold) and contrasted with its striking stimulatory effect on [U-14C]-glucose incorporation into glycogen (fourfold). When lipid labeling was assessed in longer experiments, increasing acetate concentrations in the medium stimulated the incorporation rate of [1-14C]-acetate into lipids (3.5-fold from 1 to 5 mmol/L after 36 hours) and decreased that of [U-14C]-glucose (by twofold). The stimulatory effect of insulin on the rate of lipid labeling developed with both precursors from 12 to 36 hours after insulin exposure (by approximately twofold) independently of acetate concentration and was not glucocorticoid-dependent, contrary to the glycogenic response. Addition of a glucose, load simultaneously with insulin increased the stimulation of lipogenesis when measured with [U-14C]-glucose (twofold to 3.7-fold). Besides contributing to an accumulation of larger and numerous lipid droplets in the cells, insulin increased fatty acid synthase activity by 26%, whereas malic enzyme was not affected. Thus, insulin-dependent lipogenesis in cultured fetal hepatocytes appears to be mostly regulated by a long-term mechanism, contrary to the glycogenic effect of insulin.

Involvement of lipogenesis in the lower VLDL secretion induced by oligofructose in rats

Dietary supplementation with oligofructose (OFS; 100 g/kg), a non-digestible oligomer of beta-D-fructose, decreases serum triacylglycerols in serum and VLDL of rats. In order to investigate the role of hepatic metabolism in the hypolipidaemic effect of OFS, male Wistar rats were fed on a standard diet with or without 100 g Raftilose P95/kg as OFS source for 30 d. OFS feeding (1) significantly decreased triacylglycerol and phospholipid concentrations in both blood and liver, (2) increased the glycerol-3-phosphate liver content but decreased the hepatic activity of glycerol-3-phosphate acyltransferase (EC 2.3.1.15), suggesting a decrease in acylglycerol synthesis, (3) did not affect the blood non-esterified fatty acid concentrations, but (4) reduced by 54% the capacity of isolated hepatocytes to synthesize and secrete triacylglycerols from labelled acetate; the activity of fatty acid synthase, a key lipogenic enzyme was also significantly decreased. These findings suggest that OFS decreases serum triacylglycerols by reducing de novo fatty acid synthesis in the liver; the lower insulin level in the serum of OFS-fed rats could explain, at least partly, the metabolic effect induced by such non-digestible carbohydrates.

Effect of oxaloacetate and phosphorylation on ATP-citrate lyase activity

ATP-citrate lyase (CL) catalyzes the conversion of citrate and CoA to oxaloacetate (OA) and acetyl-CoA. As the coupled malic dehydrogenase (MDH) assay is not able either to study the effect of oxaloacetate (OA) on CL activity or to measure accurately CL activity in biological samples, a new assay was developed. The CL-citrate coupled CAT assay measures the amount of acetyl-CoA formed by transferring radiolabeled acetyl-CoA synthesized from [14C]citrate to chloramphenicol with chloramphenicol acetyltransferase (CAT). Employing this assay, the rate of increase in acetyl-CoA synthesis from citrate is linear with respect to added CL. Kinetic values for ATP, CoA and citrate are similar to those obtained using the MDH assay. The effect of CL phosphorylation on enzyme activity was determined. CL phosphorylated by cAMP-dependent protein kinase or by this kinase and glycogen synthase kinase-3 (GSK-3) decreases the apparent Vmax without changing the apparent Km. The effect of OA, a product of the enzyme reaction, on CL activity was also determined. Computational analysis of the data obtained without added OA and at three concentrations of OA indicate that the apparent Km for the substrate is not altered even though the apparent Vmax is decreased. The effect of OA on the activity of phosphorylated enzyme was also determined. OA decreases the apparent Vmax of the phosphorylated enzyme to the same extent as in control CL. This assay is able to measure CL activity in cytosol from 3T3-L1 adipocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Medium-chain fatty acids as short-term regulators of hepatic lipogenesis

Short-term exposure of isolated rat hepatocytes to short- and medium-chain fatty acids led to an activation of acetyl-CoA carboxylase as measured in digitonin-permeabilized hepatocytes. Up to a certain concentration, typical for each of the fatty acids used, fatty acid-dependent activation of acetyl-CoA carboxylase coincided with an increase in the rate of fatty acid synthesis in intact hepatocytes, as determined by the incorporation of 3H from 3H2O water into fatty acids. At higher concentrations loss of stimulation of fatty acid synthesis occurred, but not the enhancement of carboxylase activity. With the fatty acids tested (C8:0-C14:0), the peak in fatty acid synthesis coincided with a peak in the level of malonyl-CoA. The onset of the stimulation of carboxylase activity coincided with the start of the peak in both fatty acid synthesis and malonyl-CoA. The longer the chain length of the fatty acid added, the lower the concentration at which the rate of fatty acid synthesis and the level of malonyl-CoA reached a peak and carboxylase activity started to become elevated. In cell suspensions incubated with increasing concentrations of fatty acids, accumulation of lactate decreased progressively. The latter observation, in combination with the fact that the activity of acetyl-CoA carboxylase is not always related to the rate of fatty acid biosynthesis, suggests that under these conditions not the activity of the carboxylase but the flux through the glycolytic sequence determines, at least in part, the rate of fatty acid synthesis de novo.

ATP citrate-lyase and glycogen synthase kinase-3 beta in 3T3-L1 cells during differentiation into adipocytes

ATP citrate-lyase (CL), acetyl-CoA carboxylase (ACC) and glycogen synthase kinase-3 beta (GSK-3 beta) levels were measured in cytosol from 3T3-L1 cells during differentiation from fibroblasts into fat-cells. Protein levels were estimated from immunoblots using specific antisera. Cytosol from confluent cells contain significant amounts of GSK-3 beta, which fell during differentiation of these cells into adipocytes. CL from confluent cells was found to be mostly in the form of a single protein band of apparent mass 110 kDa. Levels of CL and ACC increased during cell differentiation into adipocytes. During the first 3 days of differentiation, CL migration changed, and it was expressed as a complex of protein bands of apparent mass 110 kDa, 113 kDa and 115 kDa. At later stages of differentiation, when these cells had assumed the phenotype of fat-cells, they expressed CL mainly as protein bands of 110 and 113 kDa. When samples containing these bands were treated with alkaline phosphatase, the 113 kDa protein band collapsed into the 110 kDa species. This suggests that the slower-migrating species of CL is a higher-order phosphorylation state of the same protein. Furthermore, when purified CL, mostly expressed as the 110 kDa species, was phosphorylated with cyclic AMP-dependent protein kinase alone or together with GSK-3 and resolved by SDS/PAGE, the phosphorylated CL now migrated more slowly as the 113 kDa and 115 kDa forms. CL phosphorylation was hormone-regulated, since, in samples from fat-cells that had the complex two-band pattern, when cultured in medium without serum or hormones, CL migration reverted to a single band of 110 kDa, similar to confluent cells. Treatment of these 'down-regulated' cells with insulin rapidly induced substantial amounts of the 113 kDa species, with a concomitant decrease in the 110 kDa species.

Pyruvate stimulates hormonal induction of lipogenic enzymes in primary cultured rat hepatocytes

Hormonal inductions of lipogenic enzyme activities (fatty acid synthetase, malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PD) and ATP-citrate lyase) were studied in primary cultured rat hepatocytes. Insulin, triiodothyronine and dexamethasone markedly stimulated the inductions of the enzymes (particularly G6PD and ME) in the presence of pyruvate. Lactate also induced their activities. The activities of these enzymes in the presence of appropriate hormone combinations and a substrate amount of pyruvate were as high as, or higher than those in the liver of rats on high-carbohydrate, low-fat diet. The aldolase and glucokinase activities induced by these hormones were not enhanced by the addition of pyruvate. The induction by pyruvate was inhibited by actinomycin D or cycloheximide. The ATP content of rat hepatocytes was maintained without increase during culture with pyruvate for 6 days. These results indicate that the additions of pyruvate, or its metabolites to cultures of isolated hepatocytes have specific effects on the inductions of certain hepatic enzymes, possibly acting at the level of transcription. Their effects are similar to those of feeding a high-carbohydrate, low-fat diet to intact animals.

Dietary polyunsaturated fats suppress the high-sucrose-induced increase of rat liver pyruvate dehydrogenase levels

Pyruvate dehydrogenase complex (PDC) has a key role in the regulation of hepatic lipogenesis by dietary factors. We have investigated the effects of dietary carbohydrate and fat on hepatic PDC. Sucrose-based or starch-based diets were administered for 15 days. A positive correlation between PDC activity and the lipogenic potential of the diet was found. A high-sucrose, fat-free diet caused a 3-fold increase in total activity whereas a high-starch, fat-free diet caused a 1.5-fold increase, as compared with chow-fed rats. Dietary polyunsaturated fat (PUF) caused a marked inhibitory effect on total and active PDC; fish oil being more effective than corn oil. Dietary saturated fat (butter) failed to inhibit the sucrose-induced elevation in total activity, but was almost as effective as fish oil in depressing percent active enzyme. Changes in total PDC activity closely correlated with modifications in the content of enzyme quantitated by immunoblotting, indicating that increased enzyme content and not activation is the predominant mechanism underlying the adaptive response to high-sucrose feeding. This response is suppressed by dietary PUF. Inhibition of hepatic lipogenesis by PUF involves a reduction of PDC content as well as that of several lipogenic enzymes. The relevant mechanisms remain to be established.

Regulation of ATP-citrate lyase at transcriptional and post-transcriptional levels in rat liver

The amounts of ATP-citrate lyase in liver cytosol began to increase at 12 hours after refeeding a high-carbohydrate diet and further increased until 48 hours. The amounts of the ATP-citrate lyase mRNA began to increase at 6 hours and reached to a maximum level at 12 hours, followed by decrease to a very low level until 48 hours. The elevated amount of the ATP-citrate lyase mRNA reflected on the increase of ATP-citrate lyase content in the first 24 hours, but these two parameters were not paralleled thereafter. The transcriptional activity of ATP-citrate lyase gene in nuclei of rat liver began to increase at 4 hours and further increased to reach a maximum level of 24 fold at 12 hours, maintaining a high level of 17 fold until 48 hours. The elevation of transcriptional activity of ATP-citrate lyase gene preceded the increase of ATP-citrate lyase mRNA content in the liver cytosol by 2 hours, and its increasing pattern was similar to changes of mRNA content until 12 hours. However, while the transcriptional activity remained at a high level until 48 hours, the ATP-citrate lyase mRNA concentration in the cytosol decreased after 12 hours.

Effects of nutrients and hormones on gene expression of ATP citrate-lyase in rat liver

Northern-blot analyses demonstrated a strong gene expression of ATp citrate-lyase in liver and adipose tissue of rat and a weak expression in brain, heart, small intestine and muscle. After refeeding a carbohydrate/protein diet to fasted rats, the transcriptional rate had already increased within 2 h, the mRNA concentration reached a maximal level of approximately 30-fold increased in 16 h, and the enzyme induction increased sixfold in 48 h. By feeding only carbohydrate without protein, the transcriptional rate was increased threefold, and the mRNA concentration and enzyme induction comparably, to the levels in the carbohydrate/protein diet. It appears that protein feeding is not necessary to induce ATP citrate-lyase. In diabetic rats fed on a glucose diet, the transcriptional rate, mRNA concentration and enzyme level were very low in comparison with the normal. By fructose feeding, however, the transcriptional rate was more greatly increased and the mRNA concentration increased comparably to the levels reached by insulin treatment, while the enzyme induction was not so increased. Thus, it is suggested that insulin is important in regulated translation in addition to transcription. However, triiodothyronine treatment did not have much effect on the gene expression. As a result of the present experiment, it is noted that ATP citrate-lyase-gene expression was greatly dependent on carbohydrate.

Biosynthesis and secretion of triacylglycerol in rat liver after partial hepatectomy

Partially hepatectomized rats were used to investigate the mechanism of fatty-liver development in the regenerating rat liver. After partial hepatectomy the amount of hepatic triacylglycerol increased by almost 4-fold compared with sham-operated rats. The activities of both cytosolic and microsomal phosphatidate phosphohydrolase were enhanced at 12 h after surgery. The activity of diacylglycerol acyltransferase was increased at a later stage of regeneration. Analysis of plasma lipoproteins showed a significant decrease of lipids associated with very-low-density lipoproteins (VLDL). Relative to control, the rate of hepatic triacylglycerol synthesis from [3H]glycerol in vivo was stimulated at 22 h after partial liver resection. However, secretion of glycerol-labelled triacylglycerol in VLDL was the same in control and hepatectomized rats. In cultures of hepatocytes from hepatectomized donor rats, the concentration of triacylglycerol and the biosynthesis of this lipid from [3H]glycerol or from [3H]oleate were enhanced. The secretion of total triacylglycerol into the medium was not affected, resulting in a net accumulation of intracellular triacylglycerol. The rate of secretion of leucine-labelled apolipoproteins B and E associated with VLDL was similar in cell cultures from hepatectomized and sham-operated rats. The results of this study show that the enhancement of the biosynthesis of triacylglycerol in hepatectomized livers is not accompanied by an increase of the secretion of VLDL.

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Alcohol as a nutrient: interactions between ethanol and carbohydrate

This study examined metabolic interactions between two nutrients--ethanol and carbohydrate. Both nutrients are metabolized by a common pathway to fatty acids from acetyl-coenzyme A by lipogenic enzymes. The effects of ethanol and carbohydrate on the induction of lipogenic enzymes in livers of rats were examined using two types of base diets differing in carbohydrate and lipid content and using isocaloric substitutions of ethanol, carbohydrate, and fat. Three nonlipogenic enzymes were used for comparison. Isocaloric substitution of both fat and carbohydrate for ethanol was necessary to show the specific effects of alcohol on the activity of lipogenic or nonlipogenic enzymes. Carbohydrate, and not ethanol, induced lipogenic enzymes. Ethanol specifically reduced the activity of lactate dehydrogenase and malic enzyme, but did not affect those of alcohol dehydrogenase or glycerol 3-phosphate dehydrogenase. Ethanol interacted with carbohydrate to increase the activity of ATP citrate lyase. In addition, we studied the effects of ethanol and different kinds of carbohydrates on the growth of rats and on the morphology of their livers and intestines. Ethanol significantly decreased growth characteristics (weight gain, growth rate, and caloric efficiency). Fructose, either as a monosaccharide or in sucrose, decreased this alcohol effect. Sucrose was better than glucose in lowering lipid accumulation in livers of rats. Fragility of intestinal villi was found with an alcohol, low carbohydrate diet, but was not present in alcohol diets with a higher level of carbohydrate. In contrast to carbohydrate, ethanol lacked some characteristics of a nutrient, namely, it did not induce some enzymes involved in its metabolism and did not promote optimum growth.

Long-term adaptive response to dietary protein of hexose monophosphate shunt dehydrogenases in rat kidney tubules

We have studied the effects of several different macronutrients on the kinetic behaviour of rat renal glucose 6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Rats were meal-fed with high-carbohydrate/low-protein, high-protein/low-carbohydrate and high-fat diets. High-protein increased renal G6PDH and 6PDGH activities by 66 per cent and 70 per cent respectively, without significantly changing the Km values of either and each Hexose monophosphate dehydrogenase activity increased steadily, reaching a significant difference on day 4. A rise in carbohydrate or fat in the diets, produced no significant change in either the activity or the kinetic parameters, Vmax and Km of the two dehydrogenases. In addition, the administration of a high-protein diet for 8 days significantly increased both the pentose phosphate pathway flux (92.6 per cent) and the kidney weigth (35 per cent), whereas no significant changes in these parameters were found when the animals were treated with the other diets. Our results suggest that an increase in the levels of dietary protein induces a rise in the intracellular levels of these enzymes. The possible role of this metabolic pathway in the kidneys under these nutritional conditions is also discussed.

Effects of ethanol feeding on hepatic lipid synthesis

Rats were fed a high-fat, liquid diet containing either 36% of total calories as ethanol or an isocaloric amount of sucrose, for a period up to 35 days. At different time intervals we measured the effects of ethanol administration on the activities of a number of key enzymes involved in hepatic lipid synthesis. At the start of the experimental period the activities of acetyl-CoA carboxylase and fatty acid synthase, measured in liver homogenates, increased in the control as well as in the ethanol-fed group. After 35 days these enzyme activities were still elevated but there were no significant differences between the two groups. In hepatocytes isolated from controls as well as from ethanol-fed rats, short-term incubations with ethanol induced an increase in the rate of fatty acid synthesis and in the activities of acetyl-CoA carboxylase and fatty acid synthase. However, no alterations in the regulation of these enzymes by short-term modulators of lipogenesis were apparent in hepatocytes isolated from alcohol-treated animals. The results do not indicate a major role for the enzymes of de novo fatty acid synthesis in the development of the alcoholic fatty liver. The amount of liver triacylglycerols increased in ethanol-fed rats during the entire treatment period, whereas the hepatic levels of phosphatidylcholine and phosphatidylethanolamine were not affected by ethanol ingestion. Ethanol administration for less than 2 weeks increased the activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and microsomal phosphocholine cytidylyltransferase, whereas the cytosolic activity of phosphocholine cytidylyltransferase was slightly decreased. Upon prolonged ethanol administration the activities of these enzymes were slowly restored to control values after 35 days, suggesting development of some kind of adaptation. It is interesting that, although the activities of phosphatidate phosphohydrolase and diacylglycerol acyltransferase were restored to the levels found in the control rats, this effect was not accompanied by a stabilization or decrease of the concentration of hepatic triacylglycerols.

Effects of corn oil- and fish oil-supplemented diets on phospholipid fatty acid composition of rat liver nuclei

The effects of dietary fat on fatty acid compositions of phospholipids in liver nuclei were investigated. By feeding a diet containing 5% corn oil or fish oil to rats, the proportions of n -6 or n -3 polyunsaturated fatty acids (PUFA), respectively, were increased in phospholipids of the liver nuclei. By feeding a fat-free diet, endogenous PUFA were increased. Even after feeding the fat-free diet for 40 weeks, the n - 6 still remained at about 10% in both phosphatidylcholine (PC) and phosphatidylethanolamine (PE), while the n -3 PUFA remained at about 5 and 16% in PC and PE, respectively. The fatty acid compositions of phospholipids in the liver nuclei were influenced by dietary fat and were roughly similar to those in the microsomes. The proportion of n - 3 PUFA was high in PE of both the nuclear membrane and matrix. The proportion of n - 6 was higher in both PC and PE of the nuclear matrix than in those of the membrane.

Quantitation by immunoblotting of the in vivo induction and subcellular distribution of hepatic acetyl-CoA carboxylase

The in vivo induction of rat liver acetyl-CoA carboxylase (ACC) the rate-limiting enzyme of fatty acid biosynthesis, has been examined by immunoblotting, avidin blotting, and enzyme isolation. Three high-molecular-weight immunoreactive bands (Mr 220,000-260,000) were recognized in liver extracts by an anti-carboxylase polyclonal antiserum. Two bands, A and B, comigrated on sodium dodecyl sulfate polyacrylamide gels with purified acetyl-CoA carboxylase, were avidin binding, and were dramatically induced following high carbohydrate refeeding. Only band A was recognized on immunoblots using a monoclonal antibody directed against acetyl-CoA carboxylase, suggesting that band B is a proteolytic fragment in which the epitope recognized by the monoclonal antibody is absent. Following refeeding, approximately 57% of acetyl-CoA carboxylase mass (band A + band B) was present in the high-speed supernatant fraction, while 34 and 9% were in the high-speed (microsomal) and low-speed pellet fractions, respectively. Refeeding caused a large increase in total acetyl-CoA carboxylase mass, the magnitude of which differed in the various fractions. In the low-speed supernatant, a 20-fold increase in ACC mass was observed, while a 12-fold increase was seen in the high-speed supernatant. The fold increase in the high-speed pellet was even greater (greater than 27-fold). Acetyl-CoA carboxylase purified by avidin-Sepharose chromatography from fasted/refed rats had an approximate 4-fold higher Vmax and a significantly lower Ka for citrate than enzyme purified from fasted animals. The results of this study indicate that the induction of hepatic ACC that occurs during high carbohydrate refeeding of the fasted rat predominantly involves increases in enzyme content in both cytosol and microsomes, but is also accompanied by an increase in enzyme specific activity.

ATP citrate lyase activity in liver and adipose tissue of veal or ruminating calves (Bos taurus)

1. The activity of ATP citrate lyase in liver and adipose tissue and the concentrations of glucose and insulin in plasma were determined in veal and in ruminating calves. 2. The activity of ATP citrate lyase per g of tissue was substantially higher in liver and adipose tissues of veal calves. 3. Although activity of this enzyme was higher in liver than in adipose tissue on a per g of tissue basis, comparison on a per mg protein basis showed the adipose tissue levels of the enzyme to be higher. 4. Both plasma glucose and insulin levels were also higher in veal calves which agreed well with both the ATP citrate lyase activity and with data from previous studies.

Free fatty acid inhibition of the insulin induction of glucose-6-phosphate dehydrogenase in rat hepatocyte monolayers

Rat hepatocytes in monolayer culture were utilized to determine if the decrease in glucose-6-phosphate dehydrogenase (G6PD) activity resulting from the ingestion of fat can be mimicked by the addition of fatty acids to a chemically, hormonally defined medium. G6PD activity in cultured hepatocytes was induced several-fold by insulin. Dexamethasone or T3 did not amplify the insulin induction of G6PD. Glucose alone increased G6PD activity in cultured hepatocytes from fasted donors by nearly 500%. Insulin in combination with glucose induced G6PD an additional two-fold. The increase in G6PD activity caused by glucose was greater in hepatocytes isolated from 72 hr-fasted rats as compared to fed donor rats. Such a response was reminiscent of the "overshoot" phenomenon in which G6PD activity is induced well above the normal level by fasting-refeeding rats a high glucose diet. Addition of linoleate to the medium resulted in a significant suppression of insulin's ability to induce G6PD, but linoleate had no effect on the induction of G6PD activity by glucose alone. A shift to the right in the insulin-response curve for the induction of G6PD also was detected for the induction of malic enzyme and acetyl-CoA carboxylase. Arachidonate (0.25 mM) was a significantly more effective inhibitor of the insulin action than linoleate was. Apparently rat hepatocytes in monolayer culture can be utilized as a model to investigate the molecular mechanism by which fatty acids inhibit the production of lipogenic enzymes. In part, this mechanism of fatty acid inhibition involves desensitization of hepatocytes to the lipogenic action of insulin.

Dependence of ATP citrate lyase activity on cell density of isolated rat hepatocytes

The activities of two enzymes involved in the lipogenic process, ATP citrate lyase and NADP-linked malic enzyme were evaluated as a function of cell density in isolated rat hepatocytes. The activity of ATP citrate lyase was profoundly affected by cell density with the activity/cell being higher at low cell densities than at high cell densities. Malic enzyme was not similarly affected, nor was cellular ATP content. The effect was observed regardless of dietary state but was most dramatic with hepatocytes from fasted-refed rats. Both an activator and an inhibitor of ATP citrate lyase have been isolated from conditioned medium from cells at low density and at high density, respectively. The activator fraction was heat stable while the inhibitor fraction was heat labile, and both factors had molecular weights in excess of 10,000 daltons.