Coordinate control of rat liver lipogenic enzymes by insulin

Fatty acid synthase and liver triglyceride metabolism: housekeeper or messenger?

Fatty acid synthase (FAS) catalyzes the de novo synthesis of fatty acids. In the liver, FAS has long been categorized as a housekeeping protein, producing fat for storage of energy when nutrients are present in excess. Most previous studies of FAS regulation have focused on the control of gene expression. However, recent findings suggest that hepatic FAS may also be involved in signaling processes that include activation of peroxisome proliferator-activated receptor α (PPARα). Moreover, reports of rapid alterations in FAS activity as well as findings of post-translational modifications of the FAS protein support the notion that dynamic events in addition to transcription impact FAS regulation. These results indicate that FAS enzyme activity can impact liver physiology through signaling as well as energy storage and that its regulation may be complex. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Regulation of de novo hepatic lipogenesis by insulin infusion in rainbow trout fed a high-carbohydrate diet

Carbohydrate energy intake in excess of total energy expenditure is converted to fat. In fish, the liver is considered to be the main lipogenic tissue. Its regulation by insulin is not fully understood, and some of the available in vivo findings are contradictory. In this study, bovine insulin was infused for 5 d into rainbow trout fed a high-carbohydrate diet, and variables of de novo hepatic lipogenesis were measured. We found that hepatic lipogenesis in trout is stimulated by insulin, reflected in enhanced mRNA and protein abundance and enzyme activity of ATP-citrate lyase, acetyl-CoA carboxylase, and fatty acid synthase. These results were further supported by parallel changes in enzymes acting as NAD phosphate donors, especially those participating in the pentose phosphate pathway. This is the first time that the main enzymes involved in de novo hepatic lipogenesis have been studied at the molecular, protein, and activity levels in fish. We hypothesize that some of the delayed changes found in the different levels of regulation were probably related to the insulin resistance achieved by the trout liver after 5 d of insulin infusion. We assessed enzyme activity and mRNA abundance of lipid oxidation-related enzymes in the livers of insulin-infused fish in which paradoxically increased β-oxidation potential was found. The insulin-stimulated de novo hepatic lipogenesis in carbohydrate-fed trout reinforces the hypothesis that this pathway may act as an important sink for excess glucose, which could ultimately contribute to improved glucose homeostasis in this carnivorous and glucose-intolerant species when fed high-carbohydrate diets.

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.

Effects of additional Vitamin E and selenium supply on G6PDH activity in rats treated with high doses of glucocorticoid

The aim of this work was to determine the effects of dietary intake Vitamin E and selenium (Se) on glucose-6-phosphate dehydrogenase (G6PDH) activity in rats treated with high doses of prednisolone. Two hundred and fifty adult male Wistar rats were randomly divided into five groups. The rats were fed a normal diet, but groups 3, 4, and 5 received a daily supplement in their drinking water of 20mg Vitamin E, 0.3mg Se, and a combination of Vitamin E and Se, respectively, for 30 days. For 3 days subsequently, the control group (group 1) was treated with a placebo, and the remaining four groups were injected intramuscularly with 100 mg/kg body weight prednisolone. After the last administration of prednisolone, 10 rats from each group were killed at 4, 8, 12, 24, and 48 h and the activities of G6PDH enzymes in their tissues were measured. Hepatic and spleen G6PDH activities in the prednisolone treatment group began to decrease gradually at 8 h, while enzyme activities did not change in the kidney and heart. However, the administration of Vitamin E alone did not affect G6PDH activity in any of the tissues. Se supplementation had a preventive effect on the decrease of G6PDH caused by prednisolone and improved the diminished activities of G6PDH. Therefore, the present study demonstrates that a high dose of prednisolone may alter the effects of normal dose glucocorticoids and that Se is effective in reducing damage in prednisolone-treated rats. Se may prevent the changes in G6PDH activity in various tissues caused by prednisolone in various tissues.

Identification of insulin-responsive regions in the HMG-CoA reductase promoter

An insulin-responsive line of rat hepatoma cells, H4IIE, was used to investigate the basis for insulin's transcriptional regulation of HMG-CoA reductase. Insulin addition to the media of these cells resulted in at least a 10-fold increase in levels of HMG-CoA reductase protein. Adding insulin to H4IIE cells transfected with pHMGR1 (containing the proximal reductase promoter from -270 to +20 ligated to luciferase) caused greater than 10-fold increases in luciferase activity. Transfections carried out with a series of deletion constructs identified insulin responsive regions between -203 and -130 (contains the SRE sequence) and between -85 and -105 (contains a CRE sequence). Mutation of the SRE in the -203 to -130 sequence did not decrease activation by insulin. In contrast, mutation of the C at -90 of the CRE completely eliminated the insulin response. The data suggest that insulin's activation of HMG-CoA reductase involves the CRE in the -85 to -105 region and the -203 to -130 region of the promoter exclusive of the SRE.

Activity and mRNA levels of enzymes involved in hepatic fatty acid synthesis and oxidation in mice fed conjugated linoleic acid

The effects of dietary conjugated linoleic acid (CLA) on the activity and mRNA levels of hepatic enzymes involved in fatty acid synthesis and oxidation were examined in mice. In the first experiment, male ICR and C57BL/6J mice were fed diets containing either a 1.5% fatty acid preparation rich in CLA or a preparation rich in linoleic acid. In the second experiment, male ICR mice were fed diets containing either 1.5% linoleic acid, palmitic acid or the CLA preparation. After 21 days, CLA relative to linoleic acid greatly decreased white adipose tissue mass but caused hepatomegaly accompanying an approximate 10-fold increase in the tissue triacylglycerol content irrespective of mouse strain. CLA compared to linoleic acid greatly increased the activity and mRNA levels of various lipogenic enzymes in both experiments. Moreover, CLA increased the mRNA expression of Delta6- and Delta5-desaturases, and sterol regulatory element binding protein-1 (SREBP-1). The mitochondrial and peroxisomal palmitoyl-CoA oxidation rate was about 2.5-fold higher in mice fed CLA than in those fed linoleic acid in both experiments. The increase was associated with the up-regulation of the activity and mRNA expression of various fatty acid oxidation enzymes. The palmitic acid diet compared to the linoleic acid diet was rather ineffective in modulating the hepatic lipid levels or activity and mRNA levels of enzymes in fatty acid metabolism. It is apparent that dietary CLA concomitantly increases the activity and mRNA levels of enzymes involved in fatty acid synthesis and oxidation, and desaturation of polyunsaturated fatty acid in the mouse liver. Both the activation of peroxisomal proliferator alpha and up-regulation of SREBP-1 may be responsible for this.

Hepatic fatty acid oxidation enzyme activities are stimulated in rats fed the brown seaweed, Undaria pinnatifida (wakame)

The activities of hepatic enzymes involved in fatty acid synthesis and oxidation were compared in rats fed diets containing different proportions of dried powder of the brown seaweed, Undaria pinnatifida (wakame). Rats were fed diets containing 0, 0.5, 1.0, 2. 0, 5.0 and 10 g/100 g of dried wakame powder. Experimental diets were adjusted to provide consistent amounts of most nutrients, but mineral concentrations were not standardized. After the 21-d feeding period, serum and liver triacylglycerol levels in rats fed diets in which wakame constituted at least 2% were significantly lower than those in rats fed the control diet. The activity of glucose-6-phosphate dehydrogenase was significantly lower in rats fed the 5 and 10% wakame diets than in rats fed the control diet. In contrast, 10% wakame diet increased activities of enzymes involved in the beta-oxidation pathway including hepatic carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, enoyl-CoA hydratase and 2,4-dienoyl-CoA reductase. Some differences were detected in rats fed 5% wakame as well. These results suggest that alterations of the activities of enzymes involved in fatty acid metabolism in the liver are responsible for the serum triacylglycerol-lowering effect of dietary wakame. Thus, wakame may be useful as a food to prevent hyperlipidemia.

Comparative effects of alpha- and gamma-linolenic acids on rat liver fatty acid oxidation

It has been reported that both n-3 and n-6 octadecatrienoic acids can increase hepatic fatty acid oxidation activity. It remains unclear, however, whether different enzymes in fatty acid oxidation show a similar response to n-3 and n-6 octadecatrienoic acids. The activity of hepatic fatty acid oxidation enzymes in rats fed an oil mixture rich in alpha-linolenic acid (18:3n-3) and borage oil rich in gamma-linolenic acid (18:3n-6) was therefore compared to that in rats fed an oil mixture rich in linoleic acid (18:2n-6) and a saturated fat (palm oil) in this study. Linseed oil served as the source of 18:3n-3 for the oil mixture rich in this octadecatrienoic acid and contained 30.6% 18:3n-3 but not 18:3n-6. Borage oil contained 25.7% 18:3n-6 and 4.5% 18:3n-3. Groups of seven rats each were fed diets containing 15% various fats for 15 d. The oxidation rate of palmitoyl-CoA in the peroxisomes was higher in rats fed a fat mixture rich in 18:3n-3 (3.03 nmol/min/mg protein) and borage oil (2.89 nmol/min/mg protein) than in rats fed palm oil (2.08 nmol/min/mg protein) and a fat mixture rich in 18:2n-6 (2.15 nmol/min/mg protein). The mitochondrial palmitoyl-CoA oxidation rate was highest in rats fed a fat mixture rich in 18:3n-3 (1.93 nmol/min/mg protein), but no significant differences in this parameter were seen among the other groups (1.25-1.46 nmol/min/mg protein). Compared to palm oil and fat mixtures rich in 18:2n-6, a fat mixture rich in 18:3n-3 and borage oil significantly increased the hepatic activity of carnitine palmitoyltransferase and acyl-CoA oxidase. Compared to palm oil and a fat mixture rich in 18:2n-6, a fat mixture rich in 18:3n-3, but not fats rich in 18:3n-6, significantly decreased 3-hydroxyacyl-CoA dehydrogenase activity. Compared to palm oil and a fat mixture rich in 18:2n-6, borage oil profoundly decreased mitochondrial acyl-CoA dehydrogenase activity, but a fat mixture rich in 18:3n-3 increased it. 2,4-Dienoyl-CoA reductase activity was significantly lower in rats fed palm oil than in other groups. Compared to other fats, borage oil significantly increased delt3,delta2-enoyl-CoA isomerase activity. Activity was also significantly higher in rats fed 18:2n-6 oil than in those fed palm oil. It was confirmed that both dietary 18:3n-6 and 18:3n-3 increased fatty acid oxidation activity in the liver. These two dietary octadecatrienoic acids differ considerably, however, in how they affect individual fatty acid oxidation enzymes.

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.

Concept of the noncoenzymatic thiamine effect

The experimental and clinical data on different aspects of vitamin and hormone relationships have been summarized in the form of a general concept of the noncoenzymatic thiamine effect, on the basis of a number of premises: (1) discovery of tissue factors limiting the manifestation of the specific activity of administered thiamine (the presence of a tissue buffer depot of easily accessible coenzymes, and lack of free apoenzymes); (2) evidence of a thiamine effect on the pancreatic insulin-synthesizing function; (3) stimulation of metabolic thiamine effects, including the effects of insulin administration on thiamine-dependent enzymes; (4) determination of the features of hormonal control of thiamine metabolism in the body; (5) confirmation of the predictive force of the concept by clinical trials of the new strategy of thiamine therapy.

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.

Reciprocal responses to dietary diacylglycerol of hepatic enzymes of fatty acid synthesis and oxidation in the rat

The activities of hepatic enzymes of fatty acid synthesis and oxidation were compared in rats fed on diacylglycerol and triacylglycerol. In the first trial, rats were fed on diacylglycerol or triacylglycerol (rapeseed oil) for 14 d. The diacylglycerol preparation contained 65.2 g and 32.6 g fatty acids/100 g total fatty acids as 1,3-species and 1,2-species respectively. Fatty acid compositions of these dietary lipids were similar. Dietary acylglycerols were added to experimental diets to provide the same amounts of fatty acids (93.9 g/kg diet). Dietary diacylglycerol compared with triacylglycerol significantly reduced the concentrations of serum and liver triacylglycerol. The activities of enzymes of fatty acid synthesis (fatty acid synthetase, glucose 6-phosphate dehydrogenase (EC 1.1.1.49) and malic enzyme (EC 1.1.1.40)) were significantly lower in rats fed on diacylglycerol than in those fed on triacylglycerol. In contrast, the rates of mitochondrial and peroxisomal oxidation of palmitoyl-CoA in liver homogenates were higher in rats fed on diacylglycerol than in those fed on triacylglycerol. In the second trial, varying amounts of dietary triacylglycerol were replaced by diacylglycerol while the dietary fatty acid content was maintained (93.9 g/kg diet). After 21 d of the feeding period the significant reductions in serum and liver triacylglycerol levels were confirmed in groups of rats fed on the diets in which diacylglycerol supplied more than 65.8 g fatty acids/kg diet (65.8 and 93.9 g/kg). Reductions in the activities of enzymes of fatty acid synthesis and increases in palmitoyl-CoA oxidation rates by both mitochondrial and peroxisomal pathways were also apparent when diacylglycerol replaced triacylglycerol in diets to supply more than 65.8 g fatty acid/kg. Increasing dietary levels of diacylglycerol also progressively increased the activities of enzymes involved in the beta-oxidation pathway (carnitine palmitoyltransferase (EC 2.3.1.21), acyl-CoA dehydrogenase (EC 1.3.99.3), acyl-CoA oxidase (EC 1.3.3.6), enoyl-CoA hydratase (EC 4.2.1.17), 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35), 2,4-dienoyl-CoA reductase (EC 1.3.1.34) and delta 3, delta 2-enoyl-CoA isomerase (EC 5.3.3.8)) in the liver. These results suggest that alteration of fatty acid metabolism in the liver is a factor responsible for the serum triacylglycerol-lowering effect of dietary diacylglycerol.

Metabolic effects of coconut, safflower, or menhaden oil feeding in lean and obese Zucker rats

The aim of the present investigation was to study the effects of fish oil feeding in obese Zucker rats to establish its suitability as an animal model of hyperlipidaemia, and to understand the possible mechanism of fish oil-induced perturbations in cell metabolism. Lean and obese Zucker rats were fed on diets containing 180 g coconut, safflower, or menhaden oil/kg for 10 weeks. Body-weights and food intakes of lean coconut (LC), safflower (LS), and menhaden (LM) groups were similar. Obese menhaden (OM) rats had lower food intakes and body-weights compared with obese coconut (OC) and obese safflower (OS) groups, but values for all obese rats were higher than those for lean rats. Liver weights were higher in obese compared with lean rats, but on a percentage body-weight basis menhaden oil rats had higher values within genotype. Serum cholesterol and triacylglycerol levels were lower in the OM group compared with the OC and OS groups, and in the LM group compared with the LC group. Glucose and insulin levels were highest in OS rats followed by OC and OM rats and then the lean rats. Serum triiodothyronine and thyroxine were lower in OM rats compared with OC and OS rats. Liver mitochondrial state 3 rates with glutamate-malate and succinate were lower; mitochondrial beta-oxidation was unaffected and peroxisomal beta-oxidation was higher in menhaden oil rats compared with both coconut and safflower oil rats. In general, consumption of menhaden oil lowered hepatic malic enzyme (EC 1.1.1.38, 1.1.1.40), glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and glutathione peroxidase (EC 1.11.1.9) activities and elevated long-chain fatty acyl-CoA hydrolase (EC 3.1.2.2) activity when compared with the two other diets. It is concluded that obese Zucker rats do respond like human subjects to fish oil feeding but not to vegetable oils. The hypolipidaemic effect of fish oil appears to be mediated through a lowering of lipogenic enzymes, glucose-6-phosphate dehydrogenase and malic enzyme.

Effects of nutrients and insulin on transcriptional and post-transcriptional regulation of glucose-6-phosphate dehydrogenase synthesis in rat liver

The transcriptional and post-transcriptional regulation of glucose-6-phosphate dehydrogenase induction of rat liver was investigated using a cDNA cloned in our laboratory. By feeding a carbohydrate/protein diet to fasted rats, the mRNA concentration and enzyme induction of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) reached maximal levels about 10-fold those in the fasted rats at 16 h and 72 h, respectively, whereas the transcriptional rate was increased about 3-fold in 6 h. In the protein fed (without carbohydrate) group, both the mRNA concentration and enzyme induction were increased to about 60% of the levels in the carbohydrate/protein fed group and in the group fed on a carbohydrate diet (without protein) to 30-40%. Further, dietary fat significantly reduced the transcriptional rate, mRNA concentration and enzyme induction to less than half, suggesting that dietary fat primarily reduced transcription. Thus, dietary nutrients appear to be involved in the steps preceding the translation. On the other hand, in diabetic rats, the transcriptional rate was significantly decreased as compared to the normal level and restored by insulin-treatment in 4 h. The mRNA concentration was very low in diabetic rats, and was restored to the normal level by insulin treatment in 8 h, and was half restored by fructose feeding. However, the enzyme induction of glucose-6-phosphate dehydrogenase was scarcely restored by fructose, unless accompanied by insulin treatment. Thus, it is suggested that insulin is involved in translation as well as in transcription. Further, the insulin-dependent increase of glucose-6-phosphate dehydrogenase mRNA was blocked by cycloheximide, suggesting that synthesis of a peptide is required.

Effects of insulin and fructose on transcriptional and post-transcriptional regulation of malic enzyme synthesis in diabetic rat liver

Insulin action on regulation of hepatic malic enzyme has been investigated in comparison with fructose, using streptozotocin-induced diabetic rats. Insulin-treatment caused a 2.8-fold increase in the transcriptional rate of malic enzyme (EC 1.1.1.40) after 8 h, and a 5-fold increase in the mRNA concentration of the liver. In Northern blot analysis, we demonstrated that after insulin treatment, the nuclear mRNA of malic enzyme tended to increase more rapidly than the total cellular mRNA. Therefore, it is suggested that the nuclear mRNA was primarily increased by insulin. The insulin-dependent increase of malic enzyme mRNA was blocked by cycloheximide, suggesting that synthesis of a peptide is required. On the other hand, by feeding a high-fructose diet to diabetic rats, the malic enzyme mRNA concentration was considerably increased, though with a delayed peaking in comparison with the insulin-treated animals, whereas the transcriptional rate was not significantly increased. Dietary fructose may stabilize the transcripts. Fructose increased the enzyme level far less than the mRNA level. These results suggest that insulin is required in both the translational and transcriptional regulation of malic enzyme.

Carbohydrate-dependent induction of fatty acid synthase in primary cultures of rat hepatocytes

The glucose and insulin-dependent long-term regulation of fatty acid synthase was studied in primary cultures of hepatocytes from adult female rats. Under basic culture conditions, i.e. 5.5 mM glucose and 0.5 nM insulin, the enzyme activity was continuously decreased over 6 days. In the presence of 100 nM insulin this decrease was reduced but it was not prevented. Enhancement of glucose to 20 mM was followed by an increase of the enzyme activity; after 5 days of treatment the activity was three times higher than under basic culture conditions. The simultaneous presence of 20 mM glucose and of 100 nM insulin resulted in a much more pronounced increase of the activity; after 5 days of treatment the activity was eight times higher than under basic culture conditions. The enhancement was prevented by inhibition of glycolysis. This may indicate that the increase of fatty acid synthase was mediated by a metabolite of glucose rather than by glucose itself. The coordinate regulation of fatty acid synthase and of other lipogenic enzymes was specific as demonstrated by comparison with the activity of lactate dehydrogenase and with synthesis and degradation of cytosolic proteins. The enhancement of the enzyme protein, demonstrated by rocket immunoelectrophoresis, was due to an increase in the rate of enzyme synthesis by 600% as well as to a prolongation of the apparent half-life of the enzyme by 50% (45 h).

Glucose-6-phosphate dehydrogenase mRNA sequence abundance in primary cultures of rat hepatocytes. Effect of insulin and dexamethasone

Hepatic glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is subject to nutritional regulation. To assess the possible role of hormones in this regulation, the amounts of G6PDH mRNA were studied in primary cultures of rat hepatocytes treated with insulin and dexamethasone, alone or in combination. Relative concentrations of G6PDH mRNA were directly assessed by a dot-blot hybridization procedure with nick-translated cDNA probes. G6PDH sequence abundance increased when the cultures were treated with insulin or dexamethasone, but the G6PDH mRNA induced by dexamethasone was not expressed at the protein level as active enzyme. In cultures treated with insulin and dexamethasone in combination, enzyme activity and G6PDH sequence abundance were greater than those induced by insulin alone. Our results directly demonstrate that G6PDH mRNA amounts are modulated in liver by these two classes of hormones and can partially account for the dietary induction of the enzyme observed in vivo.

Secretion of lipoprotein lipid and synthesis of fatty acids, cholesterol and triacylglycerol by hepatocytes of fasted-refed rats

Synthetic rates of fatty acid, cholesterol and triacylglycerols, and contents and secretion of lipoprotein lipids, were determined in hepatocytes of rats fed ad libitum a fat-containing stock diet or of rats fasted for 48 h and then refed for 24 or 48 h with stock diet or with a glucose-rich fat-free diet. When compared with the values for the ad libitum-fed rats, fatty acid synthesis was lower in fasted rats, slightly increased in rats refed with the stock diet, but several-fold elevated after refeeding the glucose-rich fat-free diet. Cholesterol synthesis was decreased in the fasted cells, and restored to the control level upon refeeding either diet. Triacylglycerol synthesis from exogenous oleate was greatly stimulated in the cells of fasted-refed rats above the rate in cells of the ad libitum-fed rats, the increase being considerably higher after refeeding the glucose-rich fat-free diet than the stock diet. The amount of triacylglycerol secreted by the cells was also elevated by the fasting-refeeding treatment, but the difference between the two diets was much less pronounced than seen for the lipids' synthetic rates. This imbalance may underlie the huge accumulation of this lipid observed in the heptatocytes after refeeding the rats for 48 h with the glucose-rich fat-free diet.

Insulin mediates the asynchronous accumulation of hepatic albumin and malic enzyme messenger RNAs

We have compared the rate of accumulation of hepatic albumin and malic enzyme mRNAs following insulin treatment of diabetic rats to determine whether insulin coordinately increases mRNA levels or specifically induces the accumulation of individuals mRNAs. Initially, the quantities of both albumin and malic enzyme mRNAs are reduced in diabetic rats compared to normal rats as determined by RNA blot analysis using complementary DNA probes. Following insulin administration for 12 h, albumin and malic enzyme mRNA levels increase at similar rates. However, after 12 h the rate of malic enzyme mRNA accumulation increases dramatically while albumin mRNA continues to increase at its initial rate. This accelerated rate of accumulation of malic enzyme mRNA continued through 60 h of hormone treatment and was associated with the onset of hepatic lipogenesis. Thus, our results suggest that insulin regulates the accumulation of mRNAs encoding these two inducible proteins in an asynchronous manner directly related to the metabolic requirements of the animal.

Changes in lipid metabolism in diet-induced obesity

Mature male Sprague-Dawley rats fed a powdered Purina Chow diet containing corn oil and condensed milk (CM) were compared to rats fed a Purina Chow diet (control). CM rats gained more weight and consumed more calories over a 73-day period than the control rats. The increased weight gain and body fat in CM rats was accompanied by increased cell number in retroperitoneal and inguinal but not epididymal fat pads while cell size was unchanged in all three pads. After obesity had developed there was an increase in insulin levels, lipolysis, hepatic fatty acid synthesis, and fatty acid oxidation. While CM rats demonstrated hyperinsulinemia and hyperglycerolemia, they maintained normal glucagon and glucose levels. They demonstrated higher rates of fatty acid synthesis in isolated hepatocytes but not in vivo, suggesting that a greater potential for fatty acid synthesis in CM rats was masked in vivo by the inhibitory action of dietary lipids. Beta-oxidation of (1-14C) palmitate in vivo and in vitro, and in vivo ketogenesis were greater in CM than in chow fed rats. These studies demonstrate that, after the development of obesity, CM rats, like genetically obese Zucker rats, are hyperinsulinemic and have elevated levels of fatty acid synthesis. However, unlike obese Zucker rats, CM rats displayed an increase in beta-oxidation. These studies suggest that increased insulin levels and hepatic fatty acid synthesis may contribute to dietary obesity (as they do to genetic obesity), whereas increased fatty acid oxidation in dietary obesity may be a compensatory response to maintain a lower body weight.

The effect of ethanol, alone and in combination with the glucocorticoids and insulin, on glucose-6-phosphate dehydrogenase synthesis and mRNA in primary cultures of hepatocytes

The hormonal regulation of the relative rate of synthesis and mRNA of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) was studied in primary cultures of adult-rat liver parenchymal cells maintained in a chemically defined medium. Maintenance of hepatocytes from starved animals in a culture medium devoid of any hormones resulted in a 4-fold increase in the relative rate of G6PDH synthesis in 48 h. Parallel cultures treated with glucocorticoids alone exhibited a rate of G6PDH synthesis comparable with that in the control cultures, whereas insulin alone caused a 6.5-fold increase in the rate of synthesis in 48 h. However, if the cultures were treated with glucocorticoids and insulin simultaneously, a 13-fold increase in the rate of synthesis was observed. The effect of ethanol, alone and in combination with the hormones, on the relative rate of G6PDH synthesis was studied also. Ethanol alone caused an 8-fold increase in the rate of synthesis in 48 h, whereas the combination of ethanol, glucocorticoid and insulin caused a 25-fold increase. The amount of functional mRNA encoding G6PDH, as measured in a cell-free translation system, was compared with enzyme activity and relative rate of enzyme synthesis. The increases in G6PDH activity and relative rate of synthesis in primary cultures of hepatocytes treated with ethanol, alone and in combination with the glucocorticoids and insulin, were paralleled by comparable increases in G6PDH mRNA. The results of this study show that the glucocorticoids acted in a permissive manner to amplify the insulin stimulation of G6PDH synthesis and that insulin, glucocorticoids and ethanol interact to stimulate synthesis of G6PDH primarily by increasing the concentration of functional G6PDH mRNA.

The short-term regulation of fatty acid synthesis in the rat epididymal adipocytes

Lipogenesis and fatty acid synthetase (FAS) activity of isolated rat adipocytes that were treated with insulin or epinephrine were studied. Insulin stimulated incorporation of radioactivity from D-[U-14C]glucose into CO2, saponifiable and non-saponifiable fractions, whereas epinephrine promoted lipolysis and oxidation of glucose into CO2. Whereas insulin stimulated fatty acid synthesis, epinephrine had no effect. Changes in FAS specific activity of insulin- or epinephrine-treated adipocytes were insignificant and could not account for insulin-stimulated lipogenesis. Rat adipocyte FAS, unlike hepatic FAS, was not subject to short-term regulation by insulin, although fatty acid synthesis showed such a response.

Regulation of glucose-6-phosphate dehydrogenase mRNA by insulin and the glucocorticoids in primary cultures of rat hepatocytes

The levels of functional mRNA encoding glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) were examined in hepatocytes from fasted and fasted/carbohydrate-refed rats and in hepatocytes inoculated into primary culture. Functional G6PDH mRNA was assessed in a cell-free protein synthesis system in vitro. We observed that hepatocytes from fasted/carbohydrate-refed rats had a 12-fold higher level of mRNA than did hepatocytes from fasted rats. The possibility that the adrenal glucocorticoids and insulin were responsible for the increase in G6PDH mRNA in refed rats was examined by studying the effect of insulin and the synthetic glucocorticoid, dexamethasone, on the level of functional G6PDH mRNA in primary cultures of rat hepatocytes maintained in a chemically defined medium. Hepatocytes from fasted rats were inoculated into primary culture and maintained for 48 h either in the absence of hormones or in the presence of insulin alone, dexamethasone alone or both hormones together. We observed that dexamethasone alone caused a fourfold increase in G6PDH mRNA while insulin caused about a twofold increase. Both hormones together elicited an increase that was additive. A comparison of functional G6PDH mRNA levels with the effect of the hormones on G6PDH activity and relative rate of enzyme synthesis suggests that the glucocorticoid elevates the level of G6PDH mRNA within the cell without causing a concommitant increase in the rate of synthesis of the enzyme or the level of G6PDH activity. The results obtained with the primary cultures of hepatocytes indicate that insulin and the glucocorticoids are probably involved with the regulation of hepatic G6PDH mRNA. However, involvement of other hormones, such as thyroid hormone, seems likely since the induced levels of G6PDH mRNA in hepatocytes in culture was one-third of that observed in refed rats.

Glucose-dependent induction of acetyl-CoA carboxylase in rat hepatocyte cultures

The carbohydrate-dependent long-term regulation of acetyl-CoA carboxylase was studied in primary hepatocyte cultures from adult rats. (1) The enzyme activity was increased 2-fold either by elevation of the glucose concentration to 20mM or by enhancement of the insulin concentration to 0.1 microM. Simultaneous increases in glucose and insulin resulted in a 5-fold increase in the enzyme activity. (2) As shown by immunochemical titration, the enhancement of the enzyme activity was due to an increase in the enzyme protein. (3) Incorporation of [35S]methionine and immunoprecipitation of the enzyme revealed that the increase in enzyme protein was due to an increased rate of enzyme synthesis. The rate of enzyme degradation remained essentially unchanged. (4) The glucose- and insulin-dependent induction of acetyl-CoA carboxylase was prevented by the addition of alpha-amanitin (10 microM) or cordycepin (10 microM), indicating a transcriptional regulation of the enzyme level. (5) The parallel induction of acetyl-CoA carboxylase and of ATP citrate lyase indicates a co-ordinate long-term regulation of lipogenic enzymes.

Insulin and fructose regulate malic enzyme activity by different processes

A comparison of the regulatory processes controlling hepatic malic enzyme activity following treatment of diabetic rats with insulin or with a high fructose diet demonstrated several important differences. Insulin treatment caused a 50-fold increase in activity, due to a 12-fold increase in enzyme quantity and a 4-fold increase in specific activity(units/nmol). Dietary fructose caused a 3-fold increase in enzyme activity, due to a 3-fold increase in enzyme quantity, with no change in the specific activity of the enzyme. Thus, while fructose initiated a minor increase in malic enzyme activity, insulin was more effective, causing a substantially greater increase in enzyme activity and activating a hormone specific alteration in the catalytic activity of each enzyme molecule.

Rapid purification of enzymes of fatty acid biosynthesis from rat adipose tissue

Acetyl CoA carboxylase, ATP-citrate lyase and fatty acid synthetase were purified to homogeneity by a simple procedure. The purification method consists of polymerization of acetyl CoA carboxylase with citrate followed by avidin-Sepharose affinity chromatography. ATP-citrate lyase and fatty acid synthetase were isolated as by-products of acetyl CoA carboxylase purification and are separated from each other by chromatography on DE-52. ATP-citrate lyase was further purified by CoA-agarose affinity chromatography and fatty acid synthetase was purified on Bio-Gel A-1.5m. Purified ATP-citrate lyase, acetyl CoA carboxylase and fatty acid synthetase had specific activities of 9.9, 2.8 and 1.8 U/mg respectively with an over all recovery of 30, 25 and 50% respectively. Using these purified enzymes, we found that ATP-citrate lyase and acetyl CoA carboxylase were phosphorylated in vitro by both cAMP-dependent protein kinase and ATP-citrate lyase kinase whereas fatty acid synthetase was not phosphorylated by these protein kinases.

Abnormal hepatic lipid accumulation following treatment of diabetic rats with insulin and a high-carbohydrate, fat-free diet

This study correlates the morphological and biochemical events during the accumulation of hepatic lipids in diabetic rats in response to insulin treatment and a high-carbohydrate, fat-free diet. Alloxan-diabetic rats were fed a high-carbohydrate, fat-free diet and treated with insulin for 12, 36, or 60 hr or 4.5 or 6.5 days. Samples of livers were obtained for determination of malic enzyme activity and the histochemical demonstration of lipids. An increased accumulation of hepatic lipids, although delayed, was observed following insulin treatment of diabetic rats fed the special diet. Small lipid droplets were visible after 36 hr of treatment, which later increased and coalesced into larger droplets present in all hepatocytes. Maximal accumulation was observed at 4.5 days of treatment. These changes were paralleled by an increase in the activity of hepatic malic enzyme. By 6.5 days of treatment, the lipid content of the hepatocytes had decreased and a periportal pattern was discernible. In contrast, malic enzyme activity continued to increase through 6.5 days of treatment. By comparison, no hepatic lipid accumulation occurred in regular chow-fed diabetic rats receiving insulin treatment or in diabetic rats placed on the special diet alone. These results suggest that the combination of insulin treatment and a high-carbohydrate, fat-free diet caused an imbalance in the production and mobilization of hepatic lipids.

Induction of rat liver malic enzyme messenger RNA activity by insulin and by fructose

The effects of insulin treatment and fructose feeding on malic enzyme [EC 1.1.1.40] mRNA in liver of diabetic rats were investigated. A high fructose diet, even without insulin treatment, increased the mRNA activity, much more than a high starch diet, but to a lesser extent than insulin treatment. The increase of malic enzyme activity was approximately comparable to the change in mRNA activity, but occurred 12 hr and 30 hr after the increase of mRNA in the fructose-fed rats with and without insulin treatment, respectively. Thus, insulin and fructose appear to induce malic enzyme primarily by increasing the activity of translatable mRNA.

Glucose- and insulin-independent induction of ATP citrate lyase in primary cultures of rat hepatocytes

ATP citrate lyase, which is involved in the translocation of the lipogenic precursor acetyl-CoA from mitochondria to cytosol, was studied in primary cultures of hepatocytes from adult rats. After an initial decrease at the first day of culture the enzyme activity was nearly constant during the following days. It could be enhanced between 24h and 48 h in culture about 1.5-fold by elevation of the insulin concentration to 10-7mol/1.22-fold by elevation of the glucose concentration from 5 to 25 mmol/l and 3.5-fold by simultaneous elevation of insulin and glucose. The increase of activity was about linear with time for 24 h and could be blocked by cycloheximide, which inhibits protein synthesis at the translational level. Both observations suggest that the enhancement of activity was due to induction rather than to activation by interconversion. The glucose-dependent induction was furthermore evidenced by immunotitration which indicated a parallel increase of activity and enzyme protein.

Degradation of pigeon liver fatty acid synthetase in the absence of exogenous proteinases

The homogeneity of pigeon liver fatty acid synthetase has been rigorously tested by physicochemical techniques and crossed-rocket immunoelectrophoresis. The enzyme has also been incubated for 1 h at 100 degrees C in 2% sodium dodecyl sulfate and 0.1 M dithiothreitol. The number of protein components on gel electrophoresis and of dansylated amino acids increased as a function of incubation time. Furthermore, the minor proteins observed after gel electrophoresis cross-reacted with antibody raised to the synthetase. Proteolysis was not chemically mediated by the detergent, the reducing agent or the buffer conditions chosen. Several commercially prepared proteins were not degraded by this procedure, and two proteins were recalcitrant to hydrolysis when included in the same incubation mixture as the synthetase. The inclusion of certain microbial proteinase inhibitors decreased the amount of degradation. This demonstrated that hydrolysis of the synthetase is mediated by a specific vertebrate enzyme which retains activity under denaturing conditions at 100 degrees C. Further degradation is also observed after individual treatment of four limited digestion products from the pigeon liver fatty acid synthetase, suggesting the possibility of an inherent proteolytic activity within the complex.

Heterogeneous distribution of ATP citrate lyase in rat-liver parenchyma. Microradiochemical determination in microdissected periportal and perivenous liver tissue

1. A radiochemical microtest was established for the determination of ATP citrate lyase in tissue samples of 0.2-1.0 micrograms dry weight. The specificity of this test system was guaranteed by its coenzyme A dependence as well as by inhibition of the activity measured in presence of a specific antibody. 2. Using this test system ATP citrate lyase activity was determined in microdissected periportal and perivenous liver tissue of fed, fasted and refed animals. The perivenous activity was 1.8-fold and 2.4-fold higher than the periportal one in fed male and female rats respectively. 3. The perivenous to periportal gradient was decreased during starvation-dependent reduction of the ATP citrate lyase activity. On the other hand it was not only restored but enhanced up to 2.8 after refeeding-dependent enhancement of the enzyme activity. 4. The predominance of the ATP citrate lyase activity in the perivenous, mainly glycolytic zone supports the hypothesis of the coordinate zonation of the carbohydrate and the lipid metabolism in the liver parenchyma.

Induction of fatty acid synthetase and acetyl-CoA carboxylase by isolated rat liver cells

Current studies on the synthesis of long-chain fatty acids by isolated rat liver cells are largely concerned with the regulation of the activity of previously existing acetyl-CoA carboxylase and fatty acid synthetase, and with the regulation of the quantity of these enzymes. These studies have required the development of methods for obtaining high yields of viable hepatocytes that respond to hormonal treatment. Such methods have been developed over the past 10-15 years through the efforts of several laboratories. These studies have also required the development of a method to determine whether a change in the activity of an enzyme is due to a modification of preexisting enzyme or to a change in quantity of that enzyme. The most satisfactory method to use for such studies is immunotitration of enzyme activity. In recent years studies on the regulation of acetyl-CoA carboxylase have largely centered upon the effect of phosphorylation-dephosphorylation on the activity of this enzyme and whether glucagon inhibits the activity of this enzyme through this process. Much data from a number of laboratories have suggested that glucagon regulates the activity of this enzyme through phosphorylation-dephosphorylation. However, several of these studies involved the use of crude systems in which competing enzymes and substrates that can significantly interfere with acetyl-CoA carboxylase activity measurements were still present. Hence, a confirmation of these studies needs to be carried out under conditions in which the effects of competing enzymes and substrates are eliminated. Studies on changes in quantity of acetyl-CoA carboxylase and fatty acid synthetase have shown that these enzymes are induced by the fasting and refeeding of animals. They have also shown that insulin stimulates (10- to 30-fold) the induction of these enzymes. This induction appears to be due to a change in the quantity of translatable mRNA which may, in turn, be due to a change in the rate of transcription of the genes coding for these enzymes.

Induction of lipogenic enzymes in primary cultures of rat hepatocytes. Relationship between lipogenesis and carbohydrate metabolism

Using primary cultures of adult rat hepatocytes, the regulation of the following lipogenic enzymes was studied: glucose-6-phosphate dehydrogenase, malic enzyme, ATP-citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase, and stearoyl-CoA desaturase. The addition to the culture medium of either insulin or triiodothyronine produced a 2-3-fold increase in each of the individual enzyme activities whereas glucagon slightly decreased enzyme activities. The addition to the medium of 8-bromoguanosine 3,'5'-monophosphate had no effect on any of the enzyme activities unless glucose was also added to the culture medium. Glucose addition alone to the culture medium was without any effect; however, glucose enhanced the stimulation of enzyme activity due to insulin. The addition of fructose or glycerol, even in the absence of insulin, increased the activities of each of the enzymes studied 2-3-fold. The increases in enzyme activity brought about by insulin or fructose were apparently the result of de novo enzyme synthesis, as indicated by the observation that the increases were not noted in the presence of cordycepin or cycloheximide. Immunoprecipitation of ATP-citrate lyase from hepatocytes pulse-labeled with [3H]leucine indicated that the induction of this enzyme in response to the addition of fructose or glycerol to the culture medium was the result of an increase in the rate of synthesis of the enzyme. These results indicate that the activity and synthesis of individual enzymes involved in lipogenesis are increased in response to the metabolism of carbohydrate independently in part from hormonal effects.

Lack of coordinated regulation of lipogenic enzymes in a human breast cell line, SKBr3

A human breast cell line has been identified which contains prodigious levels of fatty acid synthetase but has a very low capacity for lipogenesis from glucose, lactate or acetate. The fatty acid synthetase from this cell line appears to be structurally and functionally normal, and the low lipogenic capacity of the cells appears to be due to the low activities of other lipogenic enzymes, notably acetyl-CoA carboxylase. Thus, the SKBr3 cell line appears to lack the long-term coordinated control of acetyl-CoA carboxylase and fatty acid synthetase commonly observed in normal lipogenic tissues.

Changes in the proportion of acetyl-CoA carboxylase in the active form in rat liver. Effect of starvation, lactation and weaning

1. The activity of acetyl-CoA carboxylase (EC 6.4.1.2) in extracts of freeze-clamped liver samples from fed or 24 h-starved virgin, pregnant, lactating and weaned rats was measured (i) immediately after preparation of extracts (;I activity'), (ii) after incubation of extracts with partially purified preparations of either rabbit muscle protein phosphatase 1 [Antoniw, Nimmo, Yeaman & Cohen (1977) Biochem. J.162, 423-433] or rabbit liver phosphatase [Brandt, Capulong & Lee (1975) J. Biol. Chem.250, 8038-8044] (;A activity') and (iii) after incubation with 20mm-potassium citrate before or after incubation with phosphatases (;C activity'). 2. Incubation of liver extracts at 30 degrees C without any additions resulted in activation of acetyl-CoA carboxylase that was shown to be due to dephosphorylation of the enzyme by endogenous protein phosphatase activity. This latter activity was not stimulated by Ca(2+) and/or Mg(2+) but was stimulated by 1 mm-Mn(2+). Incubation of extracts with either of the partially purified phosphatases (0.2-0.5 unit) resulted in faster dephosphorylation and activation. The activity achieved after incubation with either of the exogenously added phosphatases was similar. 3. The A and C activities increased during late pregnancy, were lower than in the virgin rat liver during early lactation and increased by 2-fold in liver of mid-lactating rats. Weaning of mid-lactating rats for 24 h resulted in no change in A and C activities but after 48 h weaning they were significantly lower than those in livers from suckled mothers. 4. The I activity followed a similar pattern of changes as the A and C activities during pregnancy and lactation such that, although the I/A and I/C activity ratios tended to be lower during late pregnancy and early lactation, there were no significant changes in I/A and I/C ratios between lactating and virgin animals. However, these ratios were significantly higher in liver from fed 24 h-weaned animals. 5. Starvation (24 h) resulted in a marked decrease in I activity for all animals studied except early-lactating rats. This was due to a combination of a decrease in the concentration of acetyl-CoA carboxylase in liver of starved animals (A and C activities) and a decrease in the fraction of the enzyme in the active form (lower I/C and I/A ratios). The relative importance of the two forms of regulation in mediating the starvation-induced fall in I activity was about equal in livers of virgin, pregnant and lactating animals. However, the decrease in I/A and I/C ratios was of dominating importance in livers of weaned animals. The A/C activity ratios were the same for livers from all animals studied. 6. The maximal activity of fatty acid synthase was also measured in livers and was highly and positively correlated with the A and C activities of acetyl-CoA carboxylase, suggesting that the concentrations of the two enzymes in the liver were controlled coordinately. 7. It is suggested that the lack of correlation between plasma insulin levels and rates of lipogenesis in the transition from the virgin to the lactating state may be explained by different effects of insulin and prolactin on the concentration of acetyl-CoA carboxylase in the liver and on the fraction of the enzyme in the active form.

Regulation of lipid synthesis in hepatocytes from lean and obese Zucker rats

Fatty acid synthesis and CO2 production were evaluated in hepatocytes from lean and obese Zucker rats in the presence of 3H2O, and several carbon precursors. The incorporation of 3H2O into fatty acids was greater in obese compared to lean rats in both the isolated hepatocyte and in vivo. The rates of incorporation of 3H2O into fatty acids and cholesterol in hepatocytes of both lean and obese rats were linear for 2 hr, in the absence or presence of 16.7 mM glucose. Rates of fatty acid synthesis were higher in the presence of 16.7 mM glucose compared to the absence of glucose in both lean and obese while rates of cholesterol synthesis were similar. The incorporation of 3H2O into fatty acids, but not into cholesterol, was correlated with increasing glucose concentration and was 2 to three-fold higher in hepatocytes of obese compared to lean rats in the presence of several carbon precursors. Differences in CO2 production between lean and obese rats suggested increased pentose phosphate shunt activity, decreased pyruvate dehydrogenase activity, and lower tricarboxylic acid cycle activity in obese rats. Fatty acid synthesis and CO2 production from 3H2O and [U-14C]glucose in hepatocytes of lean and obese rats was similarly elevated by insulin and depressed by glucagon at several concentrations, suggesting that hepatocytes of obese animals respond to these hormones. These data indicate that rates of hepatic fatty acid synthesis although higher in obese rats respond to modulation in a fashion which is similar to the response in lean rats. The present studies suggest that the oxidation of several carbon precursors in the tricarboxylic acid cycle is diminished in obese compared to lean rats, but pentose phosphate shunt activity is greater in the obese Zucker rats.

Stimulation of hepatic lipogenesis and acetyl-coenzyme A carboxylase by vasopressin

The effect of vasopressin on the short-term regulation of fatty acid synthesis was studied in isolated hepatocytes from rats fed ad libitum. Vasopressin stimulates fatty acid synthesis by 30-110%. This increase is comparable with that obtained with insulin. Angiotensin also stimulates fatty acid synthesis, whereas phenylephrine does not. The dose-response curve for vasopressin-stimulated lipogenesis is similar to the dose-response curve for glycogenolysis and release of lactate plus pyruvate. Vasopression also stimulates acetyl-CoA carboxylase activity in a dose-dependent manner. Vasopressin does not relieve glucagon-inhibited lipogenesis, whereas insulin does. The action of vasopressin on hepatic lipogenesis is decreased, but not suppressed, in Ca2+-depleted hepatocytes. The results suggest that vasopressin acts on lipogenesis by increasing availability of lipogenic substrate (lactate + pyruvate) and by activating acetyl-CoA carboxylase.

NADP-dependent dehydrogenases in rat liver parenchyma. III. The description of a liponeogenic area on the basis of histochemically demonstrated enzyme activities and the neutral fat content during fasting and refeeding

The activities of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase(6PGDH), malic enzyme (ME) and isocitrate dehydrogenase (ICDh) were investigated with optimized histochemical methods (Rieder it al 1978), and the activity of 3-hydroxybutyrate dehydrogenase (3HBDH) and neutral fat content with conventional techniques in the liver of male rats under the following experimental dietary conditions: (A) Fasting for 0, 12 and 84h; (B) 84-h fasting followed by refeeding with a low-fat, high-carbohydrate diet for 6 h and for 2, 3, 5, 7, 11 and 14 nights; (C) refeeding with standard diet for 5 nights; (D) low-fat high-carbohydrate diet for 7 an 14 nights. The activities of G6PDH, 6PGDH and ME decreased slightly during fasting primarily in zone 1 and increased dramatically on refeeding with a low-fat, high-carbohydrate diet. This activity increase was confined mainly to zone 3 during the first 3 days and was accompanied by a deposition of neutral fats that began in zone 3 and progressed to zone 1. Neutral for accumulation was maximal after 3 nights, with a uniform accumulation of large droplets in all the hepatocytes; this was followed by a release that started in zone 3 and proceeded in a periportal direction. On the other hand, G6PDH, 6PGDH and ME attained their maximum activities after 5 amd 7 nights of low-fat diet, the activities being nearly homogeneously distributed over the liver acinus in a few cases. Subsequently the activities fill mainly in zone 1, causing the activity patterns and levels to approach those of the animals in group (D). In contrast to this, the activity of ICDH increased during fasting principally in zone 1, so that the otherwise steep activity gradient in favor of zone 3 lessened. Refeeding led at first to a fall of activity below the initial value, but later the normal distribution pattern was restored. The activity of 3HBDH showed a behavior similar to that of ICDH. The findings are discussed with reference to the functional heterogeneity of the liver parenchyma, and the existence of a liponeogenic area in zone 3 is proposed.