Regulation of hepatic acetyl coenzyme A carboxylase by insulin and glucagon

Approaches to Measuring the Activity of Major Lipolytic and Lipogenic Enzymes In Vitro and Ex Vivo

Since the 1950s, one of the goals of adipose tissue research has been to determine lipolytic and lipogenic activity as the primary metabolic pathways affecting adipocyte health and size and thus representing potential therapeutic targets for the treatment of obesity and associated diseases. Nowadays, there is a relatively large number of methods to measure the activity of these pathways and involved enzymes, but their applicability to different biological samples is variable. Here, we review the characteristics of mean lipogenic and lipolytic enzymes, their inhibitors, and available methodologies for assessing their activity, and comment on the advantages and disadvantages of these methodologies and their applicability in vivo, ex vivo, and in vitro, i.e., in cells, organs and their respective extracts, with the emphasis on adipocytes and adipose tissue.

Low-Protein Diet during Lactation and Maternal Metabolism in Rats

Some metabolic alterations were evaluated in Wistar rats which received control or low-protein (17%; 6%) diets, from the pregnancy until the end of lactation: control non-lactating (CNL), lactating (CL), low-protein non-lactating (LPNL) and lactating (LPL) groups. Despite the increased food intake by LPL dams, both LP groups reduced protein intake and final body mass was lower in LPL. Higher serum glucose occurred in both LP groups. Lactation induced lower insulin and glucagon levels, but these were reduced by LP diet. Prolactin levels rose in lactating, but were impaired in LPL, followed by losses of mammary gland (MAG) mass and, a fall in serum leptin in lactating dams. Lipid content also reduced in MAG and gonadal white adipose tissue of lactating and, in LPL, contributed to a decreased daily milk production, and consequent impairment of body mass gain by LPL pups. Liver mass, lipid content and ATP-citrate enzyme activity were increased by lactation, but malic enzyme and lipid: glycogen ratio elevated only in LPL. Conclusion. LP diet reduced the development of MAG and prolactin secretion which compromised milk production and pups growth. Moreover, this diet enhanced the store of lipid to glycogen ratio and suggests a higher risk of fatty liver development.

Role of the electrophilic lipid peroxidation product 4-hydroxynonenal in the development and maintenance of obesity in mice

The lipid peroxidation product 4-hydroxynonenal (4-HNE) is a signaling mediator with wide-ranging biological effects. In this paper, we report that disruption of mGsta4, a gene encoding the 4-HNE-conjugating enzyme mGSTA4-4, causes increased 4-HNE tissue levels and is accompanied by age-dependent development of obesity which precedes the onset of insulin resistance in 129/sv mice. In contrast, mGsta4 null animals in the C57BL/6 genetic background have normal 4-HNE levels and remain lean, indicating a role of 4-HNE in triggering or maintaining obesity. In mGsta4 null 129/sv mice, the expression of the acetyl-CoA carboxylase (ACC) transcript is enhanced several-fold with a concomitant increase in the tissue level of malonyl-CoA. Also, mitochondrial aconitase is partially inhibited, and tissue citrate levels are increased. Accumulation of citrate could lead to allosteric activation of ACC, further augmenting malonyl-CoA levels. Aconitase may be inhibited by 4-HNE or by peroxynitrite generated by macrophages which are enriched in white adipose tissue of middle-aged mGsta4 null 129/sv mice and, upon lipopolysaccharide stimulation, produce more reactive oxygen species and nitric oxide than macrophages from wild-type mice. Excessive malonyl-CoA synthesized by the more abundant and/or allosterically activated ACC in mGsta4 null mice leads to fat accumulation by the well-known mechanisms of promoting fatty acid synthesis and inhibiting fatty acid beta-oxidation. Our findings complement the recent report that obesity causes both a loss of mGSTA4-4 and an increase in the level of 4-HNE [Grimsrud, P. A., et al. (2007) Mol. Cell. Proteomics 6, 624-637]. The two reciprocal processes are likely to establish a positive feedback loop that would promote and perpetuate the obese state.

Biotin biochemistry and human requirements

Human biotin turnover and requirements can be estimated on the basis of (1) concentrations of biotin and metabolites in body fluids, (2) activities of biotin-dependent carboxylases, and (3) the urinary excretion of organic acids that are formed at increased rates if carboxylase activities are reduced. Recent studies suggest that the urinary excretions of biotin and its metabolite bisnorbiotin, activities of propionyl-CoA carboxylase and beta-methylcrotonyl-CoA carboxylase in lymphocytes, and urinary excretion of 3-hydroxyisovaleric acid are good indicators of marginal biotin deficiency. On the basis of studies using these indicators of biotin deficiency, an adequate intake of 30 microg (123 nmoles) of biotin per day is currently recommended for adults. The dietary biotin intake in Western populations has been estimated to be 35 to 70 microg/d (143-287 nmol/d). Recent studies suggest that humans absorb biotin nearly completely. Conditions that may increase biotin requirements in humans include pregnancy, lactation, and therapy with anticonvulsants or lipoic acid.

Regulation by glucose and calcium of the carboxylmethylation of the catalytic subunit of protein phosphatase 2A in insulin-secreting INS-1 cells

Previously, we reported that the catalytic subunit of protein phosphatase 2A (PP2Ac) undergoes carboxylmethylation (CML) at its COOH-terminal leucine, and that inhibitors of such a posttranslational modification markedly attenuate nutrient-induced insulin secretion from isolated beta-cells. More recent studies have suggested direct inhibitory effects of glucose metabolites on PP2A activity in isolated beta-cells, implying that inhibition of PP2A leads to stimulation of insulin secretion. Because the CML of PP2Ac has been shown to facilitate the holoenzyme assembly and subsequent functional activation of PP2A, we investigated putative regulation by glucose of the CML of PP2Ac in insulin-secreting (INS)-1 cells. Our data indicated a marked inhibition by specific intermediates of glucose metabolism (e.g., citrate and phosphoenolpyruvate) of the CML of PP2Ac in INS-1 cell lysates. Such inhibitory effects were also demonstrable in intact cells by glucose. Mannoheptulose, an inhibitor of glucose metabolism, completely prevented inhibitory effects of glucose on the CML of PP2Ac. Moreover, glucose-mediated inhibition of the CML of PP2Ac was resistant to diazoxide, suggesting that glucose metabolism and the generation of glucose metabolites might control inhibition of the CML of PP2Ac. A membrane-depolarizing concentration of KCl also induced inhibition of the CML of PP2Ac in intact INS cells. On the basis of these data, we propose that glucose metabolism and increase in intracellular calcium facilitate inhibition of the CML of PP2Ac, resulting in functional inactivation of PP2A. This, in turn, might retain the key signaling proteins of the insulin exocytotic cascade in their phosphorylated state, leading to stimulated insulin secretion.

Lipoic acid reduces the activities of biotin-dependent carboxylases in rat liver

In the past, lipoic acid has been administered to patients and test animals as therapy for diabetic neuropathy and various intoxications. Lipoic acid and the vitamin biotin have structural similarities. We sought to determine whether the chronic administration of lipoic acid affects the activities of biotin-dependent carboxylases. For 28 d, rats received daily intraperitoneal injections of one of the following: 1) a small dose of lipoic acid [4.3 micromol/( kg.d)]; 2) a large dose of lipoic acid [15.6 micromol/(kg.d)]; or 3) a large dose of lipoic acid plus biotin [15.6 and 2.0 micromol/(kg.d), respectively]. Another group received n-hexanoic acid [14.5 micromol/(kg.d)], which has structural similarities to lipoic acid and biotin and thus served as a control for the specificity of lipoic acid. A fifth group received phosphatidylcholine in saline injections and served as the vehicle control. The rat livers were assayed for the activities of acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and beta-methylcrotonyl-CoA carboxylase. Urine was analyzed for lipoic acid; serum was analyzed for indicators of liver damage and metabolic aberrations. The mean activities of pyruvate carboxylase and beta-methylcrotonyl-CoA carboxylase were 28-36% lower in the lipoic acid-treated rats compared with vehicle controls (P < 0.05). Rats treated with lipoic acid plus biotin had normal carboxylase activities. Carboxylase activities in livers of n-hexanoic acid-treated rats were normal despite some evidence of liver injury. Propionyl-CoA carboxylase and acetyl-CoA carboxylase were not significantly affected by administration of lipoic acid. This study provides evidence consistent with the hypothesis that chronic administration of lipoic acid lowers the activities of pyruvate carboxylase and beta-methylcrotonyl-CoA carboxylase in vivo by competing with biotin.

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.

Regulation of mammalian acetyl-coenzyme A carboxylase

Long-chain fatty acids are involved in all aspects of cellular structure and function. For controlling amounts of fatty acids, cells are endowed with two acetyl-coenzyme A carboxylase (ACC) systems. ACC-alpha is the rate-limiting enzyme in the biogenesis of long-chain fatty acids, and ACC-beta is believed to control mitochondrial fatty acid oxidation. These two isoforms of ACC control the amount of fatty acids in the cells. Phosphorylation and dephosphorylation of ACC-alpha cause enzyme inactivation and activation, respectively, and serve as the enzyme's short-term regulatory mechanism. Covalently modified enzymes become more sensitive toward cellular metabolites. In addition, many hormones and nutrients affect gene expression. The gene products formed are heterogeneous and tissue specific. The ACC-beta gene is located on human chromosome 12; the cDNA for this gene has just been cloned. The gene for the alpha-isoform is located on human chromosome 17. The catalytic core of the beta-isoform is homologous to that of the alpha-isoform, except for an additional peptide of about 150 amino acids at the N terminus. This extra peptide sequence makes the beta-form about 10,000 daltons larger, and it is thought to be involved in the unique role that has been assigned to this enzyme. The detailed control mechanisms for the beta-isoform are not known.

Acetyl-CoA carboxylase in reuber hepatoma cells: Variation in enzyme activity, insulin regulation, and cellular lipid content

Reuber hepatoma cells are useful cultured lines for the study of insulin action, lipid and lipoprotein metabolism, and the regulation of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme of fatty acid biosynthesis. During investigations in different clonal lines of these cells, we have uncovered marked intercellular variability in the activity, enzyme content, and insulin regulation of ACC paralleled by differences in cellular neutral lipid (triglyceride) content. Two contrasting clonal lines, Fao and H356A-1, have been studied in detail. Several features distinguish these two lines, including differences in ACC activity and enzyme kinetics, the content of the two major hepatic ACC isozymes (Mr 280,000 and 265,000 Da) and their heteroisozymic complex, the extent of ACC phosphorylation, and the ability of ACC to be activated on stimulation by insulin and insulinomimetic agonists. As studied by Nile Red staining and fluorescence-activated cell sorting, these two lines also display marked differences in neutral lipid content, which correlates with both basal levels of ACC activity and inhibition of ACC by the fatty acid analog, 5-(tetradecyloxy)-2-furoic acid (TOFA). These results emphasize the importance of characterization of any particular clonal line of Reuber cells for studies of enzyme regulation, substrate metabolism, and hormone action. With respect to ACC, studies in contrasting clonal lines of Reuber cells could provide valuable clues to understanding both the complex mechanisms of intracellular ACC regulation in the absence and presence of hormones and its regulatory role(s) in overall hepatic lipid metabolism.

Long-term fat-feeding-induced insulin resistance in normal NMRI mice: postreceptor changes of liver, muscle and adipose tissue metabolism resembling those of type 2 diabetes

Postreceptor insulin resistance was studied in liver, muscle and adipose tissue from NMRI mice of both sexes made diabetic by long-term fat-feeding. Intravenous glucose tolerance tests showed a combination of impaired glucose tolerance and increased plasma insulin concentrations consistent with insulin resistance and reduced peripheral and hepatic uptake of glucose. In the morning, the fat-fed mice were normoinsulinaemic and hyperglycaemic. Liver glucokinase activity and glycogen content were reduced whereas lactate dehydrogenase activity was enhanced. Fatty acid synthase activity was decreased but glucose 6-phosphate dehydrogenase and the rate limiting enzyme in fatty acid synthesis, acetyl CoA carboxylase, were both unaffected. In muscle, the proportion of glycogen synthase in the active I-form was decreased. Total glycogen synthase activity was not affected. In isolated adipocytes, basal and insulin-stimulated glucose oxidation, as well as basal and insulin-stimulated lipogenesis from glucose were all severely inhibited, oxidation more so than lipogenesis. It is concluded that insulin resistance and postreceptor metabolic disorders in liver, muscle and adipose tissue from mice made diabetic by long-term fat-feeding are very similar to those demonstrated in human type 2 diabetics and may be studied in more detail and with more ease in this particular animal model.

Yeast acetyl-CoA carboxylase: in vitro phosphorylation by mammalian and yeast protein kinases

Acetyl-CoA carboxylase (ACC) is regulated in mammalian tissues, in part, by multisite enzyme phosphorylation. Yeast ACC (Y-ACC) has been highly purified from S. cerevisiae by monomeric avidin-Sepharose chromatography, revealing an enzyme subunit species of molecular mass 265,000 Da. Unlike mammalian enzyme, Y-ACC is citrate-independent, and reacts weakly or not at all with a panel of anti-rat liver ACC antibodies. Like rat ACC, Y-ACC is rapidly phosphorylated and inactivated by two mammalian carboxylase kinases, the cAMP-dependent protein kinase and 5'-AMP-stimulated kinase. It is also phosphorylated by rat liver casein kinase II, but without any change in catalytic activity. Three major yeast protein kinases active on ACC have been fractionated; all co-elute with kinases active on casein, but each appears to be a distinct catalytic species. Like the mammalian casein kinases, however, phosphorylation of ACC by these yeast kinases does not alter yeast ACC activity. Taken together, these data indicate that Y-ACC possesses at least two classes of phosphorylation sites, one or more of which acutely regulates enzyme activity. Alterations in Y-ACC phosphorylation in yeast, as in mammalian tissues, could be an important modulator of the rates of fatty acid synthesis.

Synergistic inhibition of hepatic ketogenesis in the presence of insulin and a cAMP antagonist

The separate or combined effects of insulin and the cAMP antagonist, the Rp-diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), were examined on fatty acid-stimulated ketogenesis in hepatocytes from normal fasted rats. Addition of 0.4 mM oleic acid or 0.4 mM octanoic acid resulted in a linear increase in ketone production measured over 60 min. When oleic acid was the substrate, incubation with 1 to 30 microns Rp-cAMPS alone or 0.1 to 10 nM insulin alone caused a variable decrease in the production of ketones which did not exceed an average value of 30% in any one experiment. The simultaneous addition of Rp-cAMPS and insulin resulted in a greater than additive inhibition which reached average values between 47-60% when the theoretical combined inhibitory effect of the insulin alone plus the Rp-cAMPS alone was less than 18%. No significant effects of either insulin or Rp-cAMPS, alone or in combination, were seen when octanoic acid was the substrate. These data imply that Rp-cAMPS can potentiate insulin inhibition of hepatic ketogenesis through inhibition of a cAMP-mediated process.

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.

Fatty acid-independent inhibition of hepatic ketone body production by insulin in humans

To investigate whether elevated plasma insulin or glucagon concentrations are capable of modifying hepatic ketogenesis independently of plasma free fatty acid (FFA) concentrations, ketone body production was determined by [3-14C]acetoacetate infusions in overnight-fasted normal subjects during exogenous supply of FFA (Intralipid and heparin infusion). When plasma FFA concentrations were elevated from 0.73 +/- 0.07 to 1.53 +/- 0.16 mmol/l during low insulin concentrations (approximately equal to 13 microU/ml) in group A (n = 7), total ketone body production increased from 3.6 +/- 0.6 to 8.2 +/- 1.0 mumol.kg-1.min-1 (P less than 0.001). When plasma FFA were similarly elevated during raised plasma insulin concentrations (approximately equal to 110 microU/ml) in group B (n = 5), total ketone body production was only 3.8 +/- 0.8 mumol.kg-1.min-1 (P less than 0.01 vs. group A). Low plasma FFA and low insulin concentrations resulted in total ketone body production of 0.70 +/- 0.18 mumol.kg-1.min-1 in group C (n = 7; P less than 0.01 vs. groups A and B). Elevation of plasma glucagon during Intralipid infusion in group D (n = 7) failed to affect ketogenesis, but the beta-hydroxybutyrate-to-acetoacetate concentration ratio decreased significantly (P less than 0.01). The data indicate that elevation of plasma insulin to high physiological concentrations restrains FFA-induced ketogenesis.

Alpha 1-adrenergic stimulation of ketogenesis and fatty acid oxidation is associated with inhibition of lipogenesis in rat hepatocytes

The effect of norepinephrine on fatty acid synthesis (3H2O incorporation into fatty acids), on fatty acid oxidation to CO2 and on ketogenesis was studied in isolated hepatocytes of fed rats. After incubation with norepinephrine (50 microM), lipogenesis was lower (5.7 +/- 1.1 nmoles 3H2O incorporated into fatty acids/mg dry weight/30 min) than in controls (7.5 +/- 1.7; n = 6, p less than 0.02). In contrast, (1-14C) palmitate conversion into total ketone bodies was increased to 10.9 +/- 1.8 nmoles/mg/30 min with norepinephrine, vs 8.5 +/- 1.6 in controls (p less than 0.05), and more (1-14C) palmitate was converted to 14CO2 with norepinephrine than in controls (1.48 +/- 0.10 nmoles/mg/30 min vs 1.06 +/- 0.11, p less than 0.05). The inhibitory effect of norepinephrine on lipogenesis was abolished by addition of the alpha 1-receptor blocker prazosin, but not by alpha 2 or beta-blockers. The results demonstrate that the ketogenic effect of norepinephrine is coupled with an inhibitory effect on lipogenesis which may be explained by diminished activity of acetyl-CoA carboxylase, diminished formation of malonyl-CoA and decreased activity of carnitine palmitoyl transferase I.

Preparation of functional acetyl-CoA carboxylase mRNA from rat mammary gland

Poly(A)+ RNA from lactating rat mammary glands was fractionated according to size by isokinetic sucrose gradient centrifugation to obtain a fraction enriched for acetyl-CoA carboxylase. In vitro translation of this RNA preparation yielded apparent full-length acetyl-CoA carboxylase with a molecular weight of 260,000. The synthesized protein was identified as acetyl-CoA carboxylase by specific immunoprecipitation. Tests with antiserum to fatty acid synthetase, revealed that the fractions containing acetyl-CoA carboxylase mRNA also contained mRNA for fatty acid synthetase; both of these mRNAs were approximately 10 kb. Fatty acid synthetase with a molecular weight of 250,000 was synthesized. Using an in vitro rabbit reticulocyte lysate translation system, we have shown that the amount of translatable acetyl-CoA carboxylase mRNA increases during lactation. On the fifth day postpartum the level of translatable acetyl-CoA carboxylase mRNA increased to a peak level seven times that on the day of parturition.

Stimulation by a tumor-promoting phorbol ester of acetyl-CoA carboxylase activity in isolated rat hepatocytes

Acetyl-CoA carboxylase (EC 6.4.1.2) in hepatocytes from meal-fed rats was activated by phorbol myristate acetate (PMA) in a time- and concentration-dependent fashion. This activation can account for the PMA-induced stimulation of de novo fatty acid synthesis. Purified rat-liver acetyl-CoA carboxylase was found to be phosphorylated and activated by protein kinase C, thus providing a possible mechanism for the metabolic action of PMA in intact hepatocytes.

Modulation of acetyl-CoA carboxylase by inhibitors of IMP dehydrogenase: implications for insulin regulation

The activity of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme of fatty acid biosynthesis, can be regulated by both adenine and guanine nucleotides in vitro. We have employed two inhibitors of IMP dehydrogenase, ribavarin and tiazofurin, to investigate a possible role for intracellular nucleotides in ACC regulation in rat adipocytes. Ribavarin, but not tiazofurin, leads to a profound time-dependent inhibition of ACC activity that is associated with a decrease in both intracellular ATP and GTP. This inactivating effect is largely reversed with guanosine, accompanied by increases in both ATP and GTP levels. Epinephrine-mediated inactivation of ACC in intact cells is not altered by ribavarin incubation. However, in these experiments, insulin-mediated activation is observed only after ribavarin-induced inhibition of the enzyme. These data suggest that nucleotides may modulate ACC activity and influence is regulation by insulin in intact cells. The possible mechanisms underlying the insulin activation of ACC and the role of intracellular nucleotides in insulin action are discussed.

Insulin effects on apolipoprotein B lipoprotein synthesis and secretion by primary cultures of rat hepatocytes

Lipoprotein synthesis and secretion were examined in primary cultures of rat hepatocytes cultured on collagen-coated plates and incubated with pharmacologic and physiologic concentrations of insulin. Media insulin concentration declined rapidly over the course of incubation indicating that hepatocytes rapidly degrade insulin. When insulin was present in the media, cellular triglyceride accumulated while lipid secretion declined. Insulin inhibited the incorporation of labeled amino acids into total secretory lipoprotein apoproteins and apolipoprotein B (apo B) as well as apo B mass as measured by monoclonal radioimmunoassay. The effect of insulin on apo B secretion occurred as early as three hours after the addition of insulin to the culture media and both apo B of higher molecular weight (apo BH) and apo B of lower molecular weight (apo BL) were affected. Cellular apo B did not accumulate within cells. The majority of secretory lipid radioactivity synthesized from acetate was in VLDL density lipoproteins. The composition of newly synthesized lipids as assessed by thin layer chromatography was not significantly altered with insulin. These studies support the finding that insulin inhibits VLDL secretion by hepatocytes while at the same time stimulating overall triglyceride synthesis. A suggested mechanism is that insulin uncouples triglyceride and apo B synthesis, which influences subsequent lipoprotein assembly and secretory pathways. These results are consistent with the concept that postprandial insulin release inhibits hepatic lipoprotein secretion while intestinal lipoprotein metabolic pathways are most active.

Regulation of acetyl-CoA carboxylase by ADP-ribosylation

Because of certain similarities between acetyl-CoA carboxylase (ACC) and tubulin, and the recent demonstration of the ADP-ribosylation of tubulin by cholera toxin, we have investigated a potential role for ADP-ribosylation in the regulation of ACC activity. Incubation of purified rat liver ACC with cholera toxin in the presence of millimolar concentrations of [adenylate-32P]NAD results in a time-dependent incorporation of ADP-ribose into ACC of greater than 2 mol/mol of enzyme subunit, accompanied by a marked inactivation of enzyme activity. This effect is not mimicked by pertussis toxin, ADP-ribose, or ribose 5-phosphate. Incubation of labeled ACC with snake venom phosphodiesterase and alkaline hydrolysis release 32P-products tentatively identified by high-performance liquid chromatography as 5'-[32P]AMP and [32P]ADP-ribose, respectively. These data are consistent with a mono-ADP-ribosylation of ACC catalyzed by cholera toxin. Phosphodiesterase treatment of inactivated ACC partially restores enzyme activity. The effects of ADP-ribosylation of ACC are expressed both as a decrease in the enzyme Vmax and as an increase in the apparent Ka for citrate. These results suggest that ACC might be a substrate for endogenous ADP-ribosyltransferases and that this covalent modification could be an important regulatory mechanism for the modulation of fatty acid synthesis in vivo.

Differential short-term effects of growth factors on fatty acid synthesis in isolated rat-liver cells

Hepatocytes in suspension, freshly isolated from meal-fed rats, were used to study the acute influence of growth factors on the rate of de novo fatty acid synthesis. Nerve growth factor (2.5 S) and epidermal growth factor caused a substantial increase in the rate of fatty acid synthesis, whereas fibroblast growth factor was inhibitory. Little effect was observed with nerve growth factor (7 S), bombesin or substance P. Transferrin did not affect hepatic fatty acid synthesis. The results are discussed in relation to the effects of insulin and tumor-promoting phorbol esters.

Effect of insulin on association of acetyl CoA carboxylase phosphatase and acetyl CoA carboxylase

Insulin promotes an association between acetyl CoA carboxylase and acetyl CoA carboxylase phosphatase. The association between rat epididymal fat tissue carboxylase and the phosphatase occurs in both a tissue culture system and in vivo and is accompanied by an increase in acetyl CoA carboxylase activity.

Relationship between lipogenesis, ketogenesis, and malonyl-CoA content in isolated hepatocytes from the obese Zucker rat adapted to a high-fat diet

The relationship between lipogenesis and ketogenesis and the concentration of malonyl coenzyme A (CoA) was investigated in hepatocytes from adult obese Zucker rats and their lean littermates fed either a control low-fat diet or a high-fat diet (30% lard in weight). With the control diet, lipogenesis--although strongly inhibited in the presence of either 1 mmol/L oleate, 10(-6) mol/L glucagon or 0.1 mmol/L TOFA (a hypolipidemic drug)--remained about fifteen-fold higher in the obese rats than in the lean rats. In contrast, ketogenesis under some conditions (oleate + TOFA) was not significantly lower (30%) as compared with the lean rats. After adaptation to the high-fat diet, lipogenesis was depressed fourfold in the lean rats and ninefold in the obese ones; however its magnitude remained significantly higher in the latter, namely at a value close to that measured in control-fed lean rats. Ketogenesis was comparable in lean and obese rats and much higher in the presence of 1 mmol/L oleate than of 0.3 mmol/L oleate, whereas lipogenesis did not vary with increasing oleate concentration in the medium. Acetyl-CoA carboxylase activity measured in liver homogenates was higher in the obese group, but was stepwise inhibited by increasing concentrations of oleyl-CoA regardless of the diet for both lean and obese rats, thus showing no abnormality of in vitro responsiveness to this inhibitor. With the control diet, hepatocyte malonyl-CoA levels were significantly higher in the obese rats, both in the basal state and after inhibition of lipogenesis by oleate and TOFA.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of acetyl-CoA carboxylase by glucagon in HeLa cells

HeLa cells cultured in medium containing lipid-free fetal bovine serum or in Waymouth's serum-free medium showed an increase in acetyl-CoA carboxylase (ACC) activity of 8- and 4-fold respectively as compared to cells cultured in medium containing fetal bovine serum. This increase in ACC activity was associated with a corresponding increase in the relative synthesis of ACC. Addition of glucagon (1.5 microgram/ml) to the HeLa cell culture medium caused a 50% decrease in ACC activity. This was accompanied by a concordant decrease in the relative synthesis of ACC as measured by immunochemical techniques.

Diurnal changes in the rate of cholesterogenesis in hepatocytes from fed and starved rats: effects of precursors and pancreatic hormones in vitro

The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) varied with a diurnal periodicity in hepatocytes prepared at different times from rats accustomed to a controlled feeding and lighting schedule. The rates of sterol synthesis varied in a similar manner but the maximum rate was not synchronous with maximum HMG-CoA reductase activity. The diurnal increase in HMG-CoA reductase activity and sterol synthesis rate started before food was offered to donor animals. Neither insulin nor glucagon had any effect on the diurnal pattern of hepatic sterol synthesis in vitro. Pyruvate inhibited sterol synthesis in hepatocytes prepared during the feeding period but had no effect at other times of day. When food was withheld from donor animals at the beginning of the normal feeding period both HMG-CoA reductase activity and the rate of sterol synthesis rapidly decreased. During this period neither insulin nor lipogenic substrates, alone or in combination, were able to restore the rates of sterol synthesis to normal values. In hepatocytes prepared from animals starved for a longer period (43 h) the decrease in the activity of HMG-CoA reductase was much less than that in the rate of sterol synthesis. In contrast to hepatocytes from fed or short-term-starved animals, the rate of sterol synthesis in these hepatocytes could be increased by glucose or pyruvate.

Effects of hormones and pyruvate on the rates of secretion of very-low-density lipoprotein triacylglycerol and cholesterol by rat hepatocytes

The secretion of very-low-density lipoprotein (VLDL) triacylglycerol and cholesterol was determined under various conditions in hepatocytes prepared from rats maintained on a controlled lighting and feeding schedule. The rate of lipogenesis in hepatocytes prepared from rats during the feeding period was 2-3-fold higher than that in cells prepared immediately before the animals had access to food. However, there were no corresponding changes in the rates of secretion of triacylglycerol and cholesterol. Pyruvate alone stimulated triacylglycerol secretion but had no effect on the secretion of cholesterol. Despite its stimulation of lipogenesis, insulin suppressed the secretion of both triacylglycerol and cholesterol. This effect on triacylglycerol secretion was more pronounced when lipogenesis was enhanced in the presence of pyruvate. Thus, insulin may act to alleviate hypertriglyceridaemia, which may arise during periods of increased hepatic lipogenesis. The inhibitory effect of glucagon on cholesterol secretion was much less pronounced than that on the secretion of triacylglycerol. The inhibitory effects of glucagon were reversed by pyruvate on cholesterol secretion differed according to whether glucagon was present or absent. These results suggest that the rate of hepatic VLDL triacylglycerol secretion is not necessarily coupled to the rate of lipogenesis in the liver; nor is there any obligatory coupling between the output of triacylglycerol and cholesterol associated with VLDL.

Short-term regulation of acetyl CoA carboxylase: is the key enzyme in long-chain fatty acid synthesis regulated by an existing physiological mechanism?

Acetyl CoA carboxylase, the rate-limiting enzyme in regulating fatty acid synthesis, is thought to be controlled by allosteric effectors, its state of aggregation, covalent modulation and protein inhibitors. It is still obscure whether citrate, a positive allosteric effector, and long-chain fatty acyl CoA esters, negative allosteric effectors, function physiologically to regulate acetyl CoA carboxylase activity. New evidence from several laboratories reveals that the covalent phosphorylation may not involve regulation of acetyl CoA carboxylase activity. Protein inhibitors from liver cytosol and a peptide from fat cells were found to regulate acetyl CoA carboxylase both in vivo and in vitro. Coenzyme A, guanosine 5-monophosphate and phosphatidylinositol 4,5-bisphosphate may have an indirect effect, but certainly no direct involvement, on carboxylase activity.

Investigations into the direct effects of insulin on hepatic ketogenesis, lipoprotein secretion and pyruvate dehydrogenase activity

In the rat liver perfused with whole rat blood containing either decreased or increased concentrations of non-esterified fatty acids, insulin decreased production of acetoacetate and 3-hydroxybutyrate and stimulated secretion of very-low-density-lipoprotein triacylglycerol. In these same livers, pyruvate dehydrogenase activity was not altered by insulin addition, although it was diminished by non-esterified fatty acids.

Isolation of three cyclic-AMP-independent acetyl-CoA carboxylase kinases from lactating rat mammary gland and characterization of their effects on enzyme activity

Three cyclic AMP-independent acetyl-CoA carboxylase kinases (A, B1 and B2) have been isolated from lactating rat mammary gland, using phosphocellulose chromatography, high performance gel filtration, and affinity chromatography on casein-Sepharose and phosvitin-Sepharose. These protein kinases have been identified with previously described kinases by the following criteria. Kinase A phosphorylates the same sites on rabbit mammary acetyl-CoA carboxylase as acetyl-CoA carboxylase kinase 2, which was originally described as a contaminant of rabbit mammary acetyl-CoA carboxylase purified by the poly(ethylene glycol)procedure. Kinase A will henceforth be referred to as acetyl-CoA carboxylase kinase-2. Kinase B1 has been identified with casein kinase II by its heparin sensitivity, elution behaviour on phosphocellulose, molecular mass, substrate specificity and subunit composition. Kinase B2 has been identified with casein kinase I by its elution behaviour on phosphocellulose, molecular mass, substrate specificity and subunit composition. The three kinases phosphorylate distinct sites on acetyl-CoA carboxylase. Phosphorylation by either casein kinase I or II does not affect enzyme activity. However, acetyl-CoA carboxylase kinase 2 inactivates acetyl-CoA carboxylase reversibly, in an identical manner to cyclic-AMP-dependent protein kinase, and phosphorylates sites located on identical peptides. Acetyl-CoA carboxylase kinase-2 can, however, be distinguished from the free catalytic subunit of cyclic-AMP-dependent protein kinase by its molecular mass, its substrate specificity, its elution behaviour on phosphocellulose, and its complete lack of sensitivity to the protein inhibitor of cyclic-AMP-dependent protein kinase. We also present evidence that phosphorylation of acetyl-CoA carboxylase by cyclic-AMP-dependent protein kinase occurs directly and not via a bicyclic cascade system as proposed by other laboratories.

Glucagon inhibits fatty acid synthesis in isolated hepatocytes via phosphorylation of acetyl-CoA carboxylase by cyclic-AMP-dependent protein kinase

If isolated rat hepatocytes are preincubated for 90 min before addition of hormone, glucagon causes a significant (50%) decrease in fatty acid synthesis without concomitant large decreases in the cellular content of the allosteric activator, citrate. We present evidence that this inhibition can be entirely accounted for by the phosphorylation of the rate-limiting enzyme, acetyl-CoA carboxylase, by cyclic-AMP-dependent protein kinase. In particular: (1) the effect is associated with a 50% decrease in acetyl-CoA carboxylase activity (measured at physiological citrate concentration) which survives purification of the enzyme; (2) the effect is associated with a selective increase in the phosphorylation of a chymotryptic peptide (peptide 1) which is identical to the peptide containing the major site phosphorylated on purified acetyl-CoA carboxylase by cyclic-AMP-dependent protein kinase; (3) the effects of glucagon on the kinetic parameters of the enzyme are very similar to the effect of phosphorylation of the purified enzyme, i.e. a decrease in V and an increase in Ka for citrate; and (4) all of these effects occur at physiological concentrations of glucagon identical to those producing inhibition of fatty acid synthesis.

Insulin-like effects of fatty acid synthesis in liver of hamsters infected with plerocercoids of the tapeworm, Spirometra mansonoides

Elevated serum lipids are associated with infections of laboratory rodents with plerocercoids of Spirometra mansonoides. The effect of infection with these larval tapeworms on triglyceride degradation and hepatic de novo fatty acid synthesis was investigated in Syrian hamsters. Serum lipoprotein electrophoresis revealed a consistent elevation in very low density lipoproteins in the infected animals. Lipoprotein lipase activity was enhanced in the infected animals. After seven days of plerocercoid infection the activity of acetyl-CoA carboxylase (E.C. 6.4.1.2) was significantly elevated after 6, 12 and 18 hours of fasting. Fatty acid synthetase was significantly increased after 0, 6, 12, 18 and 24 hours of fasting. Therefore, a chronic insulin-like activity on lipid metabolism of hamsters is associated with plerocercoid infection.

Inhibition of the hormone-sensitive lipase in adipose tissue by long-chain fatty acyl coenzyme A

The effects of free fatty acids and fatty acyl esters of coenzyme A and carnitine on the activity of a hormone-sensitive lipase preparation made from pigeon adipose tissue were determined. Oleic acid (100 microM) resulted in a 40% inhibition of lipase activity. A more potent inhibition of lipase activity was seen with long-chain fatty acyl CoA compounds. The concentration required for half-maximal inhibition with oleoyl CoA and palmitoyl CoA was 25-40 microM, whereas palmitoyl carnitine stimulated lipase activity. Activated lipase preparations (preincubated with Mg2+, ATP, cyclic AMP and protein kinase) were 4-6 times more sensitive to inhibition by oleoyl CoA than were nonactivated preparations. An increase in cellular levels of fatty acyl coenzyme A could, therefore, contribute to the feedback inhibition of lipolysis in adipose tissue.

Phosphorylation of acetyl-coenzyme A carboxylase by casein kinase I and casein kinase II

Two cAMP-independent acetyl-CoA carboxylase (ACC) protein kinases have been partially purified from rat liver cytosol and microsomal extracts. The first kinase, present in greatest activity in microsomal extracts, appears to be identical to casein kinase I by characteristic molecular size on gel filtration (Mr 40,000) and sodium dodecyl sulfate-gel electrophoresis (Mr 34,000), autophosphorylation of this single subunit, inability to efficiently utilize GTP, and resistance to inhibition by heparin and 2,3-diphosphoglycerate. The second kinase, predominant in cytosol, appears to be identical to casein kinase II by characteristic molecular size on gel filtration (Mr 150,000), an autophosphorylated subunit of Mr 25,000, a Km for GTP nearly equal to that of ATP, inhibition by heparin and 2,3 DPG, and relative substrate specificity. Despite the incorporation of up to 2 mol 32P/mol carboxylase subunit (kinase I) and 0.6 mol/subunit (kinase II), phosphorylation by either kinase causes no change in carboxylase activity. The site(s) phosphorylated by each kinase and by the cAMP-dependent protein kinase on carboxylase appear to be clustered on a Mr 16,000 cyanogen bromide peptide that is readily released on incubation with trypsin. The potential roles of these kinases in the regulation of ACC remain to be clarified.