Inhibition of Rat Liver Acetyl Coenzyme A Carboxylase by N6,O2′-Dibutyryl Cyclic Adenosine 3′:5′-Monophosphate in Vitro

Hypothalamic malonyl-CoA triggers mitochondrial biogenesis and oxidative gene expression in skeletal muscle: Role of PGC-1alpha

Previous investigations show that intracerebroventricular administration of a potent inhibitor of fatty acid synthase, C75, increases the level of its substrate, malonyl-CoA, in the hypothalamus. The "malonyl-CoA signal" is rapidly transmitted to skeletal muscle by the sympathetic nervous system, increasing fatty acid oxidation, uncoupling protein-3 (UCP3) expression, and thus, energy expenditure. Here, we show that intracerebroventricular or intraperitoneal administration of C75 increases the number of mitochondria in white and red (soleus) skeletal muscle. Consistent with signal transmission from the hypothalamus by the sympathetic nervous system, centrally administered C75 rapidly (< or =2 h) up-regulated the expression (in skeletal muscle) of the beta-adrenergic signaling molecules, i.e., norepinephrine, beta3-adrenergic receptor, and cAMP; the transcriptional regulators peroxisomal proliferator activator regulator gamma coactivator 1alpha (PGC-1alpha) and estrogen receptor-related receptor alpha (ERRalpha); and the expression of key oxidative mitochondrial enzymes, including pyruvate dehydrogenase kinase, medium-chain length fatty acyl-CoA dehydrogenase, ubiquinone-cytochrome c reductase, cytochrome oxidase, as well as ATP synthase and UCP3. The role of PGC-1alpha in mediating these responses in muscle was assessed with C2C12 myocytes in cell culture. Consistent with the in vivo response, adenovirus-directed expression of PGC-1alpha in C2C12 muscle cells provoked the phosphorylation/inactivation and reduced expression of acetyl-CoA carboxylase 2, causing a reduction of the malonyl-CoA concentration. These effects, coupled with an increased carnitine palmitoyltransferase 1b, led to increased fatty acid oxidation. PGC-1alpha also increased the expression of ERRalpha, PPARalpha, and enzymes that support mitochondrial fatty acid oxidation, ATP synthesis, and thermogenesis, apparently mediated by an increased expression of UCP3.

Epidermal growth factor (EGF) stimulation of ATP citrate lyase activity in isolated rat hepatocytes is age dependent

1. The effect of epidermal growth factor (EGF) on ATP citrate lyase activity was determined in freshly isolated hepatocytes from rats of different ages as a function of incubation time, EGF concentration and hepatocyte density. 2. The activity of this enzyme was responsive to both dose and time of incubation of EGF with a two-fold increase in ATP citrate lyase activity and a half-maximal effect between 10(-12) and 10(-11) M. 3. EGF effects were detectable by 5 min. 4. The age of the rats had a strong effect on the magnitude of the EGF effect with ATP citrate lyase activity in younger (8 weeks) rats being more responsive than in older (14 weeks) rats.

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

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

Effect of malonyl-CoA on delta 6 desaturation activity of rat liver microsomes

The effect of malonyl-CoA on linoleic acid desaturation and elongation reactions of rat liver microsomes was studied. Under strict desaturation conditions, the in vitro microsomal conversion of linoleic acid to gamma-linolenic acid is time-dependent. When malonyl-CoA was added to the aforementioned incubation medium, linoleic acid was desaturated to gamma-linolenic acid and elongated to its higher homologues. Under these conditions, delta 6 desaturation activity, calculated by adding gamma-18:3, 20:3 and 20:4 acids, was neither inhibited nor activated by malonyl-CoA. These results indicate that the elongation of gamma-linolenyl-CoA coupled to the desaturation of linoleic acid did not modify delta 6 desaturase activity.

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.

Mechanisms of catecholamine effects on ketogenesis

Ketogenesis may be controlled at several sites. Lipolysis with release of plasma nonesterified fatty acid (NEFA) substrate is the first step. Plasma NEFA are taken up by the liver in a concentration-dependent fashion and, after conversion to the acyl-CoA derivative, may either be reesterified or enter the mitochondria via the carnitine shuttle. After beta-oxidation the resultant acetyl-CoA may either be converted to ketone bodies that are then released into the circulation or be condensed with oxaloacetate and enter the tricarboxylic acid cycle, the third potential control point. In humans, infusion of epinephrine causes a transient two- to threefold increase in fatty acids, glycerol, and ketone bodies. Insulin levels show a small absolute increase. Norepinephrine has similar effects, although insulin levels tend to be suppressed and glucagon levels rise somewhat. If somatostatin is added simultaneously, the lipolytic and ketogenic effects are accentuated and prolonged. Dopamine, in a high dose, has no effect on ketone bodies alone but shows small increases in NEFA and ketone bodies in the presence of somatostatin and may play a modulatory role in ketogenesis. The ketogenic effect of catecholamines could thus be in the adipocyte or in the liver. Studies with perfused liver or hepatocytes showed only trivial effects on ketogenesis even with supraphysiological doses of catecholamines. Furthermore infusion studies in rats showed decreased rather than increased ketogenesis with no change in NEFA levels. The data suggest that a) there are species differences, and b) in humans epinephrine- and norepinephrine-induced increases in ketogenesis are secondary to increases in NEFA substrate supply.

Regulation of lipogenic enzymes in human diploid fibroblasts by hormones

The hormonal regulation of two regulatory enzymes of fatty acid synthesis acetyl-CoA carboxylase (EC 6.4.1.2) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49), has been investigated in human diploid fibroblasts. There was a 35% increase in acetyl-CoA carboxylase activity, 72 h following addition of 10 microU/ml insulin to the culture medium. Addition of 1 microgram/ml of 3,3'5-triiodothyronine for 72 h resulted in an increase in acetyl-CoA carboxylase activity to 166% of the controls. The simultaneous addition of 1 microgram/ml triiodothyronine and 10 mU/ml insulin caused the enzyme activity to rise to 240% of the controls. A dose-dependent reduction in acetyl-CoA carboxylase activity was brought about by 1 X 10(-4) to 1 X 10(-3) M dibutyryl cyclic AMP. The earliest effect of dibutyryl cyclic AMP was observed within 24 h. Glucose-6-phosphate dehydrogenase followed qualitatively the same pattern of response, whereas the constitutive enzyme, lactate dehydrogenase (EC 1.1.1.27), did not show significant changes in these experiments. The data demonstrate common features of hormonal regulation of lipogenesis in human fibroblasts with liver and adipose tissue and substantiate the growing evidence that thyroid hormones are of major importance for the regulation of this process.

Phosphorylation of proteolytically-nicked rat hepatic acetyl-CoA carboxylase

A partially-purified preparation of acetyl-CoA carboxylase was not inactivated by ATP and Mg2+ although it was phosphorylated. SDS gel electrophoresis of the phosphorylated enzyme showed phosphopeptides migrating at 140 and 40 K along with the 250 K native subunit. Phosphorylation by the catalytic subunit of cAMP-dependent protein kinase further phosphorylated an additional 120 K phosphopeptide. Neither cAMP-independent phosphorylation nor the cAMP-dependent phosphorylation of the enzyme resulted in a significant decrease in activity.

Effect of dietary state on hepatocyte size

In order to calculate cellular metabolite concentrations, it is necessary to determine accurately the cellular volume. The purpose of the experiments reported here was to determine the effect of dietary state on the volume of hepatocytes isolated from livers of fed, fasted (48 h) or fasted (48 h)-refed (48 h) male Sprague-Dawley rats. Cellular volumes of the essentially spherical hepatocytes were calculated from the diameters of the cells obtained easily and rapidly by using an optical micrometer. Fasting resulted in approximately a two-fold reduction in hepatocyte volume, and refeeding resulted in a return to 83% of the control value. The protein content/1000 cells also dropped two-fold upon fasting but returned to 70% of the control value upon refeeding. This rapid, simple method for determining hepatocyte volumes agrees well with data obtained by more laborious means.

Accelerated turnover of hepatic glucokinase in starved and streptozotocin-diabetic rat

Rat liver glucokinase synthesis and degradation was estimated in fasted/fed and diabetic/diabetic-insulin-treated rats by the radioimmunological technique. Starvation and Streptozotocin-diabetes led to basal rates of synthesis and, consequently, to low levels in enzyme activity. In addition, a decrease in the apparent half-life from about 19 h in the fed or diabetic-insulin substituted to about 11 h in the starved or diabetic rat, respectively, was observed. Injection of Bt2cAMP into glucose-fed animals reduced glucokinase synthesis to basal levels within 90 min, without affecting enzyme activity. It is concluded that in metabolic states associated with elevated levels in tissue cAMP glucokinase synthesis is reduced to basal values and, in addition, its rate of degradation is significantly enhanced.

Comparative effect of glucagon, dibutyryl cyclic AMP, and epinephrine on the desaturation and elongation of linoleic acid by rat liver microsomes

The effect of glucagon, dibutyryl cyclic adenosine 3',5'-monophosphate, and epinephrine on the biosynthesis of polyunsaturated fatty acids of the linoleic acid family was studied. The incubations were performed with rat liver microsomes and labeled linoleic acid under desaturating and elongating conditions. Under desaturating conditions linoleic acid was converted to gamma-linolenic acid, whereas under elongating conditions it was converted to 20:2omega6. Glucagon, dibutyryl cyclic AMP, and epinephrine decreased the oxidative desaturation of linoleic acid to gamma-linolenic acid while the elongating reaction was not modified in the experimental conditions tested. Consequently, the results support the hypothesis that the oxidative desaturation of linoleic acid to gamma-linolenic acid is the main controllable step in the biosynthesis of polyunsaturated fatty acids of the linoleic acid family in the microsomes.

Effect of thyroid hormones on microsomal fatty acid chain elongation synthesis in rat liver

Evidence is presented that rat liver microsomal fatty acid chain elongation synthesis and desaturation, as well as acetyl-CoA carboxylase and fatty acid synthetase, are strongly influenced by thyroid hormone level. Conversely, the fatty acid chain elongation system in mitochondria, unlike the oxidative capacity of palmitate, NADH, succinate and malate, does not seem significantly affected by the thyrotoxic state. In triiodothyronine-induced or thyroxine-induced hyperthyroidism, rat liver acetyl-CoA carboxylase, fatty acid synthetase and microsomal chain elongation and desaturation reactions are not greatly affected after the first 10 days of treatment, while after longer intervals a respective increase in these activities is shown of up to 87, 116 and 65% after 22 days. In propylthiouracil-induced hypothyroidism, all the above synthetic activities are strongly reduced immediately after three days of drug administration and diminished no further following longer periods. Although the pattern of synthesized fatty acids in the thyrotoxic state is similar to that obtained from normal subcellular rat fractions, the esterification process of fatty acids in microsomal lipids appears to be slightly inhibited in hypothyroid rats and increased following triiodothyronine or thyroxine administration. Finally, a reduction in the hepatic cyclic AMP level of about 41% is reported after 19 days of triiodothyronine-administration to rats. On the basis of the observed insensitivity of the mitochondrial fatty acid chain elongation system to the thyrotoxic state, a tentative interpretation of its role in the hepatic cell is postulated.

Secretory processes, carbohydrate and lipid metabolism in isolated mouse hepatocytes. Aspects of regulation by glucagon and insulin

The procedure of Berry and Friend for isolation of intact hepatocytes has been adapted to mouse livers. The ultrastructure of these cells was satisfactorily preserved. Isolated mouse hepatocytes secreted proteins and triacylglycerols. These secretory processes were inhibited by colchicine, indicating a likely involvement of the microtubular system for their normal occurrence. Ultracentrifugation of medium incubated with hepatocytes, followed by electrophoresis and electron microscopic examination of the floating fraction (density less than 1.006) allowed to conclude that secreted triacylglycerols were very low density lipoproteins. Glycogenolysis and lipogenesis were stimulated or inhibited, respectively, by low concentrations of glucagon (10(-10) M). Other metabolic parameters were influenced by the hormone but were less sensitive to its action. Inhibition of lipogenesis by glucagon was associated with a decrease in acetyl CoA carboxylase activity. This decrease does not appear to be related to intracellular fatty acyl-CoA accumulation secondary to hepatic lipase activation by the hormone. Insulin was effective alone or counteracted glucagon effects on lipogenesis or glycogenolysis only when exposure of cells to collagenase was held minimal. This suggests that, during isolation of hepatocytes, insulin receptors may, for unknown reasons, be more fragile than those of glucagon.

Adenosine 3',5'-monophosphate and the regulation of rat hepatic sterol synthesis: a reexamination based on Sutherland criteria

Many investigators have reported that adenosine 3',5'-monophosphate (cyclic AMP) at supraphysiologic concentrations (10(-4) M) suppresses lipogenesis in rat livers in vitro. The data in this report demonstrate that the phosphodiesterase inhibitor theophylline does not suppress sterol synthesis by itself nor does it potentiate the cyclic AMP-mediated suppression. In addition the cyclic AMP-mediated effect was not specific as many structurally-related compounds including adenosine triphosphate, adenosine 5'-diphosphate, adenosine 5'-monophosphate, adenosine, coenzyme A, HMG coenzyme A, and guanosine 3',5'-monophosphate suppressed sterol synthesis. The phenomenon of cyclic AMP-suppression of sterol synthesis does not fulfill two of the four criteria defined by Sutherland to assess whether a given metabolic effect is mediated by cyclic AMP. Therefore, we conclude that the cyclic AMP mediated suppression of sterologenesis is not a physiologic effect of the nucleotide.

Effect of maternal diet on fetal hepatic lipogenesis

The effects of: a, maternal diet; b, cyclic-3',5'-adenosinemonophosphate (cyclic AMP) and c, clofibrate on hepatic lipogenesis in fetal rats were studied. The experimental diets contained 22% protein, 40--50% carbohydrate, adequate vitamins, and minerals. In addition, the fat-containing diets were supplemented with either 15% corn oil, 25% corn oil, or 5% cholesterol + 10% oleic acid. In the clofibrate feeding studies, 0.3% (w/v) of the ethyl ester was added to a stock ration or to fat-free diet. Lipogenesis was measured in liver slices incubated with [2-14C]pyruvate, [1-14C]acetate, or 3H2O. In addition, activities of lipogenic enzymes were measured in cytosol fractions from liver homogenates. The effec-s of the experimental diets on liver composition were also examined. Lipogenic activity was higher in fetal than in maternal liver. When 15% corn oil was added to the maternal diet, fatty acid synthesis in fetal liver did not decrease as it did in maternal liver. Maternal fasting decreased fetal fatty acid synthesys by 50% when measured with 14C and less than 10% when measured with 3H2O. Although the addition of cholesterol to the maternal diet decreased cholesterol synthesis in maternal liver, no such decrease was observed in fetal liver. Changes in enzyme activities paralleled alterations in lipogenesis in maternal but not in fetal liver. Corn oil feeding or fasting increased the rate of transfer of linoleate from the dam to the fetus. However, accumulation of linoleate in fetal liver did not correlate with a decreased rate of fatty acid synthesis as it did in maternal liver. Maternal hepatic glycogen stores were depleted by fasting, but glycogen levels in fetal liver remained high under these conditions.

[Behavior of certain parameters of lipid and energy metabolism. 3. Relationships between cyclic 3',5'-adenosine monophosphate and acetyl coenzyme A in liver of growing rats deduced from model experiments with diets differeing in fat content]

Schemes of the lipid and the pyruvate metabolism serve to show that a great part of the enzymes which intervence in the metabolic pathways and are associated with the formation and the consumption of acetyl coenzyme A may be regulated by cyclic 3',5'-adenosine monophosphate (cAMP) in the sense of activation or inhibition. The cAMP increase in the liver, which has been demonstrated in the present study for a diet containing 25% of fat, opens the metabolic pathway to the formation of acetyl coenzyme A by means of fatty acid degradation and simultaneous inhibition of lipogenesis. The deficiency of insulin (which has been evidenced in previous paper) characterizes, together with the facts mentioned, a state like diabetes or the fasting state. Acetyl coenzyme A is mainly used for energy supply. The close negative correlation between cAMP and acetyl coenzyme A (which is shown in the present paper) permits to conclude that the extent and trend of the increase and decrease in the liver is subjected to intensive hormonal control in which cAMP in involved.

Inhibition of hepatic lipogenesis by adenine nucleotides

Incubation of liver slices and isolated liver cells with adenosine cyclic-3',5'-monophosphate at concentrations which inhibit lipogenesis was found to expand the pool size of the noncyclic adenine nucleotides in the intact cells of the preparations. This observation led to studies which demonstrated that adenosine and adenosine-5'-monophosphate also inhibited lipogenesis and expanded the adenine nucleotide pool size. It is proposed but not proven that the increase in intracellular nucleotides produced by adenosine-5'-monophosphate, adenosine cyclic-3',5'-monophosphate, and adenosine may have an adverse effect upon the synthesis of fatty acids. Because of the expansion of the adenine nucleotide pool size, high concentrations of adenosine cyclic-3',5'-monophosphate should not be used to investigate the mechanism responsible for hormonal regulation of lipogenesis. As an added complication, exogenous adenosine-5'-monophosphate was found to produce a small but significant increase in the intracellular concentration of adenosine cyclic-3',5'-monophosphate of isolated liver cells. This effect also may be a factor in the inhibition of lipogenesis by adenosine-5'-monophosphate. Low concentrations of N6, O2'-dibutyryl adenosine cyclic-3',5'-monophosphate were found to inhibit lipogenesis without increasing the intracellular adenine nucleotide content of either liver slices or isolated liver cells. It is concluded that studies on the mechanism of glucagon regulation of lipogenesis should be carried out with glucagon or low concentrations of N6, O2'-dibutyryl adenosine cyclic-3',5'-monophosphate.