Acetoacetyl-coenzyme A synthetase activity in rat liver cytosol: a regulated enzyme in lipogenesis

Ketogenesis supports hepatic polyunsaturated fatty acid homeostasis via fatty acid elongation

Ketogenesis is a dynamic metabolic conduit supporting hepatic fat oxidation particularly when carbohydrates are in short supply. Ketone bodies may be recycled into anabolic substrates, but a physiological role for this process has not been identified. Here, we use mass spectrometry-based 13C-isotope tracing and shotgun lipidomics to establish a link between hepatic ketogenesis and lipid anabolism. Unexpectedly, mouse liver and primary hepatocytes consumed ketone bodies to support fatty acid biosynthesis via both de novo lipogenesis (DNL) and polyunsaturated fatty acid (PUFA) elongation. While an acetoacetate intermediate was not absolutely required for ketone bodies to source DNL, PUFA elongation required activation of acetoacetate by cytosolic acetoacetyl-coenzyme A synthetase (AACS). Moreover, AACS deficiency diminished free and esterified PUFAs in hepatocytes, while ketogenic insufficiency depleted PUFAs and increased liver triacylglycerols. These findings suggest that hepatic ketogenesis influences PUFA metabolism, representing a molecular mechanism through which ketone bodies could influence systemic physiology and chronic diseases.

A hierarchical hepatic de novo lipogenesis substrate supply network utilizing pyruvate, acetate, and ketones

Hepatic de novo lipogenesis (DNL) is a fundamental physiologic process that is often pathogenically elevated in metabolic disease. Treatment is limited by incomplete understanding of the metabolic pathways supplying cytosolic acetyl-CoA, the obligate precursor to DNL, including their interactions and proportional contributions. Here, we combined extensive 13C tracing with liver-specific knockout of key mitochondrial and cytosolic proteins mediating cytosolic acetyl-CoA production. We show that the mitochondrial pyruvate carrier (MPC) and ATP-citrate lyase (ACLY) gate the major hepatic lipogenic acetyl-CoA production pathway, operating in parallel with acetyl-CoA synthetase 2 (ACSS2). Given persistent DNL after mitochondrial citrate carrier (CiC) and ACSS2 double knockout, we tested the contribution of exogenous and leucine-derived acetoacetate to acetoacetyl-CoA synthetase (AACS)-dependent DNL. CiC knockout increased acetoacetate-supplied hepatic acetyl-CoA production and DNL, indicating that ketones function as mitochondrial-citrate reciprocal DNL precursors. By delineating a mitochondrial-cytosolic DNL substrate supply network, these findings may inform strategies to therapeutically modulate DNL.

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

Transcriptional regulation of the human acetoacetyl-CoA synthetase gene by PPARgamma

In the cytosol of lipogenic tissue, ketone bodies are activated by AACS (acetoacetyl-CoA synthetase) and incorporated into cholesterol and fatty acids. AACS gene expression is particularly abundant in white adipose tissue, as it is induced during adipocyte differentiation. In order to elucidate the mechanism controlling the gene expression of human AACS and to clarify its physiological role, we isolated the human promoter, characterized the elements required to initiate transcription and analysed the expression of the gene in response to PPARgamma (peroxisome-proliferator-activated receptor gamma), an inducer of adipogenesis. We show that the human AACS promoter is a PPARgamma target gene and that this nuclear receptor is recruited to the AACS promoter by direct interaction with Sp1 (stimulating protein-1).

Utilization of carbon from dietary polyunsaturates for brain cholesterol synthesis during early postnatal development in the rat: a 13C NMR study

Incorporation of 13C from a dietary precursor into cholesterol was studied in neonatal rats. Rats were given uniformly 13C-enriched polyunsaturated fatty acids intragastrically and total lipid extracts of liver and brain were analyzed by 13C-NMR 1, 4, 8, and 15 days later. 13C-enrichment was detected in brain but not in liver cholesterol. Maximal 13C-labeling was observed 4 days after injection of the label. Spectra revealed that 70% of newly incorporated 13C had 13C as an adjacent neighbor, the other 30% had 12C as the neighbor. Double quantum NMR revealed the arrangement in the cholesterol skeleton of the 13C-13C pairs transferred from precursors to cholesterol. Desmosterol, an intermediate of cholesterol synthesis, was identified in the spectra of brain lipids. Comparison of 13C-13C unit arrangements in both cholesterol and desmosterol allowed carbons 26 and 27 of desmosterol to be unambiguously assigned.

Utilization and preferred metabolic pathway of ketone bodies for lipid synthesis by isolated rat hepatoma cells

Morris hepatoma 7777 cells freshly isolated from highly malignant tumors grown in the hindlimb of buffalo rats actively convert ketone bodies to cholesterol and fatty acids. On the basis of results obtained with (-)-hydroxycitrate, an inhibitor of the ATP citrate lyase enzyme, the metabolic pathway for acetoacetate conversion to lipids is exclusively cytoplasmic, whereas that for 3-hydroxybutyrate involves both extra- and intramitochondrial compartments. Subcellular distribution studies indicated accumulation and compartmentation of 3-hydroxybutyryl CoA primarily in the cytoplasm of hepatoma cells incubated with either ketone body. In contrast, the compartmentation of acetoacetyl CoA is dependent on whether the substrate is acetoacetate or 3-hydroxybutyrate. With acetoacetate, the acetoacetyl CoA is entirely cytoplasmic, whereas with 3-hydroxybutyrate, it is equally divided between the intra- and extramitochondrial compartments. The results are discussed in terms of the known and proposed metabolic pathways for lipid synthesis from ketone bodies, particularly that from 3-hydroxybutyrate.

Metabolism of R-beta-hydroxypentanoate and of beta-ketopentanoate in conscious dogs

R-beta-Hydroxypentanoate and beta-ketopentanoate are homologues of physiological ketone bodies R-beta-hydroxybutyrate and acetoacetate. They derive from the oxidation in liver of the R-moiety of R,S-1,3-pentanediol, a potential nutrient. This report documents the metabolism of R-beta-hydroxypentanoate and beta-ketopentanoate in conscious dogs. Whether administered by bolus or constant infusion, the two substrates are interconverted and rapidly metabolized. When beta-ketopentanoate was infused at a rate corresponding to 75% of the dog's caloric requirement, the steady-state total plasma concentration of the two substrates was only 1.3 mM. Because the substrates are precursors of propionyl-CoA, we assayed the urinary concentrations of markers of propionic acidemia. Their accumulation was minor compared with what is observed in patients suffering from propionic acidemia. We conclude that, at least during short-term experiments, R-beta-hydroxypentanoate and beta-ketopentanoate are well metabolized in the dog without apparent intolerance to a large supply of propionyl-CoA.

Lipogenesis from n-butyrate in colonocytes. Action of reducing agent and 5-aminosalicylic acid with relevance to ulcerative colitis

Cell membranes of colonic epithelial cells (CEC) in ulcerative colitis show structural abnormalities which are specific to the disease and which suggest impaired lipogenesis in CECs. Lipogenesis from [1-14C]-n-butyrate, the chief oxidative fuel of colonic epithelial cells, was measured in isolated CECs under control conditions, with or without glucose and in the presence of mercaptoacetate, a major reducing agent in the colonic lumen. Glucose significantly (p < 0.01) stimulated lipogenesis from [1-14C]-butyrate which was reversed by 5 mM mercaptoacetate. Mercaptoacetate significantly diminished CEC thiolase activity (EC 2.3.1.9). 5-Aminosalicylic acid reversed the adverse effects of mercaptoacetate in the saponifiable fraction of extracted lipids. Changes in lipogenesis due to colonic luminal reducing agents would affect the barrier function of CECs a feature relevant to the disease process of ulcerative colitis.

In vivo utilization of [3-14C]acetoacetate for lipid and amino acid synthesis in the 15-day-old chick

1. The utilization of different concentrations of acetoacetate for the in vivo synthesis of lipids and amino acids has been studied in brain, spinal cord, liver, small intestine and kidney of 15-day-old chicks. Both lipid and amino acid synthesis increased in an almost linear fashion with precursor concentration in the five tissues mentioned. 2. Lipid synthesis was very high in spinal cord, followed in decreasing order by brain, small intestine, liver and kidney. At the highest concentration assayed (48 mM) the main lipids synthesized were triglycerides in liver (75%) and kidney (52%) and cholesterol in brain (47%), spinal cord (47%) and small intestine (42%). 3. Amino acid synthesis from acetoacetate did not vary markedly among the five organs, although brain and spinal cord showed higher rates at the maximal concentrations of precursor. Glutamate was always the main amino acid formed.

Acetoacetyl-CoA ligase activity in the isolated rat hepatocyte: effects of 25-hydroxycholesterol and high density lipoprotein

The activity of acetoacetyl-CoA (AcAc-CoA) ligase (E.C.6.2.1.16) in hepatocytes from rats was shown to be the same as the activity in homogenates of their livers. In hepatocytes treated with 25-hydroxycholesterol, AcAc-CoA ligase, 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase and rates of sterol synthesis were substantially decreased. Hepatocytes treated with high density lipoprotein (HDL) exhibited a 2 to 4 fold induction of HMG-CoA reductase activity; however an accompanying increase in AcAc-CoA ligase activity and the rate of cholesterol synthesis was not observed. We conclude (a) that increases in the activity of HMG-CoA reductase when mediated by HDL in hepatocytes do not result in a corresponding change in the capacity for sterol synthesis and (b) that changes in the activity state of HMG-CoA reductase can be dissociated from that of AcAc-CoA ligase.

Diurnal rhythms in liver carbohydrate metabolism. Comparative aspects and critical review

Literature data on the diurnal rhythms of blood glucose, liver glycogen levels and key hepatic enzyme activities of glycolysis, gluconeogenesis, glycogen metabolism and lipogenesis in animals are reviewed. Materials on the diurnal rhythms of the activities of other enzymes involved in carbohydrate metabolism and related pathways such as the equilibrium glycolytic enzymes are also given. Interspecies comparison and analysis of the results and their interpretation are given.

Age-related changes in the regulation of cholesterol metabolism in rats

Activities of the hepatic cholesterol synthetic system including initial steps of the pathway and cholesterol 7 alpha-hydroxylase were all lower in adult (8 to 9-month-old) rats than in young (5 week-old) rats. The extent of diurnal fluctuation of 3-hydroxy-3-methylglutaryl coenzyme A reductase was, however, apparently greater in adult animals. When the cholesterol-enriched diet was fed to rats for 1 day, the extent of the depression of the cholesterogenic enzymes was dependent on age of animals. The enzyme activities rapidly increased on refeeding a cholesterol-free diet after the cholesterol challenge. In young rats the activity of cholesterol 7 alpha-hydroxylase exhibited a pattern inverse to that of HMG-CoA reductase whereas in adult rats it increased continuously during the entire experimental period. Cholesterol and triglyceride accumulated in the liver of adult animals, and their response to dietary cholesterol also depended on the age of the animals. The results indicate a specific modification of the cholesterol homeostatic mechanism with age.

Regulation of squalene synthetase activity in rat liver: elevation by cholestyramine, but no diurnal variation

Squalene synthetase activity in liver microsomes from rats sacrificed at three different times of the diurnal cycle showed no significant differences. Addition of 4% cholestyramine to the food resulted in a marked increase in activity (280% of control), independent of the time of killing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase activity, determined as positive controls, were also found to be elevated by cholestyramine and additionally showed a diurnal variation. On the other hand, five control enzyme activities, not directly related to cholesterol metabolism, i.e. glutamate dehydrogenase, NADPH cytochrome-c reductase, beta-hexosaminidase, catalase and acyl coenzyme A oxidase, showed neither an influence of cholestyramine feeding nor a time of sacrifice dependent variation.

Acetoacetyl-CoA synthetase specific activity and concentration in rat tissues

An antibody against acetoacetyl-CoA synthetase purified from rat liver was raised in rabbits. Utilizing the binding of antibody-antigen complexes to a nitrocellulose membrane, a sensitive enzyme-linked immunosorbent assay was developed to estimate the enzyme concentration in rat tissues. The enzyme concentration (microgram immunoreactive protein/mg protein) in rat liver cytosol was increased about 3-, 1.8- and 7-fold by feeding rats diets containing 5% cholestyramine, 0.2% ML-236B (compactin), and 5% cholestyramine plus 0.2% ML-236B for 4 days, respectively, and decreased about 1.8-fold by fasting the animals or 1.3-fold by feeding them a diet containing 5% cholesterol. Changes in the enzyme activity were almost parallel to those in the enzyme concentration, suggesting the physiological role of this enzyme in cholesterol biosynthesis. Immunoblotting of the hepatic cytosol also confirmed that the increase in enzyme concentration on cholestyramine and/or ML-236B feeding was due to an increase in an enzyme protein the same as the purified enzyme and not the isozymic protein. Among various rat tissues examined, the concentrations of immunologically crossreactive enzyme were higher in lipogenic tissues, such as brain, adipose tissue and liver, than in other tissues. The enzymes in these three tissues were identical in molecular weight determined by gel filtration and immunoblotting.

The metabolic route by which oleate is converted into cholesterol in rat hepatocytes

The effect of (-)-hydroxycitrate on the conversion of [1-14C]oleate into cholesterol was dependent on the time of day at which the cells were prepared and on the extracellular oleate concentration. In hepatocytes prepared during the light phase of the diurnal cycle (L2-hepatocytes), (-)-hydroxycitrate inhibited the conversion of L-[U-14C]lactate (2 mM) and of 0.13 mM-[1-14C]oleate into cholesterol. However, when [1-14C]oleate was present at 1.3 mM, most of the sterol carbon was derived from this source, and under these conditions (-)-hydroxycitrate had no inhibitory effect on [14C]cholesterol formation. In these cells, non-radioactive acetoacetate blocked the conversion of 1.3 mM-[1-14C]oleate, but not of 0.13 mM-[1-14C]oleate, into cholesterol. In cells prepared during the dark phase of the diurnal cycle (D6-hepatocytes), irrespective of the concentration of [1-14C]oleate, (-)-hydroxycitrate decreased its conversion into cholesterol. In both types of cell preparation, the inhibitory effect of (-)-hydroxycitrate on the conversion of L-[U-14C]lactate into cholesterol was greater than that on the overall rate of cholesterol production from all endogenous sources. These results provide evidence for the following. (1) The major metabolic route by which oleate is converted into cholesterol is dependent on its extracellular concentration. (2) When oleate is the major source of hepatic sterol carbon, the flux of substrate through citrate into cholesterol is dependent on the nutritional state of the animal. (3) When endogenous substrates are the sole source of sterol carbon, a substantial proportion of the carbon enters the cholesterol pathway through routes not involving citrate cleavage.

Isolation of rat liver microsomal short-chain beta-ketoacyl-coenzyme A reductase and trans-2-enoyl-coenzyme A hydratase: evidence for more than one hydratase

An enzyme preparation (IIIB) isolated from liver microsomes of untreated male rats was found to contain two activities--short-chain trans-2-enoyl-CoA hydratase and beta-ketoacyl-CoA reductase. The hydratase was purified more than 1000-fold, while the reductase activity was purified over 600-fold. Employing sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, a single band with a molecular weight of 76,000 was observed. Although attempts to separate these two activities have failed, it remains to be established whether the final preparation contains a single enzyme with two activities or two separate enzymes. The hydratase was most active toward crotonyl-CoA, followed by trans-2-hexenoyl-CoA (6:1) and -octenoyl-CoA (8:1); the enzyme was essentially inactive toward substrates containing more than eight carbon atoms. The Vmax for crotonyl-CoA was 2117 mumol/min/mg protein, while the Km was 59 microM. Using acetoacetyl-CoA as substrate, the Vmax for the beta-ketoacyl-CoA reductase was over 60 mumol/min/mg protein and the Km was 37 microM; the Vmax for beta-ketopalmitoyl-CoA was only 15% of that observed with acetoacetyl-CoA, although the Km was 6 microM. During the course of purification, a second short-chain hydratase was discovered (fraction IVA); unlike IIIB, this fraction catalyzed the hydration of 4:1, 6:1, and 8:1 at similar rates. The partially purified preparation yielded maximal activity with 8:1 CoA (apparent Vmax 35 mumol/min/mg), followed by 6:1 CoA, 4:1 CoA, and 10:1 CoA; longer chain CoA's were relatively poor substrates, with trans-2-hexadecenoyl CoA about 0.1 as active as 8:1 CoA. On SDS-gels, fraction IVA contained four bands, all of which were below 60,000 Mr. Proteases, such as trypsin, chymotrypsin, and subtilisin, were found to completely inactivate both enzyme fractions.

A radiochemical assay for acetoacetyl-CoA synthetase

A sensitive radiochemical assay is described for the assay of acetoacetyl-CoA synthetase activity in cytosolic extracts. Enzyme activity is measured by the incorporation of 14C from acetoacetate into acetyl carnitine as mediated by acetoacetyl-CoA synthetase, endogenous acetoacetyl-CoA thiolase, and exogenous carnitine acetyl transferase. Separation of 14C-labeled reactants from 14C-labeled acetyl carnitine is achieved by cation-exchange chromatography. The assay is sensitive with less than 10 pmol of product readily detected. Acetoacetyl-CoA synthetase activity was measured in human fibroblasts, 0.12 nmol min-1 mg cytosolic protein-1, and was found to be more than two orders of magnitude below the activity level of acetoacetyl-CoA synthetase in rat liver cytosol, 18.4 nmol min-1 mg cytosolic protein-1. An HPLC method is also described for the purification of [3-14C]acetoacetate.

Acetoacetate: a major substrate for the synthesis of cholesterol and fatty acids by isolated rat hepatocytes

Evidence is presented that isolated, intact rat hepatocytes can synthesize fatty acids and cholesterol from acetoacetate. The quantitative importance of these processes is evaluated by measuring total rates of fatty acid and cholesterol synthesis by incorporation of 3H from 3H2O. The contribution of acetoacetate varies from 14-54% and from 21-75% for de novo synthesized fatty acids and cholesterol, respectively, depending on the physiological condition of the donor rat. The relative contribution of acetoacetate to cholesterol synthesis is 1.4-2.3-times greater than to fatty acid synthesis.