Studies on the assay, activity and sedimentation behaviour of acetyl-CoA carboxylase from isolated hepatocytes incubated with insulin or glucagon

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

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

Insulin action, thermogenesis and obesity

The case for obesity per se being a major cause of insulin resistance has been made. There is evidence that each of the control points of insulin on glucose metabolism are negatively influenced by lipid oversupply, a characteristic of the obese state. The answer to the corollary, whether insulin resistance (a universal concomitant of obesity) can in turn lead to obesity via a decrease in thermogenesis, is more complex. Overall, the answer would appear to be no. On a population basis, obese individuals would not appear to have lower metabolic rates, whether expressed on a lean tissue or any other basis, than lean individuals. Even in the subpopulation of hypometabolic obese, there are no convincing data that the reduced metabolic rate is linked to particularly severe insulin resistance. Further, improving insulin action by weight loss would not appear to increase thermogenesis as would be predicted if insulin resistance impaired thermogenesis. A case can be made for reductions in a specific aspect of energy expenditure in obesity, that of meal-induced or glucose-induced thermogenesis, and this may be due to insulin resistance. However, meal-induced thermogenesis is a small component of total energy expenditure and total energy expenditure is not different between lean and obese. That leaves the intriguing possibility that a relative failure of prandial thermogenesis has an impact upon energy balance via impairment of satiety (related to reduced metabolic flux) and thus by increasing intake. While a potentially fruitful research avenue, too few data exist on this possibility for it to be anything more than speculative at this stage.

Changes in the properties of cytosolic acetyl-CoA carboxylase studied in cold-clamped liver samples from fed, starved and starved-refed rats

We have used the cold-clamping technique to study the changes in acetyl-CoA carboxylase activity that occur in the cytosolic and mitochondrial fractions of the liver of fed, starved and starved-refed rats. No evidence was found for a role of the mitochondrial enzyme as a pool from which cytosolic carboxylase could be replenished upon refeeding of starved rats. Starvation for 24 h or 48 h induced changes in the expressed (assayed at 20 mM-citrate), total (citrate- and phosphatase-treated) and citrate-independent activities of cytosolic carboxylase, as well as in its Ka for citrate. The expressed/total activity ratio was low even in the fed state and was depressed further by starvation. The effects of refeeding occurred in two phases: an acute phase (approx. 1 h) in which the starvation-induced changes in Ka and expressed/total activity ratio were rapidly reversed, and a prolonged slow phase in which the two parameters attained values that were lower and higher, respectively, than those in the normal fed state. Refeeding also resulted in a gradual increase in citrate-independent activity of acetyl-CoA carboxylase. An additional marked increase in this activity occurred only in 48 h-starved-refed rats between 24 h and 48 h of refeeding. These findings are discussed in terms of the observed time courses of changes in lipogenic rates that occur in vivo in starved-refed rats and of the possible molecular mechanisms involved.

Protein-serine kinase from rat epididymal adipose tissue which phosphorylates and activates acetyl-CoA carboxylase. Possible role in insulin action

1. Most of the cyclic-nucleotide-independent acetyl-CoA carboxylase kinase activity in an extract of rat epididymal adipose tissue was evaluated from a Mono Q column by 0.175 M-NaCl at pH 7.4. The activity of the kinase in this fraction (fraction 1) was increased after exposure of intact tissue to insulin. 2. Incubation of purified adipose-tissue acetyl-CoA carboxylase with [gamma-32P]ATP and samples of fraction 1 led to the incorporation of up to 0.4 mol of 32P/mol of enzyme subunit. Most of the phosphorylation was on serine residues within a single tryptic peptide. This peptide, on the basis of two-dimensional t.l.c. analysis, h.p.l.c. and Superose 12 chromatography, appeared to be the same as the acetyl-CoA carboxylase peptide ('I'-peptide) which exhibits increased phosphorylation in insulin-treated tissue. 3. Phosphorylation of purified acetyl-CoA carboxylase by the kinase in fraction 1 was found to be associated with a parallel 4-fold increase in activity. However, increases in both phosphorylation and activity were much diminished if fraction 1 was treated by Centricon centrifugation to remove low-Mr components. Among these components was a potent inhibitor of acetyl-CoA carboxylase activity which appeared to be necessary for the kinase in fraction 1 to be fully active. 4. The inhibitor remains to be identified, but inhibition requires MgATP, although the inhibitor itself does not cause any phosphorylation of the carboxylase. No effects of insulin were observed on the activity of the inhibitor. 5. It is concluded that the kinase probably plays an important role in the mechanism whereby insulin brings about the well-established increases in phosphorylation and activation of acetyl-CoA carboxylase in adipose tissue.

The involvement of carnitine intermediates in peroxisomal fatty acid oxidation: a study with 2-bromofatty acids

Metabolism-dependent inactivators of 3-ketothiolase I and carnitine acyltransferase I (CAT I) have been used to study the oxidation of fatty acids in intact hepatocytes. 2-Bromooctanoate inactivates mitochondrial and peroxisomal 3-ketothiolases I in a time-dependent manner. During the first 5 min of incubation, inactivation of 3-ketothiolase in mitochondria is five times faster than its inactivation in peroxisomes. Almost complete inactivation of 3-ketothiolase I in both types of organelle is achieved after incubation with 1 mM 2-bromooctanoate for 40 min. The inactivation is not affected by preincubating hepatocytes with 20 microM tetradecylglycidate (TDGA), an inactivator of CAT I, under conditions which cause greater than 95% inactivation of CAT I. 2-Bromododecanoate (1 mM) causes 60% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min. These inactivations are greatly reduced by preincubating hepatocytes with 20 microM TDGA, demonstrating that 2-bromododecanoate enters both mitochondria and peroxisomes via its carnitine ester. 2-Bromopalmitate (1 mM) causes less than 5% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min, but causes 95% inactivation of CAT I during this time. Incubation of hepatocytes with 10-200 microM 2-bromopalmitoyl-L-carnitine causes inactivation of mitochondrial and peroxisomal 3-ketothiolases I at similar rates. This inactivation is decreased by palmitoyl-D-carnitine during the first 5 min of incubation. Pretreating hepatocytes with 20 microM TDGA does not affect the inactivation of mitochondrial or peroxisomal 3-ketothiolase I by 2-bromopalmitoyl-L-carnitine. These results demonstrate that in intact hepatocytes, peroxisomes oxidize fatty acids of medium-chain length by a carnitine-independent mechanism, whereas they oxidize long-chain fatty acids by a carnitine-dependent mechanism.

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.

Measurement of acetyl-CoA carboxylase activity in isolated hepatocytes

An assay is described for acetyl-CoA carboxylase activity in isolated hepatocytes. The assay is based on two principles: The hepatocytes are made permeable by digitonin. 64 micrograms of digitonin per mg of cellular protein were most effective in exposing enzyme activity without a significant effect on mitochondrial permeability. Enzyme activity is measured by coupling the carboxylase reaction to the fatty acid synthase reaction. The advantages offered by this procedure over existing assays are: rapidity, no need to prepare cell extracts, absence of product inhibition, no interference by mitochondrial enzymes, useful in systems with bicarbonate buffers, and simple separation of radioactive substrate from labelled products. Using this coupled enzyme assay a good correlation was observed between changes in the activity of acetyl-CoA carboxylase and changes in the rate of fatty acid synthesis in hepatocytes as effected by short-term modulators.

Use of rapid gel-permeation chromatography to explore the inter-relationships between polymerization, phosphorylation and activity of acetyl-CoA carboxylase. Effects of insulin and phosphorylation by cyclic AMP-dependent protein kinase

Superose 6 chromatography was used to separate rapidly the polymeric and dimeric forms of acetyl-CoA carboxylase. With preparations of acetyl-CoA carboxylase purified by Sepharose-avidin chromatography, it is shown that citrate promotes polymerization and that the extent of polymerization is diminished, but not eliminated, after phosphorylation by cyclic-AMP-dependent protein kinase. After exposure of rat epididymal adipose tissue to insulin, evidence was obtained for a marked increase in polymerization. The polymeric form, which was active in the absence of citrate, exhibited increased phosphorylation, particularly on a tryptic peptide designated the I-peptide in an earlier study [Brownsey & Denton (1982) Biochem. J. 202, 77-86]. In contrast, in tissue exposed to the beta-agonist isoprenaline, most of the phosphorylated acetyl-CoA carboxylase appeared to be in the dimeric form if chromatography was carried out in the absence of citrate, whereas in the presence of citrate the degree of polymerization was diminished.

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.

No synergism between ionomycin and phorbol ester in fatty acid synthesis by isolated rat hepatocytes

With hepatocytes in suspension, freshly isolated from meal-fed rats, no significant effect of ionomycin on the rate of de novo fatty acid synthesis was observed, whereas phorbol myristate acetate (PMA) was strongly stimulatory. The combination of ionomycin and PMA produced the same stimulation as was seen with PMA alone. Stimulation of fatty acid synthesis by vasopressin was comparable and not additive to that observed with PMA, indicating that activation of protein kinase C is solely responsible for this metabolic effect of vasopressin. Both vasopressin and PMA increased acetyl-CoA carboxylase activity in isolated rat hepatocytes.

Fatty acid inhibition of hormonal induction of acetyl-coenzyme A carboxylase in hepatocyte monolayers

Primary cultures of adult rat hepatocytes were utilized to ascertain the impact of free fatty acids on the insulin plus dexamethasone induction of acetyl-CoA carboxylase. Lipogenesis was induced threefold by the combination of insulin and dexamethasone. The rise in fatty acid synthesis was accompanied by a comparable increase in the rate-determining enzyme acetyl-CoA carboxylase. Dexamethasone was required for the insulin induction of acetyl-CoA carboxylase. Under the permissive action of glucocorticoid, 10(-7) M insulin maximally increased enzyme activity. Half-maximum stimulation occurred with 5 X 10(-9) M insulin. Media containing 0.2 mM palmitate, oleate, linoleate, arachidonate, or docosahexaenoate significantly suppressed the hormonal induction of acetyl-CoA carboxylase. The extent of suppression was only 30-35% and did not vary with chain length or degree of unsaturation. Carboxylase activity was not suppressed further by raising the concentration of linoleate to 0.5 mM; however, 0.5 mM palmitate depleted the cells of ATP and abolished acetyl-CoA carboxylase activity. Therefore, based upon the inhibitory characteristics of the various fatty acids and the lack of a concentration dependency of the fatty acid inhibition, it would appear that fatty acid inhibition of the induction of acetyl-CoA carboxylase activity may not be a direct, physiological regulatory mechanism.

Both insulin and epidermal growth factor stimulate lipogenesis and acetyl-CoA carboxylase activity in isolated adipocytes. Importance of homogenization procedure in avoiding artefacts in acetyl-CoA carboxylase assay

Epidermal growth factor (EGF) stimulates lipogenesis by 3-4-fold in isolated adipocytes, with a half-maximal effect at 10 nM-EGF. In the same batches of cells insulin stimulated lipogenesis by 15-fold. Freezing and prolonged homogenization of adipocytes results in release of large quantities of pyruvate carboxylase from broken mitochondria, and sufficient pyruvate can be carried through into assays for this enzyme to cause significant interference with assays of acetyl-CoA carboxylase in crude adipocyte extracts. This may account for the high amount of citrate-independent acetyl-CoA carboxylase activity reported to be present in adipocyte extracts in some previous publications. This problem may be eliminated by homogenizing very briefly without freezing. By using the modified homogenization procedure, EGF treatment of adipocytes was shown to produce an effect on acetyl-CoA carboxylase activity almost identical with that of insulin. Both messengers increase Vmax. without significant effect on the Ka for the allosteric activator, citrate.