Ketogenesis in vitro

Targeting peroxisomal fatty acid oxidation improves hepatic steatosis and insulin resistance in obese mice

Obesity and diabetes normally cause mitochondrial dysfunction and hepatic lipid accumulation, while fatty acid synthesis is suppressed and malonyl-CoA is depleted in the liver of severe obese or diabetic animals. Therefore, a negative regulatory mechanism might work for the control of mitochondrial fatty acid metabolism that is independent of malonyl-CoA in the diabetic animals. As mitochondrial β-oxidation is controlled by the acetyl-CoA/CoA ratio, and the acetyl-CoA generated in peroxisomal β-oxidation could be transported into mitochondria via carnitine shuttles, we hypothesize that peroxisomal β-oxidation might play a role in regulating mitochondrial fatty acid oxidation and inducing hepatic steatosis under the condition of obesity or diabetes. This study reveals a novel mechanism by which peroxisomal β-oxidation controls mitochondrial fatty acid oxidation in diabetic animals. We determined that excessive oxidation of fatty acids by peroxisomes generates considerable acetyl-carnitine in the liver of diabetic mice, which significantly elevates the mitochondrial acetyl-CoA/CoA ratio and causes feedback suppression of mitochondrial β-oxidation. Additionally, we found that specific suppression of peroxisomal β-oxidation enhances mitochondrial fatty acid oxidation by reducing acetyl-carnitine formation in the liver of obese mice. In conclusion, we suggest that induction of peroxisomal fatty acid oxidation serves as a mechanism for diabetes-induced hepatic lipid accumulation. Targeting peroxisomal β-oxidation might be a promising pathway in improving hepatic steatosis and insulin resistance as induced by obesity or diabetes.

Fasting induces hepatic lipid accumulation by stimulating peroxisomal dicarboxylic acid oxidation

Fasting induces lipid accumulation in the liver, while the mechanisms by which fasting dysregulates liver fatty acid oxidation are not clear. Fatty acid ω-oxidation is induced in the fasting state, and administration of dicarboxylic acids to fasting animals decreases plasma ketone bodies. We hypothesized that endogenous dicarboxylic acids might play a role in controlling mitochondrial β-oxidation in fasting animals. A peroxisome proliferator-activated receptor-alpha agonist and an inhibitor for peroxisomal β-oxidation were administered to the fasting rats to investigate the role of dicarboxylic acids in liver fatty acid oxidation and lipid homeostasis. We observed that excessive β-oxidation of endogenous dicarboxylic acids by peroxisomes generated considerable levels of succinate in the liver. Excessive succinate oxidation subsequently increased the mitochondrial NADH/NAD⁺ ratio and led to an accumulation of 3-OH-CoA and 2-enoyl-CoA intermediates in the liver. This further induced feedback suppression of mitochondrial β-oxidation and promoted hepatic lipid deposition and steatosis. Specific inhibition of peroxisomal β-oxidation attenuated fasting-induced lipid deposition in the liver by reducing succinate production and enhancing mitochondrial fatty acid oxidation. We conclude that suppression of mitochondrial β-oxidation by oxidation of dicarboxylic acids serves as a mechanism for fasting-induced hepatic lipid accumulation and identifies cross talk between peroxisomal and mitochondrial fatty acid oxidation.

Physical training reverses the increased activity of the hepatic ketone body synthesis pathway in chronically diabetic rats

This study was designed to examine whether the training-induced improvement in the plasma concentration of ketone bodies in experimental diabetes mellitus could be explained by changes in the activity of the hepatic ketone body synthesis pathway and/or the plasma free fatty acid levels. Diabetes mellitus was induced by an intravenous injection of streptozotocin (50 mg/kg), and training was carried out on a treadmill. The plasma concentration of beta-hydroxybutyric acid was increased (P < 0.001) in sedentary diabetic rats, and this was partly reversed by training (P < 0.001). The plasma concentration of free fatty acids was increased (P < 0.001) in sedentary diabetic rats, and this was reversed to normal by training (P < 0.001). Diabetes was also associated with an increased activity of the hepatic ketone body synthesis pathway. When the data are expressed as per total liver, physical training decreased the activity of the hepatic ketone body synthesis pathway by 18% in nondiabetic rats (P < 0.05) and by 22% in diabetic rats (P < 0.01), the activity present in trained diabetic rats being not statistically different from that of sedentary control rats. These data suggest that the beneficial effects of physical training on the plasma beta-hydroxybutyric acid levels in the diabetic state are probably explained in part by a decrease in the activity of the hepatic ketone body synthesis pathway and in part by a decrease in plasma free fatty acid levels.

Ketone formation in the intestinal mucosa of infant rats

The intestinal mucosa of infant rats was found to produce ketones when incubated in Krebs-Ringer-Bicarbonate solution. No production was found in weaned rats. Ketogenesis could be inhibited by D-carnitine or tetradecylglycidic acid (TDGA) an inhibitor of long-chain acylcarnitine transferase, suggesting that ketone production is due to a large extent to break-down of long-chain fatty acids. It is considered possible that both ketones and glucose (also produced by the infant mucosa) serve as substrates for the muscular part of the intestine.

In vitro hepatic gluconeogenesis and ketogenesis as affected by prolonged ketonemia-glucosuria and fasting in steers

Both 1,3-butanediol, which causes ketonemia, and phlorizin, which causes glucosuria, were given to four steers for 28 days to determine effects of prolonged ketonemia and glucosuria on in vitro hepatic gluconeogenesis and ketogenesis. Treatments were: control ration; control with butanediol plus phlorizin; and fasting for 9 days. Liver slices, obtained by biopsy, were incubated with carbon-14 substrates. Substrate converted to glucose [mumol/(h X g liver)] during control, butanediol plus phlorizin, and fasting averaged 2.34, 7.21, and 12.00 for propionate; .99, 3.80, and 12.26 for lactate; .30, .76, and 2.20 for alanine; and 2.06, 5.37, and 5.78 for glycerol. Omission of calcium++ eliminated increases of gluconeogenesis caused by butanediol plus phlorizin and by fasting. Ketone bodies, octanoate, and bovine serum albumin did not affect glucose production markedly. Stearate inhibited gluconeogenesis during all periods except fasting. Production of beta-hydroxybutyrate [mumol/(h X g liver)] during control, butanediol plus phlorizin, and fasting averaged 2.07, 4.27, and 3.25 from butyrate and .06, .27, and .02 from palmitate. Results demonstrate that the gluconeogenic capacity of bovine liver is responsive to physiological and nutritional status.

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.

"In vitro" study of liver slices lipid (1-14C) acetate incorporation in experimental diabetes

The effect of insulin deficiency on lipid synthesis in the liver of normal rats, diabetic rats by alloxan and pancreatectomized rats was studied in vitro using (1-14C) acetate as lipid precursor. Insulin deficiency induces an increased incorporation of (1-14C) acetate into triglycerides in rat liver. This is particularly evident in pancreatectomized rats with respect to alloxan diabetic rats. It is concluded that in experimental diabetes an atherogenous metabolic pattern is elaborated by the liver.

Comparative effects of chenodeoxycholic acid and ursodeoxycholic acid on lipid synthesis in rat liver

Chenodeoxycholic and ursodeoxycholic acids reduce significantly the hepatic synthesis of lipids in rats. The present study has been carried out using (1-14C)acetate and evaluating its incorporation into different lipidic fractions of the liver by thin-layer radiochromatography. Ursodeoxycholic acid proved to be more active than chenodeoxycholic acid: in addition to a significant decrease of the hepatic incorporation of the acetate into cholesterol and triglycerides an increase of the hepatic incorporation of the acetate into phospholipids has been observed. The exogenous administration of bile acids diminishes the hepatic synthesis of cholesterol and therefore its biliary excretion; it enriches the bile acid and phospholipid pool in the liver and bile. By this way the action of bile acids establishes in the liver a condition which induces such an increase of availability of mixed micelles in the bile as to make it unsaturated in cholesterol.

The role of glucagon in the regulation of plasma lipids

The role of glucagon in regulating plasma lipid concentrations (nonesterified fatty acids, ketone bodies, and triglycerides) is reviewed. The effects of glucagon-induced insulin secretion upon this lipid regulation are discussed that may resolve conflicting reports in the literature are resolved. In addition, the unresolved problem concerning the pharmacologic versus physiologic effects of glucagon is stressed. Glucagon's role in stimulating lipolysis at the adipocyte serves two important functions. First, it provides plasma nonesterified fatty acids for energy metabolism and secondly, it ensures substrate for hepatic ketogenesis. In vitro, glucagon's lipolytic activity has been consistently observed, but in vivo, this activity has sometimes been obscured by the effects of glucagon-induced insulin secretion. Frequently, a biphasic response has been reported in which a direct lipolytic response is followed by a glucagon-induced insulin suppression of plasma nonesterified fatty acid concentration. When the glucagon-induced insulin secretion has been controlled by various in vivo techniques, glucagon's lipolytic activity in vivo has frequently been demonstrable. In the 1960s, in vitro liver perfusion experiments demonstrated that glucagon enhanced hepatic ketogenesis independent of glucagon's lipolytic activity. However, this direct effect of glucagon on the hepatocyte was not universally accepted because of conflicting reports in the literature. Failure to observe an in vitro ketogenic effect of the hormone in some studies may have been due to suboptimal experimental conditions. Certain factors are now known to influence the ketogenic response, such as the concentration of fatty acids in the media and the nutritional status of the animal. Under optimal in vitro conditions with liver preparations from fed animals, the ketogenic response to physiologic concentrations of glucagon has been demonstrated. However, further study is necessary to define the quantitative ketogenic role of the hormone. In spite of this early in vitro work, glucagon was not definitely shown to be ketogenic in vivo (independent of fatty acid availability) both in the rat and in diabetic man until 1975. Since these observations, several reports have confirmed the ketogenic action of glucagon in vivo by direct hepatic catheterization experiments. Glucagon's role in decreasing hepatic triglyceride synthesis and secretion in vitro has been repeatedly shown but the mechanism is unresolved. This lipid regulatory action of glucagon has been more difficult to demonstrate in vivo because of the many variables that affect triglyceride synthesis. Under specific experimental conditions, however, glucagon has been shown to decrease plasma triglyceride concentration in man at both physiologic and pharmacologic concentrations. Hepatic catheterization experiments have also confirmed this effect in man. The regulation of lipids by glucagon fits well into its role as a stress hormone...

Relative utilization of fatty acids for synthesis of ketone bodies and complex lipids in the liver of developing rats

The regulation of hepatic ketogenesis, as related to the metabolism of fatty acids through oxidative and synthetic pathways, was studied in developing rats. [1-14C] palmitate was used as a substrate to determine the proportions of free fatty acids utilized for the production of ketone bodies, CO2 and complex lipids. Similar developmental patterns of hepatic ketogenesis were obtained by measuring the production of either [14C] acetoacetate from exogenous [1-14C] palmitate or the sum of unlabeled acetoacetate and beta-hydroxybutyrate from endogenous fatty acids. The production of total ketone bodies was low during the late fetal stage and at birth, but increased rapidly to a miximum value within 24 hr after brith. The maximal ketogenic capacity appeared to be maintained for the first 10 days of life. 14CO2 production from [1-14C] palmitate increased by two- to fourfold during the suckling period, from its initial low rate seen at birth. The capacity for synthesis of total complex lipids was low at birth and had increased by day 3 to a maximal value, which was comparable to that of adult fed rats. The high lipogenic capacity lasted throughout the remaining suckling period. When ketogenesis was inhibited by 4-pentenoic acid, the rate of synthesis of complex lipids did not increase despite an increase in unutilized fatty acids. During the mid-suckling period, approximately equal amounts of [1-14C] palmitate were utilized for the synthesis of ketone plus CO2 and for complex lipid synthesis. By contrast, in adult fed rats, the incorporation of fatty acids into complex lipids was four times higher than that of ketone plus CO2. These observations suggest that stimulated hepatic ketogenesis in suckling rats results from the rapid oxidation of fatty acids and consequent increased production of acetyl CoA, but not from impaired capacity for synthesis of complex lipids.

Ketogenesis in vertebrate livers

1. The hypothesis is advanced that a gluconeogenic organ such as the liver would evolve to oxidise fatty acids as its source of ATP for gluconeogenesis. It is also argued that such an organ might, in the light of current knowledge, be expected to be ketogenic. The animals investigated were lamprey, rainbow trout, eel, toad, axolotl, lizard and rat. 2. The respiratory quotients of liver slices from all animals was close to 0.74. Ketone bodies were produced from butyrate by all livers excepting the lamprey and ketone bodies were present in all blood samples examined. 3. There was no convincing evidence that direct deacylation of acetoacetyl CoA was important in any liver. HMGCoA synthase activity could not be found in the livers of the lamprey and eel. This enzyme was present in livers of the other animals. There was a large amount of acetoacetyl CoA-succinate transferase in the livers of the rainbow trout and eel, but only small amounts in the higher animals. 4. It is suggested that, initially the transferase was the important ketogenic pathway and the HMGCoA pathway evolved later.

Lactic acid formation in crustaceans and the liver function of the midgut gland questioned

1. The possibility of the midgut gland of the crustacean (Cherax destructor) functioning as a liver has been investigated. 2. Seven species of crustaceans accumulate lactic acid in the haemolymph when exercised. The rate of disappearance of lactate in Homarus gammarus and in C. destructor is very slow when compared with man. 3. In the midgut gland of C. destructor no firm evidence was obtained for gluconeogenesis from lactate and for ketogenesis from fatty acids. 4. It is concluded that there is at present no justification for the common practice of calling the midgut gland an hepatopancreas.

Enzymes involved in acetoacetate formation in various bovine tissues

1. The activities of acetoacetyl-CoA thiolase, hydroxymethylglutaryl-CoA synthase and lyase and acetoacetyl-CoA deacylase were measured in homogenates of samples of liver, rumen epithelium (long papillae), kidney and lactating mammary gland derived from slaughtered cows. 2. The activities of the four enzymes in bovine liver were similar to the activities previously reported for the corresponding enzymes in rat liver. 3. Acetoacetyl-CoA thiolase and hydroxymethylglutaryl-CoA synthase and lyase were present in rumen epithelium. The activities of the enzymes were all lower on a wet weight basis than in liver. Only very slight deacylase activity was detected. 4. Kidney contained acetoacetyl-CoA thiolase, hydroxymethylglutaryl-CoA lyase and acetoacetyl-CoA deacylase, but only trace amounts of hydroxymethylglutaryl-CoA synthase. 5. Mammary gland contained acetoacetyl-CoA thiolase and some hydroxymethylglutaryl-CoA lyase, but virtually no hydroxymethylglutaryl-CoA synthase or acetoacetyl-CoA deacylase. 6. Since physiologically significant ketogenesis probably occurs solely via the hydroxymethylglutaryl-CoA pathway, it is evident that, of the four tissues examined, such ketogenesis must be restricted to the liver and the rumen epithelium. 7. All the enzymes except hydroxymethylglutaryl-CoA lyase were also assayed in the four tissues derived from cows suffering from bovine lactational ketosis. Ketosis did not cause a statistically significant change in the activity of any of the enzymes measured. 8. Hepatic hydroxymethylglutaryl-CoA synthase and lyase were found to be associated mainly with the particulate fraction, as in the rat. A considerably greater proportion of these enzymes was found to be present in the cytoplasmic fraction from rumen epithelium, although it was not excluded that this was due to mitochondrial damage during homogenization. 9. Appreciable hydroxymethylglutaryl-CoA synthase was also present in epithelium from the dorsal region of the rumen, from the reticulum and from the omasum, but not from the abomasum.