Compartmentation of fatty acid oxidation in liver cells

Mechanism of Free Fatty Acid Effects on Hepatocyte Insulin Receptor Binding and Processing

We determined whether the palmitate effects on hepatocyte insulin receptor binding and post-receptor trafficking were mediated by accelerated mitochondrial beta-oxidation or accumulation of intracellular fatty acyl-CoA derivatives and possibly protein acylation. Preincubation of hepatocytes with moderate concentrations of palmitate (0.5 mM) resulted in a 23% decline in cell-surface binding and proportional decreases in receptor-mediated insulin internalization and degradation. Brief pretreatment of hepatocytes with the carnitine palmityltransferase-I inhibitor, methyl palmoxirate (MP), prevented 70% of the palmitate effects. At higher palmitate concentrations (2.0 mM), cell-surface binding was reduced by 34%, whereas internalization of the receptor complex was reduced by 78%. These effects were only partially prevented by MP pretreatment. Receptor-mediated insulin degradation increased by 34% and was uninfluenced by MP pretreatment. Octanoate, which is rapidly shunted into mitochondrial oxidation, produced a dose-dependent reduction in insulin binding, with proportional decreases in internalization and degradation. Similarly preincubation with 2.0 mM oleate, which, unlike palmitate, is not known to produce protein acylation, resulted in proportional decreases in insulin receptor binding and receptor-mediated internalization and degradation. High concentrations of octanoate or oleate (2.0 mM) did not reproduce the additive post-receptor effects of palmitate. We conclude that the receptor and post-receptor effects of moderate palmitate concentrations are closely linked to accelerated fatty acid oxidation. The post-receptor effects observed at higher concentrations involve other mechanisms, possibly relating to intracellular levels of palmityl-CoA derivatives.

Stimulation of oxygen consumption following addition of lipid substrates in liver and skeletal muscle from rats fed a high-fat diet

We studied hepatic and skeletal muscle metabolic activity in rats fed a high-fat diet. Rats were fed a low-fat or high-fat diet for 15 days. At the end of the experimental period, full energy-balance determinations together with serum free triiodothyronine (FT3), leptin, and free fatty acid (FFA) measurements were performed. In addition, we assessed fatty acid-stimulated oxygen consumption in perfused liver and in skeletal muscle homogenate. Rats fed a high-fat diet showed a significant increase in energy intake but no variation in body energy gain, due to a significant increase in energy expenditure. Serum FT3 and FFA levels significantly increased in rats fed a high-fat diet versus rats fed a low-fat diet, while no variation was found in serum leptin levels. Perfused livers and skeletal muscle homogenates from rats fed a high-fat diet exhibited a significant increase in fatty acid-stimulated oxygen consumption. Our results suggest that the enhanced fatty acid oxidation rates in liver and skeletal muscle contribute to the maintenance of fat balance in response to increased fat intake, preventing excess fat deposition.

Proliferation of myocardial peroxisomes in experimental rat diabetes: a biochemical and immunocytochemical study

Myocardial peroxisomes were investigated in normal and diabetic rats. Catalase and acyl-CoA oxidase activities were increased in the diabetic rat heart and immunoblot analysis showed that both enzyme proteins were markedly enhanced in diabetic heart homogenates. After immunoenzyme staining, catalase and acyl-CoA oxidase were localized in fine granules in the myocardium, which were increased in number in diabetic rats. The numerical density of the granules stained for catalase was increased 1.7 times and that for acyl-CoA oxidase 1.8 times, compared with controls. Protein A-gold labeling for catalase and acyl-CoA oxidase was present in myocardial peroxisomes. The labeling density for both enzymes was increased in diabetic rats by 1.6 times for catalase and 1.5 times for acyl-CoA oxidase, compared with controls. The results indicate that myocardial peroxisomes are increased in the diabetic rat and that this proliferation is accompanied by an increase in catalase and acyl-CoA oxidase activities.

Peroxisomal oxidases in various tissues of diabetic rats

The effect of diabetes mellitus induced by streptozotocin on the activities of peroxisomal oxidases and H2O2-metabolizing enzymes, and lipid peroxidation in various rat tissues were investigated. Peroxisomal acyl-CoA oxidase, D-amino acid oxidase and L-alpha-hydroxyacid oxidase were measured by a sensitive spectrophotometric method using dichlorofluorescein/peroxidase as the detector of H2O2. Acyl-CoA oxidase activity was increased most markedly in the heart of diabetic rats, less markedly in the liver, and tended to be increased in the kidneys. The activities of other peroxisomal oxidases were much lower than that of acyl-CoA oxidase in the liver and kidneys, and were undetectable in the heart. Catalase activity was decreased in the liver and kidneys of diabetics, and was increased in the heart. Glutathione peroxidase activity was increased more markedly in the kidneys of the diabetics, and less markedly in the heart than in the liver. Lipid peroxide level was higher in the kidneys of the diabetics than in the controls, unchanged in the heart, and was lower in the liver of the diabetics than in the controls. Thus, peroxisomal beta-oxidation and the H2O2 production coupled with that, were activated in various tissues of diabetic rats, presumably as a part of the overall increase in lipid oxidation. However, they did not appear to contribute to the enhanced oxidative stress induced by diabetes mellitus.

On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

Octanoate applied to rat liver mitochondria respiring with glutamate plus malate or succinate (plus rotenone) under resting-state (State 4) conditions stimulates oxygen uptake and decreases the membrane potential, both effects being sensitive to oligomycin but not to carboxyatractyloside. Octanoate also decreases the rate of pyruvate carboxylation under the same conditions, this effect being correlated with the decrease of intramitochondrial content of ATP and increase of AMP. The decrease of pyruvate carboxylation and the change of mitochondrial adenine nucleotides are both reversed by 2-oxoglutarate. Fatty acids of shorter chain length have similar effects, though at higher concentrations. Addition of octanoate in the presence of fluoride (inhibitor of pyrophosphatase) produces intramitochondrial accumulation of pyrophosphate, even under conditions when oxidation of octanoate is prevented by rotenone. In isolated hepatocytes incubated with lactate plus pyruvate, octanoate also increases oxygen uptake and produces a shift in the profile of adenine nucleotides similar to that observed in isolated mitochondria. It decreases the 'efficiency' of gluconeogenesis, as expressed by the ratio between an increase of glucose production and an increase of oxygen uptake upon addition of gluconeogenic substrates (lactate plus pyruvate), and increases the reduction state of mitochondrial NAD. These effects taken together are not compatible with uncoupling, but point to intramitochondrial hydrolysis of octanoyl-CoA and probably also shorter chain-length acyl-CoAs. This mechanism probably functions as a 'safety valve' preventing a drastic decrease of intramitochondrial free CoA under a large supply of medium- and short-chain fatty acids.

Quantitation of ketogenesis in periportal and pericentral regions of the liver lobule

A method has been devised to quantitate rates of ketogenesis (acetoacetate + beta-hydroxybutyrate production) in discrete regions of the liver lobule based on changes in NADH fluorescence. In perfused livers from fasted rats, ketogenesis was inhibited nearly completely with either 2-bromoctanoate (600 microM) or 2-tetradecylglycidic acid (25 microM). During inhibition of ketogenesis, a linear relationship (r = 0.90) was observed between decreases in NADH fluorescence detected from the liver surface and decreases in ketone body production. NADH fluorescence was monitored subsequently from individual regions of the liver lobule by placing microlight guides on periportal and pericentral regions of the liver lobule visible on the liver surface. Rates of ketogenesis in sublobular regions were calculated from regional decreases in NADH fluorescence and changes in the rate of ketone body formation by the whole liver during infusion of inhibitors. In the presence of bromoctanoate, ketogenesis was reduced 80% and local rates of ketogenesis were decreased 31 +/- 4 mumol/g/h in periportal areas and 28 +/- 3 mumol/g/h in pericentral regions. Similar results were observed with tetradecylglycidic acid. Therefore, it was concluded that submaximal rates of ketogenesis from endogenous, mainly long-chain fatty acids are nearly equal in periportal and pericentral regions of the liver lobule in liver from fasted rats. Rates of ketogenesis and NADH fluorescence were strongly correlated during fatty acid infusion. Infusion of 250 microM oleate increased NADH fluorescence maximally by 8 +/- 1% over basal values in periportal regions and 17 +/- 4% in pericentral areas. Local rates of ketogenesis, calculated from these changes in fluorescence, increased 35 +/- 6 mumol/g/h in periportal areas and 55 +/- 5 mumol/g/h in pericentral regions. Thus, oleate stimulated ketogenesis nearly 60% more in pericentral than in periportal regions of the liver lobule.

Metabolic pathways for ketone body production. 13C NMR spectroscopy of rat liver in vivo using 13C-multilabeled fatty acids

The hormonal regulation of ketogenesis in the liver of living rat has been studied noninvasively with 13C nuclear magnetic resonance. The protocol involved the use of a surface coil that was placed on the skin of the rat, directly over the normal location of the liver. Signals from superficial tissue were suppressed with a 180 degrees pulse at the center of the coil. A resolution of 0.6 ppm was obtained in the 13C NMR spectra at 20.1 MHz, which was equal to or better than that observed in experiments where the liver was surgically exposed and surrounded with radiofrequency coil. The spatial selection for the liver was better than 90%, with extrahepatic adipose tissue contributing only a very small amount of signal. The metabolic activities of the liver were investigated by infusion of 13C-labeled butyrate in the jugular vein of the anesthetized rat. The rate of butyrate infusion was chosen to be close to the maximum oxidative capacity of the rat liver, and the 13C signal intensities were enhanced by using doubly labeled [1,3-13C]butyrate as a substrate. Different 13C NMR spectra and hence different metabolites were observed depending on the hormonal state of the animal. In the fasted rat, the most intense 13C signal came from the end product of the Krebs cycle, namely, HCO3, with additional resonances from glutamine and glutamate. Weak resonances of the ketone bodies 3-hydroxybutyrate and acetoacetate could also be detected and allowed an evaluation of the "redox state" of the in vivo liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of long-chain fatty acids on the inhibition by antimycin of respiration in hepatocytes and isolated mitochondria from rat liver

A previous study [Berry, M. N., Gregory, R. B., Grivell, A. R. & Wallace, P. G. (1983) Eur. J. Biochem. 131, 215-222] suggested that long-chain fatty acid (palmitate) oxidation by hepatocytes was less sensitive than short-chain fatty acid (hexanoate) oxidation to inhibition by a given concentration of antimycin. Re-examination of this phenomenon showed that palmitate oxidation by hepatocytes could be depressed by antimycin to the same degree as other NAD+-linked substrates, only if the concentration of the inhibitor was raised 2-4-fold. The presence of palmitate also reduced the sensitivity to antimycin of hepatocytes metabolizing lactate or pyruvate. Over the range of fatty acids tested, butyrate (C4) to stearate (C18), only long-chain (greater than C10) fatty acids endowed cells with decreased sensitivity towards antimycin. 2-Bromopalmitate, a non-metabolizable fatty acid, and inhibitor of fatty acid oxidation, also decreased the inhibitory effect of antimycin in cells, suggesting that long-chain fatty acids per se rather than their metabolites, reverse the inhibition by antimycin. Moreover, another inhibitor of fatty acid oxidation, 2-tetradecylglycidic acid, did not diminish the effects of palmitate. Succinate oxidation in isolated mitochondria that had been inhibited by a low concentration of antimycin could be restored by subsequent addition of palmitate or other long-chain fatty acids such as dodecanoate, tetradecanoate and oleate under conditions where fatty acid oxidation was prevented. 2-Bromopalmitate, likewise partially restored antimycin-depressed succinate oxidation. This amelioration of antimycin inhibition was counteracted by the addition of more antimycin and was not seen upon addition of defatted bovine serum albumin, palmitoylcarnitine or octanoate. The total amount of antimycin bound to mitochondria was not affected by the presence of palmitate. The data suggest that long-chain fatty acids are able to interact with the mitochondrial inner membrane in a manner which can relieve the inhibitory effect of antimycin, whether the antimycin is added to the cell or mitochondrial suspension before or after fatty acid addition.

The mechanism of the hormonal activation of respiration in isolated hepatocytes and its importance in the regulation of gluconeogenesis

The effects of hormones on the cytochrome spectra of isolated hepatocytes were recorded under conditions of active gluconeogenesis from L-lactate. Glucagon, phenylephrine, vasopressin and valinomycin, at concentrations that caused stimulation of gluconeogenesis, increased the reduction of the components of the cytochrome bc1 complex, just as has been observed in liver mitochondria isolated from glucagon-treated rats [Halestrap (1982) Biochem. J. 204, 37-47]. The effects of glucagon and phenylephrine were additive. The time courses of the increased reduction of cytochrome c/c1 and NAD(P)H/NAD(P)+ caused by hormones, valinomycin, A23187 and ethanol were measured by dual-beam spectrophotometry and fluorescence respectively. Ethanol (14 mM) produced a substantial rise in NAD(P)H fluorescence, beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios, no change in cytochrome c/c1 reduction, a 10% decrease in O2 consumption and a 60% decrease in gluconeogenesis. Glucagon, phenylephrine and vasopressin caused a substantial and transient rise in NAD(P)H fluorescence, but a sustained increase in cytochrome c/c1 reduction and the rates of O2 consumption and gluconeogenesis. The transience of the fluorescence response was greater in the absence of Ca2+, when the cytochrome c/c1 response also became transient. The fluorescence response was smaller and less transient, but the cytochrome c/c1 response was greater, in the presence of fatty acids. Both responses were greatly decreased by the presence of 1 mM-pent-4-enoate. Valinomycin (2.5 nM) caused a decrease in NAD(P)H fluorescence coincident with an increase in cytochrome c/c1 reduction and the rate of gluconeogenesis and O2 consumption. A23187 (7.5 mM) caused increases in both NAD(P)H fluorescence and cytochrome c/c1 reduction. The effects of hormones and valinomycin on the time courses of NAD(P)H fluorescence, cytochrome c/c1 reduction and light-scattering by hepatocytes were compared with those of 0.5 microM-Ca2+ or 1 nM-valinomycin on the same parameters of isolated liver mitochondria. It is concluded that hormones increase respiration by hepatocytes in a biphasic manner. An initial Ca2+-dependent activation of mitochondrial dehydrogenases rapidly increases the mitochondrial [NADH], which is followed by a volume-mediated stimulation of fatty acid oxidation and electron flow between NADH and cytochrome c. 10. Amytal (0.5 mM) was able to reverse the effects of hormones on the reduction of cytochromes c/c1 and the rates of gluconeogenesis and O2 consumption without significantly lowering tissue [ATP].(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism for the oleate stimulation of gluconeogenesis from dihydroxyacetone by hepatocytes from fasted rats

Oleate stimulates glucose production and concomitantly decreases lactate and pyruvate production by rat hepatocyte suspensions incubated with dihydroxyacetone as substrate. The actions of oleate could be blocked by D-(+)dodecanoylcarnitine, which inhibits transport of the fatty acid into the mitochondria and the subsequent oxidation. beta-Hydroxybutyrate, but not acetoacetate, also stimulated glucose synthesis and inhibited lactate and pyruvate production. Furthermore, both beta-hydroxybutyrate and oleate stimulated oxygen consumption to the same extent. This suggests that oleate stimulates glucose production by the provision of energy subsequent to mitochondrial beta-oxidation of the fatty acids. The content of ATP itself did not appear to be responsible for the effects of oleate. Crossover analysis of the gluconeogenic intermediates implicated a site of oleate action between fructose 1,6-bisphosphate and fructose 6-phosphate, suggesting phosphofructokinase and/or fructose-bisphosphatase as possible regulatory sites. Coupled with the finding that intracellular citrate accumulates upon addition of oleate or beta-hydroxybutyrate, but not acetoacetate, the results suggest that citrate inhibition of phosphofructokinase accounts for the redirection of carbon flow from lactate and pyruvate formation and towards that of glucose.

Peroxisomal fatty acid oxidation in rat and human tissues. Effect of nutritional state, clofibrate treatment and postnatal development in the rat

Oxidation of palmitate 14C-labeled in different positions was assayed in the absence and presence of antimycin and rotenone in homogenates of various rat and human tissues to determine total and peroxisomal oxidation and acetyl group production. Total and antimycin-insensitive palmitate oxidation rates were higher in rat heart, liver and quadriceps muscle than in the corresponding human tissues. The proportion of antimycin-insensitive oxidation of [1-14C]palmitate was 17-35% in tissues of starved rats and in human muscles and fibroblasts, but peroxisomal production of acetyl groups amounted only to 5-11% of that by mitochondria. The mean number of peroxisomal beta-oxidation cycles was 1.5-2.5 per palmitate molecule. The nutritional state markedly influenced the total oxidation rate and the antimycin-insensitive proportion in rat liver. Clofibrate feeding increased total and antimycin-insensitive oxidation rates in liver, heart and kidney, but not in quadriceps muscle. Total oxidation capacity was maximal in rat liver at weaning, and in rat heart at an age of 70 days. Antimycin-insensitive oxidation rates increased in rat liver and heart at postnatal development up to weaning. A marked proportion of lignocerate oxidation was antimycin-sensitive in rat tissues.

Mechanism of the stimulation of respiration by fatty acids in rat liver

The mechanism of stimulation of hepatic respiration by fatty acids was studied in isolated rat hepatocytes. Stimulation of respiration by fatty acids varied from about 35% to about 105% depending on chain length. The stimulatory effect of octanoate (1 mM) or oleate (0.5 mM) was prevented by oligomycin (2 micrograms/ml). With carboxyatractyloside (100 microM) and ouabain (2 mM) the stimulation of respiration was partially inhibited (by 50-70 and 50-60%, respectively). From these results it can be concluded that the increased rate of respiration after addition of fatty acids is coupled to ATP synthesis. A large part (50-60%) of this ATP is utilized by the (Na+ + K+)-ATPase.

Flux of palmitate through the peroxisomal and mitochondrial beta-oxidation systems in isolated rat hepatocytes

Peroxisomes catalyze the beta-oxidation of fatty acids but their quantitative role in fatty acid catabolism in the intact hepatocyte is not yet clarified. In the present study peroxisomal beta-oxidation of [1-14C]palmitate was quantitated in hepatocytes without the use of metabolic inhibitors. It was assumed that acetyl-CoA formed by peroxisomal beta-oxidation enters the cytosolic pool of acetyl-CoA, whereas that from mitochondrial beta-oxidation enters the mitochondrial pool. The labeling of the two acetyl-CoA pools was assessed by measuring the incorporation of radioactivity into cholesterol (from cytosolic acetyl-CoA) and CO2 (from mitochondrial acetyl-CoA). The system was calibrated with [1-14C]acetate and [1-14C]butyrate because butyrate undergoes beta-oxidation only in mitochondria, whereas acetate forms acetyl-CoA primarily in the cytosol. The labeling ratio, [( 14C]cholesterol X 100)/[( 14C]cholesterol + [14C]CO2), reflects the site of formation of acetyl-CoA. This ratio was 0.51 for butyrate, 1.39 for acetate and 0.79 for palmitate. The difference between palmitate and butyrate was statistically significant (P less than 0.02). This indicates that not all of the palmitate was oxidized in mitochondria. By linear interpolation it was estimated that approximately 32% of the [1-14C]palmitate oxidation began in peroxisomes.

The contribution of hepatic metabolism to diet-induced thermogenesis

In order to examine the possible contribution of the liver to diet-induced thermogenesis, we examined the metabolism of hepatocytes from rats that had been fed a varied choice of highly palatable human food items ("cafeteria feeding"). Liver cells derived from cafeteria-fed rats that had been fasted for 20 hours showed marked increases in rates of respiration and gluconeogenesis in the presence of glycerol or sorbitol. These cells were also much less sensitive to the inhibitory effects of rotenone than were hepatocytes of control animals. hepatocytes from fasted cafeteria-fed rats also demonstrated a substantially enhanced rate of fatty acid oxidation and ketogenesis, which did not appear to be correlated with cellular demands for adenosine triphosphate (ATP). This apparent fall in metabolic efficiency was confirmed by calorimetric studies, which indicated augmented cellular heat production. These changes in hepatic metabolism, associated with cafeteria-feeding, suggest that the liver may have a significant role in diet-induced thermogenesis.