High fat intake lowers hepatic fatty acid synthesis and raises fatty acid oxidation in aerobic muscle in Shetland ponies

The metabolic effects of feeding soyabean oil instead of an isoenergetic amount of maize starch plus glucose were studied in ponies. Twelve adult Shetland ponies were given a control diet (15 g fat/kg DM) or a high-fat diet (118 g fat/kg DM) according to a parallel design. The diets were fed for 45 d. Plasma triacylglycerol (TAG) concentrations decreased by 55 % following fat supplementation. Fat feeding also reduced glycogen concentrations significantly by up to 65 % in masseter, gluteus and semitendinosus muscles (P < 0.05 and P < 0.01 and P < 0.01 respectively). The high-fat diet significantly increased the TAG content of semitendinosus muscle by 80 % (P < 0.05). Hepatic acetyl-CoA carboxylase and fatty acid synthase activities were 53 % (P < 0.01) and 56 % (P < 0.01) lower respectively in the high-fat group, but diacylglycerol acyltransferase activity was unaffected. Although carnitine palmitoyltransferase-I (CPT-I) activity in liver mitochondria was not influenced, fat supplementation did render CPT-I less sensitive to inhibition by malonyl-CoA. There was no significant effect of diet on the activity of phosphofructokinase in the different muscles. The activity of citrate synthase was raised significantly (by 25 %; P < 0.05) in the masseter muscle of fat-fed ponies, as was CPT-I activity (by 46 %; P < 0.01). We conclude that fat feeding enhances both the transport of fatty acids through the mitochondrial inner membrane and the oxidative capacity of highly-aerobic muscles. The higher oxidative ability together with the depressed rate of de novo fatty acid synthesis in liver may contribute to the dietary fat-induced decrease in plasma TAG concentrations in equines.

The AMP-activated protein kinase prevents ceramide synthesis de novo and apoptosis in astrocytes

Fatty acids induce apoptosis in primary astrocytes by enhancing ceramide synthesis de novo. The possible role of the AMP-activated protein kinase (AMPK) in the control of apoptosis was studied in this model. Long-term stimulation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) prevented apoptosis. AICAR blunted fatty acid-mediated induction of serine palmitoyltransferase and ceramide synthesis de novo, without affecting fatty acid synthesis and oxidation. Prevention of ceramide accumulation by AICAR led to a concomitant blockade of the Raf-1/extracellular signal-regulated kinase cascade, which selectively mediates fatty acid-induced apoptosis. Data indicate that AMPK may protect cells from apoptosis induced by stress stimuli.

Lipid metabolism in equines fed a fat-rich diet

The hypothesis tested was that dietary fat, when compared with an isoenergetic amount of non-structural carbohydrates, stimulates lipolysis in adipose tissue and also stimulates the fatty-acid oxidative capacity in skeletal muscle from horses. Six adult horses were fed a high-fat, glucose or starch containing diet according to a 3 x 3 Latin square design with feeding periods of three weeks. The diets were formulated so that the intake of soybean oil versus either glucose or corn starch were the only variables. In accordance with previous work, whole plasma triacylglycerol (TAG) concentration decreased significantly by 58% following fat supplementation. This fat effect was accompanied by a 247% increase in lipoprotein lipase (LPL) activity in post-heparin plasma. The dietary variables did neither significantly affect the basal in vitro lipolytic rate nor the lipolytic rate after adding noradrenaline. There was no significant diet effect on the activities of hexokinase and phosphofructokinase as indicators of glycolytic flux and citrate synthase and 3-hydroxy-acyl-CoA dehydrogenase as indicators of fatty-acid oxidative capacity. The concentrations of muscle glycogen and TAG were not affected by fat supplementation. It is concluded that our hypothesis is not supported by the present results.

Consumption of olive oil has opposite effects on plasma total cholesterol and sphingomyelin concentrations in rats

The hypothesis that olive-oil consumption alters plasma sphingomyelin concentrations and hepatic sphingomyelin metabolism was tested. Rats were fed on purified, high-cholesterol diets with either coconut fat or olive-oil (180 g/kg). In accordance with previous work, olive-oil v. coconut-fat consumption significantly elevated hepatic and total plasma cholesterol concentrations. During the course of the experiment, the concentration of plasma sphingomyelin rose in the coconut-fat group and remained constant in the olive-oil group. When compared with the coconut-fat-fed group, the plasma sphingomyelin levels were significantly lower in the olive-oil-fed group after 14 and 21 d of treatment. Dietary olive oil raised the amounts of cholesterol and sphingomyelin in the VLDL density region, and this change was associated with a reduction in the cholesterol and sphingomyelin contents of the LDL and HDL density ranges. Olive-oil consumption reduced the activity of serine palmitoyltransferase, while the activities of phosphatidylcholine:ceramide cholinephosphotransferase and phosphatidylethanolamine:ceramide ethanolaminephosphotransferase were left unchanged. Dietary olive oil also enhanced the activity of acidic sphingomyelinase, but not that of neutral sphingomyelinase. The present data indicate that dietary olive oil v. coconut fat has opposite effects on total plasma cholesterol and sphingomyelin concentrations. The lower plasma sphingomyelin levels observed in olive-oil-fed, as compared with coconut-fat-fed rats, may be explained by a simultaneous elevation and reduction in sphingomyelin catabolism and synthesis respectively, as based on the measured enzyme activities.

Do cytoskeletal components control fatty acid translocation into liver mitochondria?

For two decades it has been assumed that inhibition of carnitine palmitoyltransferase I (CPT-I) by malonyl-CoA represents the main regulatory mechanism of liver ketogenesis. However, recent evidence indicates that CPT-I activity is also controlled by interactions between mitochondria and cytoskeletal components. This newly recognized mechanism emphasizes the emerging role of the cytoskeleton in the regulation of metabolic pathways.

Interrelation of fatty acid composition in adipose tissue, serum, and liver of dairy cows during the development of fatty liver postpartum

We investigated the composition of fatty acids in adipose tissue, serum, and liver of cows that were fed at restricted energy intake or were overfed during the dry period. Overfed cows had higher concentrations of serum nonesterified fatty acids and consequently accumulated greater amounts of triacylglycerols in the liver than did cows that were fed at restricted energy intake. The percentages of the different fatty acids present in adipose tissue were similar for both groups and did not change during sampling intervals. Before parturition, concentrations of the individual fatty acids present in serum were similar between groups. After parturition, concentrations of major fatty acids in serum, including palmitic, stearic, oleic, and linoleic acids significantly increased in both groups and were higher in overfed cows than in cows that were fed at restricted energy intake. The shift of concentrations of the different fatty acids present in the liver--as a result of increased lipolysis-was observed in palmitic, oleic, and linoleic acids but not stearic acid, suggesting that stearic acid is used by the liver (i.e., oxidation) or is considerably secreted through the milk, thereby not increasing in accumulation in the liver. In conclusion, different feeding regimens during the dry period do not influence the composition of fatty acids in adipose tissue. More intensive lipolysis results in increased concentrations of palmitic, stearic, oleic, and linoleic acids in the blood; subsequently, these fatty acids, excluding stearic acid, greatly accumulated in the liver.

Consumption of casein instead of soybean protein produces a transient rise in the concentration of sphingomyelin in VLDL in rats

In rats fed cholesterol-rich diets, dietary casein vs. soybean protein raises VLDL cholesterol concentrations. Because sphingomyelin may be an essential, structural component of VLDL, we tested whether casein feeding would raise VLDL-sphingomyelin. Rats were fed cholesterol-rich semipurified diets containing either soybean protein (35 g/100 g) or casein for up to 21 d. Consistent with previous work, casein consumption increased hepatic and VLDL cholesterol concentrations. Dietary casein also significantly raised the amount of sphingomyelin in the VLDL fraction, but this effect was transient. Casein feeding transiently lowered LDL- and HDL-2-sphingomyelin concentrations. We suggest that an increase in hepatic VLDL secretion after casein consumption imposed an increased demand for sphingomyelin in the liver. The activity of key enzymes of sphingomyelin synthesis, i.e., serine palmitoyltransferase, phosphatidylcholine:ceramide phosphocholinetransferase and phosphatidylethanolamine:ceramide phosphoethanolaminetransferase and sphingomyelin degradation, i.e., acid sphingomyelinase, were enhanced and depressed, respectively, by casein consumption. Again these effects were transient. Thus, these data indicate that the extra sphingomyelin needed after short-term casein feeding came about through enhanced rates of biosynthesis and reduced rates of degradation in the liver. In addition, plasma transfer of sphingomyelin from HDL-2 to VLDL might have contributed to the increase in VLDL sphingomyelin in the casein-fed rats. This study shows that dietary casein vs. soybean protein transiently influences sphingomyelin metabolism in rats.

Cloning and expression of CTP:phosphoethanolamine cytidylyltransferase cDNA from rat liver

CTP:phosphoethanolamine cytidylyltransferase (ET) is a key regulatory enzyme in the CDP-ethanolamine pathway for phosphatidylethanolamine synthesis. As a first step in the elucidation of the structure-function relationship and the regulation of ET, an ET cDNA was cloned from rat liver. The cloned cDNA encodes a protein of 404 amino acid residues with a calculated molecular mass of 45.2 kDa. The deduced amino acid sequence is very similar to that of human ET (89% identity). Furthermore, it shows less, but significant, similarity to yeast ET as well as to other cytidylyltransferases, including rat CTP:phosphocholine cytidylyltransferase and Bacillus subtilis glycerol-3-phosphate cytidylyltransferase. Like human and yeast ET, rat ET has a large repetitive internal sequence in the N- and C-terminal halves of the protein. Both parts of the repeat contain the HXGH motif, the most conserved region in the N-terminal active domain of other cytidylyltransferases, indicating the existence of two catalytic domains in ET. The hydropathy profile revealed that rat ET is largely hydrophilic and lacks a hydrophobic stretch long enough to span a bilayer membrane. There was no prediction for an amphipathic alpha-helix. Transfection of COS cells with the cDNA clone resulted in an 11-fold increase in ET activity, corresponding to an increase in the amount of ET protein as detected on a Western blot. Determination of the ET activity during liver development showed a 2. 5-fold increase between day 17 of gestation and birth (day 22) and the amount of ET protein changed accordingly. Northern blot analysis showed that this was accompanied by an increase in the amount of ET mRNA. Between day 17 of gestation and birth, the amount of mRNA in fetal rat liver increased approx. 6-fold, suggesting the regulation of ET at both pretranslational and post-translational levels during rat liver development.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis in rat-2 fibroblasts by cell-permeable ceramides

Phospholipids and sphingolipids are important precursors of lipid-derived second messengers such as diacylglycerol and ceramide, which participate in several signal transduction pathways and in that way mediate the effects of various agonists. The cross-talk between glycerophospholipid and sphingolipid metabolism was investigated by examining the effects of cell-permeable ceramides on phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) synthesis in Rat-2 fibroblasts. Addition of short-chain C6-ceramide to the cells resulted in a dose- and time-dependent inhibition of the CDP-pathways for PtdCho and PtdEtn synthesis. Treatment of cells for 4 h with 50 microM C6-ceramide caused an 83% and a 56% decrease in incorporation of radiolabelled choline and ethanolamine into PtdCho and PtdEtn, respectively. Exposure of the cells for longer time-periods (>/= 16 h) to 50 microM C6-ceramide resulted in apoptosis. The structural analogue dihydro-C6-ceramide did not affect PtdCho and PtdEtn synthesis. In pulse-chase experiments, radioactive choline and ethanolamine accumulated in CDP-choline and CDP-ethanolamine under the influence of C6-ceramide, suggesting that synthesis of both PtdCho and PtdEtn were inhibited at the final step in the CDP-pathways. Indeed, cholinephosphotransferase and ethanolaminephosphotransferase activities in membrane fractions from C6-ceramide-treated cells were reduced by 64% and 43%, respectively, when compared with control cells. No changes in diacylglycerol mass levels or synthesis of diacylglycerol from radiolabelled palmitate were observed. It was concluded that C6-ceramide affected glycerophospholipid synthesis predominantly by inhibition of the step in the CDP-pathways catalysed by cholinephosphotransferase and ethanolaminephosphotransferase.

Effect of overfeeding during the dry period on the rate of esterification in adipose tissue of dairy cows during the periparturient period

The in vitro rate of esterification of fatty acids in adipose tissue was compared between cows that were fed at restricted energy intake and cows that were overfed during the dry period. Subcutaneous adipose tissue was biopsied at -1, 0.5, 1, and 3 wk from parturition. The basal in vitro rate of esterification was quantified, as well as the rate of esterification after the addition of glucose or glucose plus insulin. The basal rate in adipose tissue from overfed cows at -1 wk was higher than in adipose tissue from cows that were fed at restricted energy intake and indicated enhanced storage of triacylglycerols in adipose tissue of overfed cows at that time. The rate of esterification after the addition of glucose or glucose plus insulin was increased in both groups at each sampling time, but the mean rates, expressed as a percentage of the basal rates, were lower for overfed cows than for cows that were fed at restricted energy intake at 0.5 and 1 wk. Although the addition of glucose or glucose plus insulin increased esterification rates in adipose tissue from both groups of cows, adipose tissue from overfed cows was less sensitive to the addition of these compounds. In conclusion, overfeeding during the dry period predisposed cows to accumulate fat in adipose tissue during the prepartum period. The smaller increase in esterification rate after the addition of glucose or glucose plus insulin in adipose tissue of overfed cows indicates a lower ability of the adipose tissue to esterify circulating fatty acids or to reesterify mobilized fatty acids, which, combined with higher rates of lipolysis postpartum, contributes to continuously elevated concentrations of circulating nonesterified fatty acids postpartum, leading to a more severe hepatic lipidosis in overfed cows.

Metabolism of trans fatty acids by hepatocytes

The present work was undertaken to study the metabolism of fatty acids with trans double bonds by rat hepatocytes. In liver mitochondria, elaidoyl-CoA was a poorer substrate for carnitine palmitoyltransferase I (CPT-I) than oleoyl-CoA. Likewise, incubation of hepatocytes with oleic acid produced a more pronounced stimulation of CPT-I than incubation with trans fatty acids. This was not due to a differential effect of cis and trans fatty acids on acetyl-CoA carboxylase (ACC) activity and malonyl-CoA levels. Elaidic acid was metabolized by hepatocytes at a higher rate than oleic acid. Surprisingly, compared to oleic acid, elaidic acid was a better substrate for mitochondrial and, especially, peroxisomal oxidation, but a poorer substrate for cellular and very low density lipoprotein triacylglycerol synthesis. Results thus show that trans fatty acids are preferentially oxidized by hepatic peroxisomes, and that the ACC/malonyl-CoA/CPT-I system for coordinate control of fatty acid metabolism is not responsible for the distinct hepatic utilization of cis and trans fatty acids.

Effect of fatty liver on hepatic gluconeogenesis in periparturient dairy cows

The purpose of this study was to compare the hepatic enzyme activities of gluconeogenesis between control cows and experimental cows that had been overfed during the dry period to induce fatty liver postpartum. Blood and liver samples were collected 1 wk before and 0.5, 1, 2, and 3 wk after parturition. Before parturition, neither the serum nonesterified fatty acid nor the liver triacylglycerol concentration differed between the two groups. After parturition, these variables were higher in experimental cows than in control cows. Liver glycogen was higher at 1 wk before parturition in experimental cows; sharply decreased after parturition in both groups; and, at 1 wk after parturition, was lower in experimental cows than in control cows. In the liver, activities of phosphoenolpyruvate carboxykinase were significantly lower at 1 wk before and at 0.5 and 2 wk after parturition in experimental cows; in addition, the activities tended to be lower at 1 wk after parturition. Activities of fructose 1,6-bisphosphatase tended to be lower, but activities of glucose 6-phosphatase tended to be higher, at 0.5 wk after parturition in experimental cows than in control cows. Our results suggest that, in fatty infiltrated liver, the rate of gluconeogenesis is not optimal, which results in prolongation of lipolysis, particularly during the first weeks after parturition.

Dietary pectin lowers sphingomyelin concentration in VLDL and raises hepatic sphingomyelinase activity in rats

There is evidence that cholesterol and sphingomyelin metabolism are interrelated, and thus the hypothesis tested was that dietary pectin, because it can alter hepatic cholesterol metabolism, would also alter hepatic sphingomyelin metabolism. For that purpose, 4-wk-old female Wistar rats were fed a diet without or with pectin (20 g/100 g) up to 21 d. In accordance with previous work, pectin consumption caused a significant (P < 0.001) reduction in hepatic (65%), whole plasma (37%), and VLDL (80%) cholesterol levels. Pectin also significantly reduced VLDL sphingomyelin concentrations (57%), but raised the amount of sphingomyelin in the high density lipoproteins (HDL)-2 fractions (58%), so that the level of sphingomyelin in whole plasma remained unaffected. Pectin did not affect the sphingomyelin concentration in the liver. Pectin consumption did not affect the hepatic sphingomyelin synthesizing enzymes, serine palmitoyltransferase, phosphatidylcholine:ceramide phosphocholine transferase, or phosphatidylethanolamine:ceramide phosphoethanolamine transferase. In contrast, dietary pectin activated both lysosomal (28%) and plasma membrane (26%) sphingomyelinase and thus may have enhanced sphingomyelin degradation. An attempt was made to describe the effects of dietary pectin on sphingomyelin metabolism in terms of altered fluxes through liver and plasma, with whole liver and whole plasma concentrations of sphingomyelin remaining unaffected.

Effect of overfeeding during the dry period on regulation of adipose tissue metabolism in dairy cows during the periparturient period

During the dry period, cows were either fed restricted amounts or were overfed to study lipolytic rates in adipose tissue. Higher lipolytic rates can result in greater accumulation of triacylglycerols in liver and, subsequently, hepatic lipidosis. Adipose tissue was biopsied at -1, 0.5, 1, 2, and 3 wk from parturition. The basal in vitro lipolytic rate was measured as well as the lipolytic rate as affected by the addition of noradrenaline, 3-hydroxybutyrate, or glucose. Liver was biopsied to quantify triacylglycerol concentrations. Blood was collected to determine insulin and nonesterified fatty acid concentrations. Basal in vitro lipolytic rates at -1 and 0.5 wk were lower in overfed cows. Lipolytic rate was enhanced in both groups of cows when noradrenaline was added, but rates at -1 and 3 wk tended to be higher in overfed cows than in cows that were fed restricted amounts. After the addition of 3-hydroxybutyrate or glucose in vitro, lipolytic rates tended to be higher in overfed cows. Liver triacylglycerol concentration was higher in overfed cows at 0.5 and 1 wk. Plasma insulin concentration tended to be higher in overfed cows at -1 wk. Plasma nonesterified fatty acid concentration was higher in overfed cows at 0.5 and 1 wk. Although overfeeding compared with restricted feeding did not significantly alter the in vitro lipolytic response to 3-hydroxybutyrate or glucose, adipose tissue from overfed cows tended to be less inhibited by these substances, which may contribute to higher lipolytic rates in vivo and a greater triacylglycerol accumulation in the liver after parturition.

Channelling of intermediates in the biosynthesis of phosphatidylcholine and phosphatidylethanolamine in mammalian cells

Previous studies with electropermeabilized cells have suggested the occurrence of metabolic compartmentation and Ca2+-dependent channeling of intermediates of phosphatidylcholine (PC) biosynthesis in C6 rat glioma cells. With a more accessible permeabilization technique, we investigated whether this is a more general phenomenon also occurring in other cell types and whether channeling is involved in phosphatidylethanolamine (PE) synthesis as well. C6 rat glioma cells, C3H10T12 fibroblasts and rat hepatocytes were permeabilized with Staphylococcus aureus alpha-toxin, and the incorporation of the radiolabelled precursors choline, phosphocholine (P-choline), ethanolamine and phosphoethanolamine (P-EA) into PC and PE were measured both at high and low Ca2+ concentrations. In glioma cells, permeabilization at high Ca2+ concentration did not affect [14C]choline or [14C]P-choline incorporation into PC. However, reduction of free Ca2+ in the medium from 1.8 mM to <1 nM resulted in a dramatic increase in [14C]P-choline incorporation into permeabilized cells, whereas [14C]choline incorporation remained unaffected. Also, in fibroblasts, reduction of extracellular Ca2+ increased [14C]P-choline and [14C]P-EA incorporation into PC and PE respectively. In hepatocytes, a combination of alpha-toxin and low Ca2+ concentration severely impaired [14C]choline incorporation into PC. Therefore, alpha-toxin-permeabilized hepatocytes are not a good model in which to study channeling of intermediates in PC biosynthesis. In conclusion, our results indicate that channeling is involved in PC synthesis in glioma cells and fibroblasts. PE synthesis in fibroblasts is also at least partly dependent on channeling.

Malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I activity. Role of Ca2+/calmodulin-dependent protein kinase II and cytoskeletal components

The mechanism of malonyl-CoA-independent acute control of hepatic carnitine palmitoyltransferase I (CPT-I) activity was investigated. In a first series of experiments, the possible involvement of the cytoskeleton in the control of CPT-I activity was studied. The results of these investigations can be summarized as follows. (i) Very mild treatment of permeabilized hepatocytes with trypsin produced around 50% stimulation of CPT-I activity. This effect was absent in cells that had been pretreated with okadaic acid (OA) and seemed to be due to the action of trypsin on cell component(s) distinct from CPT-I. (ii) Incubation of intact hepatocytes with 3, 3'-iminodipropionitrile, a disruptor of intermediate filaments, increased CPT-I activity in a non-additive manner with respect to OA. Taxol, a stabilizer of the cytoskeleton, prevented the OA- and 3, 3'-iminodipropionitrile-induced stimulation of CPT-I. (iii) CPT-I activity in isolated mitochondria was depressed in a dose-dependent fashion by the addition of a total cytoskeleton fraction and a cytokeratin-enriched cytoskeletal fraction, the latter being 3 times more potent than the former. In a second series of experiments, the possible link between Ca2+/calmodulin-dependent protein kinase II (Ca2+/CM-PKII) and the cytoskeleton was studied in the context of CPT-I regulation. The data of these experiments indicate that (i) purified Ca2+/CM-PKII activated CPT-I in permeabilized hepatocytes but not in isolated mitochondria, (ii) purified Ca2+/CM-PKII abrogated the inhibition of CPT-I of isolated mitochondria induced by a cytokeratin-enriched fraction, and (iii) the Ca2+/CM-PKII inhibitor KN-62 prevented the OA-induced phosphorylation of cytokeratins in intact hepatocytes. Results thus support a novel mechanism of short-term control of hepatic CPT-I activity which may rely on the cascade Ca2+/CM-PKII activation --> cytokeratin phosphorylation --> CPT-I de-inhibition.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.

Estrogen replacement during hypoalbuminemia may enhance atherosclerotic risk

Estrogen replacement therapy is considered antiatherosclerotic because it reduces LDL cholesterol and fibrinogen and increases HDL cholesterol concentrations. However, exogenous estrogen is also known to increase hepatic triglyceride production. Hyperlipidemia in the nephrotic syndrome is probably due to increased lipoprotein secretion into plasma and decreased clearance of lipoprotein cholesterol and triglycerides. Previously, lipid-lowering effects of ovariectomy in analbuminemic rats were observed, suggesting that in the presence of hypoalbuminemia, estrogen replacement may have adverse effects on the lipid profile. To test this hypothesis, ovariectomized control rats and rats with Adriamycin-induced nephrotic syndrome were treated with estradiol. In ovariectomized controls, estradiol reduced plasma LDL cholesterol, apolipoprotein B, and fibrinogen and increased apolipoprotein A-I and triglycerides. Nephrotic rats were characterized by a marked decrease in plasma colloid osmotic pressure, hyperfibrinogenemia, hyperlipidemia, and stimulated hepatic fatty acid synthesis. The beneficial effects of estradiol on LDL cholesterol, apolipoprotein B, and fibrinogen found in ovariectomized controls were not present in estradiol-treated nephrotic rats. This suggests that in hypoalbuminemia, downregulation of the LDL receptor overrides putative estradiol-induced increases in LDL receptor activity. Moreover, estrogen replacement in the nephrotic syndrome doubled fatty acid synthesis and triglyceride secretion, and markedly exacerbated hypertriglyceridemia, suggesting saturation of triglyceride clearance. Thus, severe hypoalbuminemia in rats induces an atherosclerotic metabolic response that is aggravated by estrogen replacement. These findings suggest that estrogen replacement in hypoalbuminemic subjects could be contra-indicated.

Studies on the intracellular localization of acetyl-CoA carboxylase

The present work was performed to identify the subcellular localization of hepatic acetyl-CoA carboxylase (ACC). Cellular organelles involved in fatty acid oxidation that contain a malonyl-CoA sensitive carnitine palmitoyltransferase (CPT) activity or that are linked to the control of this activity were analysed for the presence of ACC. No significant amount of ACC was observed in the mitochondrial fraction prepared from isolated rat hepatocytes. Furthermore, no association of ACC activity and mass with isolated hepatic peroxisomes could be detected. Incubation of isolated hepatocytes with compounds known to affect the integrity of the cytoskeleton like okadaic acid or taxol indicates that ACC is associated with this subcellular structure of the hepatocyte. Such association may allow for efficient regulation of CPT activity and thus of fatty acid oxidation.

Control of hepatic fatty acid oxidation by 5'-AMP-activated protein kinase involves a malonyl-CoA-dependent and a malonyl-CoA-independent mechanism

Incubation of rat hepatocytes with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), an activator of the 5'-AMP-activated protein kinase (AMPK), produced a twofold stimulation of palmitate oxidation and of the activity of carnitine palmitoyltransferase I (CPT-I), together with a profound decrease of the activity of acetyl-CoA carboxylase and of the intracellular level of malonyl-CoA. AICAR-induced CPT-I stimulation progressively blunted with time after cell permeabilization, pointing to reversal of conformational constraints of the enzyme in control cells due to the permeabilization-triggered dilution of intracellular malonyl-CoA. The stimulation stabilized at a steady 20-25%. This 20-25% increase in CPT-I activity survived upon complete removal of malonyl-CoA from the permeabilized cells, indicating that it was not dependent on the malonyl-CoA concentration of the cell. This malonyl-CoA-independent activation of CPT-I was not evident when mitochondria were isolated for assay of enzyme activity or when cells were disrupted by vigorous sonication. In addition, the microtubule stabilizer taxol prevented the malonyl-CoA-independent stimulation of CPT-I induced by AICAR. Hence, stimulation of hepatic fatty acid oxidation by AMPK seems to rely on the activation of CPT-I by two different mechanisms: deinhibition of CPT-I induced by depletion of intracellular malonyl-CoA levels and malonyl-CoA-independent stimulation of CPT-I, which might involve modulation of interactions between CPT-I and cytoskeletal components.

The autophagic and endocytic pathways converge at the nascent autophagic vacuoles

We used an improved cryosectioning technique in combination with immunogold cytochemistry and morphometric analysis to study the convergence of the autophagic and endocytic pathways in isolated rat hepatocytes. The endocytic pathway was traced by continuous uptake of gold tracer for various time periods, up to 45 min, while the cells were incubated in serum-free medium to induce autophagy. Endocytic structures involved in fusion with autophagic vacuoles (AV) were categorized into multivesicular endosomes (MVE) and vesicular endosomes (VE). Three types of AV--initial (AVi), intermediate (AVi/d), and degradative (AVd)--were defined by morphological criteria and immunogold labeling characteristics of marker enzymes. The entry of tracer into AV, manifested as either tracer-containing AV profiles (AV+) or fusion profiles (FP+) between AV and tracer-positive endosomal vesicles/vacuoles, was detected as early as 10 min after endocytosis. The number of AV+ exhibited an exponential increase with time. FP+ between MVE or VE and all three types of AV were observed. Among the 112 FP+ scored, 36% involved VE. Of the AV types, AVi and AVi/d were found five to six times more likely to be involved in fusions than AVd. These fusion patterns did not significantly change during the period of endocytosis (15-45 min). We conclude that the autophagic and endocytic pathways converge in a multistage fashion starting within 10 min of endocytosis. The nascent AV is the most upstream and preferred fusion partner for endosomes.

Involvement of Ca2+/calmodulin-dependent protein kinase II in the activation of carnitine palmitoyltransferase I by okadaic acid in rat hepatocytes

The present work was undertaken to study the mechanism by which okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A, stimulates carnitine palmitoyltransferase I (CPT-I) in isolated rat hepatocytes [Guzmán, Kolodziej, Caldwell, Costorphine and Zammit (1994) Biochem. J. 300, 693-699]. The OA-induced stimulation of CPT-I was abolished by the general protein kinase inhibitor K-252a as well as by KN-62, a specific inhibitor of Ca2+/calmodulin-dependent protein kinase II (Ca2+/CM-PKII). However, neither the protein kinase C-specific inhibitor bisindolylmaleimide nor the protein kinase A/protein kinase C inhibitor H-7 was able to prevent the OA-induced stimulation of CPT-I. Hepatocyte-shrinkage-induced stimulation of CPT-I as well as OA-induced hepatocyte shrinkage was prevented by KN-62. KN-62 also antagonized the OA-enhanced release of lactate dehydrogenase from digitonin-permeabilized hepatocytes. Exposure of 32P-labelled hepatocytes to OA increased the degree of phosphorylation of Ca2+/CM-PKII, as immunoprecipitated by a monoclonal antibody raised against the alpha-subunit of rat brain kinase. This effect of OA was also antagonized by KN-62. The results thus indicate that the OA-dependent stimulation of CPT-I may be mediated (at least in part) by increased phosphorylation and subsequent activation of Ca2+/CM-PKII.

Are cytoskeletal components involved in the control of hepatic carnitine palmitoyltransferase I activity?

The present work was undertaken to test whether cytoskeletal components are involved in the control of rat-liver carnitine palmitoyltransferase I (CPT-I) activity by cellular effectors. The microtubule stabilizer taxol abolished the changes in CPT-I activity induced by the effectors tested. Taxol also prevented OA-induced shrinkage of hepatocytes as well as the enhanced release of lactate dehydrogenase from digitonin-permeabilized hepatocytes. On the basis of its relative sensitivity to tautomycin and OA, the modulation of CPT-I activity seemed to involve mostly protein phosphatase 1. These data suggest that the short-term control of hepatic CPT-I by cellular effectors may involve modulation of interactions between CPT-I and cytoskeletal components.

Higher postpartum hepatic triacylglycerol concentrations in dairy cows with free rather than restricted access to feed during the dry period are associated with lower activities of hepatic glycerolphosphate acyltransferase

We investigated the activities of hepatic glycerolipid synthesizing enzymes during postpartum fatty liver development in 10 high-producing dairy cows that had free access to feed during the dry period; a parallel group of 8 control cows was fed according to recommended energy requirements. After calving, both test and control cows had free access to feed. In the period of 10-14 wk before calving, voluntary dry matter intake of the test cows was 20.6 kg/d (SEM 0.42); the restricted control cows received 7 kg/d. Postpartum triacylglycerol concentrations in liver biopsies were one- to twofold higher in the test than in the control cows. The higher plasma nonesterified fatty acid (NEFA) concentrations after parturition in the test vs. the control group were probably caused by a more negative energy balance in the test cows, which was associated with a slightly lower postpartum dry matter intake. After calving, hepatic mitochondrial glycerolphosphate acyltransferase (GPAT) activities were significantly lower in the test than in the restricted control cows. A low GPAT activity may divert fatty acids from esterification to beta-oxidation to protect the hepatocytes against further accumulation of triacylglycerols. The activities of hepatic phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and cholinephosphate cytidylyltransferase were not different in the two groups. This study indicates that in cows given free instead of restricted access to feed during the dry period have a postpartum hepatic triacylglycerol accumulation that is mainly determined by a raised hepatic uptake of plasma NEFA.

Flexibility of zonation of fatty acid oxidation in rat liver

Periportal and perivenous hepatocytes were isolated from rats subjected to different treatments that induce (starvation, cold exposure) or depress (refeeding after starvation) hepatic fatty acid oxidation. These experiments were designed to determine factors that may be involved in creating and maintaining the asymmetrical distribution of this metabolic pathway in the acinus of the liver. The uneven distribution of mitochondrial [14C]-palmitate oxidation within the acinus (i) was very flexible and changed markedly with the physiological status of the animal (periportal/perivenous ratio: 1.5, 2.0, 1.0 and 0.4 for fed, starved, refed and cold-exposed animals respectively), (ii) coincided with a similar zonation of carnitine palmitoyltransferase I activity in fed as well as in cold-exposed animals, (iii) was paralleled by a comparable zonation of mitochondrial 3-hydroxy-3-methyl-glutaryl-CoA synthase activity in starved animals, and (iv) was not determined by zonal differences in any of the following parameters: sensitivity of carnitine palmitoyltransferase I to malonyl-CoA, intracellular concentration of malonyl-CoA, fatty acid synthesizing capacity, acetyl-CoA carboxylase activity, fatty acid synthase activity or relative content of the two hepatic acetyl-CoA carboxylase isoforms. Unlike mitochondrial oxidation, peroxisomal [14C]palmitate oxidation was always zonated towards the perivenous zone of the liver irrespective of the physiological status of the animal. The data presented show that changes in the acinar distribution of mitochondrial long-chain fatty acid oxidation involve specific long-term mechanisms under different physiological conditions.

Dietary medium-chain fatty acids raise and (n-3) polyunsaturated fatty acids lower hepatic triacylglycerol synthesis in rats

The hypothesis tested was that dietary medium-chain or (n-3) polyunsaturated fatty acids, when compared with (n-6) polyunsaturated fatty acids, alter plasma triacylglycerol levels by affecting hepatic triacylglycerol synthesis as reflected by the activities of acetyl-CoA carboxylase, fatty acid synthase and diacylglycerol acyltransferase in liver. In two separate experiments rats were fed purified diets containing (n-6) polyunsaturated fatty acids in the form of corn oil and either (n-3) polyunsaturated fatty acids in the form of fish oil or medium-chain triacylglycerols (MCT). Consumption of MCT significantly raised plasma triacylglycerol concentrations, whereas fish oil feeding had a lowering effect compared with the corn oil-fed group. In individual rats, the hepatic triacylglycerol concentration was directly correlated with the plasma triacylglycerol concentration (r = 0.60, P < 0.001). The MCT oil diet vs. the corn oil diet markedly raised the activities of hepatic acetyl-CoA carboxylase, fatty acid synthase and diacylglycerol acyltransferase. In the rats fed fish oil, the activities of fatty acid synthase and diacylglycerol acyltransferase were significantly reduced, whereas the activity of acetyl-CoA carboxylase was not affected relative to activities in rats fed corn oil. The activities of the three enzymes were directly correlated with plasma triacylglycerol concentrations in individual rats (r = 0.60-0.75, P < 0.001). The type of fat in the diet probably affects the rate of hepatic triacylglycerol synthesis which is an important determinant of plasma triacylglycerol concentrations.

Time trends of plasma lipids and enzymes synthesizing hepatic triacylglycerol during postpartum development of fatty liver in dairy cows

We studied development of fatty liver in high producing dairy cows with free access to feed during the dry period and thus showed the combined effects of parturition and prepartum overfeeding. Postpartum liver triacylglycerol concentrations at 1 wk postpartum, as measured in liver biopsies, had increased more than 6-fold, which was preceded or accompanied by an increase in plasma NEFA concentrations. Concentrations of hepatic phospholipid changed only slightly. The amounts of total lipids in serum, very low density lipoproteins, and high density lipoproteins significantly decreased by .5 wk after parturition, and concentrations of high density lipoproteins rose steadily. The pattern was similar for concentrations of total cholesterol and phospholipid in serum. Total lipid concentrations in low density lipoproteins were not altered after parturition. The activity of microsomal phosphatidate phosphohydrolase in the liver showed a transient increase at .5 wk after calving, but activity of microsomal glycerolphosphate acyltransferase remained relatively constant. The activities of diacylglycerol acyltransferase had increased about twice at 1 wk after calving and remained at this high level until at least 4 wk after parturition. The rise in activity of diacyglycerol acyltransferase was probably a response to the extra influx of fatty acids to channel them into triacylglycerol. Activities of microsomal cholinephosphate cytidylyltransferase initially increased after calving and then decreased slightly. Activities of hepatic choline kinase had increased after calving. This study indicates that hepatic triacylglycerol accumulates because of the increased hepatic uptake of NEFA and the simultaneous increase in activity of diacylglycerol acyltransferase.

Ovariectomy decreases plasma triglyceride levels in analbuminaemic rats by lowering hepatic triglyceride secretion

In mutant analbuminaemic rats (NAR), females demonstrate a more marked hypertriglyceridaemia than males. Ovariectomy decreases triglyceride levels in female NAR. We measured triglyceride secretion rates in vivo as well as the activity of acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS) in hepatic cytosol obtained from female control Sprague-Dawley (SD) rats and NAR with or without ovariectomy. NAR were severely hyperlipidaemic, and triglyceride, cholesterol, apolipoprotein A-I and plasma protein concentrations levels were decreased (all P < 0.01) by ovariectomy. Only triglyceride levels were decreased by ovariectomy in the SD rats (P < 0.05). Oestradiol treatment in ovariectomized NAR restored plasma protein and triglyceride concentrations to levels similar to those observed in intact female NAR and caused a marked increase in plasma cholesterol. Ovariectomy in NAR reduced lipoprotein triglycerides and cholesterol in VLDL, IDL and LDL1, but had little effect on the triglyceride-cholesterol ratio of these particles. Both ACC and FAS activities were markedly increased in NAR vs. SD rats (P < 0.01). This increase was partially corrected by ovariectomy. There was no significant effect of ovariectomy on ACC or FAS activity in the SD rats. Triglyceride secretion rates were significantly increased in NAR vs. SD rats (135 +/- 10 vs. 103 +/- 12 nmol/min per 100 g body weight; P < 0.05). Ovariectomy markedly decreased triglyceride secretion rate in NAR to 69 +/- 6 (P < 0.01), but not in SD rats (92 +/- 8 nmol/min per 100 g body weight, NS). Oestradiol treatment in ovariectomized SD rats restored triglyceride levels but had no significant effect on triglyceride secretion rate (106 +/- 23 nmol/min per 100 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol consumption alters hepatic sphingomyelin metabolism in rats

The hypothesis that cholesterol consumption alters hepatic sphingomyelin homeostasis was tested. Rats were fed a purified diet with or without added cholesterol (1 g/100 g) for up to 21 d. In accordance with previous work, cholesterol consumption significantly increased hepatic, whole plasma and VLDL cholesterol concentrations. Dietary cholesterol also raised the amount of sphingomyelin in the VLDL fraction, which was associated with a decrease in hepatic sphingomyelin concentrations. We suggest that the increase in hepatic VLDL secretion after cholesterol consumption imposed an increase in the demand for sphingomyelin in the liver because this phospholipid is a structural component of VLDL. Determination of the activity of two key enzymes of sphingomyelin homeostasis revealed that cholesterol consumption reduced the activity of acid sphingomyelinase in the liver but did not affect that of serine palmitoyltransferase. These enzyme data indicate that the extra sphingomyelin needed after cholesterol loading results from a decrease in the rate of its catabolism in the liver.

Plasma triglyceride levels are higher in nephrotic than in analbuminemic rats despite a similar increase in hepatic triglyceride secretion

The relative contributions of increased hepatic secretion of triglyceride (TG) and decreased TG catabolism to hypertriglyceridemia in the nephrotic syndrome, and their relationship to urinary protein loss and reduced plasma colloid osmotic pressure (pi) remain unclear. We measured the activity of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), two key enzymes of fatty acid synthesis in hepatic cytosol, in fed control rats, in rats with congenital analbuminemia (NA) that are free of proteinuria, and in rats with adriamycin-induced nephrotic syndrome (ADR). Both NA and ADR rats had decreased pi (respectively 13.2 +/- 0.3 and 10.7 +/- 0.4 mm Hg vs. control rats 18.3 +/- 0.7 mm Hg, P < 0.05), but only ADR rats had increased plasma TG (5.8 +/- 2.6 mmol/liter vs. 1.5 +/- 0.2 mmol/liter in both control and NA rats, P < 0.05), and were proteinuric: 811 +/- 45 mg/day, P < 0.01 versus control and NA rats. Total cytosolic ACC activity, expressed per g body weight, was increased in both NA and ADR rats by 45% and 39%, respectively (P < 0.05). Total FAS activity was increased by 65% and 115% in NA and ADR rats, respectively (P < 0.05). Thus low pi was consistently associated with an increase in total ACC and FAS activities in the livers of fed rats. However, low pi was consistently associated with an increase in plasma TG only in ADR rats. Hepatic TG secretion rates, measured in vivo after blocking lipolysis with Triton WR-1339 in fasting animals, were increased by 33% in both ADR and NA rats as compared to controls (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Ultrastructural localization of CTP:phosphoethanolamine cytidylyltransferase in rat liver

CTP:phosphoethanolamine cytidylyltransferase (EC 2.7.7.14) (ethanolamine-phosphate cytidylyltransferase, ET) was recently purified to homogeneity from a post-microsomal supernatant of rat liver and subsequently used to raise a polyclonal antibody against the enzyme in rabbits (Vermeulen, P. S., Tijburg, L. B. M., Geelen, M. J. H., and van Golde, L. M. G. (1993) J. Biol. Chem 268, 7458-7464). In the present study, we used the affinity-purified antibody against ET for ultrastructural immunogold labeling studies on rat liver cryosections. Single-label experiments clearly demonstrated that ET label was not randomly distributed in hepatocytes. The ET label was concentrated in areas that contained cisternae of the rough endoplasmic reticulum, whereas other cellular organelles (nuclei, mitochondria, and peroxisomes) were only marginally labeled for ET. Double-label experiments for ET and established markers for either soluble or integral endoplasmic reticulum proteins suggested a bimodal distribution of ET between the RER cisternae and the cytosolic space. Complementary single-label studies for ET and the soluble marker protein showed that the fraction of ET label that was present on RER cisternae was significantly greater than that of the soluble marker, supporting the idea of an uneven distribution. These immunoelectron microscopy studies strongly suggest that the cellular organization of ET differs considerably from that reported recently for the corresponding enzyme in the CDP-choline pathway, CTP:phosphocholine cytidylyltransferase (Wang, Y., Sweiter, T. D., Weinhold, P. A., and Kent, C. (1993) J. Biol. Chem. 268, 5899-5904).

Substrate specificity of CTP: phosphoethanolamine cytidylyltransferase purified from rat liver

CTP: phosphoethanolamine cytidylyltransferase was recently purified to homogeneity from rat liver (Vermeulen, P.S. Tijburg, L.B.M., Geelen, M.J.H. and van Golde, L.M.G. (1993) J. Biol. Chem. 268, 7458-7464). The present study focuses on the specificity of this enzyme for phosphorylated bases with a varying degree of N-methylation. The apparent Km for phosphoethanolamine was 0.072 mM. As the number of N-methylated substituents on phosphoethanolamine increased, the apparent Km increased: 0.11 mM for phosphomonomethylethanolamine and 6.8 mM for phosphodimethylethanolamine. Introduction of a third N-methyl group did not further increase the Km value. The effect of N-methyl groups on the reaction velocity was far more pronounced. A decreased Vmax for the reaction was found as the number of N-methyl substituents increased: 1.52 and 0.24 mumol/min per mg protein for phosphoethanolamine and phosphomonomethylethanolamine, and 44 and 0.69 nmol/min per mg protein for phosphodimethylethanolamine and phosphocholine, respectively. Phosphomonomethylethanolamine, phosphodimethylethanolamine and phosphocholine were weak competitive inhibitors of the cytidylyltransferase catalyzed reaction when phosphoethanolamine was used as a substrate, with Ki values of 7.0, 6.8 and 52.9 mM, respectively. The results show that this cytidylyltransferase is highly specific for phosphoethanolamine. Comparison of these data with previously reported information on the substrate specificity of CTP: phosphocholine cytidylyltransferase endorses the view that the two cytidylyltransferases are functionally different.

Effects of chronic benfluorex treatment on the activities of key enzymes of hepatic carbohydrate metabolism in old Sprague-Dawley rats

Chronic effects of benfluorex on some parameters of carbohydrate metabolism have been studied in 24-month-old Sprague-Dawley rats. Treatment once a day for 14 days with 25 mg benfluorex per kg body weight lowered body weight, decreased circulating insulin and resulted in an increase in hepatic glycogen. Measurement of the activities of several important regulatory enzymes of hepatic carbohydrate metabolism showed a significant decrease in the activities of phosphoenolpyruvate carboxykinase and glycogen phosphorylase. The activity of glucose-6-phosphatase, on the other hand, was slightly increased. Taken collectively, our data offer an explanation for the observed inhibition of hepatic glucose production by chronic benfluorex treatment in cases of hyperinsulinemia.

The role of citrate in the regulation of hepatic fatty acid synthesis by insulin and glucagon

Incubation of intact rat hepatocytes with insulin and glucagon resulted in increased and decreased rates of de novo fatty acid biosynthesis, respectively. These changes were paralleled by corresponding alterations in the activity of acetyl-CoA carboxylase, an important regulatory enzyme of this pathway. The hormonal conditions imposed on the hepatocytes did not change the cellular or the cytosolic level of citrate. Incubation of hepatocytes with octanoate showed a parallel increase in the rate of fatty acid synthesis and acetyl-CoA carboxylase with a concomitant elevation of the cellular citrate level. The increase in whole-cell citrate was mainly due to a marked increase in the level of cytosolic citrate. Collectively, our data indicate that insulin and glucagon-determined changes in acetyl-CoA carboxylase are not mediated by changes in cytosolic citrate levels.

Effects of proglycosyn (LY177507) on fatty acid metabolism in rat hepatocytes

Proglycosyn (LY177507) belongs to a series of powerful agents that stabilize liver glycogen stores by promoting glycogen synthesis from different precursors and inhibiting glycogenolysis and glycolysis. In the present study we have examined the effects of proglycosyn on fatty acid metabolism in isolated hepatocytes. Preincubation of hepatocytes with medium containing proglycosyn led to a marked inhibition of fatty acid synthesis de novo and acetyl-CoA carboxylase activity without affecting fatty acid synthase. Likewise, proglycosyn depressed the synthesis of triacylglycerols and phospholipids from labeled palmitate. Although octanoate oxidation was unaffected by proglycosyn, mitochondrial palmitate oxidation was notably stimulated. This effect may be attributed to the proglycosyn-induced decrease of intracellular malonyl-CoA levels relative to control incubations and the concomitant relieve of the inhibition of the mitochondrial-outer-membrane carnitine palmitoyl-transferase by malonyl-CoA. By contrast, neither peroxisomal palmitate oxidation nor peroxisomal carnitine palmitoyltransferase activity was changed upon hepatocyte incubation with proglycosyn. Results thus indicate that proglycosyn increases the fatty-acid-oxidation efficiency of the liver at the expense of lipogenesis, and this may contribute to the proglycosyn-induced sparing of liver glycogen stores.

Immunological characterization, lipid dependence, and subcellular localization of CTP:phosphoethanolamine cytidylyltransferase purified from rat liver. Comparison with CTP:phosphocholine cytidylyltransferase

CTP:phosphoethanolamine cytidylyltransferase (ET) (ethanolamine-phosphate cytidylyltransferase, EC 2.7.7.14), which is generally considered as the rate-regulatory enzyme of phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway, was purified to homogeneity from a rat liver postmicrosomal supernatant. A polyclonal antibody was raised against the enzyme in rabbits and subsequently purified by affinity chromatography. The affinity-purified antibody recognized one single immunoreactive 49.6-kDa protein band on SDS-polyacrylamide gel. The enzyme showed an isoelectric point at a pH of 6.5 and was sensitive to various sulfhydryl reagents. Cross-reactivity experiments of ET and CTP:phosphocholine cytidylyltransferase (CT) (choline-phosphate cytidylyltransferase, EC 2.7.7.15) with their corresponding antibodies showed that these enzymes were immunologically distinct. In contrast with the well known lipid dependence of CT, the activities of both purified and cytosolic ETs were not affected by the presence of various phospholipid preparations. Differential centrifugation studies as well as release experiments with digitonin-permeabilized hepatocytes demonstrated that ET, unlike CT, is not associated with cellular organelles. However, amino acid analysis of ET revealed a high content of hydrophobic amino acids, suggesting a possible association of this enzyme with some kind of cellular structure in the hepatocyte.

Activity of carnitine palmitoyltransferase in mitochondrial outer membranes and peroxisomes in digitonin-permeabilized hepatocytes. Selective modulation of mitochondrial enzyme activity by okadaic acid

A procedure is described for the rapid measurement of the activity of mitochondrial-outer-membrane carnitine palmitoyltransferase (CPTo) and peroxisomal carnitine palmitoyltransferase (CPTp) in digitonin-permeabilized hepatocytes. CPTo activity was determined as the tetradecylglycidate (TDGA)-sensitive malonyl-CoA-sensitive CPT activity, whereas CPTp activity was monitored as the TDGA-insensitive malonyl-CoA-sensitive CPT activity. Under these experimental conditions, the respective contributions of CPTo and CPTp to total hepatocellular malonyl-CoA-sensitive CPT activity were 74.6 and 25.4%, which correlated well with the values of 76.9 and 23.1% for the respective contributions of the mitochondrial and the peroxisomal compartment to total hepatocellular palmitate oxidation. The sensitivity of CPTo to inhibition by malonyl-CoA was very similar to that of CPTp; thus 50% inhibition of CPTo and CPTp activities was achieved with malonyl-CoA concentrations of 2.6 +/- 0.5 and 3.0 +/- 0.4 microM respectively. Short-term incubation of hepatocytes with the phosphatase inhibitor okadaic acid (i) increased the activity of CPTo and the rate of mitochondrial palmitate oxidation, (ii) decreased the affinity of CPTo for palmitoyl-CoA substrate, and (iii) decreased the sensitivity of CPTo to inhibition by malonyl-CoA. By contrast, neither the properties of CPTp nor the rate of peroxisomal palmitate oxidation were changed upon incubation of cells with okadaic acid. Results indicate therefore that CPTo, but not CPTp, may be regulated by a mechanism of phosphorylation/dephosphorylation. The physiological relevance of these findings is discussed.

Secretion of lipoprotein cholesterol by perfused livers from rabbits hypo- or hyperresponsive to dietary cholesterol: greater dietary cholesterol-induced secretion in hyperresponsive rabbits

In two inbred strains of rabbits with high or low response of plasma cholesterol to dietary cholesterol, secretion of lipoprotein cholesterol by perfused livers was determined. The perfused rabbit livers secreted cholesterol into the perfusate at essentially constant rates between 30 and 120 min of perfusion. Most of the secreted cholesterol resided in VLDL. Addition of cholesterol to the diet of the donor rabbits caused a higher cholesterol:triglyceride ratio of lipoproteins secreted by the perfused liver. Such a rise was also seen in the plasma lipoproteins of cholesterol-fed rabbits. Plasma and perfusate lipoproteins differed in that the former had a higher cholesterol:triglyceride ratio. Cholesterol feeding produced a higher output of lipoprotein cholesterol by the perfused liver, the increment being greater in hyper- than in hyporesponders. Cholesterol-fed hyperresponders had higher liver cholesterol concentrations than their hyporesponsive counterparts. There was a direct relationship between the rise of liver cholesterol concentrations and the rise of hepatic secretion of lipoprotein cholesterol; this relationship was identical for hypo- and hyperresponders. We conclude that the higher cholesterolemic response to cholesterol feeding in the hyperresponders, when compared with the hyporesponders, can be explained, at least partly, by a greater cholesterol-induced hepatic secretion of lipoprotein cholesterol.

In situ measurement of HMG-CoA reductase activity in digitonin-permeabilized hepatocytes

An assay procedure for HMG-CoA reductase is described which allows rapid measurement of the activity of this enzyme in isolated rat hepatocytes. In a one step procedure digitonin permeabilizes the plasma membrane and at the same time HMG-CoA reductase activity is measured. Digitonin at a concentration of 64 micrograms per mg of cell protein was found to be optimal for exposing microsomal HMG-CoA reductase to the assay components. The enzyme assay is linear with time up til 5 min and with protein concentrations in the range of 0.06-0.6 mg of cell protein per assay. It is shown that cellular enzyme activity is affected by preincubation of intact hepatocytes with a variety of short-term modulators of hepatic cholesterogenesis.

Biosynthesis and secretion of triacylglycerol in rat liver after partial hepatectomy

Partially hepatectomized rats were used to investigate the mechanism of fatty-liver development in the regenerating rat liver. After partial hepatectomy the amount of hepatic triacylglycerol increased by almost 4-fold compared with sham-operated rats. The activities of both cytosolic and microsomal phosphatidate phosphohydrolase were enhanced at 12 h after surgery. The activity of diacylglycerol acyltransferase was increased at a later stage of regeneration. Analysis of plasma lipoproteins showed a significant decrease of lipids associated with very-low-density lipoproteins (VLDL). Relative to control, the rate of hepatic triacylglycerol synthesis from [3H]glycerol in vivo was stimulated at 22 h after partial liver resection. However, secretion of glycerol-labelled triacylglycerol in VLDL was the same in control and hepatectomized rats. In cultures of hepatocytes from hepatectomized donor rats, the concentration of triacylglycerol and the biosynthesis of this lipid from [3H]glycerol or from [3H]oleate were enhanced. The secretion of total triacylglycerol into the medium was not affected, resulting in a net accumulation of intracellular triacylglycerol. The rate of secretion of leucine-labelled apolipoproteins B and E associated with VLDL was similar in cell cultures from hepatectomized and sham-operated rats. The results of this study show that the enhancement of the biosynthesis of triacylglycerol in hepatectomized livers is not accompanied by an increase of the secretion of VLDL.

Lipoproteins do not influence cholesterol synthesis in freshly isolated rat hepatocytes, cautionary note

1. Suspensions of freshly isolated rat hepatocytes were used to study the effects of native and derivatized lipoproteins on the rate of cholesterogenesis. 2. Short-term incubation of the hepatocytes with a variety of lipoproteins failed to modify the rate of cholesterol synthesis as determined by the incorporation of tritium from tritiated water into cholesterol after separation from other lipids by thin-layer chromatography. 3. Neither an increase in the cholesterol content of the particles (beta-very-low-density lipoproteins) nor derivatization of the lipoproteins (lactosylated-low-density lipoproteins or high-density lipoproteins associated with a tris-galactoside-terminated cholesterol derivative) nor cholesterol-containing liposomes were effective in this respect. 4. Whether this behaviour represents an artefact of the isolated hepatocyte preparation is unknown yet.

A method for perfusion of the isolated rabbit liver through the portal vein and the hepatic artery in a recirculating system

We describe a method for the isolation and recirculating perfusion of rabbit livers. Livers were perfused simultaneously through the portal vein and the hepatic artery. Precise descriptions of the surgical preparation, perfusion apparatus and perfusion method are given. The viability of rabbit livers was assessed with the use of physiological, biochemical and histological parameters. We conclude that the isolated perfused rabbit livers are sufficiently viable to study their short-term metabolism.

Time course of hormonal effects on acetyl-CoA carboxylase as measured in digitonin-permeabilized rat hepatocytes

Acetyl-CoA carboxylase activity was measured in digitonin-permeabilized rat hepatocytes by coupling the carboxylase reaction to the fatty acid synthase reaction. Using this assay the activity of acetyl-CoA carboxylase was covariant with the rate of fatty acid synthesis. Insulin and the tumor promotor phorbol myristate acetate were found to stimulate, and glucagon and noradrenaline to inhibit both cellular parameters. The stimulation of acetyl-CoA carboxylase by insulin developed slowly (15 to 30 min) whereas the phorbol myristate acetate effect developed faster (within 15 min). The inhibition of the enzyme caused by glucagon was already apparent within 1 min after hormone addition. Inhibition by noradrenaline, in the presence of propranolol, was also quite rapid and occurred within 2 min after addition of the agonist.

Biosynthesis of phosphatidylethanolamine via the CDP-ethanolamine route is an important pathway in isolated rat hepatocytes

In the present study pulse-label and pulse-chase experiments with isolated rat hepatocytes in suspension were designed to investigate the effects of the presence of either serine or ethanolamine in the medium on the rate of phosphatidylethanolamine synthesis via the CDPethanolamine pathway and by decarboxylation of phosphatidylserine. Addition of serine to the medium did not affect the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamine. Pulse-label experiments showed that the incorporation of [3H]serine into phosphatidylserine decreased in the presence of ethanolamine with a corresponding decrease of the incorporation of label into the ethanolamine base moiety of phosphatidylethanolamine. However, the radioactivity in the diacylglycerol part of phosphatidylethanolamine was considerably higher in the presence of ethanolamine than in its absence. Pulse-chase experiments with labelled serine demonstrated that the conversion of phosphatidylserine to phosphatidylethanolamine was not affected by varying concentrations of ethanolamine. Our observations indicate that in the presence of ethanolamine the biosynthesis of phosphatidylethanolamine via the CDPethanolamine pathway is enhanced relative to the synthesis by decarboxylation of phosphatidylserine.

Immunochemical quantitation of fatty-acid-binding proteins. I. Tissue and intracellular distribution, postnatal development and influence of physiological conditions on rat heart and liver FABP

Antisera against rat heart and liver fatty acid-binding protein (FABP) were applied in Western blotting analysis and ELISA to assess their tissue and intracellular distribution, and the influence of development, physiological conditions and several agents on the FABP content of tissue cytosols. The data obtained are compared with the oleic acid-binding capacity. Heart FABP is found in high concentrations in heart, skeletal muscles, diaphragm and lung, and in lower concentrations in kidney, brain and spleen, whereas liver FABP is limited to liver and intestine. In heart and liver, FABP is only present in the cytosol. The FABP content of both heart and liver shows a progressive increase during the first weeks of postnatal development, in contrast to their constant oleic acid-binding capacity. The reciprocally declining alpha-fetoprotein content of both tissues may partially account for the complementary fraction of the fatty acid-binding capacity. The FABP content and the fatty acid-binding capacity of adult heart and liver were in good accordance under various physiological conditions. Addition of clofibrate to the diet induces an increase of liver FABP content, whereas feeding of cholesterol, cholestyramine, mevinolin or cholate caused a marked decrease. The significance of the combined determination of fatty acid-binding capacity and FABP content (by immunochemical quantitation and blotting analysis) is indicated.

Inhibition of phosphatidylethanolamine synthesis by glucagon in isolated rat hepatocytes

Exposure of isolated rat hepatocytes to glucagon or chlorophenylthio cyclic AMP led to an inhibition of the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamine. Pulse-chase experiments and measurement of the activities of the enzymes involved in the CDP-ethanolamine pathway provided evidence that the inhibitory effect of glucagon on the synthesis de novo of phosphatidylethanolamine was not caused by a diminished conversion of ethanolamine phosphate into CDP-ethanolamine. The observations suggested that the glucagon-induced inhibition of the biosynthesis of phosphatidylethanolamine is probably due to a decreased supply of diacylglycerols, resulting in a decreased formation of phosphatidylethanolamine from CDP-ethanolamine and diacylglycerols.