Effects of chronic modification of dietary fat and carbohydrate in rats

Enhancement of carcinogenesis and fatty infiltration in the pancreas in N-nitrosobis(2-oxopropyl)amine-treated hamsters by high-fat diet

OBJECTIVES

Obesity is associated with increased pancreatic cancer risk, although the mechanisms have yet to be detailed. This study aimed to elucidate promotion of pancreatic cancer by obesity and hyperlipidemia.

METHODS

Six-week-old female Syrian golden hamsters were treated with N-nitrosobis(2-oxopropyl)amine (BOP) and after 1 week were fed a high-fat diet (HFD) or standard diet (STD) for 6 or 17 weeks.

RESULTS

Body weight and serum levels of lipids and leptin were significantly higher in the HFD than the STD group at 14 weeks of age. Pancreatic ductal adenocarcinomas developed only in the BOP + HFD group, with an incidence of 67% (P < 0.01) at 14 weeks of age. In addition, the multiplicity was 2-fold greater in the BOP + HFD group than in the BOP + STD group (P < 0.05) at 25 weeks of age. Pancreatic fatty infiltration was increased by BOP treatment and further enhanced by the HFD, correlating with progression of BOP-induced pancreatic ductal adenocarcinoma and up-regulated expression of adipocytokines and cell proliferation-related genes in the pancreas.

CONCLUSIONS

High-fat diet is shown to increase serum lipid levels and enhance fatty infiltration in the pancreas with abnormal adipocytokine production, which may accelerate and enhance pancreatic cancer.

Effect of long-term high-fat feeding on energy balance and liver oxidative activity in rats

The purpose of this work was to study the effect of early long-term high-fat feeding on energy balance and liver oxidative activity. To this end, rats aged about 30 d were fed a high-fat or a low-fat diet for 15, 30 or 60 d. Full energy balance and energy partitioning measurements were carried out. In addition, we measured hepatic mitochondrial and peroxisomal oxidative capacities. Serum levels of free triiodothyronine (T3) and leptin were also determined. Rats fed a high-fat diet showed an increase in metabolizable energy intake as well as in energy expenditure, while lipid gain over the whole period was lower than that expected due to a decrease in metabolic efficiency. An increase in serum free T3 levels was also found in rats fed a high-fat diet after 15 and 30 d. Statistically significant correlations between serum leptin levels and body fat mass were found after 15, 30 and 60 d of high-fat feeding. Finally, no variation in hepatic mitochondrial and peroxisomal fatty acid oxidation capacity was found in rats fed a high-fat diet for 15, 30 or 60 d. In conclusion, the results of the present study show that young Wistar rats fed a high-fat diet for up to 60 d are able to counteract, at least in part, obesity development.

Acyltransferase activities in the yolk sac membrane of the chick embryo

The activities of some enzymes of glycerolipid synthesis and fatty acid oxidation were measured in subcellular fractions of the yolk sac membrane (YSM), an extra-embryonic tissue that mediates the transfer of lipid from the yolk to the circulation of the chick embryo. The activities of monoacylglycerol acyltransferase and carnitine palmitoyl transferase-1 in the YSM (respectively, 284.8+/-13.2 nmol/min/mg microsomal protein and 145.6+/-9.1 nmol/min/mg mitochondrial protein; mean +/- SE; n = 4) at day 12 of development appear to be the highest yet reported for any animal tissue. Also, the carnitine palmitoyl transferase-1 of the YSM was very insensitive to inhibition by malonyl CoA. The maximal activities of glycerol-3-phosphate acyltransferase and diacylglycerol acyltransferase in the YSM (respectively, 26.7+/-2.2 and 36.1+/-2.1 nmol/min/mg microsomal protein) were also high compared with the reported values for various animal tissues. The very high enzymic capacity for glycerolipid synthesis supports the hypothesis that the yolk-derived lipids are subjected to hydrolysis followed by reesterification during transit across the YSM. The monoacylglycerol pathway appears to be the main route for glycerolipid resynthesis in the YSM. The results also suggest that the YSM has the capacity to perform simultaneously beta-oxidation at a high rate in order to provide energy for the lipid transfer process.

Obesity on a high-fat diet: role of hypothalamic galanin in neurons of the anterior paraventricular nucleus projecting to the median eminence

Previous studies have suggested that the peptide galanin (GAL) in the hypothalamus is related to the preference of an animal for dietary fat. The present report investigates this relationship further to identify the specific GAL-synthesizing cell groups involved and to characterize their association to circulating glucose or hormones and their possible contribution to body fat deposition. Male albino Sprague Dawley rats were tested in different feeding paradigms with diets containing the macronutrients, fat, carbohydrate, or protein. These studies, using multiple techniques, identify a cell group in the hypothalamus that expresses GAL and that shows a shift in peptide activity in close relation to dietary fat, circulating glucose, and body fat. In all paradigms, a rise in fat intake, from 10 to 30%, is associated with reduced levels of insulin and corticosterone and normal glucose levels, whereas a further increase in fat ingestion (>30%) leads to hyperglycemia along with greater adiposity. In the hypothalamus, GAL gene expression, peptide production, and peptide release rise significantly (by 40%) in association with fat ingestion, showing no relation to either carbohydrate or protein ingestion. This change is highly site specific, evident predominantly in GAL-synthesizing neurons in the anterior parvocellular region of the paraventricular nucleus (aPVN) and in GAL-containing terminals in the external zone of the median eminence (ME). Positive correlations detected between mRNA abundance in the aPVN and GAL peptide in the ME support the existence of an aPVN-ME projection system related to fat intake and fat deposition. When activated by dietary fat, the contribution of this projection to body fat is suggested by consistent positive correlations between aPVN-ME GAL and either dietary fat, circulating glucose, or body fat and by significantly higher GAL levels (+30%) in obesity-prone compared with obesity-resistant rats. This evidence supports a role for this hypothalamic GAL projection system in the development of obesity produced by the overconsumption of fat.

Obesity on a high-fat diet: role of hypothalamic galanin in neurons of the anterior paraventricular nucleus projecting to the median eminence

Previous studies have suggested that the peptide galanin (GAL) in the hypothalamus is related to the preference of an animal for dietary fat. The present report investigates this relationship further to identify the specific GAL-synthesizing cell groups involved and to characterize their association to circulating glucose or hormones and their possible contribution to body fat deposition. Male albino Sprague Dawley rats were tested in different feeding paradigms with diets containing the macronutrients, fat, carbohydrate, or protein. These studies, using multiple techniques, identify a cell group in the hypothalamus that expresses GAL and that shows a shift in peptide activity in close relation to dietary fat, circulating glucose, and body fat. In all paradigms, a rise in fat intake, from 10 to 30%, is associated with reduced levels of insulin and corticosterone and normal glucose levels, whereas a further increase in fat ingestion (>30%) leads to hyperglycemia along with greater adiposity. In the hypothalamus, GAL gene expression, peptide production, and peptide release rise significantly (by 40%) in association with fat ingestion, showing no relation to either carbohydrate or protein ingestion. This change is highly site specific, evident predominantly in GAL-synthesizing neurons in the anterior parvocellular region of the paraventricular nucleus (aPVN) and in GAL-containing terminals in the external zone of the median eminence (ME). Positive correlations detected between mRNA abundance in the aPVN and GAL peptide in the ME support the existence of an aPVN-ME projection system related to fat intake and fat deposition. When activated by dietary fat, the contribution of this projection to body fat is suggested by consistent positive correlations between aPVN-ME GAL and either dietary fat, circulating glucose, or body fat and by significantly higher GAL levels (+30%) in obesity-prone compared with obesity-resistant rats. This evidence supports a role for this hypothalamic GAL projection system in the development of obesity produced by the overconsumption of fat.

Quantification in vivo of the effects of different types of dietary fat on the loci of control involved in hepatic triacylglycerol secretion

Polyunsaturated fatty acids (PUFA) have been suggested to exert their hypotriglyceridaemic effect through several possible mechanisms that would be expected to decrease the rate of hepatic very-low-density-lipoprotein-triacylglycerol secretion. We have quantified the role played in vivo by changes in the pattern of partitioning of (i) acyl-CoA between oxidation and esterification, (ii) diacylglycerol between synthesis of triacylglycerol and of the major phospholipids, and (iii) triacylglycerol between secretion and storage within the liver, in response to two dietary levels of n-6 and n-3 PUFA. In order to achieve this we used the technique of selective labelling of hepatic fatty acids in vivo. Compared with a predominantly saturated fatty acid diet, both n-6 and n-3 PUFA intake resulted in a decrease in the proportion of acyl moieties that were secreted by the liver, through an increased diversion of acyl-CoA towards oxidation and a lower fractional rate of secretion of newly synthesized triacylglycerol. In addition, a diet rich in n-3 fatty acids resulted not only in a greater magnitude of these effects but also in a doubling of the partitioning of diacylglycerol towards phospholipid labelling. It is shown that the overall 50% reduction achieved by fish oil feeding in the proportion of acyl groups that were secreted by the liver was distributed over all three branch points. The contribution of each of these adaptations was quantified. The application of such an approach, i.e. the localization and in vivo quantification of the importance of loci of control, in studies on dietary and pharmacological agents that affect lipaemia, is discussed.

Triacylglycerol biosynthetic enzymes in lean and obese Zucker rats

In the present investigation, we have compared the potential of triacylglycerol formation from sn-glycerol-3-phosphate (GP) and 2-monoacylglycerol (MG) in liver, adipose tissue and intestine from lean and obese Zucker rats. Microsomal fractions were used to measure the sn-glycerol-3-phosphate acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT) and monoacylglycerol acyltransferase (MGAT) activities and homogenates were used to measure NEM-sensitive and NEM-insensitive phosphatidate phosphohydrolase (PPH) activities. In adipose tissue and liver, the GP pathway served as the major route of glycerolipid formation, with adipose tissue being 5-20-fold more active. The activities of the GP pathway enzymes increased further in response to obesity, with some degree of organ specificity. In adipose tissue of obese rats, the activities of all the pathway enzymes increased; whereas, in liver and intestine, this response was limited to PPH and GPAT, respectively. In contrast with the GP pathway enzymes, obesity in Zucker rats was not associated with alterations in the acylation of 2-monoacylglycerol. Comparison of the activities of MGAT in different intestinal segments indicated that the MG pathway was most active in the jejunum and least active in the ileum and that this pattern did not change in response to obesity. These measurements of the individual enzyme reactions provide evidence that the entire process of esterification via sn-glycerol-3-phosphate is accelerated in the various organs from obese rats and that this perturbation in lipid metabolism may contribute significantly to the increased deposition of body fat noted in this animal model.

Stimulation of glucose utilization in 3T3 adipocytes and rat diaphragm in vitro by the sulphonylureas, glimepiride and glibenclamide, is correlated with modulations of the cAMP regulatory cascade

The long-term hypoglycemic activity of sulphonylurea drugs has been attributed, in part at least, to the stimulation of glucose utilization in extra-pancreatic tissues. The novel sulphonylurea, glimepiride, gives rise to a longer lasting reduction in the blood sugar level in dogs and rabbits compared to glibenclamide (Geisen K, Drug Res 38: 1120-1130, 1988). This cannot be explained adequately by elevated plasma insulin levels. This study investigated whether this prolonged hypoglycemic phase was based on the drug's abilities to stimulate glucose utilization and affect the underlying regulatory mechanisms in insulin-sensitive cells in vitro. It was found that in the absence of added insulin, glimepiride and glibenclamide (1-50 microM) stimulated lipogenesis (3T3 adipocytes) and glycogenesis (isolated rat diaphragm) approximately 4.5- and 2.5-fold, respectively, and reduced the isoproterenol-stimulated lipolysis (rat adipocytes) up to 40-60%. The increased glucose utilization was correlated with a 3-4-fold higher 2-deoxyglucose transport rate and amount of GLUT4 at the plasma membrane, as well as with increased activities of key metabolic enzymes (glycerol-3-phosphate acyltransferase, glycogen synthase) within the same concentration range. Furthermore, the low Km cAMP-specific phosphodiesterase was activated 1.8-fold, whereas the cytosolic cAMP level and protein kinase A activity ratios were significantly lowered after incubation of isoproterenol-stimulated rat adipocytes with the sulphonylureas. In many of the aspects studied the novel sulphonylurea, glimepride, exhibited slightly lower ED50-values than glibenclamide. This study demonstrates correlations existing between drug-induced stimulation of glucose transport/metabolism and cAMP degradation/protein kinase A inhibition as well as between the relative efficiencies of glimepiride and glibenclamide in inducing these extra-pancreatic processes. Therefore, it is suggested that the stimulation of glucose utilization by sulphonylureas is mediated by a decrease of cAMP-dependent phosphorylation of GLUT4 and glucose metabolizing enzymes. The therapeutic relevance of extra-pancreatic effects of sulphonylureas, in general, and of the differences between glimepiride and glibenclamide as observed in vitro in this work, in particular, remain to be elucidated.

Regional fat distribution and metabolism in a new mouse model (C57BL/6J) of non-insulin-dependent diabetes mellitus

It has been suggested that a genetic predisposition and an increased total fat mass, particularly a specific increase in visceral fat, contribute to the metabolic aberrations associated with human non-insulin-dependent diabetes mellitus (NIDDM). In this study, we investigated the interactions between genetic and dietary components on fat distribution and metabolism in two mouse strains, one genetically predisposed to NIDDM (BL/6) and one not (A/J), fed either a chow diet or a high-fat, high-simple carbohydrate (HFHSC) diet for 5 months. As expected, both strains of mice fed a HFHSC diet were heavier, had more fat in both the subcutaneous (inguinal [ING] and visceral (mesenteric [MES]) regions, and had larger fat cells and higher lipoprotein lipase (LPL) activities. The results of interactions between strain and diet showed important differences in fat distribution and metabolism between strains. In comparison with A/J mice, BL/6 mice fed a HFHSC diet developed hyperglycemia, hyperinsulinemia, and hypercholesterolemia, were heavier, had more overall fat, and particularly increased their MES adipose tissue. This increase in visceral fat mass was due to an increase in fat cell number. In contrast, BL/6 mice fed a chow diet had less overall fat, a smaller MES fat pad with smaller adipocytes, and lower LPL activity than A/J controls. Significant differences between BL/6 and A/J mice fed either a HFHSC or a chow diet were not observed in ING adipose tissue. These data suggest that in BL/6 mice, changes in the metabolic characteristics of visceral fat seem to be a specific characteristic associated with the genetic predisposition for NIDDM.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin effect on isolated rat hepatocytes: diacylglycerol-phosphatidic acid interrelationship

It is widely accepted that insulin action does not involve inositol phospholipid hydrolysis through the stimulation of a phosphatidylinositol-specific phospholipase C (PI-PLC). This consideration prompted us to investigate the insulin effect on the mechanism leading to the accumulation of diacylglycerol (DAG) and phosphatidic acid (PA) in rat hepatocytes. Basically, insulin induces: (i) a significant increase of both [3H]glycerol and fatty acid labelling of DAG; (ii) a significant increase of PA labelling preceding DAG labelling and paralleled by a decrease of phosphatidylcholine (PC) labelling. These observations, which suggest an insulin-dependent involvement of a phospholipase D, are strengthened by the increase of PC-derived phosphatidylethanol in presence of ethanol. Finally, the observation that the PA levels do not return to basal suggests that other mechanisms different from PC hydrolysis, such as the stimulation of direct synthesis of PA, may be activated.

Role of dietary fat in calorie intake and weight gain

This paper reviews the literature on the role of dietary fat in calorie intake and body weight gain in humans and laboratory animals. An overview of 40 animal studies which compared growth on high-fat (HF) and high-carbohydrate (HC) solid/powdered diets indicated that the HF diet elicited greater weight gain in 33 out of 40 studies. Enhanced growth on the HF diet was often, but not exclusively, attributable to greater caloric intake. Additional evidence for the growth-enhancing effect of HF diets emerges from "diet option" and "supermarket" feeding studies in rats, and experimental and epidemiological studies in humans. Three principal factors that contribute to the different responses to HF and HC diets are (a) caloric density, (b) sensory properties and palatability, and (c) postabsorptive processing. It is concluded that both calorie intake and metabolic energy expenditure are biased towards weight gain when a HF diet is consumed, and that the high caloric density of high-fat diets plays a primary role in weight gain. Humans may be biologically predisposed to gain weight when a HF diet is consumed.

Enhanced responses to stress induced by fat-feeding in rats: relationship between hypothalamic noradrenaline and blood glucose

High-fat-feeding in rats has been reported to enhance stress reactions, as assessed by elevation of blood glucose and corticosterone levels. This study was designed to investigate the relationship between changes in blood glucose and hypothalamic neuronal noradrenaline activity (HNNA), as indexed by the ratio of dihydroxyphenylethyleneglycol (DHPG) to noradrenaline (NA), following physiological stress in high-fat-fed rats. Two groups of adult male Wistar rats were fed isocaloric diets high in fat (59% of calories) or starch (70% of calories). After 3 weeks each of these groups was further subdivided into (a) control, (b) 2 min ambient temperature (20 degrees C) swim or (c) 2 min swim in ice-cold water. Animals were decapitated 20 min after commencing the swim; trunk blood and a sample of medial basal hypothalamus were obtained. Computerized gas chromatography/mass spectrometry was used to measure hypothalamic DHPG and NA concentrations. There were no differences between fat- and starch-fed rats in basal levels of serum glucose, insulin or corticosterone and no differences in DHPG, NA or DHPG/NA ratio. Compared to starch-fed rats, ambient swim stress in the fat-fed group produced significantly larger serum glucose (P less than 0.01), serum corticosterone (P less than 0.05), DHPG (P less than 0.05) and DHPG/NA (P less than 0.01) responses. Following cold swim stress similar differences between fat- and starch-fed animals were observed. In addition, serum insulin was found to be significantly suppressed in the fat-fed group (P less than 0.05) following cold swim.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin rapidly increases glycerol-3-phosphate-acyltransferase activity in rat adipocytes

Glycerol-3-phosphate acyltransferase (G3PAT) was activated by insulin in intact rat adipocytes within 1 min: this activation persisted for 10 min, and was due to a decrease in the Km of the enzyme. The addition of insulin to control adipocyte membranes also increased G3PAT activity, and this effect was mimicked by phosphatidylinositol-specific phospholipase C. Cytosol fractions from insulin-treated adipocytes stimulated G3PAT activity of control membranes, suggesting that a soluble mediator is released during insulin action, possibly through activation of a PI-specific PLC.

The effect of adriamycin on glycerolphosphate acyltransferase and lipid metabolism in rat hepatocytes in monolayer culture

Total and mitochondrial glycerolphosphate acyltransferase activities were measured after 24 hr exposure of rat hepatocytes to Adriamycin. Both activities decreased with increasing concentrations of Adriamycin. The activity of the microsomal glycerolphosphate acyltransferase, which was determined from the difference between the total and mitochondrial enzyme activity, also decreased with increasing drug concentration. The effect on glycerolphosphate acyltransferase was specific as there was no change in lactate dehydrogenase or cytochrome oxidase activity in this time period. Adriamycin did not inhibit mitochondrial glycerolphosphate acyltransferase activity in vitro. After 24 hr exposure of hepatocytes to Adriamycin no change was observed in the biosynthesis of phosphatidylcholine or triacylglycerol. Secretion of lipid into the medium was measured over the subsequent 24 hr. There was a significant reduction in very low density lipoprotein secretion as measured by triacylglycerol secretion from cells incubated with 5 microM Adriamycin. Cells were damaged by the 48 hr exposure to 1 microM and higher concentrations of Adriamycin as evidenced by a fall in lactate dehydrogenase activity in these cells. The secretion of lysophosphatidylcholine, as measured by the incorporation of [3H]glycerol into medium lysophosphatidylcholine, was significantly increased when cells were incubated with 5 microM Adriamycin. The results are discussed in relation to the effect of Adriamycin on hepatic lipid metabolism and the cardiotoxicity of the drug.

Insulin activates glycerol-3-phosphate acyltransferase (de novo phosphatidic acid synthesis) through a phospholipid-derived mediator. Apparent involvement of Gi alpha and activation of a phospholipase C

We studied the mechanism whereby insulin activates de novo phosphatidic acid synthesis in BC3H-1 myocytes. Insulin rapidly activated glycerol-3-phosphate acyltransferase (G3PAT) in intact and cell-free preparations of myocytes in a dose-related manner. The apparent Km of the enzyme was decreased by treatment with insulin, whereas the Vmax was unaffected. No activation was found by ACTH, insulin-like growth factor-I, angiotensin II, or phenylephrine, but epidermal growth factor, which, like insulin, is known to activate de novo phosphatidic acid synthesis in intact myocytes, also stimulated G3PAT activity. In homogenates or membrane fractions, the effect of insulin on G3PAT was fully mimicked by nonspecific or phosphatidylinositol (PI)-specific phospholipase C (PLC). An antiserum raised against PI-glycan-PLC completely blocked the effect of insulin on G3PAT. Although the above findings suggested involvement of a PLC in insulin-induced activation of G3PAT, neither diacylglycerol nor protein kinase C activation appeared to be involved. On the other hand, insulin stimulated the release of a cytosolic factor, which activated membrane-associated G3PAT. This cytosolic factor had a molecular weight of less than 5K as determined by Sephadex G-25 chromatography. NaF, a phosphatase inhibitor, blocked the activation of G3PAT by insulin, suggesting involvement of a phosphatase. Insulin-induced activation of G3PAT was also blocked by pretreatment of intact myocytes with pertussis toxin and by prior addition, to homogenates, of an antiserum that recognizes the C-terminal decapeptide of Gi alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased incorporation of glucose into lipids and increased lactate production by adipose tissue after long-term treatment of rats with D-fenfluramine

Male rats were treated with ten daily doses of 10 mg of D-fenfluramine/kg. Body weight decreased after days 1 and 2, but thereafter the weight gain paralleled that of the control rats. After the tenth injection there were decreases in the weights of the epididymal fat pads, their fat content, and the average size of the adipocytes after collaginase digestion. The rate of glucose uptake by incubated pieces of adipose tissue was maintained after D-fenfluramine treatment, and the production of lactate increased. The incorporation of glucose into fatty acids by adipose tissue pieces decreased by 65-74% after treatment with D-fenfluramine. This effect was not reversed by adding insulin or phenylisopropyladenosine to the incubations. D-Fenfluramine also decreased the incorporation of glucose into glyceride-glycerol, but this effect was less pronounced than that for fatty acid synthesis. Direct addition of D-fenfluramine to the incubation inhibited lipid synthesis from [14C]glucose but only at drug concentrations above 1 mM. It is concluded that the treatment of rats with D-fenfluramine modifies the metabolic balance of adipose tissue so as to direct glucose metabolism away from lipid synthesis and towards lactate production. This could be a significant mechanism in the overall loss of adipose tissue mass caused by the administration of D-fenfluramine.

Triacylglycerol synthesis in cultured rat hepatocytes. The rate-limiting role of diacylglycerol acyltransferase

The limiting role of diacylglycerol acyltransferase with respect to triacylglycerol synthesis in cultured rat hepatocytes was evaluated by following the inhibition of the overall synthetic flux by 2-bromooctanoate acting as an inhibitor of the diacylglycerol acyltransferase step. The flux-control coefficient of diacylglycerol acyltransferase in intact cultured hepatocytes amounted to 0.76 in the presence of saturating glycerol and either palmitate or oleate as the fatty acyl substrates. The flux-control coefficient of diacylglycerol acyltransferase in lysolecithin-permeabilized cultured hepatocytes amounted to 0.80 and 0.99 in the presence of saturating glycerol 3-phosphate and either palmitate or oleate as the fatty acyl substrate, respectively. Hence, triacylglycerol synthesis in liver cells under the experimental conditions employed is rate-limited by the diacylglycerol acyltransferase.

Dietary hyperphagia and obesity: what causes them?

Diets that cause animals to overeat and become obese have been used in many investigations of obesity. Most of this research, however, has concentrated on the consequences rather than the causes of overeating. Furthermore, in most studies, several nutritional variables were manipulated simultaneously, making cause and effect relationship impossible to disentangle. Consequently, progress has been slow. Diets could alter energy intake by virtue of their effects on oral-sensory, gastrointestinal or postabsorptive effects. Palatability is the most popular oralsensory hypothesis but the empirical basis for this hypothesis is particularly weak. A substantial body of evidence is consistent with the possibility that the osmotic effects of diets in the gastrointestinal tract and metabolic postabsorptive factors may play a major role in dietary hyperphagia and obesity. Suggestions for future research directions are offered.

Effects of chronic administration of benfluorex to rats on the metabolism of corticosterone, glucose, triacylglycerols, glycerol and fatty acid

(1) Rats were fed on diets enriched with sucrose, beef tallow or corn oil and treated for 11-16 days with 50 mg of benfluorex per kg of body weight. By these times the growth rate and food intake were not significantly different from those of control rats. (2) Benfluorex approximately halved the concentration of circulating triacylglycerol in rats fed the beef tallow or sucrose diets. (3) It did not significantly alter the total lipoprotein lipase activity in diaphragm, heart and adipose tissue. (4) The clearance of triacylglycerols from chylomicrons exhibited two t 1/2 values of about 0.6 and 6.9 min in rats fed the beef tallow diet. Benfluorex did not significantly alter these values. (5) Benfluorex did not significantly alter the rate of appearance of triacylglycerol in the blood of rats injected with Triton WR 1339 to block triacylglycerol uptake. It did, however, decrease the rise in circulating glucose which presumably resulted from the stress of the procedure. (6) Benfluorex decreased the extent and duration of the rise in serum corticosterone when rats maintained on the corn oil diet were fed acutely with fructose. It also decreased the circulating concentrations of glycerol, triacylglycerol and glucose after fructose feeding. (7) Rats fed on the corn oil diet and then treated with benfluorex had lower concentrations of circulating glucose, triacylglycerol, glycerol and fatty acids after being injected with 2-deoxyglucose. (8) It is proposed that some of the long-term hypoglycaemic and hypotriglyceridaemic effects of benfluorex could be mediated indirectly through changes in endocrine balance, perhaps via the serotonergic system and in particular, by decreasing the effects of stress hormones relative to insulin. The implications of these findings are discussed in relation to controlling metabolism in stress conditions and for the management of obesity, diabetes and atherosclerosis.

Di(2-ethylhexyl)phthalate enhances hepatic phospholipid synthesis in rats

The effects of di(2-ethylhexyl)phthalate, a typical peroxisomal proliferator, on the activities of key enzymes in the glycerophospholipid synthetic pathway and the incorporation of lipid precursors into liver lipids in vitro were studied periodically in rats. When di(2-ethylhexyl)phthalate was fed at the 1% level to rats, glycerol-3-phosphate acyltransferase activity increased 2-3-fold in liver homogenates and microsomes in 2-4 days. The specific activity of microsomal CTP:phosphocholine cytidylyltransferase increased by 1.5-fold, whereas the cytosolic activity was depressed. The microsomal CDPcholine:diacylglycerol cholinephosphotransferase specific activity decreased, whereas the activity in the homogenates increased, suggesting the proliferation of the hepatic endoplasmic reticulum in di(2-ethylhexyl)phthalate-treated rats. The incorporation of [1(3)-3H]glycerol or [1-14C]acetate into liver phospholipids in vitro increased in 2 days and stayed at a high level up to 12 days. The present study confirmed that di(2-ethylhexyl)phthalate induced an enhancement of phospholipid synthesis in the liver. The increase in hepatic phospholipid synthesis by this drug is presumably linked to the proliferation of peroxisomes and other intracellular membranes.

Effect of dietary fat on total and peroxisomal fatty acid oxidation in rat tissues

In this study the effect of dietary trans fatty acids on the peroxisomal and mitochondrial beta-oxidation is compared with that of saturated or cis-monounsaturated fatty acids. Oxidation of [1-14C]- and [16-14C]palmitate was assayed in the absence as well as in the presence of antimycin plus rotenone in homogenates of liver, heart and skeletal muscle of four groups of rats fed diets containing 40 energy% fat of different fatty acid composition. Three groups were given fat blends rich in C16, C18 saturated (cocoa butter), cis-monounsaturated (low-linoleic-acid olive oil) or trans fatty acids (partially hydrogenated soybean oil), respectively. The fourth group received a mixture of these fats with half the amount of trans fatty acids of the third group. Total oxidation rates of [1-14C]- and [16-14C]palmitate in the absence of antimycin were not significantly influenced by the type of dietary fat in the investigated tissues. The antimycin-insensitive [1-14C]palmitate oxidation rate and the proportion of peroxisomal oxidation of the total oxidation were lower in all tissues of those animals fed the mixed dietary fat than in those fed the other diets; both parameters were higher in the liver of cocoa butter-fed rats than in those of the other groups. Comparison of the results with literature data and with previous results obtained with a low-fat diet (Veerkamp and Van Moerkerk (1986) Biochim. Biophys. Acta 875, 301-310) indicates that high-fat diets only induce peroxisomal beta-oxidation activity if they also contain C20, C22 fatty acids. High dietary concentrations of trans C18 fatty acids do not result in a higher peroxisomal activity than that observed for other fatty acids with the same chain length.

In vivo insulin resistance in individual peripheral tissues of the high fat fed rat: assessment by euglycaemic clamp plus deoxyglucose administration

We have examined peripheral insulin action in conscious rats chronically fed high fat (60% calories as fat) or high carbohydrate (lab chow) diets using the euglycaemic clamp plus 3H-2-deoxyglucose technique. A response parameter of individual tissue glucose metabolic rate (the glucose metabolic index, based on tissue deoxyglucose phosphorylation) was used to assess diet effects in eight skeletal muscle types, heart, lung and white and brown adipose tissue. Comparing high fat with high carbohydrate fed rats, basal glucose metabolism was only mildly reduced in skeletal muscle (only diaphragm was significant, p less than 0.05), but was more substantially reduced in other tissues (e.g. white adipose tissue 61% and heart 33%). No evidence of basal hyperinsulinaemia was found. In contrast, widespread insulin resistance was found during the hyperinsulinaemic clamp (150 mU/l) in high fat fed animals; mean whole body net glucose utilization was 34% lower (p less than 0.01), and the glucose metabolic index was lower in skeletal muscle (14 to 56%, p less than 0.05 in 6 out of 8 muscles), white adipose (27%, p less than 0.05) and brown adipose tissue (76%, p less than 0.01). The glucose metabolic index was also lower at maximal insulin levels in muscle and fat, suggesting the major effect of a high fat diet was a loss of insulin responsiveness. White adipose tissue differed from muscle in that incremental responses (maximal insulin minus basal) were not reduced by high fat feeding. The heart showed an effect opposite to other tissues, with an increase in insulin-stimulated glucose metabolism in high fat versus chow fed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of insulin, glucagon and dexamethasone in controlling the activity of glycerol phosphate acyltransferase and the activity and subcellular distribution of phosphatidate phosphohydrolase in cultured rat hepatocytes

Rat hepatocytes were incubated in monolayer culture for 8 h. Glucagon (10nM) increased the total phosphatidate phosphohydrolase activity by 1.7-fold. This effect was abolished by adding cycloheximide, actinomycin D or 500 pM-insulin to the incubations. The glucagon-induced increase was synergistic with that produced by an optimum concentration of 100 nM-dexamethasone. Theophylline (1mM) potentiated the effect of glucagon, but it did not affect the dexamethasone-induced increase in the phosphohydrolase activity. The relative proportion of the phosphohydrolase activity associated with membranes was decreased by glucagon when 0.15 mM-oleate was added 15 min before the end of the incubations to translocate the phosphohydrolase from the cytosol. This glucagon effect was not seen at 0.5 mM-oleate. Since glucagon also increased the total phosphohydrolase activity, the membrane-associated activity was maintained at 0.15 mM-oleate and was increased at 0.5 mM-oleate. This activity at both oleate concentrations was also increased in incubations that contained dexamethasone, particularly in the presence of glucagon. Insulin increased the relative proportion of phosphatidate phosphohydrolase that was associated with membranes at 0.15 mM-oleate, but not at 0.5 mM-oleate. It also decreased the absolute phosphohydrolase activity on the membranes at both oleate concentrations in incubations that also contained glucagon and dexamethasone. None of the hormonal combinations significantly altered the total glycerol phosphate acyltransferase activity. However, glucagon significantly increased the microsomal activities, and insulin had the opposite effect. Glucagon also decreased the mitochondrial acyltransferase activity. There was a highly significant correlation between the total phosphatidate phosphohydrolase activity and the synthesis of neutral lipids from glycerol phosphate and 0.5 mM-oleate in homogenates of cells from all of the hormonal combinations. Phosphatidate phosphohydrolase activity is increased in the long term by glucocorticoids and also by glucagon through cyclic AMP. In the short term, glucagon increases the concentration of fatty acid required to translocate the cytosolic reservoir of activity to the membranes on which phosphatidate is synthesized. Insulin opposes the combined actions of glucagon and glucocorticoids. The long-term events explain the large increases in the phosphohydrolase activity that occur in vivo in a variety of stress conditions. The expression of this activity depends on increases in the net availability of fatty acids and their CoA esters in the liver.

Possible relationships between changes in body weight set-point and stress metabolism after treating rats chronically with D-fenfluramine. Effects of feeding rats acutely with fructose on the metabolism of corticosterone, glucose, fatty acids, glycerol and triacylglycerol

Rats were maintained on a corn oil diet and treated with D-fenfluramine at doses of 2.5 mg/kg twice a day for 11 days or with 10 mg or 25 mg/kg once a day for 12 days. The lower dose of D-fenfluramine produced no marked changes in body weight and after 11 days of treatment the weights of the rats on average were only 2% lower than the controls. The food intake of these rats was only decreased on the first day. The two higher doses of D-fenfluramine decreased the food consumption for about 3 days but thereafter it was similar to that of the control rats. The body weight of these rats fell on the first day, but after about four days the gain in body weight paralleled rather than approached that of the control rats. Increasing the dose of D-fenfluramine progressively decreased the relative size of the epididymal fat pad. At the end of the treatment period the rats were fed acutely with fructose to increase the circulating concentrations of corticosterone and to stimulate triacylglycerol synthesis. All three doses of D-fenfluramine decreased the concentration of circulating triacylglycerol after fructose feeding. The 10 mg/kg dose also decreased the basal concentration of triacylglycerol. The two higher doses of fenfluramine decreased the rises in the circulating concentrations of corticosterone, glycerol and fatty acids that are produced by fructose feeding. The basal concentrations of these compounds in the absence of fructose feeding were not significantly affected by the 10 mg/kg dose of D-fenfluramine. The possible relationship between the effect of chronic treatment with D-fenfluramine in decreasing a metabolic stress response and lipolysis is discussed relative to its hypotriglyceridaemic action and its effect on body weight-set point. The results demonstrate that D-fenfluramine produced persistent changes in metabolism at a time when the treated rats were growing at the same rate as the control rats and when they were eating similar quantities of food.

The adaptive effects of dietary fish and safflower oil on lipid and lipoprotein metabolism in perfused rat liver

To elucidate the mechanisms underlying the plasma triacylglycerol-lowering effects of certain fish oils, livers from male rats fed either a standard commercial diet (controls) or diets supplemented with 15% (w/w) fish or safflower oils were perfused with undiluted rat blood. Rates of hepatic lipogenesis, measured by the incorporation of 3H2O into fatty acids, followed the order: control greater than safflower oil greater than fish oil. Secretion of newly synthesized fatty acids in very-low-density lipoproteins was also inhibited by the feeding of both oil-supplemented diets with the greater suppression being seen in livers from animals fed fish oil. The hepatic release of very-low-density lipoprotein triacylglycerol mass was also significantly depressed in animals fed the fish oil-supplemented diet but not in those fed safflower oil. Ketogenesis did not differ between livers from rats fed the control and safflower oil diets but was significantly raised in the fish oil group. Increased ketogenesis with fish oil was paralleled by a decrease in the sensitivity of carnitine palmitoyl transferase of isolated mitochondria to inhibition by malonyl-CoA. The inhibitory effect of malonyl-CoA in the safflower oil group was intermediate between that in the fish oil and control groups. Activities of glycerophosphate acyltransferase with either palmitoyl-CoA or oleyl-CoA were increased by feeding oil-supplemented diets. Activity with palmitoyl-CoA that was suppressible by N-ethylmaleimide was also considerably diminished in both groups. The results indicate that the lowering of plasma triacylglycerols by fish oil reflects: (a) diminished lipogenesis; (b) increased fatty acid oxidation possibly in peroxisomes; and (c) diminished secretion of triacylglycerols by the liver.