Effect of excess and deficiency of vitamin A on the utilization of FFA by liver and skeletal muscle.

Fatty acid metabolism in liver and skeletal muscle has been studied in rats treated with high doses of vitamin A and in those made vitamin A-deficient. Ingestion of 30,000 IU of vitamin A for two days resulted in increased incorporation of palmitate-1-14C into triglycerides but not into phospholipids. Accumulation of hepatic triglycerides was observed in vitamin A-fed rats. Deficiency of vitamin A did not cause any change in the triglyceride or phospholipid content of the liver. The rate of hepatic fatty acid oxidation and ketogenesis was markedly increased in vitamin A-fed rats. The experimental evidence indicated that vitamin A may have a stimulatory effect on these processes apart from that exerted by the high plasma FFA level in vitamin A-fed rats. Oxidation of palmitate-1-14C into C32 by skeletal muscle (latissimus dorsi) was also increased as a result of vitamin A administration. Vitamin A deficiency did not cause any change in fatty acid oxidation by liver and skeletal muscle. Hepatic palmitoyl-CoA synthetase activity was decreased in vitamin A-deficient rats. The results presented suggest that vitamin A may be required for the uptake and utilization of fatty acids by liver and akeletal muscle.     

Early effects of excessive retinol intake on gluconeogenesis. Involvement of adrenals in the increased activities on Gluconeogenic Enzymes of rat.

A stimulation of gluconeogenesis by excessive intake of retinol is suggested on the basis of enhanced incorporation of [2-¹⁴C]glycine into liver glycogen in rats fed excess retinol. Among the key gluconeogenic enzymes studied, activities of hepatic PEP-carboxykinase, fructose-1,6-bisphosphatase, glucose-6-phosphatase, and alanine aminotransferase were markedly increased, whereas that of pyruvate carboxylase remained unaltered. However, feeding of retinol to bilaterally adrenalectomized rats or rats treated with actinomycin D failed to cause significant increase in the activities of these enzymes. It is concluded that (i) gluconeogenesis is stimulated by excess retinol due to, perhaps, increased activities of key gluconeogenic enzymes, and (ii) adrenals are directly or indirectly involved in the retinol-mediated increase in the activities of the gluconeogenic enzymes. Also, data are presented that indicate a requirement for protein synthesis for the expression of retinol-mediated alterations in the activities of gluconeogenic enzymes.     

Effect of Retinol on Liver Lipid Metabolism of Rats

Effect of feeding 4.23, 16.94 and 27.53 mg of retinol daily for 10 days on the liver lipids of adult rats has been studied. Feeding of different amounts of retinol produced dose dependent toxicity symptoms in rats. Retinol feeding resulted in significant elevations of liver total lipids, total fatty acids, and glycerides, The amounts of liver esterified cholesterol were significantly raised in rats fed different amounts of retinol. Acetate-1-¹⁴C incorporation was increased in liver total cholesterol of rats fed 27.53 mg retinol and in free cholesterol of all retinol fed rats. Total ¹⁴C activity of hepatic triglycerides of retinol fed rats was the same as that of control, but their specific activity was decreased. Significant alterations were noted in phosphatidyl serine, lysophosphatidyl choline, lysophosphatidyl ethanolamine, sphingomyelin, phosphatidyl choline, phosphatidyl ethanolamine and phosphatidic acid and polyglycerophosphate fractions in liver rats fed different amounts of retinol.     

Lipogenesis and cholesterogenesis in the liver of rats after cholesterol loading

Intensity of fatty acids and separate classes of lipids synthesis was studied in vitro in the liver of white rats at loading by cholesterol in the dose of 300 mg/kg once a day during 30 days by incubation of organ homogenate with [6-(14)C] glucose, [2-(14)C] lysine, [1-(14)C] palmitic acid with following determination of radioactivity of fatty acids, phospholipids, cholesterol, acylglycerols radioactivity was investigated. The inhibition of fatty acids and separate classes of lipids synthesis in vitro in the liver of white rats at loading by cholesterol at the use of [6-(14)C] of glucose and [2-(14)C] lysine, as predecessors of fatty acids and lipids and stimulation of lipids synthesis at the use of [1-(14)C] palmitic acid as the predecessor was established. The loading of white rats by cholesterol results in its synthesis inhibition in the liver during incubation of its homogenates with [6-(14)C] glucose and does not influence the cholesterol synthesis during incubation of homogenates with [2-(14)C] lysine and [1-(14)C] palmitic acid. Thus synthesis of fatty acids and their use in the phospholipids and acylglycerols synthesis in the liver of white rats with hypercholesterolemia sharply decreases during incubation of their homogenates with [6-(14)C] glucose and [2-(14)C] lysine, and the synthesis of cholesterol, phospholipids and acylglycerols - increases during incubation with [1-(14)C] palmitic acid.     

Incorporation of acetate-1-14C into lipids by the perfused liver of normal, X-irradiated, or partially hepatectomized rats

In order to study lipid metabolism in the liver without interference due to transport from and to the liver the isolated livers of normal, X-irradiated, and partially hepatectomized rats were perfused with acetate-1-(14)C and the distribution of radioactivity in total lipids, total fatty acids, individual lipids, and fatty acids of individual lipids was determined. In X-irradiated animals, an increased incorporation of acetate into many lipids, particularly into cholesterol, was observed. Lipids in the liver of the partially hepatectomized rats exhibited a marked increase in triglyceride content together with a decreased rate of incorporation into all but the phospholipid fractions. It is concluded that the increase usually observed in lipid pontent of the regenerating liver is due to the changes in transcort rather than to changes in synthesis. The changes observed in irradiated liver could be the result of alterations in the metabolism of precursors common to most lipids.     

Cholesterol-rich Very Low Density Lipoproteins and Fatty Liver in Rats Fee Polychlorinated Biphenyls

Feeding xenobiotics such as polychlorinated biphenyls (PCB) causes hypercholesterolemia and fatty liver in rats. The hypercholesterolemia was characterized by high levels of high density lipoproteins (HDL) and apolipoprotein A-I (apo A-I), and by very low density lipoproteins (VLDL) rich in cholesterol and apo E (designated “PCB-VLDL”). The mechanisms for the generation of “PCB-VLDL” and fatty liver, and for hyper-α-lipoproteinemia in rats fed PCB were investigated. The secretion rate of VLDL-lipids was increased by PCB on day 3, while the secretion rate of only VLDL-cholesterol and phospholipid were increased by PCB on days 8 and 57. Although all liver lipids were accumulated by PCB, the accumulation of esterified cholesterol was the most drastic. These results suggested that PCB stimulated the secretion of VLDL at the early period of PCB feeding (on day 3), and that cholesterol-rich VLDL, “PCB-VLDL”, was not generated in the circulation, but was originally secreted from the liver. In spite of the stimulation of VLDL secretion, liver lipids accumulated within 8 days on the PCB diet. On days 3 and 8, serum levels of free fatty acids were not changed by PCB feeding. These data and our previous findings that PCB induced hepatic lipogenic enzymes lead us to speculate that fatty liver induced by PCB may be attributed to a stimulation of de novo synthesis of liver lipids. Even when hepatic secretion of VLDL was blocked by orotic acid, HDL-cholesterol was increased by PCB feeding, suggesting that the increase in serum level of HDL by PCB was not due to stimulation of cholesterol transport into HDL from VLDL.     

Free Radical-Induced Liver Injury. I. Effects of Dietary Vitamin E Deficiency on Triacylglycerol Level and its Fatty Acid Profile in Rat Liver

Effects of dietary vitamin E deficiency on the fatty acid compositions of total lipids and phospholipids were studied in several tissues of rats fed a vitamin E-deficient diet for 4, 6, and 9 months. No significant differences were observed between the vitamin E deficiency and controls except in the fatty acid profiles of liver total lipids. Triacylglycerol (TAG) accumulation was found in the liver of rats fed a vitamin E-deficient diet. The levels of TAG-palmitate and -oleate increased particularly in the liver from such animals. The fatty acid compositions of hepatic phospholipids were not affected by the diet. Increased TAG observed in the liver of rats fed a vitamin E-deficient diet was restored to normal when the diet was supplemented with 20 mg α-tocopheryl acetate/kg diet. These findings indicate that dietary vitamin E deficiency causes TAG accumulation in the liver and that the antioxidant, vitamin E, is capable of preventing free radical-induced liver injury.     

Early effects of dietary orotic acid upon liver lipid synthesis and bile cholesterol secretion in rats

Dietary orotic acid is known to cause impaired fatty acid synthesis and increased cholesterol synthesis in rats. We found that the impaired fatty acid synthesis occurs during the first day of orotic acid feeding and, in studies with albumin-bound [1-14C]palmitic acid, an associated decrease in the rate of esterification of this fatty acid into triacylglycerol, phospholipid, and cholesteryl ester was observed. These changes may result from the known decreases in liver levels of adenine nucleotides or, as reported here, from decreased liver CoASH levels in orotic acid-fed rats. The increase in hepatic cholesterol synthesis occurred during the second day of orotic acid feeding. It was detected by increased incorporation of [1,2-14C]acetate into cholesterol by liver slices and by a 7-fold increase in HMG-CoA reductase activity. At the same time the biliary output of cholesterol was increased 2-fold and studies using 3H2O revealed that the output of newly synthesized cholesterol in bile was increased 5-fold. The content of cholesteryl ester in hepatic microsomes decreased during orotic acid feeding but free cholesterol was unchanged. The findings are interpreted to suggest that the increased bile cholesterol secretion caused by orotic acid is a result of impaired hepatic cholesterol esterification and that the increase in HMG-CoA reductase activity is a result of diminished negative feedback due to the depleted content of cholesteryl ester in the hepatic microsomes.     

Effect of aflatoxin B1 on lipids of rat tissues.

The effect of aflatoxin B1 on lipids of liver, kidney, adipose and plasma of rats was studied. A single dose administration (6 mg/kg body weight) increased liver and kidney weights and their total lipids within 24 h. Increase in liver lipids was confined mainly to phospholipid and cholesterol, whereas triglycerides showed a significant decrease. Adipose tissue triglycerides were, however, increased. Plasma showed decreases in triglycerides, free fatty acids and cholesterol. Incorporation studies with palmitate-1-14C revealed increased incorporation in adipose tissue lipids and decreased incorporation in liver and plasma lipids, thereby indicating an increased synthesis of lipids in adipose. Their mobilization to plasma was, however, inhibited, hence the low levels of triglyceride in liver. But the adrenals showed hypo-activity upon aflatoxin B1 administration.     

Effects of two prototypic polychlorinated biphenyls (PCBs) on lipid composition of rat liver and serum

Mature male Sprague-Dawley rats received a single IP injection of either 2,2',4,4',5,5'-hexachlorobiphenyl (HCB), 3,3',4,4'-tetrachlorobiphenyl (TCB) (300 microm/kg) in corn oil (10 ml/kg) or the corn oil vehicle alone, and were killed four days later after having been fasted overnight. The vehicle control group consisted of rats which were allowed free access to feed as well as pair-fed animals. Lipid analyses were conducted on liver, hepatic microsomes and serum. TCB- (but no HCB-) treatment resulted in a statistically significant increase in total liver lipids and triglycerides. Liver phospholipids remained unchanged. Both PCBs increased the cholesterol and phospholipids content of the liver microsomal fraction. Serum lipids measured were not statistically different from control values. While HCB had little effect on the fatty acid composition of liver lipids, TCB caused an increase in C 18:1 (n-9) and a decrease in C 20:4 (n-6). Both PCBs increased C 18:0 in the hepatic microsomal fraction, but TCB also decreased C 16:0. Neither PCB altered the fatty acid composition of serum total lipids. These data are consistent with the concept that specific alterations in lipid metabolism are dependent on the structure of the PCB.     

Effects of Dietary Polychlorinated Biphenyls and Protein Level on Liver and Serum Lipid Metabolism of Rats

The effects of polychlorinated biphenyls (PCB) and dietary protein level on the liver and serum lipid metabolism of rats were studied. Rats were fed an experimental diet containing 7 or 30% casein with or without 0.1 % PCB for 24 days. Dietary PCB increased the level of triglyceride, phospholipid and cholesterol in the liver. The accumulation of triglyceride and cholesterol in liver was markedly increased with a low protein diet. The incorporation of injected ³H₂O into liver cholesterol was increased by PCB, but not affected by the dietary level of protein. The incorporation of the tracer into liver fatty acids was not increased by PCB intake. Dietary PCB also raised serum cholesterol and phospholipid, while PCB decreased triglyceride level, especially in rats on low protein diet. In addition, PCB intake clearly raised serum high density lipoprotein and diminished very low density lipoprotein. In the low protein group, PCB markedly repressed the incorporation of ³H₂O into serum lipids. The results suggest that the hepatic lipids accumulation by the addition of 0.1 % PCB to a low protein diet might be mainly ascribed to a repression in the transport of triglyceride from liver to blood. KEY WORDS: PCB, dietary protein, liver lipids, serum lipoprotein.     

FATP1 channels exogenous FA into 1,2,3-triacyl-sn-glycerol and down-regulates sphingomyelin and cholesterol metabolism in growing 293 cells

Biosynthesis of lipids was investigated in growing 293 cells stably expressing fatty acid (FA) transport protein 1 (FATP1), a bifunctional polypeptide with FA transport as well as fatty acyl-CoA synthetase activity. In short-term (30 s) incubations, FA uptake was increased in FATP1 expressing cells (C8 cells) compared with the vector (as determined by BODIPY 3823 staining and radioactive FA uptake). In long-term (4 h) incubations, incorporation of [(14)C]acetate, [3H]oleic acid, or [(14)C]lignoceric acid into 1,2,3-triacyl-sn-glycerol (TG) was elevated in C8 cells compared with vector, whereas incorporation of radiolabel into glycerophospholipids was unaltered. The increase in TG biosynthesis correlated with an increase in 1,2-diacyl-sn-glycerol acyltransferase activity in C8 cells compared with vector. In contrast, incorporation of [(14)C]acetate into sphingomyelin (SM) and cholesterol, and [3H]oleic acid or [(14)C]lignoceric acid into SM was reduced due to a reduction in de novo biosynthesis of these lipids in C8 cells compared with vector. The results indicate that exogenously supplied FAs, and their subsequently produced acyl-CoAs, are preferentially channeled by an FATP1 linked mechanism into the TG biosynthetic pathway and that such internalized lipids down-regulate de novo SM and cholesterol metabolism in actively growing 293 cells.     

Tumor lipids and liver lipid metabolism in the model human lung carcinoma/nude mice

Tumor lipids were studied in the experimental model Human Lung Carcinoma/nude mice as well as the effect of this human neoplasm on the host liver lipid metabolism. Fatty acid profiles from tumoral lipids revealed the loss of specificity for fatty acid composition in triglycerides. Host liver fatty acid composition and cholesterol metabolism were affected by the implanted human lung tissue. A noticeable increase ratio between saturated/unsaturated fatty acids was observed in host liver fatty acid phospholipids (1.17 +/- 0.17) in comparison to control liver (0.84 +/- 0.04). Cholesterol synthesis was assessed "in vivo" by means of [14C]acetate incorporation. The specific radioactivity of [14C] cholesterol was increased by a factor of about 6 in host liver as compared with control liver. This observation along with the marked decrease in the cholesterol content of host liver and the hypocholesterolemia detected in the host mice led us to suggest an increase in the liver cholesterol catabolism promoted by the presence of the tumor.     

Fatty acid synthesis in vitro in the liver and adipose tissue of rats of various ages.

In white male Wistar rats aged 2, 4, 6, 9, 12, 18 and 32 months, receiving a standard laboratory diet (25 cal% proteins, 22 cal% fat and 53 cal% glycides) we assessed the total amount of fatty acids in the carcass and in vitro lipogenesis in liver and adipose tissue by the means of glucose-14C(U). The amount of body fat increased with age; the highest rate of increase in the percentage of body fat occurs 2 to 4 months after birth. Incorporation of 14C of glucose into total liver lipids attains maximum values in the group of rats age 2 to 4 months. By the age of 6 months lipogenesis decreased to a level observed in all the remaining age groups. In adipose tissue, the incorporation of 14C into total lipids decreased between the age of 2 to 4 months, a further decrease was observed around the age of 6 months. No appreciable changes were noted in any of the older age groups.     

Effect of Artichoke Leaf Extract on Hepatic and Cardiac Oxidative Stress in Rats Fed on High Cholesterol Diet

Hypercholesterolemia and lipid peroxidation play complementary roles in atherosclerosis. Artichoke (Cynara scolymus L., Asteraceae) leaf extract (ALE), rich in antioxidants, has cholesterol-reducing effect. We investigated the effect of ALE on serum and hepatic lipid levels and pro-oxidant–antioxidant balance in the liver and heart of hypercholesterolemic rats. Rats were fed on 4% (w/w) cholesterol and 1% cholic acid (w/w) supplemented diet for 1 month. ALE (1.5 g/kg/day) was given by gavage during the last 2 weeks. High cholesterol (HC) diet caused significant increases in serum and liver cholesterol and triglyceride levels. It increased malondialdehyde (MDA) and diene conjugate (DC) levels in both tissues. Hepatic vitamin E levels and hepatic and cardiac glutathione peroxidase (GSH-Px) activities decreased, but superoxide dismutase and glutathione transferase activities, glutathione, and vitamin C levels remained unchanged due to HC diet. Serum cholesterol and triglyceride levels and ratio of cholesterol to high-density lipoprotein (HDL)-cholesterol decreased in ALE plus HC-treated rats, but liver cholesterol and triglyceride levels remained unchanged. Significant decreases in hepatic and cardiac MDA and DC levels and increases in hepatic vitamin E and GSH-Px activities were observed in ALE-treated hypercholesterolemic rats. Our results indicate that ALE decreases serum lipids and hypercholesterolemia-induced pro-oxidant state in both tissues.     

Biosynthetic studies on mannolipids and mannoproteins of normal and vitamin A-depleted hamster livers.

The incorporation of [1-14C]mannose into hamster liver glycolipids and glycoproteins was studied in normal and vitamin A-depleted hamsters. Severly (25% weight loss) and mildly (no weight loss) deficient animals were compared to vitamin A-fed controls. The incorporation of [14C]mannose into glycolipids and glycoproteins decreased in mild and severe vitamin A deficiency by 63-90% compared to vitamin A-fed animals. These results were essentially the same whether expressed per g of wet liver, per DNA or per protein. The size of the pools of mannose, glucose and galactose and their specific radioactivity in liver were determined by gas-liquid chromatography of the boronates of the hexitols (Eisenberg, Jr, F. (1972) Methods Enzymol. XXVIIIB, 168-178) in normal and vitamin A-deficient conditions. It was found that the amount of free hexoses per g of liver was very similar in normal and vitamin A-deficient conditions. The specific radioactivities for mannose and glucose were greater in vitamin A deficiency, thus excluding the possibility that the observed severe decrease in glycopeptide and glycolipid synthesis is a reflection of a similar decrease in the specific radioactivity of the precursor pools. Quantitation of mannose in glycoprotein showed a 79% decrease in vitamin A deficiency. Specific radioactivity of mannose in glycoproteins, 20 min after injection of the label, was 187 dpm/mug of mannose in the normal and 48 kpm/mug of mannose in the vitamin A-deficient livers. It is concluded that vitamin A is necessary for the biosynthesis of liver mannose-containing glycoproteins and glycolipids.     

Effect of hypophysectomy and insulin on lipogenesis in cockerels.

Hypophysectomy increases hepatic lipogenesis in cockerels. In an attempt to find the possible hormonal mechanism for this we have examined the effects of hypophysectomy, insulin and pituitary homogenates on hepatic lipogenesis. Insulin (5 U/kg) injected intravenously, simultaneously with glucose-14C tracer, increased the amount of 14C found in liver lipids at 30 min after the injection. Similarly, insulin injected 5 min before killing, increased the incorporation of glucose-14C and acetate-14C by liver slices during a 30 min incubation. Hypophysectomy increased lipogenesis within 24 hours. The effect of insulin was most pronounced in hypophysectomized cockerels. The activity of the lipogenic enzyme, acetyl CoA carboxylase was similarly affected. A homogenate of chicken pituitaries added to the medium reduced lipid synthesis from glucose-14C by liver slices. This effect was larger in liver slices in which lipogenesis had been stimulated by insulin. The increased rate of hepatic lipgenesis in hypophysectomized cockerels may be caused partly by increased hepatic sensitivity to the lipogenic action of insulin or by the removal of a direct inhibition by pituitary hormones.     

The effect of dietary menhaden,olive, and coconut oil fed with three levels of vitamin E on plasma and liver lipids and plasma fatty acid composition in rats

The effect of dietary fats with varying degrees of unsaturation in the presence of different concentrations of vitamin E on tissue lipid levels was studied in rats. Rats were fed either menhaden oil, olive oil or coconut oil at 15% levels with either 0.1, 0.3 or 0.6 mg/g of vitamin E as alpha-tocopherol for four weeks. Rat serum and liver were analyzed for total cholesterol, HDL-cholesterol, triacylglycerol and phospholipids. In addition, fatty acid composition of serum lipids was also analyzed. Serum total cholesterol and triacylglycerol were significantly lower in rats fed menhaden oil than in those fed olive or coconut oil, while the HDL-cholesterol was significantly higher in serum of rats fed menhaden and olive oil than in those fed coconut oil. Levels of vitamin E in the diet had only a significant effect on serum cholesterol and liver phospholipids. The Pearson correlation coefficient showed a significant positive relationship between serum triacylglycerol and total cholesterol, and a negative correlation between triacylglycerol and HDL-cholesterol, and between total and HDL-cholesterol.In the liver, total cholesterol was significantly higher in rats fed coconut oil than in rats fed menhaden oil. Total liver phospholipids were lower in rats fed either coconut oil or olive oil compared to those fed menhaden oil, especially with higher levels of vitamin E intake. Higher levels of vitamin E in the diet appear to increase triacylglycerol and phospholipids in livers of rats fed menhaden oil. In the liver a significant negative correlation was observed between phospholipids and cholesterol. The type and degree of unsaturation (polyunsaturated fatty acids in menhaden oil, monounsaturated fatty acids in olive oil and saturated fatty acids in coconut oil) significantly affected plasma and tissue lipids.     

Hepatic lipids of intact or thyroidectomized rats after administration of tetracycline or ethanol]

The purpose of this work was to study the quantitative modifications of the hepatic lipids in adult thyroidectomized rats after administration of tetracycline or ethanol (acute dose or prolonged ingestion). 1. - Thyroidectomy did not inhibit the accumulation of fat in the liver of fed euthyroid or hypothyroid rats after intraperitoneal infusion of tetracycline (320 mg/body weight in 2 injections at an interval of 16 h, the diet containing 6% of lipids). 2. - Sixteen hours after the oral administration of a single large dose of ethanol (5 g/kg body weight), there were only found some small modifications of the lipid composition of the liver in fasting euthyroid or thyroidectomized rats, receiving a diet with 6% of lipids before the experiment; on the contrary, when the diet contained 19% of lipids, a fatty liver occurred in the intact rat, but not in the thyroidectomized rat. 3. - The prolonged ethanol intake (in a 20% solution in water) for 5 months with a diet containing 19% of lipids did not induce a fatty liver in intact rats but produced a decrease of hepatic non-phosphorus lipid and an increase of the cholesterol amounts. After the administration of L-thyroxin (10 mug/100 g body weight per day) to these alcoholic thyroidectomized rats during 2 weeks, it was found an increase of the hepatic non phosphorus lipids till an higher amount than in the euthyroid rats. 4. - The hepatic phospholipid amounts were relatively constant in the different experiments. These results accounting for this differential effects were discussed.