Comparison between the hypercholesterolaemia in rabbits induced by semipurified diets containing either cholesterol or casein

Rabbits were fed a semipurified diet containing soy protein for 4 weeks and subsequently transferred to a semipurified diet containing soy protein plus cholesterol (2 g/kg), or a semipurified diet containing casein as protein source. One group of rabbits was fed the soy protein diet throughout the entire experimental period. Blood samples from the animals were taken after an overnight fast. The rabbits transferred to the soy plus cholesterol diet and to the casein diet showed a significant increase in serum cholesterol concentration after 1 and 3 days, respectively. The cholesterol and protein content of the LDL1 (1.019 less than p20 less than 1.040) fraction was markedly increased after 3 days on the casein and soy plus cholesterol diets. Thereafter the cholesterol, but not the protein concentration increased in the IDL1 (1.006 less than p20 less than 1.012) and VLDL (p20 less than 1.006) fractions, the effect being earlier and more pronounced in the soy plus cholesterol-fed animals. When compared to the soy-fed animals, the casein and soy plus cholesterol-fed animals showed a marked increase in the apoprotein E content of their VLDL and IDL fractions. It is concluded that cholesterol- and casein-induced hypercholesterolaemia in rabbits develop in a similar manner. In both hypercholesterolaemias the cholesterol concentration increases first in the LDL fraction and subsequently in the IDL and VLDL fractions.

Effects of insulin and glucagon on fatty acid synthesis from acetate by hepatocytes incubated with (--)-hydroxycitrate

(--)-Hydroxycitrate is a well-known inhibitor of the citrate cleavage enzyme (EC 4.1.3.8). In isolated hepatocytes it inhibits fatty acid synthesis from glucose, but it does not affect fatty acid synthesis from acetate. In its presence, insulin stimulates and glucagon inhibits incorporation of labelled acetate into fatty acids. This is evidence that both hormones directly influence fatty acid synthesis from acetate.

Short-term regulation of hepatic triacylglycerol metabolism by insulin and glucagon

This paper presents a review of our current knowledge of short-term regulation of triacylglycerol metabolism by insulin and glucagon in isolated rat hepatocytes. Insulin is devoted to rapidly improve the lipogenic climate of the hepatocyte. The hormone stimulates glycolysis, the conversion of pyruvate into acetyl-CoA, the synthesis of fatty acids and their esterification. The opposite pathways, hydrolysis of triacylglycerols and fatty acid oxidation, are depressed by insulin. Glucagon does all but deteriorate the lipogenic climate within the liver cell as the hormone inhibits triacylglycerol synthesis and stimulates the breakdown of triacylglycerols.

Inhibition of hepatic lipogenesis by salicylate

Salicylate has been found to be an inhibitor of fatty acid synthesis in isolated rat hepatocytes. Half-maximal inhibition of fatty acid synthesis occurs at approximately 2 mM. The inhibitory effect of salicylate on fatty acid synthesis is not relieve by the addition of acetate, suggesting that salicylate inhibits the conversion of acetate into fatty acids. Acetyl-CoA carboxylase activity in homogenates of hepatocytes is not influenced by previous exposure of the intact cells to salicylate. Partially purified acetyl-CoA carboxylase, isolated and assayed in the absence of citrate, is markedly inhibited by salicylate. However, in the presence of 0.5 mM citrate, which is the concentration of this metabolite in the cytosol of the liver cell, salicylate activates the enzyme. Upon treatment of acetyl-CoA carboxylase with salicylate (in the absence or presence of citrate), followed by separation of enzyme and effector on a Sephadex G-25 column, the enzyme activity is enhanced as compared to the salicylate-free control, demonstrating that the inhibitory effect of salicylate (in the absence of citrate) is reversible, but not the stimulatory effect (in the presence of citrate). Salicylate inhibition of fatty acid synthesis by hepatocytes is not rapidly reversible; hepatocytes preincubated with salicylate followed by a wash procedure (centrifugation and resuspension) still show depressed rates of fatty acid synthesis from acetate upon further incubation. Salicylate was found to prevent pyruvate accumulation in hepatocyte suspensions observed in the absence of this compound; salicylate even induces the disappearance of pyruvate and lactate initially present in the cell suspension. This suggests that salicylate activates pyruvate and lactate consumption, which is most likely related to the well-known fact that salicylate uncouples oxidative phosphorylation. The latter action of the drug will stimulate citric acid-cycle activity. This causes an inhibition of fatty acid and cholesterol synthesis since acetyl units will be specifically channelled into the citric acid cycle and not into the lipogenic pathway. It is concluded that part of the inhibitory effect of salicylate on fatty acid biosynthesis is exerted at (a) step(s) in the conversion of acetate into fatty acids, acetyl-CoA carboxylase not being a target of this compound. In addition, salicylate prevents that pyruvate, generated by glycolysis, enters the lipogenic pathway. The latter effect of salicylate would also explain the observed inhibition of cholesterol synthesis by this compound.

Short-term inhibition of fatty acid biosynthesis in isolated hepatocytes by mono-aromatic compounds

An overview is presented of a selected number of mono-aromatic derivatives and their short-term effects on hepatic fatty acid biosynthesis. The compounds discussed in this paper are ortho-hydroxybenzoate (salicylate), meta-hydroxybenzoate, para-hydroxybenzoate, benzoate, para-t-butylbenzoate, para-aminosalicylate, clofibrate, halofenate, alpha-cyano-4-hydroxycinnamate and benfluorex. All of these drugs inhibit fatty acid biosynthesis by isolated rat liver cells, albeit with different effectiveness. In contrast, the compounds have differential effects on fatty acid esterification and oxidation by isolated hepatocytes. An attempt is made to describe in molecular terms the underlying mechanisms of the acute inhibitory effects of the mono-aromatic derivatives on hepatic lipogenesis. It is proposed that all of the drugs exert an inhibitory action at the level of acetyl-CoA carboxylase, the enzyme generally considered to catalyse the rate-limiting step in hepatic fatty acid synthesis. This inhibitory effect may be either direct, i.e. by an alteration of the enzyme's structure as a result of interaction between drug and enzyme, or indirect, i.e. through a drug-induced change in the cellular levels of allosteric effectors of acetyl-CoA carboxylase.

The effects of lactate and acetate on fatty acid and cholesterol biosynthesis by isolated rat hepatocytes

1. The present study demonstrates that lactate and acetate stimulate fatty acid synthesis and inhibit cholesterogenesis by isolated rat hepatocytes. 2. Exposure of the intact cells to lactate increases the activity of acetyl-CoA carboxylase, as can be measured in homogenates of these cells. A similar effect by acetate was not observed. 3. Both acetate and lactate drastically increase the cellular level of citrate. 4. Possible mechanisms underlying the difference in response of fatty acid and cholesterol synthesis to an increase in substrate availability are discussed. Futhermore, a mechanism is proposed for the lactate effect on acetyl-CoA carboxylase.

The effects of insulin and glucagon on the release of triacylglycerols by isolated rat hepatocytes are mere reflections of the hormonal effects on the rate of triacylglycerol synthesis

1. Isolated hepatocytes from meal-fed donor rats secrete newly synthesized very-low-density lipoproteins (VLDL) when incubated in a simple bicarbonate buffer. When incubated with 3H2O for 2 h, 72-81% of the 3H-labelled triacylglycerols secreted by the hepatocytes were recovered in VLDL. The secretion of newly synthesized triacylglycerols shows a lag phase of about 30 min. 2. Insulin stimulates the secretion of newly synthesized VLDL triacylglycerols, whereas glucagon has an inhibitory effect on this process. 3. When hepatocytes triacylglycerols were labelled by preincubating the cells with 3H2O or [1-14C]oleate and the cells were subsequently washed and further incubated in radioisotope-free buffer containing hormones, it was observed that the release of the pre-labelled triacylglycerols is not hormone-sensitive. This suggests that insulin and glucagon do not affect the release of triacylglycerols per se. 4. It is concluded that the effects of insulin and glucagon on the overall process of triacylglycerol secretion are reflections of the hormone-determined rate of triacylglycerol synthesis.

Control of glycogen metabolism by insulin in isolated hepatocytes

Isolated rat hepatocytes incubated in a simple bicarbonate buffer, previously shown to display enhanced rates of fatty acid biosynthesis upon a brief exposure to insulin, were used to study acute effects of this hormone on glycogen metabolism. In hepatocytes prepared from fed rats insulin does not increase glycogen deposition as measured chemically, but the incorporation of labelled glucose is markedly accelerated. The rate of disappearance of radioactivity from glycogen, prelabelled with D[U-14C]glucose, is lowered by insulin. In hepatocytes from starved rats insulin promotes net glycogen synthesis. All metabolic parameters studied are affected by glucagon in manner opposite to that of insulin.

Inhibition of hepatic cholesterol biosynthesis by chloroquine

Chloroquine is shown to be a potent inhibitor of cholesterol biosynthesis by isolated rat hepatocytes. Half-maximal inhibition of cholesterogenesis occurs at ca. 10 micro M chloroquine. Chloroquine does not affect fatty acid synthesis by isolated hepatocytes. This suggests that chloroquine acts on the cholesterol biosynthetic pathway beyond the cytosolic acetyl-coA branchpoint of cholesterol and fatty acid synthesis.

Inhibition of lipogenesis in isolated hepatocytes by 3-amino-1,2,4-triazole

3-Amino-1,2,4-triazole has been found to be an inhibitor of fatty acid synthesis by isolated rat hepatocytes. Half-maximal inhibition of fatty acid synthesis occurs at approximately 20mM. Acetyl-CoA carboxylase activity in homogenates of hepatocytes is not affected by previous exposure of the intact cells to 3-amino-1,2,4-triazole. The drug opposes the activation of partially purified acetyl-CoA carboxylase by citrate, but does not influence enzyme activity in the absence of citrate. As compared to fatty acid synthesis, cholesterol synthesis by the hepatocytes is more drastically depressed by incubation of the cells with 3-amino-1,2,4-triazole. Half-maximal inhibition of cholesterogenesis occurs at approximately 5 mM 8-amino-1,2,4-triazole.

Acute effects of insulin on fatty acid metabolism in isolated rat hepatocytes

Isolated rat hepatocytes, previously shown to display enhanced rates of fatty acid biosynthesis upon a brief exposure to insulin, were used to study acute effects of this hormone on other aspects of hepatic fatty acid metabolism. Insulin activates the incorporation of exogenously added fatty acids into glycerolipids and depresses their utilization in the formation of ketone bodies. Insulin increases both the activity of acetyl-CoA carboxylase and the cellular content of malonyl-CoA. Evidence is presented that malonyl-CoA plays an important role in the insulin-mediated control of both ketogenesis and de novo fatty acid synthesis. All metabolic parameters studied are affected by glucagon in a manner opposite to that of insulin.

A mathematical relationship between the fatty acid composition of the diet and that of the adipose tissue in man

Based on literature data, the hypothesis is advanced that in human subjects a direct mathematical relationship exists between the average fatty acid composition of the habitual diet and that of the lipid stores of subcutaneous adipose tissue. Since the half-life of adipose tissue fatty acids in man is in the order of 600 days, the fatty acid pattern of depot fat provides a qualitative measure of the fat intake over a period of 2 to 3 years. It is concluded that in long-term experimental and epidemiological nutritional surveys the adipose tissue fatty acid pattern of the subjects is a useful index of the average composition of their habitual dietary fat.

Opposite effects of insulin and glucagon in acute hormonal control of hepatic lipogenesis

Conditions for the isolation of rat hepatocytes that are responsive to insulin with regard to fatty acid synthesis were explored. Cells prepared according to the procedure of Ingebretsen and Wagle require the presence of fetal calf serum for insulin expression. Cells isolated by the Seglen method are the preparation of choice, since they respond to insulin in a simple, well-defined medium and, moreover, show much higher basal rates of fatty acid synthesis. In the latter cells isolated from fed male rats, the rate of fatty acid synthesis, as determined by tritium incorporation from [3H]H2O at 37 degrees C, is enhanced within 30 min after addition of insulin to the incubation medium; with glucagon, it is depressed. In the presence of insulin, the cellular content of malonyl coenzyme A is noticeably increased, whereas the concentrations of pyruvate, lactate, and citrate are not markedly affected. Glucagon, on the other hand, decreases the concentrations of all four intermediates. The activity of acetyl-CoA carboxylase is stimulated and depressed after addition of insulin and glucagon, respectively. In all conditions tested, the activity of acetyl-CoA carboxylase correlates with the rate of fatty acid synthesis, which in turn correlates with the cellular level of malonyl-CoA.