Effects of clofibrate on the main regulatory enzymes of cholesterogenesis

Influence of dipyridamole on the fatty acid composition of the main lipid classes of chick serum

We have studied the effects of dipyridamole treatment on the fatty acid composition of the main lipid classes of chick serum bearing in mind the relationship between platelet aggregation and eicosanoids production from arachidonic acid. In the free fatty acids, percentages of MUFA and n-6 PUFA fell. The effects of dipyridamole appeared to be less pronounced in triglyceride fraction, but more so in serum phospholipids and cholesterol esters. The percentage of arachidonic acid was significantly reduced by dipyridamole, as well as that of different n-3 PUFA present in phospholipid fraction. The percentage of linoleic acid in serum cholesterol esters was significantly lowered by dipyridamole, whereas that of arachidonic acid did not change significantly. Our results suggest that decreased arachidonic acid in the serum phospholipids and linoleic acid in all lipid classes may be of importance in order to dipyridamole participation in several pathologies characterized by an imbalance in the production of vasodilator and vasoconstrictor prostanoids.

Changes in plasma lipid composition induced by coconut oil. Effects of dipyridamole

The comparative effects of 10-20% coconut oil feeding on fatty acid composition of the main lipid classes of chick plasma have been studied with and without simultaneous treatment with dipyridamole in order to clarify the hypolipidemic role of this drug. Coconut oil drastically increased the percentages of lauric and myristic acids in free fatty acid and triacylglycerol fractions, whereas these changes were less pronounced in phospholipids and cholesterol esters. The percentage of arachidonic acid was higher in plasma phospholipids than in the other fractions and was significantly decreased by coconut oil feeding. Linoleic acid, the main fatty acid of cholesterol esters, was drastically increased by coconut oil feeding. Changes induced by the simultaneous administration of dipyridamole were more pronounced in the phospholipids and cholesterol esters than in the other fractions. The fall observed in linoleic acid levels after dipyridamole treatment may be of interest for a lower production of its derived eicosanoids, especially in plasma phospholipids and cholesterol esters.

Dipyridamole prevents the coconut oil-induced hypercholesterolemia. A study on lipid plasma and lipoprotein composition

For a better understanding of the hypolipidemic function of dipyridamole, we have studied the comparative effects of diet supplementation with 10% coconut oil with and without dipyridamole on the lipid plasma and lipoprotein composition in chicks. This study was performed under postprandial and food-deprivation (12h) conditions. Coconut oil induced a clear hypercholesterolemia under both feeding conditions. Simultaneous administration of dipyridamole maintained total and esterified cholesterol at levels similar to those observed in control animals sacrificed under postprandial conditions. Under these conditions, our results also show that dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. Nevertheless, it should be emphasised that the levels of total cholesterol found in intermediate- and very-low-density lipoproteins were lower than in control. All chemical components of these fractions were significantly decreased by dipyridamole. The effects were not significant in chicks deprived of food. In conclusion, our results show that the hypercholesterolemia induced by coconut oil was prevented by dipyridamole. To our knowledge, this is one of the first reports on the antihypercholesterolemic effects of dipyridamole.

Dipyridamole reduces cholesterol and triglyceride levels in plasma and lipoproteins from young chicks fed a saturated fat-enriched diet

We have studied the value of dipyridamole for the prevention of hypercholesterolemia caused by saturated fat. This study shows that supplementation of 10-20% coconut oil to the diet induced a significant hypercholesterolemia under postprandial conditions. Simultaneous administration of dipyridamole and 10% coconut oil for 2 weeks maintained plasma cholesterol and triglycerides at levels similar to control, counteracting the increase induced by coconut oil. Dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. However, it should be emphasized that cholesterol and other chemical components were more drastically reduced by dipyridamole in intermediate- and very-low-density lipoproteins than in other lipoprotein fractions.

Hypocholesterolemic activity of dipyridamole: effects on chick plasma and lipoprotein composition and arachidonic acid levels

We have studied the effects of dipyridamole treatment on chick plasma and lipoprotein composition in postprandial and fasting (12 h) conditions. Plasma cholesterol levels were higher in fasted than in fed chicks, whereas triglycerides declined during starvation. Dipyridamole treatment reduced plasma cholesterol content, mainly of the free cholesterol fraction. In postprandial conditions, total cholesterol content of high and low density lipoproteins decreased in a similar proportion to that observed in plasma. However, cholesterol and other chemical constituents of intermediate and very low density lipoproteins were more drastically reduced by dipyridamole than in plasma. Total amounts of these lipoprotein fractions were also reduced about 50%. The effects of dipyridamole in fasted animals were not significant. To our knowledge, this is one of the first reports about the response of lipoprotein cholesterol to dipyridamole treatment. A strong decrease was also found in the arachidonic acid content of plasma phospholipids and cholesterol esters fractions.

The regulation of acetyl-CoA carboxylase--a potential target for the action of hypolipidemic agents

ACC exists as two major isoforms ACC1 or ACC alpha, and ACC2 or ACC beta, and there is evidence that they play separate roles in the production of malonyl-CoA for fatty acid synthesis and the control of mitochondrial beta-oxidation, respectively. ACC alpha can be regulated at the level of gene expression, allosteric regulation of the enzyme, and reversible phosphorylation by AMP-PK. Emerging lines of research suggest that similar mechanisms of regulation exist for ACC beta. Its inactivation in heart and skeletal muscle through phosphorylation by AMP-PK is becoming well-established. ACC is an important target of certain hypolipidemic drugs such as the fibrates. This is not simply because ACC alpha inhibition decreases the synthesis of a lipid component of VLDL because fatty acids synthesized de novo in liver are not always major contributors to VLDL lipid (158); it is also because ACC beta inhibition leads to a decrease in malonyl-CoA levels and the disinhibition of fatty acid oxidation. Partitioning fatty acids towards oxidation and away from esterification is an important aspect of the lipid-lowering effects of fibrates. Fibrates could use any of the mechanisms of ACC regulation to decrease activity. They could repress ACC gene expression through the activation of PPAR alpha, and fibroyl-CoA esters could inhibit ACC allosterically just as TOFyl-CoA does. However, we have demonstrated a rapid inactivation of ACC in cultured rat hepatocytes by gemfibrozil that is mediated by activation of AMP-PK and the subsequent phosphorylation of ACC. The end result is the inhibition of hepatic fatty acid synthesis and a possible activation of beta-oxidation as evidenced by the increased production of ketone bodies. The mechanism through which fibrates activate the AMP-PK cascade, the role of PPAR alpha, the physiological responses of biosynthesis and oxidation and the use of these mechanisms by other hypolipidemic agents are areas of ongoing investigation.

Peroxisome proliferators induce apoptosis in hepatoma cells

In the AH-130 hepatoma, a poorly differentiated tumor, maintained by weekly transplantations in rats, a low percentage of cells spontaneously underwent apoptosis, mainly during the transition from logarithmic- to stationary-growth phase. It was possible to induce massive apoptosis of cells by treating them with clofibrate, a peroxisome proliferator and hypolipidemic drug. Similar results were obtained with HepG2 cells. With 1 mM clofibrate, apoptosis began to manifest itself after 1 h of treatment in vitro, and was assessed by morphological analysis, by DNA fragmentation carried out with agarose gel electrophoresis, and with flow cytometric determination of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling. The mechanisms whereby clofibrate induces apoptosis are still unclear. Since the peroxisome proliferator-activated receptor was expressed at a very low level and was not stimulated by clofibrate in the AH-130 hepatoma cells, its involvement seems unlikely. Moreover, lipid peroxidation was not increased after clofibrate treatment. Phospholipids and cholesterol were significantly decreased. The decreased cholesterol content might suggest an inhibition of the mevalonate pathway and, therefore, of isoprenylation of proteins involved in cell proliferation.

Free sterols in the rat white matter following experimental global ischemia

The pattern of free sterols occurring in the cerebral white matter was studied in an experimental model of global ischemia that was induced in rats by means of the method described by KORPACHEV et al. (1982). Sterol isolation, separation and identification was accomplished by suitable extraction procedures followed by gas-chromatography-mass spectrometry of the purified sterol extracts. It has been shown that the percentage of lanosterol happened to decrease sharply following the ischemic state and other sterols, typically occurring in maturating brains and absent in the control brain specimens from adult rats, such as 4,4-dimethylcholesterol, 4,4-dimethylcholest-5,8,24-triene-3 beta-ol and 4,4,14-trimethylcholest-5,8-diene-3 beta-ol happened to appear. The appearance of these forms of free sterols in the postischemic brain is interpreted as a biochemical exponent of regeneration processes occurring in the white matter membranes following the injury suffered during the experimental heart arrest.

The effects of probucol and clofibrate alone and in combination on hepatic cholesterol metabolism in the male rat

Male rats were fed for 10 days on a diet supplemented with either probucol or clofibrate, alone or in combination, and the effects of the drugs on hepatic cholesterol metabolism studied. Plasma triacylglycerols were significantly lowered (15.6%, P < 0.05) by the drugs in combination but not individually whereas plasma cholesterol levels were reduced by probucol alone (22.4%, P < 0.05) and the combined treatment effected a further decrease leading to a total reduction of 50.6% (P < 0.001). Probucol reduced hepatic cellular triacylglycerols (20.0%, P < 0.05) and cholesterol (15.3%, P < 0.05) but cholesteryl esters were unaffected. In combination with clofibrate, probucol accentuated the reductions in both cellular cholesterol and cholesteryl esters produced by clofibrate alone and lowered their levels by 22.8%, P < 0.01 and 38.5%, P < 0.001, respectively. Although probucol, on its own, did not affect the activity of acyl-coenzyme A:cholesterol acyltransferase (ACAT), its combination with clofibrate caused less inhibition (43.5%, P < 0.01) of this enzyme activity than clofibrate alone (65.7%, P < 0.001). Probucol had a similarly moderating effect on the clofibrate-induced reductions in microsomal cholesterol and cholesteryl esters. Neither the microsomal nor the cytosolic neutral cholesteryl ester hydrolase was affected by probucol alone although both enzymes were dramatically increased (between 350% and 550%) by clofibrate and the combined treatment. The activity of the hepatic cytosolic inhibitor of cholesteryl ester hydrolase was unaffected by clofibrate or probucol individually but the two drugs in combination increased the total activity of the inhibitor by 52.1%, P < 0.01. When allowance was made for this increased inhibitor activity, it was clear that probucol accentuated the stimulatory effect of clofibrate on the cytosolic nCEH.

The effects of clofibrate and bezafibrate on cholesterol metabolism in the liver of the male rat

Fibric acid derivatives are used to treat hyperlipidemia and have wide ranging effects on lipid metabolism. The action of these compounds on cholesterol esterification, catalyzed by acyl-coenzyme A:cholesterol acyltransferase (ACAT), has been quite widely studied, but their effect on cholesteryl ester hydrolysis and the enzyme neutral cholesteryl ester hydrolase (nCEH) has been largely ignored. Male rats were therefore fed for 10 d on a standard chow diet supplemented with either clofibrate or bezafibrate, to study their effects on plasma lipid levels and hepatic cholesterol metabolism. Plasma triacylglycerols were not significantly altered by these diets, but bezafibrate significantly lowered plasma cholesterol levels (29.7%, P < 0.01). When expressed per unit weight of DNA, both fibrates reduced the hepatic content of triacylglycerol, cholesterol and cholesteryl esters (40, 18.7, 16.5 and 66.7, 28.6, 34.2% for clofibrate and bezafibrate, respectively). ACAT activity was significantly reduced by both drugs, but clofibrate (65% inhibition) was more effective than bezafibrate (35% inhibition). The most dramatic effect of the diets was a marked increase in the activity of both the microsomal and the cytosolic nCEH. When expressed on a whole liver basis, the effect of bezafibrate on the cytosolic enzyme (13.6-fold increase in activity) was much greater than that of clofibrate (4.8-fold increase). Increases in the activity of a cytosolic protein that inhibits the activity of nCEH were also noted, but these changes were relatively small. The results suggest that the activation of nCEH, in combination with the inhibition in ACAT activity, contributes to a decrease in the cholesteryl ester content of the liver which may influence the secretion of very low density lipoprotein.

Lipid abnormalities in chronic uremic patients. Response to treatment with gemfibrozil

Seventy-four patients with end stage renal failure were studied. Forty-six of them were on hemodialysis (HD) while 28 were on continuous ambulatory peritoneal dialysis (CAPD). In addition 56 nondialysis chronic renal failure (NDCRF) patients with various degree of renal failure were also studied. In all groups serum triglyceride concentrations were significantly higher and HDL cholesterol concentrations were significantly lower compared to age- and sex-matched controls. Total and LDL cholesterol were significantly higher in the NDCRF and CAPD patients compared to controls. In 55 patients (20 on HD, 13 on CAPD and 22 NDCRF) with severe hypertriglyceridemia or diminished HDL cholesterol gemfibrozil 300 mg b.i.d. per os was given for 6 months. Drug treatment reduced significantly serum triglycerides in all groups of patients and increased the levels of HDL cholesterol in CAPD patients. Moreover, a statistically significant decrease of the levels of total and LDL cholesterol was noticed in HD and NDCRF patients. During treatment no significant side effects were observed and liver and muscle enzymes remained within normal values.

Antihypercholesterolemic effect of dipyridamole in chickens

The administration of a 4 mg/kg dose of dipyridamole daily in chickens fed a diet supplemented with 2% cholesterol reversed the hypercholesterolemic effects of the diet. In particular, it reduced the plasma cholesterol concentration in approximately 18%; the levels of very-low-density lipoproteins and intermediate-density lipoproteins and the liver cholesterol content. Although the mechanism was not fully elucidated, the increased excretion of cholesterol seemed to be responsible for the lipid lowering effect. When dipyridamole was administered in chickens fed the same diet without cholesterol no significant changes were observed. Inasmuch as the chicken lipoprotein metabolism differs in several aspects to human, the extrapolation of the hypocholesterolemic effect of dipyridamole to man must be made with care.

Modulation of lipoprotein production in Hep G2 cells by fenofibrate and clofibrate

Fenofibrate and other fibrate derivatives are commonly used to treat hyperlipidemia. It is not yet clear how they exert their modulatory effects on plasma lipoproteins. To investigate whether these drugs act on the liver to primarily inhibit very low density lipoprotein production, we utilized the highly differentiated human hepatoma cell line, Hep G2. At concentrations greater than 15 micrograms/mL, fenofibrate caused a 30% decrease in secreted apolipoprotein B (apo B) after 4 days of treatment. Pulse-chase studies demonstrated that this was not due to inhibition of apo B synthesis. Triglyceride synthesis by fenofibrate-treated Hep G2 cells was decreased by 30%, and the amount secreted into the medium was reduced by 50%. At a low concentration of drug (5 micrograms/mL), triglyceride secretion was reduced markedly while apo B secretion remained unchanged. Thus, apo B secretion is less sensitive to fenofibrate than the synthesis and secretion of triglyceride, and may be secondary to changes in the latter. Fenofibrate has also been shown to raise plasma high density lipoprotein concentrations. We found that low concentrations of fenofibrate caused a 20-101% increase in secreted apolipoprotein AI (apo AI), and pulse-chase immunoprecipitation studies showed that this was due to an increase in apo AI synthesis. Fenofibrate was compared to clofibrate to investigate whether their relative effects on lipoprotein production in Hep G2 cells were comparable to their relative effects on plasma lipoproteins. Both fibrates decreased the secretion of apo B to the same extent, but only fenofibrate increased apo AI secretion. Fenofibrate was more effective than clofibrate in inhibiting the secretion of lipids by these cells. Thus, the known effects of fenofibrate on plasma lipoproteins can be attributed to its direct modulation of lipoprotein synthesis in the liver cell. Hep G2 cells may thus be useful in testing the relative efficacy of fibric acid derivatives in vitro.

Hypolipidemic activity of dipyridamole: effects on the main regulatory enzyme of cholesterogenesis

The in vivo dipyridamole treatment for 16 days produced a significant decrease in chick plasma cholesterol, mainly due to the esterified form. This effect was especially patent in the VLDL + LDL fraction. Similar results were observed in triglyceride content. To our knowledge, this is the first report on this hypolipidemic effects of dipyridamole. Total and esterified cholesterol increased after the same treatment in chick liver, while brain cholesterol content was not affected. Hepatic 3-hydroxy-3- methylglutaryl-CoA reductase activity was drastically reduced, while other secondary regulatory enzymes such as mevalonate kinase, mevalonate 5-phosphate kinase and mevalonate 5-pyrophosphate decarboxylase did not change significantly. No significant differences were found in cholesterol and lipidic phosphorus from liver microsomes, so that the effect of dipyridamole on reductase activity cannot be due to modifications in cholesterol/lipidic phosphorus molar ratio. Neither of these enzyme activities was affected in vitro by dipyridamole.