Relationship between inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase by cholesterol feeding and short-term changes in membrane fluidity during neonatal development

Cyclic fluctuations of 3-hydroxy-3-methylglutaryl-CoA reductase in aortic smooth muscle cell cultures

The cyclic fluctuations of HMG-CoA reductase activity and mRNA are reportedly related to feeding the cells in culture or to variations in food consumption by the animals over a 24-h cycle. In this work, we demonstrate cyclic increments in HMG-CoA reductase activity in smooth muscle cells (SMC) not associated with the culture feeding. Since reductase activity also shows a marked rise preceding the S phase, one of the major goals of the present work was to evaluate this dual role of reductase activity and mRNA fluctuations related to the cell cycle and to food intake in the SMC-C/SMC-Ch cultures derived from control-fed (SMC-C) and cholesterol-fed (SMC-Ch) chicks. The period and amplitude oscillations in HMG-CoA reductase activity varied depending on culture conditions: lipoprotein-deficient serum vs. FBS, young vs. senescent cells, or confluent vs. nonconfluent cultures. The HMG-CoA reductase mRNA concentration showed a marked rise after feeding not correlated to the fluctuation activity, suggesting posttranscriptional modulation. Reductase activity and mRNA were down-regulated in SMC-Ch. Since the nutritional culture conditions were the same in both cell lines, these findings indicate that consumption of a high-cholesterol diet by the animals prior to the establishment of the SMC cultures induced changes in the HMG-CoA reductase gene expression in-aortic SMC.

Differential effects of dietary fat on chick plasma and liver composition and HMG-CoA reductase activity

The comparative effects of diet supplementation with 10% saturated fat rich in 12:0 and 14:0 fatty acids (coconut oil), without and with 1% added cholesterol, and with 10% unsaturated fat rich in n-3 polyunsaturated fatty acids (menhaden oil) on cholesterol metabolism in neonatal chicks were examined to clarify the different mechanisms of their hyper- and hypolipidemic action. Supplementation of coconut oil produced a significant hypercholesterolemia after 7 days of treatment, with a similar increase in the amount of both free and esterified cholesterol. Supplementation of coconut oil plus cholesterol produced a higher increase of plasma cholesterol levels (approximately two to three times higher than those found with standard diet). However, supplementation of menhaden oil induced a significant decrease in total cholesterol after only 2 weeks of treatment. Levels of plasma triglycerides did not change by coconut oil addition to the diet, but a significant increase was observed after coconut oil plus cholesterol feeding. Menhaden oil produced a transient decrease in plasma triglycerides. Hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity did not change with coconut oil treatment. However, both coconut oil plus cholesterol and menhaden oil supplemented diets drastically decreased reductase activity after 1 week of dietary manipulation. These results show that different nutrients with the same inhibitory effect on reductase activity produced opposite effects on plasma cholesterol content, suggesting the existence of important differences in the regulatory mechanisms implied in cholesterol biosynthesis and its accumulation in plasma.

Regulation of mevalonate 5-pyrophosphate decarboxylase in HeLa cells. Inhibition of enzymatic protein synthesis by serum lipoproteins

Mevalonate 5-pyrophosphate decarboxylase (EC 4.1.1.33) has been considered as a secondary site of regulation of cholesterogenesis. Because of this, we have studied the regulation of decarboxylase in HeLa cells by serum lipoproteins in the cell culture medium. A first group of experiments was performed with cells grown in Eagle's medium with 10% foetal calf serum. The specific activity of decarboxylase was increased when whole foetal calf serum was replaced with lipoprotein-poor serum. This increase was clearly reduced in the presence of cycloheximide. Addition of serum lipoproteins to a medium containing lipoprotein-poor serum led to a clear decrease in the decarboxylase activity. An identical decrease was observed after the addition of lipoproteins alone or in combination with cycloheximide. These results suggest for the first time that the effect of serum lipoproteins on decarboxylase activity should be a decrease in the rate of enzymatic protein synthesis, and corroborate the important role of reactions other than those catalysed by 3-hydroxy-3-methylglutaryl-CoA reductase in the regulation of cholesterogenesis.

Diurnal rhythm of the in vivo acetate metabolism to CO2 and nonsaponifiable lipids by neonatal chick

The in vivo incorporation of acetate into nonsaponifiable lipids was studied in different tissues from 14-day-old chick. Total nonsaponifiable lipids (nmol/30 min/g tissue) were mainly synthesized in testicles and liver. The in vivo CO2 production from acetate by 1-day-old chick did not exhibit diurnal variations. However, in 14-day-old chick, a maximal value was observed in the middle of the light period, while a minimal value was found 9 h after the start of the dark period. No significant diurnal differences were detected in the in vivo acetate incorporation into nonsaponifiable lipids by liver and duodenal mucosa from 1-day-old chick. Nevertheless, a clear diurnal rhythm was found in liver and duodenal mucosa from 14-day-old chick, but not in brain and kidney from animals of the same age. Distribution of radioactivity from (1-14C)acetate among the different constituents of the nonsaponifiable fraction has been also studied at 3-h intervals. Cholesterol was the major sterol formed from acetate by chick liver at any time of day. In duodenal mucosa and kidney, maximal values in the percentage of cholesterol synthesized were observed during the light period.

Homeostatic restoration of microsomal lipids and enzyme changes in HMG-CoA reductase and acyl-CoA: cholesterol acyltransferase in chick liver

We have studied the correlation between changes in the lipid composition in chick liver microsomes and the activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acyl-CoA: cholesterol acyltransferase (ACAT) by in vivo and in vitro experiments with 21-day-old chicks. A 5% cholesterol diet for 3 hr produced an increase in the microsomal and plasmatic cholesterol content, a decrease in HMG-CoA reductase activity and a concomitant increase in ACAT activity. The effect produced by the short-term treatment virtually disappeared 27 hr after ending the cholesterol diet. In vitro experiments were carried out by using vesicles constituted by phosphatidylcholine/cholesterol and phosphatidylcholine.

Induction in Gallus domesticus of experimental hypercholesterolemia by saturated fat. Effects on cholesterogenic enzyme activity

The effect of coconut oil supplementation to the diet (10 or 20%) on lipid levels in plasma and liver as well as on the cholesterogenic enzyme activity were studied in 14-day-old chicks. Treatments for 1 or 2 weeks did not interfere in the growth rate of animals nor in the liver weight. The 10% coconut oil group showed a significant increase of plasma cholesterol after 2 weeks of treatment, while after 1 week the increase was not statistically significant. The 20% coconut oil group increased plasma cholesterol from the first week. Triacylglycerol content increased after each coconut oil supplementation to the diet during the first week. Hepatic cholesterol did not change significantly after any treatment assayed. No significant difference was observed in the cholesterogenic activity, measured as hepatic 3-hydroxy-3-methylglutaryl-CoA reductase, so that this study provides a perfect model of hypercholesterolemic animals without changes in their cholesterogenic ability.

Ethanol and lipid metabolism. Differential effects on liver and brain microsomes

We have determined the effect of prolonged ethanol treatment on several enzyme activities related to lipid metabolism in chick-brain and liver microsomes. Ethanol increased microsome cholesterol levels in both organs. The treatment caused a marked increase in the hepatic HMG-CoA reductase and ACAT activities while in the brain a clear decrease was found in these enzyme activities. At the same time the activity of reacylation of phospholipids, was clearly modified in both brain and liver. Thus, while in the liver the turnover of acyl moieties of phosphatidylethanolamine, sphingomyelin and phosphatidylinositol was enhanced by ethanol consumption, in the brain only the reacylation of phosphatidylserine increased to any significant extent. These results indicate that ethanol exerts a differential action in brain and liver, namely cholesterol synthesis and esterification decreased in brain and increased in chick liver. Ethanol also induces faster phospholipid metabolism in both brain and liver microsomes.

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.

The reversal of the effects on HMG-CoA reductase activity and microsome lipid composition in chick livers caused by short-period cholesterol feeding

Chicks of different ages were fed on a cholesterol diet for 48 hours and then switched to a standard diet. HMG-CoA reductase activity and microsomal cholesterol levels, altered by the cholesterol diet, returned to control values in younger chicks after 48 hours on the standard diet. This study clearly shows that early age is an important factor to be taken into account when considering the reversibility of cholesterol-induced membrane alterations.

Effects of clofibrate on the main regulatory enzymes of cholesterogenesis

The in vivo effect of clofibrate on the main regulatory enzymes of cholesterogenesis has been comparatively studied for the first time in chick liver and brain. 3-Hydroxy-3-methylglutaryl-CoA reductase and mevalonate 5-pyrophosphate decarboxylase from chick liver were significantly inhibited by this hypocholesterolenic drug, while mevalonate kinase and mevalonate 5-phosphate kinase were not affected. No enzyme from chick brain was significantly inhibited by the in vivo treatment. However, both liver and brain reductase activity was inhibited in vitro by clofibrate, inhibition that was progressive with increasing concentrations (1.25-5.00 mM) of drug.

Distribution of 3-hydroxy-3-methylglutaryl-CoA reductase in isolated villus and crypt cells of chick duodenum, jejunum and ileum

3-Hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34), the major rate-limiting enzyme of cholesterogenesis, was studied in epithelial cells isolated in a villus to crypt gradient from chick duodenum, jejunum and ileum, in order to resolve the apparent controversy that exists on the anatomical localization of sterol synthesis in the intestine. Consistent separation was demonstrated by using the marker enzymes alkaline phosphatase, specific to the villus cells, and thymidine kinase, specific to the crypt cells. No relative difference in stability was observed, as shown by the equal distribution of acid phosphatase. Cells were 90-95 per cent viable. The highest specific activity of reductase was located in the microsomal fraction (41 per cent of the total). The mitochondria had lower specific activity (8 per cent of the total). The distribution of reductase activity in epithelial cells of the villus-crypt axis was also studied. The specific activity in each cell fraction from chick duodenum was clearly lower than that in jejunum and ileum. The jejunal and ileal crypt regions showed lower specific activity than the villus cells. About 70 per cent of total reductase activity was found in cells from the upper and the mid villus fraction in each intestinal segment.

Influence of dietary lipids on microsomal membranes from chick breast muscle

1. The effect of different dietary fat intake on the lipid composition and fluidity of microsomal membranes as well as in the enzymatic activity of the Ca2+-ATPase from chick breast muscle was investigated. 2. When a standard diet was supplemented with 10% sunflower seed oil, an increase in the relative amounts of unsaturated fatty acids and membrane fluidity and a decrease in the cholesterol content was observed. 3. The presence of 6% cholesterol in the diet does not modify the fatty acid composition and the fluidity of the membrane but increased, in a low extension, the cholesterol content. 4. The provision of the sunflower seed oil-rich diet supplemented with cholesterol just 48 hr before death promoted an increase in the relative amounts of unsaturated fatty acids and cholesterol content whereas the membrane fluidity decreased in a significant extent. 5. Despite that dietary lipids gave rise in some cases to changes in lipid composition and in the physical state of the microsomal membrane, neither the Ca2+ uptake capacity nor the ATPase activity were significantly affected.

Effects of different nutritional conditions on chick liver mevalonate-activating enzymes

The response of mevalonate kinase, mevalonate-5-phosphate kinase and mevalonate-5-pyrophosphate decarboxylase of chick liver to different dietary situations has been investigated. Fasting inhibited mevalonate kinase and mevalonate-5-pyrophosphate decarboxylase activities, while mevalonate-5-phosphate kinase remained practically unaltered. Refeeding after 72 h of starvation restored mevalonate kinase activity to normal levels after 120 h of refeeding. Likewise, decarboxylase activity reached normal levels at 72 h of refeeding the standard diet and slightly supranormal levels after 120 h. In addition, the sequential response of the three enzymes to a high cholesterol diet was followed throughout a 120 h period. Feeding a 5% cholesterol diet to 13-day-old chicks previously fed with a standard diet from hatching reduced considerably the activity of mevalonate-5-pyrophosphate decarboxylase, while the kinases were less affected. The present results support the idea of a coordinate regulation of the enzymes implied in cholesterol biosynthesis and suggest that mevalonate-5-pyrophosphate decarboxylase may play a significant role in this regulation.

Polychlorinated biphenyls interfere with the regulation of hydroxymethylglutaryl-coenzyme A reductase activity in rat liver via enzyme-lipid interaction and at the transcriptional level

Feeding a 0.05% polychlorinated biphenyl-supplemented diet to rats resulted in an increase of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase activity within 9 days, followed by a decrease towards normal levels. Polychlorinated biphenyls were incorporated into the microsomal membrane. There was a concomitant decrease in the cholesterol/phospholipid ratio in the microsomal fraction. Immunotitration studies strongly suggested that polychlorinated biphenyls modulate preexisting 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by changing the lipid environment of the enzyme. 32P-labeled cDNA probes were used to study the levels of mRNA coding for 3-hydroxy-3-methylglutaryl coenzyme A reductase during polychlorinated biphenyl feeding. Northern dot hybridization experiments showed an increase in the amount of this mRNA. This stimulation correlated with the increase in the activity of the enzyme but was more pronounced. The data suggest that polychlorinated biphenyls act on the regulation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity by enzyme-lipid interaction and at the transcriptional level.

Development of lipidic composition of neonatal chick liver and intestine microsomes: changes induced by cholesterol feeding

Changes in cholesterol and phospholipid content of chick liver and intestine microsomes were studied throughout the two first weeks of life. Differences observed throughout postnatal development were mainly due to the free cholesterol. Cholesterol feeding resulted in a clear increase of the amounts of both free and esterified cholesterol. Phospholipid content of chick liver and intestine microsomes did not change significantly after hatching. Phosphatidylcholine and phosphatidylethanolamine were found to be the major phospholipids. Although the amount of each phospholipid could be affected by cholesterol feeding, its relative percentage did not change by this treatment.

Studies on the diurnal rhythm of mevalonate metabolism by sterol and nonsterol pathways and of mevalonate-activating enzymes

Both in vivo and in vitro incorporation of mevalonic acid into nonsaponifiable lipids by 17-day-old chick liver and kidney did not show diurnal rhythm. Using 14CO2 production from MVA as an index of the shunt pathway not leading to sterols, we have demonstrated for the first time that there is no diurnal rhythm in this pathway. No significant differences were found in the specific activities of mevalonate kinase, mevalonate-5-phosphate kinase and mevalonate-5-pyrophosphate decarboxylase from chick liver and kidney throughout a period of 24 hr, using [1-14C]mevalonate as substrate. The absence of diurnal rhythm in the decarboxylase activity was corroborated by further experiments carried out using [2-14C]mevalonate-5-pyrophosphate as specific substrate of this enzyme.

Evolution of 3-hydroxy-3-methylglutaryl-CoA reductase and microsomal membrane fluidity throughout chick embryo development

The pattern of chick liver and brain 3-hydroxy-3-methylglutaryl-CoA reductase and its relationship with changes in microsomal membrane fluidity was studied during embryonic and postnatal development. A peak of brain activity was found at 19 days of embryonic development, while liver activity only increased after hatching. A significant increase in cholesterol content of brain microsomes occurred at about 14 days of incubation, decreasing afterwards. No significant variations were observed in liver microsomes during the same period. A similar profile was found in the phospholipid content of both brain and liver microsomes. The cholesterol/lipidic phosphorus molar ratio of brain and liver microsomes did not exhibit significant changes throughout embryonic and postnatal development. These results demonstrate that membrane-mediated control does not regulate the evolution of reductase activity during this developmental period.

Influence of neonatal age on changes in fatty acid composition of microsomal lipids induced by long-term and short-term cholesterol feeding

Changes observed as a function of chick age in fatty acid composition of lipids from liver microsomes were considerably small, while the unsaturation index increased throughout postnatal development. Supplementation of the diet with 2% cholesterol from hatching produced a significant decrease in the levels of palmitic acid and a clear increase in those of polyunsaturated fatty acids. Maximum effects were attained on day 19 of treatment. Alterations in the fatty acid composition were more pronounced after short-term (48 h) cholesterol feeding. Administration for 48 h of a standard diet to chicks fed a cholesterol diet for 10 days from hatching restored the levels of fatty acids to those of the controls. However, when cholesterol feeding was prolonged for 24 days from hatching, no effect was found after the same treatment. Suppression of the cholesterol diet for 48 h in animals cholesterol fed for 48 h had no effect in 12-day-old chicks while the change to a standard diet produced a reversion of the effect of cholesterol feeding in 26-day-old animals.