Influence of dietary cholesterol, saturated and unsaturated lipid on 3-hydroxy-3-methylglutaryl CoA reductase activity in rabbit intestine and liver

Biliary lipids, cholesterol and bile synthesis: different adaptive mechanisms to dietary cholesterol in lean and obese subjects

BACKGROUND

Increased biliary cholesterol secretion together with elevated cholesterol synthesis may predispose obese subjects to cholesterol gallstone formation.

AIM

To investigate whether processing of dietary cholesterol is altered in obesity, we enrolled eight lean and seven obese subjects in a double-blind crossover study.

METHODS

Cholesterol consumption was 300 mg/day on low and 1300 mg/day on high cholesterol diet. After 3 weeks on either diet, hepatic bile was collected to determine biliary lipid secretion, and bile salt composition by high-performance liquid chromatography and cholesterol saturation index was calculated. Cholesterol synthesis was measured employing mass isotopomer distribution analysis. Bile acid synthesis via neutral and acidic pathway was assessed by serum levels of 7alpha-hydroxy-4-cholesten-3-one and 27-hydroxycholesterol.

RESULTS

Cholesterol synthesis was increased in obese compared with lean and feedback inhibited only in obese. On low cholesterol diet, cholesterol secretion was doubled in obese but bile acid composition and synthesis was similar between the two groups. After high cholesterol diet, cholesterol saturation index and bile secretion were unchanged. In contrast to obese, lean increased bile acid synthesis only via the acidic pathway.

CONCLUSIONS

Dietary cholesterol appears to preferentially induce bile acid synthesis via the acidic pathway in lean, whereas cholesterol synthesis was inhibited in obese. Thus, stable cholesterol saturation index may be achieved by different mechanisms.

Cholesterol metabolism and its implications for therapeutic interventions in patients with hypercholesterolaemia

Cardiovascular diseases are the principal causes of mortality in middle-aged people and in older people. Coronary heart disease (CHD) is the most common of the cardiovascular diseases; high serum levels of cholesterol are associated with atherosclerosis and an increased risk of CHD. Cholesterol homeostasis is achieved by means of a fine balance between cholesterol intake, absorption/excretion and synthesis. All of these processes are tightly linked and a change in one of them can significantly influence the others. Results from both experimental studies and clinical trials have shown that inhibition of cholesterol synthesis with a statin increases absorption and that conversely, inhibition of cholesterol absorption increases synthesis. The tight linkage of cholesterol absorption and synthesis in maintaining cholesterol homeostasis suggests that treatment with an agent that influences only one of these two processes is likely to have distinct limits with respect to its effects on cholesterol levels. Better understanding of cholesterol homeostasis, particularly the close interrelationship between cholesterol synthesis and absorption, may result in the design of rational integrated treatment regimens that employ multiple agents with complementary actions that attack multiple mechanisms to lower cholesterol.

Fluvastatin reduces tissue factor expression and macrophage accumulation in carotid lesions of cholesterol-fed rabbits in the absence of lipid lowering

The expression of tissue factor (TF), mainly by infiltrated inflammatory cells, has been shown to be responsible for the thrombogenicity associated with atheroma. The contribution of the nonlipid-related effects of statins to the clinical benefits of statin therapy is currently under intense investigation. In this study, we evaluated the ability of fluvastatin to modulate TF expression and macrophage accumulation in rabbit carotid intimal lesions independently of cholesterol lowering. Male rabbits were fed for 30 days a 1% cholesterol-rich diet with or without fluvastatin at 5 mg/kg per day. Two weeks from the start of treatment, a silastic collar was placed around the carotid artery. Fifteen days later, the animals were killed, and carotid segments were excised and processed. The atherogenic diet caused a consistent increase in plasma cholesterol levels (610+/-231 mg/dL versus 50+/-9 mg/dL at baseline), which were not affected by fluvastatin (603+/-248 mg/dL). In the rabbits fed a high cholesterol diet without fluvastatin, an intimal lesion with macrophage accumulation and TF expression was detected. Fluvastatin significantly reduced TF and macrophage content of the lesion (-50% for both). Results indicate that fluvastatin may attenuate the inflammatory and thrombogenic potential of atherosclerotic lesions through a mechanism(s) other than cholesterol reduction, providing new insight regarding the complex mode of action of statins.

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.

Synergism between the effects of dietary cholesterol and coconut oil on plasma, liver and lipoprotein composition of neonatal chick

The nature of the synergism between dietary factors and the development of atherosclerosis has not been fully defined. Our studies showed that simultaneous supplementation of 10% saturated fat rich in 12:0 and 14:0 fatty acids (coconut oil) plus 1% cholesterol to the diet produced a sharp increase of plasma cholesterol, indicating a synergic influence of both dietary constituents. This increase was especially patent in the VLDL fraction, modifying the distribution of other lipid components between the core and the surface of these particles. These changes are consistent with the atherogenic function of VLDL and its responsiveness to dietary manipulation.

Inhibition of lipid absorption as an approach to the treatment of obesity

A reduction in fat intake may be achieved by making educated choices to reduce total calorie intake, to consume a lower quantity of total fats, or to modify the ratio of saturated-to-polyunsaturated lipids. Leptin agonists or NPY or CCK antagonists may prove to be useful to diminish appetite and thereby reduce the total intake of food. But eating has such cultural, social, and hedonistic attributes that such a single-pronged approach is unlikely to be successful. The use of fat substitutes may prove to be popular to provide a wide range of snack food options, but these are likely to be of minimal use in weight reduction programs because of their distribution of additives in only a limited number of foods. The inhibitors of lipid digestion will be modestly successful in the short term; their long-term success will be influenced by gastrointestinal adverse effects and the need to consume fat-soluble vitamin supplements to prevent the development of fat-soluble vitamin deficiencies. The inhibition of lipid absorption is an attractive targeted approach for the treatment of obesity, since this would reduce the uptake of visible as well as invisible fats, which would potentially offer convenient dosing, and could also be a means to inhibit secondarily the uptake of carbohydrate calories.

Oyster mushroom (Pleurotus ostreatus) reduces the activity of 3-hydroxy-3-methylglutaryl CoA reductase in rat liver microsomes

The effect of dried oyster mushroom (Pleurotus ostreatus) on cholesterol (C) content in serum, in lipoproteins and in liver, and on the activity of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase in liver microsomes, was studied in male rats (strain Wistar, initial body weight 75 g) fed on low-cholesterol (9 mg/100 g) and high-cholesterol (0.3%) diets. Addition of 5% oyster mushroom to both diets reduced significantly the C-content in serum (by 30%), in very-low- and low-density lipoproteins (in a 1:1 ratio to the decrease of total serum C) and in liver (by 50%), as well as the activity of HMG-CoA reductase (by more than 30%).

Antiatherosclerotic activity of inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase in cholesterol-fed rabbits: a biochemical and morphological evaluation

Atherosclerotic lesion development was assessed in the thoracic aorta and chronically denuded iliac-femoral artery of hypercholesterolemic New Zealand White rabbits using inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase which have previously been shown to possess varying degrees of hepatoselectivity in rats. Atorvastatin, previously known as CI-981 (2.5 mg/kg), PD135022 (1.0 mg/kg), simvastatin (2.5 mg/kg), lovastatin (2.5 mg/kg), PD134965 (1.0 mg/kg), pravastatin (2.5 mg/kg) and BMY22089 (2.5 mg/kg) were added to a 0.5% cholesterol, 3% peanut, 3% coconut oil diet and fed for 8 weeks. Although reductions in plasma total cholesterol of 27% to 60%, VLDL-cholesterol of 31% to 71% and plasma total cholesterol exposure of 37% to 43% were obtained, no correlation between these parameters and vascular lipid content, lesion size or monocyte-macrophage content was noted. Iliac-femoral lipid content was unchanged; however, atorvastatin and simvastatin significantly reduced the cholesterol content of the thoracic aorta by 45%-62%. Atorvastatin and PD135022 reduced the size of the iliac-femoral lesion by 67% and monocyte-macrophage content by 72%. Simvastatin, lovastatin and PD134965 decreased the monocyte-macrophage content; however, lesion size was unchanged. Pravastatin and BMY22089 had no effect on lesion size or content. No compound significantly reduced the extent of thoracic aortic lesions. We concluded that changes in plasma lipids and lipoproteins noted with the various HMG-CoA reductase inhibitors did not account for the beneficial effect on atherosclerotic lesion development. The antiatherosclerotic potential of the HMG-CoA reductase inhibitors was compound-specific and clearly not a class effect.

Cholesterol synthesis and esterification in isolated enterocytes: regulation by cholesterol and cholestyramine feeding

The purpose of the present study was to investigate the physiological control of the main regulatory enzymes of cholesterol metabolism in isolated enterocytes obtained from chick duodenum, jejunum and ileum. Cholesterol feeding resulted in an inhibition of 3-hydroxy-3-methyl-glutaryl-CoA reductase and mevalonate 5-pyrophosphate decarboxylase, while cholestyramine feeding increased reductase activity in all the regions studied and decarboxylase activity only in duodenum. Cholesterol feeding markedly increased acyl-CoA:cholesterol acyltransferase, but the effects of cholestyramine were less clear. The effects on transferase activity cannot be due to differences in the availability of acyl-CoA as exogenous substrate as no significant differences were found in acyl-CoA hydrolase activity after any of the dietary treatments. The effects of cholesterol feeding were related to changes in the cholesterol content of epithelial cells, whereas in the case of cholestyramine this relationship was less apparent.

Mevinolin, a competitive inhibitor of hydroxymethylglutaryl coenzyme A reductase, suppresses enterocyte esterification of exogenous but not endogenous cholesterol

Mevinolin (lovastatin), a competitive inhibitor of hydroxymethylglutaryl-coenzyme A reductase, directly inhibited acyl-CoA cholesteryl acyltransferase in rabbit intestinal microsomes at a dose of 20 micrograms/ml or more. Lineweaver-Burk analysis showed a competitive type of inhibition with respect to oleoyl-CoA. In cultured intestinal Caco-2 cells, mevinolin reduced [14C]oleate incorporation into cholesteryl-esters by 86% of controls at doses as low as 0.1 micrograms/ml. However, in cells whose activity of acyl-CoA cholesteryl acyltransferase was stimulated 7-fold by 10 mM mevalonolactone, a significant inhibitory effect on cholesteryl-ester formation could not be detected, even at 40 micrograms/ml of mevinolin. In contrast, cells supplied with liposomal cholesterol or cholesterol derived from low-density lipoproteins showed a marked reduction of cholesteryl-ester formation in the presence of 10 or 0.1 micrograms/ml of mevinolin, respectively. It is concluded that the observed suppressive effects of mevinolin on cholesterol esterification in cultured Caco-2 cells are indirect and possibly caused by changes in the acyl-CoA cholesteryl acyltransferase substrate pool or intracellular cholesterol transport.

Fate of exogenous and newly synthesized cholesterol in intestinal cell lines

1. The current study was undertaken to test the existence of functionally distinct intracellular pools of cholesterol depending on the origin: neosynthesis or exogenous. 2. This was performed on two subpopulations, either differentiated or undifferentiated, of the HT29 cell line. 3. A parallel study was also carried out on Caco-2 cells. 4. First we checked the ability of differentiated HT29 cells to secrete lipids into the medium and found that lipid production was efficient but less so than in Caco-2 cells. 5. In contrast, undifferentiated HT29 cells were unable to secrete lipids into the medium. 6. Then we studied the fate of [14C]cholesterol incorporated into micellar preparations and of [14C]mevalonate in the different models. 7. The data obtained with labelled exogenous cholesterol show that it enters the membrane cholesterol pool as well as, for the differentiated models, the cholesteryl ester pool. 8. Similarly, labelled newly synthesized cholesterol could be used for membrane formation as well as for incorporation into cholesteryl esters. 9. Thus, in HT29 subpopulations as well as in Caco-2 cells, the results suggest the existence of a common pool of cholesterol whatever its origin.

Nutritional and developmental regulation of glycosylation processes in digestive organs

We review the nutritional and developmental variations of the glycosylation processes in digestive organs, since glycoproteins play a prominent part as mucins or digestive enzymes in these tissues. The biosynthesis of the glycannic chains is demonstrated to be largely sensitive to various exogenous (such as nutritional) or endogenous (such as developmental) factors. Although the metabolic regulation by dietary variations appears as rather complex, according to the variety of experimental conditions and the diversity of the organs studied, available data demonstrate that this regulation does exist, depending on the quantity or sometimes the quality of the major or minor components of the diet, which induce significant variations in the glycosylation processes. The synthesis of the internal core of N-glycans is essentially regulated by diet-induced variations of the phosphoryl-dolichol level, whereas the modulation of the biosynthesis of the external part of N-glycans or the biosynthesis of O-glycans is controlled by diet-induced variations in the systems transferring fucose, galactose, sialic acid or hexosamines. Modifications in intestinal glycosylation during post-natal development in the rat control the quality of the glycannic chains of mucins and brush-border enzymes. The post-natal maturation of the intestinal rat tissue is characterized by a shift from sialylation to fucosylation, depending on coordinate changes in glycosyltransferase activities, in sugar-nucleotide breakdown or synthesis or in the activity of regulatory proteins. These activities are largely sensitive to dietary manipulations at weaning and to hormonal stimulations before weaning. However, glucocorticoid hormones do not appear as the triggering signal for the induction of these changes.

Serum lipoproteins and cholesterol metabolism in two hypercholesterolaemic rabbit models

Serum lipoproteins and key hepatic and intestinal enzymes regulating cholesterol synthesis, esterification and catabolism, namely 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase, acyl coenzyme A: cholesterol-o-acyltransferase (ACAT) and cholesterol 7 alpha-hydroxylase respectively, were compared in two hypercholesterolaemic rabbit models - the cholesterol-fed animal and the hypercholesterolaemic diabetic animal. Hypercholesterolaemia in the cholesterol-fed animals was reflected in the VLDL and LDL fractions, whereas VLDL and HDL2 cholesterol levels were elevated in the diabetic animals. The lipoproteins of the cholesterol-fed animals were enriched with cholesterol but the lipoprotein fractions in the diabetic animals were enriched with triacylglycerol. While hepatic HMGCoA reductase activity was significantly reduced in both groups, the activities of hepatic ACAT and cholesterol 7 alpha-hydroxylase were significantly increased in the cholesterol-fed animals and significantly reduced in the diabetic animals compared with controls. In the intestine, the activity of HMGCoA reductase was increased and ACAT reduced in the diabetic animals. By contrast, in the cholesterol-fed group. HMGCoA reductase activity was lower and ACAT activity was higher in comparison with the control group. These differences in lipoproteins and cellular cholesterol metabolism between the hypercholesterolaemic rabbit models may explain the differences in susceptibility to atherosclerosis, previously reported in these two animal models.

Regulation of cholesterol metabolism in the intestine

The small intestine is a major site of cholesterol biosynthesis and lipoprotein degradation. It is also the organ responsible for absorbing dietary and endogenously produced biliary cholesterol. Cholesterol metabolism in the intestine is regulated by factors that will alter cellular cholesterol requirements. Thus, during increased cholesterol flux, which occurs by bile acid-faciliated cholesterol absorption or by lipoprotein-mediated uptake of cholesterol, cholesterol synthetic rates decrease and esterification rates increase. The mechanisms by which dietary fats regulate intestinal cholesterol metabolism are complex. Dietary fats alter membrane fatty acid composition. Simultaneously, they also promote lipoprotein secretion and alter cholesterol absorption. Intestinal 3-hydroxyl-3-methylglutaryl coenzyme. A reductase activity is regulated by enzyme phosphorylation-dephosphorylation. The regulation of acylcoenzyme A-cholesterol acyltransferase activity by this mechanism remains controversial. Data on hormone regulation of intestinal cholesterol metabolism are not conclusive, although progesterone seems to be a potent inhibitor of acylcoenzyme A-cholesterol acyltransferase activity in intestinal cell culture and isolated cells. In a manner similar to the regulation of cholesterol metabolism in other cells, the enterocyte responds appropriately to factors that alter cholesterol flux. Therefore, changes that occur in the rates of cholesterol synthesis and esterification will reflect the cholesterol requirements of the cell.

Comparative effects of simvastatin (MK-733) and pravastatin (CS-514) on hypercholesterolemia induced by cholesterol feeding in rabbits

The preventive effects of simvastatin (MK-733) and pravastatin (CS-514), 3-hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase inhibitors, on hypercholesterolemia induced by 0.25% cholesterol feeding were compared in rabbits. MK-733 (6, 2 and 0.7 mg/kg) was found to prevent the increase in serum total cholesterol levels dose-dependently. High dose CS-514 (18 mg/kg) also limited the increase in the cholesterol levels, but medium (6 mg/kg) and low doses (2 mg/kg) of CS-514 were ineffective in preventing it. MK-733 inhibited the increase in VLDL and LDL cholesterol levels dose-dependently. MK-733 suppressed the increase in serum phospholipid levels. MK-733 inhibited the accumulation of cholesterol in the liver. The high dose of CS-514 also limited it. High dose MK-733 (6 mg/kg) reduced the cholesterol concentration in gallbladder bile. Neither MK-733 nor CS-514 affected bile acid excretion in the gallbladder bile. High dose MK-733 decreased the lithogenic index. MK-733 increased the number of LDL receptors, and high dose CS-514 also increased it. The suppressive effect of CS-514 on serum cholesterol levels at 18 mg/kg was found to be less than that of MK-733 at 0.7 mg/kg.

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.

Physiological and pharmacological regulation of small intestinal cholesterol synthesis

1. The small intestine is an important site of cholesterol synthesis in the body and at least in experimental animals, it also contributes to the circulating plasma pool of cholesterol. 2. Studies on synthesis regulation have been partly contradictory but it is now concluded that the cellular cholesterol balance is the basic regulatory factor of intestinal cholesterol synthesis. However, the balance is affected differently in various specialized cells and parts of the small intestine. 3. Most data on synthesis regulation are derived from experimental animals but the few human studies suggest that similar regulatory factors function in man, too.

The role of enterocyte cholesterol metabolism in intestinal cell growth and differentiation

Cholesterol is an essential constituent of all mammalian cell membranes, and its availability is therefore a prerequisite for cellular growth and other functions. To define further the role of cholesterol metabolism in the intestine both in vitro and in vivo, studies were performed. Several lines of evidence based on these studies suggest that the main purpose of local cholesterol synthesis in the gut is the support of rapid enterocyte proliferation: 1) growth was inhibited during pharmacologic suppression of cholesterol synthesis in intestinal organ or cell culture; 2) the endocrine regulation of intestinal growth was in most but not all instances accompanied by appropriate changes in cholesterol synthesis; 3) most of cholesterol synthesis and lipoprotein uptake was localized predominantly in the crypt and lower villus region; and 4) very little of the sterol synthesized by the intestinal mucosa was exported into lymph but seems rather to be incorporated into cell membranes.

Effect of catecholamines on the hepatic rate-limiting enzymes of cholesterol metabolism in normally fed and cholesterol-fed rabbits

Adrenergic control of liver cholesterol metabolism was studied in the rabbit. The effects of noradrenaline (alpha 1, alpha 2, beta 2 agonist) and isoprenaline (beta 1, beta 2 agonist) on 3-hydroxy-3-methylglutaryl coenzyme A reductase, acyl-coenzyme A: cholesterol-o-acyltransferase (cholesterol acyltransferase) and cholesterol 7 alpha-hydroxylase, the rate-limiting enzymes of cholesterol biosynthesis and esterification and bile acid synthesis, respectively, were examined in the normally fed and cholesterol-fed male New Zealand White rabbit. Isoprenaline increased the activities of hydroxymethylglutaryl CoA reductase and cholesterol acyltransferase approx. 12-fold and 5-fold, respectively, in normally fed rabbits. Noradrenaline, by contrast, produced an effect only on hydroxymethylglutaryl CoA reductase, the activity of which was increased 3-fold in these animals. Neither catecholamine had an effect on hydroxymethylglutaryl CoA reductase in the cholesterol-fed rabbit. Isoprenaline decreased the activity of cholesterol acyltransferase by approx. 40% and increased the activity of cholesterol 7 alpha-hydroxylase 2-fold in the cholesterol-fed rabbit compared to cholesterol-fed controls. Noradrenaline had no effect on either cholesterol acyltransferase or cholesterol 7 alpha-hydroxylase in either the normally fed or the cholesterol-fed rabbit. We suggest that beta 2-adrenergic stimulation by isoprenaline in the normally fed rabbit may enhance cholesterol synthesis and storage, but that in the cholesterol-fed rabbit, it facilitates the elimination of cholesterol from the body by increasing the rate of bile acid synthesis.

Effect of dietary restriction on cholesterol biosynthesis in the liver and the intestine of cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on cholesterol biosynthesis was investigated in rabbits fed either standard or cholesterol-rich diets. Accompanying the amplification of hypercholesterolemia, additional disturbances of cholesterol metabolism were observed when cholesterol feeding was associated with dietary restriction. In the intestine, underfed rabbits showed a more marked inhibition of duodenal cholesterol biosynthesis from [14C]acetate following cholesterol feeding than rabbits on normal caloric ration. In contrast liver cholesterogenesis was equally suppressed in both groups receiving cholesterol-rich diets. Cholesterol biosynthesis from [14C]mevalonate was also inhibited by cholesterol feeding particularly in the duodenum of underfed rabbits. In addition cholesterol feeding induced a marked increase of the labeled esterified: free cholesterol ratio in the liver, demonstrating intensive esterification, this was enhanced by dietary restriction. The additional cholesterol which accumulates in the plasma and in various tissues in underfed rabbits is of dietary origin since the feedback control of cholesterogenesis by exogenous cholesterol was shown to be very effective in these animals.

Regulation of cholesterol synthesis in jejunal absorptive cells of the rat

Cholesterol metabolism was studied in jejunal mucosa of the rat with special emphasis on cholesterol synthesis of villous cells, the site of intestinal lipid absorption. The type of diet and nutritional state clearly affected the cholesterologenesis of villous cells. Consequently, the incorporation of 14C-acetate into nonsaponifiable lipids (NSL; includes squalene and sterols) decreased in the following order of magnitude: 1) fat-free diet supplemented with safflower oil (FFD-SO), 2) FFD alone, 3) standard rat chow, 4) 1% cholesterol in FFD-S(, 5) total fast. Both the cholesterol feeding and the fast increased the total cholesterol concentrations in the villous cells, but the concentrations were unaffected by the other diets. In rats fed FFD-SO diet the cholesterol synthesis was significantly higher in the villous than in the crypt cells, although the cellular cholesterol concentrations were similar. The hepatic cholesterol synthesis from 14C-acetate was low in rats fed FFD-SO as compared with chow diet, whereas the incorporation of 14C-acetate into hepatic fatty acids and the incorporation of 3H-mevalonate into NSL were not affected by the diet. 5% cholestyramine had no significant effect on the cholesterol synthesis or cholesterol concentrations of the villous cells during high (FFD-SO diet) or low cholesterol synthesis (chow or cholesterol feeding). FFD-SO increased serum total cholesterol compared with chow diet and total fast, whereas cholestyramine and cholesterol feeding had no effect. Intraperitoneal administration of the hypocholesterolemic agent, 4-aminopyrazolopyrimidine, to fasting rats decreased markedly crypt cell cholesterol and increased cholesterol synthesis in the crypt cells and in the villous cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in hepatic and intestinal cholesterol regulatory enzymes. The influence of metformin

The effect of metformin (N,N-dimethylbiguanide) on the rate-limiting enzymes of cholesterol metabolism was observed. 3-Hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acyl-CoA-cholesterol acyltransferase (ACAT) activities were estimated in hepatic microsomal and intestinal cell preparations from normal and alloxan-diabetic rats. Metformin administration had no effect on either hepatic enzyme. Intestinal ACAT activity was significantly decreased in the metformin-treated rats when compared with controls (P less than 0.001). Intestinal HMG-CoA reductase activity was not significantly affected. Diabetic rats showed a significantly higher intestinal HMG-CoA reductase activity when compared with controls (P less than 0.001). Intestinal ACAT activity in the diabetic group was similar to control values. Metformin administration to diabetic rats caused a marked decrease in both intestinal HMG-CoA reductase activity (P less than 0.001) and ACAT activity (P less than 0.002). It is concluded that the effect of metformin on the intestinal cholesterol enzyme system may be responsible for its cholesterol-lowering action.

Relative rates of sterol synthesis in the liver and various extrahepatic tissues of normal and cholesterol-fed rabbits. Relationship to plasma lipoprotein and tissue cholesterol levels

The relative rates of sterol synthesis in the liver and ten extrahepatic tissues of normal and cholesterol-fed rabbits were determined by measuring the rates of incorporation of [1-14C]octanoate into digitonin-precipitable sterols by tissue slices. In normal rabbits the rate of sterol synthesis in the liver was very low compared to that in several extrahepatic tissues, particularly the small intestine. The rate of synthesis in the small intestine showed marked regional variation, with the highest rate occurring in the section proximal to the entry of the common bile duct and the lowest rate in the mid-sections of the intestine. The regional differences in intestinal sterol synthesis correlated inversely with the cholesteryl ester content of the tissue. Rabbits fed the cholesterol diet developed marked hypercholesterolemia, with much of the additional cholesterol appearing in the VLDL and LDL fractions. The cholesteryl ester content of the liver, small intestine and various other extrahepatic tissues increased significantly. Coincident with these changes was a marked suppression of sterol synthesis, not only in the liver, but also in the small intestine, adrenal gland, kidney, lung, spleen and ovary. Thus, the rabbit, like the guinea pig, normally exhibits a very low rate of hepatic sterol synthesis compared to that found in other species such as the rat, squirrel monkey and baboon and, furthermore, manifests feedback inhibition of both hepatic and extrahepatic sterol synthesis when dietary cholesterol intake is increased. This general suppression of synthesis correlates with an accumulation of cholesteryl ester in the tissues which, in turn, presumably is related to the uptake of lipoprotein cholesterol from the hypercholesterolemic plasma that develops under such dietary conditions.

3-Hydroxy-3-methylglutaryl CoA reductase in cultured hepatocytes. Regulation by heterologous lipoproteins and hormones

Regulation of the key enzyme of cholesterol synthesis, 3-hydroxy-3-methylglutaryl CoA reductase (EC: 1.1.1.34), by heterologous human lipoproteins and hormones was studied in a maintenance culture of rat hepatocytes. The liver cells were cultured under hormone and serum free conditions and maintained differentiated morphology and specific function. Under control conditions total HMG-CoA reductase increased by 50% after 24 h culture compared to 0 h values immediately after isolation. Thereafter a plateau of enzyme activity was reached lasting until 48 h, with a slight decline at 72 h. Concomitantly the "expressed" enzyme activity increased steadily, probably through dephosphorylation of latent reductase, the activation was largely complete at 48 h. During the steady state period of total reductase VLDL added to the medium at concentrations up to 50 microgram/ml protein had no effect o HMG-CoA reductase activity. In contrast, LDL suppressed the enzyme in a dose-dependent fashion to 40% of controls at 100 microgram/ml. On the other hand, HDL had the opposite effect with a significant induction up to 252% of controls at 50 microgram/ml. Insulin also caused a comparable dose-dependent stimulation of enzyme activity at 10(-8) and 10(-7)M, whereas glucagon inhibited reductase activity. Compared to the insulin action, triiodothyronine and triamcinolone prompted a minor, but still significant increase of reductase activity. Insulin and triamcinolone acted synergistically, but the combination of triamcinolone and tri-iodothyronine was only additive. All hormonal inductions of reductase could be blocked by cycloheximide. The present data establish that HMG-CoA reductase of maintenance cultured hepatocytes is subject to a complex regulation by heterologous lipoproteins as well as pancreatic, adrenal and thyroid hormones.

Bile acid induced interconversion of 3-hydroxy-3-methylglutaryl Coenzyme A reductase in cultured intestine

The effect of bile acids and bile acid/cholesterol micelles on 3-hydroxy-3-methylglutaryl coenzyme A reductase, the key enzyme of cholesterol synthesis, was investigated in cultured intestine. Glycocholic and glycodeoxycholic acid both suppressed total (fully activated) reductase activity after 3 h culture. The portion of expressed reductase, determined in the presence of NaF, was unaffected at 3 h, but decreased after 24 h of bile acid treatment. In contrast, total enzyme activity was stimulated up to 2.5-fold at 24 h; this bile acid effect was blocked by additional cholesterol. These results suggest that bile acids modulate both total reductase activity and the activation state of the enzyme in cultured intestine.

Regulation of 3-hydroxy-3-methylglutaryl CoA reductase by analogs of cholesterol and bile acids in cultured intestinal mucosa

Sodium fusidate and its glycine conjugate, which have the same detergent properties as bile acids, significantly (p less than 0.05) stimulate HMG-CoA reductase of cultured intestine below the critical micellar concentration (CMC) without affecting brush border enzymes. Above CMC, both amphiphiles are cytotoxic. At concentrations between 1 and 5 mM, sodium fusidate decreased cholesterol contents of cultured mucosa (P less than 0.05), the increase in synthesis only partially compensating for the sterol loss. Oxygenated sterols, 7-keto- and 25-hydroxycholesterol, also depleted mucosal cholesterol at 0.5 mM, exerting their effect differently by inhibiting HMG-CoA reductase (p less than 0.01). In contrast to their marked effect on total mucosal cholesterol contents, brush border cholesterol was unaffected by both cholesterol and bile acid analogs.

Regulation of 3-hydroxy-3-methylglutaryl-CoA reductase by endogenous sterol synthesis in cultured intestinal mucosa

In vitro regulation of the key enzyme of cholesterol synthesis, 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34) by compactin, a competitive inhibitor of the enzyme, and mevalonate was studied in rabbit ileum organ culture. Addition of compactin suppressed ileum homogenate reductase activity by over 80% at concentrations up to 0.5 microgram/ml. In contrast, compactin at the same concentrations added to the culture medium induced reductase activity up to 240% of controls. This increase was blocked by cycloheximide and mevalonolactone at 10 mM, but not by mevalonate (salt form) and cholesterol. Similarly, in contrast to ionized mevalonate, mevalonolactone significantly suppressed reductase activity of cultured intestine at 1 and 10 mM by 23 and 62%, respectively. A minor effect was also observed with preformed enzyme in fresh mucosal homogenate. When endogenous cholesterol synthesis was blocked by compactin, mucosal alkaline phosphatase activity decreased progressively, whereas medium activity from desquamated cells did not change. This distribution of the villous cell marker enzyme is characteristic of a decrease in crypt cell renewal and/or villous cell differentiation. This effect of compactin was also reversible with mevalonolactone. The reductase enzyme induced by compactin was probably latent intracellularly, since tissue cholesterol contents dropped sharply after blockade of endogenous sterol synthesis.