Kurt Einarsson

Influence of Age on Secretion of Cholesterol and Synthesis of Bile Acids by the Liver

Supersaturation of bile with cholesterol predisposes to the development of cholesterol gallstones. To identify the factors determining cholesterol saturation of bile, we analyzed the lipid composition of stimulated duodenal bile in 60 healthy subjects of various ages (31 men and 29 women) who were not obese and were free of gallstones. A positive correlation between age and cholesterol saturation of bile was found (P less than 0.001). To analyze the relation between age and cholesterol saturation, we studied the rates of hepatic secretion of biliary lipids and the kinetics of cholic acid and chenodeoxycholic acid in 22 and 18 of the subjects, respectively. Age was positively correlated with the cholesterol secretion rate (r = 0.48) and negatively correlated with bile acid synthesis (r = -0.60) and the size of the cholic acid pool (r = -0.54). We conclude that cholesterol saturation of bile increases with age as a consequence of enhanced hepatic secretion of cholesterol and decreased bile acid synthesis. These findings may explain why age is a risk factor for the development of cholesterol gallstones.

Hepatic Uptake of Bile Acids in Man: FASTING AND POSTPRANDIAL CONCENTRATIONS OF INDIVIDUAL BILE ACIDS IN PORTAL VENOUS AND SYSTEMIC BLOOD SERUM

This investigation was undertaken in order to (a) characterize the postprandial inflow of individual bile acids to the liver and (b) determine if peripheral venous bile acid levels always adequately reflect the portal venous concentration, or if saturation of hepatic bile acid uptake can occur under physiological conditions. In five patients with uncomplicated cholesterol gallstone disease, the umbilical cord was cannulated during cholecystectomy, and a catheter was left in the left portal branch for 5 to 7 d. The serum concentrations of cholic acid, chenodeoxycholic acid, and deoxycholic acid in portal venous and systemic circulation were then determined at intervals of 15 to 30 min before and after a standardized meal. A highly accurate and specific gas chromatographic/mass spectrometric technique was used. The sum of the fasting concentrations of the three bile acids averaged 14.04+/-4.13 mumol/liter in portal venous serum, and 2.44+/-0.31 mumol/liter in peripheral venous serum. The estimated hepatic fractional uptake of cholic acid was approximately 90%, and those of chenodeoxycholic acid and deoxycholic acid were 70-80%. This resulted in an enrichment of systemic bile acids in the dihydroxy bile acid species. In response to a standardized meal, portal venous bile acid concentrations increased two- to sixfold, with a peak seen 15-60 min after the meal. The maximum postprandial portal venous bile acid concentration averaged 43.04+/-6.12 mumol/liter, and the corresponding concentration in peripheral serum was 5.22+/-0.74 mumol/liter. The estimated fractional uptakes of the individual bile acids were not affected by the increased inflow to the liver. The peripheral venous concentrations of individual as well as total bile acids were well correlated with those in portal venous serum. The results (a) give a quantitation of postprandial bile acid inflow to the liver and (b) indicate that the hepatic uptake system for bile acids in healthy man cannot be saturated during maximal inflow of endogenous bile acids. Measurement of peripheral serum bile acids can thus give important information on the status of the enterohepatic circulation.

Importance of a Novel Oxidative Mechanism for Elimination of Intracellular Cholesterol in Humans

We have recently demonstrated that cultured human alveolar macrophages efficiently convert cholesterol into excretable 27-oxygenated products. We show here that increasing the intracellular concentration of cholesterol by a factor of 10 leads to about a twofold increase in the excretion of 27-oxygenated products from cultured macrophages. Inhibition of the sterol 27-hydroxylase caused a significant intracellular accumulation of cholesterol. A direct comparison was made between flux of cholesterol and 27-oxygenated products from macrophages preloaded with [4-14C]cholesterol. Under the specific conditions employed with fetal calf serum in the culture medium, the flux of 27-oxygenated products was about 10% of that of cholesterol. Since the sterol 27-hydroxylase, which converts cholesterol to 27-oxygenated products, is present in many cell types, we suggest that 27-oxygenation is a general mechanism for removal of intracellular cholesterol. To evaluate this hypothesis, we measured the net uptake by the human liver of circulating 27-oxygenated products, which was found to be about 20 mg/24 h. This uptake corresponds to approximately 4% of the bile acid production, assuming quantitative conversion into bile acids. It is concluded that the 27-hydroxylase pathway is of significance for elimination of extrahepatic cholesterol.

Comparative Effects of Ursodeoxycholic Acid and Chenodeoxycholic Acid on Bile Acid Kinetics and Biliary Lipid Secretion in Humans: Evidence for Different Modes of Action on Bile Acid Synthesis

The effects of ursodeoxycholic acid on biliary lipid secretion and bile acid kinetics were determined in 12 men. For comparison, eight of the subjects were also treated with chenodeoxycholic acid using a crossover study design. The daily dose of each bile acid was 15 mg/kg body wt; each treatment period lasted for 5-6 wk. Kinetics of cholic acid and chenodeoxycholic acid, hepatic secretion rates of biliary lipids, and lipid composition of concentrated fasting duodenal bile were determined before and at the end of each treatment period. The synthesis rates of cholic acid and chenodeoxycholic acid were increased by approximately 80% and 40%, respectively, during treatment with ursodeoxycholic acid. The fractional catabolic rates of the two bile acids were increased by approximately 50%, whereas the pool sizes remained unchanged. Under similar conditions, administration of chenodeoxycholic acid reduced the pool size as well as the synthesis rate of cholic acid by approximately 70%. Ursodeoxycholic acid reduced the hepatic secretion of cholesterol to a higher extent (approximately 50%) than did chenodeoxycholic acid (approximately 30%). The secretion rates of bile acids and phospholipids remained essentially unchanged during the two treatment periods. Fasting duodenal (gallbladder) bile was unsaturated with cholesterol during both regimens. It is concluded that the two bile acids exert different effects on bile acid metabolism. The enhanced conversion of cholesterol to bile acids observed during ursodeoxycholic acid treatment may at least partly explain why ursodeoxycholic acid can reduce the biliary output of cholesterol without suppressing hepatic cholesterol synthesis.

Influence of age on the metabolism of plasma low density lipoproteins in healthy males.

The plasma concentration of the atherogenic low density lipoproteins (LDL) increases with age. To clarify the mechanism of this change, we studied the kinetics of autologous 125I-LDL apolipoprotein B (apo B) in 41 normolipidemic, nonobese healthy males. For comparison, they were divided into three age groups: young, 21-39 yr (n = 18), middle-aged, 40-59 yr (n = 11), and old, 60-80 yr (n = 12). The levels of plasma LDL cholesterol and LDL apo B increased from respectively 3.4 +/- 0.1 (SEM) mmol/liter and 86 +/- 2 mg/dl in the young to 4.1 +/- 0.1 mmol/liter and 95 +/- 3 mg/dl in the old (P less than 0.01), and this increase was linked to a progressively decreased (r = -0.38, P less than 0.02) fractional catabolic rate of LDL apo B (0.348 +/- 0.010 pools per day in the young vs. 0.296 +/- 0.009 pools per day in the old, P less than 0.01). The production rate of LDL apo B did not differ significantly between the groups. The reduced fractional catabolic rate of LDL apo B in the old was not associated with a decrease in binding affinity of the LDL particle to its receptor, as judged from its ability to compete for 125I-LDL fibroblast binding. When hepatic LDL receptor expression was stimulated by cholestyramine treatment in six old males, their LDL apo B fractional catabolic rate increased to the levels observed in the young subjects. We conclude that the increase in LDL which normally occurs with age is explained by a reduced capacity for its removal, and hypothesize that this is mediated via a reduced hepatic LDL receptor expression.

Effects of interruption of the enterohepatic circulation of bile acids on the transport of very low density-lipoprotein triglycerides

An increase in plasma very low density lipoprotein-triglycerides (VLDL-TG) is seen frequently during treatment with bile acid-binding resins. The purpose of this study was to determine whether this increment in VLDL-TG is due mainly to an increase in synthesis of VLDL, or to an enhanced catabolism. Three types of patients were studied: (1) 7 normotriglyceridemic subjects. (2) 4 obese patients, and (3) 9 hypertriglyceridemic patients. Before treatment they underwent a study of VLDL-TG kinetics that employed multicompartmental analysis of specific activity curves following injection of 3H-glycerol. The patients were then treated with a bile acid-binding resin, either cholestyramine or colestipol, for several weeks to several months. At the end of the treatment period, they were readmitted to the hospital for a second study of VLDL-TG kinetics. The patients showed a variable response to resin therapy. Many had an increase in concentrations of VLDL-TG, but others had no change or even a slight decrease. However, analysis of the data showed a high correlation between change in production rates of VLDL-TG and change in concentration. Also, when the data for the 20 patients were combined, there was a statistically significant increase in both synthetic rates and concentrations of VLDL-TG; in contrast, the fractional catabolic rate (FCR) was unchanged by therapy. Therefore, our data show that when treatment with bile acid sequestrants causes an increase in plasma VLDL-TG, the increase is due to an increment in production and not to a decrease in catabolism.

The plasma level of 7α-hydroxy-4-cholesten-3-one reflects the activity of hepatic cholesterol 7α-hydroxylase in man

Circulating levels of 7α‐hydroxy‐4‐cholesten‐3‐one have been compared with activities of the rate‐limiting enzyme in bile acid synthesis, microsomal cholesterol 7α‐hydroxylase, measured in liver biopsies obtained from patients undergoing surgery for gallstone disease. Some patients were treated with cholestyramine or bile acids prior to operation in order to alter the feed‐back inhibition of the enzyme. The levels of the sterol were similar in untreated patients and in patients treated with ursodeoxycholic acid (median concentration 17 and 13 ng/ml, respectively), and so were the activities of the enzyme (median activity 7.0 and 5.5 pmol/min/mg protein, respectively). The sterol levels and enzyme activities were significantly increased in patients treated with cholestyramine (91 ng/ml and 45 pmol/min/mg protein) and decreased in patients treated with chenodeoxycholic acid (<2.0 ng/ml and 0.7 pmol/min/mg protein). There was a strong positive correlation (r=0.90, P<0.00001) between levels of 7α‐hydroxy‐4‐cholesten‐3‐one in plasma and the activities of cholesterol 7α‐hydroxylase in the whole patient group. The results show that analysis of 7α‐hydroxy‐4‐cholesten‐3‐one in plasma is a sensitive and convenient method to determine relative rates of bile acid production in man.

Bile acid sequestrants: mechanisms of action on bile acid and cholesterol metabolism

SummaryInterruption of the enterohepatic circulation of bile acids by cholestyramine or colestipol influences the hepatic metabolism of cholesterol in many ways. The synthesis of bile acids is increased, as reflected by a several-fold increase in the activity of the cholesterol 7a hydroxylase, the rate-determining enzyme in bile acid synthesis. The increased metabolism of cholesterol to bile acids causes an enhanced demand of cholesterol in the hepatocytes, which respond with both new synthesis of cholesterol, as reflected in a several-fold increase of the HMG-CoA reductase activity, and increased expression of LDL receptors. As a consequence, the plasma level of LDL-cholesterol is lowered. The hepatic secretion rate of VLDL particles is increased. Cholestyramine therapy does not affect the output of biliary lipids or the cholesterol saturation of bile, indicating that treatment with bile acid sequestrants should not be associated with any increased risk of gallstone formation.

Biliary lipid composition during treatment with different hypolipidaemic drugs

Abstract. In an attempt to clarify the possible lithogenic effects of commonly used hypolipidaemic drugs, gallbladder bile was obtained from patients with primary hyperlipoproteinaemia before and during treatment with nicotinic acid (n= 13), cholestyramine (n= 19), clofibrate (n= 11), and a combination of cholestyramine and clofibrate (n= 11). Each treatment period was minimum 6 weeks, and standardized dietary conditions were obtained.

Bile acid synthesis in humans: Regulation of hepatic microsomal cholesterol 7α-hydroxylase activity

The present work tested the hypothesis that portal venous bile acids regulate the activity of the cholesterol 7 alpha-hydroxylase and studied the influence of hepatic microsomal free cholesterol concentration on the enzyme activity. Operative liver biopsies and samples of portal venous blood were obtained from a total of 61 patients with gallstones who were undergoing cholecystectomy. Fifteen of the patients were treated with cholestyramine (16 g/day) for 2-3 wk before operation and 23 patients with chenodeoxycholic acid (15 mg/kg.day) or ursodeoxycholic acid (15 mg/kg.day) for 3-4 wk before operation. Highly accurate methods based on isotope dilution-mass spectrometry were used for assay of the cholesterol 7 alpha-hydroxylase activity, the concentration of free cholesterol in the microsomes, and the levels of individual bile acids in portal venous blood. Cholestyramine treatment increased the cholesterol 7 alpha-hydroxylase activity about sixfold, from 7.6 +/- 1.1 (mean +/- SEM) to 45.7 +/- 6.7 pmol/min.mg protein. Administration of chenodeoxycholic acid reduced the enzyme activity considerably to 1.0 +/- 0.3 pmol/min.mg protein, whereas ursodeoxycholic acid did not significantly affect the enzyme activity (7.9 +/- 2.2 pmol/min.mg protein). The concentration of microsomal free cholesterol remained essentially unchanged in spite of a 45-fold variation in enzyme activity. There was a negative correlation between the absolute as well as the relative concentration of chenodeoxycholic acid in portal blood and the activity of the cholesterol 7 alpha-hydroxylase, whereas there was no correlation between the total concentration of bile acids and the enzyme activity. It is concluded that the composition of individual bile acids may be more important than the total concentration of bile acids in the portal vein for the regulation of the cholesterol 7 alpha-hydroxylase activity in humans. It is further concluded that chenodeoxycholic acid is a considerably stronger suppressor of bile acid synthesis than ursodeoxycholic acid.