Suppression of bile acid synthesis, but not of hepatic cholesterol 7alpha-hydroxylase expression, by obstructive cholestasis in humans.

Regulation of bile acid synthesis, a key determinant of cholesterol homeostasis, is still incompletely understood. To elucidate the feedback control exerted on bile acid biosynthesis in humans with obstructive cholestasis, 16 patients with bile duct obstruction were studied. In vivo 7alpha-hydroxylation, reflecting bile acid synthesis, was assayed in 13 of them by tritium release analysis. Serum 27-hydroxycholesterol was determined by gas chromatography-mass spectrometry. In a subgroup, hepatic cholesterol 7alpha-hydroxylase mRNA was assayed by real-time polymerase chain reaction (PCR), enzyme activity was determined by isotope incorporation, and microsomal cholesterol content was assayed by gas chromatography-mass spectrometry. Age-matched control subjects were studied in parallel. Hydroxylation rates were lower in cholestatic patients (108 +/- 33 mg of cholesterol per day, mean +/- SEM; controls: 297 +/- 40 mg/d; P <.01). The reduction was proportional to the severity of cholestasis, and synthetic rates were normalized in 4 subjects restudied after resolution of biliary obstruction. Consistent findings were obtained by analysis of serum 7alpha-hydroxycholesterol levels. On the other hand, hepatic cholesterol 7alpha-hydroxylase mRNA, microsomal enzyme activity, and cholesterol content tended to be increased in cholestasis. Finally, serum 27-hydroxycholesterol levels were slightly reduced in cholestatic subjects and were not related with the severity of the disease. Suppression of in vivo bile acid synthesis with no corresponding reduction in tissue 7alpha-hydroxylase expression and activity is consistent with nontranscriptional, posttranslational levels of regulation; these may play a role in the feedback control of bile acid synthesis in particular conditions. Alteration of the alternate biosynthetic pathway seems unlikely according to the present data.

Effects of different phenotypes of hyperlipoproteinemia and of treatment with fibric acid derivatives on the rates of cholesterol 7 alpha-hydroxylation in humans.

Little is known about the relationships between hyperlipidemia and bile acid metabolism. However, hypolipidemic treatment with fibric acid derivatives has been shown to increase biliary cholesterol secretion, presumably by reducing bile acid synthesis. To clarify such relationships, we investigated the effects of different hyperlipoproteinemic conditions and of treatment with fibric acid derivatives on the rates of cholesterol 7 alpha-hydroxylation (the limiting step of bile acid synthesis) in humans. We studied 10 patients (aged 36 to 68 years) with lipoprotein phenotype IIa and with a clinical diagnosis of heterozygous familial hypercholesterolemia, a condition of reduced activity of LDL receptors, and 11 patients (aged 48 to 70 years) with lipoprotein phenotype IIb or IV and clinical diagnosis of familial combined hyperlipidemia, a condition probably related to increased hepatic lipoprotein synthesis. Cholesterol 7 alpha-hydroxylation rates were assayed in vivo by tritium release assay after an intravenous injection of [7 alpha-3H]cholesterol. The results were compared by ANOVA to the values obtained in a group of 28 normolipidemic patients (aged 34 to 83 years), with age as the covariate. Six patients were also studied after treatment with gemfibrozil (900 to 1200 mg/d for 6 to 8 weeks) and 5 patients were studied after treatment with bezafibrate (400 mg/d for 6 to 8 weeks). Hydroxylation rates were 0.82 +/- 0.22 mmol/d in the familial hypercholesterolemia group and 1.30 +/- 0.47 mmol/d in the familial combined hyperlipidemia group (P < .05 between the two groups and between patients with familial combined hyperlipidemia and control subjects; P = NS between patients with familial hypercholesterolemia and control subjects, as determined by ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of aging on cholesterol 7 alpha-hydroxylation in humans.

In order to investigate the alterations of bile acid synthesis in aging, we studied the rates of cholesterol 7 alpha-hydroxylation, the rate-limiting step, in 28 patients of different ages (34-83 years old, 14 below and 14 above the age of 65) of both sexes. Hydroxylation rates were determined by tritium release assay after an intravenous bolus of [7 alpha-3H]cholesterol. Cholesterol 7 alpha-hydroxylation was significantly decreased in the older age group, compared to middle-aged subjects, both in males and females; moreover, a significant inverse correlation between hydroxylation rates and age was found in the whole sample (r = -0.56; P < 0.01) and in females, but not in males. The percent concentration of deoxycholic acid in plasma (determined by gas-liquid chromatography) was increased in older subjects. Plasma cholesterol and triglyceride levels were not related with age even though triglyceride concentrations tended to be lower in the older age group. Triglyceride, but not cholesterol levels, were directly correlated with hydroxylation rates (r = 0.45, P < 0.05). After cholestyramine treatment (8-12 g/day for 4 weeks) a sharp increase in 7 alpha-hydroxylation rates was observed in three elderly patients, accompanied by reduced levels of dihydroxylated bile acids. Our data are consistent with a reduced rate of conversion of cholesterol to bile acids with aging, particularly in females, and suggest a coordinate reduction of triglyceride production. Alterations of the quantitative and/or qualitative pattern of the bile acid pool recirculating to the liver may be responsible, at least in part, for the changes observed.

Regulation of bile acid synthesis in humans: effect of treatment with bile acids, cholestyramine or simvastatin on cholesterol 7 alpha-hydroxylation rates in vivo.

The rates of cholesterol 7 alpha-hydroxylation (the first and rate-limiting step of bile acid synthesis from cholesterol) were evaluated in vivo in patients administered bile acids with different structural properties, cholestyramine or simvastatin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Twenty-three subjects, with normal hepatic and intestinal functions, were studied in basal conditions and after one of the following treatment schedules, lasting 4 to 6 weeks: cholestyramine, 4 and 12 gm/day (four patients); ursodeoxycholic acid, 9 to 11 mg/kg/day (four patients); chenodeoxycholic acid, 12 to 15 mg/kg/day (five patients); deoxycholic acid, 8 to 10 mg/kg/day (four patients); and simvastatin, 40 mg/day (six patients). 7 alpha-Hydroxylation of cholesterol was assayed by measuring the increase in body water tritium after intravenous bolus of cholesterol tritiated at the 7 alpha position. Plasma bile acid composition, evaluated by gas-liquid chromatography, revealed a substantial enrichment of the recirculating pool by the administered bile acid, whereas treatment with cholestyramine decreased the content of dihydroxylated bile acids. Cholesterol 7 alpha-hydroxylation increased in a dose-related manner after cholestyramine, in parallel with a decrease of cholesterol in total plasma and low-density lipoproteins (1.006 to 1.063 gm/ml). Hydroxylation rates decreased by an average of 47% with chenodeoxycholic acid and by an average of 78% with deoxycholic acid; ursodeoxycholic acid treatment did not affect 7 alpha-hydroxylation significantly. Simvastatin markedly reduced plasma total and low-density lipoprotein-cholesterol but exerted no change on 7 alpha-hydroxylation rates.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo evaluation of cholesterol 7 alpha-hydroxylation in humans: effect of disease and drug treatment.

7 alpha-Hydroxylation of cholesterol is a stereospecific reaction consisting of the replacement of the 7 alpha-hydrogen with a hydroxyl group. When cholesterol labeled with tritium at the 7 alpha position is administered, the hydroxylation of the substrate will result in the loss of tritium which in turn will label the body water. The rate of tritium enrichment of the body water could thus give a quantitative estimate of the hydroxylation rate. This study describes the validation of the procedure with some 21 studies performed on 15 subjects in different conditions. [7 alpha-3H]cholesterol was administered intravenously in 50 ml of plasma and thereafter blood was sampled at timed intervals for 4 to 5 days. The rate of the hydroxylation of cholesterol was calculated from the time course of the specific activities of plasma cholesterol and body water after tracer administration and was expressed as 7 alpha-hydroxycholesterol formed/24 hr. Calculated values of hydroxylation in three control subjects (493 +/- 206), five patients with hyperlipoproteinemia (539 +/- 168), and seven cirrhotic patients (153 +/- 136) are in good agreement with figures reported for bile acid synthesis determined with other techniques. Cholesterol 7 alpha-hydroxylation rate is reduced in patients with cirrhosis, the impairment being related to the severity of the disease. Cholestyramine administered to one subject for 4 weeks produced a threefold increase of the hydroxylation. Administration of chenodeoxycholic acid resulted in a 50% decrease, whereas that of ursodeoxycholic did not produce consistent changes of the hydroxylation rate. The results support the current view that 7 alpha-hydroxylation of cholesterol is rate-limiting in the synthesis of bile acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic cholesterol and bile acid metabolism in subjects with gallstones: comparative effects of short erm feeding of chenodeoxycholic and ursodeoxycholic acid.

The activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and 7alpha-hydroxylase, the enzymes controlling the rate of hepatic synthesis, respectively, of cholesterol and bile acids, and the microsomal cholesterol content were evaluated in 25 patients with cholesterol gallstones and 17 subjects without gallstones. The same quantities were estimated in 16 additional patients with gallstones given chenodeoxycholic (CDCA) or ursodeoxycholic acid (UDCA) at a dose of 15 mg/kg per day in order to investigate the comparative effect of a short term (7 days) administration of the two bile acids on the hepatic sterol metabolism. As compared to the controls, subjects with gallstones exhibited a 36% decrease of 7alpha-hydroxylase (26.8 +/- 6.2 versus 41.7 +/- 4.2 pmol/min per mg protein) and a 24% increase of the microsomal cholesterol (78.7 +/- 15.3 versus 63.1 +/- 18.1 nmol/mg protein). Although higher in the gallstone patients, the activity of HMG-CoA reductase did not differ significantly in the two groups of subjects. Administration of CDCA and UDCA changed the bile acid pool composition so that the fed bile acid predominated in the bile (mean CDCA 73% and mean UDCA 54%). Bile lipid composition did not appreciably change. In the eight subjects treated with CDCA the activity of HMG-CoA reductase was reduced to 45% of the value of untreated subjects (27.9 +/- 14.5 versus 63.5 +/- 25.3 pmol/min per mg protein) whereas in the eight subjects treated with UDCA the same enzyme showed a twofold increase (123.5 +/- 20.9). In the treated groups 7alpha-hydroxylase activity was somewhat decreased but the values did not differ significantly from those of the untreated subjects. Microsomal cholesterol content decreased with CDCA (64.8 +/- 11.6 nmol/mg protein) as well as with UDCA (59.1 +/- 10.1) treatment; however in the latter the difference attained statistical significance (P < 0.05). Altogether the results would suggest that in the liver of patients with gallstones the conversion of cholesterol to bile acids is somewhat reduced, and that changing the bile acid pool composition, by exogenous bile acid feeding, has disparate effects on hepatic cholesterol synthesis. The findings could represent the acute changes produced by bile acid feeding, however they could imply that the effects of two bile acids in dissolving cholesterol gallstones might not be related only to the changes in hepatic sterol metabolism.-Carulli, N., M. Ponz De Leon, F. Zironi, A. Pinetti, A. Smerieri, R. Iori, and P. Loria. Hepatic cholesterol and bile acid metabolism in subjects with gallstones: comparative effects of short term feeding of chenodeoxycholic and ursodeoxycholic acid.