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.

Review article: effect of bile salt pool composition on hepatic and biliary functions

The enterohepatic recirculation of bile salts exerts important regulatory effects on many hepatic, biliary and intestinal functions: such regulation is likely to depend, to a large extent, on the physical-chemical property of hydrophobicity of the recirculating pool. The present review summarizes the main experimental evidence carried out by our research group over the past two decades, in the attempt to investigate systematically the relationships between structural properties and biological effects of bile acids in humans. Hydrophobic bile acids (chenodeoxycholic acid, deoxycholic acid), but not hydrophilic acids (ursodeoxycholic acid), significantly suppressed hepatic activity of HMG-CoA reductase, the limiting step of cholesterol synthesis, and in vivo cholesterol 7alpha-hydroxylation, the limiting step of bile acid synthesis. The output of biliary cholesterol and phospholipid was also directly related to the hydrophobicity of the bile acid pool. Finally, treatment with chenodeoxycholic acid, but not with ursodeoxycholic acid, significantly decreased gall-bladder emptying rates. When turning to the in vitro model of HepG2 cells, hydrophobic bile acids were found to induce greater cytotoxic and pro-apoptotic effects. From this series of studies, we conclude that the regulatory effects of bile acids on the liver and biliary tract are largely dependent on the hydrophobic-hydrophilic balance of the recirculating bile acid pool.

Herpesvirus DNA is frequently detected in liver tissue from hepatitis C patients

BACKGROUND AND AIMS

Herpesviruses infect the liver and cause minor hepatitis. Our aim is to verify the presence of herpesviruses in the liver from hepatitis C patients and the possible influence of these viruses in the liver disease.

METHODS

We searched for herpesvirus DNA in liver biopsies from patients with hepatitis C and from a control group without hepatitis by means of nested polymerase chain reaction. Serological investigations were carried out as well.

RESULTS

Thirty-four liver specimens from hepatitis C patients were examined, 12 of which (35.3%) were positive for at least one herpesvirus DNA, whereas among the 19 control specimens only two were positive (10.5%; P = 0.049). Liver biopsies from seven patients, three with acute hepatitis of unknown origin, three with non-alcoholic steatohepatitis and one with autoimmune hepatitis were also investigated and three positive samples were found.

CONCLUSIONS

The prevalence of herpesvirus DNA was found higher in patients with hepatitis C than in individuals without hepatitis. The influence of herpesviruses on the clinical course of hepatitis C is considered.

Effect of liver cirrhosis on the systemic availability of naltrexone in humans

BACKGROUND/AIMS

Naltrexone is a competitive opiate antagonist with high hepatic extraction. It is used for detoxification treatment for heroin addicts and has been proposed as a possible treatment of pruritus in cholestasis. Such patients are likely to have impaired liver function, underscoring the need to understand the pharmacokinetic behavior of naltrexone in liver disease. These studies were undertaken to evaluate the effect of liver cirrhosis on the plasma time-course of naltrexone.

METHODS

A total of 18 patients were investigated: seven migraine patients with normal liver function regarded as controls and 11 patients with liver cirrhosis (six with decompensated disease and five with preserved liver function). A bolus of 100 mg of naltrexone was administered orally in the morning, after an overnight fast. Blood samples were taken in basal conditions and at fixed intervals, up to 24 h after administration. Serum levels of naltrexone and of its major active metabolite, 6 beta-naltrexol, were assayed by reversed-phase HPLC analysis.

RESULTS

In control subjects, circulating concentrations of naltrexone were always much lower than those of 6 beta-naltrexol (area under the curve: naltrexone, 200 +/- 97 ng/ml x 24 h; 6 beta-naltrexol, 2467 +/- 730 ng/ml x 24 h, p < 0.01). In severe cirrhosis serum levels of 6 beta-naltrexol increased more slowly, so that circulating levels of naltrexone during the first 2-4 h after drug intake were higher than those of 6 beta-naltrexol (6 beta-naltrexol/naltrexone ratio at 2 h: controls, 10.91 +/- 4.80; cirrhosis, 0.39 +/- 0.18, p < 0.01). The area under the curve for naltrexone (1610 +/- 629 ng/ml x 24 h) was significantly greater than in controls, whereas that for 6 beta-naltrexol (2021 +/- 955 ng/ml x 24 h) was not significantly different. Patients with compensated cirrhosis showed an intermediate pattern. No differences in elimination half-life of the two drugs were detected among the groups.

CONCLUSIONS

Our data suggest the occurrence of important changes in the systemic availability of naltrexone and 6 beta-naltrexol in liver cirrhosis; such alterations are consistent with lesser reduction of naltrexone to 6 beta-naltrexol and appear to be related to the severity of liver disease. This must be considered when administering naltrexone in conditions of liver insufficiency.

Effect of taurohyodeoxycholic acid on biliary lipid secretion in humans

This study aimed to determine the effect in humans of taurohyodeoxycholic acid, a 6alpha-hydroxylated bile acid with hydrophilic properties, on bile lipid secretion. Four cholecystectomized patients who had gallstones and an interrupted enterohepatic circulation were intraduodenally infused with taurohyodeoxycholic and tauroursodeoxycholic acids on separate occasions at a dose of 0.8 to 1 g/h for 3 hours. In hourly bile samples collected for 8 hours after the beginning of the infusion, biliary bile acid composition (by high-performance liquid chromatography), biliary lipid concentrations (by standard methods), and distribution of biliary carriers (by gel chromatography) were evaluated. Blood liver function tests were performed before and after the infusions. Taurohyodeoxycholic and tauroursodeoxycholic acids became the predominant biliary bile acids in all patients except for one infused with taurohyodeoxycholic acid. Taurohyodeoxycholic acid stimulated significantly greater (P < .05) cholesterol and phospholipid secretion per unit of secreted bile acid (0.098 and 0.451 micromol/micromol, respectively) compared with tauroursodeoxycholic acid (0.061 micromol/micromol for cholesterol and 0.275 micromol/micromol for phospholipids). The secretory ratio between phospholipid and cholesterol was significantly higher after infusion of taurohyodeoxycholic acid (3.88 micromol/micromol) compared with taroursodeoxycholic acid (3.09 micromol/micromol) (P < .05). Biliary enrichment with taurohyodeoxycholic acid was positively related with percent concentration of phospholipids but not with that of cholesterol. The opposite trend was observed in tauroursodeoxycholic acid-enriched biles. In both taurohyodeoxycholic acid- and tauroursodeoxycholic acid-rich bile, 80% to 90% of cholesterol was carried in a gel-chromatographic fraction corresponding to an apparent molecular weight of 80 to 200 kd. No alteration in liver function test results was observed after taurohyodeoxycholic acid infusion. In conclusion, taurohyodeoxycholic acid stimulates greater cholesterol and phospholipid secretion than tauroursodeoxycholic acid, but with a higher phospholipid/cholesterol secretory ratio. In bile enriched with both bile acids, biliary cholesterol is transported in non-micellar aggregates. Finally, in the conditions of our study, taurohyodeoxycholic acid was not hepatotoxic.

Growth of human herpesvirus 6 in HEPG2 cells

HepG2 cells, a well differentiated liver cell line, were shown to be permissive for both human herpesvirus 6 (HHV-6) A and B strains by three independent methods of analysis: detection of viral antigens, viral DNA sequences and infectious virus. HepG2 cell infection with HHV-6 resulted in functional damage as shown by the increased release in the culture medium of some hepatocyte markers. Cells surviving the acute infection were serially passaged without showing cytopathic effect, but, some months later, HHV-6 DNA was still present in the cells and virus induction with a phorbol ester was successful. A possible pathogenetic role of HHV-6 in liver diseases is discussed. Experiments of HepG2 infection with human herpesvirus 7 (HHV-7) were also carried out. The lack of an efficient virus replication suggested a difficulty for HHV-7 to infect hepatic cells.

[Liver metastasis from carcinoid: diagnostic imaging]

Our study was aimed at describing the diagnostic imaging patterns of carcinoid liver metastases. Six patients with liver metastases secondary to carcinoid tumor were examined. The metastases were histologically proved in each patient. All patients were examined with ultrasonography (US), Computed Tomography (CT), Magnetic Resonance Imaging (MRI) and Digital Subtraction Angiography (DSA). All patients were treated with transcatheter embolization of liver metastases. Diagnostic imaging methods showed ten to many dozen metastases in each patient. Tumor size ranged 0.5 to 14 cm. US showed hypoechoic, hyperechoic and isoechoic carcinoid liver metastases with a hypoechoic halo. Large lesions had anechoic central areas due to colliquative necrosis. Hypoechoic patterns were the most frequent ones. Precontrast CT showed hypodense metastases; very small lesions were isodense relative to surrounding liver. CT during contrast agent injection showed that the metastases were hyperdense in the arterial phase; contrast enhancement was poorer in the portal phase. Large lesions showed a hypodense central area due to necrosis which remained hypodense in the late phase. The metastases were hypointense on T1-weighted and hyperintense on T2-weighted MR images. Gradient echo dynamic imaging with Gd-DTPA showed high-signal metastases in the arterial phase and lower signal intensity in the portal phase. DSA, an essential exam before embolization, showed tortuous and elongated intra- and extrahepatic arteries and tumor neovascularization with no malignant abnormalities. In the capillary phase, tumor uptake and inhomogeneous hypervascular patterns were shown. Portal veins were only displaced and compressed, but never infiltrated by the metastases. All the techniques we used contributed to assess liver involvement by carcinoid metastases. DSA must be used only before treatment; both CT and MRI showed the hypervascular patterns of the metastases, but no technique could predict their nature.

Effect of taurohyodeoxycholic acid on biliary lipid secretion in man: preliminary report

Taurohyodeoxycholic acid and tauroursodeoxycholic acid were infused intraduodenally at a rate of 0.8 g/h for three hours in 3 cholecystectomized T-tube patients. Biliary lipid secretion and bile acid composition were evaluated before and after replacement of the endogenous bile acid pool with the two bile acids. As compared to basal values (2.78 +/- 1.67 mM/l), taurohyodeoxycholic acid induced a greater increase in the biliary concentration of phospholipids (4.12 +/- 1.23 mM/l) as compared to tauroursodeoxycholic acid (3.14 +/- 0.98 mM/l). Biliary cholesterol concentration after taurohyodeoxycholic acid (1.89 +/- 0.63 mM/l) was unchanged as compared to the pretreatment period (1.98 +/- 0.58 mM/l), while it decreased significantly after tauroursodeoxycholic acid (0.85 +/- 0.08 mM/I). Biliary cholesterol secreted per unit of bile acid was greater during taurohyodeoxycholic acid than during tauroursodeoxycholic acid, while the opposite was observed for the secretion of phospholipids.

Bile acid structure and regulation of biliary protein secretion and composition in man

To evaluate the effect of bile acid structure on total protein secretion and composition, 4 different bile acids, deoxycholic acid, chenodeoxycholic acid, cholic acid and ursodeoxycholic acid, in order of decreasing hydrophobicity, were infused intraduodenally in 5 cholecystectomized T-tube patients with interrupted enterohepatic circulation. Concentration and composition of biliary proteins were evaluated before and after acute substitution of the endogenous bile acid pool with each bile acid. Total biliary protein concentration and secretion increased progressively with increasing hydrophobicity of the infused bile acid and was highest for deoxycholic acid, followed by chenodeoxycholic acid, cholic acid and ursodeoxycholic acid. A significant increase in the 120 and 150 kDa protein bands on gel-electrophoresis was found after infusion with the more hydrophobic bile acids (deoxycholic acid and chenodeoxycholic acid). Quantitative and qualitative changes in biliary proteins, associated with the administration of bile acids that have different physico-chemical structures, may be relevant to the process of cholesterol crystal nucleation and the pathogenesis of gallstone formation.

Regulation of bile acid synthesis in humans: studies on cholesterol 7 alpha-hydroxylation in vivo

Over the last few years important progress has been made on the quantitation of cholesterol 7 alpha-hydroxylation, the rate-limiting step of bile acid synthesis. The use of a technique based on the determination of body water tritium enrichment after i.v. administration of [7 alpha-3H] cholesterol has allowed in vivo investigation of this step in humans in different experimental conditions. The cholesterol 7 alpha-hydroxylation rate was not affected by the administration of the hydrophilic bile acid ursodeoxycholic acid (UDCA) whereas it was significantly reduced by the more hydrophobic chenodeoxycholic acid (CDCA) and even more so by the strongly hydrophobic deoxycholic acid (DCA). The administration of cholestyramine induced a significant dose-related increase of 7 alpha-hydroxylation along with a correspondent decrease in plasma cholesterol. The administration of simvastatin exerted no effect on cholesterol 7 alpha-hydroxylation despite a marked decrease in serum cholesterol. Treatment with fibrates reduced plasma lipid levels and 7 alpha-hydroxylation rates. Hydroxylation rates were unchanged in familial hypercholesterolaemia and increased in familial combined hyperlipidaemia. These data suggest that in humans bile acid synthesis can be affected by quantitative and qualitative alterations of the enterohepatic circulation of bile acids. Changes in cholesterol 7 alpha-hydroxylation rates may be associated with alterations in plasma lipid levels, but such a relationship is ill-defined and seems to vary with the different experimental models.

Effect of chenodeoxycholic acid and ursodeoxycholic acid administration on acyl-CoA: cholesterol acyltransferase activity in human liver

In order to investigate the relationship between bile acid pool composition and hepatic cholesterol metabolism in humans, we studied the effect of chronic feeding of chenodeoxycholic (CDCA) or ursodeoxycholic acid (UDCA) on the hepatic activity of acyl-CoA: cholesterol acyltransferase (ACAT) evaluated "in vitro". Twenty-eight gallstone patients were admitted to the study: 15 were untreated subjects, 8 were treated with UDCA (10 mg/kg/day for 15-20 days) and 5 were treated with CDCA (15 mg/kg/day for 15-20 days). A liver specimen and a bile sample were obtained during laparotomy for elective cholecystectomy. Untreated subjects had bile supersaturated with cholesterol (mean saturation index: 1.35 +/- 0.31) whereas subjects treated with either UDCA or CDCA had bile unsaturated with cholesterol (mean saturation index: 0.66 +/- 0.1 and 0.75 +/- 0.06 respectively). In all treated subjects the bile acid administered became predominant in bile. ACAT activity was 14% lower in subjects treated with UDCA and 16% lower in those treated with CDCA compared to controls; the differences did not achieve statistical significance. Microsomal cholesterol content did not differ between the groups (75.4 +/- 7.2 nmol/mg protein in control group; 86.5 +/- 7.0 nmol/mg protein in CDCA treated group; 83.4 +/- 7.0 nmol/mg protein in UDCA treated group). Our data show that the cholesterol esterifying activity of human liver is not affected by changes in bile acid pool composition.

Short-term effects of simvastatin on bile acid synthesis and bile lipid secretion in human subjects

To test whether de novo synthesis of cholesterol is a limiting factor for bile acid synthesis, we studied the acute effect of simvastatin, an inhibitor of HMG-coenzyme A reductase (the limiting step of cholesterol synthesis) on bile acid synthesis and biliary lipid secretion in subjects with interrupted enterohepatic circulation. In these conditions bile acid synthesis is derepressed and is assumed to equal biliary bile acid secretion. Five cholecystectomized patients fitted with T-tubes were studied. All subjects were administered simvastatin (80 mg as a single dose) 5 days after surgery. Bile was collected in 3-hr intervals for 15 hr before and 30 hr after the administration of the drug. During the experiment we kept the enterohepatic circulation of bile acid interrupted by inflating an occludable balloon inserted, during cholecystectomy, in the common bile duct. Simvastatin induced significant decreases of plasma total and low density lipoprotein cholesterol concentrations, from 163 +/- 29 mg/dl and 97 +/- 24 mg/dl of the pretreatment value to 144 +/- 30 mg/dl and 82 +/- 22 mg/dl 18 hr after simvastatin administration, respectively. Bile flow tended to increase after simvastatin, and the mean values from the third to the 15th hour after simvastatin administration (22.1 +/- 1.9 ml/hr) were significantly greater than the mean values of the pretreatment period (19.9 +/- 2.8 ml/hr). Concomitantly biliary bile acid, cholesterol and phospholipid concentrations fell from basal values of 15.9 +/- 5.1, 2.3 +/- 0.3 and 5.5 +/- 0.3 mmol/L to mean values, after treatment, of 9.0 +/- 3.5, 1.9 +/- 0.5 and 3.0 +/- 0.9 mmol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevalence rates of gallstone disease in Italy. The Chianciano population study

The prevalence of gallstone disease and associated factors in the entire population of subjects aged 15-65 years born and resident in Chianciano Terme (Siena - Tuscany) was examined in the years 1985 and 1986. The investigation included gallbladder ultrasonography, administration of a questionnaire on personal and family history, physical examination and blood chemistry. A total of 1809 subjects (attendance rate 87.7%) participated in the study. Personal history and physical examination showed that Chianciano inhabitants have a low prevalence of obesity (4.3%) and only 4.4% of the female population had more than two pregnancies. Overall prevalence of gallstone disease (cholecystectomy+cholelithiasis) was 5.9% (3.7% for males and 8.4% for females). Age standardized relative risk of gallstone disease for females was 2.25 (95% confidence limits = 1.68-2.68). Prevalence of cholelithiasis was 3.5% (2.7% for males and 4.2% for females). Prevalence of gallstone disease increased with increasing age in both sexes, being extremely low in the age interval of 15-29 years (0.25%). The overall gallstones/cholecystectomy ratio was found to be lower (1:1) in females than in males (2.7:1). Although subjects with gallstones reported more frequently biliary colics and non-specific dyspeptic symptoms, the diagnostic power of all symptoms in identifying cholelithiasis was very poor due to low sensitivity. Only one third of subjects with gallstones was aware of having the disease. Age, obesity and number of pregnancies were positively associated with gallstone disease in univariate analyses. The association with obesity and parity disappeared in multivariate analysis. Blood lipids and glucose were not associated with the disease both in univariate and multivariate analyses.(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.

Transport of sulfobromophthalein and taurocholate in the HepG2 cell line in relation to the expression of membrane carrier proteins

The transport of two different classes of organic anions (cholephilic dyes; the sulfobromophthalein, BSP, and bile acids; taurocholate, TC) was investigated in the HepG2 cell line. At 37 degrees C, BSP uptake was found to be biphasic with an apparent saturative curve in the concentration range between 0-6 microM followed by a linear component up to 18 microM. Kinetic constant determination showed an apparent Km of 26.6 +/- 3.1 microM and a Vmax of 5.64 +/- 0.82 nmol BSP.min-1.mg prot-1. At 4 degrees C, uptake was linear. By subtracting this latter component from the total uptake, a saturable, carrier mediated uptake was found with an apparent Km of 3.6 +/- 1.0 microM BSP and a Vmax of 0.37 +/- 0.04 nmol BSP.min-1.mg prot-1 (m +/- SEM, n = 6). These values were fully comparable with those found in freshly isolated male hepatocyte. Immunoblot analysis of HepG2 cell plasma membrane revealed the presence of bilitranslocase when tested against a monospecific antibody against this carrier molecule. On the contrary, TC uptake was linear up to concentration of 100 microM TC. No difference was observed in the presence or absence of Na+. Immunoprecipitation analysis showed the absence of the putative carrier of TC. These data indicate that the HepG2 cell line expresses a functioning bilitranslocase-mediated system. Conversely, carrier mediated bile acid uptake is absent in line with the lack of expression of the carrier protein.

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)

Effects of bile acid pool composition on hepatic metabolism of cholesterol in man

This work reviews the evidence concerning the role of the bile acid pool composition in the regulation of the overall hepatic metabolism of cholesterol in man. It has been known that bile acids regulate bile secretion, biliary lipid transport and hepatic cholesterol metabolism. However, the intimate mechanisms of these regulatory functions are not well understood. Current thinking attributes most of this regulation to the size of the bile acid pool. A typical example is represented by the negative feed-back mechanism by which bile acids returning to the liver control their own synthesis. Recent evidence however tend to suggest that not only the size but also the composition contributes to the regulatory activity of the bile acid pool. Specifically the hydrophobic-hydrophilic balance of the pool, as resulting from the characteristics and the proportions of the individual bile acids present within the pool, seems to dictate most of the effects of bile acids on hepatic cholesterol metabolism. Thus abundance within the pool of hydrophobic bile acids, such as deoxycholic or chenodeoxycholic acid, seems to induce a greater biliary lipid secretion and to exert inhibition of cholesterol and bile acid synthesis whereas hydrophilic bile acids such as ursodeoxycholic acid seem to be uneffective. It follows that by changing the composition of the bile acid pool it is possible to influence the hepatic metabolism of cholesterol.

Bile acid synthesis. Metabolism of 3 beta-hydroxy-5-cholenoic acid to chenodeoxycholic acid

Metabolism of 3 beta-hydroxy-5-cholenoic acid to chenodeoxycholic acid has been found to occur in rabbits and humans, species that cannot 7 alpha-hydroxylate lithocholic acid. This novel pathway for chenodeoxycholic acid synthesis from 3 beta-hydroxy-5-cholenoic acid led to a reinvestigation of the pathway for chenodeoxycholic acid from 3 beta-hydroxy-5-cholenoic acid in the hamster. Simultaneous infusion of equimolar [1,2-3H]lithocholic acid and 3 beta-hydroxy-5-[14C]cholenoic acid indicated that the 14C enrichment of chenodeoxycholic acid was much greater than that of lithocholic acid. Thus, in all these species, a novel 7 alpha-hydroxylation pathway exists that prevents the deleterious biologic effects of 3 beta-hydroxy-5-cholenoic acid.

Effect of small doses of deoxycholic acid on bile cholesterol saturation in patients with liver cirrhosis

To test the hypothesis that the detergent power of each individual bile acid--that is, its separate capacity to solubilize cholesterol and to induce biliary cholesterol secretion, present in the biliary bile acid mixture might be one of the determinant factors of biliary cholesterol saturation, we studied the effect of feeding small doses of deoxycholic acid on biliary cholesterol saturation in patients with liver cirrhosis and low deoxycholic acid pool. Eleven hospitalised patients with cirrhosis of various degree of severity were put on a standard solid diet. Fasting bile rich duodenal fluid was obtained at the beginning of the study, after a three to four weeks treatment with deoxycholic acid (3 mg/kg/day, in two doses) and one month after discontinuing bile acid ingestion. Before treatment the fraction of deoxycholic acid was 5.3 +/- 4.9% (mean +/- SD); after treatment the fraction rose to 43.9 +/- 12.0 of total bile acids, but returned to the basal values after stopping bile acids. Bile cholesterol saturation increased significantly from a mean of 0.92 +/- 0.26 (before treatment) to a mean of 1.34 +/- 0.34 after deoxycholic acid feeding (p less than 0.005). One month after treatment, bile saturation was not significantly different from the basal values (0.91 +/- 0.44). We conclude that feeding low doses of deoxycholic acid to patients with liver cirrhosis induces a significant increase of the fraction of this bile acid in the total pool and this is followed by a sharp increase of bile cholesterol saturation. These data are compatible with the hypothesis that the detergent capacity of individual bile acids is one of the main determinants of bile cholesterol saturation.

Cholesterol esterase activity of human intestinal mucosa

It has been suggested that cholesterol absorption in humans is dependent on bile acid pool composition and that expansion of the cholic acid pool size is followed by an increase of the absorption values. Similar observations were reported in rats, where the increase of cholesterol absorption, after trihydroxy bile acid feeding, seems to be due to the stimulatory effect of cholic acid on the intestinal cholesterol esterase. In the present study, therefore, we investigated some general properties of human intestinal cholesterol esterase, with particular emphasis to the effect of bile acids on this enzymatic activity. Twenty-nine segments of small intestine were taken during operations; the enzymatic activity was studied by using mucosal homogenate as a source of enzyme and oleic acid, cholesterol, and 14C-labeled cholesterol as substrates. The time-activity relationship was linear within the first two hours; optimal pH for esterification ranged between 5 and 6.2. There was little difference between the esterifying activity of the jejunal and ileal mucosa. Esterification of cholesterol was observed with all the investigated fatty acids but was maximal with oleic acid. Bile acids did not affect cholesterol esterase activity when present in the incubation mixture at 0.1 and 1.0 mM; the enzymatic activity, however, was significantly inhibited when bile acids were added at 20 mM. In conclusion, this study has shown that the human intestinal mucosa possesses a cholesterol esterase activity; at variance with the rat, however, the human enzyme does not seem to be stimulated by trihydroxy bile acids. Thus, the stimulatory effect of cholic acid on cholesterol absorption induced by the administration of this bile acid does not seem to be simply due to changes of cholesterol esterase activity of the small bowel mucosa.