Role of cholesterol synthesis in regulation of bile acid synthesis and biliary cholesterol secretion in humans

We used lovastatin, a specific inhibitor of HMG-CoA reductase, to study the role of cholesterol synthesis in regulation of both bile acid synthesis, measured by release of 14CO2 from [26-14C]cholesterol, and biliary cholesterol secretion, measured by standard marked perfusion techniques, in humans. Six volunteers were studied in each of four periods: a) control; b) 6-10 hours after a single 40 mg oral dose of lovastatin to study acute effects; c) after 5-6 weeks of lovastatin 40 mg orally twice a day to study steady-state effects; and d) 24 h after cessation of chronic lovastatin. Mean bile acid synthesis fell to 69% of control (P less than 0.01) after single-dose lovastatin and remained at 83% of control after 5-6 weeks on lovastatin (P less than 0.05). After withdrawal of lovastatin, mean bile acid synthesis was 88% of control (NS). Mean biliary cholesterol secretion did not change after single-dose lovastatin (103% of control), but fell to 81% of control during chronic lovastatin treatment (P less than 0.05). After withdrawal of lovastatin, mean cholesterol secretion remained at 80% of control (P less than 0.05). These data suggest that in humans cholesterol synthesis is an immediate regulator of bile acid synthesis. Cholesterol synthesis also regulates biliary cholesterol secretion, but the effect is not immediate and therefore may be indirect.     

Mechanism and kinetic characteristics of the uncoupling by plant steroids of biliary cholesterol from bile salt output

In male Wistar rats fed diets containing different plant steroids, including sitosterols, diosgenin, digitonin and saponin from gypsophila, biliary cholesterol secretion significantly increased 50% to 300%, whereas biliary bile salt and phospholipid showed minor changes. Both cholesterol and phospholipid outputs were coupled to biliary bile salt output in a curvi-linear relationship which could be fitted by rectangular hyperbolae, in the animals fed with different plant steroids. The theoretical maximal biliary cholesterol output significantly increased by 200% in sitosterol-fed rats and 500% in diosgenin-fed animals. No changes were found in the kinetic characteristics of biliary phospholipid outputs. Adding 2% cholesterol to the diosgenin diet abolished the increment of biliary cholesterol output induced by the plant steroid. The intraperitoneal injection of 45 mumol/kg body wt per day (3 days) diosgenin, a C27-sapogenin, and 65 mumol/kg body wt. per day (3 days) tomatidin, a C27-alkaloid, incorporated in phosphatidylcholine-taurocholate liposomes significantly increased biliary cholesterol output by 70%. These experiments indicated that the plant steroid-induced biliary cholesterol output was independent of the inputs of cholesterol from the diet and from hepatic cholesterogenesis modified by the plant steroid. It was apparent that the profound changes of biliary cholesterol secretion were the consequence of direct effects of the steroids on the intrahepatocytic regulatory mechanisms of biliary cholesterol secretion. This novel effect appears to be a universal characteristic of plant steroids, since it can be elicited by sitosterols, C27-sapogenins, C27-alkaloids, and saponins of the cholanic and beta-amirinic group.     

Effects of lovastatin on biliary lipid secretion and bile acid metabolism in humans

Lovastatin, an inhibitor of HMG-CoA reductase, lowers cholesterol saturation of bile. To determine the mechanism of this effect and further define the role of cholesterol synthesis in regulation of biliary lipid metabolism, we studied ten human volunteers in a control period and again after 5-6 weeks on lovastatin, 40 mg b.i.d. Mean sterol production from acetate in mononuclear leukocytes fell from 1.18 to 0.84 pmol/min per 10(6) cells on lovastatin (P less than 0.02). Concomitantly there was reduction in mean biliary secretion of cholesterol from 143 to 96 mumol/h (P less than 0.02). On lovastatin, mean pool size of bile acids by the Lindstedt method fell from 3193 to 2917 mumol (one-sided P = 0.05) and mean pool size by the one-sample method fell from 5158 to 4091 mumol (P less than 0.002). Lovastatin had no effect on mean fractional turnover rate of either cholic acid (0.77 vs. 0.74 day-1) or chenodeoxycholic acid (0.51 vs. 0.54 day-1). Mean total bile acid synthesis was lower on lovastatin (1443 vs. 1240 mumol/day), but the difference did not quite achieve statistical significance. In humans, inhibition of cholesterol synthesis by lovastatin lowers biliary cholesterol saturation by reducing cholesterol secretion into bile. Bile acid pool size, and perhaps bile acid synthesis, are also reduced by this inhibition.     

Biliary secretion of cefoperazone and its inter-relationship with bile acid transport in the rat

Cefoperazone is a third generation cephalosporin which is secreted predominantly in bile. This study set out to examine the effect of stimulating bile choleresis on the biliary secretion of cefoperazone. Stimulation of both bile acid-dependent and independent bile flow (phenobarbitone pretreatment) hastened the peak appearance of a pulse of cefoperazone into bile. Although the biliary secretion rate of cefoperazone was enhanced by bile acid infusion, the % recovery and maximal biliary concentration were reduced. The reciprocal effect of continuous cefoperazone infusion on the rate of biliary transport of a pulse of bile acid was examined. Cefoperazone infusion hastened the biliary transport of glycocholate. Net recovery of glycocholate was unaffected.     

Biliary lipid secretion in the rat during infusion of increasing doses of unconjugated bile acids

The aim of the present study was to examine the secretion of biliary components in rats during infusion of increasing doses of either deoxycholic acid, chenodeoxycholic acid or cholic acid and to test the hypothesis that biliary phospholipids may regulate the hepatic bile acid secretory capacity. Analysis of bile samples, collected every 10 min throughout the infusion period showed that there was an elevation of bile acid, phospholipid, cholesterol and alkaline-phosphodiesterase secretion, with all the bile acids, peaking and then gradually declining. Their secretory rates maximum differed and were inversely related to their detergent strength. However, the secretory rates maximum and total output of phospholipids and cholesterol were similar for all bile acids infused. The per cent contribution of phosphatidylcholine to total bile acid-dependent phospholipid secretion was reduced from 84% (in the pre-infusion period) to 59, 46 and 13% at the end of the cholic acid, chenodeoxycholic acid and deoxycholic acid infusions, respectively. This decrease in the per cent contribution of phosphatidylcholine was associated with an increase in the contribution of both sphingomyelin and phosphatidylethanolamine. The biliary phospholipid fatty acid pattern corroborated these changes in the phospholipid classes. Since sphingomyelin and phosphatidylethanolamine are major phospholipids in bile canalicular and other hepatocellular membranes, the marked increase in their secretion in bile during the infusion of high doses of bile acids may indicate solubilization of membrane phospholipids, resulting in membrane structural changes responsible for the reduced excretory function of the liver.     

Effect of bean intake on biliary lipid secretion and on hepatic cholesterol metabolism in the rat

We studied the effect of a bean diet on biliary lipid secretion, serum cholesterol concentration, and hepatic cholesterol metabolism in the rat. Rats fed a bean diet for 10-12 days had increased biliary cholesterol output and molar percentage by 300% and 200%, respectively, compared to rats fed an isocaloric and isoprotein casein diet. Biliary phospholipid output increased 180%. Bile flow and biliary bile salt output remained in the normal range. Total serum and VLDL cholesterol concentration significantly decreased 27% and 50%, respectively, in the rats fed the bean diet. Hepatic cholesterogenesis was increased 170% in the bean-fed animals. The relative contribution of newly synthesized hepatic cholesterol to total biliary cholesterol increased 200%, and that of endogenous origin only 50%. These results suggested that newly synthesized hepatic cholesterol was preferentially channelled to the biliary cholesterol secretory pathway in bean-fed rats. Although hepatic cholesteryl ester concentration increased 240%, the incorporation of [14C]oleate into hepatic cholesteryl esters was significantly decreased by 30% in isolated hepatocytes of bean-fed animals. These results were consistent with the possibility that the availability of hepatic free cholesterol for biliary secretion was increased in the bean-fed animals. This study demonstrates that bean intake has a profound effect on the metabolic channelling and compartmentalization of hepatic cholesterol, resulting in a significant decrease in total serum and very low density lipoprotein cholesterol concentrations and a high biliary cholesterol output.     

Regulation of bile acid synthesis. III. Correlation between biliary bile salt hydrophobicity index and the activities of enzymes regulating cholesterol and bile acid synthesis in the rat

Hepatic bile acid synthesis is thought to be under negative feedback control by bile salts in the enterohepatic circulation, acting at the level of cholesterol 7 alpha-hydroxylase (C7 alpha H), the initial and rate-limiting step in the bile acid biosynthetic pathway. Bile salts also suppress the activity of the rate-limiting enzyme for cholesterol synthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA-R). The mechanisms of these regulatory effects are poorly understood, and one or both may be indirect. Previous data suggest that the hydrophilic-hydrophobic balance of bile salts, a major determinant of their cholesterol solubilizing properties, also determines their potency as regulators of bile acid and cholesterol synthesis. To further evaluate the relationship between the physicochemical and regulatory properties of bile acids, we altered the composition of the bile salt pool of rats by feeding one or more of seven different bile acids (1% w/w for 14 days). We then determined the mean hydrophilic-hydrophobic balance (hydrophobicity index) of the bile salts in bile, and correlated this with the specific activities of C7 alpha H and HMG-CoA-R, and of acyl-CoA:cholesterol acyltransferase (ACAT), a third hepatic microsomal enzyme which regulates cholesterol esterification. In all instances following bile acid feeding, conjugates of the fed bile acid(s) became the predominant bile salts in bile. Highly significant negative linear correlations (each P less than 0.0001) were found between the hydrophobicity indices of biliary bile salts and the activities of C7 alpha H (r = 0.79) or HMG-CoA-R (r = 0.63). By contrast, no significant correlation could be demonstrated between ACAT activity and the hydrophobicity index of biliary bile salts. The correlation between activities of HMG-CoA-R and C7 alpha H was also highly significant (r = 0.81; P less than 0.0001). No significant correlation existed between ACAT and either HMG-CoA-R or C7 alpha H. Microsomal free cholesterol was not consistently altered by bile acid feeding. Thus, the potency of circulating bile salts as suppressors of the enzymes regulating bile acid and cholesterol synthesis increases with increasing hydrophobicity. The hydrophobic-hydrophilic balance of the bile salt pool may play an important role in the regulation of cholesterol and bile acid synthesis.     

Cholestasis, metabolism and biliary lipid secretion during perfusion of rat liver with different bile salts.

The biological effects of bile acids depend largely upon their molecular structure. When bile acid uptake exceeds the maximal biliary secretory rate (SRm) cholestasis occurs. In order to characterize the influence of bile acid structure on its cholestatic potency we systematically studied SRm, maximal bile flow, maximal and cumulative phospholipid and cholesterol secretion with different taurine-conjugated tri-, di- and keto bile acids (Table I) in the isolated perfused rat liver. Bile acids with a high critical micellar concentration (CMC) promoted the greatest bile flow; a positive non-linear correlation between CMC and maximal bile flow was found. 3 alpha-Hydroxylated bile acids with a hydroxyl group in 6 alpha and/or 7 beta position and lacking a 12 alpha hydroxy group had a high SRm. SRm was not related to CMC or maximal bile flow, respectively. Phospholipids and cholesterol were secreted in a nearly fixed ratio of 12:1; a strong linear relationship could be observed. Cumulative phospholipid secretion over 48 min was significantly lower for non and poor micelle forming bile acids (TDHC and TUC) than for those with comparatively low CMC values (TUDC, TC, THC, THDC, TCDC) (70-140 vs. 210-450 nmol/g liver). At SRm all bile acids with good micelle forming properties showed a similar cumulative biliary lipid output. However, when biliary lipid output was related to 1 mumol bile acid secreted bile acids with a low SRm induced the highest lipid secretion (TCDC, TC). These data (1) demonstrate that a 6 alpha and/or a 7 beta hydroxy group on the steroid nucleus reduce cholestatic potency if the 12 alpha hydroxy group is absent, (2) suggest that in the case of micelle forming bile acids the total amount of phospholipids secreted in bile (depletion of cellular phospholipids) is associated with the occurrence of cholestasis whereby bile acids with a low SRm deplete the cellular phospholipid content at much lower bile acid concentrations than those with a higher SRm and (3) imply that bile acids with non and poor micelle forming properties (TDHC, TUC) presumably do not cause cholestasis (solely) by depletion of cellular phospholipids.     

Early effects of dietary orotic acid upon liver lipid synthesis and bile cholesterol secretion in rats

Dietary orotic acid is known to cause impaired fatty acid synthesis and increased cholesterol synthesis in rats. We found that the impaired fatty acid synthesis occurs during the first day of orotic acid feeding and, in studies with albumin-bound [1-14C]palmitic acid, an associated decrease in the rate of esterification of this fatty acid into triacylglycerol, phospholipid, and cholesteryl ester was observed. These changes may result from the known decreases in liver levels of adenine nucleotides or, as reported here, from decreased liver CoASH levels in orotic acid-fed rats. The increase in hepatic cholesterol synthesis occurred during the second day of orotic acid feeding. It was detected by increased incorporation of [1,2-14C]acetate into cholesterol by liver slices and by a 7-fold increase in HMG-CoA reductase activity. At the same time the biliary output of cholesterol was increased 2-fold and studies using 3H2O revealed that the output of newly synthesized cholesterol in bile was increased 5-fold. The content of cholesteryl ester in hepatic microsomes decreased during orotic acid feeding but free cholesterol was unchanged. The findings are interpreted to suggest that the increased bile cholesterol secretion caused by orotic acid is a result of impaired hepatic cholesterol esterification and that the increase in HMG-CoA reductase activity is a result of diminished negative feedback due to the depleted content of cholesteryl ester in the hepatic microsomes.     

The relationship between HDL-, LDL-, liposomes-free cholesterol, biliary cholesterol and bile salts in the rat

In order to study the relationship between bile cholesterol and free cholesterol carried by high and low density lipoproteins (HDL and LDL), 10 male Wistar rats, 11 weeks old and fed with a standard diet were divided into 3 groups which received an intravenous infusion (jugular vein) of either LDL, HDL or liposomes. Liposomes were used for comparison because they are assimilated by hepatocytes, but are not recognized by specific receptors. HDL isolated from rat sera were labeled with [14C]cholesterol by molecular exchange and LDL were labeled by exchange with [14C]cholesterol incorporated into phosphatidyl choline/cholesterol liposomes. The peaks of radioactivity appeared in bile 30 min after the HDL or liposome injection and after 210 min for the LDL injection. The kinetic behavior of the cholesterol carried by the liposomes was quite similar to that of cholesterol carried by HDL. Cholesterol carried by HDL was metabolized in bile salts faster than that carried by LDL: cholesterol-HDL or cholesterol-liposomes contributed to the same extent to the secretion of bile cholesterol (15 and 11%, respectively, of the injected dose), LDL (20% of the injected dose). However, the main part of [14C]cholesterol from HDL, LDL or liposomes was metabolized in bile salts. Thus, cholesterol from an exogenous source seemed to be used mainly as a substrate for bile salts. Our study revealed a difference between the hepatic metabolism of HDL, liposomes and LDL in the rat: the kinetic difference between the secretions of the radioactive compounds in bile may be explained by differences in assimilation, intracellular pathways or bile secretion.     

A surgical model for studying biliary bile acid and cholesterol metabolism in swine.

Techniques were developed in young growing pigs to simultaneously collect and reinfuse bile. Silastic cannulae were designed and surgically implanted in the common bile duct and the duodenum. Direct sampling of the hepatic bile was achieved by bypassing the gallbladder. The techniques allowed for steady-state studies of hepatic function to be conducted in conscious swine in two different studies. Pigs, thus surgically modified, can serve as an appropriate model for physiologic, pharmacologic, and nutritional research that involves bile sampling.     

Rapid kinetic analysis of the bile-salt-dependent secretion of phospholipid, cholesterol and a plasma-membrane enzyme into bile.

Isolated rat livers were perfused under 'one-pass' conditions and bile was collected at 1 min intervals. After 1 min pulse, taurocholate appeared in the collected bile within 2 min, peak output occurring 2 min later. In contrast, the increased output of phospholipids and cholesterol was slower, peak output occurring 6-11 min after the original pulse of taurocholate. These results suggest that mixed micelles cannot be formed inside the cell or during passage of bile salts through the membrane, since bile salt and lipids should then parallel each other. The bile salts must therefore be pumped into the lumen and the lipids added subsequently, due to the actions of the bile salts in the canalicular lumen. It is suggested that the biliary lipid is obtained from microdomains of biliary-type lipid in the canaliculus membrane, which are vesiculated and solubilized by the action of bile salts. It is also suggested that this biliary-type lipid is brought continuously to the membrane via vesicle traffic; this traffic is increased during increased bile-salt output, and is a process that can be inhibited by colchicine.     

The effect of cholesterol feeding on bile acid kinetics and biliary lipids in normolipidemic and hypertriglyceridemic subjects.

Six normolipidemic and six hypertriglyceridemic subjects were studied. The investigations were conducted before and after the basal diet (cholesterol intake about 0.8 mmol/day) was replaced by a cholesterol-rich diet (cholesterol intake about 4 mmol/day). Irrespective of the type of diet, the combined formation of cholic acid (C) and chenodeoxycholic acid (CD) was about two times higher in the hyperlipoproteinemic (mostly type IV) than the normolipidemic subjects. With the cholesterol-rich diet, the total plasma cholesterol increased in all normolipidemic and in four hyperlipidemic patients. Although total bile acid formation remained constant, there were several indications that an augmented intake of dietary cholesterol influenced bile acid metabolism. The pool size of CD increased in all but one normolipidemic subject. This group also displayed a decrease in the C/CD ratio of the bile acids produced and in the C/CD ratio of the bile acids in duodenal bile. The latter finding was also encountered in the hyperlipoproteinemic patients. On the basis of these and other data, it is suggested that the pattern of the bile acids synthesized may roughly reflect the degree of hepatic cholesterogenesis. Cholesterol feeding had no consistent effects on the molar cholesterol concentration in duodenal bile.-Andersén, E., and K. Hellstr?m. The effect of cholesterol feeding on bile acid kinetics and biliary lipids in normolipidemic and hypertriglyceridemic subjects.     

Thyroid hormone differentially augments biliary sterol secretion in the rat. II. The chronic bile fistula model.

To further define thyroid hormone effects on bile acid synthesis and biliary lipid secretion, studies were done in chronic bile fistula rats. Euthyroid and methimazole-hypothyroid rats, with and without triiodothyronine (T3) injection, had total bile diversion for timed bile collections. With interrupted enterohepatic circulation, cholesterol absorption is negligible and bile acid secretion equals bile acid synthesis rate. Hypothyroid rats had diminished levels of bile acid synthesis and biliary secretion of cholesterol and phospholipid. Single dose T3 injection produced a 13-fold increase in bile cholesterol secretion and a 3-fold increase in phospholipid secretion, both initiated 12 h after T3. Bile acid synthesis increased by 50%, but the increase did not begin until 24 h after T3. Neither hypothyroidism nor T3 treatment abolished diurnal rhythms of bile acid synthesis and biliary lipid secretion. Inhibition of cholesterol synthesis with lovastatin resulted in a persistent 33% decrease in bile acid synthesis in euthyroid and hypothyroid rats, while bile cholesterol secretion only transiently decreased. Inhibition of cholesterol synthesis did not alter T3-induced bile cholesterol secretion, with a 10-fold increase seen. However, bile acid synthesis was not stimulated by T3 in the presence of lovastatin. We conclude that facilitated bile acid synthesis and biliary cholesterol secretion are early effects of T3 and may account for the hypocholesterolemia of T3. Cholesterol synthesis does not appear to be required for the T3-induced bile cholesterol secretion.     

Bile acid metabolism in mammals. VIII. Biliary secretion of cholylarginine by the isolated perfused rat liver.

In studies of cholic acid metabolism using the isolated perfused rat liver system, an unknown conjugate of cholic acid was observed. This conjugate comprised 15-27% of the biliary bile acids in these experiments, was less polar than cholylglycine on thin-layer chromatography using butanol, acetic acid, and water, and had an apparent molecular weight greater than that of cholyltaurine on gas-liquid chromatography. Amino acid analysis of the hydrolyzed conjugate demonstrated the presence of arginine. Perfusion studies with radioactive arginine, and mass spectrometric analysis proved that the conjugate was cholylarginine. Secretion of this conjugate does not represent a deficiency of available glycine and taurine.     

Effects on apoB-100 secretion and bile acid synthesis by redirecting cholesterol efflux from HepG2 cells

This study determined the effects of apoA-I, HDL3, or hydroxy-beta-cyclodextrin on apoB-100 secretion and bile acid synthesis by HepG2 cells. The principal observations were that: 1) ApoB-100 secretion into the medium was significantly less after the addition of any of the three agents. 2) Triglyceride mass was not significantly changed from control in the medium but was significantly, although modestly, reduced in the cells. 3) Neither free cholesterol (FC) nor cholesteryl ester (CE) mass in the medium was changed; by contrast, CE mass was reduced within the cells although FC was not. 4) Although the total mass of cholesterol in the medium was unaffected, the proportion associated with apoB-100 was reduced, whereas the proportion associated with the non-apoB-100 fraction was increased. 5) There was also an unanticipated, but substantial, increase in bile acid synthesis induced by apoA-I, HDL3, or hydroxy-beta-cyclodextrin, which was time and concentration dependent, and which was associated with marked increases in cholesterol 7 alpha-hydroxylase activity. There were no significant changes in ACAT activity and only modest increases in HMG-CoA reductase activity. These findings support previous clinical observations that an elevated apoB-100 can accompany a low HDL cholesterol in normotriglyceridemic subjects. They also point to physiologically important, although still only partially understood, metabolic relationships amongst hepatic apoB-100 secretion, cholesterol efflux, and bile acid synthesis.     

The influence of estrogen on hepatic cholesterol metabolism and biliary lipid secretion in rats fed fish oil

Both estrogen and dietary n-3 polyunsaturated fatty acids are known to be hypocholesterolemic, but appear to exert their effects by different mechanisms. In this study, the interaction between dietary fish oil (rich in n-3 polyunsaturated fatty acids) and estrogen in the regulation of hepatic cholesterol metabolism and biliary lipid secretion in rats was studied. Rats fed a low fat or a fish oil-supplemented diet for 21 days were injected with 17alpha-ethinyl estradiol (5 mg/kg body weight) or the vehicle only (control rats) once per day for 3 consecutive days. Estrogen-treatment led to a marked reduction in plasma cholesterol levels in fish oil-fed rats, which was greater than that observed with either estrogen or dietary fish oil alone. The expression of mRNA for cholesterol 7alpha-hydroxylase was decreased by estrogen in rats fed a low fat or a fish oil-supplemented diet, while the output of cholesterol (micromol/h/kg b.wt.) in the bile was unchanged in both groups. Cholesterol levels in the liver were increased by estrogen in rats given either diet, but there was a significant shift from cholesterol esterification to cholesteryl ester hydrolysis only in the fish oil-fed animals. Estrogen increased the concentration of cholesterol (micromol/ml) in the bile in rats fed the fish oil, but not the low fat diet. However, the cholesterol saturation index was unaffected. The output and concentration of total bile acid was also unaffected, but changes in the distribution of the individual bile acids were observed with estrogen treatment in both low fat and fish oil-fed groups. These results show that interaction between estrogen-treatment and dietary n-3 polyunsaturated fatty acids causes changes in hepatic cholesterol metabolism and biliary lipid secretion in rats, but does not increase the excretion of cholesterol from the body.     

Biliary phospholipid secretion is not required for intestinal absorption and plasma status of linoleic acid in mice.

Biliary phospholipids have been hypothesized to be important for essential fatty acid homeostasis. We tested this hypothesis by investigating the intestinal absorption and the status of linoleic acid in mdr2 Pgp-deficient mice which secrete phospholipid-free bile. In mice homozygous (-/-) for disruption of the mdr2 gene and wild-type (+/+) mice, dietary linoleic acid absorption was determined by 72 h balance techniques. After enteral administration, [(13)C]-linoleic acid absorption was determined by measuring [(13)C]-linoleic acid concentrations in feces and in plasma. The status of linoleic acid was determined in plasma and in liver by calculating the molar percentage of linoleic acid and the triene:tetraene ratio. Although plasma concentration of [(13)C]-linoleic acid at 2 h after enteral administration was significantly lower in (-/-) compared to (+/+) mice (P相似文献