Inhibition of ileal bile acid transport and reduced atherosclerosis in apoE-/- mice by SC-435

Blocking intestinal bile acid absorption by inhibiting the apical sodium codependent bile acid transporter (ASBT) is a target for increasing hepatic bile acid synthesis and reducing plasma LDL cholesterol. SC-435 was identified as a potent inhibitor of ASBT (IC50 = 1.5 nM) in cells transfected with the human ASBT gene. Dietary administration of 3 mg/kg to 30 mg/kg SC-435 to apolipoprotein E-/- (apoE-/-) mice increased fecal bile acid excretion by >2.5-fold. In vivo inhibition of ASBT also resulted in significant increases of hepatic mRNA levels for cholesterol 7alpha-hydroxylase and HMG-CoA reductase. Administration of 10 mg/kg SC-435 for 12 weeks to apoE-/- mice lowered serum total cholesterol by 35% and reduced aortic root lesion area by 65%. Treatment of apoE-/- mice also resulted in decreased expression of ileal bile acid binding protein and hepatic nuclear hormone receptor small heterodimer partner, direct target genes of the farnesoid X receptor (FXR), suggesting a possible role of FXR in SC-435 modulation of cholesterol homeostasis. In dogs, SC-435 treatment reduced serum total cholesterol levels by 相似文献   

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.     

Dexamethasone regulates bile acid synthesis in monolayer cultures of rat hepatocytes by induction of cholesterol 7 alpha-hydroxylase.

To study the effect of steroid hormones on bile acid synthesis by cultured rat hepatocytes, cells were incubated with various amounts of these compounds during 72 h and conversion of [4-14C]cholesterol into bile acids was measured. Bile acid synthesis was stimulated in a dose-dependent way by glucocorticoids, but not by sex steroid hormones, pregnenolone or the mineralocorticoid aldosterone in concentrations up to 10 microM. Dexamethasone proved to be the most efficacious inducer, giving 3-fold and 7-fold increases in bile acid synthesis during the second and third 24 h incubation periods respectively, at a concentration of 50 nM. Mass production of bile acids as measured by g.l.c. during the second day of culture (28-52 h) was 2.2-fold enhanced by 1 microM-dexamethasone. No change in the ratio of bile acids produced was observed during this period in the presence of dexamethasone. Conversion of [4-14C]7 alpha-hydroxycholesterol, an intermediate of the bile acid pathway, to bile acids was not affected by dexamethasone. Measurement of cholesterol 7 alpha-hydroxylase activity in homogenates of hepatocytes, incubated with 1 microM-dexamethasone, showed 10-fold and 90-fold increases after 48 and 72 h respectively, as compared with control cells. As with bile acid synthesis from [14C]cholesterol, no change in enzyme activity was found in hepatocytes cultured in the presence of 10 microM steroid hormones other than glucocorticoids. Addition of inhibitors of protein and mRNA synthesis lowered bile acid production and cholesterol 7 alpha-hydroxylase activity and prevented the rise of both parameters with dexamethasone, suggesting regulation at the mRNA level. We conclude that glucocorticoids regulate bile acid synthesis in rat hepatocytes by induction of enzyme activity of cholesterol 7 alpha-hydroxylase.     

Hepatocyte-hepatoma cell hybrids. Characterization and demonstration of bile acid synthesis

Hybrids were created by fusion of primary rat hepatocytes with well-differentiated Reuber H35 rat hepatoma cells. Seventeen hybrids were screened for bile acid synthesis using [26-14C]cholesterol. As [26-14C]cholesterol was converted to bile acid, 14CO2 was released. Using this assay, four hybrids (8B, 12C, 13C, and 13D) were identified which synthesized bile acid. These four hybrids also incorporated [14C]taurine into bile acid. Bile acids were identified by capillary gas chromatography/mass spectrometry, and their rates of synthesis were quantitated by isotope dilution. Reuber H35 cells synthesized little or no bile acid. However, hybrids 8B, 12C, 13C, and 13D synthesized chenodeoxycholic acid, alpha-muricholic acid, and cholic acid and secreted them into the media. The rates of synthesis of individual bile acids varied among these hybrids. For example, the relative percentage of cholic acid ranged from 11.1% (hybrid 8B) to 50.4% (hybrid 13C). The bile acids synthesized and secreted by the most active hybrid, 12C, were greater than 93% conjugated. In summary, hybrids were created that retain the capacity to synthesize, conjugate, and secrete three major rat bile acid species. Such hybrids are unique model systems that will allow the study of the biochemical and genetic regulation of bile acid synthesis.     

The effect of natural and synthetic antioxidant polyhydroxynaphthoquinones on cholesterol metabolism in cultured rabbit hepatocytes]

Primary cultures of rabbit hepatocytes were used to examine the effect of natural and synthetic antioxidants--polyhydroxynaphthoquinones (PHNQ) and alpha-tocopherol on cholesterol and bile acid synthesis. Histochrome, one of the PHNQ, slightly decreased cholesterol synthesis at concentrations 10-100 microM, whereas alpha-tocopherol stimulated cholesterol synthesis. After administration of histochrome or alpha-tocopherol into culture medium a significant stimulation of bile acid synthesis in dose-dependent manner was observed. The increase of bile acid secretion by histochrome in the presence of physiological concentration of HDL2 was found as well. Since histochrome in contrast to alpha-tocopherol enhanced accumulation of [14C] cholesterol of HDL2 in the hepatocytes, it was concluded that histochrome stimulated bile acid synthesis as a result of increased input of HDL2 cholesterol into hepatocytes. These data suggest that histochrome may exhibit a hypocholesterolemic effect by stimulation of bile acid synthesis and inhibition of cholesterol synthesis.     

Biliary lipid secretion in hypercholesterolemia.

A report on the effects of primary bile acid ingestion alone or in combination with plant sterols on serum cholesterol levels, biliary lipid secretion, and bile acid metabolism. Biliary bile acid and cholesterol secretion were measured in four patients with type IIa hypercholesterolemia before and after randomized treatment periods. During these periods either a bile acid mixture (cholic-chenodeoxycholic 2:1, a proportion similar to that endogenously synthesized in health), at a level of 20 mg/kg, or the same mixture plus sitosterols, 200 mg/kg, was fed. Serum cholesterol and the cholesterol saturation of fasting-state bile was also measured. Pretreatment biliary lipid secretion was within normal limits. Bile acid kinetic measurements were also recorded before treatment and showed that cholic acid synthesis was disproportionately decreased relative to that of chenodeoxycholic acid, a finding previously reported by others. Administration of the bile acid mixture increased biliary bile acid secretion in 3 of 4 patients, but did not influence biliary cholesterol secretion. The combination of sitosterol-bile acid, however, caused a relative decrease in cholesterol secretion in bile, and fasting-state bile became unsaturated in all patients. No change in fecal neutral sterol excretion occurred during the beta-sitosterol-bile acid regimen, suggesting that simultaneous bile acid feeding blocks the compensatory increase in cholesterol synthesis known to be induced by beta-sitosterol feeding in hypercholesterolemic patients. Serum cholesterol levels also fell modestly during the sitosterol-bile acid regimen, the decrease averaging 15%. We conclude that the abnormally low rate of bile acid synthesis in patients with type IIa hyperlipoproteinemia does not influence biliary lipid secretion; that increasing the input of the two primary bile acids into the enterohepatic circulation does not increase biliary cholesterol secretion or lower serum cholesterol levels in such patients; and that the usual increase in cholesterol synthesis induced by beta-sitosterol feeding does not occur if bile acids are administered simultaneously.     

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.     

Feedback inhibition of bile acid synthesis in cultured pig hepatocytes

Bile acid synthesis by cultured pig hepatocytes, as measured by conversion of [14C]cholesterol to bile acids, increased during the second and third day of culture. This rise was inhibited after addition of various conjugated and unconjugated bile acids in a concentration of 100 microM. It could be completely prevented by cycloheximide, indicating that de novo protein synthesis is required for the increase in bile acid formation. No effect of exogenous bile salts on LDH release to the medium or on cellular ATP content was observed, demonstrating that hepatocyte viability was not affected. During the period in which bile acid synthesis was inhibited, pig hepatocytes were able to accumulate taurocholic acid (100 microM) up to 7-18 nmol per mg cell protein (decreasing during culture time). It is concluded that feedback regulation of bile acid synthesis is exerted by direct action of bile acids on the hepatocyte.     

The availability of different sources of cholesterol for bile acid synthesis by cultured chick embryo hepatocytes

The availability of different sources of cholesterol for bile acid synthesis by cultured chick embryo hepatocytes was studied. Mevalonolactone was taken up by the cells and converted to cholesterol, cholesterol ester and tauroconjugates of bile acids. The addition of mevalonolactone had little effect on the conversion of endogenous cholesterol to taurocholic acid; however, taurochenodeoxycholic acid synthesis was stimulated. 25-30% of the cholesterol synthesized from mevalonolactone was converted to taurochenodeoxycholic, taurocholic and two so-far unidentified bile acids. All bile acids were secreted into the incubation medium. When cholesterol was added as mixed liposomes with phosphatidylcholine, it was taken up by the cells and converted to bile acids. At low concentrations of liposomes, the greater part of the cholesterol which was taken up by the cells was converted to bile acids. At higher concentrations, considerable amounts of cholesterol and cholesterol ester accumulated inside the cells. When mevalonolactone and cholesterol liposomes was added together, both substrates were used simultaneously for bile acids synthesis. HDL cholesterol was the best substrate tested, yielding large amounts of two, so-far, unidentified bile acids (possibly allo-bile acids) and smaller amounts of taurocholic and taurochenodeoxycholic acid. Addition of HDL suppressed the conversion of endogenous cholesterol to taurocholic acid; taurochenodeoxycholic acid synthesis, however, was stimulated.     

Ileal bile acid transport regulates bile acid pool, synthesis, and plasma cholesterol levels differently in cholesterol-fed rats and rabbits

We investigated the effect of ileal bile acid transport on the regulation of classic and alternative bile acid synthesis in cholesterol-fed rats and rabbits. Bile acid pool sizes, fecal bile acid outputs (synthesis rates), and the activities of cholesterol 7alpha-hydroxylase (classic bile acid synthesis) and cholesterol 27-hydroxylase (alternative bile acid synthesis) were related to ileal bile acid transporter expression (ileal apical sodium-dependent bile acid transporter, ASBT). Plasma cholesterol levels rose 2.1-times in rats (98 +/- 19 mg/dl) and 31-times (986 +/- 188 mg/dl) in rabbits. The bile acid pool size remained constant (55 +/- 17 mg vs. 61 +/- 18 mg) in rats but doubled (254 +/- 46 to 533 +/- 53 mg) in rabbits. ASBT protein expression did not change in rats but rose 31% (P < 0.05) in rabbits. Fecal bile acid outputs that reflected bile acid synthesis increased 2- and 2.4-times (P < 0.05) in cholesterol-fed rats and rabbits, respectively. Cholesterol 7alpha-hydroxylase activity rose 33% (24 +/- 2.4 vs. 18 +/- 1.6 pmol/mg/min, P < 0.01) and mRNA levels increased 50% (P < 0.01) in rats but decreased 68% and 79%, respectively, in cholesterol-fed rabbits. Cholesterol 27-hydroxylase activity remained unchanged in rats but rose 62% (P < 0.05) in rabbits. Classic bile acid synthesis (cholesterol 7alpha-hydroxylase) was inhibited in rabbits because an enlarged bile acid pool developed from enhanced ileal bile acid transport. In contrast, in rats, cholesterol 7alpha-hydroxylase was stimulated but the bile acid pool did not enlarge because ASBT did not change. Therefore, although bile acid synthesis was increased via different pathways in rats and rabbits, enhanced ileal bile acid transport was critical for enlarging the bile acid pool size that exerted feedback regulation on cholesterol 7alpha-hydroxylase in rabbits.     

Measurement of bile acid synthesis by 14CO2: the metabolism of propionyl CoA.

The relationship between 14CO2 evolution from the catabolism of [26 or 2714C] cholesterol to bile acids was studied in rats with biliary fistulae. When equal quantities of [26 or 2714C] cholesterol and [414C] cholesterol were administered, there was a significant linear relationship between 14CO2 expiration in the breath and [414C] bile acid excreted in the bile. Bile acid synthesis calculated as the ratio of 14CO2: molar specific activity of biliary cholesterol correlated highly with biliary bile acid excretion in the bile acid depleted rat. Phenobarbital, a known inducer of gamma-amino levulenic acid formation from succinyl CoA did not alter the relationship between the 14CO2 estimation of bile acid synthesis and biliary bile acid excretion, indicating that the relationship between [26 or 2714C] cholesterol side chain cleavage and 14CO2 formation was not altered. Phenobarbital, however, did cause a reduction in bile acid synthesis measured by 14CO2 evolution and by biliary bile acid excretion. The 14CO2 method underestimated bile acid excretion. 8.7% in untreated and phenobarbital treated rats respectively. Since 11% of the radioactivity which was expired as 14CO2 was isolated as bile acids, radioactivity cleaved as [1 or 314C] propionyl CoA may enter cholesterol-bile acid biosynthesis resulting in the underestimation of bile acid synthesis. To test whether radioactivity from propionyl CoA enters steroid biosynthesis [114C] propionate and [214C] propionate were given to untreated biliary fistula rats and the biliary lipids excreted in 60 hours were analyzed. Incorporation of radioactivity into cholesterol and bile acids was greater after the administration of [214C] propionate than after [114C] propionate than after [114C] propionate, suggesting that radioactivity from propionyl CoA may enter steroid biosynthesis by metabolic events in which the methylene and carboxyl carbon atoms are differentiated. Although the use of 14CO2 expiration from [26 or 2714C] cholesterol catabolism underestimates the rate of bile acid synthesis, it should have many applications because of the constant relationship between 14CO2 formation and cholesterol side chain cleavage.     

Examination of bile acid negative feedback regulation in rats

Recent data obtained using cultured rat hepatocytes showed that bile acids do not inhibit bile acid synthesis, whereas cholesterol concentrations vary in parallel with bile acid synthesis (Davis et al. (1983. J. Biol. Chem. 258: 4079-4082). This led us to re-evaluate in vivo experiments upon which the consensus that bile acid synthesis is primarily regulated by bile acid "negative feedback" is based. Infusion of taurocholate into either the jugular vein or duodenum of bile-diverted rats stimulated biliary cholesterol secretion and bile flow, but it did not inhibit bile acid synthesis. The lack of an inhibitory effect was evident using several different infusion rates of taurocholate. Even at the greatest rate of taurocholate infusion (25 mumol/(100 g.hr] there was no significant inhibition of bile acid synthesis. In contrast, infusing mevinolin (1 mg/hr), a potent competitive inhibitor of HMG-CoA reductase, almost completely inhibited bile acid synthesis and biliary cholesterol secretion. Since mevinolin did not affect bile flow, these results cannot be ascribed to bile secretory failure. Thus, while these studies suggest that taurocholate may not regulate bile acid synthesis directly via negative feedback, cholesterol is likely to act as a positive effector of bile acid synthesis.     

Evidence for a common biliary cholesterol and VLDL cholesterol precursor pool in rat liver.

Hepatic free cholesterol levels are influenced by cholesterol synthesis and ester formation, which, in turn, might regulate cholesterol secretion into bile and plasma. We manipulated the rates of hepatic cholesterol synthesis and esterification and measured biliary and very low density lipoprotein (VLDL) cholesterol secretion, and bile acid synthesis. Mevalonate decreased HMG CoA reductase by 80%, increased acyl coenzyme A: cholesterol acyltransferase (ACAT) by 60% and increased [3H]oleate incorporation into microsomal and VLDL cholesteryl esters by 174% and 122%, respectively. Microsomal and biliary free cholesterol remained constant at the expense of increased microsomal and VLDL cholesteryl ester content. Mevalonate did not change bile acid synthesis. 25-OH cholesterol decreased HMG-CoA reductase by 39%, increased ACAT by 24%, but did not effect 7 alpha-hydroxylase. 25-OH cholesterol increased [3H]oleate in microsomal and VLDL cholesterol esters by 71% and 120%. Biliary cholesterol decreased by 40% and VLDL cholesteryl esters increased by 83%. A small and unsustained decrease in bile acid synthesis (14CO2 release) occurred after 25-OH cholesterol. After orotic acid feeding, HMG-CoA reductase increased 352%, and [3H]oleate in microsomal and VLDL cholesteryl esters decreased by 43% and 89%. Orotic acid decreased all VLDL components including free cholesterol (68%) and cholesteryl esters (55%), and increased biliary cholesterol by 160%. No change in bile acid synthesis occurred. Hepatic cholesterol synthesis and esterification appear to regulate a cholesterol pool available for both biliary and VLDL secretion. Changing cholesterol synthesis and esterification did not alter bile acid synthesis, suggesting that either this common bile/VLDL secretory pool is functionally distinct from the cholesterol pool used for bile salt synthesis, or that free cholesterol availability in this precursor pool is not a major determinant of bile acid synthesis.     

Dietary restriction influence bile formation in aging rats

Food restriction is one of the most effective interventions which increases the survival of rodents and influences a variety of physiologic and pathologic processes. Thus, we examined whether life-long caloric restriction would influence bile formation, one of the important hepatic functions. Female Sprague Dawley rats were subjected soon after weaning to a restricted diet (60% of the diet consumed by the rats fed ad libitum) and bile formation determined at 3.5, 12 and 24 months of age. Rats had their bile ducts cannulated under nembutal anesthesia and bile collected at 10 min. intervals. Bile flow rate decreased 35% between 3.5 and 24 months of age. This decrease was associated with a reduction of the bile acid dependent fraction of bile flow (BADF) up to 12 months of age, thereafter the bile acid independent fraction (BAIF) also decreased. Phospholipid and cholesterol secretion rates increased with age, but did not correlate with bile acid secretion. In rats fed the restricted diet, bile flow was about 20% higher at 3-5 months of age when compared with the ad libitum fed group. This bile flow rate remained constant until 24 months of age. The increased bile flow was attributed to higher BADF and BAIF. The phospholipid and cholesterol secretion followed that of bile acids. It thus appears that dietary restriction exerts a beneficial effect on the age related decline in bile formation.     

Transport of cholesterol into mitochondria is rate-limiting for bile acid synthesis via the alternative pathway in primary rat hepatocytes

Bile acid synthesis occurs mainly via two pathways: the "classic" pathway, initiated by microsomal cholesterol 7alpha-hydroxylase (CYP7A1), and an "alternative" (acidic) pathway, initiated by sterol 27-hydroxylase (CYP27). CYP27 is located in the inner mitochondrial membrane, where cholesterol content is very low. We hypothesized that cholesterol transport into mitochondria may be rate-limiting for bile acid synthesis via the "alternative" pathway. Overexpression of the gene encoding steroidogenic acute regulatory (StAR) protein, a known mitochondrial cholesterol transport protein, led to a 5-fold increase in bile acid synthesis. An increase in StAR protein coincided with an increase in bile acid synthesis. CYP27 overexpression increased bile acid synthesis by <2-fold. The rates of bile acid synthesis following a combination of StAR plus CYP27 overexpression were similar to those obtained with StAR alone. TLC analysis of (14)C-labeled bile acids synthesized in cells overexpressing StAR showed a 5-fold increase in muricholic acid; in chloroform-extractable products, a dramatic increase was seen in bile acid biosynthesis intermediates (27- and 7,27-hydroxycholesterol). High-performance liquid chromatography analysis showed that 27-hydroxycholesterol accumulated in the mitochondria of StAR-overexpressing cells only. These findings suggest that cholesterol delivery to the inner mitochondrial membrane is the predominant rate-determining step for bile acid synthesis via the alternative pathway.     

Effects of ursodeoxycholic acid, analogues of ursodeoxycholic acid and combination of bile acids on bile acid synthesis in cultured rat hepatocytes

The effect of individual 7 beta-hydroxy bile acids (ursodeoxycholic and ursocholic acid), bile acid analogues of ursodeoxycholic acid, combination of bile acids (taurochenodeoxycholate and taurocholate), and mixtures of bile acids, phospholipids and cholesterol in proportions found in rat bile, on bile acids synthesis was studied in cultured rat hepatocytes. Individual steroids tested included ursodeoxycholate (UDCA), ursocholate (UCA), glycoursodeoxycholate (GUDCA) and tauroursodeoxycholate (TUDCA). Analogues of UDCA (7-methylursodeoxycholate, sarcosylursodeoxycholate and ursooxazoline) and allochenodeoxycholate, a representative of 5 alpha-cholanoic bile acid were also tested in order to determine the specificity of the bile acid biofeedback. Each individual steroid was added to the culture media at concentrations ranging from 10 to 200 microM. Mixtures of taurochenodeoxycholate (TDCA) and taurocholate in concentrations ranging from 150 to 600 microM alone and in combination with phosphatidylcholine (10-125 microM) and cholesterol (3-13 microM) were also tested for their effects on bile acid synthesis. Rates of bile acid synthesis were determined as the conversion of added lipoprotein [4-14C]cholesterol or [2-14C]mevalonate into 14C-labeled bile acids and by GLC quantitation of bile acids secreted into the culture media. Individual bile acids, bile acid analogues, combination of bile acids and mixture of bile acids with phosphatidylcholine and cholesterol failed to inhibit bile acid synthesis in cultured hepatocytes. The addition of UDCA or UCA to the culture medium resulted in a marked increase in the intracellular level of both bile acids, and in the case of UDCA there was a 4-fold increase in beta-muricholate. These results demonstrate effective uptake and metabolism of these bile acids by the rat hepatocytes. UDCA, UCA, TUDCA and GUDCA also failed to inhibit cholesterol-7 alpha-hydroxylase activity in microsomes prepared from cholestyramine-fed rats. The current data confirm and extend our previous observations that, under conditions employed, neither single bile acid nor a mixture of bile acids with or without phosphatidylcholine and cholesterol inhibits bile acid synthesis in primary rat hepatocyte cultures. We postulate that mechanisms other than a direct effect of bile acids on cholesterol-7 alpha-hydroxylase might play a role in the regulation of bile acid synthesis.     

The effect of chylomicron remnants on bile acid synthesis in cultured rat hepatocytes

The effect of chylomicron remnants on bile acid synthesis in isolated rat hepatocytes in monolayer cultures was investigated. Production of bile acids by the cells in the presence of chylomicron remnants at a cholesterol concentration of 7.8-9 nmol/ml was increased by approx. 75% after 17 h and 25% after 24 h incubation. Similar concentrations of cholesterol added to the cells in the form of chylomicrons had no significant effect on bile acid synthesis. These results suggest that cholesterol taken up in chylomicron remnants may be an important source of substrate for bile acid synthesis.     

Contraceptive steroids alter the steady-state kinetics of bile acids

Contraceptive steroids increase the ratio of cholic acid to chenodeoxycholic acid in bile. This alteration may contribute to the development of cholesterol gallstones. The objective of this study was to measure the effects of contraceptive steroids on bile acid kinetics and to relate them to changes in cholesterol metabolism. Steady-state kinetics of bile acids were measured in 15 healthy women, on and off contraceptive steroids. Cholic acid synthesis increased 30.3% (P less than 0.025) and its pool increased by 37.4% (P less than 0.025). Chenodeoxycholic acid synthesis decreased 6.4% (P = 0.08) and its pool decreased by 11.8% (P less than 0.05) during use of contraceptive steroids. The fractional turnover rates of both primary bile acids did not change. The changes in kinetics of the primary bile acids were related to alterations in biliary lipid and cholesterol metabolism, separately reported. (J. Lipid Res. 1987. 28: 828-839). During use of contraceptive steroids, total bile acid pool and total bile acid synthesis correlated directly with cholesterol synthesis, assayed in mononuclear leukocytes (r = 0.50 and r = 0.54, respectively) but not with the plasma clearance of chylomicron remnants, measured with retinyl palmitate. The data indicate that contraceptive steroids directly alter the hepatic synthesis of bile acids and suggest that newly synthesized cholesterol may be a preferred substrate for bile acid synthesis during use of contraceptive steroids.     

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.     

Study of HDL2-dependent synthesis of bile acids in culture of rabbit hepatocytes: effects of oxidized cholesterol derivatives

The effect of individual oxysterols--products of auto-oxidation of cholesterol on bile acid synthesis by cultivated rabbit hepatocytes was studied. Relative rates of bile acid synthesis were measured as the conversion of 4-14C cholesterol-HDL2 into total 4-14C labeled bile acids. 7 beta-hydroxycholesterol and 3,5-cholestane-7-dione strongly inhibited bile acid synthesis at concentrations 1-10 micrograms/ml. These data support the hypothesis that oxidized cholesterol derivatives accelerate the development of hypercholesterolemia in rabbits fed on cholesterol containing diet.