Effect of catecholamines on the hepatic rate-limiting enzymes of cholesterol metabolism in normally fed and cholesterol-fed rabbits

Adrenergic control of liver cholesterol metabolism was studied in the rabbit. The effects of noradrenaline (α₁, α₂, β₂ agonist) and isoprenaline (β₁, β₂ agonist) on 3-hydroxy-3-methylglutaryl coenzyme A reductase, acyl-coenzyme A: cholesterol-o-acyltransferase (cholesterol acyltransferase) and cholesterol 7α-hydroxylase, the rate-limiting enzymes of cholesterol biosynthesis and esterification and bile acid synthesis, respectively, were examined in the normally fed and cholesterol-fed male New Zealand White rabbit. Isoprenaline increased the activities of hydroxymethylglutaryl CoA reductase and cholesterol acyltransferase approx. 12-fold and 5-fold, respectively, in normally fed rabbits. Noradrenaline, by contrast, produced an effect only on hydroxymethylglutaryl CoA reductase, the activity of which was increased 3-fold in these animals. Neither catecholamine had an effect on hydroxymethylglutaryl CoA reductase in the cholesterol-fed rabbit. Isoprenaline decreased the activity of cholesterol acyltransferase by approx. 40% and increased the activity of cholesterol 7α-hydroxylase 2-fold in the cholesterol-fed rabbit compared to cholesterol-fed controls. Noradrenaline had no effect on either cholesterol acyltransferase or cholesterol 7α-hydroxylase in either the normally fed or the cholesterol-fed rabbit. We suggest that β₂-adrenergic stimulation by isoprenaline in the normally fed rabbit may enhance cholesterol synthesis and storage, but that in the cholesterol-fed rabbit, it facilitates the elimination of cholesterol from the body by increasing the rate of bile acid synthesis.     

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.     

Cholesterol 7α-hydroxylase activity and bile acid synthesis in hepatocytes of unwearied and weaned pigs in monolayer culture

Activity of cholesterol 7α-hydroxylase (EC 1.14.13.17) in freshly isolated hepatocytes from unweaned piglets (2 to 3 weeks old) was 16-times lower as compared to hepatocytes from weaned piglets (7 to 8 weeks old). The monolayer culture activity of the enzyme remained low in unweaned piglet hepatocytes. In contrast, in cultured hepatocytes from weaned piglets, cholesterol 7α-hydroxylase activity declined during the first day of culture, but was restored during the next 2 culture days, provided that fetal bovine serum (10%) was added to the culture medium. Addition of dexamethasone (50 nM) and insulin (135 nM) to the medium, further enhanced cholesterol 7α-hydroxyease activity to values similar to those in freshly isolated hepatocytes and retarded the decline of enzyme activity after the 3rd culture day. Cultured hepatocytes from weaned and unweaned piglets synthesized similar types of bile acids from [¹⁴C]cholesterol. among which hyocholic acid (the most prominent), hyodeoxycholic acid, chenodeoxycholic acid, murocholic acid and lithocholic acid could be identified. 95% of radiolabelled bile acids synthesized was conjugated, mainly with glycine, but also with taurine, sulfate and glucuronic acid. The rate of mass production of bile acids by cultured hepatocytes of weaned piglets (as measured by gas-chromatography) parallelled cholesterol 7α-hydroxylase activity, and was low in the absence of serum, but increased in medium containing fetal bovine serum, dexamethasone and insulin to a rate lying in the range of 75% of the in vivo bile acid production during the 3rd culture day. Bile acid production by unweaned piglet hepatocytes was 3-times lower under these conditions. It is concluded that hepatocytes from young weaned pigs cultured in medium containing 10% fetal bovine serum, offer a suitable in vitro model for the study of bile acid synthesis, in view of the high cholesterol 7α-hydroxylase activities and bile acid production rates.     

Cerebrotendinous xanthomatosis: An inborn error in bile acid synthesis with defined mutations but still a challenge

Cerebrotendinous xanthomatosis [CTX] is a rare disease characterized by the accumulation of cholesterol and cholestanol in brain and tendons caused by a mutation in the sterol 27-hydroxylase gene [CYP27A1] involved in bile acid synthesis. Disruption of this gene in mice does not give rise to xanthomas. The gene defect leads to reduced bile acid synthesis with a compensatory increase in the activity of the rate-limiting enzyme in bile acid synthesis, cholesterol 7α-hydroxylase. This leads to a marked accumulation of 7α-hydroxylated bile acid precursors, in particular 7α-hydroxy-4-cholesten-3-one. The latter oxysterol passes the blood-brain barrier and is an efficient precursor to cholestanol. The activity of cholesterol 7α-hydroxylase is normalized by treatment with bile acids. Such treatment reduces the xanthomas in CTX patients in parallel with decreased cholestanol levels. The relationship between the accumulation of cholestanol and the development of cholesterol-rich xanthomas has however not been clarified and a suitable animal model is still lacking.     

Activated polyamine catabolism leads to low cholesterol levels by enhancing bile acid synthesis

Transgenic mice with activated polyamine catabolism due to overexpression of spermidine/spermine N¹-acetyltransferase (SSAT) have significantly reduced plasma total cholesterol levels. In our study, we show that low cholesterol levels were attributable to enhanced bile acid synthesis in combination with reduced cholesterol absorption. Hepatic cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme catalyzing the conversion of cholesterol to bile acids, plays an important role in the removal of excess cholesterol from the body. We suggest that by reducing activity of Akt activated polyamine catabolism increased the stability and activity of peroxisome proliferator-activated receptor γ co-activator 1α, the critical activator of CYP7A1. This is supported by our finding that the treatment with SSAT activator, N ¹,N ¹¹-diethylnorspermine, reduced significantly the amount of phosphorylated (active) Akt in HepG2 cells. In summary, activated-polyamine catabolism is a novel mechanism to regulate bile acid synthesis. Therefore, polyamine catabolism could be a potential therapeutic target to control hepatic CYP7A1 expression.     

Regulatory effects of sterols and bile acids on hepatic 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol 7alpha-hydroxylase in the rat

Specific activities of the hepatic microsomal enzymes 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase and cholesterol 7alpha-hydroxylase were studied in rats fed sterols and bile acids. The administration of bile acids (taurocholate, taurodeoxycholate, taurochenodeoxycholate) at a level of 1% of the diet for 1 wk reduced the activity of HMG CoA reductase. Taurocholate and taurodeoxycholate, but not taurochenodeoxycholate, inhibited cholesterol 7alpha-hydroxylase. Dietary sitosterol produced increases in the specific activity of HMG CoA reductase (3.6-fold) and cholesterol 7alpha-hydroxylase (1.4-fold), and biliary cholesterol concentrations in this group more than doubled. Compared with controls fed the stock diet, the simultaneous administration of sitosterol and taurochenodeoxycholate resulted in a 60% decrease of HMG CoA reductase activity and no change in cholesterol 7alpha-hydroxylase activity or biliary cholesterol concentration. Rats fed sitosterol plus taurocholate had nearly normal HMG CoA reductase activity, but cholesterol 7alpha-hydroxylase was inhibited and biliary cholesterol remained high. Bile acid secretion rates and biliary bile acid composition were similar in controls and sterol-fed animals. In all groups receiving bile acids, biliary secretion of bile acids was nearly doubled and bile acid composition was shifted in the direction of the administered bile acid. It is concluded that the composition of the bile acid pool influences the hepatic concentrations of the rate-controlling enzymes of bile acid synthesis.     

Hepatic cholesterol metabolism in normo- and hyperlipidemic patients with cholesterol gallstones.

In vivo studies have shown abnormalities in cholesterol and bile acid metabolism in primary hyperlipoproteinemia (HLP). The aim of the present investigation was to determine if the increased production of cholesterol in HLP type IV can be attributed to a correspondingly high level of the hepatic 3-hydroxy-3-methylglutaryl (HMG) CoA reductase activity and if the low cholic acid: chenodeoxycholic acid synthesis ratio in HLP type II is due to some hydroxylase deficiency. Liver biopsies from 26 normolipidemic and 25 hyperlipidemic (10 type IIa, 6 type IIb, and 9 type IV) patients undergoing elective cholecystectomy were assayed for HMG CoA reductase activity, 12 alpha-hydroxylase activity, and 25-hydroxylase activity. The HMG CoA reductase activity was normal in HLP type IIa and type IIb and was increased about twice HLP type IV (P less than 0.001). The 12 alpha- and 25-hydroxylase activities were normal in all groups of patients. The results are compatible with a normal cholesterol synthesis in the liver in HLP type II. A reduced 12 alpha- or 25-hydroxylase activity cannot explain the low production of cholic acid relative to chenodeoxycholic acid in this type of HLP. The elevated HMG CoA reductase activity found in the liver of type IV patients may, however, be part of the explanation for the elevated synthesis of cholesterol often seen in these patients.     

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the “acidic” and “neutral” pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 μM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 μM), dibutyryl-cAMP (50 μM), nor squalestatin 1 (150 nM-1.0 μM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 μM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-β-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [¹⁴C]cholesterol remained relatively high and consisted of β-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.     

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.     

缺乏抗坏血酸对豚鼠胆固醇代谢的影响

边缘性缺乏抗坏血酸之豚鼠,于三周内其肝脏及小肠粘膜3-羟-3-甲基戊二酰辅酶A还原酶(HMGR)活力均下降到原有水平的50％,但肝脏胆固醇7α-羟化酶活力尚无显著性改变。坏血病豚鼠(三周内)上述几种酶活力都下降至原有水平的50％左右。豚鼠摄取抗坏血酸不足,其血清总胆固醇浓度显著增加,而血清高密度脂蛋自胆固醇浓度显著减少,其改变程度与抗坏血酸缺乏状况一致。     

The suppressive effect of the catatoxic steroid,pregnenolone-16α-carbonitrile,on liver microsomal cholesterol-7α-hydroxylase

The effect of the catatoxic steroid, 3β-hydroxy-20-oxo-5-pregnene-16α-carbonitrile [pregnenolone-16α-carbonitrile (PCN)] on hepatic microsomal cholesterol-7α-hydroxylase, the probable rate-limiting enzyme of bile acid biosynthesis, has been studied. Short term administration (3 days) of PCN in the diet of rats resulted in a significant decrease in the liver microsomal cholesterol-7α-hydroxylase activity, in contrast to a marked stimulation of microsomal cytochrome P-450 and ethylmorphine demethylase activity. PCN significantly depressed the cholesterol-7α-hydroxylase activity in the livers of rats with elevated levels of the enzyme produced by cholestyramine feeding. The results indicate the presence of separate control mechanisms in the regulation of bile acid synthesis and drug metabolism.     

Comparative studies on lipogenesis and cholesterogenesis in lipemic BHE rats and normal Wistar rats.

The BHE strain of rat is characterized by early hyperinsulinemia and maturity onset hyperlipemia and hyperglycemia. Since we have previously shown that insulin is required for the coordinate regulation of a number of lipogenic enzymes in rat liver, a comparative study of the hepatic activities of the rate-limiting enzymes of lipid synthesis and the in vivo rates of fatty acid and cholesterol synthesis in the liver and the adipose tissue has been conducted in BHE and Wistar rats. In the liver, BHE rats had 25–28% higher acetyl-CoA carboxylase and fatty acid synthetase activities as measured in vitro but a 100% greater rate of fatty acid synthesis in vivo as compared to Wistar animals. These results strongly suggest that factors other than the amount of acetyl-CoA carboxylase, such as allosteric effectors, must be operating in vivo, thereby facilitating the carboxylase to function at its maximal capacity in BHE rats. Such a regulation of fatty acid biosynthesis by allosteric modifiers of acetyl-CoA carboxylase is already known, although the mechanism of this regulation is not fully understood. BHE rats also exhibited a twofold greater rate of fatty acid synthesis in the adipose tissue compared to the Wistar rats. Thus, increased lipogenic capacity and increased lipogenesis in BHE rats are consistent with early hyperinsulinemia in this strain. Furthermore, BHE rats had 71% more 3-hydroxy-3-methylglutaryl CoA reductase activity with a 97% greater rate of cholesterol synthesis as compared to Wistar rats. In contrast, cholesterol 7α-hydroxylase activity was only 20% greater in BHE rats compared to Wistar rats, suggesting that the BHE rat does not have the capacity to degrade cholesterol to bile acids at a rate commensurate with the increased rate of cholesterol synthesis. This difference in synthesis versus degradation might account for the hypercholesterolemia which occurs in BHE rats, but not in Wistar rats.     

EP4 emerges as a novel regulator of bile acid synthesis and its activation protects against hypercholesterolemia

Prostaglandin E receptor subtype 4 (EP4) knockout mice develops spontaneous hypercholesterolemia but the detailed mechanisms by which EP4 affects cholesterol homeostasis remains unexplored. We sought to determine the cause of hypercholesterolemia in EP4 knockout mice, focusing on the role of EP4 in regulating the synthesis and elimination of cholesterol. Deficiency of EP4 significantly decreased total bile acid levels in the liver by 26.2% and the fecal bile acid content by 27.6% as compared to wild type littermates, indicating that the absence of EP4 decreased hepatic bile acid synthesis and their subsequent excretion in stools. EP4 deficiency negatively regulate bile acid synthesis through repression of phosphorylated extracellular signal-regulated kinase 1/2 (ERK)-mediated cholesterol 7α-hydroxylase (CYP7A1) expression and that the hypercholesterolemia in EP4 knockout mice is due to a defect in cholesterol conversion into bile acids. Deficiency of EP4 also increased de novo cholesterol synthesis and altered cholesterol fluxes in and out of the liver. Treating high fat diet-challenged mice with the pharmacological EP4 agonist, CAY10580 (200?μg/kg body weight/day i.p) for three weeks effectively prevented diet-induced hypercholesterolemia, enhanced endogenous bile acid synthesis and their fecal excretion. In summary, EP4 plays a critical role in maintaining cholesterol homeostasis by regulating the synthesis and elimination of bile acids. Activation of EP4 serves as an effective novel strategy to promote cholesterol disposal in the forms of bile acids in order to lower plasma cholesterol levels.     

Assay of microsomal oxysterol 7α-hydroxylase activity in the hamster liver by a sensitive method: in vitro modulation by oxysterols

A method of assaying hepatic cytochrome P-450, oxysterol 7α-hydroxylase (CYP7B), was developed by combining the use of 25-[26,27-³H]hydroxycholesterol as a substrate and hydroxypropyl-β-cyclodextrin as a substrate vehicle. When these assay conditions were tested, an undesirable transformation was observed of the reaction product, 7α,25-dihydroxycholesterol, into 3-oxo-7α,25-dihydroxy-4-cholesten by the activity of 3β-hydroxy-Δ⁵-C₂₇ steroid oxydoreductase, a microsomal NAD⁺ and NADP⁺ dependent enzyme of bile acid metabolism. A great improvement was reached by using a continuous NADPH generating system which constantly re-transforms NADP⁺ into NADPH, thus inhibiting this activity. This improved CYP7B assay, comparable to our previously described assay for cholesterol 7α-hydroxylase (CYP7A), allowed a 3-fold increase of the apparent enzyme activity. The possibility to simultaneously measure CYP7A and CYP7B activities on the same microsomal preparation was investigated. A marked decrease (?33%) in the CYP7B activity was noticed, while that of CYP7A remained unchanged. The CYP7B activity was observed to be inhibited by cholesterol (?30%) and also by the oxysterols 7α-hydroxycholesterol (?21%), 7β-hydroxycholesterol (?25%) and epicoprostanol (?20%), and by cyclosporin A (?26%). It can be concluded that this sensible and easy to perform CYP7B assay allows to observe, at least in vitro, a modulation of the enzyme activity by oxysterols.     

Exogenously Hypercholesterolemic Rats, Compared with Their Progenitor Sprague-Dawley Rats, Have Altered mRNAs for Cholesterol 7α-Hydroxylase and Low-Density-Lipoprotein Receptor and Activities of Cholesterol 7α-Hydroxylase and Acyl-CoA:Cholesterol Acyltransferase in the Liver in Response to Dietary Cholesterol

Exogenously hypercholesterolemic (ExHC) rats promptly increase serum cholesterol concentration in response to dietary cholesterol. To examine underlying mechanism(s) for this susceptibility, responses of mRNAs for cholesterol metabolism-related proteins and their activities in the liver to dietary cholesterol were compared between ExHC rats and their progenitor Sprague-Dawley rats. ExHC rats slightly decreased the abundance of low-density-lipoprotein (LDL) receptor mRNA in response to dietary cholesterol, although the amount of LDL receptor was not influenced. The abundance of cholesterol 7α-hydroxylase mRNA and the enzyme activity in response to dietary cholesterol were greater in ExHC rats, but the fecal excretion of bile acid was comparable between the strain. Dietary cholesterol-dependent elevation of acyl-CoA:cholesterol acyltransferase activity was greater in ExHC rats. The concentration of liver triacylglycerols was markedly lower in ExHC rats. These results suggest that ExHC rats may increase serum cholesterol by increasing hepatic secretion of cholesteryl ester-rich particles.     

Cholecystectomy prevents expansion of the bile acid pool and inhibition of cholesterol 7alpha-hydroxylase in rabbits fed cholesterol

To study the effect of cholecystectomy on the regulation of classic and alternative bile acid syntheses, gallbladder-intact (n = 20) and cholecystectomized (n = 20) New Zealand White rabbits were fed either chow or chow with 2% cholesterol (3 g/day). After 10 days, bile fistulas were constructed in half of each rabbit group to recover and measure the bile acid pool and biliary bile acid flux. After cholesterol feeding, the bile acid pool size increased from 268 +/- 55 to 444 +/- 77 mg (P < 0.01) with a 2-fold rise in the biliary bile acid flux in intact rabbits but did not expand the bile acid pool (270 +/- 77 vs. 276 +/- 62 mg), nor did the biliary bile acid flux increase in cholecystectomized rabbits. Ileal apical sodium-dependent bile acid transporter protein increased 46% from 93 +/- 6 to 136 +/- 23 units/mg (P < 0.01) in the intact rabbits but did not change in cholecystectomized rabbits (104 +/- 14 vs. 99 +/- 19 units/mg) after cholesterol feeding. Cholesterol 7alpha-hydroxylase activity was inhibited 59% (P < 0.001) while cholesterol 27-hydroxylase activity rose 83% (P < 0.05) after cholesterol feeding in the intact rabbits but neither enzyme activity changed significantly in cholesterol-fed cholecystectomized rabbits. Fecal bile acid outputs reflecting bile acid synthesis increased significantly in the intact but not in the cholecystectomized rabbits fed cholesterol.Removal of the gallbladder prevented expansion of the bile acid pool after cholesterol feeding as seen in intact rabbits because ileal bile acid transport did not increase. As a result, cholesterol 7alpha-hydroxylase was not inhibited.     

Alteration of biliary ursodeoxycholic acid in guinea pig during early stages of cholestyramine feeding

The effects of cholestyramine feeding on biliary ursodeoxycholic acid, fecal excretion of bile acids and neutral sterols on cholesterol 7α-hydroxylase and hepatic HMG-CoA reductase were examined in the guinea pig. In the bile there was a 57% decrease in the concentration of ursodeoxycholic acid while an increase was observed in the concentration of chenodeoxycholic acid. Cholestyramine feeding for ten days resulted in a decrease in plasma cholesterol levels and an increase in both hepatic HMG-CoA reductase and cholesterol 7α-hydroxylase activities. The fecal excretion of both bile acids and neutral sterols was significantly increased.     

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.