Loss of nuclear receptor SHP impairs but does not eliminate negative feedback regulation of bile acid synthesis

The in vivo role of the nuclear receptor SHP in feedback regulation of bile acid synthesis was examined. Loss of SHP in mice caused abnormal accumulation and increased synthesis of bile acids due to derepression of rate-limiting CYP7A1 and CYP8B1 hydroxylase enzymes in the biosynthetic pathway. Dietary bile acids induced liver damage and restored feedback regulation. A synthetic agonist of the nuclear receptor FXR was not hepatotoxic and had no regulatory effects. Reduction of the bile acid pool with cholestyramine enhanced CYP7A1 and CYP8B1 expression. We conclude that input from three negative regulatory pathways controls bile acid synthesis. One is mediated by SHP, and two are SHP independent and invoked by liver damage and changes in bile acid pool size.

Suppression of cholesterogenesis and reduction of LDL cholesterol by dietary ginseng and its fractions in chicken liver

The effects of ginseng root powder and of serially extracted solvent fractions of ginseng on avian hepatic cholesterol metabolism and lipogenesis and on avian serum lipoprotein cholesterol levels were examined. In one study, White Leghorn females were fed for 4 weeks a corn-based diet (control) or an experimental diet in which was incorporated 0.25% Wisconsin ginseng or an equivalent quantity of a serial solvent fraction [petroleum ether (PESF), methyl alcohol (MESF), water (WASF)] or of the residue. beta-hydroxy-beta-methylglutaryl-CoA (HMG-CoA) reductase activity was significantly lower (P less than 0.01) in each of the treatment groups (31-37% of control activity) except that fed the extracted residue (90% of control, N.S.). Cholesterol 7 alpha-hydroxylase activity was lowered in parallel (45-64% of control, P less than 0.01) by all treatments except the residue (100% of control). Also with the exception of the residue treatment, each ginseng treatment effected a lowering of the serum total cholesterol level (67-83% of control, P less than 0.01) and of serum low density lipoprotein cholesterol level (53-81% of control, P less than 0.01). Lipogenic activities and serum triglycerides levels were lowered (P less than 0.01) by two of the ginseng treatments. The PESF treatment was the most effective suppressor of each parameter, 74% and 68% respectively, of the control values. The WASF also had significant impact. Not one of the experimental diets influenced the serum high density lipoprotein level. The PESF, the potent source of suppressors, effected a change in the ratio of low to high density lipoprotein cholesterol from 1.46 (control) to 0.88. The levels of cholesterol and triglycerides in liver under these conditions showed a similar pattern as that of serum. In companion studies, broiler females were fed 0.28% Chinese red ginseng root powder or its various fractions. The results confirmed those recorded above. The factor(s) responsible for lowering the serum total and low density lipoprotein cholesterol levels were generally more concentrated in the PESF and WASF of ginseng and each was significantly more effective than was ginseng root powder. Ginsenosides (saponins) are considered to be the active agents for the suppression of cholesterogenesis and lipogenesis.

Candidate genes involved in cardiovascular risk factors by a family-based association study on the island of Kosrae, Federated States of Micronesia

Altered plasma levels of lipids and lipoproteins, obesity, hypertension, and diabetes are major risk factors for atherosclerotic cardiovascular disease. To identify genes that affect these traits and disorders, we looked for association between markers in candidate genes (apolipoprotein AII (apo AII), apolipoprotein AI-CIII-AIV gene cluster (apo AI-CIII-AIV), apolipoprotein E (apo E), cholesteryl ester transfer protein (CETP), cholesterol 7alpha-hydroxylase (CYP7a), hepatic lipase (HL), and microsomal triglyceride transfer protein (MTP)) and known risk factors (triglycerides (Tg), total cholesterol (TC), apolipoprotein AI (apo AI), apolipoprotein AII (apo AII), apolipoprotein B (apo B), body mass index (BMI), blood pressure (BP), leptin, and fasting blood sugar (FBS) levels.) A total of 1,102 individuals from the Pacific island of Kosrae were genotyped for the following markers: Apo AII/MspI, Apo CIII/SstI, Apo AI/XmnI, Apo E/HhaI, CETP/TaqIB, CYP7a/BsaI, HL/DraI, and MTP/HhpI. After testing for population stratification, family-based association analysis was carried out. Novel associations found were: 1) the apo AII/MspI with apo AI and BP levels, 2) the CYP7a/BsaI with apo AI and BMI levels. We also confirmed the following associations: 1) the apo AII/MspI with Tg level; 2) the apo CIII/SstI with Tg, TC, and apo B levels; 3) the Apo E/HhaI E2, E3, and E4 alleles with TC, apo AI, and apo B levels; and 4) the CETP/TaqIB with apo AI level. We further confirmed the connection between the apo AII gene and Tg level by a nonparametric linkage analysis. We therefore conclude that many of these candidate genes may play a significant role in susceptibility to heart disease.

Gene expression of hepatic cholesterol 7 alpha-hydroxylase in the course of puromycin-induced nephrosis

Cholesterol conversion to and biliary excretion of bile acids represents the principal pathway of cholesterol catabolism in mammals. Cholesterol 7 alpha-hydroxylase (Ch-7 alpha-H) is the first and the rate limiting step in bile acid production. Recently, Ch-7 alpha-H enzymatic activity has been shown to be normal in rats with established puromycin aminonucleoside-induced nephrosis (NS). To our knowledge, the gene expression of Ch-7 alpha-H in NS has not been investigated. We measured hepatic Ch-7 alpha-H mRNA and protein (by Northern and Western blot analyses) in rats at baseline and longitudinally during the course of induction and chronic phase of puromycin (PAN) induced NS. Groups of placebo-treated (controls) and diet-induced hypercholesterolemic (DHC) rats were included for comparison. The NS and DHC animals exhibited severe hypercholesterolemia of similar magnitude. Hepatic Ch-7 alpha-H transcript and protein remained virtually unchanged throughout the study period in the NS group. In contrast, Ch-7 alpha-H gene expression was markedly up-regulated in the DHC group. These observations suggest that hepatic Ch-7 alpha-H gene expression may be inappropriately low for the degree of the associated hypercholesterolemia in the NS group. It should be noted, however, that hepatic tissue cholesterol concentration was normal in the NS group and greatly increased in the DHC group. This can account for the disparity in Ch-7 alpha-H mRNA levels between the two groups since intracellular rather than extracellular cholesterol modulates Ch-7 alpha-H gene expression. In conclusion, the present study revealed that hepatic Ch-7 alpha-H gene expression remains unchanged during the course of PAN-induced NS in rats. It thus appears that generation and maintenance of hypercholesterolemia in this model of NS does not involve significant alteration of Ch-7 alpha-H gene expression.

HMG-CoA reductase inhibition reverses LCAT and LDL receptor deficiencies and improves HDL in rats with chronic renal failure

Dyslipidemia is a prominent feature of chronic renal failure (CRF) and a major risk factor for atherosclerosis and the progression of renal disease. CRF-induced dyslipidemia is marked by hypertriglyceridemia and a shift in plasma cholesterol from HDL to the ApoB-containing lipoproteins. Several studies have demonstrated a favorable response to administration of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors (statins) in CRF. This study was intended to explore the effect of statin therapy on key enzymes and receptors involved in cholesterol metabolism. Accordingly, CRF ( nephrectomized) and sham-operated rats were randomized to untreated and statin-treated (rosuvastatin 20 mg·kg−1·day−1) groups and observed for 6 wk. The untreated CRF rats exhibited increased total cholesterol-to-HDL cholesterol ratio, diminished plasma lecithin:cholesterol acyltransferase (LCAT) and the hepatic LDL receptor, elevated hepatic acyl-CoA:cholesterol acyltransferase (ACAT), and no change in hepatic HMG-CoA reductase, cholesterol 7α-hydroxylase, or HDL receptor (SRB-1). Statin administration lowered HMG-CoA reductase activity, normalized plasma LCAT, total cholesterol-to-HDL cholesterol ratio, and hepatic LDL receptor but did not significantly change either plasma total cholesterol, hepatic cholesterol 7α-hydroxylase, total ACAT activity, or SRB-1 in the CRF animals. Statin administration to the normal control rats led to significant increases in plasma LCAT and hepatic LDL receptor, significant reductions of total cholesterol-to-HDL cholesterol ratio, hepatic HMG-CoA reductase activity, and cholesterol 7α-hydroxylase abundance with virtually no change in plasma cholesterol concentration. Thus administration of rosuvastatin reversed LCAT and LDL receptor deficiencies and promoted a shift in plasma cholesterol from ApoB-containing lipoproteins to HDL in CRF rats.

Contribution of cholesterol 7alpha-hydroxylase to the regulation of lipoprotein metabolism

Clinical studies have clearly established a relationship between bile acid synthesis and plasma LDL-cholesterol concentrations. Interruption of the enterohepatic circulation of bile acids leads to increased bile acid synthesis and a reduction in plasma LDL-cholesterol concentrations. New studies indicate that genetic variation in cholesterol 7alpha-hydroxylase activity accounts for a significant fraction of the inter-individual variation in plasma LDL-cholesterol concentrations in the general population, and a specific CYP7A1 allele associated with increased plasma LDL-cholesterol concentrations has been identified. Studies in which cholesterol 7alpha-hydroxylase was transiently overexpressed in hamsters and mice indicate that direct manipulation of cholesterol 7alpha-hydroxylase leads to changes in plasma LDL-cholesterol concentrations. Interestingly, targeted inactivation of the gene encoding cholesterol 7alpha-hydroxylase does not lead to increased plasma LDL-cholesterol concentrations in mice.

Circulating markers for biosynthesis of cholesterol and bile acids are not depressed in asymptomatic gallstone subjects

BACKGROUND/AIMS

Cholesterol gallstone disease is often associated with an increased biliary secretion rate of cholesterol, which may be due to abnormalities in hepatic cholesterol metabolism. The aim of the present study was to investigate whether gallstone subjects may have an underlying defect in hepatic cholesterol and bile acid formation.

METHODS

In 41 asymptomatic gallstone subjects, randomly selected from a population of both sexes 40 and 60 years of age, and in 72 age- and sex-matched controls, plasma levels of lathosterol (reflecting hepatic HMG CoA reductase activity) and 7alpha-hydroxy-4-cholesten-3-one (reflecting cholesterol 7alpha-hydroxylase activity) were analysed. In a subgroup of gallstone subjects and controls, plasma levels of 27-hydroxy cholesterol were also determined.

RESULTS

The gallstone subjects had normal plasma levels of cholesterol but displayed 20-25% higher plasma levels of triglycerides compared with the controls. The plasma level of lathosterol was not significantly different between the two groups of subjects whereas the plasma level of 7alpha-hydroxy-4-cholesten-3-one was about 40% higher in the gallstone subjects compared with the controls. Positive correlations were obtained between plasma levels of 7alpha-hydroxy-4-cholesten-3-one and triglycerides in both groups of subjects. The plasma level of 27-hydroxy cholesterol was similar in gallstone subjects and controls.

CONCLUSIONS

The previously reported hypersecretion of cholesterol in gallstone patients is not due to a single metabolic defect leading to increased hepatic synthesis of cholesterol or decreased catabolism of cholesterol to bile acids via 7alpha-hydroxylation or 27-hydroxylation of cholesterol.

Genetic polymorphism of cholesterol 7alpha-hydroxylase (CYP7A1) and colorectal adenomas: Self Defense Forces Health Study

Bile acids have long been implicated in colorectal carcinogenesis, but epidemiological evidence is limited. Cholesterol 7alpha-hydroxylase (CYP7A1) is the rate-limiting enzyme producing bile acids from cholesterol. A recent case-control study showed a decreased risk of proximal colon cancer associated with the CC genotype of the CYP7A1 A-203C polymorphism. The present study examined the relationship between the CYP7A1 A-203C polymorphism and colorectal adenoma, which is a well-established precursor lesion of colorectal cancer. The study subjects comprised 446 cases of colorectal adenomas and 914 controls of normal total colonoscopy among men receiving a preretirement health examination at two hospitals of the Self Defense Forces (SDF). The CYP7A1 genotype was determined by the polymerase chain reaction-restriction fragment length polymorphism method. Statistical adjustment was made for age, hospital, rank in the SDF, smoking, alcohol use, body mass index, physical activity and parental history of colorectal cancer. The CYP7A1 polymorphism was not measurably related to the overall risk of colorectal adenomas. However, the CC genotype was associated with a decreased risk of proximal colon adenomas, but not of distal colon and rectal adenomas. Adjusted odds ratios of proximal colon adenomas (95% confidence intervals) for the AC and CC genotype versus AA genotype were 0.82 (0.54-1.24) and 0.56 (0.34-0.95), respectively. The findings add to evidence for the role of bile acids in colorectal carcinogenesis. The CC genotype of the CYP7A1 A-203C polymorphism probably renders lower activity of the enzyme synthesizing bile acids.

Bile acid synthesis and biliary hydrophobicity during obstructive jaundice in rats

Hepatic cholesterol 7alpha-hydroxylase, the rate-limiting enzyme for bile acid synthesis, is regulated by bile acid returning to the liver. In the bile duct-ligated rats, however, cholesterol 7alpha-hydroxylase activity is known to increase in spite of the elevated serum bile acids. The aim of this study was to investigate the relationship between the bile acid synthesis and biliary hydrophobicity to solve the paradoxical phenomenon. Male Wister rats (250-350 g) were divided into two groups, bile duct-ligated group and sham-operated group. Rats were sacrificed on the Days 1, 2, 4, 7, and 14 after the operation. Cholesterol 7alpha-hydroxylase activity, biliary bile acid composition, and biliary hydrophobicity were analyzed. Hepatic and serum total bile acid concentrations and serum 7alpha-hydroxycholesterol levels were also determined. Bile duct ligation caused significant increases in hepatic and serum bile acid concentrations on Day 1, which persisted for 14 days after the bile duct ligation. Activities of hepatic cholesterol 7alpha-hydroxylase increased to 3.2-fold of the preoperative value on Day 2, remained significantly high until Day 7, and then decreased to the basal value on Day 14. The serum 7alpha-hydroxycholesterol level essentially behaved in a similar fashion to that of hepatic cholesterol 7alpha-hydroxylase activity with a significant (P < 0.01) positive correlation. beta-Muricholic acid was predominant in bile until Day 7 (71 vs 10% in the controls on Day 4; P < 0.05) with a concomitant decrease in the proportion of cholic acid. Biliary bile acid became less hydrophobic and hepatic cholesterol 7alpha-hydroxylase activity significantly correlated with the hydrophobicity of biliary bile acids (n = 55, r = 0.54, P < 0.01). There were no significant correlations between the activity of cholesterol 7alpha-hydroxylase and total bile acid concentrations in serum, liver, or bile. The increased cholesterol 7alpha-hydroxylase activity is accompanied by the decreased biliary hydrophobicity, which may be a rationale for the paradoxical increase in bile acid synthesis in spite of the accumulation of bile acids in the serum and liver during obstructive jaundice in rats.

Cholesterol 7 alpha-hydroxylase activity in hypothyroidism and hyperthyroidism in humans

Alterations of serum cholesterol levels are well recognized findings in hypothyroidism and hyperthyroidism. It remains unclear, whether thyroid hormones may affect serum concentrations of cholesterol through changes in the activity of cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in the catabolic conversion of cholesterol to bile acids. We determined serum concentrations of the bile acid precursor 7 alpha-hydroxy-4-cholesten-3-one, which reflects cholesterol 7 alpha-hydroxylase activity in the liver, in 19 patients with hypothyroidism and in 10 patients with hyperthyroidism before and after treatment, respectively. In patients with hypothyroidism, serum concentrations of cholesterol and LDL-cholesterol decreased by 33% (p < 0.0005) and 39% (p < 0.0005), respectively, after replacement therapy with thyroid hormones. In contrast, serum concentrations of 7 alpha-hydroxy-4-cholesten-3-one (21.7 +/- 15.8 ng/ml vs 24.5 +/- 18.1 ng/ml before treatment, n.s.) as well as serum HDL-cholesterol were unchanged during substitution therapy. In patients with hyperthyroidism, serum concentrations of cholesterol and LDL-cholesterol increased by 27% (p < 0.01) and 39% (p < 0.01) after antithyroid treatment, respectively. Again, serum concentrations of 7 alpha-hydroxy-4-cholesten-3-one did not change significantly during treatment (15.8 +/- 12.6 ng/ml vs 14.7 +/- 8.1 ng/ml before treatment, n.s.). These findings indicate that in humans, thyroid hormones influence serum lipid concentrations by other mechanisms than by affecting the activity of cholesterol 7 alpha-hydroxylase.

Does brain specific 24S-hydroxycholesterol in plasma indicate the disruption of the blood-brain barrier in patients with ischemic stroke?

Determination of plasma 24S-hydroxycholesterol, which is produced almost exclusively in the brain and is released only in small amounts into circulation under physiological conditions, might be a marker to monitor non-invasively the time course and the extent of the disintegration of the blood-brain barrier after cerebral ischemia. We investigated the plasma concentrations for 4 to 10 days of 24S-hydroxycholesterol, and compared the concentration with the liver-specific oxysterol 7alpha-hydroxycholesterol, the ubiquitously produced 27-hydroxycholesterol, and cholesterol itself in six patients who were admitted to the hospital within 24 h after symptoms of stroke. Quantification of oxysterols was performed by isotope dilution mass spectrometry and cholesterol by gas-liquid chromatography. Initial concentrations of cholesterol, 24S-, 7alpha-, and 27-hydroxycholesterol in stroke patients were not different from data of healthy controls given in the literature. During the following days, no changes could be observed in the concentrations of cholesterol nor in the other oxysterols. Particularly the brain specific 24S-hydroxycholesterol was very constant and showed only minimal changes. Furthermore, comparison of patients with extended or small lesions did not reveal any differences in the concentrations of oxysterols. Therefore, circulating levels of 24S-hydroxycholesterol are supposed to be only of limited value for monitoring the brain-blood barrier function in patients with acute ischemic stroke.

Determination of 7α-OH cholesterol by LC-MS/MS: Application in assessing the activity of CYP7A1 in cholestatic minipigs

An LC-MS/MS method was developed and validated to determine 7α-OH cholesterol in liver microsome. This method was convenient and fast with high specificity and sensitivity. Briefly, a gradient elution was performed on a Synergi polar-C18 column (50×4.6mm i.d., 3μm). The mobile phase (consisting of 0.1% HCOOH solution and acetonitrile) eluted in gradient at a flow rate of 1ml/min. MS detection was operated on APCI (+) mode; the MRM transitions for 7α-OH cholesterol and D7-cholesterol (I.S.) were 385.1≥159.1 and 376.4≥266.3, respectively. The linear response range of 7α-OH cholesterol was covered from 1.563 to 100.0ng/ml. All of the validation items meet the requirement of FDA guidance for bioanalytical method validation. This method was applied to enzymatic studies for determination of cholesterol 7alpha-hydroxylation activity catalyzed by CYP7A1 in the cholestatic minipigs liver microsomes.

Effects of internal biliary bypass on the regulation of hepatic cholesterol 7alpha-hydroxylase activity in rats

In order to re-evaluate the importance of the enterohepatic circulation of bile acid in the regulation of the activities of hepatic cholesterol 7alpha-hydroxylase, bile acid metabolism was examined in internal biliary bypass models of rats. A polyethylene tube was inserted into the common bile duct and another side of the tube was placed in the duodenum (DD), lower jejunum (JD), cecum (CeD), or transverse colon (CoD) as internal biliary bypass models and in the urinary bladder as an external biliary drainage (ED). After bile diversion for 7 days in each group, hepatic cholesterol 7alpha-hydroxylase activities, bile acid concentrations in bile, serum, and portal vein, biliary bile acid compositions, and intestinal absorption rates of infused labeled taurocholic acid were analyzed. Hepatic cholesterol 7alpha-hydroxylase activity was similar in the JD group compared with the DD group, however, it was significantly up-regulated in the CeD (227% of the DD group), CoD (312%), and ED groups (316%). Biliary, serum, and portal bile acid concentrations were not significantly changed in the DD, JD, and CeD groups but those were significantly lower in the CoD and ED groups compared with the DD group. The proportion of the secondary bile acids was significantly increased in the CeD group and was decreased in the CoD and ED groups. The absorption rate of taurocholic acid was almost 100%, 56%, and 23% in the JD, CeD, and the CoD group, respectively. As the cholesterol 7alpha-hydroxylase activity was not significantly changed in the JD group and the predominance of secondary bile acids did not suppress the enzyme activity in the CeD group, the luminal factor, which is absorbed in the presence of bile acids, and the bile acid metabolites are not likely the regulatory factor. The cholesterol 7alpha-hydroxylase activity seems to be primarily regulated by the intestinal absorption of bile acids and partly by the intestinal mucosal factor which is linked to the intestinal bile acid absorption.

Regulation of bile acid synthesis in the rat: relationship between hepatic cholesterol 7 alpha-hydroxylase activity and portal bile acids

Bile acid biosynthesis has been believed to be regulated by negative feedback control; however, recent experiments have cast considerable doubts on the concept. The aim of the study was to examine the consensus of the negative feedback regulation of bile acids by clarifying the correlation between the portal bile acids and the rate-limiting enzyme of bile acid biosynthesis, hepatic microsomal cholesterol 7 alpha-hydroxylase. We measured the enzyme activity and the portal bile acids in male Wistar rats that were orally administered three different bile acids or cholestyramine for 2 weeks. The serum level of 7 alpha-hydroxycholesterol was also determined to verify whether it would be a parameter of bile acid synthesis rate in the rat. The activity of cholesterol 7 alpha-hydroxylase increased about threefold in rats treated with cholestyramine when compared with controls. On the other hand, in rats fed ursodeoxycholic, chenodeoxycholic, and deoxycholic acids, the enzyme activities decreased to 40%, 26%, and 28%, respectively. Treatment with cholestyramine had no significant effect on the portal bile acid concentration. Administration of ursodeoxycholic and chenodeoxycholic acids resulted in a significant increase in the concentration of portal bile acids, whereas deoxycholic acid feeding did not significantly affect it. In the control group, conjugated cholic acid constituted a major part of the portal bile acids while the administered bile acid and its metabolites became predominant in each bile acid feeding group. Treatment with ursodeoxycholic acid made the portal bile acids more hydrophilic, but, by contrast, administration of chenodeoxycholic, deoxycholic acids, and cholestyramine made the portal bile acids more hydrophobic.(ABSTRACT TRUNCATED AT 250 WORDS)