Bile acid synthesis in man: assay of hepatic microsomal cholesterol 7 alpha-hydroxylase activity by isotope dilution-mass spectrometry

Biochemical profiles of rat primary cultured hepatocytes following treatment with rotenone, FCCP, or (+)-usnic acid

The metabolomic profiles of rat primary hepatocytes following treatment with rotenone, FCCP, or (+)-usnic acid were determined using liquid chromatography-mass spectrometry/mass spectrometry and gas chromatography-mass spectrometry. Significant and similar changes in the levels of 283 biochemical metabolites were associated with the three treatments compared with solvent control samples. Overall, the three treatments generated similar global biochemical profiles, with some minor differences associated with rotenone treatment. All three treatments resulted in a shift in energy metabolism as demonstrated by decreased glycogen stores and glycolysis. A reduced antioxidant response was detected in cells following all treatments. In addition, bile acid biosynthesis decreased as a potential consequence of increased oxidative stress by all three treatments. Conversely, rotenone treatment induced a number of changes after 1 hr, which were not detected in FCCP- or (+)-usnic acid-treated samples; these changes were not sustained over time and included increased NAD+ salvage and lysine degradation. In conclusion, these biochemical profiles could provide new insights into the mechanism(s) of mitochondrial toxicity.

Overexpression of transforming growth factor β induced factor homeobox 1 represses NPC1L1 and lowers markers of intestinal cholesterol absorption

BACKGROUND AND AIMS

Transforming growth factor β induced factor homeobox 1 (TGIF1) is a transcriptional repressor that limits the response to transforming growth factor ß signaling and also represses transcription independent of this pathway. Recently, we found higher serum cholesterol levels and more hepatic lipid accumulation in mice lacking Tgif1, and showed that TGIF1 can repress the expression of Soat2, the gene encoding the cholesterol esterifying enzyme acyl-Coenzyme A:cholesterol acyltransferase 2. Although there is evidence that TGIF1 plays a role in lipid metabolism, its role in this metabolic pathway is not fully characterized. Here we investigate whether overexpression of TGIF1 affects intestinal cholesterol absorption.

METHODS AND RESULTS

TGIF1 was found to repress human and mouse Niemann-Pick C1 like 1 (Npc1l1) promoter activity in intestinal Caco2 cells. We also found TGIF1 to be able to oppose the induction of the promoter activity by sterol regulatory element binding protein 2 and hepatocyte nuclear factor 1α and 4α. To validate these effects of TGIF1 in vivo, we generated transgenic mice specifically overexpressing TGIF1 in the intestine (Villin-Tgif1). We observed lower intestinal expression levels of Npc1l1 that was associated with lower expression of ATP-binding cassette transporter (Abc) a1, Abcg5, and Abcg8. Villin-Tgif1 mice fed regular chow or a high-fat diet had lower levels of markers of intestinal cholesterol absorption than wild types.

CONCLUSIONS

We suggest TGIF1 as a new player in intestinal cholesterol metabolism.

Treatment with the natural FXR agonist chenodeoxycholic acid reduces clearance of plasma LDL whilst decreasing circulating PCSK9, lipoprotein(a) and apolipoprotein C-III

BACKGROUND

The natural farnesoid X receptor (FXR) agonist chenodeoxycholic acid (CDCA) suppresses hepatic cholesterol and bile acid synthesis and reduces biliary cholesterol secretion and triglyceride production. Animal studies have shown that bile acids downregulate hepatic LDL receptors (LDLRs); however, information on LDL metabolism in humans is limited.

METHODS

Kinetics of autologous ¹²⁵ I-LDL were determined in 12 male subjects at baseline and during treatment with CDCA (15 mg kg^-1 day^-1 ). In seven patients with gallstones treated with CDCA for 3 weeks before cholecystectomy, liver biopsies were collected and analysed for enzyme activities and for specific LDLR binding. Serum samples obtained before treatment and at surgery were analysed for markers of lipid metabolism, lipoproteins and the LDLR modulator proprotein convertase subtilisin/kexin type 9 (PCSK9).

RESULTS

Chenodeoxycholic acid treatment increased plasma LDL cholesterol by ~10% as a result of reduced clearance of plasma LDL-apolipoprotein (apo)B; LDL production was somewhat reduced. The reduction in LDL clearance occurred within 1 day after initiation of treatment. In CDCA-treated patients with gallstones, hepatic microsomal cholesterol 7α-hydroxylase and HMG-CoA reductase activities were reduced by 83% and 54%, respectively, and specific LDLR binding was reduced by 20%. During treatment, serum levels of fibroblast growth factor 19 and total and LDL cholesterol increased, whereas levels of 7α-hydroxy-4-cholesten-3-one, lathosterol, PCSK9, apoA-I, apoC-III, lipoprotein(a), triglycerides and insulin were reduced.

CONCLUSIONS

Chenodeoxycholic acid has a broad influence on lipid metabolism, including reducing plasma clearance of LDL. The reduction in circulating PCSK9 may dampen its effect on hepatic LDLRs and plasma LDL cholesterol. Further studies of the effects of other FXR agonists on cholesterol metabolism in humans seem warranted, considering the renewed interest for such therapy in liver disease and diabetes.

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.

Laxative treatment with polyethylene glycol decreases microbial primary bile salt dehydroxylation and lipid metabolism in the intestine of rats

Polyethylene glycol (PEG) is a frequently used osmotic laxative that accelerates gastrointestinal transit. It has remained unclear, however, whether PEG affects intestinal functions. We aimed to determine the effect of PEG treatment on intestinal sterol metabolism. Rats were treated with PEG in drinking water (7%) for 2 wk or left untreated (controls). We studied the enterohepatic circulation of the major bile salt (BS) cholate with a plasma stable isotope dilution technique and determined BS profiles and concentrations in bile, intestinal lumen contents, and feces. We determined the fecal excretion of cholesterol plus its intestinally formed metabolites. Finally, we determined the cytolytic activity of fecal water (a surrogate marker of colorectal cancer risk) and the amount and composition of fecal microbiota. Compared with control rats, PEG treatment increased the pool size (+51%; P < 0.01) and decreased the fractional turnover of cholate (-32%; P < 0.01). PEG did not affect the cholate synthesis rate, corresponding with an unaffected fecal primary BS excretion. PEG reduced fecal excretion of secondary BS and of cholesterol metabolites (each P < 0.01). PEG decreased the cytolytic activity of fecal water [54 (46-62) vs. 87 (85-92)% erythrocyte potassium release in PEG-treated and control rats, respectively; P < 0.01]. PEG treatment increased the contribution of Verrucomicrobia (P < 0.01) and decreased that of Firmicutes (P < 0.01) in fecal flora. We concluded that PEG treatment changes the intestinal bacterial composition, decreases the bacterial dehydroxylation of primary BS and the metabolism of cholesterol, and increases the pool size of the primary BS cholate in rats.

Gut microbiota regulates bile acid metabolism by reducing the levels of tauro-beta-muricholic acid, a naturally occurring FXR antagonist

Bile acids are synthesized from cholesterol in the liver and further metabolized by the gut microbiota into secondary bile acids. Bile acid synthesis is under negative feedback control through activation of the nuclear receptor farnesoid X receptor (FXR) in the ileum and liver. Here we profiled the bile acid composition throughout the enterohepatic system in germ-free (GF) and conventionally raised (CONV-R) mice. We confirmed a dramatic reduction in muricholic acid, but not cholic acid, levels in CONV-R mice. Rederivation of Fxr-deficient mice as GF demonstrated that the gut microbiota regulated expression of fibroblast growth factor 15 in the ileum and cholesterol 7α-hydroxylase (CYP7A1) in the liver by FXR-dependent mechanisms. Importantly, we identified tauro-conjugated beta- and alpha-muricholic acids as FXR antagonists. These studies suggest that the gut microbiota not only regulates secondary bile acid metabolism but also inhibits bile acid synthesis in the liver by alleviating FXR inhibition in the ileum.

Inhibition of intestinal bile acid transporter Slc10a2 improves triglyceride metabolism and normalizes elevated plasma glucose levels in mice

Interruption of the enterohepatic circulation of bile acids increases cholesterol catabolism, thereby stimulating hepatic cholesterol synthesis from acetate. We hypothesized that such treatment should lower the hepatic acetate pool which may alter triglyceride and glucose metabolism. We explored this using mice deficient of the ileal sodium-dependent BA transporter (Slc10a2) and ob/ob mice treated with a specific inhibitor of Slc10a2. Plasma TG levels were reduced in Slc10a2-deficient mice, and when challenged with a sucrose-rich diet, they displayed a reduced response in hepatic TG production as observed from the mRNA levels of several key enzymes in fatty acid synthesis. This effect was paralleled by a diminished induction of mature sterol regulatory element-binding protein 1c (Srebp1c). Unexpectedly, the SR-diet induced intestinal fibroblast growth factor (FGF) 15 mRNA and normalized bile acid synthesis in Slc10a2−/− mice. Pharmacologic inhibition of Slc10a2 in diabetic ob/ob mice reduced serum glucose, insulin and TGs, as well as hepatic mRNA levels of Srebp1c and its target genes. These responses are contrary to those reported following treatment of mice with a bile acid binding resin. Moreover, when key metabolic signal transduction pathways in the liver were investigated, those of Mek1/2 - Erk1/2 and Akt were blunted after treatment of ob/ob mice with the Slc10a2 inhibitor. It is concluded that abrogation of Slc10a2 reduces hepatic Srebp1c activity and serum TGs, and in the diabetic ob/ob model it also reduces glucose and insulin levels. Hence, targeting of Slc10a2 may be a promising strategy to treat hypertriglyceridemia and diabetes.

Five decades with oxysterols

I have been involved in research on oxysterols since 1963 and this review is intended to cover some of the most important aspects of this work. The first project dealed with 7α-hydroxy-4-cholesten-3-one. My successful synthesis of this steroid with high specific radioactivity allowed a demonstration that it is a bile acid precursor. The mechanism of conversion of 7α-hydroxycholesterol into 7α-hydroxy-4-cholesten-3-one was investigated and I concluded that only one enzyme is required and that no isomerase is involved. Accumulation of 7α-hydroxy-4-cholesten-3-one in patients with lack of sterol 27-hydroxylase (Cerebrotendinous xanthomatosis was shown to be an important pathogenetic factor. This disease is characterized by cholestanol-containing xanthomas in tendons and brain and we could show that most of this cholestanol is formed from 7α-hydroxy-4-cholesten-3-one. We also showed that 7α-hydroxy-4-cholesten-3-one passes the blood-brain barrier. In contrast to cholesterol itself, side-chain oxidized oxysterols have a high capacity to pass lipophilic membranes. We demonstrated conversion of cholesterol into 27-hydroxycholesterol to be a significant mechanism for elimination of cholesterol from macrophages. We also showed that conversion of cholesterol into 24S-hydroxycholesterol is important for elimination of cholesterol from the brain. Side-chain oxidized oxysterols have a high capacity to affect critical genes in cholesterol turnover in vitro. Most of the published in vitro experiments with oxysteroids are highly unphysiological, however. Mouse models studied in my laboratory with high or low levels of 27-hydroxycholesterol have little or no disturbances in cholesterol homeostasis. 24S-hydroxycholesterol is an efficient ligand to LXR and suggested to be important for cholesterol homeostasis in the brain. We recently developed a mouse model with markedly increased levels of this oxysterol in circulation and brain. This overexpression had however only a very modest effect on cholesterol turnover. We concluded that oxysterols are not the master regulators of cholesterol homeostasis in vivo suggested previously.

Cholesterol-metabolizing cytochromes P450: implications for cholesterol lowering

Cardiovascular disease (CVD) continues to be a leading cause of death worldwide. Elevated serum cholesterol is one of the classical risk factors for CVD, which also include age, hypertension, smoking, diabetes mellitus, obesity and family history. Several therapeutic drug classes have been developed to treat hypercholesterolemia; yet, an important percentage of patients do not reach their treatment goals. Therefore, new cholesterol-lowering medications that have sites of action different from that of drugs available at present need to be developed. This review summarizes new information about cytochrome P450 enzymes 7A1, 27A1 and 46A1. These enzymes play key roles in cholesterol elimination and have the potential to serve as targets for cholesterol-lowering.

Dramatically increased intestinal absorption of cholesterol following hypophysectomy is normalized by thyroid hormone

BACKGROUND & AIMS

Hypopituitarism is associated with dyslipidemia, and feeding hypophysectomized rats cholesterol induces severe hypercholesterolemia. This study aimed to unravel further how hypophysectomy alters cholesterol and bile acid metabolism.

METHODS

Intact and hypophysectomized rats were studied during challenge with dietary cholesterol and ezetimibe and upon hormonal substitution with growth hormone, cortisone, and thyroid hormone.

RESULTS

Five findings were established in hypophysectomized rats: (1) The intestinal absorption of cholesterol is doubled. (2) Treatment with ezetimibe abolishes the increases in serum and liver cholesterol. (3) Only thyroid hormone treatment normalizes the increased absorption of cholesterol. (4) The intestinal gene expression of cholesterol transporters NPC1L1 and ABCG5/G8 is unaltered, whereas the hepatic expression of ABCG5/G8 is diminished but strongly stimulated by thyroid hormone. The latter mechanism was supported by measurements of biliary cholesterol and of fecal neutral steroids. (5) The reduced hepatic expression of ABCG5/G8 and Cyp7a1 was normalized by cholesterol feeding, suggesting that other nonestablished mechanisms under pituitary control are important to maintain rats resistant to dietary cholesterol.

CONCLUSIONS

The intestinal absorption of dietary cholesterol is under pituitary control largely exerted by thyroid hormone. Hepatic secretion of cholesterol and ABCG5/G8 expression are strongly stimulated in hypophysectomized rats during treatment with thyroid hormone.

Hypocholesterolemic effects of fatty acid bile acid conjugates (FABACs) in mice

Fatty acid bile acid conjugates (FABACs) prevent and dissolve cholesterol gallstones and prevent diet induced fatty liver, in mice. The present studies aimed to test their hypocholesterolemic effects in mice. Gallstone susceptible (C57L/J) mice, on high fat (HFD) or regular diet (RD), were treated with the conjugate of cholic acid with arachidic acid (FABAC; Aramchol). FABAC reduced the elevated plasma cholesterol levels induced by the HFD. In C57L/J mice, FABAC reduced plasma cholesterol by 50% (p<0.001). In mice fed HFD, hepatic cholesterol synthesis was reduced, whereas CYP7A1 activity and expression were increased by FABAC. The ratio of fecal bile acids/neutral sterols was increased, as was the total fecal sterol excretion. In conclusion, FABACs markedly reduce elevated plasma cholesterol in mice by reducing the hepatic synthesis of cholesterol, in conjunction with an increase of its catabolism and excretion from the body.

Ethanol stimulates bile acid formation in primary human hepatocytes

The conversion of cholesterol to bile acids is a key pathway for elimination of cholesterol from the body, thereby reducing the risk of arteriosclerosis. Moderate consumption of ethanol has been shown to have preventive effects on cardiovascular disease and decrease the risk of gallstone formation. In the present study primary human hepatocytes were used to investigate if ethanol affected bile acid synthesis. Hepatocytes were prepared from donor liver (n=11) and treated with ethanol, 7.7 or 50 mM, for 24 h. mRNA levels for enzymes in bile acid synthesis pathways were studied and bile acid synthesis was analyzed. Treatment with 7.7 mM ethanol increased cholic acid synthesis by 20% and treatment with 50 mM ethanol up-regulated cholic acid formation by 60%. The synthesis of cholic acid increased more than that of chenodeoxycholic acid, indicating that the classical pathway for bile acid synthesis was up-regulated. Increased bile acid levels in the cells treated with ethanol were seen after approximately 20 h. mRNA expression of CYP7A1, CYP27A1, and CYP8B1 in the hepatocytes was not affected by alcohol exposure.

Dietary myristic acid modifies the HDL-cholesterol concentration and liver scavenger receptor BI expression in the hamster

The influence of myristic acid in a narrow physiological range (0·5 to 2·4 % of total dietary energy) on the plasma and hepatic cholesterol metabolism was investigated in the hamster. The hamsters were fed on a diet containing 12·5 g fat/100 g and 0·05 g cholesterol/100 g with 0·5 % myristic acid (LA diet) for 3 weeks (pre-period). During the following 3 weeks (test period), they were divided into four dietary groups with 0·5 % (LA), 1·2 % (LM), 1·8 % (ML) or 2·4 % (M) myristic acid. Finally, half the hamsters in each group were again fed the LA diet for another 3 weeks (post-period). At the end of the test period, the hepatic expression of the scavenger receptor BI (SR-BI) was lower in the LM, ML and M groups than in the LA group whereas the hepatic cholesteryl ester concentration was higher. Cholesterol 7α hydroxylase activity was lower in the ML and M groups than in the LA and LM groups while the sterol 27 hydroxylase and 3-hydroxy-3-methyl glutaryl coenzyme A reductase activities were not modulated by dietary myristic acid. This is the first time a negative correlation has been observed between the HDL-cholesterol concentration and the hepatic mass of SR-BI (r-0·69;P<0·0001) under physiological conditions. An inverse linear regression was also shown between SR-BI and the percentage of myristic acid in the diet (r-0·75;P<0·0001). The hepatic mass of SR-BI in the M group had increased at the end of the post-period compared with the test-period values. The present investigation shows that myristic acid modulates HDL-cholesterol via a regulation of the SR-BI expression.

Regulation of cytochrome P450 by posttranslational modification

Cytochrome P450s are a family of enzymes represented in all kingdoms with expression in many species. Over 3,000 enzymes have been identified in nature. Humans express 57 putatively functional enzymes with a variety of critical physiological roles. They are involved in the metabolic oxidation, peroxidation, and reduction of many endogenous and exogenous compounds including xenobiotics, steroids, bile acids, fatty acids, eicosanoids, environmental pollutants, and carcinogens [Nelson, D. R., Kamataki, T., Waxman, D. J., Guengerich, F. P., Estabrook, R. W., Feyereisen, R., Gonzalez, F. J., Coon, M. J., Gunsalus, I. C., Gotoh, O. (1993) The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature. DNA Cell Biol. 12(1):1-51.] The development of numerous diseases and disorders including cancer and cardiovascular and endocrine dysfunction has been linked to P450s. Several levels of regulation, including transcription, translation, and posttranslational modification, participate in maintaining the proper function of P450s. Modifications including phosphorylation, glycosylation, nitration, and ubiquitination have been described for P450s. Their physiological significance includes modulation of enzyme activity, targeting to specific cellular compartments, and tagging for proteasomal degradation. Knowledge of P450 posttranslational regulation is derived from studies with relatively few enzymes. In many cases, there is only enough evidence to suggest the occurrence and a possible role for the modification. Thus, many P450 enzymes have not been fully characterized. With the introduction of current proteomics tools, we are primed to answer many important questions regarding regulation of P450 in response to a posttranslational modification. This review considers regulation of P450 in a context that describes the potential role and physiological significance of each modification.

Evaluation of 3-hydroxy-3-methylglutaryl-COA-reductase, cholesterol-7?-hydroxylase and acyl-COA:cholesterol acyltransferase activities: alternative chromatographic methods to separate metabolites

Alternative HPLC and solid-phase extraction column methods were developed to separate metabolites of enzymes involved in cholesterol metabolism in rabbit liver microsomes: hydroxyl-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase. A comparison method of thin-layer chromatography and solid-phase extraction column were assayed to separate substrate and metabolite of hydroxy-methylglutaryl-CoA reductase, whereas for cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase, this comparison was done between thin layer chromatography and HPLC. The results obtained by the new analytical chromatographic methods are not significantly different than those observed in literature. Moreover a larger percentage recovery was obtained for analysed metabolites. Our results demonstrate the reliability of these alternative chromatographic techniques and showed that they are valuable tools to precisely and rapidly measure the activity of those enzymes.

Leptin induces the hepatic high density lipoprotein receptor scavenger receptor B type I (SR-BI) but not cholesterol 7alpha-hydroxylase (Cyp7a1) in leptin-deficient (ob/ob) mice

Cholesterol elimination from the body involves reverse cholesterol transport from peripheral tissues in which the elimination of high density lipoprotein (HDL) and low density lipoprotein (LDL) cholesterol by the liver and subsequent biliary excretion as free cholesterol and bile acids are important. In situations of peripheral fat and cholesterol accumulation, such as obesity, these pathways may be overloaded, contributing to increased cholesterol deposition. Leptin has an important role in obesity, suppressing food intake and increasing energy expenditure. This hormone, which is absent in genetically obese ob/ob mice, is also thought to be involved in the coordination of lipid excretion pathways, although available data are somewhat inconsistent. We therefore studied the expression of the hepatic HDL receptor, scavenger receptor class B type I (SR-BI), and the LDL receptor as well as the rate-limiting enzyme in bile acid synthesis, cholesterol 7alpha-hydroxylase (Cyp7a1), in leptin-deficient ob/ob mice and their wild-type controls. In ob/ob mice, protein levels of both LDL receptor and SR-BI were reduced, whereas LDL receptor mRNA levels were increased and those of SR-BI were reduced, regardless of challenge with a 2% cholesterol diet. In ob/ob mice, the enzymatic activity and mRNA for Cyp7a1 were reduced, and the increase in response to dietary cholesterol was blunted. Upon short-term (2 days) treatment with leptin, a dose-dependent increase was seen in the SR-BI protein and mRNA, whereas the Cyp7a1 protein and mRNA were reduced. Our findings indicate that leptin is an important regulator of hepatic SR-BI expression and, thus, HDL cholesterol levels, whereas it does not stimulate Cyp7a1 and bile acid synthesis.

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.

Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood

We describe an accurate method for monitoring the enzymatic activity of hepatic cholesterol 7alpha-hydroxylase (C7alphaOH; CYP7A1), the rate-limiting and major regulatory enzyme in the synthesis of bile acids. Assay of 7alpha-hydroxy-4-cholesten-3-one (C4), an intermediate in bile acid synthesis, revealed that the level of C4 in peripheral blood serum or plasma showed a strong correlation to the enzymatic activity of hepatic C7alphaOH, both at steady-state conditions (r = 0.929) as well as during the rapid changes that occur during the diurnal phases. This assay should be of value in clarifying the regulation of bile acid synthesis in vivo in laboratory animals and humans since it allows for the monitoring of hepatic C7alphaOH activity using peripheral blood samples.

Pharmacological interference with intestinal bile acid transport reduces plasma cholesterol in LDL receptor/apoE deficiency

Reduction of plasma cholesterol by statins is fundamental to prevent coronary heart disease. Such therapy is often sub-optimal, however, particularly in patients with reduced LDL receptors (familial hypercholesterolemia), and novel or adjuvant therapies are therefore warranted. Cholesterol elimination is profoundly influenced by the rate of its conversion to bile acids (BA), regulated by the enzyme Cyp7a1. Induced fecal loss of BA by resin treatment reduces plasma cholesterol, presumably through induction of hepatic LDL receptors (LDLR). We here describe the effect of PR835, a drug belonging to a new class of lipid-lowering agents that inhibit the Slc10a2 protein, the intestinal transporter responsible for active uptake of BA. Treatment reduced plasma cholesterol by 40% in mice devoid of both the LDLR and its ligand, apoE, while triglycerides and HDL cholesterol were unchanged. Cyp7a1 enzyme activity and mRNA were induced several-fold, and hepatic HMG CoA reductase mRNA increased, mirroring an induced synthesis of BA and cholesterol. The addition of a statin potentiated the effect, leading to reductions of plasma total and LDL cholesterol by 64% and 70%, respectively. These effects could not be attributed to induction of other known hepatic lipoprotein receptors and indicate the presence of new points of targeting in lipid-lowering therapy.

Simultaneous evaluation of HMG-CoA reductase and cholesterol 7alpha-hydroxylase activities by electrospray tandem MS

Simultaneous evaluation of HMG-CoA reductase and cholesterol 7alpha-hydroxylase activities by electrospray tandem mass spectrometry (ES-MS-MS) was performed. The assay was based on the measurement of mevalonolactone (MVL) and 7alpha-hydroxycholesterol (7alpha-OHC) produced by the incubation of HMG-CoA with hepatic microsomes in the presence of NADPH and glucose-6-phosphate dehydrogenase. Following extraction and purification using a cyanopropyl cartridge, MVL and 7alpha-OHC were analyzed, without derivatization, by ES-MS-MS. The analysis was achieved in 5 min. Calibration curves were made for MVL and 7alpha-OHC, and were linear from 0 to 100 microg. The recovery was >97%. The procedure was validated under similar calibration and recovery experiments, by measuring the above mentioned products as dimethylethylsilyl ether derivatives using the classical technique of GC-MS. Data obtained by ES-MS-MS and GC-MS showed a good correlation, with no significant differences. ES-MS-MS is a simple and reliable method for the evaluation of HMG-CoA reductase and cholesterol 7alpha-hydroxylase activities in liver microsomal preparations.

Effects of combined treatment with pravastatin and ursodeoxycholic acid on hepatic cholesterol metabolism

BACKGROUND

Treatment with ursodeoxycholic acid and also, to some degree, statins reduces cholesterol saturation of bile. The present study aimed [1] to study the effects of combined treatment with ursodeoxycholic acid and pravastatin on hepatic cholesterol metabolism and [2] to evaluate if the addition of pravastatin to ursodeoxycholic acid treatment has beneficial effects on the lipid composition of gallbladder bile in gallstone patients.

MATERIALS AND METHODS

Nineteen patients with cholesterol gallstones were subjected to combined treatment with ursodeoxycholic acid (500 mg bid) and pravastatin (20 mg bid) for three weeks before cholecystectomy. Eleven patients received ursodeoxycholic acid only and 20 untreated gallstone patients served as controls. Gallbladder bile was collected, and for both the patients receiving combined treatment and the controls a liver biopsy was also obtained peroperatively.

RESULTS

The cholesterol saturation of bile averaged 59% in the patients on combined treatment, 60% in the ursodeoxycholic acid-treated patients, and 130% in the untreated controls. In the patients receiving ursodeoxycholic acid, this bile salt constituted approximately 60% of all bile salts. The patients receiving combined treatment had reduced cholesterol synthesis, as reflected by a 45% reduction in serum lathosterol. The activity and the mRNA levels of cholesterol 7 alpha-hydroxylase and the mRNA levels for the low density lipoprotein-receptor were not significantly affected.

CONCLUSIONS

Pravastatin does not further reduce the cholesterol saturation of bile in gallstone patients treated with ursodeoxycholic acid, although hepatic cholesterol synthesis is inhibited. The study supports the important concept that de novo synthesized cholesterol is not particularly important for biliary cholesterol secretion in humans.

Growth hormone reduces plasma cholesterol in LDL receptor-deficient mice

Growth hormone (GH) has pleiotropic effects on cholesterol and lipoprotein metabolism. Pituitary GH is important for the normal regulation of hepatic LDL receptors (LDLR), for the enzymatic activity of bile acid regulatory cholesterol 7alpha-hydroxylase (C7alphaOH), and for the maintenance of resistance to dietary cholesterol. The present study aimed to determine whether GH has beneficial effects on plasma lipids and hepatic cholesterol metabolism in mice devoid of LDLR. Compared with wild-type controls, LDLR-deficient mice had approximately 250% elevated plasma total cholesterol and approximately 50% increased hepatic cholesterol levels; hepatic HMG CoA reductase activity was reduced by 70%, whereas C7alphaOH activity was increased by 40%. In LDLR mice, GH infusion reduced plasma cholesterol and triglycerides up to 40%, whereas HMG CoA reductase and C7alphaOH activities were stimulated by approximately 50% and 110% respectively. GH also stimulated HMG CoA reductase and C7alphaOH activities in control mice, whereas hepatic LDLR and plasma lipoproteins were unchanged. The effects of cholestyramine and atorvastatin on C7alphaOH in LDLR-deficient mice were potentiated by GH, and this was associated with a further reduction in plasma cholesterol. GH treatment reduces plasma cholesterol and triglycerides and stimulates C7alphaOH activity in mice devoid of LDLR, particularly in combination with resin or statin treatment. The potential of GH therapy in patients with homozygous familial hypercholesterolemia should be evaluated.

Biphasic effects of the natural estrogen 17beta-estradiol on hepatic cholesterol metabolism in intact female rats

The protective influence of estrogens in cardiovascular disease is believed to be partly due to beneficial effects on cholesterol metabolism. Much of the experimental data are based on models in which synthetic estrogens have been used in pharmacological doses, and therefore, the physiological role of estrogens in cholesterol metabolism is uncertain. To evaluate this important issue, we performed experiments in intact female rats with use of the natural estrogen 17beta-estradiol (E2) administered either subcutaneously or orally. After physiological doses of E2 (< or =0.04 mg. kg(-1). d(-1)) were administered, plasma levels of high density lipoprotein (HDL) cholesterol and apolipoprotein (apo) A-I were increased. In the liver, 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7alpha-hydroxylase activities were increased, as well as cholesterol 7alpha-hydroxylase mRNA levels. These effects were abolished during treatment with higher doses of E2, whereas apo A-I mRNA increased in a dose-dependent way. After treatment with pharmacological doses of E2 (> or =0.2 mg. kg(-1). d(-1)), the number of hepatic low density lipoprotein receptors increased and plasma cholesterol was reduced. These effects were similar after both oral and subcutaneous administration of E2. Our results show that the responses to E2 are biphasic: plasma HDL, apo A-I, and hepatic enzyme activities governing bile acid and cholesterol synthesis increased only at physiological doses of E2. At pharmacological doses of E2, hepatic low density lipoprotein receptors are stimulated and plasma cholesterol is reduced. Therefore, under physiological conditions, E2 exerts its major effects on hepatic cholesterol metabolism through mechanisms other than stimulation of low density lipoprotein receptor expression.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Deoxycholic acid treatment in patients with cholesterol gallstones: failure to detect a suppression of cholesterol 7alpha-hydroxylase activity

Hillebrant C-G, Nyberg B, Angelin B, Axelson M, Björkhem I, Rudling M, Einarsson C (Huddinge University Hospital and Karolinska Hospital, Karolinska Institute, Stockholm, Sweden). Deoxycholic acid treatment in patients with cholesterol gallstones: failure to detect a suppression of cholesterol 7alpha-hydroxylase activity. J Intern Med 1999; 246: 399-407.

OBJECTIVES

Based on animal studies, hydrophobic bile acids have been postulated to be particularly strong inhibitors of bile acid synthesis. The present study was undertaken to characterize in humans the effects of one of the most hydrophobic of the common bile acids, deoxycholic acid (DCA), on the transcriptional regulation and activity of the cholesterol 7alpha-hydroxylase, on hepatic cholesterol metabolism and on biliary lipid metabolism and plasma lipids. DESIGN, SUBJECTS AND SETTINGS: Thirteen patients with cholesterol gallstone disease were treated with DCA (750 mg day-1) for 3 weeks prior to cholecystectomy. Blood samples were collected before and during treatment. At operation, a liver biopsy and gallbladder bile were obtained. Twenty-eight untreated gallstone patients undergoing cholecystectomy served as controls. The study was carried out at a university hospital.

RESULTS

Deoxycholic acid comprised 72 +/- 6% (mean +/- SEM) of total biliary bile acids in DCA-treated patients (n = 8), and 21 +/- 2% in the controls (n = 16; P < 0.001). Cholesterol saturation of gallbladder bile averaged 102% in both treated (n = 7) and untreated (n = 16) patients. Cholesterol 7alpha-hydroxylase and HMG CoA reductase activities and mRNA levels were not different between DCA-treated and untreated gallstone patients. The LDL receptor mRNA levels were similar in both groups of patients. Plasma levels of total cholesterol were lowered by 10% upon DCA treatment (P < 0.05).

CONCLUSIONS

Treatment with DCA did not significantly affect mRNA levels and activity of hepatic cholesterol 7alpha-hydroxylase or HMG CoA reductase in patients with cholesterol gallstones. There was no effect on the saturation of gallbladder bile, Thus, the present study could not verify that the hydrophobicity of the bile acid pool is a major factor regulating human hepatic cholesterol 7alpha-hydroxylase activity.

Cholesterol and lipoprotein metabolism in aging: reversal of hypercholesterolemia by growth hormone treatment in old rats

Plasma cholesterol levels increase with age, as does the incidence of coronary heart disease. The mechanisms responsible for the age-related hypercholesterolemia are not well understood. An interesting hypothesis suggests that the relative deficiency in growth hormone (GH), which occurs with aging, contributes to the development of the age-related hypercholesterolemia, because GH has beneficial effects on cholesterol metabolism. In the present work, we tested this hypothesis by the administration of GH to normal rats of varying ages. Plasma lipids and hepatic cholesterol metabolism were characterized in 2-, 12-, and 18-month-old male Sprague-Dawley rats. In 2-month-old rats, GH specifically stimulated the hepatic low density lipoprotein (LDL) receptor expression in a dose-dependent way, both at the protein level and at the mRNA level. Concomitantly, plasma cholesterol increased by approximately 30% within the large high density lipoprotein and LDL fractions. In 12-month-old animals, cholesterol 7alpha-hydroxylase (C7alphaOH) activity was reduced, whereas hepatic LDL receptors and plasma total cholesterol were unchanged. GH treatment (1 mg. kg-1. d-1) normalized the activity of C7alphaOH and had effects on plasma cholesterol and LDL receptors similar to those seen in 2-month-old animals. In 18-month-old rats, plasma cholesterol was increased 2-fold, whereas hepatic LDL receptor expression and C7alphaOH activity were similar to those of the 12-month-old animals. Infusion of GH to 18-month-old rats had similar effects on hepatic C7alphaOH and LDL receptors as seen in 12-month-old rats. However, GH treatment strongly reduced the hypercholesterolemia in 18-month-old animals. We conclude that the age-dependent increase of plasma cholesterol in rats can be reversed by the administration of GH, presumably through the pleiotropic effects of this hormone on lipoprotein metabolism.

Determination of 7alpha-hydroxycholesterol in dog plasma by high-performance liquid chromatography with fluorescence detection

A new method was developed for the determination of 7alpha-hydroxycholesterol (7-HC) in dog plasma by means of high-performance liquid chromatography (HPLC) with fluorescence detection. 7-HC extracted with organic solvent from plasma was purified with Bond Elut 2OH and converted to a sensitive fluorescent derivative containing double coumarin groups at the C-3 and C-7 positions of the steroid nucleus with 7-methoxycoumarin-3-carbonyl azide. After removal of the excess reagent with Bond Elut NH2, the 7-HC derivative was separated by reverse-phase HPLC method. The detection limit of the authentic 7-HC-3,7-coumarin derivative was 4 pg (S/N = 5), approximately four times less than that of the 7-HC-3-anthroyl derivative yielded by reaction of 7-HC with 1-anthroylcyanide. The newly developed method was used to investigate the effects of consecutive oral administrations of cholestyramine (CA) on 7-HC levels in dog plasma. The plasma 7-HC levels of the CA-treated group were two times greater than those of the control group.

Improved assay of hepatic microsomal cholesterol 7 alpha-hydroxylase activity by the use of hydroxypropyl-beta-cyclodextrin and an NADPH-regenerating system

Cholesterol 7 alpha-hydroxylase, the key enzyme in bile acid synthesis, has been implicated in atherosclerosis and gallstone disease. The aim of this study was to check if the use of hydroxypropyl-beta-cyclodextrin (HPBCD), a vehicle for solubilizing cholesterol, augmented the rate of 7 alpha-hydroxycholesterol formation in hamster liver microsomes compared to classical assays in which labeled cholesterol was delivered in Tween 80. We observed that [14C]cholesterol carried by HPBCD enhanced the sensitivity of the assay tenfold. However, linearity of 7 alpha-hydroxycholesterol formation with time was short because of the rapid transformation of 7 alpha-hydroxycholesterol into 7 alpha-hydroxy-cholesten-3-one and 7 alpha,12 alpha-dihydroxy-cholesten-3-one when NADPH alone was present in the incubation medium. In order to avoid the transformation of 7 alpha-hydroxycholesterol into 7 alpha-hydroxy-cholesten-3-one, which is essentially NAD(+)-dependent, but is also NADP(+)-dependent, NADPH (1 mmol/l) plus an NADPH-regenerating system must be present in the medium. In this improved assay, the optimal pH was 7.4 and the apparent Km for control and cholestyramine-fed hamsters had a similar value of 315 mumol/l; linearity in the formation of 7 alpha-hydroxycholesterol was also apparent after a relatively short time period (10 min), but with a markedly greater slope of the curve. With a short incubation time (6 min), microsomes from livers of hamsters (five and nine weeks old) that were fed with a commercial ground diet yielded rates of 7 alpha-hydroxycholesterol formation of 115 +/- 10 and 150 +/- 16 pmol/min.mg protein, respectively, whereas microsomes from hamsters fed with a lithogenic sucrose-rich diet (five weeks old) yielded rates of 7 alpha-hydroxycholesterol formation of 77 +/- 7 pmol/min.mg protein, which were significantly lower (-33%) than those of corresponding control hamsters. This improved cholesterol 7 alpha-hydroxylase assay is very sensitive, simple and rapid, and does not necessitate sophisticated equipment. It can be particularly useful for determining cholesterol 7 alpha-hydroxylase activity in liver biopsies from dyslipidemic or lithiasic patients.

Effect of selenium deficiency on hepatic lipid and lipoprotein metabolism in the rat

Since experimental Se deficiency results in a significant increase in plasma cholesterol concentration the present investigation was undertaken to assess further the influence of this deficiency on the expression of proteins involved in hepatic lipid metabolism. Se deficiency was induced by feeding weanling male Wistar rats on a deficient diet for 6 weeks. Hypercholesterolaemia associated with Se deficiency was related to increased 3-hydroxy-3-methylglutaryl-coA (HMG-CoA) reductase (EC 1.1.1.34) activity in liver microsomes as compared with control animals. Hepatic lipoprotein receptor levels (LDL-receptor and HDL-binding proteins, HB1 and HB2) were not significantly affected by Se deficiency, as assessed by immunoblotting. Plasma triacylglycerol concentrations tended to decrease in Se-deficient rats in concert with their reduced post-Triton secretion. There was no significant effect of Se deficiency on the hepatic synthesis of apolipoproteins. These results point to the need for further investigations into the mechanism related to the increased activity of HMG-CoA reductase and the enhanced cholesterogenesis in the liver of Se-deficient rats likely to result from this.

Cholesterol and bile acid metabolism after selective portal vein ligation

AIM

To examine the regulatory effect of bile acid level on bile acid synthesis in the liver.

METHODS

The portal branch perfusing left lateral and median lobes of the liver was ligated in rats and the activities of hepatic microsomal cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme of bile acid synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and intrahepatic concentrations of cholesterol and bile acids were determined in the liver lobes deprived of and supplied with portal blood on Days 0, 1, 2, 4, and 7 after selective portal vein ligation (SPVL).

RESULTS

In the portal vein (PV)-ligated lobes, liver weight decreased, hepatic cholesterol concentration was unchanged, and microsomal cholesterol concentration increased after SPVL. In the PV-nonligated lobes, liver weight increased, hepatic cholesterol concentration increased, and microsomal cholesterol concentration was unchanged. There were no significant differences in the activities of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase among the PV-ligated and PV-nonligated lobes and the sham-operated controls. Intrahepatic bile acid level increased significantly in the PV-nonligated lobes for 4 days after SPVL, whereas those were essentially constant in the PV-ligated and the sham-operated control liver. Despite significant changes in the concentrations of intrahepatic cholesterol and bile acid, no significant correlations were observed between these concentrations and the activities of cholesterol 7 alpha-hydroxylase.

CONCLUSIONS

SPVL causes atrophy and hypertrophy of the PV-ligated and nonligated liver lobes, respectively, without any significant changes in cholesterol and bile acid synthesis. Intrahepatic concentrations of bile acids and cholesterol have no regulatory effect on cholesterol 7 alpha-hydroxylase activity in the SPVL rat model.

Lithogenic diet and gallstone formation in mice: integrated response of activities of regulatory enzymes in hepatic cholesterol metabolism

Supersaturation of bile with cholesterol is a prerequisite of the development of gallstones. With the intention to study the integrated response of enzymes regulating hepatic cholesterol metabolism during gallstone formation we used an established model for the induction of cholesterol gallstone disease in mice. Ten mice were fed on a lithogenic diet containing 10 g cholesterol/kg and 5 g cholic acid/kg for 8 weeks and were compared with ten mice fed on a standard pellet diet. Cholesterol crystals or gallstones developed in 90% of gallbladders in treated mice. The lithogenic diet had an inhibitory effect on the rate-limiting enzyme of cholesterol biosynthesis, hepatic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88) activity, 39.6 (SEM 2.8) v. 171.0 (SEM 47.3) pmol/min per mg protein. Cholesterol 7 alpha-hydroxylase (EC 1.14.13.17) activity, regulating bile acid synthesis, was decreased by 80%, and this was assumed to be due to cholic acid in the diet. The cholesterol-enriched diet also induced a tenfold increase in cholesterol esterification rate in the liver, i.e. acyl-CoA:cholesterol acyl transferase (ACAT; EC 2.3.1.26) activity. The total, as well as esterified, cholesterol contents of liver homogenates were significantly higher in cholesterol- and cholic acid-treated mice and correlated well with the ACAT activity (rs 0.72 (P < 0.005), and rs 0.68 (P < 0.01) respectively). A significantly higher ACAT activity was obtained in mice given cholesterol and cholic acid even when the enzyme was saturated with exogenous cholesterol, thus indicating an increased amount of the enzyme. The formation of gallstones is dependent on a delicate balance between lithogenic factors (increased absorption of cholesterol and reduced secretion of bile acids) and defence mechanisms (decreased synthesis and increased esterification of cholesterol). In the specific animal model studied here the two defence mechanisms cannot compensate for the increased absorption of cholesterol and the reduced synthesis of bile acids.

Concentration of unsulfated lithocholic acid in portal and systemic venous plasma: evidence that lithocholic acid does not down regulate the hepatic cholesterol 7 alpha-hydroxylase activity in gallstone patients

It has been proposed that lithocholic acid may have a physiological role for the regulation of bile acid synthesis in humans. In this study, the portal concentration and hepatic uptake of unsulfated lithocholic acid was determined in 21 gallstone patients-untreated, cholestyramine-treated and chenodeoxycholic acid-treated-at cholecystectomy. Lithocholic acid was analyzed by a combined gas-liquid mass-fragmentographic technique. In most of the patients a liver biopsy was obtained for assay of the cholesterol 7 alpha-hydroxylase activity. The portal venous concentration of unsulfated lithocholic acid averaged 0.32 mumol/l in untreated patients, constituting about 4% of the total bile acids. The apparent hepatic uptake of lithocholic acid averaged 78%, being as high as that of cholic acid. No significant correlation was obtained between the portal venous concentration of unsulfated lithocholic acid and the hepatic cholesterol 7 alpha-hydroxylase activity. This study thus confirms an enterohepatic circulation of lithocholic acid in humans. No evidence was obtained that the portal venous inflow of small amounts of lithocholic acid to the liver is of regulatory importance for the cholesterol 7 alpha-hydroxylase activity.

Comparison of the effects of cyclic AMP analogues on cholesterol metabolism in cultured rat and hamster hepatocytes

The effects of two cell-permeable cyclic AMP analogues, 8-chloro cyclic AMP (8-Cl cAMP) and 8-(4-chlorophenylthio) cyclic AMP (8-CPT cAMP), on cholesterol esterification, cholesteryl ester hydrolysis and bile acid synthesis were compared in cultured rat and hamster hepatocytes. Cholesterol esterification, as measured by the incorporation of [3H]oleate into cholesteryl ester, was increased by 58-88% by the analogues in rat hepatocytes and by 33-43% in hamster cells. The response in rat hepatocytes, however, was observed after a relatively short incubation time (28% increase after 1 hr), whereas that in hamster cells required a longer period (36% after 12 hr) to become apparent. The activity of the cytosolic neutral cholesteryl ester hydrolase in rat hepatocytes was also stimulated by both cyclic AMP analogues (31-37%, but the microsomal activity was unaffected. In hamster hepatocytes, however, microsomal cholesteryl ester hydrolase activity was increased (47-80%) in the presence of 8-Cl cAMP or 8-CPT cAMP. Bile acid synthesis was increased by 8-CPT cyclic AMP in rat cells (approximately 25%) but was unchanged by both analogues in hamster hepatocytes. These results indicate significant differences in the way in which cholesterol metabolism responds to cyclic AMP in cultured rat and hamster hepatocytes.

Antilithiasic and hypocholesterolemic effects of diets containing autoclaved amylomaize starch in hamster

The prevention of cholelithiasis by dietary manipulation was investigated in hamsters receiving a fat-free lithogenic (L) diet or this diet in which sucrose was replaced by 12 (group AS12), 36 (group AS36), 48 (group AS48), or 72.5% (group AS72.5) of autoclaved amylomaize starch for seven weeks. All hamsters (6/6) had cholesterol gallstones in groups L and AS12, while only 3/6 hamsters in group AS36 had gallstones. None were present in groups AS48 and AS72.5. Except in group AS12, biliary cholesterol level and lithogenic index (LI) decreased significantly in hamsters receiving amylomaize starch. Plasma cholesterol concentration was reduced by 31 and 54%, respectively, in groups AS48 and AS72.5 as compared to group L. The concentration of esterified cholesterol in the liver was also reduced significantly in all groups receiving amylomaize starch. Hepatic cholesterogenesis was decreased by 74 and 65%, respectively, in groups AS48 and AS72.5 as compared to group L. The transformation of cholesterol to bile acids was increased in group AS72.5 (+152%) as compared to L, while fecal cholesterol excretion was strongly lowered (-31%). Amylomaize starch reduced the microbial transformation of cholesterol to coprosterol and epicoprosterol, and in group AS72.5 it decreased the degradation of cholic acid. Thus, this autoclaved amylomaize starch, which could be used in human nutrition, prevents cholelithiasis and lowers cholesterolemia.

Total parenteral nutrition stimulates hepatic cholesterol synthesis in the rat

Cholesterol synthesis was studied in parenterally fed rats, as compared to orally fed rats with or without saline infusion. Conditions of total parenteral nutrition (TPN) involved the intravenous infusion of a nutritive mixture containing 20% Intralipid as the lipid source (50% of non-protein energy) at the continuous rate of 2 ml per h, for five days. In rats maintained in isotopic steady state by daily injections of [3H]cholesterol, isotope dilution indicated that the endogenous plasma cholesterol input was significantly higher (+15%, P < 0.05) in TPN than in orally fed rats, which suggested a slight stimulation of whole body cholesterogenesis. Cholesterol synthesis was assessed in TPN and orally fed rats by the in vivo incorporation of [1,2-13C]- and [1-14C]acetate into hepatic and intestinal sterols, and by the activity of HMG-CoA reductase in microsomes isolated from liver and small intestine. Both methods demonstrated that TPN markedly stimulated the hepatic cholesterol synthesis, since the radioactivity of liver sterols was 6- to 10-fold higher, and the activity of HMG-CoA reductase 5-fold higher, in TPN than in orally fed rats. Despite the weight reduction of the small intestine, by about 20% after TPN, the incorporation of exogenous [14C]acetate into intestinal sterols was similar in TPN and orally fed rats. As the liver and intestine are the main organs responsible for the appearance of endogenous cholesterol in plasma, it may be concluded that the increased endogenous plasma cholesterol input was mainly due to a strong stimulation of hepatic cholesterol synthesis in TPN rats.

Expression and distribution of cholesterol 7 alpha-hydroxylase in rat liver

The hydroxylation of cholesterol by cholesterol 7 alpha-hydroxylase (CYP7) to 7 alpha-hydroxycholesterol is the rate-limiting step in the production of bile acids. An anti-peptide antibody targeted to the C-terminus of CYP7 was produced by immunising rabbits with the synthetic peptide Tyr-Lys-Leu-Lys-His. The antibody bound to a single band of 54 kDa from rat hepatic microsomal fractions. The intensity of the band was subject to a diurnal variation and showed a significant increase (P < 0.01) in apoprotein at night. Treatment of rats with cholestyramine increased CYP7 apoprotein in the morning (P < 0.005) and at night (P < 0.005), but diurnal variation was maintained. CYP7 catalytic activity, measured using a specific gas chromatography/mass spectrometry assay, showed similar changes in the pattern of diurnal variation and induction. The distribution of CYP7 in rat liver tissue sections was investigated by immunocytochemistry. In sections from rats treated with cholestyramine, there was an even distribution of immunoreactivity, except in the proximal perivenous hepatocytes where immunoreactivity was slightly more intense. A similar distribution was found in sections from untreated rat liver, except immunoreactivity was overall slightly less intense. This study shows that the C-terminus of CYP7 is a useful epitope for the targeting of anti-peptide antibodies.

Effects of hyodeoxycholic acid and alpha-hyocholic acid, two 6 alpha-hydroxylated bile acids, on cholesterol and bile acid metabolism in the hamster

The effects of hyodeoxycholic (HDCA) and alpha-hyocholic acids (alpha-HCA), on cholesterol, bile acid and lipoprotein metabolism, were studied in hamsters. The animals were fed a low cholesterol control diet supplemented with 0.1% HDCA or alpha-HCA for 3 weeks. In both treated groups, the LDL-cholesterol concentration was significantly lowered and was associated with a global hypocholesterolemic effect. Moreover, hepatic cholesterol ester storage was reduced and HMGCoA reductase activity was respectively enhanced 13.5-times and 7.7-times in HDCA and alpha-HCA groups compared to controls. In contrast, cholesterol 7 alpha-hydroxylase activity and LDL-receptor activity and mass were not modified. In bile, the cholesterol saturation index was increased 5-fold (HDCA group) and 2-fold (alpha-HCA group) as a consequence of an enlarged proportion of biliary cholesterol. The two 6-hydroxylated bile acids induced an enhanced fecal excretion of neutral sterols (HDCA group: 11.6-times, alpha-HCA group: 3.2-times versus controls) which was consistent with a 59% decrease in intestinal cholesterol absorption in the HDCA group. The major effects due to bile acid treatments were a decrease in LDL-cholesterol concentration, a strong stimulation of hepatic cholesterol biosynthesis and an excessive loss of cholesterol in feces. These perturbations might be the result of the enrichment of bile with hydrophilic bile acids, leading to a limited return of endogenous cholesterol from the intestine to the liver.

Influence of bezafibrate on hepatic cholesterol metabolism in gallstone patients: reduced activity of cholesterol 7 alpha-hydroxylase

Bezafibrate is a hypolipidemic fibric acid derivative known to induce cholesterol supersaturation of bile. To characterize its effects on hepatic cholesterol metabolism, 31 normolipidemic, normal-weight patients with gallstones undergoing cholecystectomy were studied. Eleven patients (5 men) were randomized to treatment with bezafibrate, 200 mg three times daily for 4 weeks before operation; the remaining 20 patients (5 men) served as nontreatment controls. At operation, a liver biopsy specimen was obtained under standardized conditions and several important parameters of cholesterol metabolism were assayed. Bezafibrate treatment lowered total plasma cholesterol and triglycerides 30% and 37%, respectively. The hepatic cholesterol 7 alpha-hydroxylase activity was reduced by approximately 60% in the bezafibrate treated patients compared with the controls, whereas the acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was similar in the two groups. The total 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity was increased twofold in the treated patients, whereas the active enzyme remained about the same as in the controls. The low-density lipoprotein (LDL) receptor binding activity was unaffected by the treatment. Bezafibrate treatment significantly reduces cholesterol 7 alpha-hydroxylase activity, and it is suggested that this may play an important role for the development of supersaturated bile during such therapy.

Effect of ursodeoxycholic acid on hepatic cholesterol metabolism

Oral administration of ursodeoxycholic acid (UDCA) renders bile unsaturated with cholesterol by reducing the hepatic output of cholesterol. Theoretically, several mechanisms may be of importance. In the present overview, the effect of treatment with UDCA on hepatic cholesterol metabolism is evaluated, in particular the influence on hepatic cholesterol synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity, bile acid synthesis, 7 alpha-hydroxylation of cholesterol, and esterification of cholesterol--acyl coenzyme A: cholesterol acetyltransferase (ACAT) activity. It is apparent that UDCA treatment does not inhibit the hepatic HMG CoA reductase activity. Neither is ACAT activity or the cholesteryl ester content changed by UDCA. The catabolism of cholesterol to bile acids is unaffected or slightly increased during administration of UDCA. It is concluded that a stimulated degradation of cholesterol to bile acids may partly explain the decrease in hepatic secretion of cholesterol obtained during UDCA administration. It is suggested that the reduction in cholesterol absorption from the intestine seen during UDCA therapy may also be of importance.

Simultaneous determination of mevalonate and 7 alpha-hydroxycholesterol in human plasma by gas chromatography-mass spectrometry as indices of cholesterol and bile acid biosynthesis

A very sensitive and specific method for the simultaneous determination of mevalonate and 7 alpha-hydroxycholesterol in human plasma is described. The assay is based on isotope dilution mass spectrometry: the extracts from plasma were treated with benzylamine followed by dimethylethylsilylimidazole, then the resulting dimethylethylsilyl ether derivatives of mevalonylbenzylamide and 7 alpha-hydroxycholesterol were determined by gas chromatography-mass spectrometry using high-resolution selected-ion monitoring. Simple regression analysis revealed significant correlations between the plasma level of mevalonate and the hepatic activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.34) (r = 0.83, P < 0.01) and between the plasma level of free 7 alpha-hydroxycholesterol and the hepatic activity of cholesterol 7 alpha-hydroxylase (EC 1.14.13.7) (r = 0.76, P < 0.05).

Hepatic cholesterol metabolism in estrogen-treated men

Operative liver biopsies were obtained from two male patients who developed gallstone disease during estrogen treatment of metastatic prostatic carcinoma. The heparin-sensitive binding of 125I-low-density lipoprotein (LDL) to liver homogenates (reflecting the expression of the LDL receptor) was determined, together with the activities of the rate-limiting enzymes in cholesterol synthesis [3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase], bile acid production (cholesterol 7 alpha-hydroxylase), and cholesterol esterification (acyl CoA:cholesterol acyl transferase). The results were related to data available in 18 patients (5 male, 13 female) who underwent cholecystectomy because of gallstone disease. The hepatic 125I-LDL-binding activity was increased threefold compared with five controls, and the activity of HMG-CoA reductase was increased twofold. There was no major difference in the activities of cholesterol 7 alpha-hydroxylase or acyl CoA:cholesterol acyl transferase. The concentration of free and total cholesterol in liver microsomes was approximately 30% lower in the estrogen-treated men than in 11 controls. The results indicate that estrogen at pharmacological doses stimulates hepatic LDL-receptor expression and HMG-CoA reductase activity in men. The increased LDL-receptor expression could in part explain the enhanced plasma clearance of injected 125I-LDL and hence the reduction in plasma LDL cholesterol previously shown to occur in estrogen-treated men.

7 alpha-hydroxylation of 27-hydroxycholesterol in human liver microsomes

Human liver microsomes were found to catalyze 7 alpha-hydroxylation of 27-hydroxycholesterol at a rate of up to about 0.2 nmol/mg protein per min. The product of the reaction, 5-cholestene-3 beta, 7 alpha, 27-triol, was identified by means of combined gas chromatography-mass spectrometry. Liver microsomes from two patients with an upregulated cholesterol 7 alpha-hydroxylase, did not have higher 7 alpha-hydroxylase activity towards 27-hydroxycholesterol than those from untreated patients, suggesting that the 7 alpha-hydroxylase active towards 27-hydroxycholesterol is not the same as that active towards cholesterol. The mitochondrial fraction of liver from untreated patients and patients treated with cholestyramine, had negligible 7 alpha-hydroxylase activity towards 27-hydroxycholesterol less than 0.01 nmol/mg protein per min). The results are in accord with the possibility that there is a pathway to bile acids in human liver in which the first step is a 27-hydroxylation of cholesterol.