Regulation of hepatic cholesterol metabolism in humans: stimulatory effects of cholestyramine on HMG-CoA reductase activity and low density lipoprotein receptor expression in gallstone patients

Identification of an individual with a SYGNAP1 pathogenic mutation in India

Exome sequencing is a prominent tool to identify novel and deleterious mutations which could be non-sense, frameshift, and canonical splice-site mutations in a specific gene. De novo mutations in SYNGAP1, which codes for synaptic RAS-GTPase activating the protein, causes Intellectual disability (ID) and Autism Spectrum Disorder (ASD). SYNGAP1 related ASD/ID is one of the rare diseases that are detrimental to the healthy neuronal developmental and disrupts the global development of a child. We report the first SYNGAP1 heterozygous patient from Indian cohort. We report a case of a child of 2-year old with global developmental delay, microcephaly subtle dysmorphism, absence seizures, disrupted sleep, delay in learning a language, and eating problems. Upon further validation, the child has a few traits of ASD. Here, based on focused exome sequencing, we report a de novo heterozygous mutation in SYNGAP1 exon 11 with c. 1861 C > T (p.arg621ter). Currently, the child is on Atorvastatin, a RAS inhibitor, already available in the market for the treatment of hypercholesterolemia and has shown considerable improvement in global behaviour and cognitive development. The long-term follow up of the child's development would contribute to the already existing knowledge of the developmental trajectory in individuals with SYNGAP1 heterozygous mutation. In this report, we discuss the finding of a novel mutation in one of the genes, SYNGAP1, implicated in ASD/ID. Besides, we discuss the current treatment prescribed to the patient and the progress of global developmental of the child.

Asynchronous rhythms of circulating conjugated and unconjugated bile acids in the modulation of human metabolism

BACKGROUND AND OBJECTIVES

Bile acids (BAs) traversing the enterohepatic circulation (EHC) influence important metabolic pathways. By determining individual serum BAs in relation to markers of metabolic activity, we explored how diurnal variations in their EHC relate to hepatic metabolism in normal humans.

METHODS

Serum BAs, fibroblast growth factor 19 (FGF19), lipoproteins, glucose/insulin and markers of cholesterol and BA syntheses were monitored for 32 h in 8 healthy males. Studies were conducted at basal state and during initiation of cholestyramine treatment, with and without atorvastatin pretreatment. Time series cross-correlation analysis, Bayesian structural model and Granger causality test were applied.

RESULTS

Bile acids synthesis dominated daytime, and cholesterol production at night. Conjugated BAs peaked after food intake, with subsequent FGF19 elevations. BA synthesis was reduced following conjugated BA and FGF19 peaks. Cholestyramine reduced conjugated BAs and FGF19, and increased BA and cholesterol production; the latter effects attenuated by atorvastatin. The relative importance of FGF19 vs. conjugated BAs in this feedback inhibition could not be discriminated. Unconjugated BAs displayed one major peak late at night/early morning that was unrelated to FGF19 and BA synthesis, and abolished by cholestyramine. The normal suppression of serum triglycerides, glucose and insulin observed at night was attenuated by cholestyramine.

CONCLUSIONS

Conjugated and unconjugated BAs have asynchronous rhythms of EHC in humans. Postprandial transintestinal flux of conjugated BAs increases circulating FGF19 levels and suppresses BA synthesis. Unconjugated BAs peak late at night, indicating a non-postprandial diurnal change in human gut microflora, the physiological implications of which warrants further study.

Understanding the mechanisms of faecal microbiota transplantation

This Review summarizes mechanistic investigations in faecal microbiota transplantation (FMT), which has increasingly been adapted into clinical practice as treatment for Clostridium difficile infection (CDI) that cannot be eliminated with antibiotics alone. Administration of healthy donor faecal microbiota in this clinical situation results in its engraftment and restoration of normal gut microbial community structure and functionality. In this Review, we consider several main mechanisms for FMT effectiveness in treatment of CDI, including direct competition of C. difficile with commensal microbiota delivered by FMT, restoration of secondary bile acid metabolism in the colon and repair of the gut barrier by stimulation of the mucosal immune system. Some of these mechanistic insights suggest possibilities for developing novel, next-generation CDI therapeutics. FMT might also have potential applications for non-CDI indications. The gut can become a reservoir of other potential antibiotic-resistant pathogens under pressure of antibiotic treatments, and restoration of normal microbial community structure by FMT might be a promising approach to protect against infections with these pathogens as well. Finally, FMT could be considered for multiple chronic diseases that are associated with some form of dysbiosis. However, considerable research is needed to optimize the FMT protocols for such applications before their therapeutic promise can be evaluated.

Effects of dietary probiotic supplementation on LXRα and CYP7α1 gene expression, liver enzyme activities and fat metabolism in ducks

1. The objective of this study was to investigate the effects of dietary probiotic supplementation on liver X receptor alpha (LXRα) and cholesterol 7α-hydroxylase (CYP7α1) mRNA levels, protein enzymatic activities and fat metabolism in Cherry Valley Pekin ducks. 2. A total of 750 one-day-old Cherry Valley Pekin ducks were randomly divided into 5 groups with three replicates of 50 ducks each in a completely randomised experiment. Each group was fed on a basal diet supplemented with 0, 500, 1000, 1500 or 2000 mg probiotics/kg. 3. Body rate and feed conversion ratio were highest and abdominal subcutaneous fat % was lowest at 1000 mg probiotic/kg. 4. The mRNA levels of LXRα and CYP7α1 in liver tissue was estimated by RT-PCR; serum triglyceride (TG) and total cholesterol (TC) concentrations were measured by ELISA. 5. The expression levels and enzyme activity of LXRα and CYP7α1 increased in conjunction with decreases in TG and TC concentrations following probiotic supplementation to a maximum at 1000 mg probiotics/kg and decreased thereafter. 6. It is concluded that dietary probiotics can enhance LXRα and CYP7α1 enzyme activities in the liver and reduce lipid concentrations and fat deposition in ducks.

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.

Cholesterol-lowering effects of oat β-glucan

Elevated total and low-density lipoprotein (LDL) cholesterol levels are considered major risk factors for cardiovascular disease. Oat β-glucan, a soluble dietary fiber that is found in the endosperm cell walls of oats, has generated considerable interest due to its cholesterol-lowering properties. The United States Food and Drug Administration (FDA) approved a health claim for β-glucan soluble fiber from oats for reducing plasma cholesterol levels and risk of heart disease in 1997. Similarly, in 2004 the United Kingdom Joint Health Claims Initiative (JHCI) allowed a cholesterol-lowering health claim for oat β-glucan. The present review aims to investigate if results from more recent studies are consistent with the original conclusions reached by the FDA and JHCI. Results of this analysis show that studies conducted during the past 13 years support the suggestion that intake of oat β-glucan at daily doses of at least 3 g may reduce plasma total and low-density lipoprotein (LDL) cholesterol levels by 5-10% in normocholesterolemic or hypercholesterolemic subjects. Studies described herein have shown that, on average, oat consumption is associated with 5% and 7% reductions in total and LDL cholesterol levels, respectively. Significant scientific agreement continues to support a relationship between oat β-glucan and blood cholesterol levels, with newer data being consistent with earlier conclusions made by the FDA and JHCI.

Beta-glucan- or rice bran-enriched foods: a comparative crossover clinical trial on lipidic pattern in mildly hypercholesterolemic men

BACKGROUND/OBJECTIVES

There has been growing interest in using dietary intervention to improve the lipid profile. This work aims at analyzing the effects and the comparison of the enrichment of a diet with beta-glucans or rice bran in mildly hypercholesterolemic men.

SUBJECTS/METHODS

The subjects initially consumed a 3-week Step 1 American Heart Association diet with rice bran-enriched foods. After this adaptation period, volunteers were randomly assigned to follow a crossover, controlled trial that consisted of two treatment with beta-glucan- or rice bran-enriched foods, each of 4 weeks, with a 3-week wash-out, like the adaptation period, between periods. Fasted blood samples were collected on days 0, 21, 49, 70 and 98 in both study arms for measuring low-density lipoprotein (LDL)-cholesterol (primary outcome), total cholesterol, high-density lipoprotein (HDL)-cholesterol, triglycerides, apolipoprotein (apo) A-I, apo B and glucose levels.

RESULTS

Twenty-four men (mean age: 50.3±5.3, mean body mass index: 24.9±1.9) completed the 14-week trial. Subjects in the 3-week adaptation period experienced significant reductions in the mean level of LDL cholesterol, total cholesterol, total cholesterol/HDL cholesterol, LDL cholesterol/HDL cholesterol, apo A-I, apo A-I/apo B and glucose. During the intervention diet periods, a difference was found between treatment groups for the mean change in LDL (0.21 (95% confidence interval (CI): 0.02-0.40), P=0.033) and total cholesterol (0.34 (95% CI: 0.20-0.47), P<0.001). Other parameters evaluated were not significantly affected by the diet consumed.

CONCLUSIONS

The results of the present crossover clinical trial showed that beta-glucan-enriched foods are more effective in lowering serum LDL levels, compared with rice bran-enriched foods.

Hepatic gene expression related to lower plasma cholesterol in hamsters fed high-fat diets supplemented with blueberry peels and peel extract

This study analyzed plasma lipid profiles, genes related to cholesterol and bile acid metabolism, and inflammation in liver as well as adipose tissue from Syrian Golden hamsters fed high-fat diets supplemented with blueberry (BB) pomace byproducts including 8% dried whole blueberry peels (BBPWHL), 2% dried extract of peels (BBPX; 95% ethanol extract), and 6% residue from extracted peel (BBPEXT) compared to a diet containing 5% (w/w) microcrystalline cellulose (control). All BB diets significantly lowered plasma very low density lipoprotein cholesterol and total cholesterol concentrations. Interestingly, BB diets increased fecal lipid excretion. Hepatic CYP7A1 expression was up-regulated by all BB diets, and the expression of CYP51 was up-regulated by BBPX and BBPEXT diets, suggesting that both bile acid and cholesterol synthesis were increased. No significant changes in adipocyte gene expression related to inflammatory markers were observed with any BB diet. These data suggest that hepatic modulation of bile acid and cholesterol synthesis primarily contributes to the cholesterol-lowering effect of BB pomace byproducts.

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.

CYP7A1 promoter polymorphism -203A>C affects bile salt synthesis rate in patients after ileal resection

Cholesterol 7alpha-hydroxylase (CYP7A1) plays a crucial role in cholesterol metabolism and has been implicated in genetic susceptibility to atherosclerosis. Thus, an understanding of its transcriptional regulation is of considerable importance. We evaluated the effect of a common -203A>C polymorphism in the CYP7A1 promoter region on the activity of CYP7A1, estimated as the ratios of serum 7alpha-hydroxycholest-4-en-3-one (C4) to either total or non-HDL-cholesterol. The study was performed on patients after resection of the distal ileum, leading to upregulation of CYP7A1 activity (n = 65). Healthy volunteers served as the control group (n = 66). Whereas higher CYP7A1 activity was associated with the -203A allele in the patient group (C4/cholesterol ratio, 29.0 vs. 14.8 microg/mmol, P = 0.032; C4/non-HDL-cholesterol ratio, 53.3 vs. 21.3 microg/mmol in -203AA and -203CC, P = 0.017, respectively), no differences were observed in the healthy controls. We conclude that under physiological conditions, the -203A>C polymorphism in the CYP7A1 gene promoter region does not seem to have any clinically relevant effect. However, in patients with severe bile salt malabsorption, this polymorphism markedly affects CYP7A1 activity.

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.

Bile acids and lipoprotein metabolism: effects of cholestyramine and chenodeoxycholic acid on human hepatic mRNA expression

Modulation of bile acid synthesis in human by cholestyramine or by chenodeoxycholic acid (CDCA) treatment affects lipoprotein metabolism leading to altered plasma lipid levels. The molecular changes caused by these treatments, which in turn influence lipoprotein metabolism, are still not entirely known in humans. In this study, mRNA levels were analyzed using real time RT-PCR in liver tissue from patients undergoing cholecystectomy due to gallstone disease. The patients were treated with either CDCA (n=6) or cholestyramine (n=5) for three weeks prior to surgery, six patients received no treatment and served as controls. Cholestyramine increased the expression of the LDL receptor (LDLR) by about 65% and that of proprotein convertase subtilisin kexin 9 (PCSK9) by 70%. After CDCA the levels of both LDLR and hydroxy-methyl-glutaryl coenzyme A reductase mRNA decreased approximately by 50%. The expression of PCSK9 was not changed. The mRNA levels of PCSK9, LDLR, and HMGCoAR were significantly correlated to those of sterol regulatory element binding protein 2 (SREBP2), indicating that SREBP2 is of importance in the regulation of the expression of these genes also in human liver.

Simultaneous intake of beta-glucan and plant stanol esters affects lipid metabolism in slightly hypercholesterolemic subjects

Intake of food products rich in water-soluble fiber beta-glucan and products enriched with plant stanol esters lower serum cholesterol. Combining 2 functional food ingredients into one food product may achieve additional reductions of serum cholesterol. Our objective was to investigate the effects of a simultaneous intake of beta-glucan plus plant stanol esters on lipid metabolism in mildly hypercholesterolemic volunteers. In a randomized, controlled, 3-period crossover study, 40 mildly hypercholesterolemic men and women received muesli in random order twice a day for 4 wk, which provided, in total, 5 g control fiber from wheat (control muesli), 5 g oat beta-glucan (beta-glucan muesli), or 5 g oat beta-glucan plus 1.5 g plant stanols (combination muesli). beta-Glucan muesli decreased serum LDL cholesterol by 5.0% compared with control muesli (P = 0.013). Combination muesli reduced LDL cholesterol by 9.6% compared with control muesli (P < 0.001), and by 4.4% compared with beta-glucan muesli (P = 0.036). Serum HDL cholesterol and triacylglycerol concentrations did not differ after the 3 treatments. Compared with control muesli, beta-glucan muesli increased bile acid synthesis (P = 0.043) and decreased cholesterol absorption (P = 0.011). Addition of plant stanols did not influence bile acid synthesis but decreased cholesterol absorption (P < 0.001) and raised cholesterol synthesis (P = 0.016) compared with control muesli, and the plant stanols decreased cholesterol absorption compared with beta-glucan muesli (P = 0.004). The combination muesli decreased serum concentrations of sitostanol compared with control muesli (P = 0.010). Plasma concentrations of lipid-soluble antioxidants did not differ after the 3 treatments. beta-Glucan muesli effectively lowered serum LDL cholesterol concentrations. The addition of plant stanol esters to beta-glucan-enriched muesli further lowered serum LDL cholesterol, although effects were slightly less than predicted.

Inhibition of 3-Hydroxy-3-Methylglutaryl Coenzyme A (HMG CoA) Reductase Blunts Factor VIIa/Tissue Factor and Prothrombinase Activities via Effects on Membrane Phosphatidylserine

OBJECTIVE

3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors (statins) exhibit antithrombotic properties that are independent of reductions in circulating LDL cholesterol. We hypothesized that these antithrombotic properties are mediated by membrane alterations secondary to disrupted lipid metabolism.

METHODS AND RESULTS

EA.hy926 cells were incubated in the presence of 1 micromol/L atorvastatin supplemented with fetal bovine serum or lipid-depleted serum mixtures. Lipid restriction alone had no effect on cell lipid composition but when atorvastatin was included, phosphatidylserine, sphingomyelin, and cholesterol were reduced by 50% while ceramide content decreased by 70%. These changes in lipid composition did not alter the association of decay accelerating factor or tissue factor with lipid rafts. Atorvastatin in combination with lipid restriction reduced factor VIIa/tissue factor activity by as much as 75% but did not alter tissue factor expression. Prothrombinase activity was reduced to an extent similar to factor VIIa/tissue factor. Mevalonic acid but not LDL reversed the observed changes in lipid content and prothrombinase activity induced by atorvastatin. These findings were confirmed in primary cells.

CONCLUSIONS

Inhibition of HMG-CoA reductase limits exposure of phosphatidylserine at the cell surface by restricting the cellular pool of mevalonate-derived isoprenoids. This membrane alteration restricts the activity of proteolytic enzyme complexes that propagate the coagulation cascade.

Regulation of hepatic cholesterol metabolism in CETP/LDLr mice by cholesterol feeding and by drugs (cholestyramine and lovastatin) that lower plasma cholesterol

1. The hepatic mechanisms involved in the simultaneous regulation of plasma cholesterol concentration and cholesteryl ester transfer protein (CETP) activity were investigated by sharply modifying the hepatic rates of cholesterol synthesis. This was accomplished by cholestyramine, lovastatin and cholesterol feeding in human CETP transgenic mice cross-bred with low-density lipoprotein receptor (LDLr)-knockout mice, generating CETP(+/-)/LDLr(+/-) mice, which present a plasma lipoprotein profile resembling that of humans. 2. Analyses of pooled data showed that the plasma CETP activity correlated positively with plasma total cholesterol concentration, hepatic CETP mRNA and the liver microsomal cholesterol content; a negative correlation was found between plasma CETP activity and the liver 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and LDLr mRNA levels. These coordinated events represent an efficient control system that stabilizes the cell cholesterol content. 3. Nonetheless, not all cholesterol metabolism regulatory systems seem to fit into a coherent pattern of responses, suggesting that other unknown cellular mechanisms play roles depending on the type of pharmacological intervention. 4. For example, microsomal cholesterol content was not affected by cholestyramine, but was increased on cholesterol feeding (as predicted), and, surprisingly, on lovastatin treatment. Furthermore, although both plasma cholesterol-lowering drugs increased CYP7A1 mRNA and had no effect on CYP27 mRNA, other metabolic components were differentially modified. Cholestyramine and lovastatin, respectively, did not modify and increased both HMG-CoA and sterol responsive element binding protein 1c mRNA, did not modify and lowered liver X receptor alpha mRNA, lowered and increased ATP binding cassette A1 mRNA and lowered and did not modify scavenger receptor B1 mRNA. 5. That is, different to unabsorbed cholestyramine, lovastatin, as an absorbed plasma cholesterol-lowering drug, may have modified the activity of other unknown genes that play roles in the interaction of CETP with the metabolism of hepatic cholesterol.

Cytochrome P450s and cholesterol homeostasis

By participating in pathways of cholesterol biosynthesis and elimination, different cytochrome P450 (P450 or CYP) enzymes play an important role in maintenance of cholesterol homeostasis. CYP51 is involved in cholesterol biosynthesis, whereas CYP 7A1, 27A1, 46A1, 7B1, 39A1, and 8B1 are the key enzymes in cholesterol catabolism to bile acids, the major route of cholesterol elimination in mammals. Cholesterol transformations to steroid hormones are also initiated by the P450 enzyme CYP11A1. Finally, one of the major drug-metabolizing P450s CYP3A4 seems to contribute to bile acid biosynthesis as well. The 9 P450s will be the focus of this review and assessed as drug targets for cholesterol lowering.

Role of cholesterol 7alpha-hydroxylase (CYP7A1) in nutrigenetics and pharmacogenetics of cholesterol lowering

The relationship between dietary composition/cholesterol-lowering therapy and final plasma lipid levels is to some extent genetically determined. It is clear that these responses are under polygenic control, with multiple variants in many genes participating in the total effect (and with each gene contributing a relatively small effect). Using different experimental approaches, several candidate genes have been analyzed to date.Interesting and consistent results have been published recently regarding the A-204C promoter variant in the cholesterol 7alpha-hydroxylase (CYP7A1) gene. CYP7A1 is a rate-limiting enzyme in bile acid synthesis and therefore plays an important role in maintaining cholesterol homeostasis. CYP7A1-204CC homozygotes have the greatest decrease in total cholesterol level in response to dietary changes in different types of dietary intervention studies. In contrast, one study has reported that the effect of statins in lowering low-density lipoprotein (LDL)-cholesterol levels was slightly greater in -204AA homozygotes. The CYP7A1 A-204C variant accounts for a significant proportion of the genetic predisposition of the response of plasma cholesterol levels.

Use of an indirect effect model to describe the LDL cholesterol-lowering effect by statins in hypercholesterolaemic patients

Statins are the most commonly prescribed agents for the treatment of hypercholesterolaemia. This is due to their efficacy in reducing low-density lipoprotein cholesterol (LDL) level which is the primary goal of the treatment especially for patients with multiple risk factors or with established coronary heart diseases. The purpose of this study was to develop a pharmacokinetic/pharmacodynamic (PK/PD) model that describes the LDL-lowering process in patients with hypercholesterolaemia treated with atorvastatin, fluvastatin or simvastatin. A total of 100 patients were studied retrospectively. They received atorvastatin (n = 57), fluvastatin (n = 26) or simvastatin (n = 17). As no pharmacokinetic data were available, the absorption rate was fixed to 1/h and atorvastatin, simvastatin and fluvastatin elimination half-lives were fixed to 14, 2 and 2.5 h respectively. A total of 309 LDL levels were measured and the data were analysed by nonmem v. The time course of the LDL-lowering effect of statins was described by an indirect-response model with precursor (LDL synthesis, input rate K(in)) and response (circulating LDL, input and output rates K) compartments. The following parameters were estimated: LDL input rate (K(in)) 0.14 +/- 0.015 g/L/day (mean +/- SD); inhibition fraction of K(in) (INH) 0.21 +/- 0.017; and dose producing 50% increase of LDL removal (D50), 26 +/- 7.8, 1.3 +/- 0.48 and 15 +/- 5.25 mg for atorvastatin, simvastatin and fluvastatin, respectively. Gender, bodyweight, age, calories/day, sugar/day, lipids/day, hyperlipidaemia types and waist/hip circumference, renal and hepatic functions had no effect on the pharmacodynamic parameters. The pharmacodynamic parameters for the three statins were accurately estimated. The PK/PD model developed successfully predicted the time course of the LDL-lowering effect of statins.

Guinea pigs: a suitable animal model to study lipoprotein metabolism, atherosclerosis and inflammation

Numerous animal models have been used to study diet effects on cholesterol and lipoprotein metabolism. However, most of those models differ from humans in the plasma distribution of cholesterol and in the processing of lipoproteins in the plasma compartment. Although transgenic or knock-out mice have been used to study a specific pathway involved in cholesterol metabolism, these data are of limited use because other metabolic pathways and responses to interventions may differ from the human condition. Carbohydrate restricted diets have been shown to reduce plasma triglycerides, increase HDL cholesterol and promote the formation of larger, less atherogenic LDL. However, the mechanisms behind these responses and the relation to atherosclerotic events in the aorta have not been explored in detail due to the lack of an appropriate animal model. Guinea pigs carry the majority of the cholesterol in LDL and possess cholesterol ester transfer protein and lipoprotein lipase activities, which results in reverse cholesterol transport and delipidation cascades equivalent to the human situation. Further, carbohydrate restriction has been shown to alter the distribution of LDL subfractions, to decrease cholesterol accumulation in aortas and to decrease aortic cytokine expression. It is the purpose of this review to discuss the use of guinea pigs as useful models to evaluate diet effects on lipoprotein metabolism, atherosclerosis and inflammation with an emphasis on carbohydrate restricted diets.

CHOLESTEROL-METABOLIZING CYTOCHROMES P450

By catalyzing the first steps in different pathways of cholesterol degradation, cytochromes P450 (P450s) 7A1, 27A1, 11A1, and 46A1 play key roles in cholesterol homeostasis. CYP7A1 is a microsomal liver-specific enzyme that converts cholesterol to 7alpha-hydroxycholesterol. CYP27A1 is a ubiquitously expressed mitochondrial P450 that metabolizes cholesterol to 27-hydroxycholesterol. CYP11A1 also resides in mitochondria but is expressed mainly in steroidogenic tissues, where it catalyzes the conversion of cholesterol to pregnenolone. Finally, CYP46A1 is a brain-selective microsomal monooxygenase producing 24S-hydroxycholesterol from cholesterol. Catalytic efficiencies of cholesterol-metabolizing P450s vary significantly and probably reflect physiological requirements of different organs for the rate of cholesterol turnover. P450s 7A1, 27A1, 11A1, and 46A1 represent a unique system for elucidation of how different enzymes have adapted to fit their specific roles in cholesterol elimination. Studies of cholesterol-metabolizing P450s suggest that their activities could be modulated post-translationally and that they should also be considered as targets for regulation of cholesterol homeostasis.

CYP7A1 A-278C polymorphism affects the response of plasma lipids after dietary cholesterol or cafestol interventions in humans

The response of plasma lipids to dietary cholesterol and fat varies among individuals. Variations in genes involved in cholesterol metabolism can be important in these interindividual differences. The rate-limiting enzyme in the conversion of cholesterol into bile acids is cholesterol 7alpha-hydroxylase (CYP7A1). We investigated the effect of the A278-C promoter polymorphism in the CYP7A1 gene on responses of plasma lipids to an increased intake in dietary cholesterol (742 +/- 114 mg/d), cafestol (57 +/- 6 mg/d), saturated fat [change of 8-9 energy percent/d (en%/d)] and trans fat (change of 10-11 en%/d) in 496 normolipidemic subjects. These responses were measured in 26 previously published dietary trials. After adjustment for the apolipoprotein E genotype effect, AA-subjects consuming a cholesterol-rich diet had a smaller increase in plasma HDL cholesterol than CC-subjects (0.00 +/- 0.02 vs. 0.17 +/- 0.04 mmol/L; P < 0.001). Upon intake of cafestol, AA-subjects had a smaller increase in plasma total cholesterol than CC-subjects (0.69 +/- 0.10 vs. 1.01 +/- 0.10 mmol/L; P = 0.028). No effects of the polymorphism were found in the saturated and trans fat interventions. In conclusion, the CYP7A1 polymorphism has a small but significant effect on the increase in plasma HDL cholesterol and plasma total cholesterol after an increased intake of dietary cholesterol and cafestol, respectively.

Guinea pigs as models to study the hypocholesterolemic effects of drugs

Guinea pigs are useful models to investigate the mechanisms of the hypocholesterolemic effects of drugs. Like humans, guinea pigs are one of the few species that carry the majority of cholesterol in LDL. This animal model has also been shown to develop atherosclerosis when challenged with hypercholesterolemic diets. In addition, plasma lipid profiles in males, females and ovariectomized guinea pigs, a model for menopause, follow similar patterns to those observed in humans. In this report, drugs aimed at lowering plasma cholesterol and triglycerides in hyperlipidemic individuals are reviewed. Studies analyzing the hypolipidemic effect of HMG-CoA reductase inhibitors, acyl CoA cholesterol acyltransferase inhibitors, fibrates, bile acid resins, apical sodium bile acid transporter inhibitors, and others show that guinea pigs and humans have comparable responses to drug therapy. In addition, results from the limited clinical reports addressing specific effects of drugs on LDL catabolism or VLDL synthesis are in agreement with observations in guinea pigs. From the review of these studies, it is apparent that the guinea pig is a useful animal model to further explore the mechanisms of action of lipid lowering drugs including effects on specific receptors and regulatory enzymes involved in cholesterol metabolism and on early atherosclerosis development.

ABBREVIATIONS

ACAT, acyl-CoA:cholesterol acyltransferase; ASBT, apical sodium co-dependent bile acid transporter; ApoB, apolipoprotein B; CHD, coronary heart disease; CYP7, cholesterol 7alpha-hydroxylase; HDL, high density lipoprotein; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; FCR, free catabolic rate; LDL, low density lipoprotein; PPAR, peroxisome proliferators-activated receptor; TC, total cholesterol; TG, triglycerides; VLDL, very low density lipoprotein.

Polymorphisms in CYP-7A1, not APOE, influence the change in plasma lipids in response to population dietary change in an 8 year follow-up; results from the Czech MONICA study

OBJECTIVES

To evaluate the influence of variation in the genes for apolipoprotein E (APOE; epsilon2, epsilon3, epsilon4) and cholesterol-7alpha hydroxylase (CYP-7A1; -204A-->C) on plasma lipid level changes.

DESIGN AND METHODS

131 males for whom dietary composition markedly changed and total cholesterol decreased (from 6.21 +/- 1.31 mmol/L in 1988 - 5.43 +/- 1.06 mmol/L in 1996) over an 8 yr follow-up study. Polymorphisms were investigated using PCR.

RESULTS

APOE genotype influenced plasma total and LDL cholesterol, with carriers of the epsilon4 having the highest and epsilon2 carriers the lowest levels, this reached borderline significance for cholesterol in 1988 (p = 0.06) and strongly affected the 1996 levels of LDL cholesterol (p = 0.008). However, APOE did not influence the change in these measures over time. In contrast, the CYP-7A1 -204A-->C polymorphism did not affect lipid measures per se but was strongly associated with a decrease in plasma total cholesterol [AA -0.38 (+/- 0.20) mmol/L, AC -0.65 +/- (0.08), CC -1.33 (+/- 0.3) mmol/L, p = 0.01] over the 8 yr time period.

CONCLUSIONS

Variation in the CYP-7A1 gene may play an important role in an individual's sensitivity to dietary composition.

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.

Oat bran stimulates bile acid synthesis within 8 h as measured by 7alpha-hydroxy-4-cholesten-3-one

BACKGROUND

Oat bran contains soluble fibers, such as beta-glucan, that increase bile acid excretion and thus decrease serum cholesterol. Bile acid synthesis correlates with serum concentrations of the metabolite 7alpha-hydroxy-4-cholesten-3-one (alpha-HC).

OBJECTIVE

The objective was to investigate whether consumption of beta-glucan from oat bran increases bile acid synthesis, as measured by the serum alpha-HC concentration, within hours after consumption in response to the loss of bile acids from the liver.

DESIGN

In a randomized, single-blind, wheat bran-controlled study with crossover design, 8 subjects were served a controlled diet during 2 periods of 3 d each, with an 11-d washout between the periods. Breakfast included either 75 g extruded oat bran, of which 11 g was beta-glucan, or 75 g wheat bran, of which 1 g was beta-glucan. Alpha-HC was measured by HPLC on each day at 0, 12, and 24 h after breakfast and also at 8 h after breakfast on the first day.

RESULTS

After 8 and 12 h of the oat bran diet period, the serum alpha-HC concentration was 84% (P = 0.012) and 92% (P = 0.017) higher, respectively, than that before breakfast. Serum concentrations returned to the baseline value after 24 h. Wheat bran did not influence serum alpha-HC concentrations.

CONCLUSIONS

Consumption of beta-glucan from oat bran nearly doubled the serum alpha-HC concentration within 8 h, indicating increased bile acid synthesis. alpha-HC in serum could be used as a marker of increased bile acid excretion induced by the diet.

Guinea pigs as models for cholesterol and lipoprotein metabolism

Guinea pigs carry the majority of their plasma cholesterol in LDL, making them a unique animal model with which to study hepatic cholesterol and lipoprotein metabolism. In this review, the benefits and advantages of using this particular model are discussed. How dietary factors such as soluble fiber, cholesterol and fatty acids that vary in saturation and chain length affect hepatic cholesterol homeostasis and influence the synthesis, intravascular processing and catabolism of lipoproteins is reviewed. In addition, alterations in hepatic cholesterol metabolism and plasma lipoproteins as affected by treatment with cholestyramine or 3-hydroxyl-3-methylglutaryl coenzyme A reductase inhibitors, exercise, marginal intake of vitamin C, ovariectomy (a model for menopause) and similarities to the human situation are addressed. A review of guinea pigs as models for early atherosclerosis development is also presented.

Acute intraduodenal bile salt depletion leads to strong gallbladder contraction, altered antroduodenal motility and high plasma motilin levels in humans

Cholecystokinin is the main hormone involved in postprandial gallbladder contraction. There is also considerable gallbladder contraction in the fasting state, associated with phase III of the gastrointestinal migrating motor complex and release of the hormone motilin. It has been proposed that intraduodenal bile salts exert a negative-feedback control on postprandial cholecystokinin release and resulting gallbladder contraction. We wanted to elucidate whether a similar control mechanism on gallbladder contraction exists in the fasting state. We therefore performed gallbladder ultrasonography and 24-h antroduodenal motility registrations and determined plasma cholecystokinin and motilin levels in six healthy subjects before and after acute (4 g) and chronic (8 days; 8 g day(-1)) oral cholestyramine. Acute cholestyramine strongly decreased gallbladder volumes and increased motilin without changed cholecystokinin levels. There was a negative relationship between gallbladder volumes and plasma motilin levels. Although there was a persistent fasting pattern of antroduodenal motility, its cycle length was increased (P < 0.03) with markedly longer phase II (P < 0. 005). Fasting gallbladder volumes 24 h later were still strongly decreased but gradually increased to pretreatment levels. Before and after 8 days cholestyramine, interdigestive and postprandial gallbladder emptying, intestinal migrating motor complex and hormone levels did not differ. We conclude that acute (but not chronic) intraduodenal bile salt depletion with cholestyramine affects gallbladder and antroduodenal motility, possibly partly related to motilin release.

Gender and hormonal status affect the hypolipidemic mechanisms of dietary soluble fiber in guinea pigs

The objective of this study was to assess the effects of gender on the secondary mechanisms by which dietary soluble fiber lowers plasma LDL cholesterol. For that purpose, male, female and ovariectomized (to mimic menopause) guinea pigs (8-10 per group) were allocated to two dietary treatments. Diets were identical in composition except for the fiber source: the control diet contained 10 g/100 of cellulose and 2.5 g/100 g of guar gum, while the soluble fiber (SF) diet contained 5 g/100 of psyllium, 5 g/100 of pectin and 2.5 g/100 g of guar gum. SF intake resulted in 44% lower plasma LDL cholesterol, 64% lower apo B and 22% lower plasma triacylglycerol (TAG) concentrations (P < 0.01) compared to guinea pigs fed the control diet. However, ovariectomized guinea pigs had higher plasma cholesterol, apo B and TAG concentrations (P < 0.01) compared to males and females, even those fed SF. Plasma HDL-cholesterol was higher in females than in males (P < 0.05). LDL size, as measured by LDL composition and fast protein liquid chromatography, was larger in females than males. Guinea pigs fed SF had smaller LDL than controls. LDL susceptibility to oxidation was 80% lower in male and females fed the SF diet (P < 0.001) than in controls, while there was no effect of diet in ovariectomized guinea pigs. Hepatic free cholesterol and TAG were lower, and activities of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase and cholesterol 7alpha-hydroxylase were higher in guinea pigs fed SF (P < 0.05) than in controls. These results indicate that gender plays an important role in the metabolic responses to dietary soluble fiber and that estrogen deprivation leads to a detrimental lipoprotein profile.

Effects of cholestyramine on hepatic cholesterol 7alpha-hydroxylase and serum 7alpha-hydroxycholesterol in the hamster

Cholestyramine increases activities of hepatic cholesterol 7alpha-hydroxylase and serum levels of 7alpha-hydroxycholesterol. To examine if serum 7alpha-hydroxycholesterol levels parallel with enzyme activity, 0, 0.5, 1, 2, 4, and 10% of cholestyramine was administered to female golden Syrian hamsters for 28 d in the dose-dependent study, and 2% cholestyramine for 0, 1, 3, 7, 14, 21, and 28 d in the time-dependent study. In the dose-dependent study, hepatic and serum cholesterol levels were significantly decreased dose-dependently when more than 0.5% of cholestyramine was fed for 28 d. Cholestyramine increased the cholesterol 7alpha-hydroxylase activity in a dose-dependent manner, while the serum 7alpha-hydroxycholesterol level was essentially unchanged. No correlation was found between the serum level and the hepatic enzyme activity. In the time-dependent study, hepatic and serum cholesterol levels markedly decreased when 2% cholestyramine was fed for longer than 3 d. The serum triglyceride level increased significantly for the first 7 d and then decreased. Cholesterol 7alpha-hydroxylase activity increased significantly as early as day 1, reached maximum activity level on day 7, and then kept the significantly high values until day 28. The serum 7alpha-hydroxycholesterol level significantly increased for the first 7 d and decreased to the pretreatment level thereafter. 7Alpha-hydroxycholesterol levels significantly correlated with serum cholesterol and triglyceride levels. We conclude that the serum 7alpha-hydroxycholesterol level does not always reflect the activity of hepatic cholesterol 7alpha-hydroxylase, when cholesterol metabolism is severely disturbed by cholestyramine.

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.

The level of 7-dehydrocholesterol in plasma reflects the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in the human liver

Plasma levels of the cholesterol precursor 7-dehydrocholesterol (7-DHC) were compared with activities of the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase assayed in liver biopsies from patients undergoing cholecystectomy. Some patients were treated with cholestyramine, deoxycholic acid or chenodeoxycholic acid prior to surgery in order to alter the activity of the enzyme. The median level of 7-DHC and the activity of HMG-CoA reductase in the untreated group were 55 ng/ml and 98 pmol/min/mg protein, respectively. The sterol levels and enzyme activities were increased in patients treated with cholestyramine (85 ng/ml and 439 pmol/min/mg protein) and deoxycholic acid (86 ng/ml and 173 pmol/min/mg protein) and decreased in patients treated with chenodeoxycholic acid (38 ng/ml and 51 pmol/min/mg protein). There was a strong positive correlation (rs=0.75, P<0.0005) between the plasma levels of 7-DHC and the activities of hepatic HMG-CoA reductase in these patients. This correlation was further improved when the plasma levels of 7-DHC were expressed relative to those of cholesterol (rs=0.90, P<0.0001). The results show that the level of 7-DHC in plasma reflects the activity of HMG-CoA reductase in the liver.

Overexpression of cholesterol 7alpha-hydroxylase (CYP7A) in mice lacking the low density lipoprotein (LDL) receptor gene. LDL transport and plasma LDL concentrations are reduced

This study was undertaken to determine the effect of transient overexpression of hepatic cholesterol 7alpha-hydroxylase on low density lipoprotein (LDL) cholesterol transport in mice lacking LDL receptors (LDL receptor-/-). Primary overexpression of hepatic 7alpha-hydroxylase in LDL receptor-/- mice was accompanied by a dose-dependent decrease in the rate of LDL cholesterol appearance in plasma (whole body LDL cholesterol transport) and a corresponding reduction in circulating LDL cholesterol levels. The increase in hepatic 7alpha-hydroxylase activity necessary to achieve a 50% reduction in plasma LDL cholesterol concentrations was approximately 10-fold. In comparison, cholestyramine increased hepatic 7alpha-hydroxylase activity approximately 3-fold and reduced plasma LDL cholesterol concentrations by 17%. This study demonstrates that augmentation of hepatic 7alpha-hydroxylase expression is an effective strategy for lowering plasma LDL concentrations even in animals with a genetic absence of LDL receptors.

Effects of dietary cholesterol and triglycerides on lipid concentrations in liver, plasma, and bile

Dietary cholesterol (CHL) and triglycerides (TG) can influence plasma, hepatic, and biliary lipid composition, but effects on lipids in these three compartments during the early stages of CHL gallstone formation have not been studied in parallel. We fed prairie dogs diets containing one of four test oils (safflower, coconut, olive, or menhaden) at either 5 or 40% of calories, in the presence of 0 or 0.34% CHL, for 3 wk. In the absence of dietary CHL, increases in dietary TG produced 50-200% increases in the concentrations of biliary CHL and hepatic cholesteryl ester (CE), while the concentrations of hepatic free CHL (FC) as well as plasma FC and CE remained relatively unchanged. Increasing dietary CHL to 0.34% resulted in increases in hepatic FC of approximately 50% for all four fats regardless of whether they were supplied at 5 or 40% of calories. CHL supplementation caused more pronounced increases in biliary CHL (200-400%), hepatic CE (50-200%), plasma FC (up to 100%), and plasma CE (up to 150%), and these increases were exacerbated by concurrent supplementation of dietary fat and CHL (biliary CHL: 300-700%; hepatic CE: 100-250%; plasma FC: up to 165%; plasma CE: 100-350%). These results indicate that enhanced secretion of biliary CHL and, to a lesser extent, increased synthesis of hepatic CE, may be primary mechanisms for maintaining the hepatic FC pool. Furthermore, dietary CHL and high levels of fat intake are independent risk factors for increasing biliary CHL concentrations, and adverse effects on lipid concentrations in plasma and bile tend to be exacerbated by ingestion of diets rich in both fat and CHL.

Hepatic adenine nucleotides and microsomal cholesterol 7 alpha-hydroxylase activity in the obstructed and freely draining lobes of the liver after selective bile duct obstruction

BACKGROUND

The effect of selective bile duct obstruction (SBDO) on hepatic reserve function of the bile duct obstructed (BDO) and nonobstructed freely draining (FD) lobes of the liver is obscure.

METHODS

The bile duct branches draining from the left lateral and median lobes of the liver were ligated for 4 and 10 days in rats, and hepatic reserve functions in BDO and FD lobes were assessed by microsomal cholesterol 7 alpha-hydroxylase activities and by hepatic adenine nucleotides and energy charge levels. The values were compared with those in the sham-operated control liver. Cholesterol 7 alpha-hydroxylase activities were determined by gas-liquid chromatography--mass spectrometry, and hepatic adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) levels with high-pressure liquid chromatography.

RESULTS

The histological examination of the BDO lobes showed proliferation and formation of new bile ductules and fibrous connective tissues linking portal areas. Microsomal cholesterol 7 alpha-hydroxylase activities, hepatic energy charge and each adenine nucleotide level did not differ between FD and BDO lobes, and the values were similar to those in the sham-operated liver.

CONCLUSIONS

Selective bile duct obstruction shows no adverse effects on microsomal and mitochondrial functions in both the BDO and FD lobes of the liver.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Hepatic bile acid synthesis and DNA synthetic rate after partial hepatectomy

The relationship between hepatic DNA synthetic rate and activity of cholesterol 7 alpha-hydroxylase, the rate limiting enzyme for bile acid synthesis, was examined in regenerating liver after partial hepatectomy or sham operation in rats. Hepatic cholesterol 7 alpha-hydroxylase activity was significantly (P < 0.001) suppressed on days 1 and 2, returned to the control level on day 3, and was significantly raised on day 7 after hepatectomy. The rate of DNA synthesis was significantly (P < 0.001) activated during the first 3 days after hepatectomy and returned to the control level on day 7. Enzyme activities regulating hepatic bile acid synthesis and DNA synthesis change inversely during liver regeneration after hepatectomy.

Effects of simvastatin and cholestyramine on bile lipid composition and gall bladder motility in patients with hypercholesterolaemia

Although the effects of 3-hydroxy, 3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors and bile acid sequestrants on bile lipid composition have been studied separately, no data are available on combination therapy of these drugs. Moreover, the effects of prolonged (four weeks) administration of these drugs on gall bladder motility, an important determinant of cholesterol gall stone formation, have not been studied so far. A prospective study was therefore performed with eight patients who had hypercholesterolaemia (age 53 (5) (SEM), body mass index 27.4 (1.1) kg m-2, low density lipoprotein cholesterol 5.9 (0.3) mmol/l). They received treatment during three periods of four weeks with simvastatin 20 mg/day, cholestyramine 4 g twice daily, and a combination of both in random order, each treatment period separated by a two week wash out period. Before treatment and after each treatment period, postprandial gall bladder motility was studied with ultrasound, followed by duodenal bile sampling. Serum cholesterol decreased in all subjects in any treatment period illustrating good compliance. Molar percentages in duodenal bile of cholesterol, phospholipids, and bile salts were unchanged during simvastatin and cholestyramine treatment. During combined therapy percentage bile salts was lower (72.5 (2.9)% v 77.8 (1.7)% at baseline, p < 0.05) whereas phospholipids were higher (21.2 (2.4)% v 16.4 (1.3)% at baseline, p < 0.05). As a result cholesterol saturation index (CSI) did not change in any treatment period. No cholesterol crystals were detected in any bile sample, taken at baseline and after each treatment period. Bile salt hydrophobicity index during cholestyramine (0.19 (0.02)) and combined treatment (0.22 (0.01)) decreased strongly compared with baseline (0.34 (0.01), p < 0.001, p < 0.01, respectively), resulting from increased proportions of glycocholate (59.4 (3.9)% (cholestyramine), 55.6 (2.4)% (combination), and 28.2 (2.2) (baseline), p < 0.001)) and decreased proportions of deoxycholic acid and chenodeoxycholic acid. Fasting gall bladder volume was increased during simvastatin (28.7 (2.8) ml) v baseline (23.2 (2.3) ml, p < 0.01) whereas, residual volume did not differ (5.7 (0.9) ml (simvastatin) v 5.9 (0.7) (baseline). During cholestyramine and combined treatment, no significant differences in gall bladder motility were seen. In conclusion, this study suggests that HMG-CoA reductase inhibitors alone and combined with cholestyramine do not affect major determinants of cholesterol gall stone formation, for example, CSI and gall bladder emptying. In addition cholestyramine alone and combined with simvastatin leads to a strong decrease of bile salt hydrophobicity, which may be beneficial in the prevention of nucleation of cholesterol crystals.

ApoB metabolism in familial hypercholesterolemia. Inconsistencies with the LDL receptor paradigm

The biology of the low-density lipoprotein (LDL) receptor has been examined in detail, and a paradigm for LDL metabolism has evolved from comparative studies of cholesterol metabolism in a variety of cells cultured from normal individuals and subjects with familial hypercholesterolemia (FH). Cultured cells from patients with homozygous FH lack a functional LDL receptor and show diminished LDL clearance, induction of the enzyme hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase, increased cholesterol synthesis, decreased cholesterol ester production, and depleted cholesterol ester stores. The observed decrease in the fractional catabolic rate (FCR) of LDL is attributed to the mutated LDL receptor gene. However, in the experimental animal model of this disease, the Watanabe heritable hyperlipidemic (WHHL) rabbit, cholesterol ester stores are increased, while hepatic cholesterol synthesis is decreased. Furthermore, in humans HMG-CoA reductase is suppressed, and the LDL apolipoprotein (apo) B production rate is increased in patients with FH. These findings raise questions about the adequacy of the paradigm in understanding hepatic cholesterol metabolism in vivo. In humans, apoB metabolism is believed to be principally determined by the liver, where apoB is both synthesized and catabolized. Assuming the neutral lipid content of the liver is the major determinant of apoB metabolism, we postulated that the changes in apoB metabolism in FH are predictable when based on the assumption of an increase in hepatic cholesterol and cholesterol ester content, as observed both in the WHHL rabbit and in humans. We examined this hypothesis in vivo in patients with heterozygous FH by using tracer kinetic methodology and have used similar data from normal and hypertriglyceridemic (HTG) subjects as controls. Whereas normal and HTG subjects secrete apoB primarily as large, triglyceride-enriched very-low-density lipoprotein (VLDL), heterozygous FH patients have an absolute decrease in apoB production and secrete almost 40% of apoB as smaller intermediate-density lipoprotein (IDL)/LDL. In normal humans, about half of secreted apoB is catabolized rather than being converted to LDL. In HTG subjects two thirds of apoB follows this same route, by which VLDL remnants remaining after triglyceride hydrolysis are largely returned to the liver. In contrast, in FH subjects secreted apoB is fully converted to LDL. Thus, although total apoB secretion is reduced in FH subjects, total LDL production is greater than in either normal or HTG subjects. Under basal conditions the elevated LDL in heterozygous FH is due to both decreased LDL receptor-mediated catabolism and increased LDL production. However, the number of LDL receptors actually expressed is suppressed below the number of potentially functional receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo regulation of hepatic lipase activity and mRNA levels by diets which modify cholesterol influx to the liver

The aim of this study was to assess whether diets enriched in cholesterol, sodium cholate and drugs known to modify liver cholesterol biosynthesis can modulate hepatic lipase (H-TGL) expression and activity in vivo. Female lean Zucker rats, known to be good responders to cholesterol, were fed for 7 days with a control C diet or the C diet supplemented (w/w) with either 2% cholesterol, 0.5% sodium cholate, 2% cholestyramine or simvastatin (0.1%) added to the cholestyramine diet or given by gavage (10 mg/rat) for 3 days. H-TGL activity decreased by 34% with cholesterol, and by 27% when both cholesterol and cholate were administered to the rats. Under these conditions, H-TGL mRNA decreased by 34% and 87%, respectively. The sharp decrease in H-TGL expression was associated with a strong increase in cholesteryl ester in total liver and in the liver microsome fraction. H-TGL activity decreased by 33% with cholestyramine and the mRNA level decreased by 47%. Simvastatin lowered H-TGL activity by 55% when added to the cholestyramine diet, probably because of a reduction in food intake. When administrated by gavage, simvastatin increased both the H-TGL activity (by 28%) and mRNA (by 23%). These variations may be linked to the availability of mevalonate-derived sterol and non-sterol products.

Effects of pravastatin and ursodeoxycholic acid on cholesterol and bile acid metabolism in patients with cholesterol gallstones

To investigate the effects of pravastatin and ursodeoxycholic acid (UDCA) on cholesterol and bile acid metabolism in humans, 41 patients with cholesterol gallstone disease were allocated to four groups and treated with pravastatin (20 mg/day), UDCA (600 mg/day), both pravastatin and UDCA, or neither drug (control) for 1-2 weeks prior to elective cholecystectomy. Cholesterol 7 alpha-hydroxylase activity and serum levels of total 7 alpha-hydroxycholesterol were significantly increased by pravastatin and unaffected by UDCA. 3-Hydroxy-3-methylglutaryl coenzyme A reductase activity was markedly increased by pravastatin and decreased by UDCA. UDCA significantly decreased biliary cholesterol concentration and the cholesterol saturation index and prolonged the nucleation time; however, pravastatin alone had little effect on biliary lithogenicity. Serum total and low-density lipoprotein (LDL)-cholesterol levels were reduced most by the combined administration of pravastatin and UDCA. In conclusion, at a dose of 20 mg/day, pravastatin increased bile acid synthesis but did not decrease biliary lithogenicity. UDCA had no significant effect on bile acid synthesis, but markedly decreased biliary lithogenicity.

Effects of ursodeoxycholic acid on plasma lipids

Interference with the enterohepatic circulation leads to changes in plasma lipoprotein metabolism. Thus, increased bile acid synthesis (such as after cholestyramine therapy or biliary diversion) stimulates hepatic triglyceride production and increases the number of low density lipoprotein (LDL) receptors in the liver. However, treatment with chenodeoxycholic acid reduces triglyceride production and appears to reduce LDL catabolism. Ursodeoxycholic acid therapy, which has minor effects on bile acid synthesis, results in relatively minor changes in lipoprotein metabolism. A tendency for lowered LDL cholesterol levels may be related to the fact that ursodeoxycholic acid interferes with the enterohepatic circulation of the normal bile acids, particularly when administered at a high dosage.