Comparative regulation of hepatic sterol 27-hydroxylase and cholesterol 7alpha-hydroxylase activities in the rat, guinea pig, and rabbit: effects of cholesterol and bile acids

Update on the Benefits and Mechanisms of Action of the Bioactive Vegetal Alkaloid Berberine on Lipid Metabolism and Homeostasis

Elevation of circulating levels of blood cholesterol, especially LDL cholesterol, and/or the decrease of HDL cholesterol levels have long been recognized as primary risk factors for developing atherosclerosis that leads to cardiovascular and cerebrovascular disease. Hypertriglyceridemia is an independent risk factor that is known to contribute to the development of atherosclerosis. Thus, various interventional efforts aimed at reducing hypercholesterolemia and hypertriglyceridemia have been practiced clinically for decades to reduce morbidity and mortality risk associated with deleterious cardiovascular and cerebrovascular events. As such, many drugs have been developed and clinically used to treat hypocholesteremia and/or hypertriglyceridemia; however, dietary approaches including supplements along with changes in nutrition and lifestyle have become increasingly attractive and acceptable methods used to control borderline or moderately increased levels of blood cholesterol and triacylglycerols. In this regard, the use of a plant/herbal bioactive compound, berberine (BBR), has recently been studied extensively in terms of its efficacy as well as its mechanisms of action and safety as an alternative intervention that beneficially modulates blood lipids. The aim of this review is to provide a comprehensive update on BBR research, new concepts and directions in terms of product development and current challenges, and future prospects of using BBR to manage diseases and complications associated with dyslipidemia.

Liver radiofrequency ablation compromises the biological gut barrier

AIM

Liver radiofrequency ablation (RFA) has been shown to disrupt the mechanical component of the gut barrier. The aim of the present study was to investigate the consequences of liver RFA on the biological gut barrier in terms of the effects of bile production rate and bowel inflammatory state on intestinal microflora balance.

METHOD

A total of 25 New Zealand rabbits were assigned to five groups (n = 5 per group): group CBD: subjected to common bile duct (CBD) extracorporeal bypass; group CBD-RFA: subjected to CBD bypass plus one session of open liver RFA; group RFA: subjected to liver RFA; group sham: subjected to sham operation; and group TBD: subjected to total bile deviation (TBD). In groups CBD and CBD-RFA, bile production rate was assessed for 48 h. In groups sham and RFA, measurement of biliary glycine conjugates of cholic and deoxycholic acid levels, histopathologic examination of the non-ablated liver tissue, morphometric analysis, and histopathologic examination of the terminal ileum and microbiological analysis of fecal and tissue samples collected from the jejunum and the cecum (and in group TBD) were performed at 48 h post-operation.

RESULTS

One session of liver RFA resulted in ablation of 18.7 ± 2.7% of liver weight. Following liver RFA, bile production rate was reduced, while the levels of biliary bile salts were not affected. There was mild injury of the non-ablated liver parenchyma, mild intestinal wall inflammation, intestinal mucosa atrophy, and intestinal microbial population overgrowth.

CONCLUSION

Reduced in bile production and mild bowel inflammation secondary to liver RFA impaired the biological gut barrier as manifested by intestinal microflora imbalance.

The bile acid synthesis pathway is present and functional in the human ovary

BACKGROUND

Bile acids, end products of the pathway for cholesterol elimination, are required for dietary lipid and fat-soluble vitamin absorption and maintain the balance between cholesterol synthesis in the liver and cholesterol excretion. They are composed of a steroid structure and are primarily made in the liver by the oxidation of cholesterol. Cholesterol is also highly abundant in the human ovarian follicle, where it is used in the formation of the sex steroids.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe for the first time evidence that all aspects of the bile acid synthesis pathway are present in the human ovarian follicle, including the enzymes in both the classical and alternative pathways, the nuclear receptors known to regulate the pathway, and the end product bile acids. Furthermore, we provide functional evidence that bile acids are produced by the human follicular granulosa cells in response to cholesterol presence in the culture media.

CONCLUSIONS/SIGNIFICANCE

These findings establish a novel pathway present in the human ovarian follicle that has the capacity to compete directly with sex steroid synthesis.

Neonatal dietary cholesterol and alleles of cholesterol 7-alpha hydroxylase affect piglet cerebrum weight, cholesterol concentration, and behavior

This experiment was designed to test the effect of polymorphism in the cholesterol 7-alpha hydroxylase (CYP7) gene locus and dietary cholesterol (C) on cerebrum C in neonatal pigs fed sow's milk formulas. Thirty-six pigs (18 male and 18 female) genetically selected for high (HG) or low (LG) plasma total C were weaned at 24-36 h after birth and assigned in a 2 x 2 x 2 factorial arrangement of treatments with 2 diets (0 or 0.5% C), 2 sexes, and 2 genotypes (HG and LG). Individually housed pigs consumed diets ad libitum for 42 d. Open-field behavior was tested at wk 2 and 4. All pigs were killed at 42 d of age, the cerebrum was weighed, and C content and concentration measured. All data were analyzed by general linear model ANOVA. Cerebrum weight was greater in HG than LG pigs (P < 0.03) but was not affected by diet or sex. Pigs fed C tended to have a higher cerebrum C concentration than those deprived (P = 0.12). At 2 wk, LG pigs explored a novel open-field environment less often (P < 0.001) than did HG pigs. At 4 wk, some LG pigs explored the open field but fewer (P < 0.001) vs. HG pigs retreated back to the safe area. There were no genotype x diet, genotype x sex, or diet x sex interactions affecting cerebrum weight, or C content or concentration. Polymorphism in the CYP7 gene locus affected cerebrum weight and behavior and dietary C tended to increase cerebrum C concentration in neonatal pigs. These findings in neonatal pigs have considerable potential importance in human infant nutrition and behavioral development.

Exploitation of bile acid transport systems in prodrug design

The enterohepatic circulation of bile acids is one of the most efficient recycling routes in the human body. It is a complex process involving numerous transport proteins, which serve to transport bile acids from the small intestine into portal circulation, from the portal circulation into the hepatocyte, from the hepatocyte into the bile, and from the gall bladder to the small intestine. The tremendous transport capacity and organ specificity of enterohepatic circulation combined with versatile derivatization possibilities, rigid steroidal backbone, enantiomeric purity, availability, and low cost have made bile acids attractive tools in designing pharmacological hybrid molecules and prodrugs with the view of improving intestinal absorption, increasing the metabolic stability of pharmaceuticals, specifically targeting drugs to organs involved in enterohepatic circulation, as well as sustaining therapeutically reasonable systemic concentrations of active agents. This article briefly describes bile acid transport proteins involved in enterohepatic circulation, summarizes the key factors affecting on the transport by these proteins, and reviews the use of bile acids and their derivatives in designing prodrugs capable of exploiting the bile acid transport system.

Amyloid beta-protein and lipid metabolism

Lipids play an important part as risk or protective factors for Alzheimer's disease. This review summarizes the current findings in which lipids influence Alzheimer's disease and introduces the molecular mechanism how these lipids are linked to amyloid production. Besides the pathological impact of amyloid in Alzheimer's disease, amyloid has a physiological function in regulating lipid homeostasis in return. The understanding of the resulting regulatory cycles between amyloid precursor protein processing and lipids provides a platform for the development of new causal therapeutic approaches for Alzheimer's disease.

Roles of nuclear receptors in the up-regulation of hepatic cholesterol 7alpha-hydroxylase by cholestyramine in rats

Nuclear receptors are involved in regulating the expression of cholesterol 7alpha-hydroxylase (CYP7A1), however, their roles in the up-regulation of CYP7A1 by cholestyramine (CSR) are still unclear. In the present study, male Wistar rats were divided into four groups and fed [high sucrose + 10% lard diet] (H), [H + 3% CSR diet] (H + CSR), [H + 0.5% cholesterol + 0.25% sodium cholate diet] (C), or [C + 3% CSR diet] (C + CSR) for 2 weeks. Cholestyramine decreased serum and liver cholesterol levels significantly in rats fed C-based diets, but had no effect on these parameters in rats fed H-based diets. Cholestyramine raised hepatic levels of CYP7A1 mRNA and activity in both groups. The gene expression of hepatic ATP-binding cassettes A1 and G5, regulated by liver X receptor (LXR), were unchanged and down-regulated by cholestyramine, respectively. The mRNA levels of the hepatic ATP-binding cassette B11 and short heterodimer partner (SHP), regulated by farnesoid X receptor (FXR), were not changed by cholestyramine. C-based diets, which contained cholesterol and cholic acid, increased SHP mRNA levels compared to H-based diets. Consequently, in rats fed the C+CSR diet, hepatic FXR was activated by dietary bile acids, but the hepatic CYP7A1 mRNA level was increased 16-fold compared to that in rats fed an H diet. These results suggest that cholestyramine up-regulates the expression of CYP7A1 independently via LXR- or FXR-mediated pathways in rats.

Validation of using gene expression in mononuclear cells as a marker for hepatic cholesterol metabolism

HMG-CoA reductase and the LDL receptor are ubiquitously expressed in major tissues. Since the liver plays a major role in regulating circulating LDL, it is usually of interest to measure the effects of drug or dietary interventions on these proteins in liver. In humans, peripheral blood mononuclear cells have been used as a surrogate for liver to assess regulation of these genes, although there is concern regarding the validity of this approach. The purpose of this study was to evaluate the relationship between liver and mononuclear cell expression of HMG-CoA reductase and the LDL receptor in guinea pigs, a well established model for human cholesterol and lipoprotein metabolism. We extracted RNA from liver and mononuclear cells of guinea pigs from a previous study where the effects of rapamycin, an immunosuppresant drug used for transplant patients, on lipid metabolism were evaluated. Guinea pigs were assigned to three different diets containing the same amount of fat (15 g/100 g) and cholesterol (0.08 g/100 g) for a period of 3 weeks. The only difference among diets was the concentration of rapamycin: 0, 0.0028 or 0.028 g/100 g. There were no differences in plasma LDL cholesterol (LDL-C) among groups. Values were 78.4 ± 14.3, 65.8 ± 17.2 and 68.4 ± 45.4 mg/dL (P > 0.05) for guinea pigs treated with 0, low or high doses of rapamycin, respectively. The mRNA abundance for the LDL receptor and HMG-CoA reductase was measured both in liver (n = 30) and mononuclear cells (n = 22) using reverse transcriptase PCR. In agreement with the finding of no changes in plasma LDL-C, there were also no differences for the expression of HMG-CoA reductase or the LDL receptor among groups. However, a positive correlation was found between liver and mononuclear cells for both HMG-CoA reductase (r = 0.613, P < 0.01) and the LDL receptor (r = 0.622, P < 0.01). These correlations suggest that monocytes can be used in humans as an index for liver to assess diet and drug effects on the expression of HMG-CoA reductase and the LDL receptor.

CETP expression enhances liver HDL-cholesteryl ester uptake but does not alter VLDL and biliary lipid secretion

The aim of this work was to study how CETP expression affects whole body cholesterol homeostasis. Thus, tissue uptake and plasma removal rates of labeled HDL-cholesteryl ester (CE), VLDL secretion rates, and biliary lipid secretion and fecal bile acid content were compared between human CETP transgenic (Tg) and non-transgenic (nTg) mice fed with a standard diet. CETP Tg mice exhibited increased HDL-CE plasma fractional catabolic rate and uptake by the liver, adrenals, adipose tissue and spleen. HDL fractions from both CETP Tg and from nTg mice were removed faster from the plasma of CETP expressing than from nTg mice, suggesting a direct role of CETP in accelerating tissue CE uptake. However, neither hepatic output of VLDL cholesterol and triglycerides nor biliary lipid and fecal bile acid excretion were changed in CETP Tg compared to nTg mice. CETP Tg mice also showed enhanced hepatic cholesterol content. Steady state cholesterol homeostasis was probably preserved through the downregulation of hepatic HMG-CoA reductase and LDL receptor expression. In conclusion, although CETP expression facilitates cholesteryl ester tissue uptake, it does not alter biliary lipid and fecal bile acid excretion, the mandatory final step of the reverse cholesterol transport.

Regulation of cholesterol 7alpha-hydroxylase mRNA expression in C57BL/6 mice fed an atherogenic diet

The nuclear receptors liver X receptor (LXR) alpha and farnesoid X receptor (FXR) are positive and negative regulators of cholesterol 7alpha-hydroxylase (CYP7A1) transcription, respectively. To clarify their roles in the regulation of CYP7A1 in mice, we investigated mRNA expression of their target genes in the livers of C57BL/6 mice fed the following five diets for 2 weeks: a standard diet, cholic acid, cholesterol, cholesterol+high fat, or an atherogenic diet (cholic acid+cholesterol+high fat). The mRNA level of ATP-binding cassette transporter (ABC) A1 gene, one of LXRalpha target genes, significantly increased on the diets containing cholic acid and/or cholesterol+high fat, but not on the diet containing cholesterol alone. On the other hand, the mRNA levels of the FXR target genes ABCB11, ABCC2, and short heterodimer partner increased only on the diet containing cholic acid with or without cholesterol+high fat. Surprisingly, cholesterol alone or cholesterol+high fat did not affect CYP7A1 mRNA level, whereas cholic acid with or without cholesterol+high fat greatly reduced the level. Thus, in the atherogenic diet-fed mice, cholic acid component is needed for the FXR activation, and FXR dominantly regulates CYP7A1 transcription.

Comparative regulation of major enzymes in the bile acid biosynthesis pathway by cholesterol, cholate and taurine in mice and rats

These enzymes play important roles in the biosynthesis of bile acids. They are cholesterol 7alpha-hydroxylase (CYP7A1), the rate limiting enzyme in the classic pathway, sterol 12alpha-hydroxylase (CYP8B1), the key enzyme for synthesis of cholic acid (CA), and sterol 27-hydroxylase (CYP27), the initial enzyme in the alternative pathway. In the present study, the susceptibility of these three enzymes to dietary cholesterol and cholate, and the cholesterol lowering effect of taurine were determined in male C57BL/6 mice and Wistar rats. Both mice and rats were divided into 6 groups: control group (N), high cholesterol diet group (C), high cholesterol and cholate diet group (CB), and their 1% taurine-supplemented groups (NT, CT, CBT, respectively). After animals were fed with the respective diets for one week, the mRNA levels of CYP7A1 increased in the C-group compared with those of the N-group, and decreased in the CB-group compared with those of the C-group in both mice and rats. But the extent of decrease is different between the two species. CYP8B1 was also markedly repressed by cholate in mice, but not in rats. These results are consistent with the changes in serum and liver cholesterol concentrations. Taurine significantly increased CYP7A1 mRNA levels in the CBT-group compared with the CB-group in both animal models, with a subsequent decrease in serum and liver cholesterol levels and increase in fecal bile acid excretion. Up-regulated CYP8B1 was also observed after taurine supplementation in the CBT-group in mice. No increase in CYP7A1 was produced by taurine in the CT-group compared with that of the C-group in mice, although the changes of serum and liver cholesterol and fecal bile acids indicated taurine showed an efficient cholesterol lowering effect. In addition, CYP27 was induced in both C- and CB-groups of rats but not of mice, and no changes were produced by taurine. The overall results suggest that there are differences between mice and rats in susceptibility of the three enzymes to dietary cholesterol and cholate, and taurine induced CYP7A1 to produce its cholesterol-lowering effect only in the presence of cholate in the cholesterol diet.

Species-specific mechanisms for cholesterol 7alpha-hydroxylase (CYP7A1) regulation by drugs and bile acids

The gene encoding cholesterol 7alpha-hydroxylase (CYP7A1) is tightly regulated in order to control intrahepatic cholesterol and bile acid levels. Ligands of the xenobiotic-sensing pregnane X receptor inhibit CYP7A1 expression. To retrace the evolution of the molecular mechanisms underlying CYP7A1 inhibition, we used a chicken hepatoma cell system that retains the ability to be induced by phenobarbital and other drugs. Whereas bile acids regulate CYP7A1 via small heterodimer partner and liver receptor homolog-1, mRNA expression of these nuclear receptors is unchanged by xenobiotics. Instead, drugs repress chicken hepatic nuclear factor 4alpha (HNF4alpha) transcript levels concomitant with a reduction in CYP7A1 expression. Importantly, no reduction of HNF4alpha levels is found in mouse liver in vivo and in human primary hepatocyte cultures, respectively. Thus, besides the importance of HNF4alpha in CYP7A1 regulation in all species, birds and mammals use different signaling pathways to adjust CYP7A1 levels after exposure to xenobiotics.

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.

High doses of simvastatin, pravastatin, and cholesterol reduce brain cholesterol synthesis in guinea pigs

Recent epidemiological studies suggest that inhibitors of 3-hydroxy-3-methyl-glutaryl CoA reductase, so-called statins, are effective in lowering the prevalence of Alzheimer's disease. Whether the effect of statins is due to a local inhibition of cholesterol synthesis in the brain or whether it is mediated by the reduced levels of cholesterol in the circulation is not known. In the present work, we tested the possibility that high doses of lipophilic and hydrophilic statins, simvastatin and pravastatin, respectively, or a diet high in cholesterol could affect cholesterol homeostasis in the brain of guinea pigs. The total brain cholesterol levels were not affected by high-dose simvastatin or pravastatin treatment. Significantly lower levels of the cholesterol precursor lathosterol and its ratio to cholesterol were found in the brains of simvastatin and pravastatin-treated animals. 24S-Hydroxycholesterol, the transportable form of cholesterol across the blood-brain barrier, was significantly lower in the brain of pravastatin-treated animals. Excessive cholesterol feeding resulted in higher serum cholesterol levels but did not affect total brain cholesterol level. However, de novo cholesterol synthesis in the brain seemed to be down-regulated, as indicated by lower absolute levels and cholesterol-related ratios of lathosterol compared with controls. The passage of deuterium-labeled cholesterol across the blood-brain barrier in one animal was found to be approximately 1%. Our results suggest that brain cholesterol synthesis in guinea pigs can be slightly, but significantly, influenced by high doses of lipophilic and hydrophilic statins as well as by high dietary cholesterol intake, while total brain cholesterol content and thus, cholesterol homeostasis is maintained.

Programming of initial steps in bile acid synthesis by breat-feeding vs. Formula-feeding in the baboon

We tested the hypothesis that breast- vs. formula-feeding differentially affects the enzymatic activity of three sterol hydroxylases critical in the initial steps of bile acid formation. Thirty baboons were either breast-fed or formula-fed for the first 14 wk of life before weaning to baboon chow. At 14 and 34 wk of age, liver biopsies were assayed for cholesterol 7alpha-hydroxylase (CYP7A1), 27-hydroxycholesterol-7alpha-hydroxylase (CYP7B1), and cholesterol 27-hydroxylase (CYP27A1). We also determined the kinetics of 3H-27-hydroxycholesterol (27-OHC) turnover in vivo at both ages. At 14 wk of age, hepatic CYP7A1 activity was low but sevenfold higher among formula-fed vs. breast-fed baboons. By 34 wk, CYP7A1 activity had increased nearly 10-fold in both infant diet groups, and the sevenfold difference in CYP7A1 between previously breast- and formula-fed animals persisted. There were no differences in CYP7B1 activities between infant diet groups at either 14 or 34 wk of age although the activity increased in both groups by about 50% from 14 to 34 wk. CYP27A1 activity also increased between 14 and 34 wk of age, and, compared with CYP7A1, relatively small differences in CYP27A1 activity due to infant diet were observed at each age. Plasma 27-OHC turnover had a half-time of 2-4 min. We had previously reported that after weaning, the total bile acid synthesis rate was higher among baboons that were formula-fed than among breast-fed animals. The present results suggest that this difference is most likely due to significantly higher CYP7A1 activity among formula-fed vs. breast-fed animals.

Effects of dietary soybean lecithin on plasma lipid transport and hepatic cholesterol metabolism in rats

Dietary lecithin can stimulate bile formation and biliary lipid secretion, particularly cholesterol output in bile. Studies also suggested that the lecithin-rich diet might modify hepatic cholesterol homeostasis and lipoprotein metabolism. Therefore, we examined hepatic activities of 3-hydroxy-3 methylglutaryl coenzyme A reductase "HMG -CoA reductase", cholesterol 7 alpha-hydroxylase and acyl-CoA: cholesterol acyltransferase "ACAT" as well as plasma lipids and lipoprotein composition in rats fed diets enriched with 20% of soybean lecithin during 14 days. We also evaluated the content of hepatic canalicular membrane proteins involved in lipid transport to the bile (all P-glycoproteins as detected by the C 219 antibody and the sister of P-glycoprotein "spgp" or bile acid export pump) by Western blotting. As predicted, lecithin diet modified hepatic cholesterol homeostasis. The activity of hepatic HMG-CoA reductase and cholesterol 7 alpha-hydroxylase was enhanced by 30 and 12% respectively, while microsomal ACAT activity showed a dramatic decrease of 75%. As previously reported from ACAT inhibition, the plasma level and size of very low-density lipoprotein (VLDL) were significantly decreased and bile acid pool size and biliary lipid output were significantly increased. The canalicular membrane content of lipid transporters was not significantly affected by dietary lecithin. The current data on inhibition of ACAT activity and related metabolic effects by lecithin mimic the previously reported effects following drug-induced inhibition of ACAT activity, suggesting potential beneficial effects of dietary lecithin supplementation in vascular disease.

Regulation of oxysterol 7alpha-hydroxylase (CYP7B1) in primary cultures of rat hepatocytes

Conversion of cholesterol into 7alpha-hydroxylated bile acids is a principal pathway of cholesterol disposal. Cholesterol 7alpha-hydroxylase (CYP7A1) is the initial and rate-determining enzyme in the "classic" pathway of bile acid synthesis. An "alternative" pathway of bile acid synthesis is initiated by sterol 27-hydroxylase (CYP27) with subsequent 7alpha-hydroxylation of 27-hydroxycholesterol by oxysterol 7alpha-hydroxylase (CYP7B1). The regulation of CYP7B1, possibly a rate-determining enzyme in the alternative pathway, has not been thoroughly studied. The aims of this study were to (1) study the regulation of liver CYP7B1 by bile acids, cholesterol, adenosine 3', 5'-cyclic monophosphate (cAMP), and phorbol myristate acetate (PMA) in primary rat hepatocytes and (2) determine the effect of CYP7B1 overexpression on rates of bile acid synthesis. The effects of different bile acids (3-150 micromol/L), cAMP (50 micromol/L), PMA (100 nmol/L; protein kinase C stimulator), cholesterol (200 micromol/L), and squalestatin (1 micromol/L; cholesterol synthesis inhibitor) on CYP7B1 expression in primary rat hepatocytes were studied. Taurocholic acid and taurodeoxycholic acid decreased CYP7B1 activity by 45% +/- 10% and 36% +/- 7%, respectively. Tauroursodeoxycholic acid and taurochenodeoxycholic acid did not alter CYP7B1 activity. Inhibition of cholesterol synthesis with squalestatin decreased CYP7B1 activity by 35%, whereas addition of cholesterol increased activity by 39%. Both PMA and cAMP decreased CYP7B1 activity by 60% and 34%, respectively, in a time-dependent fashion. Changes in CYP7B1 messenger RNA (mRNA) levels correlated with changes in specific activities. Overexpression of CYP7B1 led to a marked increase in CYP7B1 mRNA levels and specific activity but no change in rates of bile acid synthesis. In conclusion, in the rat, CYP7B1 specific activity is highly regulated but does not seem to be rate limiting for bile acid synthesis.

LXR alpha is the dominant regulator of CYP7A1 transcription

Cholesterol 7 alpha-hydroxylase (CYP7A1) catalyzes the rate-limiting step in the classic pathway of bile acid biosynthesis. Dietary cholesterol stimulates CYP7A1 transcription via activation of oxysterol receptor, LXR alpha, whereas bile acids repress transcription through FXR-mediated induction of SHP protein. The aim of this study was to determine the quantitative role that LXR- and FXR-regulated pathways play in regulating CYP7A1 and SHP in both rat and hamster models. In rats fed a 2% cholesterol diet, both SHP and CYP7A1 mRNA levels were elevated. The inability to induce CYP7A1 mRNA levels by cholesterol feeding in hamsters led to a decline in SHP mRNA levels. Elimination of hepatic bile acid flux by cholestyramine or bile fistula resulted in a marked repression of rat SHP mRNA levels. These results suggest that under conditions of both SHP and LXR alpha activation, stimulatory effect of LXR alpha overrides the inhibitory effect of FXR and results in an induction of rat CYP7A1 mRNA levels.

High and low responding strains of laboratory opossums differ in sterol 27-hydroxylase and acyl-coenzyme A:cholesterol acyltransferase activities on a high cholesterol diet

Two partially inbred strains of laboratory opossums exhibit extremely high or low levels of VLDL+LDL cholesterol concentrations, respectively, when challenged with a high cholesterol and high fat diet. The present studies were conducted to determine whether the high and low responding strains differ in activities of important enzymes that have been shown to affect lipemic responsiveness to diet. We measured plasma 27-hydroxycholesterol and hepatic activities of 27-hydroxylase and 7alpha-hydroxylase in high and low responding opossums while consuming the basal diet and cholesterol-enriched diets. Plasma 27-hydroxycholesterol concentration and 27-hydroxylase activity in liver did not differ between groups on the basal diet, but both were significantly higher in low responders than in high responders on the cholesterol-enriched diet with unsaturated fat (10.79 +/- 0.56 in low vs. 7.31 +/- 0.50 &mgr;g/dl in high responders for 27-hydroxycholesterol and 14.14 +/- 0.79 in low vs. 10.07 +/- 0.80 pmol/mg protein/min in high responders for 27-hydroxylase activity). On the other hand, 7alpha-hydroxylase activity was significantly higher in high responding opossums (75.72 +/- 6.81 pmol/mg protein/min) than in low responding opossums (51.39 +/- 6.18 pmol/mg protein/min) on the basal diet, but it did not differ on the high cholesterol and high fat diet. We measured hepatic ACAT and extrahepatic hepatic 27-hydroxylase activities in high and low responding opossums on the cholesterol enriched diet. Hepatic ACAT activity was significantly higher in high responding opossums (137.00 +/- 18.33 pmol/mg protein/min) than in low responding opossums (47.67 +/- 2.71 pmol/mg protein/min), whereas extrahepatic 27-hydroxylase activity was higher in low responding opossums (33.00 +/- 2.10 pmol/mg protein/min in lungs and 3.69 +/- 0.20 in kidneys) than in high responding opossums (21.17 +/- 1.54 pmol/mg protein/min in lungs and 2.82 +/- 0.31 in kidneys). We also compared the composition of bile between high and low responders. The concentration of taurine conjugates of cholic acid in bile of both groups was similar, but concentration of taurine conjugates of chenodeoxycholic acid in bile of low responding animals was higher than in high responding animals (124.9 +/- 17.3 in low vs. 59.2 +/- 13.2 &mgr;mol/ml in high responders). The results of these studies suggest two enzymes may affect the lipemic response to diet in laboratory opossums: sterol 27-hydroxylase and ACAT. Each of these enzymes may influence diet-induced hyperlipidemia at a different step of lipoprotein metabolism.

Potency of select statin drugs in a new mouse model of hyperlipidemia and atherosclerosis

Poloxamer-407 (P-407) is a nonionic surfactant that induces atheroma formation in the aortas of C57BL/6 mice with long-term (14 weeks) administration. The objectives of the present study were to determine the mechanism(s) responsible for the induction of hypercholesterolemia as well as to determine whether this animal model may be of potential use in rank ordering the efficacy (lipid lowering) of various statin drugs. The effect of long-term (16 weeks) administration of P-407 on the catalytic activities of rate-limiting enzymes of cholesterol biosynthesis [HMG-CoA reductase (HMGR)] and catabolism [microsomal cholesterol 7alpha-hydroxylase (C7alphaH) and mitochondrial sterol 27 hydroxylase (S27H)] was assessed in C57BL/6 mice. Effects of P-407 on these enzymes were compared in mice fed an atheroma-inducing diet (high-cholesterol, supplemented with cholic acid) and animals maintained on a basal diet and injected with saline (controls) after 16 weeks. The mean value for the activities of C7alphaH in P-407-injected mice was 24.3+/-3.8 pmol min(-1) mg(-1) and was significantly (P<0.05) less than the mean value determined for sham-injected control animals (37.0+/-14.3 pmol min(-1) mg(-1)). In contrast, the mean values for the catalytic activities of S27H and HMGR did not change with P-407 administration. Neither C7alphaH nor S27H activity in mice fed the high-cholesterol diet differed from values for control animals, whereas the mean HMGR activity was drastically reduced (-94%, P<0.05). The hypercholesterolemic effect of P-407 is not due to altered cholesterol biosynthesis, but is mediated by reduced cholesterol catabolism due to decreased activity of the rate limiting enzyme (C7alphaH) in the classic bile acid synthetic pathway. Plasma triglyceride lowering resulting from the oral administration of equal doses of various statin drugs appeared, in general, to be positively correlated with their relative aqueous solubility and paralleled the efficacy of these agents to lower low-density-lipoprotein-associated cholesterol (LDL-C) in humans. The plasma triglyceride lowering effect of the five statin drugs tested produced the following rank order; pravastatin sodium (-44%)>atorvastatin calcium (-36%)>simvastatin (-33%)>lovastatin (-25%)>fluvastatin sodium (-19%). While reductions in plasma total cholesterol following administration of the statin drugs was not as profound as that observed with triglycerides, the relative rank order or trend was preserved. The percent reduction in plasma triglycerides in the present model appears to be a useful parameter with which to predict the relative reduction in plasma LDL-C expected for these agents in humans.

Hamsters predisposed to sucrose-induced cholesterol gallstones (LPN strain) are more resistant to excess dietary cholesterol than hamsters that are not sensitive to cholelithiasis induction

We compared the effects of cholesterol feeding in male hamsters from two strains with different propensities to sucrose-induced cholelithiasis; Laboratoire de Physiologie de la Nutrition (LPN) hamsters are predisposed to developing biliary cholesterol gallstones, whereas Janvier (JAN) hamsters are not. When fed a basal control diet, LPN hamsters had a lower cholesterolemia (-21%, P = 0.01) than JAN hamsters, and a higher activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase in liver (+148%, P = 0.018) and intestine (+281%, P < 0.0001). After feeding the same diet enriched with 0.3% cholesterol for 5 wk, cholesterolemia increased more dramatically in JAN hamsters (+235%, P < 0.001) than in LPN hamsters (+108%, P < 0.001), as did the liver concentration of cholesterol, which reached 152.30 +/- 13.00 and 44.41 +/- 9.06 micromol/g, respectively. Only JAN hamsters displayed hepatomegaly, with an increased cholesterol saturation index of the gallbladder bile (+100%, P < 0.01), due to the cholesterol challenge. In liver, cholesterol feeding reduced cholesterol 7alpha-hydroxylase activity and mRNA level, and stimulated sterol 27-hydroxylase and oxysterol 7alpha-hydroxylase activities. Hepatic levels of LDL receptor decreased by approximately 60% in both strains, whereas HDL receptor scavenger class B type 1 (SR-BI) levels were unaffected by dietary cholesterol. The greater resistance of LPN hamsters to the hypercholesterolemic diet can be explained by a lower capacity to store cholesterol in the liver and greater efficiency in reducing the activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase in response to cholesterol feeding [from 11263 to 261 pmol/(min x organ) in LPN hamsters and from 4530 to 694 pmol/(min x organ) in JAN hamsters]. These results highlight the usefulness of this two-strain model, which offers some analogy with the inverse association between the predisposition to cholelithiasis and the risk of atherosclerosis in humans.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Simvastatin strongly reduces levels of Alzheimer's disease beta -amyloid peptides Abeta 42 and Abeta 40 in vitro and in vivo

Recent epidemiological studies show a strong reduction in the incidence of Alzheimer's disease in patients treated with cholesterol-lowering statins. Moreover, elevated Abeta42 levels and the varepsilon4 allele of the lipid-carrier apolipoprotein E are regarded as risk factors for sporadic and familial Alzheimer's disease. Here we demonstrate that the widely used cholesterol-lowering drugs simvastatin and lovastatin reduce intracellular and extracellular levels of Abeta42 and Abeta40 peptides in primary cultures of hippocampal neurons and mixed cortical neurons. Likewise, guinea pigs treated with high doses of simvastatin showed a strong and reversible reduction of cerebral Abeta42 and Abeta40 levels in the cerebrospinal fluid and brain homogenate. These results suggest that lipids are playing an important role in the development of Alzheimer's disease. Lowered levels of Abeta42 may provide the mechanism for the observed reduced incidence of dementia in statin-treated patients and may open up avenues for therapeutic interventions.

The role of orphan nuclear receptors in the regulation of cholesterol homeostasis

Cholesterol balance is maintained by a series of regulatory pathways that control the acquisition of cholesterol from endogenous and exogenous sources and the elimination of cholesterol, facilitated by its conversion to bile acids. Over the past decade, investigators have discovered that a family of membrane-bound transcription factors, sterol regulatory element-binding proteins (SREBPs), mediate the end-product repression of key enzymes of cholesterol biosynthesis. Recently orphan members of another family of transcription factors, the nuclear hormone receptors, have been found to regulate key pathways in bile acid metabolism, thereby controlling cholesterol elimination. The study of these orphan nuclear receptors suggests their potential as targets for new drug therapies.

Regulation of cholesterol 7alpha-hydroxylase gene (CYP7A1) transcription by the liver orphan receptor (LXRalpha)

The cholesterol 7alpha-hydroxylase gene (CYP7A1) plays an important role in regulation of bile acid biosynthesis and cholesterol homeostasis. Oxysterol receptor, LXR, stimulates, whereas the bile acid receptor, FXR, inhibits CYP7A1 transcription. The goal of this study was to investigate the role of LXRalpha on the regulation of rat, human and hamster CYP7A1 transcription in its native promoter and cellular context. Cotransfection with LXRalpha and RXRalpha expression plasmids strongly stimulated rat CYP7A1/luciferase reporter activity in HepG2 cells and oxysterol was not required. However, LXRalpha had much less effect on hamster and no significant effect on human CYP7A1 promoter activity in HepG2 cells. In Chinese hamster ovary cells, cotransfection with LXRalpha stimulated reporter activity by less than 2-fold and addition of 22(R)-hydroxycholesterol caused a small but significant stimulation of rat, human and hamster CYP7A1 promoter activity. At least two direct repeats of AGGTCA-like sequences with 4-base spacing (DR4) and five-base spacing (DR5), in previously identified bile acid response elements of the rat CYP7A1 were able to bind LXRalpha/RXRalpha and confer LXRalpha stimulation. However, LXRalpha did not bind to the corresponding sequences of the human gene and bound weakly to hamster and mouse DR4 sequences. Therefore, rats and mice have the unusual capacity to convert cholesterol to bile acids by LXRalpha-mediated stimulation of CYP7A1 transcription, whereas other species do not respond to cholesterol and develop hypercholesterolemia on a diet high in cholesterol.

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.