Effect of cholestyramine treatment on biliary lipid secretion rates in normolipidaemic men

Bile acid sequestration normalizes plasma cholesterol and reduces atherosclerosis in hypercholesterolemic mice. No additional effect of physical activity

AIMS

Bile acid sequestrants (BAS) and physical activity (RUN) decrease incidence of cardiovascular events. Both treatments are often prescribed, yet it is not known whether their beneficial effects are additive. We assessed the effects of BAS treatment alone and in combination with RUN on cholesterol metabolism, heart function and atherosclerotic lesion size in hypercholesterolemic mice.

METHODS

Male Ldlr-deficient mice remained either sedentary (CONTROL), were treated with Colesevelam HCl (BAS), had access to a running wheel (RUN), or were exposed to BAS and RUN (BAS RUN). All groups were fed a high cholesterol diet for 12 weeks. Then, feces, bile and plasma were collected. Atherosclerotic lesion size was determined in the aortic arch and heart function by echocardiography.

RESULTS

BAS RUN ran more than RUN (6.4 ± 1.4 vs. 3.5 ± 1.0 km/day, p < 0.05). BAS and BAS RUN displayed ~3-fold reductions in plasma cholesterol levels (p < 0.001), ~2.5-fold increases in fecal neutral sterol (p < 0.001) and bile acid (p = 0.01) outputs, decreases in biliary secretions of cholesterol (~6-fold, p < 0.0001) and bile acids (~2-fold, p < 0.001) vs. CONTROL while no significant effects were observed in RUN. Compared to CONTROL, lesion size decreased by 78% in both BAS and BAS RUN, (p < 0.0001).

CONCLUSION

BAS reduce atherosclerosis in Ldlr-deficient mice, coinciding with a switch from body cholesterol accumulation to cholesterol loss. RUN slightly modulated atherosclerotic lesion formation but the combination of BAS and RUN had no clear additive effects in this respect.

Statins as potential treatment for cholesterol gallstones: an attempt to understand the underlying mechanism of actions

INTRODUCTION

Statin therapy is widely used across the globe for the treatment and prevention of cardiovascular disease (CVD). It is well established that statin therapy is associated with significant decreases in low-density lipoprotein cholesterol (LDL-C) and plasma cholesterol levels. Cholesterol gallstones are a common problem, resulting in hospital admission and surgery, throughout western healthcare systems.

AREAS COVERED

This review describes the mechanisms, and addresses the potential, for statins to be used as a treatment for gallstones. Medline was searched for the risk factors and treatment of cholesterol gallstones.

EXPERT OPINION

Obesity, metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), insulin resistance and high-fat diets (unsaturated fats) rich in cholesterol are all associated risk factors for cholesterol gallstones. In view of the high prevalence of cholesterol gallstones, there is an urgent need to understand whether pharmacological therapies can be harnessed for the treatment of cholesterol gallstones. Gallstones are shown to be associated with an increased risk, not only of mortality, but also of CVD. Statins, widely used in prevention of CVD and hypercholesteremia, have been shown to dissolve cholesterol gallstones in animal models and human studies, highlighting the potential for a pharmacological therapy for gallstones. More studies are required to understand the role of statins in the treatment of gallstones and for comparison with current treatment strategies.

Cholecystectomy increases hepatic triglyceride content and very-low-density lipoproteins production in mice

BACKGROUND & AIMS

Bile acid (BA) pool size remains unchanged after cholecystectomy (XGB) but it circulates faster, exposing the enterohepatic system to an increased flux of BA. Triglyceride (TG) and BA metabolisms are functionally inter-related. We investigated whether ablation of the gallbladder (GB) modifies hepatic TG metabolism.

METHODS

Male mice were subjected to XGB and fed a normal diet. In some experiments, mice received a 1% nicotinic acid diet to block lipolysis. Parameters of BA and TG metabolism, and microsomal triglyceride transfer protein (MTTP) activity were measured 1-2 months after XGB. Serum parameters, hepatic lipids and mRNA expression of genes of lipid metabolism were determined.

RESULTS

BA pool size and synthesis were normal, but biliary BA secretion doubled during the diurnal light phase in XGB mice. Serum and hepatic TG concentrations increased 25% (P<0.02), and hepatic very-low-density lipoproteins (VLDL)-TG and apoB-48 productions increased 15% (P<0.03) and 50% (P<0.01), respectively, after XGB. Feeding a 1% nicotinic acid did normalize VLDL production. MTTP activity increased 15% (P<0.005) after XGB. Hepatic free fatty acid (FFA) synthesis and content, and mRNA levels of lipid metabolism-related genes remained normal in XGD mice.

CONCLUSIONS

XGB increased serum and hepatic TG levels, and VLDL production, which were restored to normal by nicotinic acid. The results suggest that FFA flux from adipose tissue to the liver is increased in XGB mice. They support the hypothesis that the GB has a role in the regulation of hepatic TG metabolism and that XGB may favour the accumulation of fat in the liver.

Modulating effect of the A-278C promoter polymorphism in the cholesterol 7alpha-hydroxylase gene on serum lipid levels in normolipidaemic and hypertriglyceridaemic individuals

The rate-limiting enzyme in the conversion of cholesterol into bile acids is cholesterol 7alpha-hydroxylase (CYP7A1). An A to C substitution 278 bp upstream in the promoter of the CYP7A1 gene was found to be associated with variations in serum lipid levels in normolipidaemic populations. In the present study, we investigated the involvement of this polymorphism in four different lipid disorders: hypertriglyceridaemia (HTG), combined hyperlipidaemia (CH), familial dysbetalipoproteinaemia (FD) and familial hypercholesterolaemia (FH). In a normolipidaemic male population, homozygous for the apoE3 isoform, an association was found between the AA genotype and higher levels of serum triglycerides (AA: +34%, P = 0.036). In HTG patients, the AA genotype was associated with significantly higher concentrations of total cholesterol (+23%, P = 0.005). There was a tendency towards increased levels of serum triglycerides (+39%, P = 0.06), VLDL-triglycerides (+48%, P = 0.053) and VLDL-cholesterol (+35%, P = 0.059). No significant associations were found between serum lipid levels and the CYP7A1 polymorphism in patients with CH, FD and FH. Our results show that the A-278C polymorphism in the CYP7A1 gene has an effect on triglyceride levels in normolipidaemic males and on cholesterol levels in patients with hypertriglyceridaemia..

Cholesterol 7α-Hydroxylase Deficiency in Mice on an APOE*3-Leiden Background Impairs Very-Low-Density Lipoprotein Production

OBJECTIVE

Cholesterol 7alpha-hydroxylase (cyp7a1) catalyzes the rate-limiting step in conversion of cholesterol to bile acids. To study the relationship between bile acid biosynthesis and triglyceride metabolism, we cross-bred mice lacking cyp7a1 on a hyperlipidemic APOE*3-Leiden background.

METHODS AND RESULTS

Female mice received a chow or lipogenic diet. On both diets, fecal bile acid excretion was 70% decreased concomitantly with a 2-fold increased neutral sterol output. The differences in bile acid biosynthesis did not change plasma cholesterol levels. However, plasma triglyceride levels decreased by 41% and 38% in the cyp7a1-/-. APOE*3-Leiden mice as compared with APOE*3-Leiden mice on chow and lipogenic diet, respectively. Mechanistic studies showed that very-low-density lipoprotein (VLDL)-apolipoprotein B and VLDL-triglyceride production rates were reduced in cyp7a1-/-. APOE*3-Leiden mice as compared with APOE*3-Leiden mice (-34% and -35%, respectively). Cyp7a1 deficiency also increased the hepatic cholesteryl ester and triglyceride content (2.8-fold and 2.5-fold, respectively). In addition, hepatic anti-oxidative vitamin content, which can influence VLDL-production, was lower. Hepatic mRNA analysis showed decreased expression of genes involved in lipogenesis including srebf1.

CONCLUSIONS

Cyp7a1 deficiency in APOE*3-Leiden mice decreases the VLDL particle production rate, as a consequence of a strongly reduced bile acid biosynthesis, leading to a decrease in plasma triglycerides. These data underscore the close relationship between bile acid biosynthesis and triglyceride levels.

Role of the enterohepatic circulation of bile salts in lipoprotein metabolism

The enterohepatic circulation of bile salts and cholesterol plays a central role in maintaining whole body cholesterol homeostasis. Hepatic lipoprotein metabolism is reviewed and the role of disturbances in bile salt metabolism in the pathogenesis of dyslipidemias is discussed. Further, the manipulation of bile salt metabolism to treat dyslipidemia is reviewed.

Complex genetic control of HDL levels in mice in response to an atherogenic diet. Coordinate regulation of HDL levels and bile acid metabolism

Inbred strains of mice differ in susceptibility to atherogenesis when challenged with a high fat, high cholesterol diet containing 0.5% cholic acid. Studies of recombinant inbred (RI) strains derived from the susceptible strain C57BL/6J (B6) and the resistant strains C3H/HeJ (C3H) and BALB/cJ have revealed an association between fatty streak lesion size and a decrease in high density lipoprotein (HDL) levels on the diet. To better understand the genetic factors contributing to HDL metabolism and atherogenesis in response to the diet, we studied mice derived from an intercross between B6 and C3H using a complete linkage map approach. A total of 185 female progeny were typed for 134 genetic markers spanning the mouse genome, resulting in an average interval of about 10 cM between markers. A locus on distal chromosome 1 containing the apolipoprotein AII gene was linked to HDL-cholesterol levels on both the chow and the atherogenic diets, but this locus did not contribute to the decrease in HDL-cholesterol in response to the diet. At least three distinct genetic loci, on chromosomes 3, 5, and 11, exhibited evidence of linkage to a decrease in HDL-cholesterol after a dietary challenge. Since a bile acid (cholic acid) is required for the diet induced changes in HDL levels and for atherogenesis in these strains, we examined cholesterol-7-alpha hydroxylase (C7AH) expression. Whereas B6 mice exhibited a large decrease in C7AH mRNA levels in response to the diet, C3H showed an increase. Among the intercross mice, multiple loci contributed to the regulation of C7AH mRNA levels in response to the diet, the most notable of which coincided with the loci on chromosomes 3, 5, and 11 controlling HDL levels in response to the diet. None of these loci were linked to the C7AH structural gene which we mapped to proximal chromosome 4. These studies reveal coordinate regulation of C7AH expression and HDL levels, and they indicate that the genetic factors controlling HDL levels are more complex than previously suggested by studies of RI strains. Furthermore, we observed that two of the loci for C7AH expression contributed to differences in gallstone formation between these strains.

Effects of cholestyramine and parenteral nutrition on hepatic metabolism of lidocaine: a study using isolated rat liver perfusion

BACKGROUND

The effect of an oral bile salt binder, cholestyramine, on parenteral nutrition-related hepatic dysfunction and lidocaine metabolism was studied in rats.

METHODS

Rats were randomly assigned to one of three treatment groups: the PN group received infusions of dextrose and amino acids; the PNC group was treated the same as the PN group, but also received oral cholestyramine; CF group animals were fed rat food and water. Lidocaine metabolism was studied in livers isolated from animals after 7 days of parenteral nutrition.

RESULTS

No differences in liver function test values of PN and PNC groups were detected compared with group fed rat food. However, lidocaine metabolism was found to be significantly reduced in both the PN and PNC groups. Significant reductions were observed in the hepatic extraction ratio (23% and 15%) and in intrinsic clearance (61% and 53%) in PN and PNC animals, respectively (p < .05). Material balance at steady state showed that recovery of lidocaine was threefold higher in the PN group and twofold higher in the PNC group than the rat food group (p < .05). Metabolite-to-drug ratios were determined for each lidocaine metabolite and this revealed significant reductions in N-dealkylation (64% and 57%) and aryl methyl hydroxylation (92% and 86%) in PN and PNC animals, respectively (p < .05).

CONCLUSIONS

Histologic findings suggest that cholestyramine feeding prevented liver dysfunction, possibly through interruption of secondary bile salt reabsorption. However, lidocaine metabolism was still impaired after cholestyramine ingestion; the impairment mechanism remains unknown.

Apparent selective bile acid malabsorption as a consequence of ileal exclusion: effects on bile acid, cholesterol, and lipoprotein metabolism

A new model has been developed to characterise the effect of a standardised ileal exclusion on bile acid, cholesterol, and lipoprotein metabolism in humans. Twelve patients treated by colectomy and ileostomy for ulcerative colitis were studied on two occasions: firstly with a conventional ileostomy and then three months afterwards with an ileal pouch operation with an ileoanal anastomosis and a protective loop ileostomy, excluding on average 95 cm of the distal ileum. The ileostomy contents were collected during 96 hours and the excretion of bile acids and cholesterol was determined using gas chromatography-mass spectrometry. Fasting blood and duodenal bile samples were collected on two consecutive days. After the exclusion of the distal ileum, both cholic and chenodeoxycholic acid excretion in the ileostomy effluent increased four to five times without any change in cholesterol excretion. Serum concentrations of lathosterol (a marker of cholesterol biosynthesis) and 7 alpha-hydroxycholesterol (a marker for bile acid biosynthesis) were increased several fold. Plasma concentrations of total VLDL triglycerides were also increased whereas the concentrations of total and LDL cholesterol, and apolipoprotein B were decreased. There were no changes in biliary lipid composition or cholesterol saturation of bile. The results show that the exclusion of about 95 cm of distal ileum causes malabsorption of bile acids but apparently not of cholesterol. The bile acid malabsorption leads to increased synthesis of both bile acids and cholesterol in the liver. It is suggested that bile acids can regulate cholesterol synthesis by a mechanism independent of the effect of bile acids on cholesterol absorption. The enhanced demand for cholesterol also leads to a decrease in plasma LDL cholesterol and apolipoprotein B concentrations. The malabsorption of bile acids did not affect biliary lipid composition or cholesterol saturations of VLDL triglycerides.