Activation of liver X receptor sensitizes mice to gallbladder cholesterol crystallization

Gallstone disease is a hepatobiliary disorder due to biochemical imbalances in the gallbladder bile. In this report, we show that activation of nuclear receptor liver X receptor (LXR) sensitized mice to lithogenic diet-induced gallbladder cholesterol crystallization, which was associated with dysregulation of several hepatic transporters that efflux cholesterol, phospholipids, and bile salts. The combined effect of increased biliary concentrations of cholesterol and phospholipids and decreased biliary concentrations of bile salts in LXR-activated mice led to an increased cholesterol saturation index and the formation of cholesterol crystals. Interestingly, the lithogenic effect of LXR was completely abolished in the low-density lipoprotein receptor (Ldlr) null background or when the mice were treated with Ezetimibe, a cholesterol-lowering drug that blocks intestinal dietary cholesterol absorption. These results suggest that LDLR-mediated hepatic cholesterol uptake and intestinal cholesterol absorption play an essential role in LXR-promoted lithogenesis. Conclusion: The current study has revealed a novel lithogenic role of LXR as well as a functional interplay between LXR and LDLR in gallbladder cholesterol crystallization and possibly cholesterol gallstone disease (CGD). We propose that LXR is a lithogenic factor and that the LXR transgenic mice may offer a convenient CGD model to develop therapeutic interventions for this disease.     

Identification of cholelithogenic enterohepatic helicobacter species and their role in murine cholesterol gallstone formation

BACKGROUND & AIMS: Helicobacter spp are common inhabitants of the hepatobiliary and gastrointestinal tracts of humans and animals and cause a variety of well-described diseases. Recent epidemiologic results suggest a possible association between enterohepatic Helicobacter spp and cholesterol cholelithiasis, chronic cholecystitis, and gallbladder cancer. To test this, we prospectively investigated the effects of Helicobacter spp infection in cholesterol gallstone pathogenesis in the highly susceptible C57L/J mouse model. METHODS: Helicobacter spp-free adult male C57L mice were infected with several different enterohepatic Helicobacter spp or left uninfected and fed either a lithogenic diet or standard mouse chow for 8 and 18 weeks. At the conclusion of the study, bile was examined microscopically and diagnostic culture and polymerase chain reaction were performed. RESULTS: Mice infected with Helicobacter bilis or coinfected with Helicobacter hepaticus and Helicobacter rodentium and fed a lithogenic diet developed cholesterol gallstones at 80% prevalence by 8 weeks compared with approximately 10% in uninfected controls. Monoinfections with H hepaticus , Helicobacter cinaedi , and H rodentium gave a cholesterol gallstone prevalence of 40%, 30%, and 20%, respectively; the latter 2 groups did not differ significantly from uninfected animals. Neither infected nor uninfected mice fed a chow diet developed cholesterol gallstones. CONCLUSIONS: These findings, along with prior epidemiologic studies, suggest that Helicobacter spp play a major role in the pathophysiology of cholesterol gallstone formation in mice and perhaps humans.     

Biliary lipid secretion,bile acid metabolism,and gallstone formation are not impaired in hepatic lipase-deficient mice

Whereas hepatic lipase (HL) has been implicated in lipoprotein metabolism and atherosclerosis, its role in controlling biliary lipid physiology has not been reported. This work characterizes plasma lipoprotein cholesterol, hepatic cholesterol content, bile acid metabolism, biliary cholesterol secretion, and gallstone formation in HL-deficient mice and C57BL/6 controls fed standard chow, a cholesterol-supplemented diet, or a lithogenic diet. Compared with C57BL/6 controls, HL knockout mice exhibited increased basal plasma high-density lipoprotein (HDL) cholesterol as well as reduced cholesterol levels transported in large lipoproteins in response to cholesterol-enriched diets. Hepatic cholesterol content and biliary cholesterol secretion of chow-fed HL knockout and wild-type mice were not different and increased similarly in both strains after feeding dietary cholesterol or a lithogenic diet. There were no differences in biliary bile acid secretion, bile acid pool size and composition, or fecal bile acid excretion between HL-deficient and control mice. HL knockout mice had a similar prevalence of gallstone formation as compared with control mice when both strains were fed with a lithogenic diet. In conclusion, the deficiency of HL has no major impact on the availability of lipoprotein-derived hepatic cholesterol for biliary secretion; HL expression is not essential for diet-induced gallstone formation in mice.     

Apolipoprotein A‐I deficiency does not affect biliary lipid secretion and gallstone formation in mice

Background/Aims: Apolipoprotein A‐I (apo A‐I) is the main protein component of plasma high‐density lipoproteins (HDL) and a key determinant of HDL cholesterol levels and metabolism. The relevance of HDL in controlling the traffic of cholesterol from plasma into bile has been partially addressed. The aim of this study was to evaluate the role of apo A‐I expression in controlling the secretion of biliary lipids as well as the risk of gallstone disease in vivo. Methods: We evaluated biliary lipid secretion and bile acid homeostasis in mice deficient for apo A‐I compared with wild‐type animals when fed with low‐ or high‐cholesterol diets. In addition, we assessed the importance of murine apoA‐I expression for gallstone formation after feeding a lithogenic diet. Results: Bile acid pool size and faecal excretion were within the normal range in chow‐ and cholesterol‐fed apo A‐I knockout (KO) mice. Basal biliary cholesterol secretion was comparable and increased similarly in both murine strains after cholesterol feeding. Lithogenic diet‐fed apo A‐I KO mice exhibited an impaired hypercholesterolaemic response owing to a lower increase in cholesterol levels transported in large lipoproteins. However, the lack of apo A‐I expression did not affect biliary cholesterol precipitation or gallstone formation in lithogenic diet‐fed mice. Conclusions: These findings indicate that biliary lipid secretion, bile acid metabolism and gallstone formation are independent of apo A‐I expression and plasma HDL cholesterol levels in mice.     

Impaired biliary cholesterol secretion and decreased gallstone formation in apolipoprotein E-deficient mice fed a high-cholesterol diet

BACKGROUND & AIMS: Because apolipoprotein E (apoE) is a key cholesterol transport molecule involved in the hepatic uptake of chylomicron cholesterol, it may play a critical role in controlling bile cholesterol elimination and cholesterol gallstone formation induced by dietary cholesterol. To test this hypothesis, we studied biliary lipid secretion and gallstone formation in apoE-deficient mice fed cholesterol-rich diets. METHODS: Bile lipid outputs and gallstone sequence events were analyzed in apoE-deficient mice fed a high-cholesterol diet or a lithogenic diet compared with control animals. RESULTS: A high-cholesterol diet increased biliary cholesterol secretion and gallbladder bile cholesterol concentration in wild-type mice; the increase in bile cholesterol secretion was significantly attenuated in apoE-deficient mice. ApoE knockout mice fed a high-cholesterol lithogenic diet had a markedly lower frequency of gallbladder bile cholesterol crystal and gallstone formation than wild-type mice, which was most likely a result of the decreased cholesterol saturation index found in gallbladder bile of apoE-deficient mice. CONCLUSIONS: These results show that apoE expression is an important factor for regulating both biliary secretion of diet-derived cholesterol as well as diet-induced cholesterol gallstone formation in mice.     

Cholangiocyte bile salt transporters in cholesterol gallstone-susceptible and resistant inbred mouse strains

Background and Aim: We investigated the dietary and gender influences on the expression and functionality of cholangiocyte bile salt transporters and development of biliary hyperplasia in cholesterol gallstone‐susceptible C57L/J and resistant AKR/J mice. Methods: C57L and AKR mice were fed chow, a lithogenic diet, or a cholic acid‐containing diet for 14 days. Expression of cholangiocyte bile salt transporter proteins ASBT (SLC10A2), ILBP (FABP6), and MRP3 (ABCC3) were studied by Western blot analysis. Taurocholate uptake studies were performed using microperfusion of isolated bile duct units. The pre‐ and post‐perfusion taurocholate concentrations were analyzed by high performance liquid chromatography. Biliary proliferation in liver sections was scored. Results: The lithogenic diet induced ductular proliferation in C57L mice. On chow, SLC10A2 and ABCC3 were overexpressed in male and female C57L compared to AKR mice. A lithogenic diet reduced the expressions of FABP6 in both male and female C57L mice, SLC10A2 in female C57L mice, and ABCC3 in male C57L mice. These alterations in transporter expressions were not associated with changes in taurocholate uptake. The lithogenic diet induced biliary hyperplasia and reduced bile salt transporter expressions in C57L mice. Conclusions: Although bile salt uptake was not increased in the bile duct unit, we speculate that the biliary hyperplasia on the lithogenic diet may lead to an increase in intrahepatic bile salt recycling during cholesterol cholelithogenesis.     

Effects of sphingolipid synthesis inhibition on cholesterol gallstone formation in C57BL/6J mice

Background: Sphingolipids play a very important role in cell membrane formation, signal transduction and plasma lipoprotein metabolism. The first rate‐limiting step in the sphingolipid biosynthetic pathway is catalyzed by serine palmitoyltransferase (SPT), and myriocin is a potent and specific inhibitor of SPT. We investigated the impact of SPT inhibition on cholesterol gallstone formation in C57BL/6J mice. Methods: Three groups of eight‐week‐old C57BL/6J mice were utilized. Each group consisted of 20 mice; group A, B, and C were fed normal chow, lithogenic diet with phosphate buffered saline, and lithogenic diet with myriocin (0.3 mg/kg), respectively, for 6 weeks. The ceramide levels in both serum and bile were assessed by high performance liquid chromatography analysis. Protein expression of ERK, JNK and p38 in the extracted gallbladder were determined by Western‐blot analysis. Results: Myriocin treatment caused a significant decrease in the rate of cholesterol gallstone formation. The lithogenic diet mice (group B) showed the highest ceramide activities in both the serum and bile among all the tested groups and there was significant suppression of the ceramide levels in both the serum and bile of the myriocin‐treated mice (group C, p < 0.05). Phosphorylation of p38 in the gallbladder was increased in the lithogenic‐diet mice and the expression of phosphorylated p38 was significantly suppressed in the myriocin treated mice. Conclusions: SPT inhibition by myriocin suppressed gallstone formation and the levels of ceramide in both the serum and bile. p38 in the cellular signaling pathways might be associated with cholesterol gallstone formation.     

Deficiency of Niemann‐Pick C1 protein protects against diet‐induced gallstone formation in mice

Background/aims: Receptor‐mediated endocytosis is a critical cellular mechanism for the uptake of lipoprotein cholesterol in the liver. Because Niemann‐Pick C1 (NPC1) protein is a key component for the intracellular distribution of cholesterol originating from lipoprotein endocytosis, it may play an important role in controlling biliary cholesterol secretion and gallstone formation induced by a lithogenic diet. Methods: We studied biliary cholesterol secretion, gallbladder lipid composition and gallstone formation in NPC1‐deficient mice fed a low‐fat lithogenic diet (1.5% cholesterol and 0.5% cholic acid) compared with control animals under the same diet. Results: The lipid secretion response to the lithogenic diet was impaired in NPC1 (?/?) mice, leading to a decreased cholesterol output and an increased hepatic cholesterol concentration compared with the lithogenic diet‐fed wild‐type mice. A decreased cholesterol saturation index was found in the gallbladder bile of NPC1 (+/?) and (?/?) mice after lithogenic diet feeding. Consequently, mice with a partial or a total deficiency of NPC1 had a drastically lower frequency of gallbladder cholesterol crystals and a reduced prevalence of gallstones. Conclusion: Hepatic NPC1 expression is an important factor for regulating the biliary secretion of diet‐derived cholesterol as well as for diet‐induced cholesterol gallstone formation in mice.     

Biliary lipids and cholesterol crystal formation in leptin-deficient obese mice

Background. Obesity is often associated with increased biliary cholesterol secretion resulting in cholesterol gallstone formation. We have previously demonstrated that leptin-deficient C57Bl/6J Lep ob obese mice have abnormal biliary motility and are prone to cholesterol crystal formation. In addition, others have demonstrated that leptin-deficient mice when fed a lithogenic diet for eight weeks are not prone to gallstone formation. However, the biliary lipid and in vivo cholesterol crystal response of homozygous and heterozygous leptin-deficient mice to four weeks on a lithogenic diet has not been studied. Therefore, we tested the hypothesis that lithogenic diets influence gallbladder bile composition, serum lipids and cholesterol crystal formation in homozygous and heterozygous leptin-deficient mice compared to normal lean controls. Methods. 319 female lean control mice, 280 heterozygous lep ob obese mice and 117 homozygous lep ob obese mice were studied. Mice were fed either a lithogenic or control non-lithogenic chow diet for four weeks. Gallbladder volumes were measured, and bile was pooled to calculate cholesterol saturation indices. Serum cholesterol, glucose, and leptin levels were determined. Hepatic fat vacuoles were counted, and bile was observed microscopically for cholesterol crystal formation. Results. The lithogenic diet and mouse strain influenced body and liver weights, gallbladder volume, cholesterol crystal formation, serum cholesterol, glucose and leptin levels and hepatic fat vacuole numbers. However, only diet, not strain, altered biliary cholesterol saturation. Conclusion. The association among obesity, leptin, and gallstone formation may be primarily related to altered gallbladder motility and cholesterol crystal formation and only secondarily to biliary cholesterol saturation.     

Biliary cholesterol hypersecretion in gallstone-susceptible mice is associated with hepatic up-regulation of the high-density lipoprotein receptor SRBI

Enhanced hepatocellular trafficking of cholesterol to the bile canaliculus and cholesterol hypersecretion appears critical for gallstone formation. Therefore, we studied in more detail the hepatic cholesterol transport pathways in a mouse model of cholesterol gallstone disease. Biliary lipid secretion rates, plasma lipoprotein levels, hepatic expression of lipoprotein receptors, lipid regulatory enzymes, and putative cholesterol transporting proteins were analyzed in gallstone-susceptible C57L/J and gallstone-resistant AKR/J mice, which were fed a lithogenic diet. Biliary cholesterol hypersecretion in C57L mice was associated with decreased plasma high-density lipoprotein (HDL) cholesterol levels and significant hepatic induction of the HDL receptor (SRBI) and cholesteryl ester hydrolase. In response to the lithogenic diet, fatty-acid binding protein of liver (FABPL) was markedly induced in both mouse strains. Caveolin 1 was elevated only in plasma membranes of gallstone-susceptible C57L mice, which also failed to down-regulate cholesterol synthesis. These data suggest a role of the reverse cholesterol transport pathway for genetically determined gallstone susceptibility in the mouse.     

Transgenic overexpression of human lecithin: cholesterol acyltransferase (LCAT) in mice does not increase aortic cholesterol deposition

Results from several atherosclerosis studies using morphometric procedures have proven controversial with regard to whether over-expression of human LCAT in transgenic (Tg) mice is atherogenic. The purpose of the present study was to determine the effect of 10-fold over-expression of human LCAT on aortic free and esterified cholesterol (EC) deposition as well as plasma lipoprotein cholesteryl ester (CE) fatty acid composition in mice fed an atherogenic diet containing cholic acid. C57Bl/6 (control) and human LCAT-Tg mice were fed chow or an atherogenic diet (15% of calories from palm oil, 1.0% cholesterol and 0.5% cholic acid) for 24 weeks before measurement of aortic cholesterol content. Compared with the chow diet, control and LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in plasma total, free and EC, a 7-fold increase in plasma apoB lipoprotein cholesterol, and a 40-50-fold increase in hepatic cholesterol content. The aortic EC content was increased in control (0.7 vs. 1.2 mg/g protein) and LCAT-Tg (0.3 vs. 1.5 mg/g protein) mice fed the atherogenic diet compared with those consuming the chow diet; however, there was no difference in aortic free (14.4+/-6.8 vs. 18.5+/-7.7 mg/g protein) or esterified (1.2+/-1.0 vs. 1.5+/-1.2 mg/g protein) cholesterol content between atherogenic diet-fed control and LCAT-Tg mice, respectively. LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in the ratio of saturated+monounsaturated to polyunsaturated CE species in plasma apoB lipoproteins compared with control mice (9.4+/-2.4 vs. 4.9+/-0.7). We conclude that over-expression of human LCAT in Tg mice fed an atherogenic diet containing cholic acid does not result in increased aortic cholesterol deposition compared with control mice, even though the CE fatty acid saturation index of plasma apoB lipoproteins was doubled.     

Effect of Beta-sitosterol on Cholesterol-cholic Acid-induced Gallstone Formation in Mice

Beta-sitosterol has been shown to prevent gallstone formation in mice fed 1.2% cholesterol and 0.5% cholic acid (lithogenic diet). The incidence of gallstone formation in the mouse by the addition of 2.5% sitosterol in the lithogenic diet is about 35.5% in male and 25% in female. The condition of the liver, whether fatty or normal, did not correlate with the presence or absence of cholelithiasis. The serum and liver cholesterol levels of mice fed either sitosterol and cholesterol or sitosterol and cholic acid is lower than those of mice fed cholesterol or cholic acid alone. Elevation, of liver phospholipid concentration was noticed in mice fed sitosterol or a combination of sitosterol with cholesterol or cholic acid or both cholesterol and cholic acid.     

Relation between hepatic expression of ATP-binding cassette transporters G5 and G8 and biliary cholesterol secretion in mice

BACKGROUND/AIM: Mutations in genes encoding the ATP-binding cassette (ABC)-transporters ABCG5 and ABCG8 underlie sitosterolemia, which is characterized by elevated plasma levels of phytosterols due to increased intestinal absorption and impaired biliary secretion of sterols. The aim of our study was to correlate the expression levels of Abcg5 and Abcg8 to biliary cholesterol secretion in various (genetically-modified) mouse models. METHODS: Bile was collected from genetically-modified mice fed a chow diet, or from mice fed either a chow diet, or chow supplemented with either 1% diosgenin, 0.1% simvastatin, or a synthetic liver X receptor agonist, for determination of biliary lipids. Livers and small intestines were harvested and expression levels of Abcg5, Abcg8 and Abcb4 were determined by real-time polymerase chain reaction. RESULTS: Intestinal expression of Abcg5 and Abcg8 did not show much variation between the various models. In contrast, a linear correlation between hepatic expression levels of Abcg5 and Abcg8 and biliary cholesterol secretion rates was found. This relation was independent of Abcb4-mediated phospholipid secretion. However, in diosgenin-fed mice showing cholesterol hypersecretion, hepatic Abcg5 and Abcg8 expression levels remained unchanged. CONCLUSIONS: Our results strongly support a role for Abcg5 and Abcg8 in regulation of biliary cholesterol secretion, but also indicate the existence of a largely independent route of cholesterol secretion.     

LXRβ activation increases intestinal cholesterol absorption,leading to an atherogenic lipoprotein profile

Abstract. Hu X, Steffensen KR, Jiang Z‐Y, Parini P, Gustafsson J‐Å, Gåfvels M, Eggertsen G (Karolinska Institutet, Huddinge, Stockholm, Sweden; Karolinska Institutet, Huddinge, Stockholm, Sweden; Shanghai Institute of Digestive Surgery, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China; and University of Houston, Houston, TX, USA). LXRβ activation increases intestinal cholesterol absorption, leading to an atherogenic lipoprotein profile. J Intern Med 2012; 272: 452–464. Objectives. Liver X receptors (LXRs) are essential for the regulation of intestinal cholesterol absorption. Because two isoforms exist, LXRα and LXRβ, with overlapping but not identical functions, we investigated whether LXRα and LXRβ exert different effects on intestinal cholesterol absorption. Design. Wild‐type (WT), LXRα^?/? and LXRβ^?/? mice were fed control diet, 0.2% cholesterol‐enriched diet or 0.2% cholesterol‐enriched diet plus the LXR agonist GW3965. Results. When fed a control diet, all three genotypes showed similar levels of cholesterol absorption. Of interest, a significant increase in cholesterol absorption was found in the LXRα^?/? mice, but not in the WT or LXRβ^?/? animals, when fed a diet enriched with 0.2% cholesterol or 0.2% cholesterol + GW3965. Reduced faecal neutral sterol excretion and a hydrophobic bile acid profile were also observed in LXRα^?/? mice. Greater increases in the apolipoprotein (apo)B‐containing lipoproteins in serum were seen in the LXRα^?/? mice. A 0.2% cholesterol + GW3965 diet suppressed intestinal Npc1l1 protein expression to the same extent for all genotypes, while Abca1 and Abcg5 were elevated to the same degree. Conclusions. In the intestine, LXRα and LXRβ seem to exert similar effects on expression of cholesterol‐transporting proteins such as Npc1l1. Selective activation of LXRβ may generate effects such as increased cholesterol absorption and elevated serum levels of apoB‐containing lipoproteins, which seem to be counteracted by LXRα. Therefore, an intestinal LXRβ‐specific pathway might exist in terms of cholesterol transportation in addition to the main pathway.     

Gallstone prevention in prairie dogs: comparison of chow vs. semisynthetic diets

The effects of a standard rodent chow were compared with those of a semisynthetic diet of known composition (with and without added cholesterol) in the prairie dog model of cholesterol cholelithiasis. Gallstone incidence was 40% higher in animals fed a semisynthetic diet plus cholesterol compared to chow plus cholesterol. The semisynthetic diet plus cholesterol caused significant increases in tissue cholesterol levels (serum, liver and bile) and lithogenic index, but significant decreases in the activity of hepatic 3-hydroxy-3-methyl-glutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase compared to chow plus cholesterol. Histologic study of liver sections revealed that the semisynthetic diet plus cholesterol resulted in moderate to marked portal tract changes characterized by bile duct proliferation, inflammatory infiltration and fibrosis, whereas the cholesterol-supplemented chow diet caused only slight bile duct proliferation with minimal inflammation and fibrosis in the portal areas. Dietary hyodeoxycholic acid prevented cholesterol gallstones and biliary cholesterol crystals when added to either chow plus cholesterol or semisynthetic plus cholesterol diets. The hyodeoxycholic acid supplements also prevented the development of severe histopathologic alterations along the portal tracts. Biliary cholesterol levels were elevated in prairie dogs fed cholesterol plus hyodeoxycholic acid; these animals had liquid crystals in the bile, and hyodeoxycholic acid and its 6 beta-isomer became the major biliary bile acids. A semisynthetic diet plus cholesterol is superior to a high cholesterol chow diet for gallstone formation and prevention studies, but in prolonged feeding experiments, the potential hepatotoxicity of this diet in the prairie dog must be appreciated.     

Modeling plasma lipoprotein-bile lipid relationships: Differential impact of psyllium and cholestyramine in hamsters fed a lithogenic diet

Hamsters fed a lithogenic diet become hyperlipemic with elevated very-low-density lipoprotein (VLDL) and high-density lipoprotein 2 (HDL₂) cholesterol pools and develop lithogenic bile in which chenodeoxycholate (cheno) typically predominates. The relationship between these distorted lipoprotein and bile lipid profiles and gallstone induction was investigated in male Syrian hamsters fed for 5 weeks a gallstone-inducing purified diet (5% butter, 0.4% cholesterol) or the same diet supplemented with 5% psyllium or 1% cholestyramine, agents known to alter bile acid metabolism. The gallstone diet essentially doubled plasma cholesterol level, whereas psyllium decreased it to near normal, and cholestyramine to a subnormal level, while correcting the distorted distribution of cholesterol among lipoproteins. Both the gallstone diet and psyllium produced cholesterol-laden livers, in contrast to subnormal values produced by cholestyramine. Fecal bile acid excretion was increased eightfold with cholestyramine and fourfold with psyllium relative to the value produced by the gallstone diet and a literature control value. Supersaturated bile developed with the gallstone diet (lithogenic index [LI], 2.3 ± 0.6), whereas the LI was decreased by psyllium (1.2 ± 0.4) and cholestyramine (0.7 ± 0.3). The gallstone diet decreased the concentration of bile acids in gallbladder bile, but greatly increased the percentage of taurochenodeoxycholic acid, whereas psyllium preferentially decreased all taurine-conjugated bile acid levels and expanded glycocholate output. Cholestyramine greatly decreased the secretion of biliary cholesterol and cheno independent of its conjugation. Accordingly, psyllium increased the glycine to taurine ratio of gallbladder bile fivefold, whereas cholestyramine did not affect this ratio, but increased the cholate to cheno ratio dramatically (25-fold) as compared with a threefold increase with psyllium. This combination of biliary lipid and bile acid alterations induced coordinated responses in the LI and the hydrophobicity index (HI) such that cholesterol gallstones developed in 11 of 12 hamsters fed the gallstone diet, whereas only one of 11 of the psyllium-fed and none of 12 cholestyramine-fed hamsters had cholesterol stones. Thus, psyllium and cholestyramine differentially increased bile acid excretion, which improved the lipoprotein profile and inhibited cholesterol gallstone formation. Both agents operated by different means to decrease biliary cholesterol secretion and the percentage of cheno, which decreased the LI and HI, respectively.     

Reduced susceptibility to cholesterol gallstone formation in mice that do not produce apolipoprotein B48 in the intestine

It has been found that polymorphisms in the apolipoprotein (APO)-B gene are associated with cholesterol gallstones in humans. We hypothesized that APO-B plays a major regulatory role in the response of biliary cholesterol secretion to high dietary cholesterol and contributes to cholesterol gallstone formation. In the present study, we investigated whether lack of expression of intestinal Apob48 or Apob100 reduces susceptibility to cholesterol gallstones by decreasing intestinal absorption and biliary secretion of cholesterol in male mice homozygous for an "APO-B48 only" allele (Apob(48/48)), an "APO-B100 only" allele (Apob(100/100)), or a wild-type APO-B allele (Apob+/+) before and during an 8-week lithogenic diet. We found that cholesterol absorption was significantly decreased as a result of the APO-B48 deficiency in Apob(100/100) mice compared with wild-type and Apob(48/48) mice, regardless of whether chow or the lithogenic diet was administered. Consequently, hepatic cholesterol synthesis was significantly increased in Apob(100/100) mice compared with wild-type and Apob(48/48) mice. On chow, the APO-B100 deficiency in Apob(48/48) mice with reduced plasma levels of LDL/VLDL--but not HDL cholesterol--induced relative hyposecretion of biliary bile salts and phospholipids accompanying normal biliary cholesterol secretion. Compared with Apob(48/48) and wild-type mice, lithogenic diet-fed Apob(100/100) mice displayed significantly lower secretion rates of biliary cholesterol, but not phospholipid or bile salts, which results in significant decreases in prevalence rates, numbers, and sizes of gallstones. In conclusion, absence of expression of intestinal Apob48, but not Apob100, reduces biliary cholesterol secretion and cholelithogenesis, possibly by decreasing intestinal absorption and hepatic bioavailability.     

Increased fecal neutral sterol loss upon liver X receptor activation is independent of biliary sterol secretion in mice

BACKGROUND & AIMS: Reverse cholesterol transport (RCT) is defined as high-density lipoprotein (HDL)-mediated flux of excess cholesterol from peripheral cells to liver, followed by secretion into bile and disposal via the feces. Various steps of this pathway are controlled by the liver X receptor (LXR). We addressed the role of the intestine in LXR-dependent stimulation of fecal cholesterol excretion. METHODS: To segregate biliary from intestine-derived cholesterol, wild-type and Mdr2 P-glycoprotein-deficient mice ( Mdr2 -/- ), which are unable to secrete cholesterol into bile, were treated with the LXR agonist GW3965. RESULTS: Treatment with GW3965 increased biliary cholesterol secretion by 74% in wild-type mice but had no effect in Mdr2 -/- mice. LXR activation increased fecal neutral sterol excretion 2.1-fold in wild-type mice. Surprisingly, an identical increase was observed in Mdr2 -/- mice. Fractional cholesterol absorption was reduced on LXR activation in both strains but was more pronounced in Mdr2 -/- mice, coinciding with reduced Npc111 expression. Intestinal gene expression of ATP-binding cassette transporters (Abc) Abca1 , Abcg1 , Abcg5 , and Abcg8 was strongly induced upon LXR activation in both strains, whereas expression of HMGCoA reductase , controlling cholesterol synthesis, remained unaffected. Additionally, LXR activation stimulated the excretion of plasma-derived [ 3 H]cholesterol into the fecal neutral sterol fraction in Mdr2 -/- mice. CONCLUSIONS: Increased fecal cholesterol loss upon LXR activation is independent of biliary cholesterol secretion in mice. An important part of excess cholesterol is excreted directly via the intestine, supporting the existence of an alternative, quantitatively important route for cholesterol disposal.