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.     

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.     

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 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.     

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.     

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.     

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.     

Hyperhomocysteinemia from trimethylation of hepatic phosphatidylethanolamine during cholesterol cholelithogenesis in inbred mice

Because hyperhomocysteinemia can occur in cholesterol gallstone disease, we hypothesized that this may result from trimethylation of phosphatidylethanolamine (PE), which partakes in biliary phosphatidylcholine (PC) hypersecretion during cholesterol cholelithogenesis. We fed murine strains C57L/J, C57BL/6J, SWR/J, AKR/J, PE N-methyltransferase (PEMT) knockout (KO), PEMT heterozygous (HET), and wildtype (WT) mice a cholesterol/cholic acid lithogenic diet (LD) for up to 56 days and documented biliary lipid phase transitions and secretion rates. We quantified plasma total homocysteine (tHcy), folate, and vitamin B12 in plasma and liver, as well as biliary tHcy and cysteine secretion rates. Rate-limiting enzyme activities of PC synthesis, PEMT and cytidine triphosphate: phosphocholine cytidylyltransferase (PCT), S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured in liver homogenates. Other potential sources of plasma tHcy, glycine N-methyltransferase (GNMT) and guanidinoacetate N-methyltransferase (GAMT), were assayed by gene expression. Plasma tHcy and PEMT activities became elevated during cholelithogenesis in gallstone-susceptible C57L, C57BL/6, and SWR mice but not in the gallstone-resistant AKR mice. Persisting in C57L mice, which exhibit the greatest Lith gene burden, these increases were accompanied by elevated hepatic SAM/SAH ratios and augmented biliary tHcy secretion rates. Counter-regulation included remethylation of Hcy to methionine concurrent with decreased folate and vitamin B12 levels and Hcy transsulfuration to cysteine. Concomitantly, methylenetetrahydrofolate reductase (Mthfr), betaine-homocysteine methyltransferase (Bhmt), and cystathionine-β-synthase (Cbs) were up-regulated, but Gnmt and Gamt genes were down-regulated. PEMT KO and HET mice displayed biliary lipid secretion rates and high gallstone prevalence rates similar to WT mice without any elevation in plasma tHcy levels. CONCLUSION: This work implicates up-regulation of PC synthesis by the PEMT pathway as a source of elevated plasma and bile tHcy during cholesterol cholelithogenesis.     

Lith1, a major gene affecting cholesterol gallstone formation among inbred strains of mice.

The prevalence of cholesterol gallstones differs among inbred strains of mice fed a diet containing 15% (wt/wt) dairy fat, 1% (wt/wt) cholesterol, and 0.5% (wt/wt) cholic acid. Strains C57L, SWR, and A were notable for a high prevalence of cholelithiasis; strains C57BL/6, C3H, and SJL had an intermediate prevalence; and strains SM, AKR, and DBA/2 exhibited no cholelithiasis after consuming the diet for 18 weeks. Genetic analysis of the difference in gallstone prevalence rates between strains AKR and C57L was carried out by using the AKXL recombinant inbred strain set and (AKR x C57L)F1 x AKR backcross mice. Susceptibility to gallstone formation was found to be a dominant trait determined by at least two genes. A major gene, named Lith1, mapped to mouse chromosome 2. When examined after 6 weeks on the lithogenic diet, the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.88) was downregulated as expected in the gallstone-resistant strains, AKR and SJL, but this enzyme failed to downregulate in C57L and SWR, the gallstone-susceptible strains. This suggests that regulation of the rate-limiting enzyme in cholesterol biosynthesis may be pivotal in determining the occurrence and severity of cholesterol hypersecretion and hence lithogenicity of gallbladder bile. These studies indicate that genetic factors are critical in determining gallstone formation and that the genetic resources of the mouse model may permit these factors to be identified.     

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.     

Estrogen receptor alpha, but not beta, plays a major role in 17beta-estradiol-induced murine cholesterol gallstones

BACKGROUND & AIMS: Cholesterol gallstones are more common in women than men, and exposure to oral contraceptive steroids and conjugated estrogens increases the risk for gallstones. It is hypothesized that estrogen enhances cholesterol cholelithogenesis by augmenting functions of hepatic estrogen receptors (ERs). METHODS: To investigate molecular mechanisms of how estrogen influences cholesterol gallstones, we studied gonadectomized AKR/J mice of both genders that were implanted subcutaneously with pellets releasing 17beta-estradiol at 0, 3, or 6 microg/day and that were fed a lithogenic diet for 12 weeks. To test the hypothesis that ERs play a pivotal role in mediating lithogenic actions of estrogen and to dissect the potential pathophysiologic roles of each receptor subtype, ERalpha and ERbeta, in the formation of gallstones, we investigated gonadectomized mice treated with synthetic ER subtype-selective agonists or antagonists. RESULTS: 17beta-estradiol promoted gallstone formation by up-regulating hepatic expression of ERalpha but not ERbeta, and the lithogenic actions of estrogen can be blocked completely by the antiestrogenic ICI 182,780. The ERalpha-selective agonist propylpyrazole, but not the ERbeta-selective agonist diarylpropionitrile, augmented hepatic cholesterol output that resulted in cholesterol supersaturated bile and gallstones. Similar to the 17beta-estradiol treatment, tamoxifen significantly increased biliary cholesterol secretion and gallstone prevalence in both gonadectomized females and males. CONCLUSIONS: The hepatic ERalpha, but not ERbeta, plays a critical role in 17beta-estradiol-induced cholesterol gallstones. Our findings may offer a new approach to treat gallstones by inhibiting hepatic ER activity with a liver-specific, ERalpha-selective antagonist.     

Biliary lipid outputs in young women with cholesterol gallstones

Hepatic secretions of biliary lipids were determined in eight young women with cholesterol gallstones and 14 white women without gallstones. All of the gallstone patients were non-Indian; seven were white and one was black. Hourly outputs of biliary cholesterol were significantly greater in gallstone patients than in white controls. This increased cholesterol output was a major factor in the production of lithogenic bile. The greater cholesterol output in gallstone patients was apparently related to obesity. Despite an increased hepatic secretion of cholesterol, secretion rates of bile acids were relatively low in gallstone patients. However, there was considerable overlap between secretion rates of bile acids in subjects with and without stones, and it was not demonstrated that an absolute deficiency of bile acids existed in gallstone patients. Nevertheless, the contribution of an increased output of biliary cholesterol to the formation of lithogenic bile was clearly evident in our patients.     

Cholelithiasis: genetic hypothesis

In inbred mice gallstone susceptibility is determined by Lith (lithogenic) genes which promote cholesterol hypersecretion in bile as a response to a high-fat diet. At least three major classes of proteins can be considered as candidate genes: (a) enzymes involved in cholesterol metabolism regulation; (b) trans-membrane carrier proteins from hepatocyte into bile; (c) cytosolic transfer proteins which regulate intrahepatocyte trafficking. The main candidates are: Spgp, a transmembrane protein which produces bile salt transport. Its gene map in Lith 1 region (Chromosome 2) and its expression is increased in susceptible inbred strains of mice (C57L) of inbred mice. Cmoat is a carrier protein that promotes the secretion of conjugated substances in bile. Its gene map in Lith 2 region (Chromosome 19) and its expression is increased in susceptible inbred strains of mice fed on a lithogenic diet. HGMR is the enzyme that regulates de novo synthesis of cholesterol in the liver. Its activity increases in resistant strains fed on a lithogenic diet and unvaries in susceptible strains. Its gene does not reside in any Lith region, but one Lith gene could be responsible for its activity regulation. To date, the hypothesis of a genetic basis of cholelithiasis in men has only been investigated in one study, in which the association of cholelithiasis and a mutation in the C7AH gene was documented in a group of Mexican patients.     

Inactivation of hepatic microsomal triglyceride transfer protein protects mice from diet-induced gallstones

BACKGROUND & AIMS: Microsomal triglyceride transfer protein (MTTP) is critical for the production of very-low-density lipoproteins (VLDL). The current studies were undertaken to examine the in vivo role of MTTP in hepatic cholesterol and fatty acid metabolism, as well as in biliary lipid secretion. We also tested whether MTTP plays a role in diet-induced cholelithiasis in mice. METHODS: We used mice in which Mttp had been inactivated in the liver (Mttp(Delta/Delta) mice). We measured several parameters of cholesterol metabolism, fatty acid synthesis, and biliary lipid levels in mice fed a normal or a lithogenic diet. We also assessed the incidence of diet-associated gallstones. RESULTS: Hepatic Mttp inactivation markedly decreased plasma triglyceride and cholesterol levels and increased biliary cholesterol and bile acid output. Hepatic cholesterogenesis and fatty acid synthesis were significantly decreased in Mttp(Delta/Delta) mice compared with control mice. The incidence of gallstones decreased from 90% in control mice to 33% in Mttp(Delta/Delta) mice after 8 weeks of a lithogenic diet (P < .0001). The mechanism of the protective effect appears to be increased biliary phospholipid output in Mttp(Delta/Delta) mice, leading to significant unsaturation of gallbladder bile. CONCLUSIONS: These results indicate that modulation of Mttp expression in the liver affects hepatic lipid synthesis and storage as well as biliary lipid secretion. Our findings further indicate that inhibition of hepatic MTTP activity decreases the risk of experimental cholelithiasis by favoring phospholipid output into the bile.     

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.     

Induction of bile acid synthesis by cholesterol and cholestyramine feeding is unimpaired in mice deficient in apolipoprotein AI

High density lipoprotein (HDL) cholesterol is believed to be preferentially utilized for bile acid synthesis and biliary secretion. In mice, the deletion of apolipoprotein AI (apo AI), the major apolipoprotein in HDL, results in very low plasma HDL-cholesterol levels. This article describes bile acid metabolism in apo AI-deficient (Apo AI(-/-)) mice and their C57BL/6 (Apo AI(+/+)) controls fed either a basal rodent diet alone or containing cholesterol or cholestyramine. Basal plasma HDL-cholesterol levels in the (-/-) mice (<10 mg/dL) were less than 20% of those in their (+/+) controls, but there were no phenotypic differences in either the relative cholesterol content of gallbladder bile, bile acid pool size and composition, fecal bile acid excretion or the activity of, or mRNA level for, cholesterol 7alpha-hydroxylase. However, compared with their (+/+) controls, the (-/-) mice absorbed more cholesterol (33 vs. 24%) and manifested lower rates of hepatic sterol synthesis (534 vs. 1,019 nmol/h per g). Cholesterol feeding increased hepatic cholesterol levels in the (+/+) animals from 2.7 to 4.4 mg/g and in the (-/-) mice from 2.6 to 8.1 mg/g. Bile acid synthesis increased 70% in both genotypes. Cholestyramine feeding stimulated bile acid synthesis 3.7 fold in both (-/-) and (+/+) mice. We conclude that the virtual loss of HDL-cholesterol from the circulation in apo AI deficiency has no impact on the ability of the hepatocyte to adapt its rate of bile acid synthesis in concert with the amount of cholesterol and bile acid returning to the liver from the small intestine.     

Sterol carrier protein 2 gene transfer changes lipid metabolism and enterohepatic sterol circulation in mice

BACKGROUND & AIMS: Sterol carrier protein 2 (SCP-2) enhances sterol cycling and facilitates cholesterol translocation between intracellular organelles and plasma membrane in cultured cells, including hepatocytes. We examined the role of SCP-2 in hepatic cholesterol and lipid trafficking through the sinusoidal and canalicular secretory pathways of the liver in vivo. METHODS: Recombinant adenovirus-mediated SCP-2 gene transfer was used to obtain hepatic overexpression of SCP-2 in C57BL/6 mice. RESULTS: SCP-2 overexpression in the mouse liver resulted in an 8-fold increase of SCP-2 protein levels and determined various effects on lipid metabolism. It decreased high-density lipoprotein cholesterol and increased low-density lipoprotein (LDL) cholesterol concentrations. The expressions of hepatic LDL receptor, apolipoprotein (apo) A-I, apoB, and apoE were decreased. SCP-2 overexpression also increased hepatic cholesterol concentration, associated with decreased cholesterol neosynthesis. Increased biliary cholesterol and bile acid secretion, bile acid pool size, and intestinal cholesterol absorption were also observed. CONCLUSIONS: These results indicate that modulation of SCP-2 expression in the liver determines important modifications on lipoprotein metabolism, hepatic cholesterol synthesis and storage, biliary lipid secretion, bile acid metabolism, and intestinal cholesterol absorption.     

Insulin injections enhance cholesterol gallstone incidence by changing the biliary cholesterol saturation index and apo A-I concentration in hamsters fed a lithogenic diet.

BACKGROUND/AIMS: A link between insulin and cholesterol gallstone disease has often been suspected but never demonstrated. The aim was to evaluate the direct implication of insulin in the gallbladder cholesterol gallstone formation process. METHODS: Hamsters fed with a soft-inducing lithogenic diet, enriched with sucrose, were injected daily, for 1 week, either with long-acting insulin or saline (controls). RESULTS: Insulin injections doubled the cholesterol gallstone incidence. The cholesterol saturation index (CSI) of bile significantly increased (+19%) and biliary apolipoprotein A-I (apo A-I) decreased, both in concentration (-71%) and the proportion relative to the total biliary proteins (-25%). No modifications in the biliary bile acid composition were noticed. Hepatic HMGCoA reductase activity was higher (+341%), CYP7A1 activity was lower (-52%), whereas CYP27A1 and CYP7B1 were not affected. The hepatic low-density liprotein (LDL)-receptor and SR-BI masses did not vary. The hepatic total cholesterol content increased (+42%). Fasting plasma phospholipid and triglyceride concentrations significantly decreased (-15 and -60%, respectively), but the cholesterol concentration remained constant. CONCLUSIONS: These results suggest that insulin injections enhance cholesterol gallstone incidence by increasing the CSI of bile and decreasing the concentration and proportion of a biliary anti-nucleating protein, apo A-I. Insulin modulates the major enzymes of cholesterol and bile acid metabolisms in vivo.