Evidence for a common biliary cholesterol and VLDL cholesterol precursor pool in rat liver

Determinants of transhepatic cholesterol flux and their relevance for gallstone formation

Cholesterol available for bile secretion is controlled by a wide variety of proteins that mediate lipoprotein cholesterol uptake and cholesterol transport and metabolism in the liver. From a disease perspective, abnormalities in the transhepatic traffic of cholesterol from plasma into the bile may influence the risk of cholesterol gallstone formation. This review summarizes some recent progress in understanding the hepatic determinants of biliary cholesterol secretion and its potential pathogenic implications in cholesterol gallstone disease. This information together with new discoveries in this field may lead to improved risk evaluation, novel surrogate markers and earlier diagnosis, better preventive approaches and more effective pharmacological therapies for this prevalent human disease.

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.

Impaired biliary lipid secretion in obese Zucker rats: leptin promotes hepatic cholesterol clearance

Human obesity is associated with elevated plasma leptin levels. Obesity is also an important risk factor for cholesterol gallstones, which form as a result of cholesterol hypersecretion into bile. Because leptin levels are correlated with gallstone prevalence, we explored the effects of acute leptin administration on biliary cholesterol secretion using lean (FA/-) and obese (fa/fa) Zucker rats. Zucker (fa/fa) rats become obese and hyperleptinemic due to homozygosity for a missense mutation in the leptin receptor, which diminishes but does not completely eliminate responsiveness to leptin. Rats were infused intravenously for 12 h with saline or pharmacological doses of recombinant murine leptin (5 microg x kg(-1) x min(-1)) sufficient to elevate plasma leptin concentrations to 500 ng/ml compared with basal levels of 3 and 70 ng/ml in lean and obese rats, respectively. Obesity was associated with a marked impairment in biliary cholesterol secretion. In biles of obese compared with lean rats, bile salt hydrophobicity was decreased whereas phosphatidylcholine hydrophobicity was increased. High-dose leptin partially normalized cholesterol secretion in obese rats without altering lipid compositions, implying that both chronic effects of obesity and relative resistance to leptin contributed to impaired biliary cholesterol elimination. In lean rats, acute leptin administration increased biliary cholesterol secretion rates. Without affecting hepatic cholesterol contents, leptin downregulated hepatic activity of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, upregulated activities of both sterol 27-hydroxylase and cholesterol 7alpha-hydroxylase, and lowered plasma very low-density lipoprotein cholesterol concentrations. Increased biliary cholesterol secretion in the setting of decreased cholesterol biosynthesis and increased catabolism to bile salts suggests that leptin promotes elimination of plasma cholesterol.

Enhancement of fatty acid and cholesterol synthesis accompanied by enhanced biliary but not very-low-density lipoprotein lipid secretion following sustained pravastatin blockade of hydroxymethyl glutaryl coenzyme A reductase in rat liver

A 3-week treatment of rats with pravastatin (PV) augmented biliary cholesterol and phospholipid output 3.6- and 2.2-fold over controls, while bile acid (BA) output and kinetics were unchanged. No major changes were detected in hepatic and serum cholesterol concentrations despite the PV inhibitory property on hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase. To evaluate the mechanisms of this adaptive phenomenon, several parameters of hepatic lipid homeostasis were assessed. Biliary cholesterol changes could not be attributed to an increased influx of lipoprotein cholesterol to the liver and bile. Hepatic low-density lipoprotein (LDL) receptor content, as inferred from Western blot analysis, was unchanged, as was the biliary excretion of labeled cholesterol derived from chylomicron remnants. In vivo 3H2O-incorporation studies showed an 80% increase in hepatic cholesterol synthesis, evidence for bypass of the PV block. Remarkably, fatty acid synthesis was also stimulated twofold, providing substrate for hepatic triglycerides, which were slightly enhanced. However, serum triglycerides decreased 52% associated with a 22% decrease in hepatic very-low-density lipoprotein (VLDL) secretion. Thus, the biochemical adaptation following PV treatment produces complex alterations in hepatic lipid metabolism. An enhanced supply of newly synthesized cholesterol and fatty acids in association with a limited VLDL secretion rate augments the biliary lipid secretion pathway in this experimental model.

The mechanism underlying the hypocholesterolemic effect of chronic fish oil feeding in rats is not due to increased excretion of dietary cholesterol

The role of the excretion of dietary cholesterol in the hypocholesterolaemic effect of chronic fish oil feeding in rats was investigated. The hepatic uptake and processing of [3H]cholesterol carried in chylomicrons derived from fish oil was studied in vivo in rats fed a low fat diet or a diet supplemented with fish oil for 21 days. In addition, the effects of the fish oil diet on cholesterol esterification, cholesteryl ester hydrolysis, bile acid synthesis and biliary lipid secretion were determined. In rats fed the fish oil as compared to the low fat diet, the uptake of [3H]cholesterol from the blood and its secretion into bile as bile acids was significantly slower, and this was entirely due to a decrease in the bile acid fraction. Biliary bile acid mass secretion was unchanged by fish oil feeding, while biliary cholesterol and phospholipid secretion was increased. No significant differences were observed either in the expression of mRNA for cholesterol 7alpha hydroxylase or the secretion of bile acids into bile after 20 h biliary drainage between the fish oil and low fat diet groups, suggesting that bile acid synthesis is not affected. These results indicate that the access of chylomicron cholesterol to the hepatic substrate pool for bile acid formation is decreased in the fish oil fed rats, and this, together with its slower uptake from the blood, accounts for the retardation of its excretion via the bile. Thus, the hypocholesterolemic effect of dietary fish oil in rats is not due to more rapid metabolism of cholesterol originating from the diet.

The effects of cholesterol uptake from high-density lipoprotein subfractions on biliary sterol secretion in rats with essential fatty-acid deficiency

High-density lipoprotein (HDL) participates in the transfer of cholesterol to the liver, in which it is subsequently excreted into bile as bile acid and cholesterol. In this study, the effect of essential fatty-acid (EFA) deficiency on cholesterol contribution from HDL subfractions to bile was investigated. Rats that were rendered EFA-deficient over 4 weeks displayed changes in their plasma HDL subfractions and liver tissue fatty acids. Plasma linoleic (18:2n6), linolenic (18:3n3,) and arachidonic (20:4n6) acids decreased, whereas palmitoleic (16:1n7) and eicosatrienoic (20:3n9) acids increased. EFA deficiency was confirmed by an elevation of the 20:3(n-9)/20:4(n-6) index. To examine the hepatic handling of lipoprotein-derived cholesterol, HDL2 and HDL3 from donor rats were isolated, labeled with [14C]-cholesterol, and injected iv into EFA-deficient and normal rats with a bile fistula. In HDL subfractions from control rats, no significant variations were noted in the specific activity of cholesterol output in both groups of EFA recipient rats; however, the output of biliary bile acids was significantly decreased in EFA-deficient rats following the administration of labeled HDL3. In HDL2 and HDL3 originating from EFA-deficient rats, a decrease in the specific activity of both biliary cholesterol and bile acid output was recorded in EFA-deficient rats. Concomitant with the defective HDL2- and HDL3-[14C] cholesterol translocation into bile of EFA-deficient rats, increased hepatic very-low-density lipoprotein (VLDL)-[14C] cholesterol secretion was observed in vivo. HDL2 and HDL3 particles, derived from EFA-deficient rats, had an altered composition including a depletion in apo A-I and an enrichment in apo E isoforms, which are the the two major HDL apolipoproteins involved in the delivery of cholesterol to the liver. Taken together, these results show that normal EFA status is necessary for efficient HDL-cholesterol processing by the liver.

Oral acetylsalicylic acid induces biliary cholesterol secretion in the rat

Several agents can alter biliary cholesterol secretion, critical for cholesterol excretion and gallstone formation. Although salicylate effects on bile formation and gallstones have been studied, biliary lipid secretion has not been measured during oral aspirin treatment. We examined whether oral acetylsalicylic acid affects bile lipid secretion. Three groups of young rats were fed chow for 3 wk. Two of the groups then received aspirin at either 1.67 or 3.33 g/kg diet for 4 d. Serum, hepatic, and bile lipids were measured, as were enzymes of cholesterol synthesis and esterification. With oral aspirin, bile cholesterol secretion increased by 42% and hepatic cholesteryl ester content decreased by 40%. Serum cholesterol and hepatic free cholesterol did not change. To evaluate mechanisms of the cholesterol hypersecretion, hypothyroid animals fed low-fat or fish oil diets and repleted with triiodothyronine were also studied. Aspirin stimulated cholesterol secretion to a degree similar to triiodothyronine. An additive response was seen in fish oil-fed rats. Aspirin did not appear to have a primary action on 3-hydroxy-3-methylglutaryl-CoA reductase or acyl CoA:cholesterol acyltransferase activities, and had no direct effect on esterification of cholesterol by isolated hepatocytes. Aspirin may directly increase cholesterol transport into bile or have cell membrane effects which alter cholesterol transport. It remains to be determined whether the observed alterations in bile cholesterol secretion are specific to the rat or also apply to humans.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Thyroid hormone is required for dietary fish oil to induce hypersecretion of biliary cholesterol in the rat

In the rat, both fish oil diet and thyroid hormone replacement are reported to augment bile cholesterol secretion out of proportion to bile flow or secretion of other bile lipids. We sought common mechanisms for these effects and evaluated the role of phospholipid fatty acid composition in the process. Methimazole-treated hypothyroid rats were fed low-fat chow or chow supplemented with 10% corn oil or fish oil, and were studied before and after thyroid hormone treatment. Serum, hepatic, and bile lipids were measured, phospholipid fatty acid composition determined, and hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity assayed. Fish oil diet stimulated cholesterol secretion into bile only after thyroid hormone was given, and this action was synergistic with that of thyroid hormone. Reduced serum cholesterol in fish oil-treated rats was associated with increased biliary cholesterol secretion and diminished hepatic cholesterol content. This suggests that augmented biliary cholesterol secretion may contribute to the fish oil-induced reduction of serum cholesterol. No definite relationship between hepatic or biliary phospholipid fatty acid composition and biliary secretion was apparent, although high bile cholesterol secretion was associated with a low percentage of hepatic and bile phospholipid linoleic acid.

Cholesterol gallstone formation in man and potential treatments of the gallbladder motility defect

Cholelithiasis affects 10-15% of the adult population in Western society, and about 75% of gallstones are of cholesterol type. Hepatic hypersecretion of cholesterol with the formation of instable cholesterol-rich vesicles in bile, an imbalance between nucleation-inhibiting and nucleation-promoting proteins with further aggregation of cholesterol crystals in a gallbladder with a motility defect (stasis), all play a role in the pathogenesis of cholesterol gallstones. Experimental animal models suggest that gallstone formation can be prevented by improving gallbladder emptying. Thus, a better understanding of the causes underlying the impaired gallbladder motor function in patients with gallstones might lead to the selection of therapeutic approaches for those individuals who are at increased risk for the formation or recurrence of gallstones. The present article focuses on current concepts and theories on the pathogenesis of cholesterol gallstones with emphasis on the gallbladder motility defect. Several treatment strategies for the correction of gallbladder hypomotility are also discussed.

Evaluation in vivo of the differential uptake and processing of high-density lipoprotein unesterified cholesterol and cholesteryl ester in the rat

The uptake and processing of high-density lipoprotein (HDL) unesterified and esterified cholesterol were compared in vivo in the rat. HDL labelled with 3H in either unesterified cholesterol or cholesteryl ester was administered intravenously, and the clearance of radioactivity from the blood, its distribution in plasma lipoprotein density fractions, uptake by tissues, and appearance in bile were studied at intervals up to 180 min. 3H in HDL unesterified cholesterol was cleared more rapidly from the blood than that in HDL cholesteryl ester, and this difference was mainly due to rapid sequestration of [3H]unesterified cholesterol by the liver, with 58.2% of the administered dose found in this tissue after 10 min, compared to 6.8% of the [3H]cholesteryl ester dose. Non-hepatic tissues took up only a small proportion of the administered label from both HDL unesterified and esterified cholesterol, but on a per gram wet weight basis, the specific uptake of HDL cholesteryl ester in the adrenal glands and the spleen was higher than in the liver, particularly in the first 60 min. The distribution of radioactivity in the plasma lipoprotein density fractions remained constant between 10 and 180 min when [3H]unesterified cholesterol was used, but the proportion of plasma radioactivity from HDL labelled in esterified cholesterol in the very-low-density lipoprotein (VLDL) fraction increased from 0% to 26%, while in HDL there was a shift in the distribution of radioactivity from the most (d 1.125-1.250 g/ml) to the least (d 1.050-1.085 g/ml) dense sub-fractions. A greater percentage of the administered label from HDL unesterified cholesterol (8.8%) than from HDL cholesteryl ester (3.3%) was secreted into bile during 180 min, but the proportions secreted in bile acids and unesterified cholesterol were similar with both labels. These findings indicate that there are significant differences in the uptake and processing of HDL unesterified as compared to esterified cholesterol in the rat in vivo.