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Phosphatidylethanolamine N-methyltransferase (PEMT) is a hepatic integral membrane protein localized to the endoplasmic reticulum (ER). PEMT catalyzes approximately 30% of hepatic phosphatidylcholine (PC) biosynthesis. Pemt(-/-) mice fed a high-fat diet (HFD) develop steatohepatitis. Interestingly, portions of the ER located close to the canaliculus are enriched in PEMT. Phospholipid balance and asymmetrical distribution by adenosine triphosphatase phospholipid transporting 8B1 (ATP8B1) on the canalicular membrane is required for membrane integrity and biliary processes. We hypothesized that PEMT is an important supplier of PC to the canaliculus and that PEMT activity is critical for the maintenance of canalicular membrane integrity and bile formation following HFD feeding when there is an increase in overall hepatic PC demand. Pemt(+/+) and Pemt(-/-) mice were fed a chow diet, an HFD, or a choline-supplemented HFD. Plasma and hepatic indices of liver function and parameters of bile formation were determined. Pemt(-/-) mice developed cholestasis, i.e, elevated plasma bile acid (BA) concentrations and decreased biliary secretion rates of BAs and PC, during HFD feeding. The maximal BA secretory rate was reduced more than 70% in HFD-fed Pemt(-/-) mice. Hepatic ABCB11/bile salt export protein, responsible for BA secretion, was decreased in Pemt(-/-) mice and appeared to be retained intracellularly. Canalicular membranes of HFD-fed mice contained fewer invaginations and displayed a smaller surface area than Pemt(+/+). mice. Choline supplementation (CS) prevented and reversed the development of HFD-induced cholestasis. Conclusion: We propose that hepatic PC availability is critical for bile formation. Dietary CS might be a potential noninvasive therapy for a specific subset of patients with cholestasis.

Background & Aims: Starvation induces massive perturbations in metabolic pathways involved in energy metabolism, but its effect on the metabolism of lipids, particularly cholesterol, is little understood. Methods: A comparative genomic analysis of the gut and the liver in response to fasting was performed, with intestinal perfusion and lipid profiling of the plasma, bile, liver, intestinal tissue, perfusate, and faeces in FVB mice. Results: The expression profiles suggested increased cholesterol trafficking in the liver and decreased trafficking in the small intestine. Plasma cholesterol concentrations significantly increased, and triglycerides decreased in fasting. Surprisingly, in prolonged fasting, the biliary bile salt and lipid output rates increased, with increased hepatic and intestinal lipid turnover, and enhanced trans-intestinal cholesterol excretion. In contrast, faecal sterol loss declined sharply. To investigate whether the increased biliary phospholipid secretion could nourish the intestinal epithelium, we studied the histology of the small intestines upon fasting in multidrug resistant protein 2 deficient mice with scarce biliary phospholipids. Their adaptive biliary response to fasting was lost, while the shortage of biliary phospholipids strongly induced apoptosis and proliferation in the small intestine and increased the number of mucin-producing cells. Conclusion: Even with no dietary fat, lipid levels remain remarkably constant in the murine liver and intestines during prolonged fasting. The biliary system, always assumed to be coupled to the postprandial response, shows a paradoxical increase in activity. We hypothesise that biliary lipids are mobilised to supply the enterocytes with luminal fuel and to stabilise transport systems in the intestine for ensuring a rapid recovery when the food supply resumes. (C) 2010 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.

Human bile salt export pump ( BSEP) mutations underlie progressive familial intrahepatic cholestasis type 2 (PFIC2). In the PFIC2 animal model, Bsep−/−mice, biliary secretion of bile salts (BS) is decreased, but that of phospholipids (PL) and cholesterol (CH) is increased. Under physiological conditions, the biliary secretion of PL and CH is positively related (“coupled”) to that of BS. We aimed to elucidate the mechanism of increased biliary lipid secretion in Bsep−/−mice. The secretion of the BS tauro-β-muricholic acid (TβMCA) is relatively preserved in Bsep−/−mice. We infused Bsep−/−and Bsep+/+(control) mice with TβMCA in stepwise increasing dosages (150–600 nmol/min) and determined biliary bile flow, BS, PL, and CH secretion. mRNA and protein expression of relevant canalicular transporters was analyzed in livers from noninfused Bsep−/−and control mice. TβMCA infusion increased BS secretion in both Bsep−/−and control mice. The secreted PL or CH amount per BS, i.e., the “coupling,” was continuously two- to threefold higher in Bsep−/−mice ( P < 0.05). Hepatic mRNA expression of canalicular lipid transporters Mdr2, Abcg5, and Abcg8 was 45–55% higher in Bsep−/−mice (Abcg5; P < 0.05), as was canalicular Mdr2 and Abcg5 protein expression. Potential other explanations for the increased coupling of the biliary secretion of PL and CH to that of BS in Bsep−/−mice could be excluded. We conclude that the mechanism of increased biliary lipid secretion in Bsep−/−mice is based on increased expression of the responsible canalicular transporter proteins.

Transgenic mice expressing human ABCG5 (G5) and ABCG8 (G8) have decreased fractional absorption and increased biliary secretion of cholesterol, but their plasma cholesterol levels are unchanged (males) or modestly reduced (females). To determine whether increased expression of G5 and G8 can ameliorate hypercholesterolemia in mice lacking LDL receptors (LDLRs), we examined the effects of G5G8 transgene expression on cholesterol metabolism and atherosclerosis in Ldlr-/- mice. In chow-fed Ldlr-/- mice, the G5G8 transgene reduced fractional absorption of dietary cholesterol by 50% and increased biliary cholesterol levels by 60% but did not affect plasma cholesterol levels. On a Western diet (21% fat, 0.2% cholesterol), G5G8Tg; Ldlr-/- mice had a 30% reduction in the level of hepatic cholesterol and 45% lower plasma cholesterol levels than the Ldlr-/- mice. After 6 months on the Western diet, the atherosclerotic lesion area in the aortic root and arch was approximately 70% lower in the G5G8Tg;Ldlr-/- than in the Ldlr-/- mice and was correlated with the plasma cholesterol levels. These results demonstrate that increased expression of G5 and G8 attenuates diet-induced hypercholesterolemia in Ldlr-/- mice, resulting in a significant reduction in plasma levels of cholesterol and aortic atherosclerotic lesion area.

The effect of cholic acid pool expansion on biliary lipid composition was investigated in 21 subjects with cholesterol gallstones. All subjects were fed cholic acid (15 mg/kg/day) and ampicillin (2 g/day) in order to depress the intestinal degradation of cholic to deoxycholic acid. Five additional subjects were given ampicillin alone. The treatment lasted 2-3 wk. Parameters investigated included biliary and plasma lipid, biliary bile acid composition, and total and individual bile acid pool size. In 11 of 21 subjects (group I) cholic acid pool expanded two-threefold, whereas deoxycholic acid pool increased only slightly. In this group mean saturation index fell from 1.32 +/- 0.27 (mean +/- SD) to 0.93 +/- 0.25 (p less than 0.001), and plasma cholesterol increased from 185 +/- 45 mg/dl before to 213 +/- 37 after treatment (p less than 0.01). In the remaining 10 subjects (group II) the increase of the deoxycholic acid pool for exceeded that of cholic acid, and in these subjects the saturation index rose from a mean value of 1.07 +/- 0.27 to 1.42 +/- 0.22 after treatment (p less than 0.01). In this group plasma cholesterol tended to decrease (from 213 +/- 57 to 197 +/- 51 after treatment). In the 5 subjects treated with ampicillin alone deoxycholic acid pool was greatly reduced, and the saturation index fell from 1.25 +/- 0.25 to 0.95 +/- 0.35. The results suggest that cholic acid pool expansion reduces bile cholesterol saturation, whereas the increase of deoxycholic acid pool tends to supersaturate the bile. It is concluded that a determinant of bile cholesterol saturation might be the detergent power of the bile acid pool.