Utilization of different fatty acids for hepatic and biliary phosphatidylcholine formation and the effect of changes in phosphatidylcholine molecular species on biliary lipid secretion

Lipidomic QTL in Diversity Outbred mice identifies a novel function for α/β hydrolase domain 2 (Abhd2) as an enzyme that metabolizes phosphatidylcholine and cardiolipin

We and others have previously shown that genetic association can be used to make causal connections between gene loci and small molecules measured by mass spectrometry in the bloodstream and in tissues. We identified a locus on mouse chromosome 7 where several phospholipids in liver showed strong genetic association to distinct gene loci. In this study, we integrated gene expression data with genetic association data to identify a single gene at the chromosome 7 locus as the driver of the phospholipid phenotypes. The gene encodes α/β-hydrolase domain 2 (Abhd2), one of 23 members of the ABHD gene family. We validated this observation by measuring lipids in a mouse with a whole-body deletion of Abhd2. The Abhd2KO mice had a significant increase in liver levels of phosphatidylcholine and phosphatidylethanolamine. Unexpectedly, we also found a decrease in two key mitochondrial lipids, cardiolipin and phosphatidylglycerol, in male Abhd2KO mice. These data suggest that Abhd2 plays a role in the synthesis, turnover, or remodeling of liver phospholipids.

Lipidomic QTL in Diversity Outbred mice identifies a novel function for α/β hydrolase domain 2 ( Abhd2 ) as an enzyme that metabolizes phosphatidylcholine and cardiolipin

We and others have previously shown that genetic association can be used to make causal connections between gene loci and small molecules measured by mass spectrometry in the bloodstream and in tissues. We identified a locus on mouse chromosome 7 where several phospholipids in liver showed strong genetic association to distinct gene loci. In this study, we integrated gene expression data with genetic association data to identify a single gene at the chromosome 7 locus as the driver of the phospholipid phenotypes. The gene encodes α/β-hydrolase domain 2 ( Abhd2 ), one of 23 members of the ABHD gene family. We validated this observation by measuring lipids in a mouse with a whole-body deletion of Abhd2 . The Abhd2 KO mice had a significant increase in liver levels of phosphatidylcholine and phosphatidylethanolamine. Unexpectedly, we also found a decrease in two key mitochondrial lipids, cardiolipin and phosphatidylglycerol, in male Abhd2 KO mice. These data suggest that Abhd2 plays a role in the synthesis, turnover, or remodeling of liver phospholipids.

Plasma Lipid Profiling of Three Types of Drug-Induced Liver Injury in Japanese Patients: A Preliminary Study

Drug-induced liver injury (DILI) is a major adverse event caused by drug treatment, which can be categorized into three types: hepatocellular, mixed, and cholestatic. Although nearly every class of drugs can cause DILI, an overall understanding of lipid profiles in DILI patients is lacking. We used lipidomics to analyze the plasma lipid profiles of patients to understand their hepatic pathophysiology and identify DILI biomarkers. We identified 463 lipids and compared their levels between the acute and recovery phases of the three types of DILI patients. Mixed and cholestatic types demonstrated specific plasma lipid alterations between the phases, but the hepatocellular type did not. Moreover, as specific indicators of mixed-type DILI, levels of several ceramides increased in the acute phase, while those of arachidonic acid-containing ether-linked phosphoglycerolipids decreased. In contrast, as specific indicators of cholestatic-type DILI, levels of palmitic acid-containing saturated or monounsaturated phosphatidylcholines increased in the acute phase, while those of arachidonic acid- or docosahexaenoic acid-containing ether-linked phosphoglycerolipids and phosphatidylinositols decreased. We also identified lipids with a relatively high capacity to discriminate the acute phase from the recovery phase and healthy subjects. These findings may help with understanding the pathophysiology of different DILI types and identify candidate biomarkers.

Knockout of arsenic (+3 oxidation state) methyltransferase results in sex-dependent changes in phosphatidylcholine metabolism in mice

Arsenic (+3 oxidation state) methyltransferase is the key enzyme in the methylation pathway for inorganic arsenic. We have recently shown that As3mt knockout (KO) has a profound effect on metabolomic profiles in mice. Phosphatidylcholine species (PCs) were the largest group of metabolites altered in both plasma and urine. The present study used targeted analysis to investigate the KO-associated changes in PC profiles in the liver, the site of PC synthesis. Results show that As3mt KO has a systemic effect on PC metabolism and that this effect is sex dependent.

Screening phosphatidylcholine biomarkers in mouse liver extracts from a hypercholesterolemia study using ESI-MS and chemometrics

When fed a high-fat, high-cholesterol diet (HFD), homozygous LDL receptor knockout mice exhibit extremely high levels of plasma cholesterol that are expected to influence liver metabolism. One step in the investigation of potential hepatic alterations was the analysis of organic extracts of livers from these and control mice by electrospray mass spectrometry (ESI-MS). Chemometrics (bioinformatics) analysis shows that the sample spectra cluster into two groups: one from mice with plasma cholesterol levels in excess of 900 mg dL(-1) and one from animals with cholesterol levels of 60-250 mg dL(-1). The loadings plot of the first PC in the principal-components analysis (PCA) reveals the chemical basis for clustering, i.e., biomarkers present at different concentrations in the different groups. The exact masses of the key peaks in this loadings plot indicate these species are phosphatidylcholines (PtdChos). This assignment is confirmed by tandem MS. Partial least-squares (PLS) with variable selection shows that the spectra are well correlated with plasma total cholesterol, HDL cholesterol, and triglyceride (TG) levels.

Dietary garlic and onion reduce the incidence of atherogenic diet-induced cholesterol gallstones in experimental mice

Mice fed with diet containing 0·5 % cholesterol for 10 weeks resulted in cholesterol supersaturation in gallbladder bile which promoted the formation of cholesterol gallstones (CGS). In this study, dietary hypocholesterolaemic spices, garlic and onion (both raw or heat-processed) were examined for their antilithogenic potential by including at 0·6 and 2·0 % level, respectively, along with lithogenic (LG) diet for 10 weeks. Dietary garlic and onion reduced the CGS incidence by 15–39 %, the effect being maximum in the heat-processed onion group. Dietary garlic and onion markedly reduced biliary cholesterol. The cholesterol:phospholipid ratio which was 1·58 in the LG diet group was reduced to 0·73–0·96 in the garlic and onion groups. The biliary cholesterol saturation index was 0·92, 1·25, 1·09 and 0·86, respectively, in the heat-processed onion, raw garlic, heat-processed garlic and raw onion groups, while it was 1·9 in the LG group. The hydrophobicity index of bile was − 0·08, − 0·079, − 0·032 and − 0·073, respectively, in the heat-processed onion, raw garlic, heat-processed garlic and raw onion groups, while it was +0·054 in the LG group. Hepatic hydroxymethyl glutaryl-CoA reductase activity was lowered in the LG diet-fed group, while dietary garlic or onion countered this alteration and also increased the activities of hepatic cholesterol 7α-hydroxylase and sterol 27-hydroxylase. Serum and liver cholesterol were decreased by feeding garlic or onion compared to the LG diet. Thus, dietaryAlliumspices exerted antilithogenic influence by decreasing the cholesterol hyper-secretion into bile and increasing the bile acid output thus decreasing the formation of lithogenic bile in experimental mice.

Spray-dried milk supplemented with alpha-linolenic acid or eicosapentaenoic acid and docosahexaenoic acid decreases HMG Co A reductase activity and increases biliary secretion of lipids in rats

In our earlier study, we have shown that rats fed spray-dried milk containing alpha-linolenic acid (LNA 18:3 n-3) or eicosapentaenoic acid (EPA 20:5 n-3) and docosahexaenoic acid (DHA 22:6 n-3) had significantly lower amounts of serum and liver cholesterol. To evaluate the mechanism for hypocholesterolemic effect of n-3 fatty acids containing milk formulation, we fed male Wistar rats with spray-dried milk containing linseed oil (LSO) (source of LNA) or fish oil (FO) (source of EPA+DHA) for 8 weeks. Feeding n-3 fatty acid containing milk formulation lowered the hepatic 3-hydroxy-methylglutaryl coenzyme A (HMG Co A) activity by 17-22% compared to rats given control diet devoid of n-3 fatty acids. The cholesterol level in liver microsomes was found to be decreased by 16% and 20%, respectively, in LSO and FO containing formulation fed rats. The bile flow was enhanced to an extent of 19-23% in experimental groups compared to control animals. The biliary cholesterol and phospholipid secretion was increased to an extent of 49-55% and 140-146%, respectively, in rats fed n-3 fatty acid containing formulation. The increase in the total bile acids secretion in bile was mainly reflected on an increase in the levels of taurine conjugated bile acids. These results indicated that n-3 fatty acid containing spray-dried milk formulation would bring about the hypocholesterolemic effect by lowering HMG Co A reductase activity in liver and by increasing the secretion of bile constituents.

The incorporation of fatty acids of different chain length into liver and biliary lipids in the perfused rat liver

In an attempt to correlate the incorporation of fatty acids (FA) of different chain length into liver and biliary lipids, isolated rat livers were perfused for 2 h with Krebs-Ringer bicarbonate containing 1% albumin and 10 mumol of [1-14C]-labeled FA: C2, C8, C10, C12, C16, and C18:1. One to 1.36 mumol of medium-chain fatty acids (MCFA, C8, C10, and C12) and 6.6 mumol of long-chain FA (LCFA) were incorporated into liver lipids, 40% of the latter into phosphatidylcholine (PC). 14C-acetate (13 nmol) was incorporated into biliary cholesterol; 14C-MCFA contributed only 3.2-5 nmol; LCFA did not lead to newly synthesized cholesterol. Newly synthesized liver PC (2.75 to 3.25%) and newly synthesized liver cholesterol (6.5 to 10%) were secreted into bile. The specific radioactivity of biliary PC after infusion of all-saturated FA was 3.8-6.8 times higher than that of liver PC; for C18:1 it was only 1.7-fold. The specific radioactivity of biliary cholesterol, as compared to liver cholesterol, was 12 times higher for C2 and five times higher for MCFA. This indicates that a considerable proportion of the newly synthesized lipids was secreted into bile prior to significant mixing with preexisting liver PC and cholesterol pools. Liver PC contained 8% of unchanged 14C-C12; while 14C-C10 was not detected. Biliary PC, in contrast, contained 18% of unchanged 14C-C12 and 3% 14C-C10. These results suggest that after prolonged infusion of medium-chain triacylglycerols/long-chain triacylglycerols to patients, biliary PC may become enriched with MCTA. In addition, the oxidation of these FA may provide C-2 units which increase cholesterol synthesis.

Determinants of the selection of phosphatidylcholine molecular species for secretion into bile in the rat

Certain phosphatidylcholine (PC) molecular species appear to be secreted into bile preferentially, but the mechanism for this selection remains obscure. We used multivariate analysis to examine the relationship between PC structure and the odds of secretion for individual PC species secreted into bile. PC was isolated from Folch extracts of bile and liver from rats, and individual molecular species of PC were quantified with reverse-phase high-performance liquid chromatography (HPLC). The odds of secretion for a given PC species were quantified as the ratio of its mole% in bile/mole% in liver. Regression analysis indicated that the odds of secretion were significantly related to length of both the sn-1 and sn-2 acyl chains (P < .0001 for both) and to relative hydrophobicity as determined by reverse-phase HPLC (P < .0001). In addition, the relationship between odds of secretion and sn-1 chain length was best described by a parabolic function. Considered together, these characteristics accounted for 88% of the observed differences in odds of secretion. This relationship between PC structure and odds of secretion was strikingly similar to the relationship between PC structure and affinity for bovine PC transfer protein. When multivariate models were used to predict both the odds of secretion and the affinity for PC transfer protein for a set of biologically plausible PC species, there was a linear relationship between the two. The likelihood of a given PC species being secreted into bile can be related to the structural characteristics of the acyl chains without having to postulate the existence of a special pool of PC destined for biliary secretion. Second, the structural characteristics that dictate selection of PC species for secretion into bile are similar to those that determine binding affinity for PC transfer protein, suggesting that the likelihood of a PC being secreted into bile is, in fact, closely related to its binding affinity for PC transfer protein (PC-TP).

Increased saturation of the fatty acids in the sn-2 position of phospholipids reduces cholesterol crystallization in model biles

Changes in the molecular structure of biliary phospholipids were shown to have major effects on cholesterol solubility, carriers and crystallization in human and model biles. This study investigated systematically the effects of varying saturation of the phosphatidylcholine (PC) sn-2 fatty acid on the cholesterol crystallization process in 3 different model biles. Twenty % of the egg PC (EPC) in these biles were replaced by synthetic PC's with 16:0-18:0, 16:0-18:1, or 16:0-18:2 fatty acyl chains. With 18:0 in the sn-2 position, the crystal observation time (COT) was prolonged from 2 days in the control EPC solution to 14 days (p<0.05). The crystal growth rate (CGR) was reduced from 0.1 OD/day to unmeasurable levels, and the total crystal mass on day 14 decreased by 86%. The introduction of one (18:1), and two (18:2) double bonds in the sn-2 fatty acid rapidly reversed these effects. Ultracentrifugal analysis showed precipitable cholesterol as monohydrate crystals. In the 16:0-18:0 test solution, most of the precipitable cholesterol remained in the supersaturated multilamellar vesicles. Saturation of the biliary PC sn-2 fatty acyl chain prolongs the COT, slows the CGR, reduces the crystal mass, and extends cholesterol solubility in multilamellar vesicles. Desaturation of the sn-2 fatty acid reverses these effects.

Evidence for secretory coupling of phosphatidylcholine molecular species to cholesterol in rat bile

BACKGROUND/AIMS

Hepatocytes secrete cholesterol into bile within lipid vesicles of selected phosphatidylcholines, mainly palmitoyl-linoleoyl-phosphatidylcholines, palmitoleoyl-oleoyl-phosphatidylcholines and palmitoleoyl-arachidonyl-phosphatidylcholines, which could in part determine the secreted amount of cholesterol.

AIMS

To study whether increased secretion of cholesterol, as caused by manipulation of cholesterol synthesis rate, changes the composition of phosphatidylcholines secreted in bile.

METHODS

Livers from control rats (Control), rats fed pravastatin for 7 days (Pravastatin) and livers isolated 5-7 or 8-11 hours after pravastatin had been withdrawn (Rebound5-7h; Rebound8-11h) were isolated perfused during infusion of taurocholic acid (400 nmol/min/100 g rat), to study biliary secretion of bile salts, cholesterol and phosphatidylcholine molecular species.

RESULTS

Bile salt secretion rate was similar in all four groups, secretion of cholesterol and phosphatidylcholines was similar in Control and Pravastatin. With duration of pravastatin withdrawal the secretion rates of phosphatidylcholine and cholesterol progressively increased by +38% and +122% in Rebound5-7h and by +70% and +300% in Rebound8-11h (vs Control), respectively. In parallel, the secretion rates of palmitoleoyl-oleoyl- and palmitoleoyl-arachidonyl-phosphatidylcholines rose up to sixfold and twofold, respectively, while the secretion rate of palmitoyl-linoleoylphospatidylcholines remained constant. The secretion rate of cholesterol was correlated (p < 0.01) with the secretion rates of palmitoleoyl-oleoyl-phosphatidylcholines (r = 0.83) and palmitoleoyl-arachidonyl-phosphatidylcholines (r = 0.81). Bilirubin ditaurate or taurodehydrocholate reduced (p < 0.05) biliary secretion of phosphatidylcholines (-33%; -72%) without changes in cholesterol/phosphatidylcholine secretory ratio or phosphatidylcholine species.

CONCLUSIONS

The secretion of the major molecular species of phosphatidylcholine in bile could be coregulated with the amount of cholesterol destined for biliary secretion.

Dietary fat alters biliary lipid secretion in the hamster

Dietary fat has been found to alter the incidence of cholesterol gallstones in hamsters: butterfat intensifies while safflower oil reduces lithiasis. WE not report how dietary fat affects bile flow and biliary lipid secretion in this model. Male hamsters were fed one of three experimental diets: a control diet (containing 0.3% cholesterol); control diet + 4.0% butterfat; or control diet + 4.0% safflower oil. After three weeks, bile samples were collected via an external biliary fistula. The endogenous bile acid pool was depleted for 120 min followed by increasing rates of taurocholate infusion for 160 min. Basal secretion of biliary lipids was measured during the bile acid depletion period. Basal bile flow and bile acid output were not significantly different in the three groups. Dietary butterfat increased basal cholesterol output compared to the control diet (0.037 vs. 0.025 mumol/min.kg, respectively); safflower oil did not change cholesterol output (0.027 mumol/min.kg). Hamsters fed butterfat or safflower oil secreted more phospholipid (0.171 and 0.178 mumol/min.kg, respectively) than controls (0.131 mumol/min.kg). The cholesterol/phospholipid output ratio of the butterfat group was higher than the safflower oil group (0.220 vs. 0.153, respectively). Effects of dietary fat on several relationships between file flow and biliary lipid secretion were analyzed by linear regression using the data for the entire bile collection period (bile acid depletion and taurocholate infusion). Butterfat and safflower oil did not change either bile acid dependent or bile acid independent bile flow. Hamsters fed butterfat had a higher linkage coefficient (slope) of cholesterol vs. bile acid output than the safflower oil group (0.023 vs. 0.009, respectively). The linkage coefficient of phospholipid vs. bile acid output of the butterfat group was higher than the controls (0.278 vs. 0.185, respectively). In summary, butterfat induced a high cholesterol and phospholipid secretion with a high cholesterol/phospholipid output ratio; safflower oil induced a high phospholipid secretion with a low cholesterol/phospholipid output ratio. Butterfat and safflower oil have different effects on biliary lipid secretion. These differences in biliary lipid secretion may explain, in part, how butterfat and safflower oil differ in affecting gallstone formation in hamsters.

The effects of dietary phospholipids enriched with phosphatidylethanolamine on bile and red cell membrane lipids in humans

The role of phospholipids in biliary cholesterol solubilization and crystallization has only recently begun to be appreciated. Phospholipid vesicles are believed to be the metastable carrier from which cholesterol nucleates. Cholesterol crystallization is influenced by the phospholipid species in bile. Feeding rats and hamsters with diets enriched in phospholipids or their precursors, especially ethanolamine, resulted in reduced cholesterol saturation of bile. Although whole phospholipids are normal dietary constituents, the effects and safety of phospholipid components have not been tested in humans. In the present study, we have evaluated the effects of a dietary phospholipid mixture, enriched with phosphatidylethanolamine, on human bile and red blood cell membrane lipid composition. Five ambulatory volunteers having a chronic indwelling T-tube, with an intact enterohepatic circulation, were investigated. Thirty-six grams of phospholipids (54% phosphatidylethanolamine, 54% linoleyl acyl chains) were added to their daily diet for fourteen days. Biliary nucleation time, cholesterol carriers, as well as plasma, red blood cell membrane, and bile lipid compositions, were monitored. Following phospholipid supplementation, the proportion of linoleyl chains (18:2) in biliary phospholipids increased significantly from 31.1 +/- 1.2 to 37.7 +/- 5.3%, while that of oleyl chains (18:1) decreased from 11.4 +/- 1.6 to 9.6 +/- 1.1%. These changes were accompanied by an increase of linoleate and its metabolite, arachidonate, in red cell membranes. Phospholipid feeding did not cause any side effects, and no significant changes in biliary nucleation time, cholesterol, phospholipid, or bile salt concentrations, or in the distribution of cholesterol within micelles or vesicles. We conclude that phospholipid feeding is safe, and can be effective as a vehicle for lecithin fatty acyl chain modulation of bile and lipid membranes. These findings may provide a basis for a controlled modulation of biliary phospholipids to increase cholesterol solubility in bile.

Effect of dietary phospholipids and their constituents on bile composition in rats and hamsters

The formation of cholesterol gallstones is a result of an interaction between bile lipid concentration, composition and other factors. The solubilization of cholesterol in bile is affected by the nature of the fatty acids and head groups of biliary phospholipids. In this study we tried to modulate the composition of biliary lipids in the rat and hamster by means of dietary supplementation of whole phospholipids or their constituents (at 3% to 5% of food weight). A striking effect of ethanolamine feeding in rats and hamsters was demonstrated: Biliary cholesterol concentration and cholesterol saturation index were significantly lower (p < 0.03). In rats, bile acid concentration was increased, contributing to decreased bile lithogenicity. Ethanolamine also increased biliary phospholipid output. Other test substances had less marked effects. The addition of lecithin to rat diet significantly increased phospholipid concentration (p < 0.05) compared with controls. Cholesterol and bile acid concentrations were reduced in palmitic acid-fed hamsters (NS). Choline supplementation insignificantly increased the cholesterol saturation index in hamsters. Despite the clear effect of ethanolamine on biliary lipid composition, no change could be demonstrated in the proportion of phosphatidylethanolamine in bile. It is postulated that ethanolamine was methylated in the liver to choline. Our data demonstrate that biliary lipid composition can be modulated by dietary phospholipids or their components.

Dietary N-3 polyunsaturated fatty acids decrease biliary cholesterol saturation in gallstone disease

Because fatty acid composition of biliary phospholipids influences cholesterol secretion into bile, we investigated whether replacement of n-1 monounsaturated or n-6 polyunsaturated fatty acids with n-3 polyunsaturated fatty acids in biliary phosphatidylcholines reduces supersaturation with cholesterol and prevents precipitation of cholesterol crystals in bile of gallstone patients. Seven patients with radiolucent gallstones in functioning gallbladders were studied before (control) and after 5 wk of dietary supplementation with marine fish oil (11.3 gm/day = 3.75 gm n-3 polyunsaturated fatty acids/day). Duodenal bile was collected for analysis during intravenous infusion of cholecystokinin. Gallbladder emptying in response to cholecystokinin was comparable before and during intake of n-3 polyunsaturated fatty acids. Intake of n-3 polyunsaturated fatty acids increased (p less than 0.001) the fractions of eicosapentaenoic and docosahexaenoic acids and decreased the fractions of linoleic (p less than 0.001) and arachidonic acids (p less than 0.02) in biliary phospholipids. Concomitantly, the molar ratio of cholesterol to phospholipids decreased (-19%; p less than 0.05). As a consequence, the cholesterol saturation index was reduced by -25% (p = 0.01), from 1.60 +/- 0.44 to 1.24 +/- 0.38. However, in vitro nucleation time of duodenal bile was not prolonged. The decrease in cholesterol saturation was not sufficient to prevent nucleation of cholesterol crystals in bile of gallstone patients. In conclusion, our data suggest that cholesterol saturation can be influenced by the fatty acid composition of the phosphatidylcholines secreted in bile.