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

Pathways of cholesterol crystallization in model bile and native bile

Hypersecretion of hepatic cholesterol, chronic supersaturation of bile with cholesterol and rapid precipitation of cholesterol crystals in the gallbladder from cholesterol-enriched vesicles represent the primum movens in cholesterol gallstone formation. Physical-chemical factors and pathways leading to cholesterol crystallization can be investigated in artificial model biles and ex vivo in fresh human bile. Depending on modulatory factors (i.e., lipid concentration, bile salt or phospholipid species, humidity, mucins, etc.), cholesterol can precipitate in several forms (i.e., monohydrate, anhydrous) and habits (i.e., plate-like, needle-like, intermediate arcs, filaments, tubules, spirals). Careful analysis of biliary cholesterol crystals includes biochemical analysis of precipitated crystals, polarizing quantitative light microscopy, and turbidimetric methods. In this paper, recent concepts on cholesterol crystallization in artificial model biles as well as in human bile will be reviewed.

Effects of hydrophobic and hydrophilic bile salt mixtures on cholesterol crystallization in model biles

The hydrophilic bile salt ursodeoxycholate is frequently used to dissolve cholesterol gallstones. We have now quantitated crystallization as a function of bile salt hydrophobicity, phospholipid content, cholesterol saturation and total lipid concentration (TLCo).

METHODS

Crystallization in supersaturated model biles with low phospholipid contents (left two-phase-micelles and crystal-containing-zone) was assessed during 21 days by microscopy and chemical measurement of crystal mass. For model biles with higher phospholipid contents (central three-phase-micelles, vesicles and crystal-containing-zone), lipid distribution into various phases was determined by combined ultracentrifugation-filtration-dialysis methodology (Biochim. Biophys. Acta 1532 (2001) 15-27).

RESULTS

In the left two-phase zone, crystal numbers and masses were highest in case of more hydrophilic bile salt composition (TUDC 100%>TC/TUDC 70%/30%>TC 100%>TC/TDC 70%/30%>TDC 100%) and decreased with increasing phospholipid contents, lower TLCo and lower cholesterol saturation index (CSI). In contrast, in the presence of vesicles (three-phase zone), crystallization decreased at increasing bile salt hydrophilicity, with concomitant increased vesicular cholesterol solubilization.

CONCLUSIONS

Presence of vesicular phases is a prerequisite for inhibition of cholesterol crystallization by tauroursodeoxycholate.

Dietary phospholipid alters biliary lipid composition in formula-fed piglets

Plasma cholesterol, arachidonic acid (AA, 20:4n-6), and docosahexaenoic acid (DHA, 22:6n-3) are higher in breast-fed infants than in infants fed formula without cholesterol, AA, or DHA. This study investigated differences in plasma, hepatic, and bile lipids and phospholipid fatty acids, and expression of hepatic proteins involved in sterol metabolism that result from feeding formula with cholesterol with egg phospholipid to provide AA and DHA. For this study, three groups of piglets were evaluated: piglets fed formula with 0.65 mmol/L cholesterol, the same formula with 0.8% AA and 0.2% DHA from egg phospholipid, and piglets fed sow milk. Piglets fed the formula with phospholipid AA and DHA had higher plasma high density lipoprotein, but not apoprotein (apo) B cholesterol or triglyceride; higher bile acid and phospholipid concentrations in bile; and higher liver and bile phospholipid AA and DHA than piglets fed formula without AA and DHA (P < 0.05). Hydroxy methylglutaryl (HMG)-CoA reductase and 7-alpha-hydroxylase, the rate-limiting enzymes of cholesterol and bile acid synthesis, respectively, and low density lipoprotein receptor mRNA levels were not different between piglets fed formula without and with phospholipid AA and DHA, but HMG-CoA reductase and 7alpha-hydroxylase mRNA were higher, and plasma apo B containing lipoprotein cholesterol was lower in all piglets fed formula than in piglets fed milk. These studies show that supplementing formula with AA and DHA from egg phospholipid alters bile metabolism by increasing the bile AA and DHA, and bile acid and phospholipid.

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.