Role of plasma and liver cholesterol- and lipoprotein-metabolism determinants in LpX formation in the mouse

Cholestasis is characterized by hypercholesterolemia and the appearance of an abnormal lipoprotein, lipoprotein X (LpX), in plasma. The mechanisms responsible for this cholestatic plasma lipid phenotype are not fully understood. We used ATP-binding cassette A1 (ABCA1)-/- and scavenger receptor class B type I (SR-BI)-/- mice to test the hypothesis that hepatic sinusoidal cholesterol transporters contribute to LpX formation and hypercholesterolemia during cholestasis. Bile-duct ligation (BDL) of both ABCA1-/- and SR-BI-/- mice, as well as their respective controls, induced a dramatic increase in plasma cholesterol and phospholipid concentrations. Plasma fractionation revealed the presence of LpX in plasma of cholestatic mice, irrespective of their genetic background. We observed that the presence of HDL before cholestasis, a decrease in the activity of LCAT, and an increase in VLDL synthesis were not required for hypercholesterolemia and lipoprotein modifications induced by obstructive cholestasis in mice. In addition, murine cholestasis resulted in increased hepatic cholesterol synthesis that may contribute to the higher plasma free cholesterol levels found during the early hours after BDL. Together these findings indicate that hypercholesterolemia and LpX formation associated with obstructive cholestasis are correlated with an increase in hepatic cholesterol synthesis and are independent of plasma HDL levels, LCAT activity, VLDL synthesis, and ABCA1 and SR-BI expression.

Catabolism of lipoprotein-X induced by infusion of 10% fat emulsion

The clinical significance of lipoprotein-X (Lp-X) induced by intravenous infusion of 10% fat emulsion was assessed, with special reference to atherogenesis, by in vitro experiment using purified Lp-X from the sera of patients receiving Intralipid 10%. Lp-X appeared after long-term intravenous infusion of 10% fat emulsion in the patients with intestinal fistula due to the anastomotic leakage. To clarify the role of Lp-X in terms of atherogenicity, the cholesterol metabolism of Lp-X in macrophages as scavenger cells and in hepatocytes as parenchymal cells was studied. When [3H]cholesterol-labeled Lp-X or oxidized low-density lipoprotein (o-LDL) was incubated with J-774 macrophages, the incorporation of Lp-X into macrophages was negligible compared with o-LDL. When Lp-X or high-density lipoprotein (HDL) was incubated with J-774 macrophages laden with [3H]cholesterol, the release of cholesterol from macrophages was enhanced by Lp-X as well as HDL. When [3H]cholesterol-labeled Lp-X LDL or HDL was incubated with the human hepatoma cell line of Hep G2 cells, the incorporation of Lp-X into Hep G2 cells was less than that of LDL, but similar to that of HDL. From these findings, it is suggested that the catabolism of Lp-X cholesterol generated with intravenous 10% fat emulsion was mediated by hepatocytes rather than by macrophages, indicating that the hyperlipidemia due to increased Lp-X may not be atherogenic.

[Catabolism of lipoprotein-X (Lp-X) induced by infusion of 10% intralipid]

In order to clarify the metabolism of Lipoprotein X (Lp-X) induced by intravenous Intralipid 10%, in vitro experiments using purified Lp-X from the sera of the patients receiving Intralipid 10% were carried out. 1) Lp-X or high density lipoprotein (HDL) was incubated with J-774 macrophages laden with [3H] cholesterol. Marked extraction of cholesterol from macrophages by Lp-X as well as HDL was observed. 2) [3H] cholesterol labelled Lp-X or oxidized LDL (o-LDL) was incubated with J-774 macrophages. Incorporation of Lp-X into macrophages was negligible comparing to o-LDL. 3) [3H] cholesterol labelled Lp-X, low density lipoprotein (LDL), or HDL was incubated with Hep G2 cells was less than LDL, but similar to that of HDL. These results indicated that Lp-X extracted cholesterol from peripheral tissues during its formation, and it was not catabolized by the scavenger pathway, but catabolized by the LDL pathway of hepatocytes.