Secretion of lipoprotein-X by perfused livers of rats with cholestasis.

Abstract

The major abnormal plasma lipoprotein of cholestasis (LP-X) was isolated from bFood plasma and from perfusates of isolated livers of rats with biliary obstruction. In both cases LP-X was composed mainly of about equimolar parts of phospholipids and unesterified cholesterol; the small protein component was primarily the arginine-rich apolipoprotein. By electron microscopy, LP-X appeared as a unilamellar liposome (690 A mean diameter, range 400-1000 A) with the trilaminar staining image typical of phospholipid bilayers.Extensive block staining of cholestatic livers for 48 hr with warmed uranyl acetate (370) permitted the visualization of vesicles indistinguishable from LP-X within hepatic parenchyma. These trilaminar-staining vesicles occurred predominantly within bile canaliculi. They also were seen in nearby cytoplasmic vacuoles or invaginations between hepatocytes and in the space of Disse. Similar vesicles were not seen in the endoplasmic reticulum or Golgi cisternae. These observations raise the possibility that the vesicles are formed within bile canaliculi and are transported from the canaliculi to the space of Disse within pinocytotic vacuoles. Cholestasis may be defined as the interruption of bile flow from the biliary passages of the liver with consequent appearance of biliary constituents in blood plasma. In humans, prolonged obstruction to bile flow causes a progressive and often equimolar rise in plasma unesterified cholesterol and phospholipid (1-4). This unique hyperlipidemia results largely from the accumulation in plasma of a major abnormal lipoprotein that is separated with normal low density lipoproteins (LDL) by ultracentrifugation. This abnormal lipoprotein, which differs greatly in lipid and protein composition from normal LDL, is commonly called LP-X (3,5). Most normal plasma lipoproteins are pseudomicellar (6) with a core of relatively nonpolar triglycerides and cholesteryl esters covered by a monomolecular surface of phospholipids, unesterified cholesterol, and apoproteins, whereas LP-X is largely a unilamellar liposome of about 400-700 A diameter (4, 7-9). It probably exists in vivo as a bilayer vesicle of equimolar phospholipids and unesterified cholesterol containing small amounts of plasma proteins (mainly albumin) in its internal aqueous compartment together with some apolipoproteins adsorbed on its surface (4,6,8,9). Several other abnormal plasma lipoproteins have also been reported in patients with cholestasis (8, 10-13).The origin of LP-X is unknown, although it is generally assumed to be produced by regurgitation of bile components into the blood (for review, see ref. 14). Recent studies have shown that LP-X occurs in the plasma of animals after surgical interruption of the common bile duct (15)(16)(17)(18) (19)(20)(21). The perfusion medium, which was pumped at a rate of 1 ml min-1 g-1 of liver, was composed of three parts of Krebs-Ringer bicarbonate buffer, pH 7.4, containing 1.5 mg of glucose per ml and 1 part of thrice-washed rat erythrocytes. The bile duct was cannul...

Class III P-glycoproteins mediate the formation of lipoprotein X in the mouse.

“…An important conclusion from our study is therefore that Mdr2 Pgp is essential for the formation of LpX. This is in line with the hypothesis (3,5) that LpX is derived from biliary vesicles that in some way end up in the plasma.…”

“…According to several criteria, we found that LpX is indeed present in cholestatic murine plasma and actually represents the major lipoprotein under these conditions. Agarose electrophoresis, gradient ultracentrifugation, and gel filtration of cholestatic plasma demonstrated that murine LpX behaves very similarly as reported for other species (3,4,21). Electron microscopy proved that murine LpX is a 40-100 nm liposomal particle.…”

“…This hypercholesterolemia is mainly due to unesterified cholesterol and is accompanied by the appearance of an abnormal lipoprotein, lipoprotein X (LpX) 1 (1). Several decades ago, the aberrant mobility of plasma lipoproteins from cholestatic patients was already demonstrated via agarose electrophoresis (2) and gradient ultracentrifugation (3). Upon agarose electrophoresis, human LpX migrates toward the anode more slowly than all normal lipoproteins, and on agar electrophoresis it may even migrate toward the cathode (4,5).…”

“…Upon agarose electrophoresis, human LpX migrates toward the anode more slowly than all normal lipoproteins, and on agar electrophoresis it may even migrate toward the cathode (4,5). Upon gradient ultracentrifugation, LpX sediments in the LDL fraction (3). LpX from cholestatic humans and rats appears to have very similar characteristics (3).…”

“…Upon gradient ultracentrifugation, LpX sediments in the LDL fraction (3). LpX from cholestatic humans and rats appears to have very similar characteristics (3). Using smallangle X-ray scattering, it was shown that LpX, in contrast to the normal serum lipoproteins, is a unilamellar vesicle with an aqueous lumen (6,7).…”