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

Abstract

Cholestasis is associated with hypercholesterolemia and appearance of the abnormal lipoprotein X (LpX) in plasma. Using mice with a disrupted Mdr2 gene, we tested the hypothesis that LpX originates as a biliary lipid vesicle. Mdr2-deficient mice lack Mdr2 P-glycoprotein, the canalicular translocator for phosphatidylcholine, and secrete virtually no phospholipid and cholesterol in bile. Bile duct ligation of Mdr2 ϩ / ϩ mice induced a dramatic increase in the plasma cholesterol and phospholipid concentration. Agarose electrophoresis, density gradient ultracentrifugation, gel permeation, and electron microscopy revealed that the majority of phospholipid and cholesterol was present as LpX, a 40-100 nm vesicle with an aqueous lumen. In contrast, the plasma cholesterol and phospholipid concentration in Mdr2 Ϫ / Ϫ mice decreased upon bile duct ligation, and plasma fractionation revealed a complete absence of LpX. In mice with various expression levels of Mdr2 or MDR3 , the human homolog of Mdr2 , we observed that the plasma level of cholesterol and phospholipid during cholestasis correlated very closely with the expression level of these canalicular P-glycoproteins. These data demonstrate that during cholestasis there is a quantitative shift of lipid secretion from bile to the plasma compartment in the form of LpX. The concentration of this lipoprotein is determined by the activity of the canalicular phospholipid translocator. ( J. Clin. Invest. 1998. 102:1749-1757.)

Cholestasis and hypercholesterolemia in SCD1-deficient mice fed a low-fat, high-carbohydrate diet

“…2/2 mice is reminiscent of lipoprotein X, an abnormal VLDL-sized lipoprotein that is largely devoid of apolipoproteins and triglycerides (17,23). Agarose gel electrophoresis of plasma from VLF SCD1 2/2 mice indicated the presence of a slow-mobility band consistent with the mobility of lipoprotein X (Fig.…”

“…Plasma bile acids were measured by a colorimetric enzyme cycling assay (Bioquant). Bile was collected by aspiration of the gallbladder or upon gallbladder cannulation and analyzed for cholesterol and phospholipids as described (17). Briefly, biliary cholesterol was measured by an enzymatic assay, using the cholesterol oxidase reaction coupled to fluorimetric determination of hydrogen peroxide.…”

“…Probes), and proteins were imaged by Fluorimager quantitation (ImageQuant; Molecular Dynamics). For detection of lipoprotein X, fresh plasma was stained with Sudan black (0.02%, v/v) and subjected to agarose gel electrophoresis as described elsewhere (17,23), except for the substitution of a Tris-Tricine buffer for barbital (24). Hepatic triglyceride secretion rates were determined by measuring the temporal increase in plasma triglyceride after inhibition of lipoprotein lipase by intraperitoneal injection of poloxamer 407 (P-407; BASF Corp.), as described by Millar et al (25).…”

Diet-induced lipid accumulation in phospholipid transfer protein-deficient mice: its atherogenicity and potential mechanism

“…This is reminiscent of other scenarios in which biliary lipid outputs are impeded either through bile duct ligation ( 30,31 ) or due to biliary cholestasis ( 28,42 ), and plasma free cholesterol and phospholipids are consequently elevated. Under those circumstances, the excess free cholesterol and phospholipids circulate on an abnormal lipoprotein termed "lipoprotein X," which appear as lamellar structures under the electron microscope ( 42 ).…”

A Novel in Vivo Lecithin-Cholesterol Acyltransferase (LCAT)-Deficient Mouse Expressing Predominantly LpX Is Associated with Spontaneous Glomerulopathy

“…13 To obtain more specific in vivo evidence for the causative role of LpX, it is of great interest to create an LCAT-deficient mouse model that eliminates all other circulating lipoprotein fractions but, at the same time, accumulates significant LpX in blood. In light of a recent report by Elferink et al, 14 demonstrating that LpX may be hepatic in origin, we hypothesize that breeding the lcat knockout mice into the sterol regulatory element binding protein (SREBP)1a transgenic mouse background may fulfill this goal. The SREBP1a-transgenic (Sϩ) mice were generated with a constitutive overexpression of the NH 2 -terminus segment of the SREBP1a variant, resulting in a dramatic increase in hepatic lipogenesis and overproduction of VLDL.…”