Selective Uptake of Low Density Lipoprotein-Cholesteryl Ester Is Enhanced by Inducible Apolipoprotein E Expression in Cultured Mouse Adrenocortical Cells

Snehasikta Swarnakar,Mary E Reyland,Jiatai Deng,Salman Azhar,David L Williams

doi:10.1074/jbc.273.20.12140

Abstract

Apolipoprotein (apo) E is expressed at high levels by steroidogenic cells of the adrenal gland, ovary, and testis. The cell surface location of apoE in adrenocortical cells suggests that apoE may facilitate the uptake of lipoprotein cholesterol by either the endocytic or the selective uptake pathways, or both. To examine these possibilities, the human apoE gene was expressed in murine Y1 adrenocortical cells under control of an inducible tetracycline-regulated promoter. The results show that induction of apoE yielded a 2-2.5-fold increase in the uptake of low density lipoprotein-cholesteryl ester (LDL-CE) but had little effect on high density lipoprotein-CE uptake. Analysis of lipoprotein uptake pathways showed that apoE increased LDL-CE uptake by both endocytic and selective uptake pathways. In terms of cholesterol delivery to the adrenal cell, the apoE-mediated enhancement of LDL-CE selective uptake was quantitatively more important. Furthermore, the predominant effect of apoE expression was on the low affinity component of LDL-CE selective uptake. LDL particles incubated with apoE-expressing cells contained 0.92 +/- 0.11 apoE molecules/apoB after gel filtration chromatography, indicating stable complex formation between apoE and LDL. ApoE expression by Y1 cells was necessary for enhanced LDL-CE selective uptake. This result may indicate an interaction between apoE-containing LDL and cell surface apoE. These data suggest that apoE produced locally by steroidogenic cells facilitates cholesterol acquisition by the LDL selective uptake pathway.

Highlights

Apolipoprotein E1 is a prominent component of plasma lipoproteins and serves to mediate endocytic uptake of remnant lipoproteins by members of the LDL receptor family [1,2,3,4,5,6]
The major finding in this study is that Apolipoprotein E (apoE) expression markedly increased LDL-cholesteryl ester (CE) uptake into adrenocortical cells via both endocytic and selective uptake pathways
The enhancement of LDL-CE uptake by apoE expression may play a quantitatively important role in LDL-CE uptake into adrenal cells in vivo since apoE expression occurs in adrenal cells of all mammalian species examined [7, 8, 10, 13, 16]

Summary

Introduction

Apolipoprotein E (apoE) is a prominent component of plasma lipoproteins and serves to mediate endocytic uptake of remnant lipoproteins by members of the LDL receptor family [1,2,3,4,5,6]. ApoE mRNA is expressed in adrenocortical zona fasciculata and zona reticularis cells, the sites of steroid production and cholesteryl ester storage in rat adrenal gland [17]. Immunolocalization studies in rat adrenocortical cells show apoE intracellularly within multivesicular bodies of the endocytic pathway and on cell surface microvillar channels [19]. The presence of apoE within multivesicular bodies of adrenocortical cells and in microvillar channels may indicate that locally synthesized apoE acts to facilitate the uptake of HDLand/or LDL-CE by either the endocytic or selective uptake pathways, or both [19]. Analysis of lipoprotein uptake pathways showed that apoE increased LDL-CE uptake by both endocytic and selective uptake pathways These data suggest that apoE produced locally by steroidogenic cells facil-

Methods

Results

Conclusion