SR-BI-mediated High Density Lipoprotein (HDL) Endocytosis Leads to HDL Resecretion Facilitating Cholesterol Efflux

Tamara A Pagler,Sebastian Rhode,Angelika Neuhofer,Hildegard Laggner,Wolfgang Strobl,Claudia Hinterndorfer,Ivo Volf,Margit Pavelka,Erik R.M Eckhardt,Deneys R Van Der Westhuyzen,Gerhard J Schütz,Herbert Stangl

doi:10.1074/jbc.m510261200

Abstract

The high density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake from lipoproteins into liver and steroidogenic tissues but also cholesterol efflux from macrophages to HDL. Recently, we demonstrated the uptake of HDL particles in SR-BI overexpressing Chinese hamster ovarian cells (ldlA7-SRBI) using ultrasensitive microscopy. In this study we show that this uptake of entire HDL particles is followed by resecretion. After uptake, HDL is localized in endocytic vesicles and organelles en route to the perinuclear area; many HDL-positive compartments were classified as multivesiculated and multilamellated organelles by electron microscopy. By using 125I-labeled HDL, we found that approximately 0.8% of the HDL added to the media is taken up by the ldlA7-SRBI cells within 1 h, and almost all HDL is finally resecreted. 125I-Labeled low density lipoprotein showed a very similar association, uptake, and resecretion pattern in ldlA7-SRBI cells that do not express any low density lipoprotein receptor. Moreover, we demonstrate that the process of HDL cell association, uptake, and resecretion occurs in three physiologically relevant cell systems, the liver cell line HepG2, the adrenal cell line Y1BS1, and phorbol myristate acetate-differentiated THP-1 cells as a model for macrophages. Finally, we present evidence that HDL retroendocytosis represents one of the pathways for cholesterol efflux.

Highlights

high density lipoprotein (HDL) Retroendocytosis Linked to Cholesterol Efflux onstrated that cholesterol transfer from HDL particles to polarized hepatocytes via SR-BI was accompanied by a transport of HDL particles to the endosomal recycling compartment
We present evidence that holo-HDL particle uptake facilitated by SR-BI is followed by resecretion in cell lines derived from tissues of central importance in cholesterol metabolism
HDL Cell Association and Holoparticle Uptake in SR-BI Overexpressing CHO Cells—By using ultrasensitive microscopy, we have shown previously that holo-HDL particle uptake can be observed in CHO cells that lack the LDL receptor and overexpress SR-BI (40)

Summary

Introduction

Besides its role in cholesterol delivery, SR-BI mediates cholesterol efflux for example from macrophages to HDL particles (17–22). Silver et al (38) described that selective cholesteryl ester uptake mediated by SR-BI is linked to this holo-HDL uptake process. HDL Retroendocytosis Linked to Cholesterol Efflux onstrated that cholesterol transfer from HDL particles to polarized hepatocytes via SR-BI was accompanied by a transport of HDL particles to the endosomal recycling compartment. This is in contrast to findings of Nieland et al (39) suggesting that endocytosis is not required for selective lipid uptake mediated by SR-BI. The present study was designed to: 1) analyze HDL holoparticle uptake, its relation to selective uptake, and the role of SR-BI for this uptake process; 2) describe the intracellular trafficking of HDL and LDL during retroendocytosis in more detail; and 3) study the resecretion of HDL and its role for cholesterol efflux

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