Caveolin-1-Mediated Apolipoprotein A-I Membrane Binding Sites Are Not Required for Cholesterol Efflux

Ludger Johannes,Soazig Le Lay,Katharina Gaus,Thomas Grewal,Wendy Jessup,Macarena Rodriguez,Siân Cartland,Carles Rentero,Qiong Li

doi:10.1371/journal.pone.0023353

Ludger Johannes, Soazig Le Lay + Show 7 more

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https://doi.org/10.1371/journal.pone.0023353

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Abstract

Caveolin-1 (Cav1), a structural protein required for the formation of invaginated membrane domains known as caveolae, has been implicated in cholesterol trafficking and homeostasis. Here we investigated the contribution of Cav1 to apolipoprotein A-I (apoA-I) cell surface binding and intracellular processing using mouse embryonic fibroblasts (MEFs) derived from wild type (WT) or Cav1-deficient (Cav1−/−) animals. We found that cells expressing Cav1 have 2.6-fold more apoA-I binding sites than Cav1−/− cells although these additional binding sites are not associated with detergent-free lipid rafts. Further, Cav1-mediated binding targets apoA-I for internalization and degradation and these processes are not correlated to cholesterol efflux. Despite lower apoA-I binding, cholesterol efflux from Cav1−/− MEFs is 1.7-fold higher than from WT MEFs. Stimulation of ABCA1 expression with an LXR agonist enhances cholesterol efflux from both WT and Cav1−/− cells without increasing apoA-I surface binding or affecting apoA-I processing. Our results indicate that there are at least two independent lipid binding sites for apoA-I; Cav1-mediated apoA-I surface binding and uptake is not linked to cholesterol efflux, indicating that membrane domains other than caveolae regulate ABCA1-mediated cholesterol efflux.

Highlights

Expression of caveolin-1 (Cav1) and cavin proteins in mammalian cells drives the formation of small (50–100 nm) invaginations of the plasma membrane known as caveolae [1,2,3]
ApoA-I mediated-cholesterol export critically depends on the expression of ATP-binding cassette transporter A1 (ABCA1), a transporter that associates with non-raft domains [47]
We previously reported that membraneassociated cholesterol levels are similar in WT and Cav12/2 mouse embryonic fibroblasts (MEFs) [51] and that ABCA1 is exclusively found in non-raft factions on this gradient [47]

Summary

Introduction

Expression of caveolin-1 (Cav1) and cavin proteins in mammalian cells drives the formation of small (50–100 nm) invaginations of the plasma membrane known as caveolae [1,2,3]. Caveolae are regarded as a subtype of a group of specialized membrane microdomains, i.e. lipid rafts [4] that are characterized by their enrichment in cholesterol and sphingolipids [5,6]. Both the expression of Cav and the formation of caveolae are dependent on cellular cholesterol levels [7]. It has been suggested that Cav promotes the transport of newly synthesized cholesterol from the endoplasmic reticulum to outer leaflet of the plasma membrane, where it is accessible to extracellular cholesterol oxidase [15,16]

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