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

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 125 I-labeled HDL, we found that ϳ0.8% 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.125 I-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.Numerous studies have demonstrated the protective role of HDL 4 in the development of atherosclerosis and coronary artery disease (for review see Ref. 1). HDL exerts this atheroprotective effect mainly by transporting cholesterol from peripheral tissues back to the liver for biliary secretion, in a process referred to as "reverse cholesterol transport" (2). Moreover, HDL represents an important source of cholesterol for adrenal steroid hormone synthesis. The molecular details of the efflux of cellular cholesterol in the periphery and of cholesterol delivery to hepatocytes and adrenal cells are not completely understood. In particular, the fate and route of HDL particles taken up by cells and the physiological relevance of this process have not been delineated.The scavenger receptor class B, type I (SR-BI), a cell surface glycoprotein that binds HDL, LDL, very low density lipoprotein, modified LDL, and anionic phospholipids (3-6), can mediate the last step in reverse cholesterol transport, namely the delivery of cholesteryl esters from HDL to liver without HDL degradation, termed selective cholesteryl ester uptake (7-9). SR-BI is highly expressed in liver, adrenals, and ovaries with the highest mass of SR-BI protein localized in the liver (7). In addition to cholesterol uptake, SR-BI participates in the internalization of hepatitis C virus particles (10, 11) and lipopolysaccharide (12, 13). Serum amyloid A, a ligand of SR-BI, blocks the selective cholesteryl ester uptake (14). Furthermore, several small chemical inhibitors termed BLTs (block lipid transports) have been described to enh...

Recent advances in physiological lipoprotein metabolism

“…Other studies suggest that transfection mediated SR-BI overexpression by COS cells can alter the lipid organization of cell membranes and that the proposed binding independent SR-BI-mediated efflux might be a consequence of lipid organization [293,294]. Pagler et al [295] suggest that retroendocytosis of HDL is mediated by SR-BI and that resecretion of HDL particle is accompanied by an efflux of cellular cholesterol.…”

Cholesterol efflux via HDL resecretion occurs when cholesterol transport out of the lysosome is impaired

“…20). Our earlier data demonstrate that HDL uptake and resecretion mediated by SR-BI provide an alternative form of cellular cholesterol efflux (2). Here, we hypothesize that cholesterol efflux mediated by HDL resecretion may be important in cells with disturbed cholesterol efflux, in which HDL might serve as the central cholesterol-distributing platform.…”

“…Several mechanisms for cholesterol efflux have been described in detail, such as cell surface cholesterol efflux to nascent and mature HDL (1). Recently, we demonstrated that HDL retroendocytosis can be associated with cholesterol efflux (2). HDL uptake, visualized by single-dye tracing (3), and resecretion occurred in hepatocytes, macrophages, and adrenocortical cells (2).…”

“…Recently, we demonstrated that HDL retroendocytosis can be associated with cholesterol efflux (2). HDL uptake, visualized by single-dye tracing (3), and resecretion occurred in hepatocytes, macrophages, and adrenocortical cells (2). HDL uptake was facilitated by the scavenger receptor class B type I (SR-BI) (2).…”

“…HDL uptake, visualized by single-dye tracing (3), and resecretion occurred in hepatocytes, macrophages, and adrenocortical cells (2). HDL uptake was facilitated by the scavenger receptor class B type I (SR-BI) (2). However, the relevance of HDL uptake and resecretion for cholesterol homeostasis in cells and tissues needs to be established.…”