Murine SR-BI, a High Density Lipoprotein Receptor That Mediates Selective Lipid Uptake, Is N-Glycosylated and Fatty Acylated and Colocalizes with Plasma Membrane Caveolae

Abstract

The class B, type I scavenger receptor, SR-BI, was the first molecularly well defined cell surface high density lipoprotein (HDL) receptor to be described. It mediates transfer of lipid from HDL to cells via selective lipid uptake, a mechanism distinct from receptor-mediated endocytosis via clathrin-coated pits and vesicles. SR-BI is expressed most abundantly in steroidogenic tissues (adrenal gland, ovary), where trophic hormones coordinately regulate its expression with steroidogenesis, and in the liver, where it may participate in reverse cholesterol transport. Here we have used immunochemical methods to study the structure and subcellular localization of murine SR-BI (mSR-BI) expressed either in transfected Chinese hamster ovary cells or in murine adrenocortical Y1-BS1 cells. mSR-BI, an ϳ82-kDa glycoprotein, was initially synthesized with multiple high mannose N-linked oligosaccharide chains, and some, but not all, of these were processed to complex forms during maturation of the protein in the Golgi apparatus. Metabolic labeling with [ 3 H]palmitate and [ 3 H]myristate demonstrated that mSR-BI was fatty acylated, a property shared with CD36, another class B scavenger receptor, and other proteins that concentrate in specialized, cholesterol-and glycolipid-rich plasma membrane microdomains called caveolae. OptiPrep density gradient fractionation of plasma membranes established that mSR-BI copurified with caveolin-1, a constituent of caveolae; and immunofluorescence microscopy demonstrated that mSR-BI colocalized with caveolin-1 in punctate microdomains across the surface of cells and on the edges of cells. Thus, mSR-BI colocalizes with caveolae, and this raises the possibility that the unique properties of these specialized cell surface domains may play a critical role in SR-BI-mediated transfer of lipids between lipoproteins and cells. High density lipoprotein (HDL)1 and low density lipoprotein (LDL) play important roles in the movement of cholesterol and cholesteryl esters through the plasma and in receptor-mediated sterol uptake by tissues. The mechanisms by which HDL delivers cholesterol and cholesteryl ester to steroidogenic (3) and hepatic (4) cells, although not well understood, clearly differ from the classic LDL receptor pathway, which involves clathrin-coated pits, coated vesicles, endosomes, and lysosomes (1, 2). HDL particles bind to cell surface receptors, but the intact particles are neither transported to lysosomes nor degraded (hydrolyzed). Instead, the core cholesteryl esters are selectively transferred into cells, and the lipid-depleted apolipoprotein-containing shell is released into the extracellular fluid, a process known as selective cholesterol uptake (5-11). The detailed molecular mechanisms underlying selective cholesterol uptake from HDL have not yet been determined.Recent in vitro and in vivo studies have suggested that the class B, type I scavenger receptor, SR-BI, is a cell surface HDL receptor that mediates physiologically relevant selective cholesterol uptake (12)(13)(14). SR-BI ...

Biliary cholesterol hypersecretion in gallstone-susceptible mice is associated with hepatic up-regulation of the high-density lipoprotein receptor SRBI

“…Taking into account the colocalization of SRBI and CAV1, 34 this indicates enhanced HDL cholesteryl ester uptake in the gallstone-susceptible strain.…”

Lower Plasma Levels and Accelerated Clearance of High Density Lipoprotein (HDL) and Non-HDL Cholesterol in Scavenger Receptor Class B Type I Transgenic Mice

“…Although numerous studies have revealed many steps involved in LDL metabolism, significantly less is known about HDL metabolism. Novel insights concerning the metabolism of HDL have come from a recent series of experiments suggesting that the scavenger receptor class B type I (SR-BI) is an HDL receptor (3)(4)(5)(6)(7)(8)(9). SR-BI, a member of the CD36 superfamily, was originally cloned as a receptor for modified LDL (10 -12).…”

Novel forms of protein glycosylation

“…For cxample, on hulnan thyroglobulin, 16 ()tit of the 20 putativc sites for .\~g13 cosylation havc been confirmed as being carbohydrate-bearing sites [30]. It has also been shown that routine SR-BI, a high-density lipoprntcin rcccptor that mediates selcctivc lipid uptake, is highl T N-glycosylated with multiple oligomannose chains [31].…”