Scavenger receptor Bl and cholesterol trafficking

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Scavenger receptor class B, type I (SR-BI) shows a variety of effects on cellular cholesterol metabolism, including increased selective uptake of high density lipoprotein (HDL) cholesteryl ester, stimulation of free cholesterol (FC) efflux from cells to HDL and phospholipid vesicles, and changes in the distribution of plasma membrane FC as evidenced by increased susceptibility to exogenous cholesterol oxidase. Previous studies showed that these multiple effects require the extracellular domain of SR-BI, but not the transmembrane and cytoplasmic domains. To test whether 1) the extracellular domain of SR-BI mediates multiple activities by virtue of discrete functional subdomains, or 2) the multiple activities are, in fact, secondary to and driven by changes in cholesterol flux, the extracellular domain of SR-BI was subjected to insertional mutagenesis by strategically placing an epitope tag into nine sites. These experiments identified four classes of mutants with disruptions at different levels of function. Class 4 mutants showed a clear separation of function between HDL binding, HDL cholesteryl ester uptake, and HDLdependent FC efflux on one hand and FC efflux to small unilamellar vesicles and an increased cholesterol oxidase-sensitive pool of membrane FC on the other. Selective disruption of the latter two functions provides evidence for multiple functional subdomains in the extracellular receptor domain. Furthermore, these findings uncover a difference in the SR-BI-mediated efflux pathways for FC transfer to HDL acceptors versus phospholipid vesicles. The loss of the cholesterol oxidasesensitive FC pool and FC efflux to small unilamellar vesicle acceptors in Class 4 mutants suggests that these activities may be mechanistically related.Scavenger receptor class B, type I (SR-BI) 1 is an ϳ82-kDa cell-surface glycoprotein that was first identified by its sequence homology to CD36 (1, 2) and later characterized as one of the first physiologically relevant receptors for high density lipoprotein (HDL) particles (Ref. 3; for review,. Early analyses of SR-BI knockout mice revealed altered plasma HDL metabolism and reduced adrenal gland cholesteryl ester (CE) accumulation (7,8). Several recent reports have shown that alterations in murine SR-BI expression can have profound effects on biliary cholesterol excretion, red blood cell development, female infertility, atherosclerosis, and the development of coronary heart disease (9 -17), all of which seem to be related to changes in HDL and/or cholesterol. Taken together, these studies are consistent with SR-BI playing a major role in HDL cholesterol metabolism in vivo.SR-BI mediates its effects on HDL CE metabolism by facilitating the transport of lipids to cells in a process termed selective uptake (3-5, 18 -23). Contrary to the classic low density lipoprotein receptor endocytic pathway, in which the entire lipoprotein is internalized in clathrin-coated pits and degraded (24), HDL binds SR-BI, and the core CE is delivered to the plasma membrane without the concomitant upta...

Section: Discussionmentioning

confidence: 99%

Separation of Lipid Transport Functions by Mutations in the Extracellular Domain of Scavenger Receptor Class B, Type I

Connelly

Llera-Moya

Peng

et al. 2003

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“…It is not clear whether the HDL binding domain of SR-BI is involved in selective CE uptake. Williams and coworkers (66,67) recently proposed that SR-BI may form "a lipid channel" that facilitates transfer of lipid between cells and lipoproteins. Therefore, it is possible that AGE-BSA could inhibit SR-BI-mediated CE uptake by binding to the lipid channel rather than by binding to the HDL binding domain.…”

Section: Discussionmentioning

confidence: 99%

Scavenger Receptor Class B Type I-mediated Reverse Cholesterol Transport Is Inhibited by Advanced Glycation End Products

Ohgami

Nagai

Miyazaki

et al. 2001

154

105

Cellular interactions of advanced glycation end products (AGE) are mediated by AGE receptors. We demonstrated previously that class A scavenger receptor types I and II (SR-A) and CD36, a member of class B scavenger receptor family, serve as the AGE receptors. In this study, we investigated whether scavenger receptor class B type I (SR-BI), another receptor belonging to class B scavenger receptor family, was also an AGE receptor. We used Chinese hamster ovary (CHO) cells overexpressed hamster SR-BI (CHO-SR-BI cells).125 I-AGE-bovine serum albumin (AGE-BSA) was endocytosed in a dose-dependent fashion and underwent lysosomal degradation by CHO-SR-BI cells.125 I-AGE-BSA exhibited saturable binding to CHO-SR-BI cells (K d ‫؍‬ 8.3 g/ml). Endocytic uptake of 125 I-AGE-BSA by CHO-SR-BI cells was completely inhibited by oxidized low density lipoprotein (LDL) and acetylated LDL, whereas LDL exerted only a weak inhibitory effect (<20%). Cross-competition experiments showed that AGE-BSA had no effect on HDL binding to these cells and vice versa. Interestingly, however, SR-BI-mediated selective uptake of HDL-CE was completely inhibited by AGE-BSA in a dose-dependent manner (IC 50 <10 g/ml). Furthermore, AGE-BSA partially inhibited (by <30%) the selective uptake of HDL-CE in human hepatocarcinoma HepG2 cells (IC 50 <30 g/ml). In addition, [ 3 H]cholesterol efflux from CHO-SR-BI cells to HDL was significantly inhibited by AGE-BSA in a dose-dependent manner (IC 50 <30 g/ml). Our results indicate that AGE proteins, as ligands for SR-BI, effectively inhibit both SR-BI-mediated selective uptake of HDL-CE and cholesterol efflux from peripheral cells to HDL, suggesting that AGE proteins might modulate SR-BI-mediated cholesterol metabolism in vivo.In the Maillard reaction, proteins react with glucose to form Schiff base and Amadori products. After long term incubation, these early products are converted to advanced glycation end products (AGE), 1 which are characterized physicochemically by fluorescence, brown color, and intra-or inter-molecular crosslinking (1, 2), and biologically by specific recognition by AGE receptors. The presence of AGE in several human tissues suggests that they may be involved in the aging process, diabetic complications, and atherosclerosis (3-11).The physiological significance of AGE has been analyzed primarily using AGE structure(s) expressed in vivo and AGEbinding proteins or AGE receptors, through which AGE are believed to elicit several biological phenomena in monocytes/ macrophages (12-17), endothelial cells (18,19), and mesangial cells (20,21). Several AGE receptors have been characterized (22-25), one of which is a novel 35-kDa protein (called RAGE) from bovine lung endothelium that belongs to the immunoglobulin superfamily (23). Two AGE-binding proteins of 60-and 90-kDa (called p60 and p90) were also identified from the rat liver (24).Recently, galectin-3, a lectin-like protein with a high binding affinity for galactose-containing glycoproteins, was identified as a component of p90 (25). We have re...

“…For example, SR-BI is believed to be localized to cholesterol/sphingolipidenriched membrane microdomains or caveolae that appear to be the site of CE uptake from HDL particles (9,55,56). In contrast, the LRP/␣ 2 M receptor and other members of the LDL receptor family are targeted for endocytosis via coated pits (57).…”

Section: Discussionmentioning

confidence: 99%

The Apolipoprotein E-dependent Low Density Lipoprotein Cholesteryl Ester Selective Uptake Pathway in Murine Adrenocortical Cells Involves Chondroitin Sulfate Proteoglycans and an α2-Macroglobulin Receptor

Swarnakar

Beers

Strickland

et al. 2001

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Cells acquire lipoprotein cholesterol by receptor-mediated endocytosis and selective uptake pathways. In the latter case, lipoprotein cholesteryl ester (CE) is transferred to the plasma membrane without endocytosis and degradation of the lipoprotein particle. Previous studies with Y1/E/tet/2/3 murine adrenocortical cells that were engineered to express apolipoprotein (apo) E demonstrated that apoE expression enhances low density lipoprotein (LDL) CE uptake by both selective and endocytic pathways. The present experiments test the hypothesis that apoE-dependent LDL CE selective uptake is mediated by scavenger receptor, class B, type I (SR-BI). Surprisingly, SR-BI expression was not detected in the Y1/E/tet/2/3 clone of Y1 adrenocortical cells, indicating the presence of a distinct apoE-dependent pathway for LDL CE selective uptake. ApoE-dependent LDL CE selective uptake in Y1/E/tet/2/3 cells was inhibited by receptor-associated protein and by activated ␣ 2 -macroglobulin (␣ 2 M), suggesting the participation of the LDL receptor-related protein/␣ 2 M receptor. Reagents that inhibited proteoglycan synthesis or removed cell surface chondroitin sulfate proteoglycan completely blocked apoE-dependent LDL CE selective uptake. None of these reagents inhibited SR-BI-mediated LDL CE selective uptake in the Y1-BS1 clone of Y1 cells in which LDL CE selective uptake is mediated by SR-BI. We conclude that LDL CE selective uptake in adrenocortical cells occurs via SR-BI-independent and SR-BI-dependent pathways. The SR-BI-independent pathway is an apoE-dependent process that involves both chondroitin sulfate proteoglycans and an ␣ 2 M receptor.