Expression of Scavenger Receptor BI in COS-7 Cells Alters Cholesterol Content and Distribution

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• 作者：G. Kellner-Weibel ; M. de la Llera-Moya ; M. A. Connelly ; G. Stoudt ; A. E. Christian ; M. P. Haynes ; D. L. Williams ; and G. H. Rothblat
• 刊名：Biochemistry
• 出版年：2000
• 出版时间：January 11, 2000
• 年：2000
• 卷：39
• 期：1
• 页码：221 - 229
• 全文大小：188K
• 年卷期：v.39,no.1(January 11, 2000)
• ISSN：1520-4995

文摘

Previous studies have shown that scavenger receptor BI (SR-BI) stimulates the bidirectionalflux of free cholesterol (FC) between HDL and SR-BI-expressing cells. A major component of the enhancedFC flux appears to occur independently of HDL binding to SR-BI and may be due to changes in membranelipid domains resulting from SR-BI expression (1). In the present study, the impact of SR-BI on cellularcholesterol metabolism was determined by examining SR-BI-mediated changes in cellular cholesterolmass, the esterification of HDL-derived FC, and changes in membrane lipid pools. Growth of SR-BI-expressing cells in medium containing HDL led to increased cellular cholesterol mass, most of whichaccumulated as ester. The esterification of HDL-derived FC was enhanced by SR-BI-expression to a fargreater extent than the SR-BI mediated increase in FC uptake, suggesting an SR-BI-mediated effect oncholesterol utilization in the cell. This observation was tested by comparing FC esterification rates inSR-BI positive and negative cells when equivalent amounts of extracellular FC were taken up viacyclodextrins or apolipoprotein AI/phospholipid disks, neither of which contained cholesteryl ester. Underthese conditions, SR-BI did not preferentially stimulate cholesterol esterification. These results indicatethat the enhanced esterification of HDL-derived FC in SR-BI-expressing cells is due to the expandedpool of cellular FC and not to a specific effect of SR-BI on cholesterol utilization. Two approaches wereused to test the effects of SR-BI expression on membrane lipid organization. In the first, the sensitivityof cellular FC to exogenous cholesterol oxidase was tested under conditions in which there is a preferentialoxidation of caveolar cholesterol. SR-BI-expression was found to greatly increase the fraction of cellularcholesterol available to the oxidase as compared to either vector-transfected cells or cells expressing therelated class B scavenger receptor CD36. These results suggest that SR-BI expression alters the distributionof membrane-free cholesterol to a caveolar fraction or alters the accessibility of this membrane fractionto exogenous cholesterol oxidase. In the second approach, the efflux of cellular FC to high concentrationsof cyclodextrins was monitored under conditions where desorption of FC from the plasma membrane israte limiting for efflux. SR-BI-expressing cells showed a shift in the distribution of FC between twokinetic pools with more FC in the fast pool and less in the slow pool. These data support a model inwhich SR-BI expression leads to a redistribution of cholesterol to membrane domains that serve to facilitatethe flux of FC between cells and lipoproteins.