Scavenger receptor class B type I affects cholesterol homeostasis by magnifying cholesterol flux between cells and HDL

Results from several laboratories clearly indicate that expression of scavenger receptor class B type I (SR-BI) enhances the bidirectional flux of cholesterol between cells and lipoproteins. Because the activity of HMG-CoA reductase, the key enzyme in cholesterol biosynthesis, is regulated by cell cholesterol content, we designed experiments to investigate the effect of SR-BI expression on the activity of this enzyme and on net cellular cholesterol mass. In addition, we compared the function of SR-BI with its human homolog, CD36 and LIMPII analogous 1. Our experiments demonstrate that both receptors enhance the flux of unesterified or free cholesterol bidirectionally, down a concentration gradient. Receptor-mediated cholesterol flux can effectively modulate multiple aspects of cellular cholesterol metabolism, including the pool that regulates the activity of HMG-CoA reductase. We also found that constitutive expression of SR-BI alters the steady state level of cellular cholesterol and phospholipid when SR-BI-expressing cells are maintained in medium containing serum lipoproteins. All of these effects are proportional to the level of receptor on the cell surface. These data indicate that the level of SR-BI expression determines both the rate of free cholesterol flux and the steady state level of cellular cholesterol.

Binding and cross-linking studies show that scavenger receptor BI interacts with multiple sites in apolipoprotein A-I and identify the class A amphipathic alpha-helix as a recognition motif

Scavenger receptor, class B, type I (SR-BI) mediates the selective uptake of high density lipoprotein (HDL) cholesteryl ester without the uptake and degradation of the particle. In transfected cells SR-BI recognizes HDL, low density lipoprotein (LDL) and modified LDL, protein-free lipid vesicles containing anionic phospholipids, and recombinant lipoproteins containing apolipoprotein (apo) A-I, apoA-II, apoE, or apoCIII. The molecular basis for the recognition of such diverse ligands by SR-BI is unknown. We have used direct binding analysis and chemical cross-linking to examine the interaction of murine (m) SR-BI with apoA-I, the major protein of HDL. The results show that apoA-I in apoA-I/palmitoyl-oleoylphosphatidylcholine discs, HDL(3), or in a lipid-free state binds to mSR-BI with high affinity (K(d) congruent with 5-8 microgram/ml). ApoA-I in each of these forms was efficiently cross-linked to cell surface mSR-BI, indicating that direct protein-protein contacts are the predominant feature that drives the interaction between HDL and mSR-BI. When complexed with dimyristoylphosphatidylcholine, the N-terminal and C-terminal CNBr fragments of apoA-I each bound to SR-BI in a saturable, high affinity manner, and each cross-linked efficiently to mSR-BI. Thus, mSR-BI recognizes multiple sites in apoA-I. A model class A amphipathic alpha-helix, 37pA, also showed high affinity binding and cross-linking to mSR-BI. These studies identify the amphipathic alpha-helix as a recognition motif for SR-BI and lead to the hypothesis that mSR-BI interacts with HDL via the amphipathic alpha-helical repeat units of apoA-I. This hypothesis explains the interaction of SR-BI with a wide variety of apolipoproteins via a specific secondary structure, the class A amphipathic alpha-helix, that is a common structural motif in the apolipoproteins of HDL, as well as LDL.

Expression of scavenger receptor BI in COS-7 cells alters cholesterol content and distribution

Previous studies have shown that scavenger receptor BI (SR-BI) stimulates the bidirectional flux of free cholesterol (FC) between HDL and SR-BI-expressing cells. A major component of the enhanced FC flux appears to occur independently of HDL binding to SR-BI and may be due to changes in membrane lipid domains resulting from SR-BI expression (1). In the present study, the impact of SR-BI on cellular cholesterol metabolism was determined by examining SR-BI-mediated changes in cellular cholesterol mass, 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 which accumulated as ester. The esterification of HDL-derived FC was enhanced by SR-BI-expression to a far greater extent than the SR-BI mediated increase in FC uptake, suggesting an SR-BI-mediated effect on cholesterol utilization in the cell. This observation was tested by comparing FC esterification rates in SR-BI positive and negative cells when equivalent amounts of extracellular FC were taken up via cyclodextrins or apolipoprotein AI/phospholipid disks, neither of which contained cholesteryl ester. Under these conditions, SR-BI did not preferentially stimulate cholesterol esterification. These results indicate that the enhanced esterification of HDL-derived FC in SR-BI-expressing cells is due to the expanded pool of cellular FC and not to a specific effect of SR-BI on cholesterol utilization. Two approaches were used to test the effects of SR-BI expression on membrane lipid organization. In the first, the sensitivity of cellular FC to exogenous cholesterol oxidase was tested under conditions in which there is a preferential oxidation of caveolar cholesterol. SR-BI-expression was found to greatly increase the fraction of cellular cholesterol available to the oxidase as compared to either vector-transfected cells or cells expressing the related class B scavenger receptor CD36. These results suggest that SR-BI expression alters the distribution of membrane-free cholesterol to a caveolar fraction or alters the accessibility of this membrane fraction to exogenous cholesterol oxidase. In the second approach, the efflux of cellular FC to high concentrations of cyclodextrins was monitored under conditions where desorption of FC from the plasma membrane is rate limiting for efflux. SR-BI-expressing cells showed a shift in the distribution of FC between two kinetic pools with more FC in the fast pool and less in the slow pool. These data support a model in which SR-BI expression leads to a redistribution of cholesterol to membrane domains that serve to facilitate the flux of FC between cells and lipoproteins.