Human scavenger receptor class B type II (SR-BII) and cellular cholesterol efflux

Abstract

Although studies in recombinant cells indicate that scavenger receptor class B, type I (SR-BI) can promote cholesterol efflux, investigations in transgenic mice overexpressing or deficient in SR-BI endorse its physiological function as selectively sequestering cholesteryl esters from high-density lipoproteins (HDLs). Less clear is the role of SR-BII, a splice variant of the SR-B gene that differs only in the C-terminal cytoplasmic domain. Here, we identify several putative signalling motifs in the C-terminus of human SR-BII, which are absent from SR-BI, and hypothesize that these motifs interact with signalling molecules to mobilize stored cholesteryl esters and/or promote the efflux of intracellular free cholesterol. 'Pull-down' assays using a panel of tagged SH3 (Src homology 3) domains showed that cytoplasmic SR-BII, but not cytoplasmic SR-BI, bound the SH3 domain of phospholipase C-gamma1; this interaction was not, however, detected under more physiological conditions. Specific anti-peptide antisera identified SR-BII in human monocyte/macrophage THP-1 cells and, in recombinant cells, revealed receptor localization to caveolae, a plasma membrane microdomain that concentrates signal-transducer molecules and acts as a conduit for cholesterol flux between cells and lipoproteins. Consistent with its caveolar localization, expression of human SR-BII in recombinant Chinese hamster ovary cells (CHO-SR-BII) was associated with increased HDL-mediated cholesterol efflux. Nevertheless, when CHO-SR-BII cells were pre-loaded with cholesteryl [(3)H]oleate and incubated with HDL, cholesteryl ester stores were not reduced compared with control cells. We conclude that although human SR-BII is expressed by macrophages, contains cytoplasmic signalling motifs and localizes to caveolae, its ability to stimulate cholesterol efflux does not reflect enhanced hydrolysis of stored cholesteryl esters.

Quantification of apolipoprotein E receptors in human brain-derived cell lines by real-time polymerase chain reaction

“…These findings contrast with previous reports of the proportional distribution of these receptors in other tissues [30], and with our present survey across a wide range of cell types.…”

“…It was the first authentic HDL binding protein to be characterized [51], although other ligands including apoE are now recognized [52]. A splice variant, termed SR-BII, was subsequently identified; this results from exon 12 skipping [30,53] to produce a receptor with a different C-terminal cytoplasmic tail, but identical extracellular binding region, to that of SR-BI. This implies differential functions; for example, the interaction between C-terminal SR-BI and PDZK1, a scaffolding protein [54], would not occur with SR-BII.…”

“…This implies differential functions; for example, the interaction between C-terminal SR-BI and PDZK1, a scaffolding protein [54], would not occur with SR-BII. On the other hand, cytoplasmic SR-BII contains six PxxP motifs, two being conserved in rat, mouse and hamster, which are potential ligands for tyrosine kinases or adaptor proteins with Src homology 3 (SH3) domains [30]. The presence of these motifs, which are also seen in cytoplasmic apoER2 [55], suggests that SR-BII may initiate signalling pathways that cannot be mediated by SR-BI; an exciting possibility given the preferential expression of SR-BII in many brain-derived cell lines (Fig.…”

A novel model of cholesterol efflux from lipid-loaded cells

“…Further studies have shown that the extracellular domain (ECD) of SR-B1 not only binds to ligands but also forms a hydrophobic channel for the trafficking of cholesterol esters (CE). It has been reported that caveolae present in cell membranes are the initial receiving sites of SR-B1-mediated CE intake, and 80% of CE accumulates in caveolae [24] . After discriminating HDL, SR-B1 forms a dimer via a leucine zipper region to construct a hydrophobic channel [25] .…”

The predominance of one of the SR-BI isoforms is associated with increased esterified cholesterol levels not apoptosis in mink testis

“…SR-BII, a spliced variant of the SR-B gene that differs from SR-BI only in the C-terminal cytoplasmic domain, also binds HDL with high affinity (27). In contrast to SR-BI, SR-BII does not mobilize intracellular cholesteryl ester stores and is less efficient at promoting cellular cholesterol efflux (27), but it effluxes 2-fold more FC than SR-BI in macrophages (28). Nonetheless, SR-BI and SR-BII both colocalize with cholesterolrich plasma membrane caveolae (28)(29)(30)(31), which influence cholesterol homeostasis by acting as conduits for cellular flux (32) and concentrate signaling proteins for signal transduction (33).…”