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-γ1; 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 [3H]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.
- cholesteryl ester
- intracellular signalling
- lipid raft
- scavenger receptor class B type I
- Src homology 3 (SH3)
↵1 Present address: Neurology Centre of Excellence for Drug Discovery, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow CM19 5AW, U.K.
Abbreviations used: ABCA1, ATP-binding cassette transporter A1; apo, apolipoprotein; CHO, Chinese hamster ovary; CLM, caveolin-rich light membrane; ECL, enhanced chemiluminescence; eNOS, endothelial nitric oxide synthase; FBS, fetal bovine serum; GST, glutathione S-transferase; HDL, high-density lipoprotein; LDL, low-density lipoprotein; PLC, phospholipase C; SH, Src homology; SR-BI/II, scavenger receptor class B, type I/II.
- The Biochemical Society, London ©2004