Apolipoprotein AII Enrichment of HDL Enhances Their Affinity for Class B Type I Scavenger Receptor but Inhibits Specific Cholesteryl Ester Uptake

Abstract

Abstract-Apolipoproteins of high density lipoprotein (HDL) and especially apolipoprotein (apo)AI and apoAII have been demonstrated as binding directly to the class B type I scavenger receptor (SR-BI), the HDL receptor that mediates selective cholesteryl ester uptake. However, the functional relevance of the binding capacity of each apolipoprotein is still unknown. The human adrenal cell line, NCI-H295R, spontaneously expresses a high level of SR-BI, the major apoAI binding protein in these cells. As previously described for murine SR-BI, free apoAI, palmitoyl-oleoylphosphatidylcholine (POPC)-AI, and HDL are good ligands for human SR-BI. In vitro displacement of apoAI by apoAII in HDLs or in Lp AI purified from HDL by immunoaffinity enhances their ability to compete with POPC-AI to bind to SR-BI and also enhances their direct binding capacity. The next step was to determine whether the higher affinity of apoAII for SR-BI correlated with the specific uptake of cholesteryl esters from these HDLs. Free apoAII and, to a lesser extent, free apoAI that were added to the cell medium during uptake experiments inhibited the specific uptake of [ 3 H]cholesteryl esters from HDL, indicating that binding sites on cells were the same as cholesteryl ester uptake sites.In direct experiments, the uptake of [ 3 H]cholesteryl esters from apoAII-enriched HDL was highly reduced compared with the uptake from native HDL. These results demonstrate that in the human adrenal cell line expressing SR-BI as the major HDL binding protein, efficient apoAII binding has an inhibitory effect on the delivery of cholesteryl esters to cells. iver and adrenal selective uptake of HDL cholesteryl esters (CEs) appears to be a receptor-mediated process. The class B type I scavenger receptor (SR-BI), a molecularly well-defined cell surface HDL receptor for selective cholesterol uptake, has recently been identified. 1 First described in rodents, its human analogue, initially called CLA-1, has a protein sequence identity close to that of the rodent receptor. 2,3 Selective uptake of CE does not involve endocytic uptake and lysosomal degradation of lipoprotein particles. The first step in the cellular mechanism of this pathway involves HDL binding and then the incorporation of HDLderived CE into the plasma membrane. 4 After uptake, CEs are directed intracellularly to a nonlysosomal destination for degradation. 5 In mice, SR-BI is largely expressed in liver, and overexpression of the SR-BI level in vivo on hepatocytes induces the disappearance of plasmatic HDL and the doubling of biliary cholesterol 6 ; inversely targeted disruption of the SR-BI gene induces a 2.2-fold increase in plasma cholesterol concentration. 7 All these results strongly suggest that SR-BI plays a key role in hepatic HDL metabolism in rodents. Also in rodents, SR-BI seems to play an important role in the maternal-fetal lipoprotein transport system during embryogenesis. 8 Azhar et al 9 have shown that the induction of the SR-BI receptor and the HDL-selective cholesterol uptake pathway in r...

Apolipoprotein A-II Modulates the Binding and Selective Lipid Uptake of Reconstituted High Density Lipoprotein by Scavenger Receptor BI

“…ApoA-II enrichment of HDL was found to actually increase cell association of HDL but, in an inverse manner, to decrease selective uptake. The positive correlation of apoA-II content with cell association contrasts with our finding of reduced association, as does the decreased selective CE uptake from particles containing apoA-II (43). The explanation for this is not clear.…”

“…1-3) indicate that apoA-II serves as a high -affinity ligand for SR-BI but binds somewhat less effectively than apoA-I. This finding is in agreement with previous reports by Xu et al (30) and Pilon et al (43). However, in those studies the rHDL preparations were heterogeneous in size, and their apolipoprotein content, in terms of number of apolipoprotein molecules per particle, was not defined.…”

“…Rinninger et al (14) reported that LpAI/AII was associated to a lesser extent to HepG2 cells and fibroblasts and promoted less selective uptake than LpAI. Pilon et al (43) compared, in an adrenal cell line, selective uptake from HDL and HDL that were enriched by the addition of purified apoA-II in vitro. ApoA-II enrichment of HDL was found to actually increase cell association of HDL but, in an inverse manner, to decrease selective uptake.…”

Discovery of chemical inhibitors of the selective transfer of lipids mediated by the HDL receptor SR-BI

“…A number of studies have explored the influence of the protein composition of HDL particles on binding and lipid transfer (26,29,(31)(32)(33). They showed that variations in the structure͞composition of HDL particles could affect the efficiency of selective uptake or efflux relative to lipoprotein binding and thus support the concept of productive binding.…”

Scavenger Receptor Class B, Type I-mediated Uptake of Various Lipids into Cells

“…Cross-linking studies have demonstrated that the class A amphipathic ␣-helix present in apolipoproteins is a recognition motif for the receptor (22), but multiple sites in apolipoproteins may be recognized by SR-BI (23). Furthermore, there are conflicting reports on the role of apoA-I and apoA-II in mediating HDL binding and selective CE uptake from HDL; Pilon et al (24) have suggested that apoA-II is a better ligand for SR-BI than apoA-I but that apoA-II has an inhibitory effect on CE uptake. On the contrary, de Beer et al (25) obtained results showing that apoA-II binds less well to SR-BI than apoA-I but is more efficient at promoting CE uptake than apoA-I.…”