Apolipoprotein AII enrichment of HDL enhances their affinity for class B type I scavenger receptor but inhibits specific cholesteryl ester uptake

Apolipoproteins of high density lipoprotein (HDL) and especially apolipopro tein (apo)AI and apoAII have been demonstrated as binding directly to the c lass B type I scavenger receptor (SR-BI), the HDL receptor that mediates se lective 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 m ajor apoAI binding protein in these cells. As previously described for muri ne SR-BI, free apoAI, palmitoyl-oleoyl-phosphatidylcholine (POPC)-AI, and H DL are good ligands for human SR-BI. In vitro displacement of apoAI by apoA II in HDLs or in Lp AI purified from HDL by immunoaffinity enhances their a bility to compete with POPC-AI to bind to SR-BI and also enhances their dir ect binding capacity. The next step was to determine whether the higher aff inity of apoAII for SR-BI correlated with the specific uptake of cholestery l esters from these HDLs. Free apoAII and, to a lesser extent, free apoAI t hat were added to the cell medium during uptake experiments inhibited the s pecific uptake of [H-3]cholesteryl esters from HDL, indicating that binding sites on cells were the same as cholesteryl eater uptake sites. In direct experiments, the uptake of [H-3]cholesteryl esters from apoAII-enriched HDL was highly reduced compared with the uptake hom native HDL. These results demonstrate that in the human adrenal cell line expressing SR-BI as the maj or HDL binding protein, efficient apoAII binding has an inhibitory effect o n the delivery of cholesteryl esters to cells.