Cell association of liposomes with high fluid anionic phospholipid content is mediated specifically by LDL and its receptor, LDLr.

We have sought to confirm indications in our recent studies suggesting that association of liposomes composed of 75-100 mol % egg phosphatidylglycerol (ePG), a fluid anionic phospholipid, with cells is mediated by low density lipoprotein (LDL) and the classical LDL receptor (LDLr). In the present study, binding of liposomes composed of 75-100 mol % ePG to CV1-P cells, either in serum-supplemented medium or in defined medium supplemented with LDL, is blocked by the presence of either of two monoclonal antibodies. The first is immunoglobulin (Ig)G C7, an antibody specific for LDLr. The second is IgG 5E11, an antibody specific for domain 3441-3569 of apolipoprotein B100. CHOldlA7, a cell line known to lack the LDLr and previously shown by us to associate minimally with 75-100 mol % ePG liposomes, was transfected with the human LDLr. The transfected cells bound 75-100 mol % ePG liposomes at high levels, and this binding was blocked by IgG C7. Previously, we have shown that serum, but not LDL or high density lipoprotein, induces association of 25-50 mol % ePG liposomes with both CV1-P and CHO wild type cells, but not CHOldlA7. In the present study, IgG C7 does not block this interaction, and transfected CHOldlA7 cells do not show this interaction. Hence, this form of liposome binding appears not to involve LDL or LDLr, but requires a receptor, currently unknown, and a serum component other than LDL or high density lipoprotein. The unknown receptor, in addition to LDLr, is missing from CHOldlA7.