Displacement of apo A-I from HDL by apo A-II or its C-terminal helix promotes the formation of pre-β1 migrating particles and decreases LCAT activation

The displacement of apolipoprotein (apo) A-I by apo A-II is a major event in the remodelling of high density lipoproteins (HDL), In the present study, we investigated the displacement of apo A-I both from native and reconstituted HDL (rHDL) by either ape A-II or by the C-terminal helical peptide (i.e. residues 53-70). We studied the remodelling process of the original particles, the changes in size and composition and in their lecithin:cholesterol acyltransferase (LCAT) activating properties. Using gel filtration, we show that, at low ape A-II/AI ratios, the initial lipid-apolipoprotein complex containing 2 mol apo A-I is remodelled into a mixed complex containing ape A-I and apo A-II, involving the displacement of one apo A-I by apo A-II. Upon addition of a larger amount of apo A-II, the rHDL particles become more heterogeneous and of larger size. Immunoblotting of the particles separated by non denaturing gradient gel electrophoresis shows that most of the apo A-I remains associated with the largest particles. The LCAT activation properties of the remodelled complexes decrease upon addition of either apo A-II or its C-terminal helix. This decrease is more pronounced when rHDL are incubated with the apo A-II C-terminal helix than with native ape A-II, as V-max decreases from 28 to 16 and 7 nmol cholesteryl ester/ml per h respectively, whereas K-m remains unchanged. The displacement of apo A-I observed with rHDL also occurred with native HDL particles as demonstrated by two-dimensional gel electrophoresis, using pyrene-phospholipid labeled HDL. Displacement of apo A-I generates pre-beta(1) migrating particles containing ape A-I and phospholipids. We therefore propose that apo A-II has a dual effect on the role of HDL in reverse cholesterol transport: displacement of ape A-I from rHDL results in a negative control of the LCAT activity, while generation of pre-P, migrating particles enhances the formation of potential accepters of cellular cholesterol. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.