Unique Epitope of Apolipoprotein A-I Expressed in Pre-.beta.-1 High-Density Lipoprotein and Its Role in the Catalyzed Efflux of Cellular Cholesterol

Abstract

The ability of mouse anti-apolipoprotein A-I (apo A-I) monoclonal antibodies to recognize pre-beta-HDL species in native plasma was determined. An antibody identifying residues 137-144 of the mature protein uniquely recognized pre-beta-1 HDL, an HDL species of low molecular weight implicated in early cholesterol transport from cell membranes to plasma [Castro, G. R., & Fielding, C. J. (1988) Biochemistry 27, 25-29]. Incubation of plasma with this antibody significantly inhibited the efflux of labeled cholesterol from cultured fibroblast monolayers. A second antibody, binding to residues 93-99 of apo A-I, recognized a second pre-beta-HDL species (pre-beta-2 HDL) but not pre-beta-1 HDL and did not inhibit cholesterol efflux. Several other antibodies had broad specificity for HDL (including pre-beta-1 HDL). This research suggests that apo A-I residues 137-144 are adjacent to or part of a structural site in pre-beta-1 HDL active in promoting the efflux of cellular cholesterol and that this site is not exposed in other HDL species.

Reagent or myeloperoxidase-generated hypochlorite affects discrete regions in lipid-free and lipid-associated human apolipoprotein A-I

“…Several groups have suggested that specific amino acid sequences participate in HDL-mediated cholesterol efflux [4,5,43,44]. In addition it has been suggested that the presence of amphipathic α-helices with high lipid-binding affinity determines the ability of apoproteins to participate in the membrane microsolubilization of cellular phospholipids and cholesterol [45].…”

“…We now have evidence that the modification of DPPC-associated apo A-I by HOCl results in impaired cholesterol efflux from J774 macrophages even at low modification rates [46] with no detectable formation of cross-linked, highmolecular-mass apo A-I. This could result from the modification of T9, T11, T16 and T22, peptides overlapping with specific amino acid sequences contributing to cellular cholesterol efflux [4,5,43,44]. The initial efflux of cellular unesterified cholesterol to HDL occurs by one of two independent mechanisms [47,48], depending on the degree of apo A-I lipidation.…”

“…In particular the amphipathic nature of these helices is thought to contribute to the ability of apo A-I to associate with lipids and membrane surfaces, as well as to activate LCAT. Several distinct apo A-I epitopes responsible for the unique features were identified : defined amino acid stretches contribute to cellular cholesterol efflux [4,5] …”

A Novel Mutant, ApoA-I Nichinan (Glu235→0), Is Associated With Low HDL Cholesterol Levels and Decreased Cholesterol Efflux From Cells

“…47 The best characterized of the lipid-poor forms of apoA-I is pre-␤1 HDL, a small particle containing only apoA-I and some phospholipid. 1, 46 Fielding et al 6 have reported that apoA-I residues 137 to 144 are adjacent to or part of a structural site in pre-␤1 HDL that is active in promoting the efflux of cellular cholesterol. Another study has attempted to demonstrate that monoclonal antibodies specific to apoA-I residues 140 to 150 are able to inhibit cellular cholesterol efflux from intracellular or plasma membrane pools of cholesterol.…”

“…The fourth exon contains homologous sequences that code for ten 11-or 22-amino acid repeats to form amphipathic helixes that mediate biological functions. 5 However, the domains of apoA-I necessary to form distinct HDL subclasses, 6 activate LCAT, 7 bind to cellsurface receptors, 8 and promote cholesterol efflux from cells 9 are not clearly understood. Monoclonal antibodies with epitopes recognizing discontinuous groups of amino acids in residues 143 to 164 were demonstrated to inhibit LCAT activation and cellular cholesterol efflux.…”