ApoA-I Dissociated From Human HDL Retains its Acceptor Properties in ABCA1-mediated Cholesterol Efflux From RAW 264.7 Macrophages in Coronary Artery Disease.

BACKGROUND: The significance of cholesterol efflux as a predictor of coronary artery disease (CAD) remains controversial. The intracellular cholesterol export via the ABCA1 transporter involves the acceptance of cholesterol by both lipid-free apolipoprotein A-I and high-density lipoproteins (HDL). An estimate of the efficiencies of two reactions is thus required.

METHODS: HDL from the plasma of 63 control and 76 male CAD patients was obtained by the precipitation of apoB-containing lipoproteins and denatured by urea. We measured apoA-I dissociation concomitant with HDL denaturation by agarose gel electrophoresis followed by immunodetection and the expression of 65 preselected genes in blood mononuclear cells by real-time PCR. The total cholesterol efflux capacity (CEC) of ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol efflux from RAW 264.7 macrophages, when preβ-HDL and α-HDL act as competitive inhibitors of each other for the binding to ABCA1 transporter, was measured with intact HDL and pre-denatured HDL as a source of lipid-free apoA-I.

RESULTS: The phospholipid:apoA-I and cholesterol:apoA-I ratios in HDL from CAD patients were higher than those for control patients across the full range of plasma HDL-cholesterol levels. ApoA-I partitioned 1.5-fold higher into the water phase for HDL from CAD patients relative to controls. In CAD patients, the dissociation parameter D was inversely correlated with absolute and normalized per apoA-I phospholipid and cholesterol levels in HDL. For control patients, the D parameter was positively correlated with ABCA1 gene expression. For CAD patients, the D parameter was positively correlated with PLTP and inversely with CUBN and ALB gene expression. ApoA-I functionality in ABCA1-mediated cholesterol efflux from RAW 264.7 macrophages to lipid-free apoA-I generated from urea-induced HDL denaturation was similar for HDL from control and CAD groups. The retained CEC of lipid-free apoA-I in CAD may be masked by competition with α-HDL, which has a lower CEC, for ABCA1 binding to preβ-HDL.

CONCLUSIONS: The enrichment of HDL with cholesterol and phospholipids may contribute to the increased apoA-I dissociation from HDL in CAD. Estimates of both lipid-free apoA-I and intact HDL may be a prerequisites for a detailed study of ABCA1-mediated cholesterol efflux, which could allow these apoA-I forms to be identified as CAD predictors.