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

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.

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.

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.

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.