Abstract 102: Profound Exclusion of Apolipoprotein A-II from Selective Hepatic Cholesteryl Ester Uptake

Abstract

Reverse cholesterol transport (RCT), the transfer of cholesterol from peripheral tissues, including the subendothelial space of the arterial wall, to the liver for disposal, is a current model of HDL atheroprotection. The final RCT step, selective hepatic HDL-cholesteryl ester (CE) uptake, is mediated by scavenger receptor class B type I (SR-BI). The net receptor reaction of SR-BI vs. HDL is distinct from that of LDL vs. the LDL receptor. LDL holo particle uptake is succeeded by steps that breakdown apo B-100 and hydrolyze and recycle the CE. In contrast, HDL-CE uptake is selective, occurring without a concomitant net uptake of the major HDL protein, apo A-I and even though apo E and apo A-I bind equally well to SR-BI, apoA-I-containing particles mediate 2-fold more selective CE uptake. The reaction of HDL with SR-BI is similar to the activity of a streptococcal serum opacity factor (SOF) against HDL_both reactions selectively remove CE from HDL leaving remnants. In addition, SOF catalyzes the displacement of apo A-I leaving an apo A-II-rich neo HDL, an effect that was assigned to the greater lipophilicity of apo A-II vs. apo A-I. Thus, we tested the hypothesis that the same occurs during the interaction of HDL with SR-BI, i.e., that apo A-II vs. apo A-I is selectively excluded from cellular uptake via SR-BI. Herein, we compare the selective uptake of HDL-CE vs. HDL-apo A-I and apo A-II. Cellular uptake of HDL-[ 3 H]CE labeled with [ 125 I]apo A-I or [ 125 I]apo A-II was compared in CHO-K1 and CHO-ldlA7 cells (LDL-R -/- ) with and without over expression of mouse SR-BI, and Huh7 human hepatocytes. Cell-associated 125 I and 3 H were determined by γ- and β-counting respectively. Uptake of CE, apo A-I, and apo A-II SR-BI-over expressing CHO cells was 32,800 ± 4800, 9.3 ± 2.7, and 2.5 ± 0.2 nmol/mg cell protein. The corresponding values for Huh7 cells were 9,700 ± 1,800, 15 ± 2.4, and 7.6 ± 0.9 nmol/mg cell protein. Relative to CE, both apo A-I and apo A-II were excluded from uptake by all cells. However, relative to the apo A-I and apo A-II contents of HDL, uptake of apo A-I was twice that of apo A-II, thus supporting the hypothesis that the more lipophilic apo A-II is selectively excluded from cellular uptake via SR-BI and retained in the neo HDL remnant.