Abstract 113: Streptococcal Serum Opacity Factor Activity Diverts Human High Density Lipoprotein (HDL)-Cholesteryl Esters (CE) to the LDL-Receptor Pathway in Human Huh7 Hepatocytes

Abstract

Plasma concentrations of HDL-cholesterol (C) are negatively correlated with atherosclerotic cardiovascular disease. Nevertheless, current evidence suggests that this correlation is not axiomatic and that some forms of HDL are dysfunctional and atherogenic. Moreover, clinical trials of interventions that raise HDL-C have not been uniformly successful suggesting that mechanisms by which HDL-C is increased determine its anti atherogenic potency. Additionally, mice overexpressing the HDL receptor, SR-BI, have lower plasma HDL-C concentrations and less atherosclerosis. Thus, new strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. Serum opacity factor (SOF) disrupts HDL and forms three products_lipid-free apo A-I, a CE-depleted remnant neo HDL, and a cholesteryl ester-rich microemulsion (CERM) containing apo E and the CE of up to ~400,000 HDL particles. Hepatic CE uptake in Huh7 and HepG2 cells is faster when delivered by CERM than from the parent HDL, and cleared in vitro , in part, via the LDL-receptor (LDLR). We investigated the therapeutic potential of SOF in promoting RCT by comparing the uptake of HDL-, LDL- and CERM-CE in the final RCT steps_hepatocyte uptake, CE metabolism to cholesterol and bile salts, and their secretion and transfer to bile. HDL and LDL with [ 14 C]CE were labeled with [ 14 C]CE by incubation with lipoprotein deficient serum and the lipoproteins recovered by ultracentrifugation. CERM-labeled with [ 14 C]CE was obtained from SOF activity vs. HDL labeled with [ 14 C]CE. Incorporation efficiencies for HDL and LDL were 67% and 52% while [ 14 C]CE specific activities were 3.93 x 10 -4 and 3.04 x 10 -4 mCi/mg CE, respectively. [ 14 C]Metabolites from the hepatic uptake of HDL, LDL, and CERM were quantified by a two-solvent TLC system that resolves CE, C, and the major bile salts. The [ 14 C]CE of all three particles was taken up by cells and converted to free cholesterol with half-times of 2.7, 3.8, and 3.7 h respectively from HDL, CERM and LDL. Rates of bile salt synthesis and secretion are currently being investigated. These data show that SOF diverts potentially toxic HDL-CE to the hepatic LDLR pathway where it is hydrolyzed.