Abstract 12463: Apolipoprotein (Apo) AI Stability Determines High Density Lipoprotein Function

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Introduction Atherosclerosis is characterized by the accumulation of free and esterified cholesterol in the subendothelial space of the arterial wall. Reverse cholesterol transport reduces the burden of cholesterol in the arterial wall in three steps including selective uptake by hepatic scavenger receptor class B member 1 (SR-BI). The mechanism for HDL-lipid uptake is distinct from that of low density lipoproteins, which are removed by endocytosis of the holo particle; in contrast, SR-BI selectively extracts lipids from HDL and leaves remnant HDL as an obligatory lipid-poor particle. Hypothesis HDL resides in a kinetic trap from which it escapes via perturbations that transfer apo AI into the surrounding aqueous phase as a lipid-free (LF) protein. Given that apo AI but not other HDL apos nor apo B-100 is labile and readily released by HDL, we hypothesized that apo AI lability is necessary or at least permitting for HDL function, including selective CE uptake. Methods HDL-protein lipophilicity was increased by lysine acylation. HDL stability was tested by chaotropic perturbation with guanidinium chloride (GdmCl), and with Streptococcal serum opacity factor (SOF) and phospholipid transfer protein (PLTP), which are functional perturbants of HDL structure. Production of LF apo AI was quantified by size exclusion chromatography; SOF reaction kinetics vs. HDL was determined turbidimetrically; selective [ 3 H]CE HDL uptake was determined in human hepatoma Huh7 cells, and in SR-BI over expressing Chinese hamster ovary (CHO-SR-BI) and the parent CHO-ldlA7 cells. Results Hexanoylation of HDL profoundly reduces release of apo AI vs. the chemical perturbant–GdmCl, and biochemical perturbants–SOF and PLTP. The rate of [ 3 H]CE uptake from polyhexanoylated HDL by Huh7 and CHO-SR-BI cells was respectively 32% and 38% lower than that of control HDL. The rate of [ 3 H]CE uptake by the parent CHO-ldlA7 cells was 7% to 13% of that of CHO-SR-BI cells and similar for control and polyhexanoylated HDL. Conclusions We have developed and validated an HDL mechanism, reaction with hexanoic anhydride, which reduces the loss of LF apo AI. Our data support the hypothesis that apo AI lability in HDL is a permitting factor in multiple HDL functions, including selective hepatic CE uptake.