Molecular mechanisms of a dominant form of type III hyperlipoproteinemia and the biogenesis of apolipoprotein E-containing high density lipoprotein.

摘要

Apolipoprotein E (apoE) mediates the clearance of lipoprotein remnants from the circulation, promotes the formation of apoE-containing high density lipoprotein (HDL), and is atheroprotective. Certain mutations in apoE are associated with type III hyperlipoproteinemia. The first objective of this thesis was to elucidate the molecular etiology of a dominant form of type III hyperlipoproteinemia caused by the arginine to cysteine substitution at residue 142 in apoE4[R142C]. The second objective was to identify the domains of apoE required for the biogenesis of apoE-containing HDL.;Using adenovirus-mediated gene transfer in apoE-/- mice, low doses of apoE4 cleared cholesterol, whereas apoE4[R142C] increased plasma cholesterol and apoE, caused severe hypertriglyceridemia, and promoted the formation of discoidal HDL. Coexpression of apoE4 [R142C] with lecithin cholesterol acyltransferase (LCAT) or lipoprotein lipase (LPL) in apoE-/- mice partially corrected the apoE4[R142C]-induced dyslipidemia. High doses of the N-terminal 202 amino acids of apoE4 or apoE4[R142C] cleared cholesterol in apoE-/- mice and promoted formation of discoidal HDL.;The ability of C-terminally truncated apoE variants to form HDL was investigated in vitro and in vivo. ApoE4-185, -202, -229, or -259 promoted ATP-binding cassette transporter A1-dependent cholesterol efflux in vitro, but had diminished capacity to activate LCAT. Formation of HDL was investigated following gene transfer in apolipoprotein A-I-/- x apoE-/- mice. Fast protein liquid chromatography of plasma showed that the truncated apoE forms, except apoE4-185, generated an apoE-containing HDL peak. Two-dimensional gel electrophoresis and transmission electron microscopy of HDL showed that these truncated apoE forms generated distinct HDL subpopulations and promoted the formation of discoidal HDL that could be converted to spherical by excess LCAT.;The findings establish that apoE4[R142C] causes accumulation of apoE in plasma, and has an impact on the function of LCAT and LPL, thus resulting in severe dyslipidemia. Deletion of the C-terminal 203-299 amino acid region appears to unmask the receptor-binding domain of apoE4[R142C] and allows in vivo clearance of lipoproteins.;This represents the first report establishing that truncated apoE forms can direct in vivo the formation of mostly discoidal HDL particles and that the domains of apoE required for HDL formation reside within residues 1 to 185.