A unique protease-sensitive high density lipoprotein particle containing the apolipoprotein A-IMilano dimer effectively promotes ATP-binding cassette A1-mediated cell cholesterol efflux

Carriers of the apolipoprotein A-I-Milano (A-I-M) variant present with severe reductions of plasma HDL levels, not associated with premature coronary heart disease (CHD). Sera from 14 A-I-M. carriers and matched controls were compared for their ability to promote ABCA1-driven cholesterol efflux from J774 macrophages and human fibroblasts. When both cell types are stimulated to express ABCA1, the efflux of cholesterol through this pathway is greater with A-I-M than control sera (3.4 +/- 1.0% versus 2.3 +/- 1.0% in macrophages; 5.2 +/- 2.4% versus 1.9 +/- 0.1% in fibroblasts). A-I-M and control sera are instead equally effective in removing cholesterol from unstimulated cells and from fibroblasts not expressing ABCA1. The A-I-M sera contain normal amounts of apoA-I-containing pre beta-HDL and varying concentrations of a unique small HDL particle containing a single molecule of the A-I-M, dimer; chymase treatment of serum degrades both particles and abolishes ABCA1-mediated cholesterol efflux. The serum content of chymase-sensitive HDL correlates strongly and significantly with ABCA1-mediated cholesterol efflux (r = 0.542, p = 0.004). The enhanced capacity of A-I-M serum for ABCA1 cholesterol efflux is thus explained by the combined occurrence in serum of normal amounts of apoA-I-containing pre beta-HDL, together with a unique protease-sensitive, small HDL particle containing the A-I-M dimer, both effective in removing cell cholesterol via ABCA1.