High density lipoprotein particle size restriction in apolipoprotein A-I-Milano transgenic mice

Human carriers of apolipoprotein A-I(Milano) (Arg173 → Cys substitution in apolipoprotein A-I) are characterized by an HDL deficiency in which small, dense HDL accumulate in plasma. Because affected individuals are heterozygous for this mutation, the full impact of apolipoprotein A-I(Milano) (apoA-I(Milano)) on HDL-cholesterol metabolism is unknown. In this study, apoA-I(Milano) transgenic mice were used to evaluate the extent of apoA- I(Milano) dimerization and HDL particle size restriction in the absence of wild-type apoA-I. Murine apoA-I knockout mice were utilized to express apoA- I(Milano) and human apoA-II in the presence of wild-type, human apoA-I (apoA- IMilano/A-Iwt/A-II) cud in its absence (apoA-IMilano/A-II). Plasma HDL- cholesterol concentrations were similar (30 mg/dl) in both lines of apoA- I(Milano) transgenic mice. In the apoA-IMilano/A-Iwt/A-II phenotype, 14% of the apoA-I(Milano) formed homodimers and 33% formed heterodimers with apoA- II. ApoA-I(Milano) homodimers increased by 71% in the apoA-IMilano/A-II transgenics and was associated with an abundance of small, 7.6-nm HDL3- sized particles compared to the 9.5, 8.3, and 7.6-nm-sized particles in apoA- IMilano/A-Iwt/A-II mice. The unesterified cholesterol/cholesteryl ester mole ratio of HDL was elevated by 45% in apoA-IMilano/A-Iwt/A-II mice and by 90% in apoA-IMilano/A-II transgenics compared to Mid-type (human apoA-I/A-II). Both apoA-I(Milano) transgenics possessed normal levels of plasma LCAT activity, but endogenous cholesterol esterification rates were reduced by 50% compared to controls. Thus, HDL particle size restriction was not the result of impaired LCAT activation; rather, dimerization of apoA-I(Milano) limited the esterification of cholesterol on endogenous HDL. In the absence of wild- type apoA-I, the more extensive dimerization of apoA-I(Milano) severely limited cholesteryl ester accumulation on plasma HDL accounting for the abundance of small, 7.6-nm HDL3 particles in apoA-IMilano/A-II mice.