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High density lipoprotein particle size restriction in apolipoprotein A-I(Milano) transgenic mice
J K Bielicki1, T M Forte, M R McCall
1Ernest Orlando Lawrence Berkeley National Laboratory, Life Sciences Division, University of California, Berkeley 94720, USA.
Abstract:
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) and 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 wild-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.
Insights
Apolipoprotein A-I(Milano) (apoA-I(Milano)) dimerization restricts HDL particle size and cholesterol esterification. This dimerization, especially without wild-type apoA-I, leads to an abundance of small HDL3 particles.
Area of Science:
- Biochemistry
- Genetics
- Cardiovascular Science
Background:
- Human carriers of apolipoprotein A-I(Milano) (apoA-I(Milano)) exhibit HDL deficiency with small, dense HDL particles.
- The heterozygous nature of the mutation in humans limits understanding of apoA-I(Milano)'s full impact on HDL metabolism.
Purpose of the Study:
- To investigate the dimerization of apoA-I(Milano) and its effect on HDL particle size.
- To assess HDL-cholesterol metabolism in transgenic mice lacking wild-type apoA-I.
Main Methods:
- Generated apoA-I(Milano) transgenic mice using murine apoA-I knockout models.
- Expressed apoA-I(Milano) and human apoA-II with and without wild-type human apoA-I.
- Analyzed plasma HDL-cholesterol concentrations, apoA-I dimerization, HDL particle size, and cholesterol esterification rates.
Main Results:
- ApoA-I(Milano) dimerization increased in the absence of wild-type apoA-I, correlating with smaller HDL3 particles.
- The unesterified cholesterol/cholesteryl ester ratio in HDL was elevated in apoA-I(Milano) transgenic mice.
- Reduced endogenous cholesterol esterification rates were observed, independent of LCAT activity.
Conclusions:
- Dimerization of apoA-I(Milano) restricts HDL particle size and limits cholesterol esterification.
- The absence of wild-type apoA-I exacerbates apoA-I(Milano) dimerization, leading to a higher prevalence of small HDL particles.
- These findings elucidate the mechanism behind HDL abnormalities associated with apoA-I(Milano).