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) 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.
Bielicki
J K
JK
Ernest Orlando Lawrence Berkeley National Laboratory, Life Sciences Division, University of California, Berkeley 94720, USA.
Forte
T M
TM
McCall
M R
MR
Stoltzfus
L J
LJ
Chiesa
G
G
Sirtori
C R
CR
Franceschini
G
G
Rubin
E M
EM
D000818:Animals; D016632:Apolipoprotein A-I / Q000097:blood / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016633:Apolipoprotein A-II / Q000097:blood; D002499:Centrifugation, Density Gradient; D002784:Cholesterol / Q000378:metabolism; D002788:Cholesterol Esters / Q000097:blood; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008075:Lipoproteins, HDL / Q000097:blood / Q000737:chemistry* / Q000235:genetics; D051379:Mice; D008822:Mice, Transgenic; D016297:Mutagenesis, Site-Directed; D010316:Particle Size; D010641:Phenotype; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000097:blood
D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D002788:Cholesterol Esters; D008075:Lipoproteins, HDL; C080631:apolipoprotein A-I Milano; D002784:Cholesterol; D007862:Phosphatidylcholine-Sterol O-Acyltransferase