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High-density lipoproteins can be separated into distinct particles based on their apolipoprotein content. In the present study, the in vivo metabolism of apoE within the apoE-containing HDL particles LpE, LpE:A-I, LpE:A-II and LpE:A-I:A-II was assessed in control subjects and in patients with abetalipoproteinemia (ABL), in whom HDL are the sole plasma lipoproteins. The metabolism of apoE within these HDL subspecies was investigated in three separate studies which differed by donor or recipient status: (1) particles purified from normolipidemic plasma and reassociated with 125I or 131I-labeled apoE injected into normolipidemic subjects (study 1); (2) particles purified from ABL plasma injected into normolipidemic subjects (study 2); and (3) particles purified from ABL plasma injected into ABL subjects (study 3). The plasma residence times (RT, hours) in study 1 were 14.3+/-2.9, 11.3+/-3.4, and 9.1+/-1.2 for apoE within LpE:A-I:A-II, LpE:A-II and LpE:A-I, respectively, while those in study 2 were 10.1+/-2.2, 9.7+/-2.4, 7.9+/-1.0 and 7.3+/-0.8 for apoE within LpE:A-I:A-II, LpE:A-II, LpE:A-I and LpE, respectively. In study 3, RTs for apoE within LpE:A-I:A-II and LpE were 8.7+/-0.9 and 6.8+/-0.9, respectively. In comparison, RT for apoA-I on LpA-I:A-II has been reported to be 124.1+/-5.5 h and that for apoA-I on LpA-I 105.8+/-6.2 h. Thus, apoE within the different apoE-containing HDL particles was metabolized rapidly and at a similar rate in control and ABL subjects. The plasma RT of apoE was longest when injected on LpE:A-I:A-II particles and shortest when injected on LpE. In summary, our data show that: (1) the plasma RT of apoE within HDL is approximately ten times shorter than that of apoA-I within HDL, and (2) apoE within HDL is metabolized at a slower rate when apoproteins A-I and A-II are present (LpE:A-I:A-II RTLpE:A-IILpE:A-ILpE). These differences were related to the lipid and apolipoprotein composition of the HDL subspecies, and, in control subjects, to the transfer of apoE from HDL subspecies to apoB-containing lipoproteins as well.

We have established a mouse model for human LCAT deficiency by performing targeted disruption of the LCAT gene in mouse embryonic stem cells. Homozygous LCAT-deficient mice were healthy at birth and fertile. Compared with age-matched wild-type littermates, the LCAT activity in heterozygous and homozygous knockout mice was reduced by 30 and 99%, respectively. LCAT deficiency resulted in significant reductions in the plasma concentrations of total cholesterol, HDL cholesterol, and apoA-I in both LCAT -/- mice (25, 7, and 12%; p < 0. 001 of normal) and LCAT +/- mice (65 and 59%; p < 0.001 and 81%; not significant, p = 0.17 of normal). In addition, plasma triglycerides were significantly higher (212% of normal; p < 0.01) in male homozygous knockout mice compared with wild-type animals but remained normal in female knockout LCAT mice. Analyses of plasma lipoproteins by fast protein liquid chromatography and two-dimensional gel electrophoresis demonstrated the presence of heterogenous prebeta-migrating HDL, as well as triglyceride-enriched very low density lipoprotein. After 3 weeks on a high-fat high-cholesterol diet, LCAT -/- mice had significantly lower plasma concentrations of total cholesterol, reflecting reduced levels of both proatherogenic apoB-containing lipoproteins as well as HDL, compared with controls. Thus, we demonstrate for the first time that the absence of LCAT attenuates the rise of apoB-containing lipoproteins in response to dietary cholesterol. No evidence of corneal opacities or renal insufficiency was detected in 4-month-old homozygous knockout mice. The availability of a homozygous animal model for human LCAT deficiency states will permit further evaluation of the role that LCAT plays in atherosclerosis as well as the feasibility of performing gene transfer in human LCAT deficiency states.