Characterization of apolipoprotein E7 (Glu244→Lys, Glu245→Lys), a mutant apolipoprotein E associated with hyperlipidemia and atherosclerosis

Taku Yamamura,Li-Ming Dong,Akira Yamamoto

doi:10.1016/s0022-2275(20)33364-2

Taku Yamamura, Li-Ming Dong + Show 1 more

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https://doi.org/10.1016/s0022-2275(20)33364-2

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Abstract

Previously, a mutant apolipoprotein (apo) E, apolipoprotein E7 (Glu244→Lys, Glu245→Lys), has been identified in association with hyperlipidemia and atherosclerosis. To investigate the effects of its structural changes on lipoprotein metabolisms and its correlation with atherosclerosis, we characterized this mutant apoE with respect to its receptor-binding, heparin-binding, and lipoprotein association. In a competitive binding assay, apoE7·dimyristoylphosphatidylcholine displayed a defective binding to the low density lipoprotein (LDL) receptor. The concentration of apoE7 required for 50% displacement of 125I-labeled LDL was 0.223 μg/ml, while that for apoE3 was 0.048 μg/ml. ApoE7 possesses only 23% of normal binding activity. To investigate the lipoprotein preference of apoE7, we determined the relative amounts of apoE7 in plasma lipoprotein fractions obtained by ultracentrifugation or gel filtration. Like human apoE4, apoE7 was preferentially associated with the very low density lipoproteins (VLDL). For determination of heparin-binding activity, apoVLDL was applied to a heparin-Sepharose affinity column and the bound materials were eluted with a salt gradient. The apoE7 was eluted at a higher NaCl concentration (157 mm) than apoE3 (126 mm), indicating that this mutant has a higher affinity for heparin than does apoE3. While the reduced receptor-binding activity indicates delayed clearance of the triglyceride-rich lipoproteins, the preferential association of apoE7 with larger-size lipoproteins and the stronger interaction with heparin may compensate, to some extent, for the delayed clearance of triglyceride-rich lipoproteins. The strong interaction with proteoglycans in the arterial wall seems to be one of the possible explanations for the association of apoE7 with atherosclerosis.—Yamamura, T., L-M. Dong, and A. Yamamoto. Characterization of apolipoprotein E7 (Glu244→Lys, Glu245→Lys), a mutant apolipoprotein E associated with hyperlipidemia and atherosclerosis. J. Lipid Res. 1999. 40: 253–259.

Highlights

A mutant apolipoprotein E, apolipoprotein E7 (Glu244 ̈Lys, Glu245 ̈Lys), has been identified in association with hyperlipidemia and atherosclerosis
The receptor-binding activity was estimated by measuring the ability of apoE и DMPC complexes to compete with 125I-labeled low density lipoprotein (LDL) for binding to the LDL receptor on human fibroblasts
We have characterized the apoE7 with respect to its receptor binding, heparin binding, and lipoprotein association

Summary

Introduction

A mutant apolipoprotein (apo) E, apolipoprotein E7 (Glu244 ̈Lys, Glu245 ̈Lys), has been identified in association with hyperlipidemia and atherosclerosis. To investigate the effects of its structural changes on lipoprotein metabolisms and its correlation with atherosclerosis, we characterized this mutant apoE with respect to its receptor-binding, heparin-binding, and lipoprotein association. The polymorphism of apoE influences the plasma cholesterol and low density lipoprotein (LDL) concentrations. Several other identified apoE variants with defective receptor-binding ability have been shown to have associations with impaired catabolism of Abbreviations: apo, apolipoprotein; DMPC, dimyristoylphosphatidylcholine; ELISA, enzyme-linked immunosorbent assay; HDL, high density lipoprotein; IDL, intermediate density lipoprotein; LDL, low density lipoprotein; VLDL, very low density lipoprotein. In apoE4, substitution of cysteine with arginine at position 112 in the aminoterminus influences its lipid-binding carboxyl-terminal domain, thereby resulting in apoE4’s preference for VLDL (in contrast to the apoE3 preference for HDL) (6, 7, and 17). Removal of the carboxyl-terminal domain in apoE2 increases its receptor-binding activity by 10-fold, suggesting that the carboxyl-terminal domain modulates the receptor-binding amino-terminal domain [20]

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