Apolipoprotein E Mediates Binding of Normal Very Low Density Lipoprotein to Heparin but Is Not Required for High Affinity Receptor Binding

P E Fielding,Y Ishikawa,C J Fielding

doi:10.1016/s0021-9258(18)63881-5

Abstract

The relationship between the cholesteryl ester content of normal human very low density lipoprotein (VLDL) and its ability to bind to apolipoprotein E (apoE), heparin, and the low density lipoprotein (LDL) receptor have been compared. Plasma VLDL were separated by heparin affinity chromatography into two fractions: one with apoE and one without. Both fractions had the same cholesteryl ester content relative to apolipoprotein B (apoB). LDL, on the other hand, had a greater cholesteryl ester content. VLDL were modified by lipolysis to express the ability to bind apoE (Ishikawa, Y., Fielding, C. J., and Fielding, P. E. (1988) J. Biol. Chem. 263, 2744-2749). Lipolyzed VLDL with or without apoE were compared for their ability to bind to heparin or the up-regulated fibroblast LDL receptor. Lipolyzed VLDL bound with the same affinity to the receptor whether or not the particles contained apoE. ApoB, not apoE, appears then to be the important ligand for normal VLDL. On the other hand, modified VLDL without apoE, even though binding to the LDL receptor, did not bind to heparin. These data suggest that apoE mediates heparin binding in normal VLDL, that apoB mediates receptor binding, and that the cholesteryl ester content of VLDL is not a factor in the induction of the ability to bind apoE.

Highlights

Apolipoprotein E Mediates Binding of Normal Very Low Density Lipoprotein to Heparin but Is Not Required for High Affinity Receptor Binding*
The Cholesteryl Ester to ApoBand Free Cholesterol to ApoB Ratio of VLDL and lowdensity lipoprotein (LDL)-VLDL fractions and LDL were isolated from plasma by heparin-agarose chromatography
In each case, the CE/apolipoprotein B (apoB) mass ratio of the two different VLDL fractions was essentially identical. This findtinengtisndaincdatmesettahbaotlidcepsrpoitpeetrhtieesvoefryVdLiDffLere-ntEtraingldyVceLriDdeLc+onE+ [7, 8], these differences are not mediated by the cholesteryl ester contentof the particles; that is, the apoE onVLDL E was not bound as a result of an increased content of cholesteryl esters.On the otherhand, the same ratio, determined in LDL, demonstrated that thecholesteryl ester contentof LDL was in all cases greater than that of either VLDL fraction (Fig. 1)

Summary

RESULTS

The Cholesteryl Ester to ApoBand Free Cholesterol to ApoB Ratio of VLDL and LDL-VLDL fractions and LDL were isolated from plasma by heparin-agarose chromatography. The ratio between cholesteryl ester content and apoB mass was determined using the techniques described under “Experimental Procedures.”. In each case, the CE/apoB mass ratio of the two different VLDL fractions was essentially identical. Earlier estimates of VLDL and LDL cholesteryl ester content were made using centrifugally isolated lipoproteins (4, 5 ) .For this reason, the same plasma samples were fractionated both by heparin-agarose chromatography or by direct ultracentrifugal flotation, and the CE/apoB mass ratios obtained were compared. The slightly lower ratio of free cholesterol to apoB in VLDL lipoprotein fractions prepared by direct ultracentrifugation (Table I) may indicate the transfer of free cholesterol out of VLDL during isolation. The ratios for autologous lipoproteins (VLDL - E, VLDL E, LDL) from each individual plasma donor are joined by continuous lines

Free andesterified cholesterol content of VLDL and LDL

Another function sometimes ascribed to apoE in normal

DISCUSSION

The sequence of apoB contains two regions of strong local