Quantification and immunolocalization of apolipoprotein E in experimental atherosclerosis.

We developed a radioimmunoassay for rabbit apolipoprotein E (apo E) for studying their plasma apo E levels and its accumulation in the aorta of rabbits fed a cholesterol diet. Delipidation of plasma did not increase the apo E immunoreactivity and this immunoreactivity was indistinguishable from that in an apo E-phospholipid complex. The antigenic determinants of apo E in lipoprotein particles were therefore fully reacted with our goat anti-apo E antibodies. In our assay system, the non-ionic detergent Tween-20 was found to be necessary to significantly reduce the non-specific binding of 125I-labeled apo E to polystyrene tubes, and yet not interfere with the assay. In rabbits (n = 6) fed a high cholesterol (1%) diet, plasma apo E increased at least 10-fold above baseline levels and reached maximal levels within 17-20 days after the onset of cho-diet feeding. These levels were sharply reduced only 10 days after resuming a normal diet. Plasma total cholesterol levels went through a similar pattern. Thus, the plasma cholesterol concentration can simply be used to monitor the increase of apo E in cholesterol-fed rabbits. All the cholesterol-fed rabbits developed atherosclerotic fatty streak lesions and apo E located mostly in the thoracic region and was significantly correlated with the accumulation of lipids in the areas of lesion. In addition, the apo E deposition was limited to the aortic areas where lipids were present. On the other hand, apo A-I was not detectable in any lesion area. Our data suggest that apo E or apo E-containing lipoproteins, may be involved in the development of atherosclerosis in cholesterol-fed rabbits.