Evidence for Two Binding Sites with Different Potentials for Promoting Cholesterol Accumulation

Previous studies from our laboratory (J Upld Res 1988;29:643-656) have shown that thioglycolate-eliclted peritoneal macrophages from White Carneau and Show Racer pigeons, like mammalian macrophages, have on their surfaces specific receptors for acetyiated low density llpoproteln (acLDL) and ^-migrating very low density lipoprotelns (0-VLDL). The binding kinetics of 0-VLDL were complex, however, suggesting more than one binding site. The purpose of the present study was to further characterize these 0-VLDL binding sites. Scatchard analysis of l-0-VLDL blndlng curves indicated at least two classes of binding sites. The first binds pigeon /3-VLDL and LDL with high affinity (Kd approximately 7 jig/ml), Is down-regulated by cholesterol loading, requires calcium, and Is destroyed by the proteolytlc enzyme, pronase. This pigeon 0-VLDL receptor Is specific for pigeon /3-VLDL and LDL and does not recognize HDL, acLDL, methyl LDL, cynomolgus monkey LDL, or rabbit 0-VLDL. Like the mammalian macrophage /3-VLDL receptor, the "pigeon /3-VLDL receptor" has many of the characteristics of an LDL receptor. The second class of binding sites Is relatively nonspecific, recognizing both pigeon and rabbit 0-VLDL, LDL, acLDL, methyl LDL, and HDL Binding to this site Is not altered by Incubation of macrophages with pronase or by cholesterol loading. This binding site has low affinity for /3-VLDL (Kd approximately 100 f*glm\), but high capacity. We have called this the "llpoproteln binding site," a term used by others to describe similar llpoproteln binding characteristics on a variety of cells. Not only does binding to this site promote the internallzation and degradation of lipoprotelns, but It may also facilitate the Independent uptake of cholesterol. This conclusion Is based on the observation that more cholesterol accumulates In cells incubated with rabbit 0-VLDL, which binds only to the llpoproteln binding site, than can be accounted for by 0-VLDL uptake and degradation. Since the llpoproteln binding site recognizes a variety of normal, as well as abnormal, lipoprotelns, It would not require the generation of abnormal llpoprotein products, as must occur with the scavenger receptor, to promote the accumulation of cholesteryl esters in macrophages of atherosclerotic lesions. This, coupled with the fact that the lipoproteln binding site is not down-regulated by cholesterol loading, suggests that It could provide an alternative mechanism to the scavenger receptor pathway for the formation of foam cells. (Arteriosclerosis 9:673-683, September/October 1989)