Characterization of alpha(2)-macroglobulin receptor low density lipoprotein receptor-related protein (alpha(2)MR/LRP) in White Carneau pigeon peritoneal macrophages: Its role in lipoprotein metabolism

White Carneau pigeons develop atherosclerosis naturally, and at an accelerated rate with cholesterol feeding. Macrophages play a central role in the pathogenesis of atherosclerosis in pigeons, as they do in man. The purpose of this study was to determine whether pigeon macrophages express the alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha(2) MR/LRP) and whether this receptor would recognize beta-VLDL, the major cholesterol-transporting lipoprotein in cholesterol-fed pigeons. The binding of I-125-methylamine-treated alpha(2)M (I-125-alpha(2)M(+)) at 4 degrees C was saturable (> 10 nM), specific, Ca2+ dependent, was competed for by the receptor-associated protein (RAP), and had a K-d of binding of 1-5.6 nM, similar to mouse peritoneal macrophages studied simultaneously. At 37 degrees C the bound I-125-alpha(2)M(+) rapidly internalized and degraded in lysosomes. The binding of alpha(2)M(+) was not down-regulated with cholesterol loading, as is the LDL receptor on pigeon macrophages. At 4 degrees C there was no competition for binding of I-125-alpha(2)M(+) by either pigeon or rabbit beta-VLDL, nor was binding of I-125-pigeon or rabbit beta-VLDL competed for by alpha(2)M(+). Stimulation of cholesterol esterification by rabbit or pigeon beta-VLDL was unaffected by RAP, lactofenin, or alpha(2)M(+). Metabolism of I-125-pigeon or rabbit beta-VLDL was not competed by RAP, lactoferrin, or alpha(2)M(+) even in the presence of lipoprotein lipase. Pigeon macrophages, and a 500 kDa membrane protein isolated from them, were recognized by several antihuman alpha(2)MR/LRP monoclonal antibodies. The 500 kDa membrane protein also bound Ca-45. These data suggest considerable sequence homology with the human alpha(2)MR/LRP. This is the first study to characterize a functional alpha(2)MR/LRP on peritoneal macrophages from an avian species. There was no evidence, however, that the alpha(2)MR/ uptake of beta-VLDL by pigeon macrophages.