β-VLDL and acetylated-LDL binding to pigeon monocyte macrophages

Abstract

Blood-derived monocytes are an important source of foam cells in atherosclerotic lesions of White Carneau pigeons. Based upon studies with cultured blood monocytes (monocyte macrophages) and peritoneal macrophages from a variety of mammalian species, it has been proposed that these cells become loaded with cholesteryl esters through the uptake of lipoproteins including, β-migrating very low density lipoproteins (β-VLDL) and low density lipoproteins that have been chemically modified in a manner analogous to experimental acetylation (Ac-LDL). The purpose of this study was to determine whether similar mechanisms functioned in pigeon monocyte macrophages. Radioiodinated pigeon β-VLDL and Ac-LDL were incubated with White Carneau pigeon monocyte macrophages that had been maintained in culture for 7 days. Scatchard analysis of the specific binding data revealed the presence of specific and saturable receptors for both β-VLDL and Ac-LDL. β-VLDL receptors had both low and high affinity binding components, whereas Ac-LDL receptors displayed a single class of high affinity binding sites. β-VLDL binding remained relatively constant from 3 to 10 days in culture while Ac-LDL binding increased with time in culture. Competition studies demonstrated a high degree of binding specificity for 125I-Ac-LDL, but less for 125I-β-VLDL. Binding of 125I-β-VLDL was not competed for by Ac-LDL, but was by β-VLDL and by low density lipoproteins from both normal and hypercholesterolemic pigeons. Following binding of β-VLDL and Ac-LDL, the lipoproteins were rapidly internalized and degraded. Although the majority of degradation was secondary to internalization by the monocyte macrophages, approx. 5% of the degradation resulted from enzymatic activity in the culture medium, presumably due to secretion of proteolytic enzymes by the cells. As measured by esterification of [ 14C]oleate to cholesterol, it was shown that the cholesterol liberated from the degradation of both β-VLDL and Ac-LDL stimulated cholesteryl ester synthesis in pigeon monocyte macrophages. These studies confirm the existence of specific β-VLDL and Ac-LDL receptors on the surface of pigeon monocyte macrophages which facilitate both internalization of the lipoproteins and subsequent stimulation of cholesteryl ester synthesis. This is the first demonstration of β-VLDL and Ac-LDL receptors on monocyte macrophages from an avian species, and the findings support the potential role for the receptor-mediated uptake of a variety of abnormal lipoproteins in the formation of monocyte-derived foam cells in the arterial wall of White Carneau pigeons during the development of atherosclerosis.