Binding of apolipoprotein A-I and apolipoprotein A-IV to cultured bovine aortic endothelial cells

Adult bovine aortic endothelial (ABAE) cells specifically bound 125I-labelled high density lipoprotein3 (125I-HDL3), and saturation of the binding was observed at a concentration of about 50 to 100 micrograms protein/ml. The binding of purified apolipoproteins (apo) A-I and A-IV to cultured ABAE cells and the correlation of this binding with the binding of HDL to the cells was studied. ABAE cells bound 125I-labelled apo A-I (125I-apo A-I) and after a 3-hour exposure to the ligand, most of the radioactivity (82%) was associated with the cell surface and was accessible for trypsinization. Saturation of 125I-apo A-I binding was observed at a concentration of 3 micrograms/ml; each cell possessed 1.38 X 10(5) high affinity binding sites (Kd = 2.04 X 10(-8) M). The cultures also bound 125I-labelled apo A-IV (125I-apo A-IV), and saturation of the binding was observed at a concentration of 2 micrograms/ml. Each cell possessed 5.4 X 10(4) high affinity binding sites for apo A-IV (Kd = 1.45 X 10(-8) M). Addition of either excess unlabelled apo A-I or excess unlabelled apo A-IV competed with the binding of both iodinated apo A-I and apo A-IV. It suggests that apo A-I and A-IV share the same binding site on endothelial cells. HDL also successfully competed with the binding of apo A-I and A-IV to ABAE cells