Free apolipoproteins A‐I and A‐IV present in human plasma displace high‐density lipoprotein on cultured bovine aortic endothelial cells

Abstract

Adult bovine aortic endothelial (ABAE) cells, exposed to serum‐free medium, specifically bind 125I‐labeled human high‐density lipoprotein (125I‐HDL). Addition of human lipoprotein‐deficient serum (LPDS) reduces the specific binding of 125I‐HDL in a concentration‐dependent manner, such that LPDS at a concentration of 6 mg protein/ml almost completely inhibits the specific binding of 125I‐HDL. ABAE cultures exposed to 125I‐labeled LPDS (125I‐LPDS) specifically bind two peptides, which appear as minor iodinated components in 125I‐LPDS. The binding of these two components is abolished in the presence of excess amounts of unlabeled LPDS or HDL. Preincubation of ABAE cells with 25‐hydroxycholesterol (25‐HC) results in an increase in the binding of the two 125I‐LPDS components, similar to the increase observed in 125I‐HDL binding in the presence of 25‐HC. These two LPDS components comigrate on sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS‐PAGE) with apolipoproteins A‐I and A‐IV of molecular masses 28 kDa and 43 kDa respectively. Furthermore, these two proteins were transfered from the SDS gel to nitrocellulose paper and interacted specifically with anti‐(A‐I) and anti‐(A‐IV) sera respectively. When ABAE cultures, pretreated with 25‐HC in the presence of LPDS, are subjected to cell‐surface iodination, the A‐IV appears as one of the major proteins on the cell surface accessible to iodination. The interaction of A‐IV with the cell surface of 25‐HC‐treated cells is not specific to ABAE cells and appears also in human skin fibroblasts. Analysis of the relative amounts of various apolipoproteins in the 125I‐HDL bound to ABAE cells demonstrates a decrease in the relative amount of iodinated A‐II concomitant with increase in the relative amounts of the other iodinated apolipoproteins, when compared to the composition of the native 125I‐HDL. These changes are similar whether the binding is done in the presence or absence of LPDS. It indicates that the decrease in 125I‐HDL binding in the presence of LPDS is not due to displacement of the iodinated apolipoproteins A‐I and A‐IV in the 125I‐HDL by unlabeled A‐I and A‐IV present in LPDS. The results indicate that free apolipoproteins A‐I and A‐IV, present in LPDS, can displace HDL on the cell surface of ABAE cells. Thus, free A‐I and A‐IV, present in plasma, control the binding of HDL to endothelial cells and may regulate the process of cholesterol removal from the cells performed by HDL. Copyright © 1987, Wiley Blackwell. All rights reserved