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

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. [ABSTRACT FROM AUTHOR]