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 in a serum-free medium specifically bind [125I]-labeled high-density lipoprotein ([125I]HDL). Addition of lipoprotein-deficient serum (LPDS) to the incubation medium interferes with the binding in a concentration-dependent manner such that at a concentration of 6 mg protein/ml of LPDS almost a complete inhibition of the specific [125I]HDL binding is observed. Cultures of ABAE cells exposed to [125I]-labeled LPDS ([125I]LPDS) specifically bind two peptides that appear as minor iodinated components in the [125I]LPDS. The binding of the two iodinated LPDS 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 these two [125I]LPDS components similarly to the increase in [125I]HDL binding. These two LPDS components comigrated on sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with apolipoproteins (apos) A-I and A-IV with molecular weights of 28,000 and 43,000, respectively. The apo A-IV is also found in commercial preparations of human and bovine albumin, and indeed, these albumin preparations interfere with [125I]HDL binding to ABAE cultures similarly to the effect of LPDS.
When ABAE cultures are treated with 25-HC and subjected to cell surface iodination followed by analysis on SDS-PAGE, the apo A-IV appears as one of the major proteins on the cell surface accessible to iodination. The interaction of apo A-IV with the cell surface of 25-HC-treated cells is not specific only to ABAE cells and appears also in human skin fibroblasts.
The results indicate that free apos A-I and A-IV present in LPDS can displace HDL on the cell surface of ABAE cells. It therefore indicates that the free apos A-I and A-IV present in plasma control the binding of HDL to endothelial cells and may regulate the important process of cholesterol removal from the cells by HDL.
KeywordsHuman Skin Fibroblast Bovine Albumin Bovine Aortic Endothelial Cell Cholesterol Removal Aortic Endothelial
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