Binding, internalization and transport of apolipoprotein A-I by vascular endothelial cells

High density lipoprotems (HDL) and their main protein constituent, apolipoprotein A-I (apoA-I), exert potentially anti -atherogenic properties within the arterial wall. However, it is unknown how they are transported from the blood stream into the vascular wall. Here we investigated the interaction of apoA-I with enclothelial cells. At 4 degrees C endothelial cells bound I-125-apoA-I with high affinity, K-d=2.1 mu g/ml and in a saturable manner (B-max of 35 ng/mg cell protein). At 37 degrees C, the cell association of apoA-l revealed similar affinity as at 4 degrees C (K-d=2.2 mu g/ml) but the maximum specific cell association was much enhanced (B-max=360 ng/mg cell protein). Binding and cell association was competed by excess unlabeled apoA-I and HDL but not by albumin. Biotinylation experiments and electron microscopy studies showed that endothelial cells internalize labeled apoA-I. Only minor amounts of the internalized apoA-I were degraded. Cultivated in a Transwell system, the cells transported a fraction of I-125-apoA-I from the apical to the basolateral compartment in a competable and temperature-sensitive manner. Furthermore, after specific transport the originally prebeta-mobile and lipid-free apoA-I was recovered as particles which have electrophoretic alpha-mobility. We conclude that endothelial cells transcytose and lipidate lipid-firee apoA-I. (c) 2006 Elsevier B.V. All rights reserved.