Interactions of high-density lipoprotein 3 with brain capillary endothelial cells

Abstract

High-density lipoprotein 3 (HDL 3) binds to capillary endothelial cells when their lumen surfaces are exposed to 125I-HDL 3 by post-mortem perfusion of whole brain. Kinetic studies of binding of HDL 3 to isolated membranes show that HDL 3 binds only to endothelial membranes with high affinity( K d = 7 μg/ml). Trypsin treatment of membranes abolishes HDL 3 binding. High-affinity binding sites for HDL 3 were recovered when endothelial cells from bovine brain capillaries were maintained in culture ( K d = 13 μg/ml HDL 3 protein). The characteristics of the binding were preserved up to the 6th passage. Competition experiments using isolated luminal membranes or cultured endothelial cells indicate that only HDL 3 and not LDL or methylated LDL, are able to compete binding of 125I-HDL 3. Furthermore, the inhibition of 125I-HDL 3 binding by lipoprotein A-I and lipoprotein A-I:A-II strongly suggests that apolipoprotein A-I is implicated in the formation of HDL 3-receptor complexes. The binding is increased by loading cells with free cholesterol or LDL cholesterol. In addition, surface-bound 125I-HDL 3 remains sensitive to mild trypsin treatment after subsequent incubation of BBCE at 37°C. HDL 3 bound to the cell surface is not endocytosed, but rather rapidly released into the medium after binding( t 1/2 = 5min).