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

High-density lipoprotein 3 (HDL3) binds to capillary endothelial cells when their lumen surfaces are exposed to 125I-HDL3 by post-mortem perfusion of whole brain. Kinetic studies of binding of HDL3 to isolated membranes show that HDL3 binds only to endothelial membranes with high affinity (Kd = 7 micrograms/ml). Trypsin treatment of membranes abolishes HDL3 binding. High-affinity binding sites for HDL3 were recovered when endothelial cells from bovine brain capillaries were maintained in culture (Kd = 13 micrograms/ml HDL3 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 HDL3 and not LDL or methylated LDL, are able to compete binding of 125I-HDL3. Furthermore, the inhibition of 125I-HDL3 binding by lipoprotein A-I and lipoprotein A-I:A-II strongly suggests that apolipoprotein A-I is implicated in the formation of HDL3-receptor complexes. The binding is increased by loading cells with free cholesterol or LDL cholesterol. In addition, surface-bound 125I-HDL3 remains sensitive to mild trypsin treatment after subsequent incubation of BBCE at 37 degrees C. HDL3 bound to the cell surface is not endocytosed, but rather rapidly released into the medium after binding (t1/2 = 5 min).