Albumin-binding proteins function in the receptor-mediated binding and transcytosis of albumin across cultured endothelial cells.

The functional significance of the endothelial albumin-binding proteins (ABP) was tested on cultured bovine aortic endothelial cells (BAEC) incubated with radiolabeled albumin ([125I]Alb) alone or carrying fatty acids (oleic acid (OA) or arachidonic acid (AA)) or triiodothyronine. The [125I]Alb binding was estimated on BAEC grown on 96-well plates, and the transport was evaluated on BAEC cultured in a dual chamber system. The probe interaction with the monolayer was monitored as a function of concentration and temperature in the presence or absence of either unlabeled Alb or an anti-albumin anti-idiotypic antibody (Ab2), which was previously demonstrated to specifically recognize the ABP of endothelial cell surface. Cultured BAEC bound specifically and with high affinity [125I]Alb. The binding of fluorescein isothiocyanate (FITC)-Alb to endothelial cells was inhibited by Ab2 in immunofluorescence studies. As compared to albumin, the binding of albumin carrying either OA or AA was higher and was diminished by Ab2. Transport of [125I]Alb across BAEC grown on gelatin-coated filters increased with time, and after 60 min, approximately 30% of [125I]Alb was transported from the upper to the lower compartment; unlabeled Alb or Ab2 reduced this process by approximately 75%. Colchicine decreased transcytosis of [125I]Alb by approximately 80%, whereas chloroquine by approximately 27%. Transendothelial transport of [125I]Alb carrying fatty acids was 40% and 20% higher for OA and AA, respectively, as compared to that of defatted albumin. The results suggest the coexistence of a receptor-mediated and a receptor-independent transcytosis of albumin across cultured endothelial cells; ABP of the endothelial cell surface appear to be involved in the specific binding and transport of albumin.