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 protein s (ABP) was tested on cultured bovine aortic endothelial cells (BAEC) incubated with radiolabeled albumin ([I-125]Alb) alone or carrying fat ty acids (oleic acid (OA) or arachidonic acid (AA)) or triiodothyronin e. The [I-125]Alb binding was estimated on BAEC grown on 96-well plate s, and the transport was evaluated on BAEC cultured in a dual chamber system. The probe interaction with the monolayer was monitored as a fu nction 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 o f endothelial cell surface. Cultured BAEC bound specifically and with high affinity [I-125[Alb. The binding of fluorescein isothiocyanate (F ITC)-Alb to endothelial cells was inhibited by Ab2 in immunofluorescen ce studies. As compared to albumin, the binding of albumin carrying ei ther OA or AA was higher and was diminished by Ab2. Transport of [I-12 5]Alb across BAEC grown on gelatin-coated filters increases with time, and after 60 min, approximately 30% of [I-125]Alb was transported fro m the upper to the lower compartment; unlabeled Alb or Ab2 reduced thi s process by approximately 75%. Colchicine decreased transcytosis of [ I-125]Alb by approximately 80%, whereas chloroquine by approximately 2 7%. Transendothelial transport of [I-125]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-m ediated and a receptor-independent transcytosis of albumin across cult ured endothelial cells; ABP of the endothelial cell surface appear to be involved in the specific binding and transport of albumin.