Antibodies to SPARC inhibit albumin binding to SPARC, gp60, and microvascular endothelium

Albumin, through its binding to the endothelial glycocalyx, functions as a major determinant of capillary permeability and as a carrier for various small molecules in its transcytosis across continuous endothelium via plasma-lemmal vesicles. Several albumin-binding proteins (ABP) have been identified: three membrane-associated ABP, which we call gp60, gp30, and gp18, and one secreted protein, acidic and rich in cysteine (SPARC). In this study, we used antiserum raised against bovine SPARC (BON) to investigate the possible interrelationships among ABP to better understand their role in binding and transcytosis. BON not only interacted with SPARC secreted by cultured endothelium but also recognized gp60 in lysates of cultured rat, human, and bovine endothelial cells. Purified SPARC inhibited BON interaction with gp60. BON immunoglobulin (Ig)G specifically inhibited albumin binding to both SPARC and gp60 extracts. This effect was eliminated by preabsorption of BON to immobilized SPARC. BON also significantly inhibited albumin binding to cultured microvascular endothelial cells via its interaction with gp60. Anti-SPARC peptide sera were also tested, and one serum raised against a peptide encompassing an NH2-terminal region of SPARC recognized both SPARC and gp60 but did not inhibit albumin binding; gp30 and gp18 were not recognized by any of these anti-SPARC antibodies. These results suggest that SPARC and gp60 are functionally and immunologically related ABP that may share a common albumin-binding domain. gp60 appears to be the major mediator of albumin binding to microvascular endothelium.