SPARC (secreted protein acidic and rich in cysteine) regulates endothelial cell shape and barrier function

SPARC (secreted protein acidic and rich in cysteine) can be selectively expressed by the endothelium in response to certain types of injury and induces rounding in adherent endothelial cells in vitro. To determine whether SPARC might influence endothelial permeability, we studied the effect of exogenous SPARC on the movement of 14C-labeled bovine serum albumin across postconfluent bovine pulmonary artery endothelial cells. SPARC increased (P < 0.02) transendothelial albumin flux in a dose-dependent manner at concentrations > or = 0.5 microgram/ml. At a fixed dose (15 micrograms/ml), exposure times > or = 1 h augmented (P < 0.005) albumin flux by 1.3- to 3.6-fold; this increase was blocked by anti-SPARC antibodies but not by inhibition of protein synthesis. Barrier dysfunction was not associated with loss of cell viability. Monolayers exposed to SPARC exhibited a rounded morphology and intercellular gaps. Prior stabilization of F-actin with phallicidin protected against the changes in barrier function (P = 0.0001) that were otherwise induced by SPARC. Bovine aortic and retinal microvascular endothelia also responded to SPARC. We propose that SPARC regulates endothelial barrier function through F-actin-dependent changes in cell shape, coincident with the appearance of intercellular gaps, that provide a paracellular pathway for extravasation of macromolecules.