SPARC MEDIATES FOCAL ADHESION DISASSEMBLY IN ENDOTHELIAL-CELLS THROUGH A FOLLISTATIN-LIKE REGION AND THE CA2-BINDING EF-HAND()

SPARC is a one of a group of extracellular matrix proteins that regula te cell adhesion through a loss of focal adhesion plaques from spread cells. We previously reported that SPARC reduced the number of bovine aortic endothelial (BAE) cells positive for focal adhesions [Murphy-Ul lrich et al. (1991): J Cell Biol 115:1127-1136]. We have now character ized the effect of SPARC on the cytoskeleton of BAE cells. Addition of SPARC to spread BAE cells caused a dose-dependent loss of focal adhes ion-positive cells, that was maximal at similar to 1 mu g/ml (0.03 mu M). Consistent with the loss of adhesion plaques as detected by interf erence reflection microscopy, vinculin appeared diffuse and F-actin wa s redistributed to the periphery of cells incubated with SPARC. Howeve r, the distribution of the integrin alpha(v) beta(3) remained clustere d in a plaque-like distribution. These data, and the observation that SPARC binds to BAE cells but not to the extracellular matrix, indicate that SPARC acts via interactions with cell surface molecules and not by steric/physical disruption of integrin-extracellular matrix ligands . To determine the region(s) of SPARC that mediate a loss of focal adh esions, we tested peptides from the four distinct regions of SPARC. Th e cationic, cysteine-rich peptide 2.1 (amino acids 54-73) and the Ca2-binding EF-hand-containing peptide 4.2 (amino acids 254-273) were act ive in focal adhesion disassembly. Furthermore, antibodies specific fo r these regions neutralized the focal adhesion-labilizing activity of SPARC. These results are consistent with previous data showing that pe ptide 2.1 and 4.2 interact with BAE cell surface proteins and indicate that the loss of focal adhesions from endothelial cells exposed to SP ARC is a receptor-mediated event. (C) 1995 Wiley-Liss, Inc.