THE COUNTERADHESIVE PROTEIN SPARC REGULATES AN ENDOTHELIAL PARACELLULAR PATHWAY THROUGH PROTEIN-TYROSINE PHOSPHORYLATION

SPARC (Secreted Protein Acidic and Rich in Cysteine) regulates the tra nsendothelial flux of macromolecules through a paracellular pathway. W e now have demonstrated that SPARC-induced increments in albumin flux across postconfluent endothelial cell (EC) monolayers are mediated, in part, through protein tyrosine phosphorylation. SPARC increased tyros ine phosphorylation of EC proteins up to 12-fold within 1 h. The phosp hotyrosine-containing proteins were immunolocalized to the intercellul ar boundaries. Two substrates for SPARC-induced tyrosine phosphorylati on were identified as beta-catenin and paxillin. Inhibition of tyrosin e kinases with herbimycin A or genistein reversed the barrier dysfunct ion induced by SPARC by 71% and 49%, respectively. Herbimycin A also p rotected against SPARC-induced intercellular gap formation. In contras t, inhibition of tyrosine phosphatases with sodium orthovanadate or ph enylarsine oxide enhanced the loss of barrier function associated with SPARC treatment by 120% and 88%, respectively. These data indicate th at SPARC influences EC-EC interactions through a tyrosine phosphorylat ion-dependent signaling pathway. (C) 1998 Academic Press.