Thrombospondin-1 induces tyrosine phosphorylation of adherens junction proteins and regulates an endothelial paracellular pathway

Thrombospondin-1 (TSP) induces endothelial cell (EC) actin reorganization a nd focal adhesion disassembly and influences multiple EC functions. To dete rmine whether TSP might regulate EC-EC interactions, we studied the effect of exogenous TSP on the movement of albumin across postconfluent EC monolay ers. TSP increased transendothelial albumin flux in a dose-dependent manner at concentrations greater than or equal to 1 mu g/ml (2.2 nM). Increases i n albumin flux were observed as early as 1 h after exposure to 30 mu g/ml ( 71 nM) TSP. Inhibition of tyrosine kinases with herbimycin A or genistein p rotected against the TSP-induced barrier dysfunction by >80% and >50%, resp ectively. TSP-exposed monolayers exhibited actin reorganization and interce llular gap formation, whereas pretreatment with herbimycin A protected agai nst this effect. Increased staining of phosphotyrosine-containing proteins was observed in plaque-like structures and at the intercellular boundaries of TSP-treated cells. Ln the presence of protein tyrosine phosphatase inhib ition, TSP induced dose- and time-dependent increments in levels of phospho tyrosine-containing proteins; these TSP dose and time requirements were com patible with those defined for EC barrier dysfunction. Phosphoproteins that were identified include the adherens junction proteins focal adhesion kina se, paxillin, gamma-catenin, and p120(Cas). These combined data indicate th at TSP can modulate endothelial barrier function, in part, through tyrosine phosphorylation of EC proteins.