The Src-cortactin pathway is required for clustering of E-selectin and ICAM-1 in endothelial cells

Adhesion molecules such as E-selectin and intercellular adhesion molecule-1 (ICAM-1) expressed on endothelial cells (ECs) at sites of inflammation play an important role in the recruitment of leukocytes from the bloodstream into extravascular tissue. However, little is known about the signaling pathways that are initiated in ECs following adhesion molecule engagement. Here, we report that an 85-kDa protein becomes tyrosine phosphorylated in human ECs following leukocyte adhesion or upon antibody-induced clustering of E-selectin or ICAM-1. Through immunoprecipitation experiments, this protein was identified as cortactin, a cytoskeleton-binding molecule and prominent src substrate involved in cell adhesion. Following adhesion molecule clustering, cortactin phosphorylation was inhibited by the src family kinase inhibitor PP2. Both src and tyrosine-phosphorylated cortactin were found to be associated with E-selectin and ICAM-1 following adhesion of antibody-coated beads to ECs. PP2 did not inhibit the association of cortactin with E-selectin and ICAM-1; however, PP2 inhibited adhesion between paraformaldehyde-fixed THP-1 cells and ECs. This decrease in adhesion correlated with inhibition of adhesion molecule clustering on PP2-treated ECs at sites of THP-1 attachment. These findings implicate src and cortactin as mediators of leukocyte/EC interactions at sites of inflammation by regulating adhesion molecule clustering on ECs.