VCAM-1-mediated Rac signaling controls endothelial cell-cell contacts and leukocyte transmigration

Department of Experimental Immunohematology, Sanquin Research at CLB and Laboratory for Clinical and Experimental Immunology, Academic Medical Center, 1066 CX Amsterdam, The Netherlands Submitted 4 February 2003 ; accepted in final form 13 April 2003 Leukocyte adhesion is mediated totally and transendothelial migration partially by heterotypic interactions between the 1 - and 2 -integrins on the leukocytes and their ligands, Ig-like cell adhesion molecules (Ig-CAM), VCAM-1, and ICAM-1, on the endothelium. Both integrins and Ig-CAMs are known to have signaling capacities. In this study we analyzed the role of VCAM-1-mediated signaling in the control of endothelial cell-cell adhesion and leukocyte transendothelial migration. Antibody-mediated cross-linking of VCAM-1 on IL-1 -activated primary human umbilical vein endothelial cells (pHUVEC) induced actin stress fiber formation, contractility, and intercellular gaps. The effects induced by VCAM-1 cross-linking were inhibited by C3 toxin, indicating that the small GTPase p21Rho is involved. In addition, the effects of VCAM-1 were accompanied by activation of Rac, which we recently showed induce intercellular gaps in pHUVEC in a Rho-dependent fashion. With the use of a cell-permeable peptide inhibitor, it was shown that Rac signaling is required for VCAM-1-mediated loss of cell-cell adhesion. Furthermore, VCAM-1-mediated signaling toward cell-cell junctions was accompanied by, and dependent on, Rac-mediated production of reactive oxygen species and activation of p38 MAPK. In addition, it was found that inhibition of Rac-mediated signaling blocks transendothelial migration of monocytic U937 cells. Together, these data indicate that VCAM-1-induced, Rac-dependent signaling plays a key role in the modulation of vascular-endothelial cadherin-mediated endothelial cell-cell adhesion and leukocyte extravasation. human umbilical vein endothelial cells; vascular-endothelial cadherin; F-actin; reactive oxygen species; p38 mitogen-activated protein kinase; vascular cell adhesion molecule Address for reprint requests and other correspondence: P. L. Hordijk, Sanquin Research at CLB, Dept. of Experimental Immunohematology, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands (E-mail: p.hordijk{at}sanquin.nl ).