VCAM-1 IS INTERNALIZED BY A CLATHRIN-RELATED PATHWAY IN HUMAN ENDOTHELIAL-CELLS BUT ITS ALPHA(4)BETA(1) INTEGRIN COUNTER-RECEPTOR REMAINS ASSOCIATED WITH THE PLASMA-MEMBRANE IN HUMAN T-LYMPHOCYTES

Lymphocyte extravasation involves a step(s) of de-adhesion to allow tr ans-and subendothelial migration in response to inflammatory signals. We show here that ligated VCAM-1 was rapidly internalized (t(1/2) 14.5 min) in ECV 304 endothelial cells and in TNF-alpha-primed human umbil ical vein-derived endothelial cells (t(1/2) 11.2 min). The process req uired energy (ATP), intracellular Ca2+, an intact cytoskeletal network and active protein kinases. The internalization of VCAM-1 involved a clathrin-dependent pathway based on the observations that 1) it was in hibited in cells treated with lysosomotropic agents or with a hyperton ic concentration of sucrose, and 2) internalized VCAM-1 colocalized wi th clathrin. In contrast, the cross-linked alpha(4) beta(1) integrin c ounter-receptor of VCAM-1 remained associated with the plasma membrane of purified peripheral T and Jurkat cells. Our results suggest a mode l where VCAM-1 would initially participate in the retention of T cells to the endothelium by binding alpha(4) beta(1) integrin. Lymphocyte d e-adhesion would be facilitated as a result of the internalization of VCAM-1. The persistent cell surface expression of alpha(4) beta(1) int egrin would allow the migrating T cells to interact with and receive s ignal(s) from its fibronectin ligand of the extracellular matrix.