VCAM-1 is internalized by a clathrin-related pathway in human endothelial cells but its α4β1 integrin counter-receptor remains associated with the plasma membrane in human T lymphocytes

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Abstract
Lymphocyte extravasation involves a step(s) of de‐adhesion to allow trans‐ and subendothelial migration in response to inflammatory signals. We show here that ligated VCAM‐1 was rapidly internalized (t1/2 14.5 min) in ECV 304 endothelial cells and in TNF‐α‐primed human umbilical vein‐derived endothelial cells (t1/2 11.2 min). The process required 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 inhibited in cells treated with lysosomotropic agents or with a hypertonic concentration of sucrose, and 2) internalized VCAM‐1 colocalized with clathrin. In contrast, the cross‐linked α4β1 integrin counter‐receptor of VCAM‐1 remained associated with the plasma membrane of purified peripheral T and Jurkat cells. Our results suggest a model where VCAM‐1 would initially participate in the retention of T cells to the endothelium by binding α4β1 integrin. Lymphocyte de‐adhesion would be facilitated as a result of the internalization of VCAM‐1. The persistent cell surface expression of α4β1 integrin would allow the migrating T cells to interact with and receive signal(s) from its fibronectin ligand of the extracellular matrix.